Project Abstract Prostate cancer (PC) patient although initially respond to androgen deprivation therapy, most patients develop the resistance developing a stage referred to as the Castration Resistant Prostate Cancer (CRPC). Prostate cancer is a non-inflamed or “Immune desert” tumor where no immune infiltrate is observed, suggesting that failure has occurred somewhere in the process of T-cell priming, or T-cell trafficking back to the tumor. Not surprisingly, prostate cancer is highly refractory to immune checkpoint blockade (ICB) therapies exhibiting marginal efficacy in clinical trials, both as a single agent or in combination with other agents. Precisely how prostate cancer enforce evasion of anti-tumor immune response is not fully understood. Previously, we uncovered that a non-receptor tyrosine kinase, ACK1 deposited novel pY88-H4 epigenetic marks in AR gene enhancer, regulating AR/AR-V7 expression. Building on this discovery, we developed a new ACK1 small molecule inhibitor, (R)-9b, which suppressed enzalutamide-resistant tumor growth. To examine ACk1 signaling further, we generated a viable conditional ACK1 knockout (KO) mice, and noticed a significant increase in activated CD4+ and CD8+ T cells in KO mice, causing loss of syngeneic prostate tumor growth. The subsequent studies have revealed a crucial role for ACK1 kinase in the initiation of T cell antigen receptor (TCR) signaling by phosphorylation of CSK at a previously unknown site, Tyr18. CSK phosphorylated LCK at Tyr505 promoting auto-inhibition, inhibiting an adaptive immune response. Thus, ACK1 KO mice, or the (R)-9b injected mice exhibited increased CD4+ and CD8+ T cells activation and inhibition of tumor growth. In addition, (R)-9b functionally reinvigorated peripheral blood mononuclear cells (PBMCs) of the CRPC patients to mount robust immune response against CRPC organoids. Together, these data indicate that (R)-9b fulfills a unique niche, wherein it not only suppresses AR/AR-V7 within the tumor cells, but also activates host immune system to mount a robust `dual’ anti-tumor response. The specific aims of this project are: Aim 1: Examine roles of ACK1-mediated CSK Tyr18-phosphorylation in T cell quiescence Aim 2: Interrogate role of renewed pACK1/pY18-CSK/pY505-LCK signaling in silencing of anti-tumor immune response Aim 3: Evaluate therapeutic efficacy of (R)-9b in models of prostate cancer