Inducing Stably Persistent Transplantation Tolerance: A Mechanistic Perspective

项目摘要/摘要 供者特异性移植耐受，根据停止免疫抑制治疗后的移植物接受定义，可以在无特定病原体的啮齿动物中实现，但在人类中很少实现。 同种异体反应性T细胞的缺失、无能和抑制是诱导小鼠移植耐受最常见的3种细胞机制。已有研究表明，CD25+调节性T细胞(Tregs)通过共刺激阻断诱导移植耐受所必需的是删除同种异体反应性T细胞和抑制剩余的同种异体反应性T细胞。然而，T细胞无能对于诱导和/或维持移植耐受是否必要和/或充分尚不清楚。 此外，目前还不清楚这三种T细胞耐受机制(以及其他潜在的机制)是否需要同时共同诱导才能保持耐受性并长期成功维持。如果它们确实需要联合诱导，人类通常缺乏移植耐受性，这可能是因为目前的免疫抑制方案无法同时诱导/维持这三条途径。 我们的初步实验表明，耐受不是一个全有或全无的现象，异体移植接受度的读数不是移植耐受功能稳健性的敏感指标。 事实上，我们观察到，耗尽Tregs或阻断PD-L1并不能消除已建立的耐受性，但能够在先前感染了单核细胞增多性李斯特菌但保留了移植物的耐受小鼠中引发急性同种异体心脏移植排斥反应。这表明，感染后维持移植物接受的机制的质量或数量发生了改变，简单的急性排斥反应的缺乏并不能说明耐受状态的稳定性，也不能说明同种异体免疫反应与排斥阈值的接近程度。 我们认为“移植耐受”是一种状态的连续体，在这种状态下，抑制同种异体免疫的抗炎过程的数量和强度逐渐被促炎信号(感染发作或其他)侵蚀，直到达到排斥阈值。虽然这一概念是直观的，但尚未以任何系统的方式进行研究，并在临床移植中具有重要意义：它可以解释为什么达到移植耐受的患者，即在没有免疫抑制药物的情况下功能稳定的患者，甚至在最低限度免疫抑制下保持稳定的患者，可能会突然拒绝他们的移植物；耐受水平的稳健性也可能决定是否可以停用免疫抑制药物，以及耐受是否可以持续。 我们特别假设，只有当T细胞耗竭、T细胞调节和T细胞无能同时和最佳地被同时诱导时，移植耐受状态才能达到强大的、持续的和抵抗炎性损伤的状态。从这些机制分析中获得的见解将在临床上具有重要意义，在临床上，移植人群的基因多样性几乎保证了接受相同治疗的个体将出现不同程度的缺失、无能和抑制。 这一假设将在以下特定目标的背景下进行检验：目标1：确定在诱导和维持移植耐受过程中对T细胞无能的需求。目的：评估T细胞耐受机制在建立强大的移植耐受中的必要性和充分性。

Project Summary/Abstract Donor-specific transplantation tolerance, as defined by graft acceptance after discontinuation of immunosuppressive therapies, can be achieved in specific pathogen-free rodents but rarely in humans. Deletion, anergy and suppression of alloreactive T cells are 3 cellular mechanisms most often associated with the induction of transplantation tolerance in mice. It has been shown that deletion of alloreactive T cells and suppression of remaining alloreactive T cells by CD25+ regulatory T cells (Tregs) are necessary for the induction of transplantation tolerance by costimulation blockade. However, whether T cell anergy is necessary and/or sufficient for the induction and/or the maintenance of transplantation tolerance is not known. Furthermore, it is not clear if these 3 mechanisms of T cell tolerance (and others potentially) need to be coinduced simultaneously for tolerance to be robust and maintained long-term successfully. If they do need to be co-induced, the usual lack of development of transplantation tolerance in humans could be due to the inability of current immunosuppressive regimens to induce/maintain these 3 pathways concurrently. Our preliminary experiments indicate that tolerance is not an all or none phenomenon and that a readout of allograft acceptance is not a sensitive measure of the functional robustness of transplantation tolerance. Indeed, we observed that depletion of Tregs or blockade of PD-L1 does not abrogate established tolerance, but is able to precipitate acute cardiac allograft rejection in tolerant mice that had been previously infected with Listeria monocytogenes but retained their graft. This suggests that the quality or quantity of mechanisms that maintain graft acceptance are modified after infection and that a simple lack of acute rejection does not inform on the stability of the tolerant state or how close the alloimmune response is to the rejection threshold. We propose that "transplantation tolerance" is a continuum of states in which the number and intensity of anti-inflammatory processes that constrain alloimmunity are progressively eroded by pro-inflammatory signals (infectious episodes or others) until a rejection threshold is reached. Although intuitive, this concept has not been investigated in any systematic manner and has important implications in clinical transplantation: it may explain why patients who achieve transplantation tolerance, i.e., years of stable function without immunosuppressive drugs, or even patients stably maintained with minimal immunosuppression, may suddenly reject their grafts; the robustness of the tolerance level may also determine whether immunosuppressive drugs can be withdrawn and whether tolerance can persist. We specifically hypothesize that a state of transplantation tolerance that is robust and that can persist and resist Inflammatory insults may only be achieved when T cell depletion, T cell regulation and T cell anergy are all co-induced simultaneously and optimally. Insights gained from these mechanistic analyses will be of importance in the clinic, where the genetic diversity of the transplant population all but guarantees that individuals receiving the same treatment will develop different degrees of deletion, anergy and suppression. This hypothesis will be tested in the context of the following specific aims: Aim 1: To determine the requirement for T cell anergy in the induction and maintenance of transplantation tolerance. Aim 2: To assess the necessity and sufficiency of T cell mechanisms of tolerance in the establishment of robust transplantation tolerance.