Macrophage-mediated abscopal effects of radiation therapy

Radiation therapy is a mainstay of cancer treatment, with more than 50% of all cancer patients receiving radiation during the course of their disease. Tumor irradiation can activate both innate and adaptive immune responses, and these responses can be pro- or anti-tumor growth . These observations have led to the search for antitumor approaches combining radiotherapy and specific immunotherapies, most commonly strategies promoting the systemic activation of T cells. Thus far, however, many cancer patients still suffer from local recurrence and/or untreatable metastatic disease after radiotherapy. Here we combine radiotherapy with activation of macrophage-mediated phagocytosis via blockade of the ?don?t-eat-me? cell surface molecule CD47 in small-cell lung cancer (SCLC), a highly metastatic form of lung cancer for which treatment options remain limited. We found that irradiation of SCLC cells in culture results in the secretion of inflammatory cytokines that results in increased migration and phagocytosis by macrophages. In vivo, CD47 blockade potently enhances the local antitumor effects of radiation therapy in murine and human pre-clinical models of SCLC. Strikingly, CD47 blockade also stimulates abscopal antitumor effects inhibiting the growth of non-irradiated SCLC tumors in mice receiving radiation. Similar abscopal antitumor effects were observed in colon cancer and lymphoma models. Surprisingly, these abscopal effects are completely independent of T cells but require macrophages that migrate into the non-irradiated tumor sites in response to inflammatory signals mediated by radiation and are locally activated by CD47 blockade to eliminate cancer cells. The systemic activation of antitumor macrophages following radiotherapy and CD47 blockade may be particularly important in cancer patients who suffer from metastatic disease. Overall design: Single-cell RNA sequencing (scRNA-seq) of CD45+ leukocytes at non-irradiated sites in mice treated either with radiotherapy alone or radiotherapy and CD47