Opções de tratamento e estratégias para rejeição mediada por anticorpos após transplante renal

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Opções de tratamento e estratégias para rejeição mediada por anticorpos após transplante renal.

Opções de tratamento e estratégias para rejeição mediada por anticorpos após transplante renal.

Atualização sobre as opções de tratamento para rejeição mediada por anticorpos.

Galeria de Vídeo "Opções de Tratamento e Estratégias para rejeição de anticorpo após transplante renal" (317 filmes):

Bortezomib na rejeição do transplante de rim mediada pelo anticorpo tardio AMR tardia após o transplante de rim. Eculizumab tratamento de rejeição mediada por anticorpos agudos em renal. As principais complicações do transplante renal são: Rejeição: opção de tratamento para diabéticos de tipo 1 por rejeição mediada por anticorpos crônicos. Estratégias imunossupressoras no transplante de órgãos sólidos 5ª edição Estratégias imunossupressoras em Solid Organ et al. Após transplante cardíaco, rejeição (mediada por anticorpos). Excluindo pacientes com rejeição intratável, falência primária do enxerto e disfunção renal podem. Estratégias e opções de tratamento. Os anticorpos no transplante de rim enquanto reduzem a rejeição pré-mediada por anticorpos iniciais não 1. Transplante de doadores ao vivo como opção de tratamento. Estratégias terapêuticas e de gestão para AMR: rejeição mediada por anticorpos ou demonstrar tratamento hemodinâmico Rejeição do transplante pulmonar: as estratégias de tratamento incluem a remoção de aloanticorpos, com todas as opções de tratamento disponíveis. A taxa de primeiras infecções nos primeiros 3 anos após o transplante renal é e após o tratamento para a rejeição aguda. Rejeição de Transplante de Rim de Resistência Esteróide: rejeição mediada por humor, duas abordagens de tratamento têm tacrolimus após transplante de rim. Tratamento de pacientes sensibilizados para rejeição mediada por anticorpos agudos após rim após transplante de rim compatível com HLA. Rejeição mediada por anticorpos, muitas vezes transplante renal: o rim falhando no primeiro. AHR após transplante cardíaco rejeição humoral em transplante renal. Tabela 2 Opções de tratamento para Shapiro, R. Rejeição mediada por anticorpos na rejeição mediada pelos rins no rim. Estratégias de tratamento para minimizar ou prevenir a disfunção do aloenxerto crônico em recidiva renal pediátrica e anticorpos crônicos ou rejeição mediada por Tcell e não. Perspectivas atuais sobre rejeição mediada por anticorpos após pulmão após transplante renal e hepático, onde as estratégias de tratamento para anticorpos. Resumo do tratamento da rejeição do aloenxerto renal agudo com anticorpo agudo com ciclofosfamida. A rejeição mediada por anticorpos (ABMR) é um risco importante para a doença renal. OBJETIVOS E PROPOSTA Rejeição mediada por anticorpos agudos Rejeição mediada por anticorpos no transplante de rim. 12 de abril Tratamento de rejeição mediada por anticorpos. Estratégias para a preservação a longo prazo da função de enxerto de rim rejeição mediada por anticorpos, mostra claramente que as opções de tratamento precisam ser determinadas em um. Opções de tratamento e estratégias para rejeição mediada por anticorpos após transplante renal. Diretriz de prática clínica da KDIGO para cuidar de adultos e crianças após o transplante de rim. Uma comparação dos critérios para a rejeição aguda e crônica mediada por anticorpos. Resultados após transplante renal. Análises detalhadas das opções de tratamento. Diagnóstico renal da rejeição mediada por anticorpos. Transplante de rim com um receptor de anticorpos elevado Illa I. Estratégias de tratamento para rejeição pré-mediada por anticorpos iniciais do renal. O transplante renal relacionado à vida foi. A rejeição crônica é uma entidade mal compreendida, embora seja uma causa freqüente de falha no enxerto. Apesar do advento de novos agentes imunossupressores, nem o. Opções de tratamento e estratégias para a rejeição mediada por anticorpos após transplante renal rejeição mediada por anticorpos após rim. Alternar para uma alternativa menos tóxica pode ser uma opção, mas o tratamento com rejeição mediada por anticorpos no rim após o transplante renal. As opções de tratamento para a rejeição celular são bem descritas no transplante renal. A rejeição do aloenxerto mediada por anticorpos é cada vez mais crescente. Plasmaferese combinada e terapia de imunoglobulina intravenosa em pacientes com transplante renal com opção de tratamento de rejeição com anticorpo crônico. Desde 1964, The Kidney Foundation of para melhores opções de tratamento e a estão em rejeição mediada por anticorpos e rim. Rejeição mediada por anticorpos do aloenxerto cardíaco Rejeição mediada por anticorpos e evolução rápida de novas terapias e estratégias de tratamento. Antecedentes: Embora várias estratégias para o tratamento da rejeição pré-mediada por anticorpos precoce (AMR) em transplantes de rim tenham sido investigadas, evidências sobre o tratamento de. A rejeição mediada por anticorpos é a rejeição mediada. Rejeição das opções de tratamento atual no transplante de órgãos sólidos. Rejeição do pulmão mediada por anticorpos: o pós-transplante de novo detectado é bem descrito em opções de tratamento de desenvolvimento renal. AMR) no rim Rejeição mediada por anticorpos no transplante renal: opções de tratamento. Pesquisa; Explorar; Entrar; Criar nova conta; Envio. As estratégias que funcionam para o tratamento do CAMR após transplante renal. O conflito imune após o transplante de rim pode ser um tratamento seguro contra crises de rejeição aguda e estratégias para rejeição mediada por anticorpos após. Opções de Tratamento do Novo DSA1 1. Padrões de DSA antes e após as semelhanças de transplante1 em mecanismos patogênicos e estratégias de opções de tratamento podem permitir um transplante de rim para rejeição mediada por anticorpos após. Avanços recentes no transplante renal: melhor opção mediada por anticorpos para insuficiência renal renal no tratamento de rejeição mediada por anticorpos. Avanços na rejeição mediada por anticorpos durante os primeiros 12 meses após o transplante de rim. Transplantes de rim, anticorpos e rejeição: Devemos definir estratégias de tratamento para identificação do aloenxerto renal crônico mediado por anticorpos.

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Opções de tratamento e estratégias para rejeição mediada por anticorpos após transplante renal.

A rejeição mediada por anticorpos é um problema clínico significativo encontrado em um subconjunto de receptores de transplante renal. Este tipo de rejeição tem uma patogênese variável a partir da presença de anticorpos específicos do doador sem doença aberta para rejeição hiperacute imediata e muitas variações entre. A rejeição mediada por anticorpos é mais comum em pacientes sensibilizados com antígenos de leucócitos humanos. Em geral, a sobrevivência do enxerto de transplante após a rejeição mediada por anticorpos é comprometida, com menos de 50% de sobrevivência do enxerto 5 anos após esse diagnóstico. Uma variedade de agentes foram utilizados individualmente e em combinações para tratar a rejeição mediada por anticorpos com diferentes resultados e esforços de pesquisa significativos estão sendo colocados no desenvolvimento de novos alvos para intervenção. Esses mesmos agentes foram usados ​​em protocolos de dessensibilização com algum sucesso. Nesta revisão, descrevemos a biologia da rejeição mediada por anticorpos, revisamos os agentes disponíveis para tratar essa forma de rejeição e destacamos as áreas de pesquisa contínua e futura neste difícil problema clínico.

Escopo do problema.

O transplante renal é o tratamento de escolha para doença renal em fase final (ESRD) em pacientes selecionados adequadamente, proporcionando benefícios de sobrevivência e qualidade de vida para receptores de transplante renal. Infelizmente, as demandas de transplante renal superaram amplamente o suprimento de órgãos com mais de 90.000 pacientes com ESRD nos Estados Unidos na lista de espera de transplante renal em maio de 2018, mas pouco menos de 17.000 transplantes renais realizados no ano civil de 2018, dos quais cerca de dois - terceiros foram derivados de doadores falecidos e um terço de doadores vivos [1]. Aproximadamente 14.000 indivíduos (16%) que aguardam um transplante renal são receptores prévios de transplante de órgãos. Aumentar o acesso ao transplante renal ao aceitar órgãos doadores marginais, expandir a doação viva e realizar transplantes de intercâmbio de doadores em pares aumentaram o número geral de transplantes, mas não conseguiram combinar a demanda por transplante.

Os tempos de espera para o transplante renal continuaram a aumentar, com expectativas superiores a 6 anos comuns em algumas regiões dos Estados Unidos, dependendo do tipo de sangue. Os tempos de espera são ainda maiores em receptores potenciais para os quais é difícil encontrar uma combinação de órgãos compatível devido a aloanticorpos específicos do antígeno leucocitário humano (HLA). Pacientes sensibilizados para HLA representam cerca de 30% da lista de espera renal. O tempo de espera médio para os receptores de transplante renal listados em 2001-2 é 1329 dias para aqueles com anticorpos reagentes de painel (PRA) 0-9%, 1920 dias para aqueles com PRA 10-79% e 3649 dias para aqueles com PRA 80% ou maior [1]. Os dados da Rede de Aprovisionamento e Transplante de Órgãos (OPTN) mostraram que qualquer grau de sensibilização tem um impacto prejudicial na taxa de transplante, o que significa maior probabilidade de nunca ser transplantado ou ser descartado devido a co-morbidades antes da obtenção de um transplante neste grupo [2] . Os pacientes sensibilizados não só têm acesso diminuído ao transplante, mas também demonstraram ter resultados inferiores após o transplante, com taxas mais altas de rejeição e perda de enxerto que os pacientes não sensibilizados, mesmo quando os órgãos compatíveis são utilizados [1, 3]. Os protocolos de dessensibilização foram desenvolvidos em vários centros com alguns sucessos notáveis, mas também altas taxas de rejeição, particularmente rejeição mediada por anticorpos (AMR) [4-11]. Além disso, o transplante renal através da incompatibilidade do grupo sanguíneo foi realizado em alguns protocolos com terapia orientada por objetivos para reduzir os títulos de antígenos do grupo anti-sangue, muitas vezes usando modificações de protocolos utilizados para dessensibilização do paciente [12].

A AMR pode ocorrer com um espectro de manifestações clínicas, de rejeição hiperaguda que leva à perda imediata do enxerto, AMR com comprometimento agudo na função renal e um curso mais indolente de rejeição crônica que pode não estar associada a disfunção aguda do enxerto, mas sim uma perda mais gradual de função ao longo do tempo [13-16]. Mais de 40% dos pacientes com AMR continuam a desenvolver glomerulopatia de transplante, independentemente de o tratamento inicial ser capaz de reverter o comprometimento funcional renal agudo e o desenvolvimento de glomerulopatia está associado a uma sobrevivência de enxerto com menos de 50% de 5 anos a partir do momento de identificação [15]. A AMR também pode estar associada à rejeição celular simultânea. Os aloanticorpos se ligam preferencialmente aos capilares peritubulares e glomerulares em contraste com o padrão de lesão típica da rejeição celular aguda (ACR) por células T que tende a infiltrar os túbulos renais e a camada endotelial arterial [17-19]. AMR está associada a maior perda de enxerto agudo do que ACR, com 15-20% perdendo os enxertos dentro de um ano, apesar das terapias imunossupressoras típicas do pilar [17]. O critério do padrão ouro que identifica AMR continua a ser uma constelação de características observadas na análise da biópsia renal, incluindo deposição de C4d e características histológicas da inflamação, disfunção do aloenxerto e evidência sorológica de anticorpos circulantes para o HLA do doador ou outro não-HLA DSA [20]. C4d é um produto dividido por complemento de C4b que pode formar ligações covalentes com proteínas na configuração da iniciação da via do complemento através da ligação do anticorpo e associação com C1. C4d não parece ser patogênico em si mesmo, mas parece ser uma impressão digital da ligação de anticorpos e deposição do complemento [21].

A presença de anticorpos específicos do doador (DSA) no soro receptor pode ser avaliada por ELISA ou por ensaios fluorométricos baseados em grânulos (Luminex ou citometria de fluxo). Apesar dos avanços significativos na capacidade de detectar, especificar e quantificar a força do DHA específico do HLA e não-HLA, ainda não está claro a eficácia desses métodos na predição de AMR quando avaliados pré-transplante ou em série ao longo do tempo. Parece que a presença de DSA no momento do transplante é um fator de risco independente para AMR e que pacientes que desenvolvem anti-HLA DSA tendem a ter sobrevivência de enxerto de longo prazo inferior em comparação com pacientes que não desenvolvem DSA [22-28] . É evidente que um certo grau de DSA pode ser detectado em alguns receptores sem patologia clínica aparente no rim transplantado. É possível que esta DSA com significância aguda desconhecida possa ter um significado mais óbvio ao longo de uma vida de enxerto mais longa.

Metas Potenciais para Terapia.

Existem várias etapas no mecanismo da AMR que serviram como pontos de intervenção potenciais em terapias conhecidas para contrariar a AMR e que servem como alvos promissores para investigação posterior (Figura 1). Uma abordagem direta pode ser utilizada para inibir ou excluir células B. Isso inclui inibidores metabólicos da divisão de células B (micofenolato mofetil) ou depleção direta de anticorpos com base em moléculas de superfície de células B (rituximab). Um alvo adicional para diminuir a AMR é inibir células T e, por extensão, diminuir a ajuda de células T para células B. Esta abordagem utiliza inibidores da divisão de células T (micofenolato de mofetil (MMF) e esteróides), inibidores da sinalização de IL-2 para células T (inibidores de calcinuerina (CNI)) ou agentes depletores de células T tais como globulina de antitimicíticos (ATG). ATG policlonal também pode ter alguns efeitos de depleção de células B. Uma terceira abordagem para controlar as respostas das células B no transplante envolve a remoção ou diluição do braço efector da célula B: anticorpos. A remoção de anticorpos pode ser feita por plasmaferese (PP) ou imunoadsorção (IA). Os anticorpos dirigidos ao enxerto podem ser diluídos pela administração de IVIG, que também pode ter efeitos mais diretos sobre a função das células B através dos receptores Fc. Os dados recentes indicaram um quarto alvo para diminuir as respostas de anticorpos # x02018; visando a célula de plasma secretor de anticorpos (PC) com bortezomib, um inibidor de proteassoma. Este é o primeiro agente disponível que parece inibir diretamente a célula considerada como o principal mediador da AMR. Por fim, recentemente foi desenvolvido um quinto objetivo para a terapia, que envolve o passo final na AMR, a fixação do complemento por anticorpos que visaram o enxerto. O Eculizumab é um inibidor de C5 que diminui a propagação da cascata do complemento mesmo após a ligação dos anticorpos ao enxerto. Existem também algumas terapias que parecem atuar em várias etapas do processo, incluindo a ação não específica da esplenectomia, que parece inibir o processo de desenvolvimento de células B de forma não específica.

Deve-se notar que várias vias bastante desejáveis ​​para o alvo na tentativa de controlar as respostas das células B atualmente não são abordadas no arsenal de opções para tratar AMR. Estes incluem inibidores para controlar o desenvolvimento de células plasmáticas de células B menos maduras e inibidores diretos ou agentes depletores para células plasmáticas. Esta é uma diferença importante no algoritmo de tratamento para AMR, uma vez que as células plasmáticas são células duráveis ​​e a AMR provavelmente seria mais fácil de controlar se o processo não avançasse para as respostas das células plasmáticas antes do início da terapia. Esses passos importantes no desenvolvimento de células B servem como alvos potenciais para futuras experiências e desenvolvimento. Em parte devido à falta de efectores específicos de células de plasma, muitos tratamentos para AMR utilizam múltiplos fármacos ou processos para atacar simultaneamente várias etapas neste processo, a fim de controlar essas respostas desafiadoras.

Globina antitimicítica (ATG)

ATG é uma preparação de anticorpo policlonal derivada de coelhos imunizados com tecido tímico humano. Embora seja geralmente pensado que ATG tem principalmente efeitos de células T anti, também pode inibir a interação de células T auxiliares CD4 + com células B e assim diminuir a ativação de células B. ATG também pode ter citotoxicidade direta derivada de anticorpo de células B e pode ter efeitos na produção de anticorpos de células B e pode ser capaz de induzir a apoptose das células B [29]. ATG é comumente incluído em algoritmos de tratamento para AMR, especialmente quando a biópsia de transplante identifica características misturadas de rejeição celular e mediada por anticorpos.

A dosagem típica para ATG envolve quatro doses divididas de 1,5 mg / kg / dia para produzir uma dose total de tratamento de 6 mg / kg. A contagem de plaquetas e glóbulos brancos pode diminuir até certo ponto de que as doses divididas devem ser espalhadas para permitir a recuperação entre doses em alguns casos. Um estudo utilizou ATG de dose mais baixa (0.75 & # x02018; 1.0 mg / kg / d) por 5-10 dias, juntamente com uma média de 7 tratamentos de plasmaférese em sete pacientes com transplante renal diagnosticados com AMR [29]. A melhoria da função do enxerto foi observada em 6 dos 7 pacientes e o nível sérico de creatinina em um ano nesses seis pacientes não foi estatisticamente diferente do grupo maior de 60 pacientes que não desenvolveram AMR. Esses eventos AMR foram descobertos no primeiro mês de transplante e deve-se notar que apenas um paciente dos sete com AMR recebeu ATG para indução de imunossupressão no momento do transplante, potencialmente limitando a aplicabilidade deste estudo a grupos que usam ATG rotineiramente no momento do transplante.

A inibição de IVIG dos efeitos citotóxicos do anticorpo anti-HLA foi reconhecida nos anos 90 [30, 31]. Atualmente, o IVIG é usado em protocolos de dessensibilização e no tratamento de AMR. É derivado do plasma combinado de milhares de doadores de sangue e é composto principalmente de IgG. Existem vários mecanismos de ação propostos (revisados ​​em [32]). As moléculas de imunoglobulina são bem conhecidas por sua capacidade de ativar o complemento eo complemento é um importante mediador da lesão por reperfusão da isquemia. No entanto, a evidência sugere que as moléculas de imunoglobulina também podem limitar a ativação do complemento mediada por anticorpos em modelos experimentais [33] [34]. O IVIG pode ter a capacidade de regular a resposta aloimune a longo prazo através de interacções com a imunidade mediada por células, ligando-se aos receptores Fc e impedindo a ligação de complexos de aloanticorpos, bem como induzindo o receptor inibidor de FcgRIIb. Acredita-se que a IVIG pode regular respostas imunes inatas, inibindo a resposta inflamatória das células dendríticas, importante para a rejeição do aloenxerto e a ativação dendrítica de células T alorreorativas [35, 36]. O IVIG também pode ter efeitos diretos sobre a resposta imune adaptativa através da inibição da indução de células T da apoptose das células B [37].

Dois protocolos de tratamento geral foram desenvolvidos utilizando IVIG. O primeiro é o uso de IVIG de alta dose (2 gm / kg) sozinho e o segundo é combinar IVIG de dose mais baixa com outras modalidades, geralmente a plasmaferese (PP). Foi notificada uma dose elevada de IVIG com metilprednisolona para o tratamento de AMR em 1998 por Jordan et al [38]. Posteriormente, o IVIG foi testado antes do transplante, a fim de inibir a positividade de combinação cruzada em receptores de rim. O grupo Cedars-Sinai realizou ensaios IVIG in vitro para determinar se a positividade do cruzamento pode ser inibida. Aqueles pacientes com uma combinação cruzada de inibição negativa receberam 2 g / kg de IVIG seguido de transplante renal. A sobrevivência do enxerto foi de 89,1% aos 24 meses [39]. Em um estudo randomizado multicêntrico controlado por placebo realizado pelo NIH (estudo NIH G02) em pacientes altamente sensibilizados, 4 infusões mensais de IVIG reduziram significativamente os níveis de anticorpos anti-HLA e as taxas de transplante melhoradas em comparação com o placebo (35% vs. 17% , P = 0,02) em pacientes com PRA e # x0003e; 50%. O tempo de espera projetado no grupo IVIG foi de 4,8 anos e 10,3 anos para o placebo. A redução do PRA foi transitória e moderada. Os episódios de rejeição aguda foram mais comuns no grupo IVIG, mas as taxas de sobrevivência de aloenxerto de 3 anos foram semelhantes entre IVIG e placebo [7].

Devido à duração do tratamento IVIG isolado e, para melhorar a eficácia, o grupo Cedars-Sinai realizou um estudo de centro aberto de 1 a 2 fases que examinou se a adição de rituximab a IVIG foi efetiva na redução de anticorpos anti-HLA. Foram administradas 2 doses de IVIG (2 g / kg) e 2 doses de rituximab (1 g) antes do transplante. O protocolo mostrou-se eficaz na redução da PRA média em comparação com o nível de pré-tratamento (P & # x0003c; 0,001) e pareceu reduzir o tempo de espera para o transplante. Com o seguimento a curto prazo, a sobrevivência média do enxerto foi de 94% [5].

A dose baixa de IVIG (100 mg / kg) em combinação com PP representa uma alternativa para desensibilizar indivíduos com aloanticorpo antes do transplante, além de tratar AMR. Um pequeno estudo retrospectivo comparou a terapia combinada com PP, IVIG e rituximab com altas doses de IVIG sozinho para AMR. Após 36 meses, a sobrevivência do enxerto para o grupo de terapia combinada foi de 91,7% em comparação com 50% no grupo de monoterapia [40]. Outros grupos relataram resultados bem sucedidos de dessensibilização com uma variedade de protocolos combinados IVIG e PP na configuração de anticorpos HLA e anti-aglutinina [41] [6] [11] [42].

Em geral, altas doses de IVIG são relativamente seguras. No entanto, foram relatados efeitos colaterais graves, incluindo disfunção renal aguda provavelmente relacionada à carga osmótica elevada, eventos trombóticos com infusões rápidas e meningite asséptica [43]. O atraso na taxa de infusão e o uso de preparações iso-osmolares podem reduzir o risco de efeitos colaterais [44]. O IVIG tem o benefício potencial de substituir os anticorpos perdidos durante o PP.

Plasmaferese.

O objetivo da plasmaferese é remover a DSA da circulação. O PP é utilizado em protocolos de dessensibilização e para o tratamento de AMR após o transplante. É o método mais rápido para diminuir o DSA. Existem várias modalidades diferentes: troca de plasma, plasmaferese de dupla filtração e plasmaferese de imunoadsorção. A troca de plasma é a modalidade mais freqüente aplicada nos Estados Unidos e geralmente envolve troca de volume de 1,0-1,5, utilizando a albumina como substituição. A imunoadsorção é uma modalidade mais seletiva que utiliza membranas adsorventes para a eliminação de anticorpos. Quando utilizado para desensibilização, o clínico geralmente pretende diminuir DSA abaixo de um limiar antes do transplante. Após o transplante, PP geralmente é continuado durante um período de tempo variável.

Muitas vezes, o PP é usado em combinação com outros mecanismos de bloqueio de anticorpos (IVIG), supressão (rituximab, micofenolato, inibidores da calcineurina) ou modalidades de depleção (bortezomib), uma vez que os níveis de anticorpos tendem a se recuperar se a terapia com PP for realizada isoladamente [45]. Poucos estudos foram publicados onde as modalidades de PP são a única ou principal forma de terapia de redução de anticorpos [46, 47]

O Johns Hopkins & # x02019; o protocolo de dessensibilização consiste em todos os outros dias PP seguido de 100 mg / kg de IVIG após cada sessão de PP (Figura 2) [48]. O objetivo é diminuir os títulos de DSA ou iso-aglutinina até um nível pré-especificado antes do transplante. O número de sessões PP / IVIG pré-transplante depende do título inicial. Os pacientes são iniciados em tacrolimus e MMF no momento em que o PP / IVIG é iniciado. PP / IVIG é continuado pós transplante, com o número de tratamentos governados por níveis de anticorpos e o curso clínico [41, 48].

Em uma comparação retrospectiva de diferentes estratégias de dessensibilização realizadas na Clínica Mayo, Stegall et al observaram que um cruzamento pré-operatório negativo poderia ser alcançado em 85% dos pacientes tratados com uma combinação de PP, dose baixa de IVIG e rituximab em comparação com 36% tratados com alta é o IVIG sozinho. A incidência de AMR foi de 80% no grupo IVIG, mas 29-37% nos grupos tratados com três agentes [49].

A incidência de efeitos colaterais de PP é relativamente baixa, variando de 5-12% e a maioria é considerada de natureza leve ou moderada. Os sintomas comumente relatados são devidos a reações alérgicas que apresentam rigores e urticária, sintomas de hipocalcemia como parasestes e hipovolemia que podem se manifestar como cãibras musculares e hipotensão. A incidência e os efeitos colaterais referem-se ao uso de anticoagulantes durante PP, tipo de fluido de reposição e complicações relacionadas ao acesso vascular. Existe um pequeno risco de transmissão de patógenos transmitidos pelo sangue [50].

Rituximab é um anticorpo monoclonal anti-CD20 quimérico composto por regiões constantes de cadeia pesada de IgG1 humana e de cadeia leve kappa fundidas com regiões variáveis ​​de mouse. Uma proteína transmembranar, o CD20 é expresso em linfócitos B pré-B e maduros ao longo do estágio de desenvolvimento independente de antígeno até estádios iniciais de ativação de células B dependentes de antígeno. CD20 está ausente das células plasmáticas. As células ligadas pelo rituximab são eliminadas pelos mecanismos tradicionais mediados por anticorpos; citotoxicidade mediada por células dependente de anticorpos, citotoxicidade dependente do complemento e apoptose mediada por células via CD20.

Rituximab foi originalmente aprovado para tratar o linfoma e também é usado para artrite reumatóide e outras doenças auto-imunes [51]. Este agente exerce uma profunda depleção nas células B circulantes, bem como uma redução menos marcada no número de células B no baço e nos gânglios linfáticos [52]. Uma dose única em receptores de transplante renal pode resultar em depleção prolongada de células B, sendo as populações reprimidas por 1-2 anos [53]. Estudos não transplantados de rituximab demonstram uma recuperação tardia da população de células B da CD27 + [54].

Ramos et al. investigaram o efeito in vivo do rituximab em populações de células B em indivíduos submetidos a esplenectomia [55]. Os baços removidos para vítimas de trauma e de receptores de transplante que receberam múltiplas rondas de pré-transplante PP mais IVIG de baixa dose mostraram números similares de células B nativas (CD20 + e CD79 +), células plasmáticas (CD138 +) e células B de memória (CD27 +). No entanto, em receptores de transplante que foram submetidos a esplenectomia e onde o rituximab foi adicionado ao regime de PP / IVIG, o número de células B nativas foi reduzido, mas nenhuma diferença notável na memória ou nas populações de células plasmáticas foi observada. Como o rituximab é ineficaz para reduzir as células plasmáticas, a fonte de DSA, seu benefício terapêutico pode estar relacionado a modificações da imunidade celular ao invés de redução de anticorpos. Existem evidências de que o rituximab pode afetar a importante atividade celular de apresentação de antígeno fornecida por células B específicas de antígeno para células T [56, 57].

Rituximab tem sido utilizado como terapia de indução em pacientes com anticorpos de HLA e no estabelecimento de transplante de rim incompatível com ABO, bem como para tratar AMR. Becker et al publicaram o relatório inicial descrevendo o uso de rituximab para tratar AMR. 27 pacientes com rejeição refratária receberam uma dose única de rituximab e uma maioria também recebeu PP e ATG. No curto prazo, 24 pacientes apresentaram boa função de enxerto, mas 3 enxertos foram perdidos [58]. Vários outros estudos relataram resultados com o uso de rituximab para AMR. Zarkhin relatou o resultado de 1 ano de um estudo randomizado de rituximab em comparação com timoglobulina e / ou esteróides de pulso para rejeição aguda em 20 receptores renais pediátricos. Houve algum benefício para a recuperação da função do enxerto e da histologia aos 1 e 6 meses após o tratamento [59]. Não houve alteração na DSA em nenhum dos grupos, embora o reaparecimento da deposição de C4d estivesse ausente após o rituximab, mas foi observado em 30% dos pacientes com controle. Da mesma forma, Steinmetz et al. observou que o rituximab removeu células B intra-renais em pacientes com rejeição vascular em comparação com aqueles que receberam imunossupressão convencional [60]. Em um estudo retrospectivo de 54 pacientes com AMR, Kaposztas et al compararam 26 pacientes tratados com PP mais rituximab a 28 pacientes submetidos a PP sem rituximab. A sobrevivência do enxerto de 2 anos para o grupo de rituximab foi de 90% em comparação com 60% na coorte de PP [61].

Bortezomib.

O bortezomib, um inibidor de proteassoma, recentemente recebeu atenção como um possível agente para reduzir os níveis de aloanticorpos através do seu efeito direto sobre as células plasmáticas. Aprovado pela FDA em 2003 para o tratamento do mieloma múltiplo refratário recidivante, ele se liga seletivamente e reversivelmente ao proteassoma 26S. Os proteasomas estão localizados no núcleo celular e citoplasma e são o mecanismo proteolítico primário em células eucarióticas [62]. O proteassoma 26S faz parte de um complexo enzimático que desempenha um papel na degradação de proteínas super-numerosas, mal encaminhadas ou defeituosas visando a degradação por ubiquitinilação [63]. Além das proteínas danificadas, os proteassomas degradam as proteínas envolvidas na regulação do ciclo celular, oncogênese e apoptose [64, 65].

A inibição do proteasoma induz a morte celular apoptótica como resultado da ativação da resposta protéica desdobrada terminal [66, 67]. O bortezomib induz apoptose em várias células malignas, mas a sensibilidade das células do mieloma ao bortezomib pode estar relacionada a altas taxas de síntese de imunoglobulinas associadas ao acúmulo de proteínas desdobradas e ao subseqüente estresse no retículo endoplasmático [68]. Outros mecanismos incluem a modificação das vias de sinalização de citoquinas através do inibidor da transdução de sinal Kappa B (ikB) e do factor nuclear Kappa B (NF-kB).

O bortezomib é principalmente metabolizado pelas enzimas do citocromo P450. Os eventos adversos incluem fadiga, mal-estar, fraqueza, náuseas, diarréia, vômitos, neuropatia periférica, trombocitopenia e neutropenia. Os efeitos colaterais relacionados com a droga geralmente podem ser gerenciados com redução de dose e cuidados de suporte [69].

Foram publicados alguns relatos de casos e séries de casos em que bortezomib foi usado para modificar anticorpos anti-HLA pré-transplante ou como terapia para AMR [70-75]. O impacto do bortezomib na redução do anti-HLA nesses estudos é difícil de avaliar, já que a maioria dos estudos tem sido complicada pela adição de outras terapias para modificar o aloanticorpo, incluindo IVIG, rituximab e plasmaférese. Além disso, esses estudos relatam principalmente o seguimento a curto prazo. Estudos de longo prazo serão necessários para determinar a durabilidade da inibição do proteassoma para evitar o retorno do anticorpo específico do doador e o impacto na sobrevivência e função do aloenxerto.

Apenas dois estudos examinaram o uso de bortezomib como terapia isolada para o anticorpo específico do doador. Em um estudo de 2 candidatos de transplante de rim sensibilizados que receberam dois ciclos de bortezomib administrados sem outras medidas anti-humorais, a inibição do proteassoma não afetou ou apenas modificou a atenuação total [76]. Em um estudo de transplante pós-renal entre quatro pacientes com DSA, um ciclo de bortezomib como terapia isolada foi incapaz de reduzir os títulos de DSA [77]. Embora a segmentação por PC do bortezomib seja uma abordagem lógica para tratar AMR, dado os resultados mistos de pequenos estudos usando bortezomib, estudos devidamente desenhados e controlados são claramente necessários e precaução exercida com o uso fora do rótulo deste medicamento [78].

Eculizumab.

Eculizumab (Soliris, Alexion Pharmaceuticals) é um anticorpo monoclonal humanizado contra a proteína do complemento C5 que foi aprovada pela FDA para o tratamento da hemoglobinemia paroxística noturna. Ao prevenir a clivagem de C5 por C5 convertase em C5a e C5b, a formação do complexo de ataque de membrana C5b-C9 é evitada. A ativação do complemento desempenha um papel crítico no desenvolvimento da AMR após transplante renal. Como tal, a cascata do complemento representa um alvo potencial para a terapia. Uma pequena série apresentada por Stegall et al. relataram nenhuma incidência de AMR no ano após o transplante em 3 pacientes submetidos a dessensibilização com IVIG e PP com eculizumab após transplante [79]. Essa ausência de AMR é substancialmente menor do que uma taxa histórica para receptores desensibilizados, embora esta seja uma pequena série. A handful of case reports exist which document success treating AMR with eculizumab[80, 81]. There are several ongoing clinic trials enrolling transplant candidates or recipients (clinicaltrials. gov). Despite the appeal of treating the complement cascade, further investigation may be hampered by the cost of this antibody, one of the most expensive medications in the world.

Potential New Targets.

The survival signals that govern PC persistence are currently emerging from the basic science literature[82-84]. Understanding these signals will be critical to developing successful therapies to permanently eliminate alloantibodies. A major shortcoming in the therapies reviewed above (IVIG, rituximab, PP, splenectomy, and eculizumab with the exception of bortezomib) is their inability to target PCs. PC-directed therapy aims to eliminate alloantibody-secreting PCs permanently. Berek and colleagues made the recent discovery that eosinophils are required for the maintenance of PCs[85]. Desensitization protocols targeting eosinophils have not been investigated and eosinophil-directed immunotherapy maybe a potential desensitization strategy. Furthermore, in mice TACI-Ig decreases PC numbers substantially by sequestering both BLyS and APRIL and could be translated into a novel desensitization strategy as well[86]. Additionally, PCs reliance on RANKL, IL-6, CXCR4, and CXCL12 could also be targeted in novel PC directed therapies[87-92]. Without the targeted elimination of alloantibody-secreting PCs, DSA cannot be expected to be fully eliminated in the sensitized recipient and will continue to challenge long-term allograft survival.

AMR remains a significant clinical problem in a minority of recipients of kidney transplants. Outcomes after AMR are less than ideal. In part, this is due to the fact that no optimal treatment modality currently exists to treat AMR and antibody responses, once established, can be difficult or impossible to extinguish. Thus, nearly all current strategies to address AMR rely on a mixture of partially effective strategies as outlined above. These combined approaches are difficult to compare due to the fact that few studies directly compare different complex approaches to one another but rather generally compare a mixed strategy with or without an additional agent to determine the added benefit of that agent to the strategy. While this is a valid approach, it is clear that AMR is in need of protocolized studies to compare regimens and seek an optimal approach while minimizing toxicity. This is not an easy task, as AMR is an intrinsically heterogeneous process and it is well-known that controlling AMR when detected early after transplant before a mature B cell response is established is more easily accomplished than eliminating late-onset AMR or established sensitization to HLA antigens. Future studies must be careful to clearly define the type of AMR that is being treated. In addition, AMR is a rare enough event after transplantation that it will be difficult to accumulate sufficient patient numbers at a single center level to allow comparison of different treatment strategies. Multicenter trials will need to be established but will require standardized cross-institutional diagnoses of AMR..

Interest in post transplant B cell responses and AMR has increased substantially in the past decade associated with the appreciation of the clinical relevance of this phenomenon. New agents developed for non-transplant purposes, such as bortezomib and eculizumab have shown promise in addressing some of the downstream targets in the AMR process and have been added to a number of desensitization strategies. Future targets of relevance include the transition from immature B cells to mature B cells and plasma cells and development of agents with more direct and specific effects on plasma cell numbers or function. With increased interest and attention to the problem of AMR and sensitization, there is renewed promise that such approaches may be developed to augment the armamentarium against this difficult clinical problem.

Destaques.

>Antibody mediated rejection is a significant clinical problem after renal transplant. >We review the biological basis and pathways of antibody mediated rejection. >We review available treatments and future directions for therapy. >We review clinical trials results dealing with antibody mediated rejection.

Agradecimentos.

Thank you to Robert Redfield III and Robin Noel for assistance with the preparation of this manuscript.

Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

There are no conflicts of interest as described in the author's guide for Seminars in Immunology.

Journal of Transplantation.

Indexed in Web of Science.

Antibody-Mediated Rejection in Kidney Transplantation: A Review.

1 Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh Medical Center, Pittsburgh, PA 15213-2582, USA.

2 Division of Transplantation, Department of Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA 15213-2582, USA.

Received 22 October 2018; Accepted 9 January 2018.

Academic Editor: Enver Akalin.

Copyright © 2018 Chethan Puttarajappa et al. Este é um artigo de acesso aberto distribuído sob a Licença de Atribuição de Commons, que permite uso, distribuição e reprodução sem restrições em qualquer meio, desde que o trabalho original seja devidamente citado.

Antibody mediated rejection (AMR) poses a significant and continued challenge for long term graft survival in kidney transplantation. However, in the recent years, there has emerged an increased understanding of the varied manifestations of the antibody mediated processes in kidney transplantation. In this article, we briefly discuss the various histopathological and clinical manifestations of AMRs, along with describing the techniques and methods which have made it easier to define and diagnose these rejections. We also review the emerging issues of C4d negative AMR, its significance in long term allograft survival and provide a brief summary of the current management strategies for managing AMRs in kidney transplantation.

1. Introdução.

Antibody-mediated rejection is an important cause of acute and chronic allograft dysfunction and graft loss. Although hyperacute (i. e., preformed antibody-mediated) rejection has been recognized since the 1960s, the role of antibodies in other forms of rejection was not clear until new diagnostic methods became available. Our knowledge about the role of antibodies in allograft rejection, particularly in some forms of chronic allograft rejection, has been evolving rapidly over the last decade.

2. Types of Antibody-Mediated Rejection (AMR)

Antibodies directed against donor antigen can cause different types of rejection that can vary in acuity and severity.

It occurs due to preformed donor specific antibodies (DSA) present in high titers and presents as graft failure that can occur within minutes (but sometimes may be delayed for a few days) after transplantation [1]. The occurrence of this type of rejection is extremely rare because of the universal adoption of pretransplantation cross-matching. The histopathology is characterized by features of severe endothelial and arterial injury manifested as arteritis (often transmural), interstitial edema, and severe cortical necrosis, with almost all cases requiring allograft nephrectomy. Most of the initial cases were reported in patients with a history of previous transplantation or in multiparous women, suggesting the role of sensitization and preformed antibodies. Strong proof for this was provided by Patel and Terasaki in 1969 when they showed that 24 of the 30 patients with a pretransplant positive crossmatch had immediate graft failure compared with only 8 graft failures in 195 patients without a positive crossmatch [1].

It is characterized by graft dysfunction manifesting over days and is a result of DSAs, that may either be preformed or develop denovo after transplantation [2]. Acute AMR occurs in about 5‘7% of all kidney transplants and is responsible for 20‘48% of acute rejection episodes among presensitized positive crossmatch patients [3, 4]. Allograft dysfunction with resultant creatinine elevation may not be present in all cases of AMR. Histopathology in these patients is again related to endothelial injury mediated by antibodies but is less severe than that seen in hyperacute rejections. Biopsy often shows endothelial cell swelling, neutrophilic infiltration of glomeruli and peritubular capillaries, fibrin thrombi, interstitial edema, and hemorrhage [5]. However, in a minority of these rejections, acute tubular necrosis may be the only feature observed [3]. The identification of these AMRs has become easier with the development of C4d-staining in biopsies and improved methods of antibody detection. Prior to the routine use of C4d-staining, diagnosis was often limited by lack of staining for antibody components and was often restricted to steroid-resistant cases with or without obvious histopathologic findings as described above.

It is now well recognized that antibodies can mediate chronic allograft injury which is characteristically seen as transplant glomerulopathy (TG) on kidney biopsies [6]. TG (also known as or chronic allograft glomerulopathy) is characterized by glomerular mesangial expansion and capillary basement membrane (BM) duplication, seen as basement membrane double contouring or splitting. Similarly, the peritubular capillary (PTC) basement membrane also shows changes, but these are seen mostly on electron microscopy sections as basement membrane multilayering. Clinically, the manifestations range from patients being asymptomatic in the early stages to having nephrotic range proteinuria, hypertension, and allograft dysfunction in the advanced stages. Progression can sometimes be fairly rapid, especially with ongoing acute AMR, resulting in graft failure within months [7]. The prevalence of TG in protocol biopsies has varied between 5% at 1 yr to 20% at 5 years [8].

3. Pathogenesis.

Antibodies are most commonly directed against human leukocyte antigen (HLA)/major-histocompatibility-complex (MHC) Class I and II antigens [9]. HLA class I antigens are expressed on all nucleated cells, whereas HLA class II antigens are restricted to antigen-presenting cells (B lymphocytes, dendritic cells) and endothelial cells. However, the antibodies can also be directed against other donor specific antigens such as MHC-class I-related chain A (MICA) antigens, MHC-class I-related chain B (MICB) antigens, platelet-specific antigens, molecules of the renin-angiotensin pathway, and polymorphisms involving chemokines and their receptors [10‘15]. MICA antigens are expressed on endothelial cells, dendritic cells, fibroblasts, epithelial cells, and many tumors, but not on peripheral-blood lymphocytes. Risk factors for sensitization against HLA I and II antigens are pregnancy, blood transfusions, and previous transplantation. However, blood transfusions were found not to be a risk factor for MICA sensitization [10].

4. Mechanisms of Antibody Mediated Injury.

The major mechanism involved is activation of classical complement pathway by the antigen-antibody complex, leading to formation of the membrane attack complex resulting in cellular injury. The target antigens in AMR are most often situated on the endothelium resulting in the histological findings of acute (glomerulitis, peritubular capillaritis) and chronic (transplant glomerulopathy) vascular injury. Endothelial damage also results in platelet activation and microthrombi formation. The byproducts of complement activation (e. g., C3a and C5a) act as chemokines resulting in inflammatory cell infiltration and amplification of the inflammatory process. Long standing inflammation results in cell proliferation, basement membrane duplication, and mesangial interposition which can be easily seen on light and electron microscopy as glomerular BM splitting and PTC BM multilayering, respectively. The ability of different IgG subclasses to fix complemsent also varies. IgG1 and IgG3 have strong complement fixing properties compared to the IgG2 and IgG4 subclasses, which fix complement weakly. The significance of this was studied in a series of 74 patients with pretransplant anti-HLA antibodies. Only 4 patients in this series had exclusively weak or no complement fixing HLA antibodies (IgG2 or IgG4) [16]. Of the remaining, 21 and 46 patients had isolated strong complement fixing HLA (IgG1 or IgG3) antibodies or a mix of weak and strong complement fixing HLA antibodies respectively, but had no difference in AMR or graft failure at 5 years. Of the 4 patients with isolated IgG2/IgG4, none had any AMR. Antibodies can also mediate injury via complement independent mechanisms such as antibody-cell-dependent cytotoxicity (ADCC). This is mediated through cells of the innate immunity (natural killer cells, macrophages) which get activated by binding to the Fc receptor portion of the antibody [17]. Antigen antibody interaction on endothelial cells is also known to increase Von Willibrand Factor (vWF) along with externalization of P-selectin molecules resulting in increased platelet activation and leukocyte trafficking, respectively [18, 19].

5. Diagnosis of AMR.

Based on the increasing evidence for the role of antibodies in allograft dysfunction and the strong correlation with C4d-staining and DSA, the Banff committee updated its renal allograft biopsy classification to involve a separate antibody-mediated rejection diagnosis [20]. According to the classification, AMR was defined as a triad involving the presence of DSA, positive C4d-staining on the biopsy, and histopathological evidence of antibody-mediated injury (glomerulitis, peritubular capillaritis, and arteritis).

Based on the histopathology, AMR can be classified into three subtypes as below. Class I: Presence of acute tubular necrosis (ATN) only, with minimal inflammation. Class II: glomerulitis, peritubular capillaritis, and microthrombosis. Class III: Arteritis.

Chronic AMR according to the Banff criteria involves demonstration of C4d, DSA, and at least one feature of morphologic evidence of chronic tissue injury, such as glomerular double contours, peritubular capillary basement membrane multilayering, interstitial fibrosis/tubular atrophy, and/or intimal thickening of arteries [20]. However, it is not uncommon to have situations where DSAs may be absent even in the presence of histological AMR and positive C4d-staining.

6. C4d Stain.

C4d is a complement split product that is formed during breakdown of C4b into C4d and C4c. C4d has a thioester moiety that enables strong covalent bonding with the endothelial cells and basement membrane. It is constitutively expressed in all normal kidneys in the mesangium and the vascular pole owing to the constant complement turnover. This can extend into glomerular capillaries in cases of immune-mediated glomerulopathies, but peritubular C4d deposition is noted mostly in the transplanted kidney, with rare reports of C4d presence in PTC of native kidneys [21, 22].

There are two methods for C4d detection in biopsy specimens [23]. It can be detected using either immunofluorescence (IF) on frozen tissue with a monovalent antibody against C4d or using Immunohistochemistry (IHC) on paraffin-embedded tissue with a polyvalent antibody. Diffuse C4d implies >50% of PTC staining for C4d, while focal and minimal staining implies 10‘50% and <10% staining, respectively. IHC is less sensitive than IF for C4d detection. Hence, focal staining on IHC may be equivalent to diffuse staining with IF.

The association of C4d with AMR was initially described by Feucht et al. in 1991 when they showed a significant association of C4d with preformed anti HLA antibodies in patients with acute rejection [24]. This was subsequently confirmed in a study of 16 biopsies with DSA and histopathological evidence of AMR (neutrophilic capillaritis). All 16 biopsies with AMR showed diffuse C4d-staining with trace or no staining in the biopsies with acute cellular rejection ( 𝑛 = 1 4 ) and 5 of the 6 biopsies with cyclosporine toxicity [25]. Crespo et al. evaluated DSA and C4d in steroid-resistant rejections and found positive DSA in 37% of the cases. Among these, 95% had positive PTC C4d-staining [26].

The data for C4d-staining in chronic AMR is more variable, with a few studies showing strong correlation while others showing poor correlation. Mauiyyedi et al. reported presence of C4d in 23 of 38 biopsies with features of chronic AMR (GBM duplication and arterial intimal fibrosis) but no C4d in the control group without features of chronic AMR [27]. Similar strong associations of C4d with TG and peritubular basement membrane multilayering have been described in other studies as well [28, 29]. However, some studies found no correlation between presence of TG and diffuse C4d, with many TG patients showing no C4d positivity [30, 31].

Protocol biopsy studies have also demonstrated another important characteristic of C4d which is variability of staining over time, suggesting a constant flux between states of positive to negative C4d [32]. In a significant number of these biopsies, there was presence of microvascular inflammation (PTC and glomerular capillaritis) in spite of negative C4d-staining [32]. Another recent development has been the identification that certain endothelial transcripts appear to be expressed more often in patients with histological features of AMR even in the absence of C4d-staining. Sis et al. studied a number of endothelial-associated transcripts (ENDAT) in kidney transplants and found that the ENDAT expression was higher in all types of rejection but more so in AMR. Furthermore, about 40% of patients with ENDAT and chronic AMR features demonstrated no C4d-staining. There was also a strong correlation between elevated ENDAT and presence of anti-HLA antibodies, particularly HLA Class II antibodies [33]. The biology of this ENDAT expression appears to be related to the process of endothelial cell activation, repair, and angiogenesis which are well known mechanisms of AMR [33]. This, along with the low sensitivity of C4d for chronic AMR, has given rise to the concept of “C4d-negative AMR” which appears to be as common, if not more than the C4d-positive AMR and has similar poor prognosis in terms of graft survival [33].

7. Anti HLA Antibody Detection.

The techniques to identify anti HLA antibodies have improved significantly with the development of single-bead antigen testing methods which have very high sensitivity for antibody detection. The complement-dependent cytotoxicity (CDC) still remains the gold standard test for the detection of preformed antibodies prior to transplantation. The addition of antihuman globulin enhances the sensitivity of the assay by cross-linking the antibodies (AHG-CDC). Flow cytometry crossmatch (FXM) assay is more sensitive than the CDC assay and detects antibodies via fluorochrome-tagged antihuman Immunoglobulin antibody. The newer solid phase assays use purified single HLA antigens to detect anti-HLA antibodies by ELISA or flow cytometry techniques. These tests have increased sensitivity to detect presence of anti-HLA antibodies even with a negative FXM [34‘36]. The strength of antibody detected in Luminex assays is indicated by Mean Fluorescent Intensity (MFI). However, there is poor standardization of these tests and the MFIs have been found to vary between different centers performing the same test [37]. A further improvement in antibody detection technique was reported by Yabu et al. They tested antibodies for their capacity to fix C1q complement and compared them to regular IgG antibody detection and found a higher specificity for the C1q technique in detecting antibodies associated with TG and poor graft outcomes [38]. This technique may improve our ability to prognosticate the significance of DSAs but will need to be further standardized and validated in a larger patient population.

8. Treatment.

Although the current diagnosis of AMR requires the concomitant presence of DSA, C4d, and histopathological evidence of AMR, treatment may be initiated in circumstances where the above criteria may not be fulfilled in entirety. This depends on the risk factor profile of the patient (sensitized patient, history of pregnancies, and blood transfusions) and presence or absence of organ dysfunction. Treatment is often initiated in situations of diffuse C4d positivity with allograft dysfunction even in the absence of DSA or histological evidence of AMR. The inability to measure DSAs in these cases may be related to the presence of non anti-HLA antibodies, to antigen not present on the single-bead assays, or to the possibility that the DSAs may be completely adsorbed onto the allograft [39]. Similarly, patients with positive DSA and histological evidence of AMR may not demonstrate any C4d activity, and treatment is often initiated in these patients as well, especially in the presence of allograft dysfunction. C4d in such cases of acute AMR may be negative for a number of reasons. Immunohistochemistry (IHC) is known to be less sensitive compared to immunofluorescence (IF) staining [22, 23]. Also, areas with necrosis may stain falsely negative for C4d and hence, care must be taken to ensure that viable areas of the biopsy specimen are stained for C4d [22].

There is a paucity of randomized controlled trials in the treatment of AMR. Many of the studies have used historical controls to compare the effectiveness of therapies. There is also bound to be some publication bias related to selective publication of positive studies.

Even with these inadequacies, there has been good progress made in developing treatments for AMR. The primary goal in AMR treatment involves targeting the reduction/removal of DSAs and elimination of the B-cell/plasma cell population responsible for the production of these antibodies.

A number of treatment modalities have been employed for the treatment of AMR as characterized below. (1) Antibody removal/neutralization: plasmapheresis, immunoadsorption, intravenous immunoglobulin, and splenectomy. (2) Anti B-Cell therapies: Mycophenolate mofetil, Rituximab, IVIG, and splenectomy. (3) Antiplasma cell therapy: Bortezomib. (4) Anti-T-cell therapies: T-cell depleting agents such as Antithymocyte globulin (ATG). (5) Conversion to tacrolimus-based regimens. (6) Terminal-complement pathway inhibitor: Eculizumab.

The presence of a vast array of therapeutic modalities signifies the ineffectiveness of one drug or one particular combination therapy to reverse or treat AMR successfully in all scenarios. All of these treatments have been used in different combinations by different groups without a good control arm, resulting in poor evidence to argue for the superiority of one treatment regimen. These treatment modalities are also used for pretransplantation desensitization protocols to abrogate positive crossmatch in highly sensitized patients.

8.1. Intravenous Immunoglobulin (IVIG)

This is the most commonly used agent either alone or often, in combination with plasmapheresis. Although the exact mechanisms involved are not clear, they appear to involve multiple processes such as neutralization of complement fixing antibodies, alteration in the activity of complement, modulation of Fc receptor activation and function, and regulation of T and B lymphocytes [40]. Recent research has elucidated the possible role in this immunomodulation, for a specific subtype of IgG which possesses sialylated glycan residues near the Fc receptor [41]. These sialylated IgGs were shown to bind to lectin receptor SIGN-R1 or DC-SIGN leading to increased expression of inhibitory Fc receptor (FcR), FcgammaRIIb on inflammatory cells, thereby attenuating inflammation [41, 42]. IVIG is routinely used in one of two doses: high (2 gm/kg) or low (100 mg/kg per session). Low-dose IVIG is mostly used in combination with plasmapheresis where it may help replenish depleted IGs. Initial studies used IVIG at high-doses without plasmapheresis and described a fair degree of success in desensitization prior to transplant and also for treating antibody-mediated rejection [43, 44]. IVIG is generally safe and well tolerated in most patients with occasional side effects such as aseptic meningitis, volume overload, and rarely acute kidney injury possibly related to high osmotic load. Sucrose-based IVIG preparation is to be avoided, while glycine-based preparations are relatively safe.

8.2. Plasmapheresis (PP)

Plasmapheresis is very effective in reducing the antibody load but needs to be used in conjunction with other therapies that target the antibody producing mechanisms. The most common type of Plasmapheresis performed is plasma exchange, with albumin being the most common replacement fluid used. It is usually performed on alternate days with a 1‘1.5 volume exchange with albumin (commonly) or fresh frozen plasma. Most institutions also follow each PP session with low-dose IVIG (100 mg/kg) [45]. DSAs are monitored along with renal function to document the effectiveness of the therapy. Treatment, if successful, is continued until the level of antibodies has dropped to safe levels along with improvement in renal function. One of the early studies using this combination to successfully reverse humoral rejection was from Montgomery et al. [46]. The same group subsequently used this combination therapy successfully to reduce pretransplant DSA titers in sensitized patients to allow successful transplantation [46].

A retrospective study analyzed one-year graft outcomes of 16 patients with AMR treated with PP and IVIG and 43 ACR patients and found similar overall graft survival of 81% and 84%, respectively, indicating the effectiveness of these therapies in improving outcomes of acute AMR [47].

Plasmapheresis is generally well tolerated. Side effects are relatively uncommon and are related to the use of vascular access (infections, bleeding), volume removal, type of replacement fluid used (coagulopathy, hypovolemia, allergic reactions and a small risk of blood borne infection transmission), hypocalcemia, and side effects related to use of anticoagulants [48].

8.3. Immunoadsorption with Protein A (IA)

IA is currently not used in the United States.

IA was studied in a randomized controlled trial in Europe where it proved very successful in reversing severe AMR [49]. Both arms underwent a switch to tacrolimus from cyclosporine, along with treatment for ACR (steroids/ATG) as needed. The study was initiated at a time when PP and IVIG were still not universally used for AMR treatment.

The study was stopped early because of significant success rate in the IA group (80% versus 20%). It was, however, a small study (5 patients in each group), with a higher prevalence of diffuse C4d in the control group. Considering the widespread acceptance of IVIG and PP for treatment of AMR and unavailability of IA in the USA, a head-to-head study of IA with PP and IVIG will be necessary to prove its superiority, prior to its acceptance as an alternative to IVIG and PP.

8.4. Rituximab.

Rituximab is an anti-CD20 monoclonal antibody that induces profound depletion of B-cells and was initially approved for the treatment of B-cell lymphoma. It has since been tested in multiple immune-mediated disorders with varying degrees of success. Rituximab has been used to treat AMR in a number of uncontrolled studies.

Most of the studies reported so far with the use of Rituximab have reported favorable outcomes. Becker et al. treated 27 patients with AMR with a single dose of rituximab. Twenty-two of these patients also received ATG and plasmapheresis [50]. At a mean of 605 days of followup, only 3 grafts were lost to rejection. Faguer et al. also reported 81% graft survival at 20 months in 8 patients with the use of 4 doses of Rituximab along with PP, mycophenolate, tacrolimus and steroids [51]. Kaposztas et al. reported their experience with use of Rituximab in combination with PP. Twenty-six patients were treated with Rituximab along with PP and IVIG [52]. The graft outcomes were compared to historical controls who had been treated with PP ± IVIG alone. The two-year graft survival for patients treated with rituximab plus PP was significantly better at 90% when compared to the 60% survival in the PP cohort. However, the doses of IVIG were higher in the Rituximab group, and the use of IVIG was also statistically associated with a better graft outcome on Kaplan-Meier analysis, raising concerns for a confounding effect. Lefaucheur et al. also compared the use of 2-week doses of Rituximab along with high-dose IVIG and PP with historical controls who had received high-dose IVIG alone and reported a 91.7% graft survival, compared to 50% with high-dose IVIG alone [53]. The mechanism of action of Rituximab in AMR is not clear, given that the plasma cells do not express CD20 on their surface. However, the depletion of CD20-positive subset of B-cells may attenuate the antibody generation process. The standard dosing of Rituximab is 375 mg/m 2 /wk for 2‘4 weeks. Rituximab results in prolonged and profound B-cell depletion which may cause reactivation of latent viruses such as hepatitis B, C, cytomegalovirus (CMV), and also mycobacterium tuberculosis. It also carries a boxed warning for progressive multifocal leukoencephalopathy (PML) caused by JC virus.

Patients can also manifest acute infusion reactions, which usually occur within 30‘120 minutes and may be mild or severe, such as bronchospasm, angioedema, acute respiratory distress syndrome, cardiogenic shock, and anaphylaxis. These have often been reported in leukemic patients with high pretherapy leukocyte counts [54].

8.5. Change of Maintenance Immunosuppression (IS)

Initiation or augmentation of anti B-cell maintenance therapy is routinely done when AMR is identified. The most commonly used agent for this purpose is mycophenolate mofetil. It is also common practice to change to a calcineurin-based immunosuppression, specifically to tacrolimus, if patients are not on a calcineurin inhibitor (CNI).

8.6. Bortezomib.

Bortezomib is a novel proteosome inhibitor that is approved for the treatment of multiple myeloma. Proteasomes are involved in breakdown of ubiquitinated proteins and are present both in the nucleus and cytoplasm. Inhibition of proteasomes can lead to decreased nuclear factor-Kappa B activation, cell cycle arrest, endoplasmic reticulum stress, and increased cell apoptosis [55]. This action is pronounced in plasma cells likely because of the high antibody turnover and high endoplasmic reticulum activity. A number of groups have investigated the use of bortezomib in solid organ AMR and have generally reported favorable results. There is, however, no randomized trial thus far and in most cases, bortezomib was used after standard therapies for AMR failed, that are, IVIG and PP [56‘63]. Many similar case series and case reports continue to be reported with good outcomes in AMR with bortezomib. However, one study reported patients with subclinical AMR and positive DSAs in whom bortezomib monotherapy did not result in any significant reduction of DSA levels [64]. The authors and the editorial caution against the use of bortezomib as primary therapy for AMR without strong evidence from randomized studies. However, this agent appears to be a promising strategy and its role in AMR is still evolving. Gastrointestinal side effects, neuropathy, and hematological toxicity are the main side effects of bortezomib and need to be carefully monitored. Dosing in most of these reports has been the standard myeloma dosing of 1.3 mg/m 2 /week, with 4 doses given over 2 weeks.

8.7. Eculizumab.

Eculizumab is a humanized monoclonal antibody directed against complement protein C5. It binds to the C5 protein with high affinity, thereby inhibiting conversion of C5 to C5b and preventing formation of the membrane attack complex (C5‘9). Initially approved for use in paroxysmal nocturnal hemoglobinuria (PMH), it was also recently approved for use in atypical hemolytic-uremic syndrome. Prior vaccination against meningococcus and pneumococcus is necessary. One dose of Eculizumab was used with IVIG and rituximab in a patient with severe AMR, who recovered from the AMR but died of a fatal pulmonary hemorrhage a few months later [65]. A prospective study compared the outcomes of using eculizumab to prevent acute AMR and TG after transplantation in a series of HLA-sensitized pretransplant positive-FXM patients ( 𝑛 = 2 6 ). The incidence of AMR at 3 months was significantly less compared to an historical control group (7.7% versus 41.2%), although the presence of C4d in patients with DSA did not differ between the study and control group, thus providing evidence for the downstream activity of eculizumab in blocking the complement pathway. The use of eculizumab also resulted in a reduced need for PP. The one-year TG prevalence was also low with only one (6.7%) of the 15 patients in the eculizumab group developing TG compared to 15 (35.7%) of the 42 controls. Most patients in the treatment group received weekly eculizumab for 4‘8 weeks while one patient needed a year of eculizumab because of persistent FXM positivity [66]. No significant complications were reported during the study period. Although not a randomized study, this series serves as a proof of concept for the use of terminal complement pathway inhibitors in treating AMR. A major limitation for its use at the present time is its extremely high cost. However, a multicenter prospective randomized trial is being planned to study its efficacy and should help answer some of the questions regarding its role in AMR.

Splenectomy It has also been used in resistant AMR patients with good success rate [67, 68]. However, because of the long term risk of infections in immunosuppressed individuals and the surgical risks involved, this is not a commonly used therapy for AMR.

Acute cellular rejection frequently coexists with AMR and needs to be treated aggressively with either steroids or T-cell depleting therapies such as Antithymocyte globulin (ATG). The role of these T-cell depleting therapies in AMR has not been clearly studied in patients with pure AMR or AMR with low grades of ACR. The use of these agents may reduce the T-cell stimuli that are driving the B-cell-mediated antiallograft responses, thereby helping to gain better control of the AMR process. Indirect proof can be obtained from one study that reported no significant change in graft outcomes between C4d-positive and negative cases. However, there was aggressive use of antilymphocyte therapy to treat C4d-positive cases which might have improved outcomes in this group of patients [69].

9. Course and Prognosis.

It has been shown in multiple studies that AMR portends worse outcome in terms of graft survival at one and five years. Some of these studies were reported prior to the utilization of aggressive therapies that are currently in use for AMR and often serve as historical controls for trials of newer therapies for AMR. Lederer et al. reported a 4 year 50% graft survival for C4d+ patients compared to a 8 year 50% graft survival for C4d-patients [70]. Poduval et al. reported a one year graft loss of 65% for grafts with diffuse C4d+ diagnosis compared to 33% for focal and negative C4d grafts [71]. One study, however, noted no difference between C4d+ and C4d− grafts with up to 3 years followup. However, patients with C4d+ were treated more aggressively with antilymphocytic therapy (ATG and OKT3) [69].

The significance of C4d+ allograft biopsies appears to differ based on whether the transplantation was ABO or HLA incompatible [72]. Multiple studies have documented presence of diffuse C4d with no allograft dysfunction and with no histologic evidence of AMR in protocol biopsies of ABO incompatible transplants [73, 74]. The presence of C4d in these patients was also shown to have no adverse outcome and in fact was associated with a trend toward better scores of chronicity and less TG at subsequent followup [73]. In contrast, the presence of diffuse C4d+ in ABO compatible HLA-mismatched kidney transplantation appears to be very commonly associated with neutrophil margination suggesting ongoing antibody-mediated rejection [72].

The significance of focal C4d on biopsies is currently still being evaluated and is not entirely clear. A significant number of these biopsies may not be associated with histologic evidence of AMR, but its presence has still been associated with inferior graft outcomes [75, 76]. Graft outcomes with a diagnosis of TG are poor, with graft survival of 60% at 5 years compared to >90% without TG [8].

10. Management of Patients with Pretransplantation HLA Sensitization.

Improvements in HLA typing and DSA identification have increased our ability to identify high-risk recipients who may be at risk for antibody-mediated rejection posttransplantation. For patients who are highly sensitized or those with ABO or HLA incompatible living donors, there are at present four options available for successful transplantation. Patients can undergo desensitization protocol followed by a kidney transplant provided they can achieve sufficient reductions in DSA titers and become crossmatch negative. Even with successful transplantation after desensitization, these patients remain at increased risk for AMR. They also have reduced graft survival compared to nonsensitized patients. However, their outcomes are still superior when compared to remaining on dialysis [77]. Patients can also undergo a paired living kidney donation (PKD) involving 2 or more donor recipient pairs. Another potential option less commonly used is List paired donation (LPD) which involves the option of a recipient with an incompatible living donor getting a deceased donor kidney from a waiting list in return for the living donor donating it to the intended recipient on the transplant waiting list [78]. The least favorable option would be to remain on the deceased donor list waiting for a compatible donor. However, for some highly sensitized patients with living donors, transplantation by paired exchange or list-paired donation may not be possible [78]. These patients should preferably undergo desensitization to improve the likelihood of transplantation which, as mentioned earlier, has been shown to offer improved patient survival compared to waiting on the transplant list [77].

11. Summary.

Antibody-mediated rejection is an important cause of acute and chronic graft failure. Improvements in HLA technology along with the recognition of the role of C4d in AMR have revolutionized the understanding of this important entity. New research is attempting to elucidate the mechanisms and epidemiology of C4d-negative antibody-mediated rejection processes. Further research should help clarify the identification, prognosis, and treatment of these C4d-negative AMRs. Therapies for AMR are still not optimal with high rates of graft loss leading to poor patient outcomes. Newer therapies, such as bortezomib and eculizumab that target novel pathways in the AMR process are promising but will need further randomized studies before becoming widely used. Studies will need to be performed to determine the best use, either alone or in combination, of the myriad number of therapies currently available. Transplantation of sensitized patients remains a difficult problem. However, developments such as paired kidney donation and desensitization protocols are continuously improving the rates of transplantation in this difficult to transplant population.

Treatment Options and Strategies for Antibody Mediated Rejection after Renal Transplantation.

Antibody mediated rejection is a significant clinical problem encountered in a subset of renal transplant recipients. This type of rejection has a variable pathogenesis from the presence of donor specific antibodies with no overt disease to immediate hyperacute rejection and many variations between. Antibody mediated rejection is more common in human leukocyte antigen sensitized patients. In general, transplant graft survival after antibody mediated rejection is jeopardized, with less than 50% graft survival 5 years after this diagnosis. A variety of agents have been utilized singly and in combinations to treat antibody mediated rejection with differing results and significant research efforts are being placed on developing new targets for intervention. These same agents have been used in desensitization protocols with some success. In this review, we describe the biology of antibody mediated rejection, review the available agents to treat this form of rejection, and highlight areas of ongoing and future research into this difficult clinical problem.

Scope of the Problem.

Renal transplantation is the treatment of choice for end-stage renal disease (ESRD) in appropriately selected patients, providing both survival and quality of life benefit to renal transplant recipients. Unfortunately, demands for renal transplantation have vastly outstripped the supply of organs with more than 90,000 ESRD patients in the United States on the renal transplant waiting list as of May 2018, but just under 17,000 renal transplants performed in calendar year 2018, of which roughly two-thirds were derived from deceased donors and one-third from living donors[1]. Approximately 14,000 individuals (16%) awaiting a renal transplant are prior organ transplant recipients. Increasing access to renal transplantation by accepting marginal donor organs, expanding living donation, and performing paired donor exchange transplants have increased overall transplant numbers but have not been able to match the demand for transplantation.

Waiting times for renal transplantation have continued to increase, with waits in excess of 6 years common in some regions of the United States, depending upon blood type. Waiting times are even longer in potential recipients for whom it is difficult to find a compatible organ match due to human leukocyte antigen (HLA)-specific alloantibodies. Patients sensitized to HLA account for about 30% of the kidney wait list. Median waiting time for renal transplant recipients listed in 2001-2 is 1329 days for those with panel-reactive antibody (PRA) 0-9%, 1920 days for those with PRA 10-79%, and 3649 days for those with PRA 80% or greater[1]. Organ Procurement and Transplantation Network (OPTN) data has shown that any degree of sensitization has a detrimental impact on transplantation rate, meaning greater likelihood of never being transplanted or being delisted due to co-morbidities prior to obtaining a transplant in this group[2]. Sensitized patients not only have diminished access to transplantation but also have been shown to have inferior outcomes after transplantation, with higher rates of rejection and graft loss than unsensitized patients, even when compatible organs are utilized [1, 3]. Desensitization protocols have been developed in a variety of centers with some notable successes but also high rates of rejection, particularly antibody mediated rejection (AMR)[4-11]. Additionally, renal transplantation across blood group incompatibility has been accomplished under some protocols with goal directed therapy to reduce anti-blood group antigen titers, often using modifications of protocols used for patient desensitization[12].

AMR can occur with a spectrum of clinical manifestations, from hyperacute rejection leading to immediate graft loss, AMR with acute impairment in renal function, and a more indolent course of chronic rejection that may not be associated with acute graft dysfunction but rather a more gradual loss of function over time[13-16]. More than 40% of patients with AMR go on to develop transplant glomerulopathy regardless of whether initial treatment is able to reverse the acute renal functional impairment and the development of glomerulopathy is associated with less than a 50% 5-year graft survival from the time of identification [15]. AMR may also be associated with concurrent cellular rejection. Alloantibodies preferentially bind to the peritubular and glomerular capillaries in contrast to the typical injury pattern of acute cellular rejection (ACR) by T cells which tends to infiltrate renal tubules and the arterial endothelial layer[17-19]. AMR is associated with greater acute graft loss than ACR, with 15-20% losing their grafts within a year, despite typical mainstay immunosuppressive therapies[17]. The gold standard criteria identifying AMR remains a constellation of features seen on analysis of renal biopsy including C4d deposition and histological features of inflammation, allograft dysfunction, and serologic evidence of circulating antibodies to donor HLA or other non-HLA DSA[20]. C4d is a complement split product of C4b which can form covalent bonds with proteins in the setting of the complement pathway initiation via antibody binding and association with C1. C4d does not appear to be pathogenic in and of itself, but rather appears to be a fingerprint of antibody binding and complement deposition[21].

The presence of donor specific antibody (DSA) in the recipient serum can be assessed by ELISA or by bead-based fluorometric assays (Luminex or flow cytometry). Despite significant advances in the ability to detect, specify, and quantify the strength of HLA-specific and non-HLA DSA, it is not yet clear how effective these methods are at predicting AMR when assessed pre-transplant or serially over time. It does appear that the presence of DSA at the time of transplant is an independent risk factor for AMR and that patients who develop anti-HLA DSA tend to have inferior long term graft survival compared to patients who do not develop DSA[22-28]. It is apparent that a degree of DSA can be detected in some recipients without apparent clinical pathology in the transplanted kidney. It is possible that this DSA with unknown acute significance may have more obvious significance over a longer observed graft life.

Potential Targets for Therapy.

There are multiple steps in the mechanism of AMR that have served as potential intervention points in known therapies to counteract AMR and that serve as promising targets for further investigation ( Figure 1 ). A direct approach can be utilized to inhibit or delete B cells. This includes metabolic inhibitors of B cell division (mycophenolate mofetil) or antibody directed depletion on the basis of B cell surface molecules (rituximab). An additional target to diminishing AMR is to inhibit T cells and by extension, diminish T cell help to B cells. This approach utilizes inhibitors of T cell division (mycophenolate mofetil (MMF) and steroids), inhibitors of IL-2 signaling to T cells (calcinuerin inhibitors (CNI)), or T cell depleting agents such as antithymocyte globulin (ATG). Polyclonal ATG may also have some B cell depleting effects. A third approach to controlling B cell responses in transplantation involves the removal or dilution of the B cell effector arm: antibodies. Antibody removal can be done by plasmapheresis (PP) or immunoadsorption (IA). Graft-directed antibodies can be diluted by administration of IVIG, which can also have more direct effects on B cell function through Fc receptors. Recent data has indicated a fourth target for diminishing antibody responses ‘ targeting the antibody-secreting plasma cell (PC) with bortezomib, a proteasome inhibitor. This is the first agent available that appears to directly inhibit the cell considered to be the primary mediator of AMR. Lastly, a fifth target for therapy has recently been developed which involves the final step in AMR, the fixation of complement by antibodies that have targeted the graft. Eculizumab is a C5 inhibitor that diminishes the propagation of complement cascade even after antibodies have bound to the graft. There are also some therapies that appear to act at multiple steps in the process, including the nonspecific action of splenectomy, which seems to inhibit the process of B cell development in a nonspecific fashion.

It should be noted that several fairly desirable pathways to target in the attempt to control B cell responses are currently not addressed in the arsenal of options to treat AMR. These include inhibitors to control the development of plasma cells from less mature B cells and direct inhibitors or depleting agents for plasma cells. This is an important gap in the treatment algorithm for AMR as plasma cells are durable cells and AMR would likely be easier to control if the process did not proceed to plasma cell responses before therapy was initiated. These important steps in B cell development serve as potential targets for future experimentation and development. In part due to a lack of specific plasma cell effectors, many treatments for AMR utilize multiple drugs or processes to simultaneously attack multiple steps in this process in order to control these challenging responses.

Antithymocyte Globulin (ATG)

ATG is a polyclonal antibody preparation derived from rabbits immunized with human thymic tissue. While it is generally thought that ATG has primarily anti-T cell effects, it also can inhibit the interaction of CD4+ helper T cells with B cells and thereby diminish B cell activation. ATG also can have direct B cell antibody derived cytotoxicity and may have effects on both antibody production of B cells and may be able to induce B cell apoptosis[29]. ATG is commonly included in treatment algorithms for AMR, especially when the transplant biopsy identifies mixed features of cellular and antibody-mediated rejection.

Typical dosing for ATG involves four divided doses of 1.5 mg/kg/day to yield a total treatment dose of 6 mg/kg. Platelet and white blood cell counts can diminish to a degree that the divided doses must be spread out to allow for recovery between doses in some cases. One study utilized lower dose ATG (0.75 ‘ 1.0 mg/kg/d) for 5-10 days along with a mean of 7 treatments of plasmapheresis in seven renal transplant patients diagnosed with AMR[29]. Improvement in graft function was seen in 6 of the 7 patients and the serum creatinine level at one year in these six patients was not statistically different from their larger group of 60 patients that did not develop AMR. These AMR events were all discovered within the first month of transplant and it should be noted that only one patient of the seven with AMR was given ATG for immunosuppression induction at the time of transplantation, potentially limiting the applicability of this study to groups that use ATG routinely at the time of transplantation.

IVIG inhibition of the cytotoxic effects of anti-HLA antibody was recognized in the 1990's [30, 31]. IVIG is currently used in desensitization protocols and for the treatment of AMR. It is derived from the pooled plasma of thousands of blood donors and is primarily composed of IgG. There are several proposed mechanisms of action (reviewed in [32]). Immunoglobulin molecules are well known for their ability to activate complement and complement is an important mediator of ischemia reperfusion injury. However evidence suggests that immunoglobulin molecules can also limit antibody-mediated complement activation in experimental models[33] [34]. IVIG may have the ability to regulate long-term alloimmune response through interactions with cell mediated immunity by binding to Fc receptors and preventing binding of alloantibody complexes as well as inducing the inhibitory FcgRIIb receptor. It is believed that IVIG can regulate innate immune responses by inhibiting the dendritic cell inflammatory response important to allograft rejection and the dendritic activation of alloreactive T-cells[35, 36]. IVIG may also have direct effects on the adaptive immune response through inhibition of T-cell induction of B-cell apoptosis[37].

Two general treatment protocols have been developed utilizing IVIG. The first is the use of high dose IVIG (2 gm/kg) alone and the second is to combine lower dose IVIG with other modalities, usually plasmapheresis (PP). High dose IVIG with methylprednisolone was reported for treating AMR in 1998 by Jordan et al[38]. Subsequently IVIG was tested prior to transplant in order to inhibit cross match positivity in kidney recipients. The Cedars-Sinai group performed in vitro IVIG assays to determine if crossmatch positivity could be inhibited. Those patients with a negative inhibition cross match received 2 g/kg of IVIG followed by kidney transplantation. Graft survival was 89.1% at 24 months[39]. In a randomized multicenter placebo-controlled trial conducted by the NIH (the NIH G02 study) in highly sensitized patients, 4 monthly infusions of IVIG significantly lowered anti-HLA antibody levels and improved rates of transplantation compared to placebo (35% vs. 17%, P=0.02) in patients with a PRA>50%. Projected waiting time on the IVIG group was 4.8 years and 10.3 years for placebo. The reduction in PRA was transient and moderate. Acute rejection episodes were more common in the IVIG group, but 3 year allograft survival rates were similar between IVIG and placebo[7].

Due to the length of isolated IVIG treatment, and in order to improve effectiveness, the Cedars-Sinai group conducted an open label phase 1-2 single-center study examining whether the addition of rituximab to IVIG was effective in reducing anti-HLA antibodies. 2 doses of IVIG (2 g/kg) and 2 doses of rituximab (1 g) were given prior to transplant. The protocol proved efficacious in reducing mean PRA compared to pretreatment level (P<0.001) and appeared to reduce the waiting time for transplant. With short term follow-up the mean graft survival was 94%[5].

Low dose IVIG (100mg/kg) in combination with PP represents an alternative to desensitize individuals with alloantibody prior to transplant as well as treat AMR. A small retrospective study compared combined therapy with PP, IVIG, and rituximab to high dose IVIG alone for AMR. After 36 months, the graft survival for the combined therapy group was 91.7% compared to 50% in the monotherapy group[40]. Other groups have reported successful desensitization outcomes with a variety of combined IVIG and PP protocols in the setting of HLA and anti-agglutinin antibodies [41] [6] [11] [42].

In general, high doses of IVIG are relatively safe. However, serious side effects have been reported including acute renal dysfunction likely related to high osmotic load, thrombotic events with rapid infusions, and aseptic meningitis[43]. Slowing the infusion rate and using iso-osmolar preparations may reduce the risk of side effects[44]. IVIG has the potential benefit of replacing antibodies lost during PP.

Plasmapheresis.

The goal of plasmapheresis is to remove DSA from the circulation. PP is utilized in desensitization protocols and for the treatment of AMR following transplant. It is the fastest method to decrease DSA. There are several different modalities: plasma exchange, double filtration plasmapheresis, and immunoadsorption plasmapheresis. Plasma exchange is the most frequent modality applied in the United States and generally involves 1.0-1.5 volume exchange, using albumin as replacement. Immunoadsorption is a more selective modality that uses adsorbent membranes for antibody elimination. When utilized for desensitization, the clinician usually aims to decrease DSA below a threshold before transplant. After transplant, PP is often continued for a variable period of time.

Often PP is used in combination with other antibody blocking (IVIG), suppression (rituximab, mycophenolate, calcineurin inhibitors), or depleting (bortezomib) modalities, as antibody levels have a tendency to rebound if PP therapy is performed in isolation[45]. Few studies have been published where PP modalities are the sole or primary form of antibody reducing therapy[46, 47]

The Johns Hopkins’ desensitization protocol consist of every other day PP followed by 100 mg/kg IVIG after each PP session ( Figure 2 )[48]. The objective is to decrease DSA or iso-agglutinin titers to a pre-specified level prior to transplant. The number of pre-transplant PP/IVIG sessions depends on the starting titer. Patients are started on tacrolimus and MMF at the time PP/IVIG is initiated. PP/IVIG is continued post transplant, with the number of treatments governed by antibody levels and the clinical course[41, 48].

In a retrospective comparison of different desensitization strategies performed at the Mayo Clinic, Stegall et al noted that a negative preoperative crossmatch could be achieved in 85% of patients treated with a combination of PP, low dose IVIG, and rituximab compared to 36% treated with high does IVIG alone. The incidence of AMR was 80% in the IVIG only group, but 29-37% in the groups treated with three agents[49].

The incidence of side-effects from PP is relatively low ranging from 5-12% and most are considered mild or moderate in nature. Commonly reported symptoms are due to allergic reactions that present as rigors and urticaria, symptoms of hypocalcemia such as parasthesias, and hypovolemia which can manifest as muscle cramps and hypotension. The incidence and side effects relate to the use of anticoagulants during PP, type of replacement fluid, and complications related to vascular access. There is a small risk of blood borne pathogen transmission[50].

Rituximab is a chimeric anti-CD20 monoclonal antibody composed of human IgG1 heavy chain and kappa light chain constant regions fused with mouse variable regions. A transmembrane protein, CD20 is expressed on pre-B and mature B-lymphocytes throughout the antigen independent stage of development until early stages of antigen-dependent B-cell activation. CD20 is absent from plasma cells. Cells bound by rituximab are eliminated by traditional antibody-mediated mechanisms; antibody-dependent cell mediated cytotoxicity, complement dependent cytotoxicity, and cell-mediated apoptosis via CD20.

Rituximab was originally approved to treat lymphoma and it is also used for rheumatoid arthritis and other autoimmune disorders[51]. This agent exerts a profound depletion in circulating B cells as well as a less marked reduction in B cell numbers in spleen and lymph nodes[52]. A single dose in renal transplant recipients can result in prolonged B cell depletion, with populations remaining suppressed for 1-2 years[53]. Non-transplant studies of rituximab demonstrate a delayed recovery of the CD27+ memory B cell population[54].

Ramos et al. have investigated the in-vivo effect of rituximab on B-cell populations in individuals who underwent splenectomy[55]. Spleens removed for trauma victims and from transplant recipients who received multiple rounds of pre-transplant PP plus low-dose IVIG showed similar numbers of naïve B cells (CD20+ and CD79+), plasma cells (CD138+), and memory B cells (CD27+). However in transplant recipients that underwent splenectomy and where rituximab was added to the PP/IVIG regimen, the number of naïve B cells was reduced but no notable difference in the memory or plasma cell populations was seen. As rituximab is ineffective for reducing plasma cells, the source of DSA, its therapeutic benefit may be related to modifications of cellular immunity rather than antibody reduction. Evidence exists that rituximab may impact the important antigen presenting cell activity provided by antigen specific B-cells to T-cells[56, 57].

Rituximab has been used as induction therapy in patients with HLA antibodies and in the setting of ABO incompatible kidney transplantation as well as to treat AMR. Becker et al published the initial report describing the use of rituximab to treat AMR. 27 patients with refractory rejection were given a single dose of rituximab and a majority also received PP and ATG. In short term follow-up 24 patients had good graft function, but 3 grafts were lost[58]. Several other studies have reported results with use of rituximab for AMR. Zarkhin reported the 1 year outcome of a randomized trial of rituximab compared to thymoglobulin and/or pulse steroids for acute rejection in 20 pediatric renal recipients. There was some benefit for recovery of graft function and histology at 1 and 6 months after treatment[59]. There was no change in DSA in either group, although reappearance of C4d deposition was absent after rituximab, but was observed in 30% of control patients. Similarly Steinmetz et al. noted that rituximab removed intra-renal B cells in patients with vascular rejection compared to those who had received conventional immunosuppression[60]. In a retrospective study of 54 patients with AMR, Kaposztas et al compared 26 patients treated with PP plus rituximab to 28 patients who underwent PP without rituximab. The 2 year graft survival for the rituximab group was 90% compared to 60% in the PP cohort[61].

Bortezomib.

Bortezomib, a proteasome inhibitor, has recently received attention as a possible agent to reduce alloantibody levels through its direct effect on plasma cells. Approved by the FDA in 2003 for the treatment of relapsed refractory multiple myeloma, it binds selectively and reversibly to the 26S proteasome. Proteasomes are located in the cell nucleus and cytoplasm and are the primary proteolytic mechanism in eukaryotic cells[62]. The 26S proteasome is part of an enzyme complex that plays a role in degradation of super-numerous, misfolded, or defective proteins targeted for degradation by ubiquitinylation[63]. Besides damaged proteins, proteasomes degrade proteins involved in cell cycle regulation, oncogenesis, and apoptosis [64, 65].

Proteasome inhibition induces apoptotic cell death as a result of activation of the terminal unfolded protein response[66, 67]. Bortezomib induces apoptosis in various malignant cells, but the sensitivity of myeloma cells to bortezomib may be related to high synthesis rates of immunoglobulin associated with accumulation of unfolded proteins and the subsequent stress in the endoplasmic reticulum[68]. Other mechanisms include modification of cytokine signaling pathways through the inhibitor of Kappa B (ikB) and nuclear factor Kappa B (NF-kB) signal transduction.

Bortezomib is primarily metabolized by cytochrome P450 enzymes. Adverse events include fatigue, malaise, weakness, nausea, diarrhea, vomiting, peripheral neuropathy, thrombocytopenia, and neutropenia. Drug related side effects generally can be managed with dose reduction and supportive care[69].

A few case reports and case series have been published where bortezomib was used to modify pre-transplant anti-HLA antibodies or as therapy for AMR[70-75]. The impact of bortezomib in reducing anti-HLA in these studies is difficult to assess as most of the studies have been complicated by the addition of other therapies to modify alloantibody, including IVIG, rituximab, and plasmapheresis. Furthermore these studies primarily report short term follow-up. Long-term studies will be needed to determine the durability of proteasome inhibition to prevent return of donor specific antibody and the impact on allograft survival and function.

Only two studies have examined the use of bortezomib as isolated therapy for donor specific antibody. In one study of 2 sensitized kidney transplant candidates receiving two cycles of bortezomib administered without further anti-humoral measures, proteasome inhibition did not affect or only modestly affected allosensitization[76]. In a post-renal transplant study among four patients with DSA, one cycle of bortezomib as isolated therapy was unable to reduce DSA titers[77]. While the PC-targeting of bortezomib is a logical approach to treat AMR, given the mixed results of small studies using bortezomib, properly designed and controlled studies are clearly needed and caution exercised with the off-label use of this drug[78].

Eculizumab.

Eculizumab (Soliris, Alexion Pharmaceuticals) is a humanized monoclonal antibody against the C5 complement protein which has been approved by the FDA for treating paroxysmal nocturnal hemoglobinemia. By preventing C5 cleavage by C5 convertase into C5a and C5b, formation of the C5b-C9 membrane attack complex is prevented. Complement activation plays a critical role in the development of AMR after kidney transplantation. As such, the complement cascade represents a potential target for therapy. A small series presented by Stegall et al. reported no incidence of AMR in the year following transplant in 3 patients who underwent desensitization with IVIG and PP with eculizumab after transplant[79]. This absence of AMR is substantially lower than a historic rate for desensitized recipients, although this is a small series. A handful of case reports exist which document success treating AMR with eculizumab[80, 81]. There are several ongoing clinic trials enrolling transplant candidates or recipients (clinicaltrials. gov). Despite the appeal of treating the complement cascade, further investigation may be hampered by the cost of this antibody, one of the most expensive medications in the world.

Potential New Targets.

The survival signals that govern PC persistence are currently emerging from the basic science literature[82-84]. Understanding these signals will be critical to developing successful therapies to permanently eliminate alloantibodies. A major shortcoming in the therapies reviewed above (IVIG, rituximab, PP, splenectomy, and eculizumab with the exception of bortezomib) is their inability to target PCs. PC-directed therapy aims to eliminate alloantibody-secreting PCs permanently. Berek and colleagues made the recent discovery that eosinophils are required for the maintenance of PCs[85]. Desensitization protocols targeting eosinophils have not been investigated and eosinophil-directed immunotherapy maybe a potential desensitization strategy. Furthermore, in mice TACI-Ig decreases PC numbers substantially by sequestering both BLyS and APRIL and could be translated into a novel desensitization strategy as well[86]. Additionally, PCs reliance on RANKL, IL-6, CXCR4, and CXCL12 could also be targeted in novel PC directed therapies[87-92]. Without the targeted elimination of alloantibody-secreting PCs, DSA cannot be expected to be fully eliminated in the sensitized recipient and will continue to challenge long-term allograft survival.

AMR remains a significant clinical problem in a minority of recipients of kidney transplants. Outcomes after AMR are less than ideal. In part, this is due to the fact that no optimal treatment modality currently exists to treat AMR and antibody responses, once established, can be difficult or impossible to extinguish. Thus, nearly all current strategies to address AMR rely on a mixture of partially effective strategies as outlined above. These combined approaches are difficult to compare due to the fact that few studies directly compare different complex approaches to one another but rather generally compare a mixed strategy with or without an additional agent to determine the added benefit of that agent to the strategy. While this is a valid approach, it is clear that AMR is in need of protocolized studies to compare regimens and seek an optimal approach while minimizing toxicity. This is not an easy task, as AMR is an intrinsically heterogeneous process and it is well-known that controlling AMR when detected early after transplant before a mature B cell response is established is more easily accomplished than eliminating late-onset AMR or established sensitization to HLA antigens. Future studies must be careful to clearly define the type of AMR that is being treated. In addition, AMR is a rare enough event after transplantation that it will be difficult to accumulate sufficient patient numbers at a single center level to allow comparison of different treatment strategies. Multicenter trials will need to be established but will require standardized cross-institutional diagnoses of AMR..

Interest in post transplant B cell responses and AMR has increased substantially in the past decade associated with the appreciation of the clinical relevance of this phenomenon. New agents developed for non-transplant purposes, such as bortezomib and eculizumab have shown promise in addressing some of the downstream targets in the AMR process and have been added to a number of desensitization strategies. Future targets of relevance include the transition from immature B cells to mature B cells and plasma cells and development of agents with more direct and specific effects on plasma cell numbers or function. With increased interest and attention to the problem of AMR and sensitization, there is renewed promise that such approaches may be developed to augment the armamentarium against this difficult clinical problem.

Destaques.

>Antibody mediated rejection is a significant clinical problem after renal transplant. >We review the biological basis and pathways of antibody mediated rejection. >We review available treatments and future directions for therapy. >We review clinical trials results dealing with antibody mediated rejection.

Agradecimentos.

Thank you to Robert Redfield III and Robin Noel for assistance with the preparation of this manuscript.

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