All-trans retinoic acid overcomes solid tumor radioresistance by inducing inflammatory macrophages

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Science Immunology  11 Jun 2021:
Vol. 6, Issue 60, eaba8426
DOI: 10.1126/sciimmunol.aba8426

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Radiosensitizer recipe for tumors

Cancer radiotherapy stimulates immunogenic cell death of tumor cells, thereby boosting antitumor T cell immunity. However, cancer radiotherapy simultaneously elicits local production and recruitment of immunosuppressive molecules and cells. Rao et al. investigated combining oral all-trans retinoic acid (ATRA) with tumor irradiation in mouse cancer models as a means of reprogramming intratumoral myeloid cells and enhancing systemic antitumor immunity. Supplementing radiotherapy with a course of ATRA increased the induction of inflammatory macrophages and IFN-γ–producing CD4+ and CD8+ T cells within tumors. ATRA-assisted reprogramming of the tumor microenvironment promoted local and systemic T cell–mediated effects on tumors including abscopal effects on distal nonirradiated tumors. These findings provide a rationale for designing human clinical trials to test ATRA as a radiosensitizer to boost antitumor T cell responses after radiation therapy.


Radiotherapy is an important anticancer treatment modality that activates innate and adaptive immune responses. When all-trans retinoic acid (RA) was administered with radiation, we observed superior antitumor responses compared with ionizing radiation (IR) alone or RA alone. The superior antitumor effects of combination treatment were accompanied by a marked increase of tumor necrosis factor–α– and inducible nitric oxide synthase–producing inflammatory macrophages in local and distal nonirradiated tumors. Inflammatory macrophages are essential for the therapeutic efficacy of combination treatment by inducing effector T cell infiltration and enhancing the effector T cell to regulatory T cell ratio in local and distal tumors. T cells and T cell–derived interferon-γ are crucial for increasing inflammatory macrophage levels in IR- and RA-treated tumors. Whereas CD8+ T cells are required for the antitumor response to IR, CD4+ T cells are required for the effectiveness of the IR + RA combination. Combination treatment with RA enhanced the abscopal response when radiation and programmed cell death-ligand 1 blockade were used together. The synergistic positive feedback loop of inflammatory macrophages and adaptive immunity is required for the antitumor efficacy of IR + RA combination treatment. Our findings provide a translational and relatively nontoxic strategy for enhancing the local and systemic antitumor effects of IR.

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