Regulatory T cells mediate resistance to radiotherapy in head and neck squamous cell carcinoma.

Abstract

70 Background: Head and neck tumors are highly enriched in regulatory T cells which dampen the response to radiotherapy by creating an immune-inhibitory microenvironment. We explored mechanisms of Treg infiltration and assessed their modulation by RT in murine models of HNSCC. Methods: Mechanisms of Treg infiltration were investigated in murine HNSCC tumors using whole genome sequencing and flow cytometry. Mice were treated with anti-CTLA-4, anti-CD-25 and/or anti-PD-L1 alone and in combination with RT. Tumor growth and survival were assessed. Flow cytometry was used to assess phenotypic and functional changes in intratumoral T cell populations. Multiplex ELISA was performed for assessment of cytokines. RNA Sequencing was performed to interrogate mechanisms of response and resistance to treatment. Results: Treatment with anti-CD-25 concurrently with RT led to significant tumor growth delay, enhanced T cell cytotoxicity, decreased Tregs and improved survival. In contrast CTLA-4 blockade did not affect tumor growth or survival. Treg depletion induced an influx of CD8 and CD4 T cells when combined with RT. In addition, Treg depletion in combination with RT transformed myeloid populations decreasing M2 macrophages and MDSCs and increasing M1 macrophages. Mechanistically, tumors secrete CCL20, a potent Treg chemoattractant responsible for creating a highly immunuosuppressive tumor microenvironment and potentially responsible for treatment resistance. Conclusions: These data reveal a critical role for regulatory T cells in mediating resistance to RT. Targeted depletion of Tregs represents an important mechanism of sensitizing tumors to RT. Our data support the design of clinical trials integrating targeted Treg inhibitors in the standard of care for cancer patients receiving RT.