New aspect of immunosuppressive treatment in liver transplantation. How could you induce tolerance in liver transplantation?

Considerations and experience driving expansion of combined heart-liver transplantation

PURPOSE OF REVIEW

Heart transplantation concomitant with a liver transplant may be warranted when end-stage heart failure results in irreversible liver failure. Previously reported outcomes have been excellent yet the specific immunoprotective role of the liver allograft is not known. We review the current literature about the immunologic benefit for combined heart and liver transplantation (CHLT).

RECENT FINDINGS

The total number of combined heart and liver transplants continues to increase and accounts for approximately 25 cases per year. Familial amyloid polyneuropathy with cardiac cirrhosis is the most common indication for CHLT while adult congenital heart disease (CHD) with associated cirrhosis is increasing in frequency. The majority of recent registry data suggest a statistically equivalent to modestly improved survival advantage for CHLT compared with isolated heart transplantation. Direct mechanisms accounting for this survival advantage are not proven, but combined heart and liver transplants experience lower rates of acute cardiac rejection and cardiac allograft vasculopathy (CAV).

SUMMARY

Combined heart and liver transplants remain a small percentage of the total heart transplants worldwide, but the majority of recent literature confirms the safety and viability of this option for patients with end-stage heart and liver disease. Equivalent to modestly improved survival outcomes, lower rates of acute cardiac rejection and CAV warrant further investigation into the liver allograft's immunoprotective effect on the transplanted heart. The key mechanisms of tolerogenicity have important implications for surgical technique and immunosuppression requirements. Future directions include development of criteria for heart-liver transplant candidacy and identification of equitable allocation protocols.

The Immunological Basis of Liver Allograft Rejection

Liver allograft rejection remains a significant cause of morbidity and graft failure in liver transplant recipients. Rejection is caused by the recognition of non-self donor alloantigens by recipient T-cells. Antigen recognition results in proliferation and activation of T-cells in lymphoid tissue before migration to the allograft. Activated T-cells have a variety of effector mechanisms including direct T-cell mediated damage to bile ducts, endothelium and hepatocytes and indirect effects through cytokine production and recruitment of tissue-destructive inflammatory cells. These effects explain the histological appearances of typical acute T-cell mediated rejection. In addition, donor specific antibodies, most typically against HLA antigens, may give rise to antibody-mediated rejection causing damage to the allograft primarily through endothelial injury. However, as an immune-privileged site there are several mechanisms in the liver capable of overcoming rejection and promoting tolerance to the graft, particularly in the context of recruitment of regulatory T-cells and promotors of an immunosuppressive environment. Indeed, around 20% of transplant recipients can be successfully weaned from immunosuppression. Hence, the host immunological response to the liver allograft is best regarded as a balance between rejection-promoting and tolerance-promoting factors. Understanding this balance provides insight into potential mechanisms for novel anti-rejection therapies.

Revisiting the liver's role in transplant alloimmunity

The transplanted liver can modulate the recipient immune system to induce tolerance after transplantation. This phenomenon was observed nearly five decades ago. Subsequently, the liver's role in multivisceral transplantation was recognized, as it has a protective role in preventing rejection of simultaneously transplanted solid organs such as kidney and heart. The liver has a unique architecture and is home to many cells involved in immunity and inflammation. After transplantation, these cells migrate from the liver into the recipient. Early studies identified chimerism as an important mechanism by which the liver modulates the human immune system. Recent studies on human T-cell subtypes, cytokine expression, and gene expression in the allograft have expanded our knowledge on the potential mechanisms underlying immunomodulation. In this article, we discuss the privileged state of liver transplantation compared to other solid organ transplantation, the liver allograft's role in multivisceral transplantation, various cells in the liver involved in immune responses, and the potential mechanisms underlying immunomodulation of host alloresponses.

Inducción de tolerancia inmunológica: alotrasplantes compuestos vascularizados y trasplantes de órgano sólido

Introducción. La inducción de tolerancia inmunológica solucionaría los problemas asociados con la inmunosupresión de por vida, necesaria para evitar el rechazo de aloinjertos.Objetivos. Revisar aspectos inmunológicos, modelos clínicos utilizados y resultados obtenidos en la tolerancia y comparar los resultados obtenidos con trasplante de órgano sólido y alotrasplante compuesto vascularizado.Materiales y métodos. Se realizó una búsqueda en la base de datos PubMed que arrojó 299 resultados; se revisaron las bibliografías de los artículos y se consultaron las referencias pertinentes. Al final se seleccionaron 83 artículos.Resultados. Existen mecanismos centrales y periféricos para mantener la tolerancia a antígenos propios; en la práctica clínica, la tolerancia central ha sido más utilizada, esto se ha hecho mediante estrategias que utilizan trasplante conjunto de medula ósea. Varios ensayos clínicos, la mayoría en pacientes con trasplante renal, han mostrado resultados prometedores pero inconsistentes.Conclusiones. En trasplantes renales fue posible suspender de forma exitosa la inmunosupresión, mientras que en trasplantes de mano se logró disminuirla considerablemente. El quimerismo inmunológico parece ser indispensable para el desarrollo de tolerancia a aloinjertos, por lo que es necesario desarrollar protocolos para inducir quimerismo mixto persistente.

Liver-kidney recipients with chronic viral hepatitis C treated with interferon-alpha

Antiviral therapy with interferon-alpha (IFN-alpha) and pegylated IFN-alpha (PEG-IFN-alpha) for chronic hepatitis C (HCV)-infected kidney recipients remains controversial. IFN-alpha is not recommended in most cases because it induces severe acute graft rejection. However, IFN-alpha, as PEG-IFN-alpha, is associated with a more pronounced immune response, and is well tolerated in HCV-infected liver recipients without causing graft rejection. In combined liver-kidney transplant (LKT) recipients, IFN-alpha has been occasionally used and appears to be well tolerated. All LKT recipients with a functioning kidney and liver having a HCV replication and who needed IFN-alpha therapy have been included in the study. The occurrence of liver and/or renal acute rejection as well as the HCV replication has been collected. A total of 12 LKT patients treated with PEG-IFN-alpha plus ribavirin have been studied. No acute rejection was observed. Renal function remained stable during and after discontinuing treatment, without any graft dysfunction. Two patients had a partial viral response and four had a sustained viral response. All patients, whatever their viral response, had decreased liver-enzyme levels. Response to PEG-IFN-alpha therapy was correlated with steroid dose and transaminase level when PEG-IFN-alpha was started. These data suggest that the combination therapy of PEG-IFN-alpha plus ribavirin did not have a higher risk of acute kidney-graft rejection after liver-kidney transplantation.

Long-term follow-up of donor chimerism and tolerance after human liver transplantation

We aimed to quantify peripheral donor chimerism (DC) and to analyze its association with graft and recipient outcome. Forty-two liver transplant recipients and their respective donors were studied, providing a total of 148 posttransplantation serum samples. DC was assessed with real-time quantitative polymerase chain reaction (qPCR) to detect polymorphic markers. DC did not decrease with time post-transplantation and was higher in child recipients versus adults and in recipients of deceased donor liver transplants versus recipients of live donor liver transplants. Higher levels of DC were detected in Rh-positive blood group donors, in O blood group recipients versus A blood group recipients, and in recipients with hepatitis C virus versus recipients with alcoholic cirrhosis. High DC was associated with patients with organ damage due to recurrent disease and rejection. Stable, high levels of DC, in the absence of other major clinical events, may thus be a marker of transplantation tolerance, and this knowledge may help to tailor immunosuppressive treatment. In conclusion, qPCR is a useful technique for DC follow-up in liver transplantation, although the evolution of DC levels should be analyzed in accordance with the clinical outcome of the patient.