Although spontaneous tolerance after liver transplantation can be achieved in immunosuppression withdrawal studies, its broad application is limited by the lack of knowledge about the mechanism of tolerance, markers for tolerance, and therefore the lack of rational targets for immune induction therapy to increase the rate of tolerance. To address these gaps, we developed the only nonhuman primate liver transplant model in the United States. Preliminary data in rodent, human, and our primate liver transplant model indicate that memory CD8 T cells are the greatest barrier to tolerance after stopping immunosuppression. We now propose to control the response of memory cells by early deletion or by regulation at the time of immunosuppression withdrawal. Our long-term goal is to develop clinically-applicable protocols for tolerance induction early after liver transplant and to define the mechanism by which tolerance develops. Our central hypothesis is that control of anti-donor CD8 T cells through deletion or regulation will lead to stable graft function with normal histology after immunosuppression withdrawal. The rationale for defining the immune response to liver transplants is that this will allow us to optimize immune induction for immunosuppression withdrawal. The proposed research is innovative because it utilizes a translational animal model to develop a clinically-applicable tolerance induction protocol. It will also use innovative techniques to characterize the donor-specific immune response to liver transplants, and the interaction of graft-infiltrating lymphocytes and recipient antigen-presenting cells.
In Aim 1, we will test the ability of targeted memory T-cell depletion at the time of transplant to reduce donor-specific T-cell responses and thus lead to tolerance. We will also test the ability of ex vivo expanded donor-specific Tregs given at the time of immunosuppression withdrawal to suppress alloresponses. Finally, we will perform control transplants that receive standard clinical immunosuppression prior to early withdrawal to ensure our interventions are achieving their hypothesized results.
In Aim 2, we will adapt our laboratory's high-throughput TCR sequencing platform to the cynomolgus model to track donor-reactive effector and regulatory T-cell clones after transplant. Since mouse studies suggest the liver participates in tolerance induction by inducing anergy or apoptosis of T cells, we will also digest transplanted liver wedge biopsies to isolate and characterize graft-infiltrating T cells and APCs to determine if (and how) the liver promotes tolerance. These mechanistic studies will have a positive impact by revealing potential biomarkers of tolerance that could be used to withdraw immunosuppression on patients currently maintained on standard medications. They will also allow us to refine our induction protocol to minimize toxicities. Upon successful completion of the proposed research, we expect to have developed a tolerance induction regimen that can be rapidly translated to a clinical trial and to have gained insight into the mechanisms that contribute to rejection and tolerance. These achievements would be significant as successful withdrawal of immunosuppression early after liver transplantation would result in improved patient survival and quality of life.
The proposed research is relevant to human health since, although liver transplantation is the only treatment for end-stage liver disease, transplant outcomes are limited by rejection, infection, increased risk of cancer and toxicity related to immunosuppressive medications. This project uses novel strategies to induce and define the mechanism of tolerance to liver transplants in our unique nonhuman primate liver transplant model. This project is relevant to NIAIDs mission as it seeks to develop a clinically-applicable tolerance induction protocol and to define the contributions of deletion and regulation to the overall immune response that occurs following liver transplantation.
