Abstract

Tamoxifen has been commonly used in endocrinotherapy for estrogen receptor (ER)-positive breast cancer. In recent years, the immunomodulatory effects of high-dose tamoxifen have been discovered. The immunosuppressive tumor microenvironment (TME), however, remains a major obstacle to the efficacy of radiotherapy. Whether high-dose tamoxifen can reprogram TME to synergistically enhance radiation efficacy is still ambiguous. Here, we have found high-dose tamoxifen could dramatically enhance radiation-induced antitumor effects without significant side effects in immunocompetent mice. Flow cytometry and multiplex immunofluorescence experiments revealed that radiation combined with tamoxifen resulted in significant enrichment of effector CD8 + T cells and M1 tumor associated macrophages (TAMs). While depletion of TAMs and CD8 + T cells impaired the synergistic antitumor effects mediated by combination treatment of radiation and tamoxifen. Furthermore, spatial proximity analysis demonstrated a dramatically reduced nearest-neighbor distance between CD8 + T cells and M1-like TAMs in the combination group, indicating enhanced cellular interaction within the TME. In vitro experiments demonstrated that tamoxifen directly acted on TAMs, rather than CD8 + T cells, to promote M1 polarization and subsequently enhanced the activation and effector function of CD8 + T cells, regardless of ER expression in tumor cells and macrophages. Mechanistically, RNA-sequencing and experimental validation uncovered that tamoxifen and tumor cell-derived TNF-α and IL-1β after irradiation synergistically activated the JNK/c-JUN pathway and promoted M1 polarization of TAMs. In conclusion, our study revealed the immunomodulatory effects of high-dose tamoxifen in the context of radiation and provided preclinical evidence for combination therapy of high-dose tamoxifen and radiotherapy in both ER-positive and ER-negative cancers.