Project Summary Prostate cancer (PCa) incidence and mortality rates are the highest in African American (AA) men compared to any other racial/ethnic population. These differences persist even after accounting for socioeconomic factors, suggesting genetics and unknown biological factors may contribute to PCa health disparities. However, common genetic alterations, such as TMPRRSS2-ERG gene fusions and PTEN loss, were found to occur much less frequently in AA PCa than in European American (EA) PCa. Instead, prominent differences in tumor immunobiology between AA vs. EA men were reported in several studies, including a clinical trial with a cancer vaccine, Sipuleucel-T, which AA men had a median nine-month of overall survival advantage over EA men. To mechanistically dissect the immunological and/biological factors that determine tumor cell sensitivity and resistance to immunotherapy of different races, we have developed primary cultures of AA and EA PCa patient-derived tumor organoids, normal organoids, carcinoma associated fibroblasts (CAFs) and benign- associated fibroblasts (BAFs) from many patients and cryopreserved their peripheral blood lymphocytes (PBLs) from the same patient over past years. Our preliminary data show that AA CAFs secrete increased levels of active TGF- in the culture medium than EA CAFs. In addition, we are the first to show that Glycoprotein A repetitions predominant (GARP), the docking receptor for the release of active TGF-β, is over expressed in the adjacent stroma of AA PCa compared to adjacent stroma of EA PCa and AA PCa tissues. Interestingly, the adjacent stroma of AA PCa has increased infiltration of cytotoxic CD8+T cells compared to the adjacent stroma of EA PCa and to the distant stroma of AA PCa, suggesting that immune response is higher in AA stroma but may not be effective due to the increased TGF-β1 and GARP. Our preliminary data of co-culture studies with CAFs and T cells lends further support to the scenario as we observed increased TGF-β1 and reduced IFN- in these co-cultures. These results suggest that although AA PCa patients may be more responsive to immunotherapies, GARP/TGFβ signaling represents a vulnerable point in AA PCa and may be used as a target for developing more effective immunotherapies. Therefore, we hypothesize that the interaction between tumor and stroma in AA and EA PCa differentially affect the tumor reactivity of T cells and that GARP/TGF-β signaling contribute to the differences in the T cell tumor reactivity among patients. To test the hypothesis, first we will determine whether T cells from AA and EA PCa patients display differences in tumor reactivity in co-cultures with autologous tumor organoids and/or CAFs. Second, our preliminary data have shown that Dabigatran etexilate, an anticoagulant drug for preventing stroke in people with atrial fibrillation, effectively blocks GARP/TGF-1 signaling and that its combination with anti-CTLA4 results in a durable regression of My...