Thymosin α-1 Reverses M2 Polarization of Tumor-Associated Macrophages during Efferocytosis

The immunologic effects of chemotherapy-induced tumor cell death are not completely understood. Accumulating evidence suggests that phagocytic clearance of apoptotic tumor cells, also known as efferocytosis, is an immunologically silent process, thus maintaining an immunosuppressive tumor microenvironment (TME). Here we report that, in the breast tumor microenvironment, thymosin α-1 (Tα-1) significantly reverses M2 polarization of IL10-producing tumor-associated macrophages (TAM) during efferocytosis induced by apoptotic cells. Mechanistically, Tα-1, which bound to phosphatidylserine on the surface of apoptotic tumor cells and was internalized by macrophages, triggered the activation of SH2-containing inositol 5'-phosphatase 1 (SHIP1) through the lysosomal Toll-like receptor 7 (TLR7)/MyD88 pathway, subsequently resulting in dephosphorylation of efferocytosis-activated TBK1 and reduction of efferocytosis-induced IL10. Tα-1 combined with epirubicin chemotherapy markedly suppressed tumor growth in an in vivo breast cancer model by reducing macrophage-derived IL10 and enhancing the number and function of tumor-infiltrating CD4+ and CD8+ T cells. In conclusion, Tα-1 improved the curative effect of chemotherapy by reversing M2 polarization of efferocytosis-activated macrophages, suggesting that Tα-1 injection immediately after chemotherapy may contribute to highly synergistic antitumor effects in patients with breast cancer. SIGNIFICANCE: Thymosin α-1 improves the curative effect of chemotherapy by reversing efferocytosis-induced M2 polarization of macrophages via activation of a TLR7/SHIP1 axis.