Purpose: Radiation-mediated immune suppression limits efficacy and is a barrier in cancer therapy. Radiation induces negative regulators of tumor immunity including regulatory T cells (Treg). Mechanisms underlying Treg infiltration after radiotherapy (RT) are poorly defined. Given that dendritic cells (cDC) maintain Treg we sought to identify and target cDC signaling to block Treg infiltration after radiation.
Experimental design: Transcriptomics and high dimensional flow cytometry revealed changes in murine tumor cDC that not only mediate Treg infiltration after RT, but associate with worse survival in human cancer datasets. Antibodies perturbing a cDC-CCL22-Treg axis were tested in syngeneic murine tumors. A prototype interferon-anti-epidermal growth factor receptor fusion protein (αEGFR-IFNα) was examined to block Treg infiltration and promote a CD8+ T cell response after RT.
Results: Radiation expands a population of mature cDC1 enriched in immunoregulatory markers that mediates Treg infiltration via the Treg-recruiting chemokine CCL22. Blocking CCL22 or Treg depletion both enhanced RT efficacy. αEGFR-IFNα blocked cDC1 CCL22 production while simultaneously inducing an antitumor CD8+ T cell response to enhance RT efficacy in multiple EGFR-expressing murine tumor models, including following systemic administration.
Conclusions: We identify a previously unappreciated cDC mechanism mediating Treg tumor infiltration after RT. Our findings suggest blocking the cDC1-CCL22-Treg axis augments RT efficacy. αEGFR-IFNα added to RT provided robust antitumor responses better than systemic free interferon administration, and may overcome clinical limitations to interferon therapy. Our findings highlight the complex behavior of cDC after RT and provide novel therapeutic strategies for overcoming RT-driven immunosuppression to improve RT efficacy.