SUMMARY/ABSTRACT The overall objective of this project is to determine how androgen receptor antagonism and other androgen deprivation therapies used in prostate cancer affect human T cell function. Androgen deprivation is a cornerstone of prostate cancer therapy, in which tumor cells are killed by depriving them of androgen receptor signaling, a key growth signal. Androgens have pleiotropic effects on numerous physiological systems. In the immune system, they are generally considered immune inhibitory and are thought to contribute to the sex differences observed between men and women in rates of autoimmunity and cancer. Yet the effect of androgen deprivation on T cell function in men with prostate cancer is not known. This is highly relevant to cancer treatment, because immunotherapy in combination with androgen deprivation was recently shown to induce a durable response in a subset of patients with metastatic prostate cancer who were progressing on androgen deprivation therapy alone. Nonetheless, the majority of patients receiving this combination still endured cancer progression. It is important to understand the mechanisms by which androgen deprivation affects T cell function in order to improve combination therapy options for metastatic prostate cancer, as well as other cancers driven by androgen receptor signaling (e.g., androgen receptor positive breast cancer). The aims of this project are to i) determine the effect of the androgen receptor antagonist, enzalutamide, on human T cell function in vitro, and ii) determine the effect of androgen-deprivation therapy on T cell responses in prostate cancer patients. In aim 1, we will test the hypothesis that antagonizing androgen receptors within T cells increases T cell proliferation and type 1 cytokine secretion and decreases establishment and/or maintenance of T cell exhaustion. These studies will be performed using T cells isolated from peripheral blood of healthy volunteers. T cells will be treated in vitro with the clinically relevant androgen receptor antagonist, enzalutamide. In aim 2, we will test the hypothesis that androgen deprivation in men with prostate cancer alters the proportions of T cell subsets, changes the transcriptional profile of T cells, and increases the magnitude of antigen-specific T cell responses. In these studies, we will isolate T cells from peripheral blood of metastatic prostate cancer patients collected both before and 6-12 weeks after initiation of androgen deprivation therapy. These paired samples will allow us to clearly establish the effect of androgen deprivation therapy on T cell phenotype and function in cancer patients. This project will reveal important information about how androgen receptor antagonism impacts the adaptive immune response, and thus potentially alters anti-tumor immunity.