<h3>Purpose/Objective(s)</h3> Radiation therapy (RT) has been demonstrated to generate an in situ vaccination effect in murine models and in cancer patients, however this has not routinely translated into enhanced clinical response to immune checkpoint inhibition (ICI). We investigated whether the commonly used vaccine adjuvant, monophosphoryl lipid A (MPL) could augment the in situ vaccination regimen consisting of combination RT and ICI. <h3>Materials/Methods</h3> We used syngeneic murine models of melanoma (B78) and prostate cancer (Myc-CaP). Mice bearing B78 or Myc-CaP tumors were randomized to receive RT (12 Gy), RT + anti-CTLA4 (C4; 100 µg IP injection days 3, 6, 9 after RT), MPL (20 µg IT injection days 5, 7, 9 after RT), RT+C4+MPL, or PBS control. To model metastatic disease, 250,000 B16 melanoma cells (parental to B78) were injected via tail vein immediately following RT. To evaluate the effect of MPL on the irradiated tumor microenvironment, a separate cohort of B78 tumor bearing mice from each treatment group were sacrificed 15 days following RT, and tumor together with tumor draining lymph nodes were harvested for immune cell infiltration analysis and cytokine profiling and serum was collected for analysis of anti-tumor antibody populations. <h3>Results</h3> Combination RT+C4+MPL significantly reduced tumor growth, increased survival and complete response rate compared to RT+C4 in both B78 and Myc-CaP models. MPL favorably reprogrammed the irradiated tumor-immune microenvironment towards M1 macrophage and Th1 CD4 T cell polarization. Furthermore, MPL significantly increased intratumoral expression of several Th1 and M1 associated proinflammatory cytokines. T cells co-cultured with MPL-stimulated macrophages significantly increased expression of the activation marker CD69 in CD8 T cell populations and the Th1 polarization marker CXCR3 in CD4 T cell populations. MPL treatment significantly increased production of Th1-associated, IgG2c anti-tumor antibodies which were required for and predictive of anti-tumor response to RT and MPL and enabled macrophage-mediated antibody-dependent direct tumor cell killing by MPL-stimulated macrophages. Macrophage-mediated tumor cell killing was dependent on FcγR expression. In metastatic models, RT and MPL generated a systemic anti-tumor immune response that augmented response to ICIs. This was dependent on macrophages and CD4<sup>+</sup> but not CD8<sup>+</sup> T cells. <h3>Conclusion</h3> We report the potential for MPL to augment the in situ vaccination effect of combination RT+C4 through FcγR, macrophage, and CD4 dependent mechanisms. To our knowledge this is the first report describing generation of a CD8<sup>+</sup> T cell-independent, Th1 polarized, systemic anti-tumor immune response with subsequent generation of immunologic memory. These findings support the potential for vaccine adjuvants to enhance the efficacy of in situ tumor vaccine approaches.