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In situ vaccination is an emerging cancer treatment strategy that uses local therapies to stimulate a systemic antitumor immune response. We previously reported an in situ vaccination effect when combining radiation (RT) with intratumor (IT) injection of tumor-specific immunocytokine (IC), a fusion of tumor-specific antibody and IL2 cytokine. In mice bearing two tumors, we initially hypothesized that delivering RT plus IT-IC to the "primary" tumor would induce a systemic antitumor response causing regression of the "secondary" tumor. To test this, mice bearing one or two syngeneic murine tumors of B78 melanoma and/or Panc02 pancreatic cancer were treated with combined external beam RT and IT-IC to the designated "primary" tumor only. Primary and secondary tumor response as well as animal survival were monitored. Immunohistochemistry and quantitative real-time PCR were used to quantify tumor infiltration with regulatory T cells (Treg). Transgenic "DEREG" mice or IgG2a anti-CTLA-4 were used to transiently deplete tumor Tregs. Contrary to our initial hypothesis, we observed that the presence of an untreated secondary tumor antagonized the therapeutic effect of RT + IT-IC delivered to the primary tumor. We observed reciprocal tumor specificity for this effect, which was circumvented if all tumors received RT or by transient depletion of Tregs. Primary tumor treatment with RT + IT-IC together with systemic administration of Treg-depleting anti-CTLA-4 resulted in a renewed in situ vaccination effect. Our findings show that untreated tumors can exert a tumor-specific, Treg-dependent, suppressive effect on the efficacy of in situ vaccination and demonstrate clinically viable approaches to overcome this effect. Untreated tumor sites antagonize the systemic and local antitumor immune response to an in situ vaccination regimen. This effect is radiation sensitive and may be mediated by tumor-specific regulatory T cells harbored in the untreated tumor sites. Cancer Immunol Res; 6(7); 825-34. ©2018 AACR ., (©2018 American Association for Cancer Research.)