This Program Project grant will establish the fully integrated interdisciplinary programs of research and core facilities that are necessary to develop a fundamental understanding of the complex interplay between two rapidly emerging fields in cancer therapy: 1) targeted radionuclide therapy (TRT) and 2) immunotherapy. Our overarching objectives are to develop a detailed mechanistic understanding of the immunomodulatory capacity of TRT agents and to evaluate and compare the ability of these agents to elicit cooperative therapeutic interactions in combination with immunotherapies. To achieve this, we will employ a representative variety of TRT agents to deliver radiation in vivo using clinically relevant murine tumor models and companion canines that have spontaneously developed cancers. We will determine the precise dosimetry for each radionuclide-,TRT vector-, and tumor model. This will facilitate studies of dose-, dose-rate-, and dose-range-dependent effects on the host immune system, tumor infiltrating immune cells, cytokine expression in the tumor microenvironment, immune susceptibility of tumor cells, mechanisms of immune suppression, and the development of antigenspecific adaptive immunity. We will determine the functional consequences of these effects by testing the broad hypothesis that, by modulating tumor immune tolerance and functional immunogenicity at all tumor sites, TRT will increase response to certain cancer immunotherapies.