Anti-PD-1 antibody combined with chemotherapy suppresses the growth of mesothelioma by reducing myeloid-derived suppressor cells.

• CDDP/PEM enhanced anti-tumor effects of anti-PD-1 antibody against MPM in mice. • CDDP/PEM decreased intratumoral myeloid-derived suppressor cells (MDSCs). • CDDP/PEM reduced VEGF in MPM cells, which is a chemotactic factor for MDSCs. The combination of anti-PD-1/PD-L1 antibody with chemotherapy has been approved for the first-line therapy of lung cancer. However, the effects against malignant mesothelioma (MPM) and the immunological mechanisms by which chemotherapy enhances the effect of targeting PD-1/PD-L1 in MPM are poorly understood. We utilized syngeneic mouse models of MPM and lung cancer and assessed the therapeutic effects of anti-PD-1 antibody and its combination with cisplatin (CDDP) and pemetrexed (PEM). An immunological analysis of tumor-infiltrating cells was performed with immunohistochemistry. We observed significant therapeutic effects of anti-PD-1 antibody against MPM. Although the effect was associated with CD8+ and CD4+ T cells in tumors, the number of Foxp3+ cells was not reduced but rather increased. Consequently, combination with CDDP/PEM significantly enhanced the antitumor effects of anti-PD-1 antibody by decreasing numbers of intratumoral myeloid-derived suppressor cells (MDSCs) and vessels probably through suppression of VEGF expression by CDDP + PEM. The combination of anti-PD-1 antibody with CDDP + PEM may be a promising therapy for MPM via inhibiting the accumulation of MDSCs and vessels in tumors. [ABSTRACT FROM AUTHOR]