PROJECT SUMMARY – PROJECT 4: Prostate cancer is a significant health problem worldwide for which new treatments are needed. Radiation therapy is a standard therapy for localized prostate cancer, delivered as either external beam radiation therapy or brachytherapy. Targeted radionuclide therapy (TRT) agents have been approved for advanced, metastatic prostate cancer, and others are in clinical testing. To date, the majority of studies using these agents have focused on identifying maximum doses that can eliminate tumor cells while having minimal effects on normal cells. The concept of using these types of agents to prime the tumor microenvironment for immunotherapy has been relatively unexplored. The overarching goal of this P01 is to evaluate TRT as a means to modulate the tumor microenvironment to enable immunotherapy treatments. Project 2 will evaluate TRT in combination with T-cell checkpoint inhibitor treatments, treatments which alone have been less successful in the treatment of prostate cancer. Project 3 will evaluate TRT in combination with intratumoral delivery of immune therapies for immunologically “cold” tumors, an approach not feasible for most prostate cancers given the patterns of metastatic spread. While prostate cancer is generally considered to be an immunologically cold tumor, devoid of large numbers of tumor-infiltrating T cells, it nonetheless remains the only human cancer type to date for which an anti-tumor vaccine has been FDA-approved, likely on the basis of its ability to elicit tumor-specific T cells. This current Project will focus on prostate cancer and evaluate TRT in combination with antigen-specific anti-tumor vaccination. The hypothesis to be tested, based on existing preliminary data, is that TRT can modulate the tumor microenvironment by depleting immunosuppressive cell populations and promote infiltration of activated CD8+ T cells, and this may be modulated by the use of different TRT vectors, vaccination, and androgen deprivation therapy. This approach is complementary to the other Projects and Project 4 will inform the other Projects by permitting the direct evaluation of effects of TRT on the number and function of tumor antigen-specific CD8+ T cells, a level of analysis not possible in other Projects in which the targeted tumor antigens are not known. The work proposed will rely heavily on the RPR Core 1 for TRT vector production, AID Core 2 for dosimetry studies, and BB Core 3 for statistical and bioinformatics support. The underlying hypothesis will be tested with the following Aims: 1) to determine the effects of different TRT agents on the composition and effector function of immune infiltrating cells in murine prostate cancer models; 2) to determine whether antigen-specific tumor vaccination, when combined with different TRT agents, elicits greater numbers of tumor-specific infiltrating CD8+ T cells; and 3) to determine if CD8+ T cell infiltration and anti- tumor efficacy elicited with antigen-specific...