Virotherapy-recruited PMN-MDSC infiltration of mesothelioma blocks antitumor CTL by IL-10-mediated dendritic cell suppression.

Antitumor cytotoxic T lymphocytes (CTLs) are essential for immune surveillance, yet the blockade of eliciting such CTLs during oncolytic virotherapy remains incompletely understood. Here, we show that oncolysis of mesothelioma by modified vaccinia Tiantan (MVTT) induces damage-associated molecular patterns exposure. Although MVTT leads to regression of established mesothelioma dose-dependently, antitumor CTLs are rarely induced. Mechanistically, MVTT virotherapy generates C-X-C chemokines that recruit CXCR2-expressing polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) into tumor microenvironment, where they suppress dendritic cells (DCs) by producing IL-10 and halt CTL responses. During the virotherapy, however, depletion of PMN-MDSCs but not of monocytic (M)-MDSCs results in the induction of potent antitumor CTLs that not only eradicate established mesothelioma but also prevent the second tumor challenge. Our findings suggest that vaccinia virotherapy may combine strategies that prevent the chemotactic recruitment of PMN-MDSCs, block their suppression on DCs or deplete PMN-MDSCs in order to induce potent CTLs for tumor eradication.