NAD+ Metabolism Reprogramming Mediates Irradiation-Induced Immunosuppressive Polarization of Macrophages.

Purpose: Radiation therapy stands as an important complementary treatment for head and neck squamous cell carcinoma (HNSCC), yet it does not invariably result in complete tumor regression. The infiltration of immunosuppressive macrophages is believed to mediate the radiation therapy resistance, whose mechanism remains largely unexplored. This study aimed to elucidate the role of immunosuppressive macrophages during radiation therapy and the associated underlying mechanisms.

Methods and materials: Male C3H mice bearing syngeneic SCC-VII tumor received irradiation (2 × 8 Gy). The impact of irradiation on tumor-infiltrating macrophages was assessed. Bone marrow-derived macrophages were evaluated in differentiation, proliferation, migration, and inflammatory cytokines after treatment of irradiated tumor culture medium and irradiated tumor-derived extracellular vesicles (irTEVs). A comprehensive metabolomics profiling of the irTEVs was conducted using liquid chromatography-mass spectrometry, whereas key metabolites were investigated for their role in the mechanism of immunosuppression of macrophages in vitro and in vivo.

Results: Radiation therapy on SCC-VII syngeneic graft tumors increased polarization of both M1 and M2 macrophages in the tumor microenvironment and drove infiltrated macrophages toward an immunosuppressive phenotype. Irradiation-induced polarization and immunosuppression of macrophages were dependent on irTEVs which delivered an increased amount of niacinamide (NAM) to macrophages. NAM directly bound to the nuclear factor kappa-B transcriptional activity regulator USP7, through which NAM reduced translocation of nuclear factor kappa-B into the nucleus, thereby decreasing the release of cytokines interleukin 6 and interleukin 8. Increased enzyme activity of NAM phosphoribosyl transferase which is the rate-limiting enzyme of NAD+ metabolism, contributed to the irradiation-induced accumulation levels of NAM in irradiated HNSCC and irTEVs. Inhibition of NAM phosphoribosyl transferase decreased NAM levels in irTEVs and increased radiation therapy sensitivity by alleviating the immunosuppressive function of macrophages.

Conclusions: Radiation therapy could induce NAD+ metabolic reprogramming of HNSCC cells, which regulate macrophages toward an immunosuppressive phenotype. Pharmacologic targeting of NAD+ metabolism might be a promising strategy for radiation therapy sensitization of HNSCC.