ABSTRACT The adaptive immune system relies on stringent immune tolerance mechanisms to ensure that self-tissues are protected from autoimmune attack. The failure and success of such immune regulation have important implications in the prevention of autoimmune diseases and the efficacy of anti-tumor immune therapies. Thus, there is great interest in defining the prevailing mechanisms that regulate T cell responses specific for self- antigens, in the hopes that these processes can be manipulated for clinical benefit. While many autoreactive T cells are thought to be purged from the conventional T (Tconv) cell repertoire by clonal deletion, substantial evidence suggests that this process is imperfect. In this regard, little is known about the nature of self-reactive T cells present in the endogenous repertoire. For example, it is unclear whether most self-specific T cells are reactive to widespread antigens or tissue-restricted antigens, and whether these cells are restricted at steady state by cell-intrinsic mechanisms such as functional inactivation or dominant mechanisms such as Treg- mediated suppression. Moreover, in the context of cancer, it has been difficult to define whether self-specific T cells contribute to the repertoire of tumor-infiltrating lymphocytes (TILs), or whether most TILs are non-specific T cells that are recruited to the tumor by TCR-independent inflammatory signals. In this proposal, we will address these unanswered questions by pursuing the following specific aims. In Aim 1, we will identify CD4+ T cell clones in the endogenous T cell repertoire that infiltrate the prostate following Treg cell ablation, and determine the nature of the self-antigens recognized by these cells. In Aim 2, we will define the tolerance mechanisms regulating these Tconv cell clones. In Aim 3, we will determine the contribution of these self- specific T cell clonotypes to the tumor infiltrate in oncogene-driven mouse prostate tumors. We will achieve these aims by testing the central hypothesis that the endogenous T cell repertoire contains a pool of self- specific Tconv cells reactive to prostate-specific antigens, and that the suppression of prostate and prostate tumor infiltration by these cells is dependent on Treg-mediated suppression. It is expected that the work outlined in this proposal will demonstrate that thymic and peripheral deletion does little to impede many self- specific Tconv clonotypes, and that Treg-mediated suppression plays a pivotal role in restricting autoimmune tissue infiltration. In addition, we anticipate that the Tconv cells infiltrating the prostate following Treg depletion will exhibit reactivity to organ-specific prostatic antigens rather than widespread self-antigens. Finally, it is expected that self-specific Tconv cells will constitute a substantial proportion of the prostate tumor-infiltrating T cell repertoire. In all, our work is expected to yield new insights in our understanding of the mechanisms underlying immune tolerance and anti-tumor immunity.

PROJECT NARRATIVE The proposed study will address fundamental questions regarding the nature of self-specific conventional T cells and will reveal the predominant tolerance mechanisms that regulate these cells. This research is relevant to public health because an understanding of the mechanisms regulating self-specific T cells will inform strategies aimed at manipulating these processes for clinical benefit in the context of autoimmunity, transplantation, and cancer. Therefore, this work will directly support the NIH mission to seek fundamental knowledge that will contribute to the understanding, prevention, and treatment of autoimmunity and cancer.