Role of HCV-induced immune responses on the establishment of operational tolerance following liver transplantation in humans

Immune responses induced by ongoing and/or past infections prevent the establishment of transplantation tolerance in experimental animal models. How host-pathogen interactions influence allograft tolerance in humans has not been investigated before. The spontaneous development of operational tolerance in liver transplant recipients with chronic hepatitis C virus (HCV) infection constitutes a unique setting to address this question. We conducted a clinical trial of immunosuppression withdrawal in stable HCV-infected liver recipients to elucidate: i) the mechanisms through which allograft tolerance is established in the presence of an ongoing inflammatory response; and ii) how is influenced by anti-HCV heterologous immune responses. Enrolled patients gradually discontinued their immunosuppressive drugs over 6-9 months, and those who maintained normal allograft status 12 months after drug withdrawal were considered operationally tolerant. Successful drug withdrawal was associated with intra-hepatic over-expression of type I interferon and immune-regulatory genes, and correlated with an expansion of exhausted PD1/CTLA4/2B4-positive HCV-specific circulating CD8+ T cells. These findings were already present before immunosuppression was discontinued and were specific for HCV infection. In contrast, the magnitude of HCV-induced inflammatory gene expression and the scope of anti-HCV effector T cell responses did not influence drug withdrawal outcome. Our data indicate that in humans persistent viral infections do not necessarily preclude the development of transplantation tolerance. At least in HCV-infected liver allografts, mechanisms associated with the capacity of the virus to evade adaptive immunity could contribute to the restraining of alloimmune responses and the establishment of transplantation tolerance. Transcriptomic study from the following liver tissue samples: 12 tolerant before immunosuppression (IS), 13 non-tolerant before IS, 4 from non-tolerant at the time of rejection, 13 from tolerant patients 12 months after IS discontinuation. Additionally, 8 liver tissue samples obtained from healthy living liver donors undergoing partial hepatectomy were included as non-transplanted controls.

在实验动物模型中，现症或既往感染所诱导的免疫应答会阻碍移植耐受（transplantation tolerance）的建立。截至目前，尚无针对宿主-病原体互作（host-pathogen interactions）如何影响人类同种异体移植耐受的相关研究。慢性丙型肝炎病毒（HCV）感染的肝移植受者自发出现的功能性移植耐受（operational tolerance），为解答这一问题提供了独特的研究场景。
我们针对病情稳定的HCV感染肝移植受者开展了一项免疫抑制撤除临床试验，以阐明两个核心科学问题：其一，在持续炎症应答存在的条件下，同种异体移植耐受（allograft tolerance）的建立机制；其二，抗HCV异源免疫应答如何对其产生调控作用。入组患者在6至9个月内逐步停用免疫抑制药物（immunosuppressive drugs），于药物撤除后12个月仍维持移植器官功能正常者，被判定为达到功能性移植耐受。
成功撤除免疫抑制药物与肝内I型干扰素（type I interferon）及免疫调控基因的过表达显著相关，且与耗竭型PD1/CTLA4/2B4阳性HCV特异性循环CD8+ T细胞的扩增存在关联。上述特征在免疫抑制撤除前即已存在，且仅特异性出现于HCV感染群体。与之相反，HCV诱导的炎症基因表达水平以及抗HCV效应T细胞应答的范围，并未对药物撤除结局产生显著影响。
本研究数据表明，在人类群体中，持续性病毒感染并不必然排除移植耐受的发生。至少在HCV感染的肝同种异体移植场景中，与病毒规避适应性免疫（adaptive immunity）能力相关的分子机制，可能参与了同种异体免疫应答的抑制以及移植耐受的建立。
本研究的转录组学分析所用肝组织样本包括：免疫抑制（IS）撤除前的耐受患者样本12例、免疫抑制（IS）撤除前的非耐受患者样本13例、发生排斥反应时的非耐受患者样本4例，以及免疫抑制（IS）撤除后12个月的耐受患者样本13例。此外，本研究纳入8例接受部分肝切除术的健康活体肝供者的肝组织样本作为非移植对照。