Abstract
IL-12 is a potent NK cell-stimulating cytokine, but the presence of immunosuppressive myeloid cells such as myeloid-derived suppressor cells (MDSC) can inhibit IL-12-induced NK-cell cytotoxicity. Thus, we hypothesized that trabectedin, a myeloid cell-depleting agent, would improve the efficacy of IL-12 in triple-negative breast cancer (TNBC). In vitro treatment of healthy donor NK cells with trabectedin increased expression of the activation marker CD69 and mRNA expression of T-box transcription factor (Tbx21), the cytotoxic ligands TNF-related apoptosis-inducing ligand (TNFSF10), Fas ligand (FASLG), and the dendritic cell (DC)-recruiting chemokine lymphotactin (XCL1). The combination of IL-12 and trabectedin increased NK-cell cytotoxicity and activation and production of IFN-γ, TNF-α, and granzyme B in the presence of human TNBC cells. Treatment of 4T1 and EMT6 tumor-bearing mice with IL-12 and trabectedin led to a significant reduction in tumor burden compared with single-agent controls and the highest levels of plasma IFN-γ, intratumoral CD8+ T cells, and conventional type 1 DC. MDSC and M2-like macrophages were significantly decreased with combination therapy. NK-cell depletion abrogated the effects of combination therapy, as did the elimination of CD8+ T cells. NK-cell depletion led to lower levels of the NK cell-derived chemokine CCL5 and the DC-derived chemokine CXCL10, higher tumor burden, and decreased intratumoral CD8+ T cells. IL-12 and trabectedin also significantly enhanced the response of TNBC to anti-PD-L1 therapy. These data suggest that MDSC depletion augments the ability of IL-12-activated NK cells to drive the infiltration of DC and CD8+ T cells into TNBC for an antitumor effect.