Background Malignant peripheral nerve sheath tumor (MPNST) is a rare, aggressive soft-tissue sarcoma with a poor prognosis and is insensitive to immune checkpoint blockade (ICB) therapy. Loss-of-function of the histone-modifying polycomb repressive complex 2 (PRC2) components, EED or SUZ12, is one of the main mechanisms of malignant transformation. Our ongoing collaborative work revealed that in a murine model of MPNST, PRC2-loss tumors have an ‘immune desert’ phenotype and intratumoral (IT) delivery immunogenic modified vaccinia virus Ankara (MVA) sensitized the PRC2-loss tumors to ICB. IT MQ833, a second-generation recombinant modified vaccinia virus Ankara virus, results in neutrophil recruitment and activation and neutrophil-dependent tumor killing. MQ833 was engineered by deleting three viral immune evasion genes, E5R, E3L, and WR199, and expressing three transgenes, including the two membrane-bound Flt3L and OX40L, and IL-12 with an extracellular matrix anchoring signal. In this study, we explored strategies to enhance anti-tumor effects of MQ833 by co-administration of granulocyte colony-stimulating factor (G-CSF) and elucidated the mechanisms of action.

Methods We first assessed the innate immune effects and transgene expression of MQ833 infection in murine and human MPNST cell lines. We then evaluated antitumor effects of MQ833 in immune-competent mice. Wild-type C57BL/6J mice were implanted subcutaneously with PRC2-loss SKP605 cells (Nf1^-/-Cdkn2a^-/- Cdkn2b^-/-Eed^{- /-}). IT MQ833 was performed twice weekly. Tumor sizes were measured and survival of mice was monitored. For antibody depletion experiment, mice were treated with anti-CD4, CD8, NK1.1, CSF1, or Ly6G antibodies to deplete CD4, CD8, NK, macrophages, and neutrophils. To assess whether G-CSF co-administration improves MQ833 therapeutic efficacy, G-CSF was co-delivered with MQ833 twice weekly into the tumors. In addition, to evaluate how co-administration of G-CSF with MQ833 alters the immunosuppressive tumor microenvironment, we analyzed the tumor-infiltrating myeloid cells and T cells by flow cytometry.

Results MQ833 infection potently induced type I IFN production in human and murine MPNST cells. IT MQ833 induced CD4 and CD8 T cell activation and neutrophil recruitment into PRC2-loss tumors. Antibody depletion experiment revealed the critical role of neutrophils, macrophages and CD4⁺ T cells in MQ833-induced antitumor immunity. In addition, co-administration of MQ833 and human G-CSF generated more potent antitumor efficacy in PRC2-wt tumors than either agent alone. Finally, our results show that MQ833 and hG-CSF combination treatment increased activated neutrophils in vivo.

Conclusions In conclusion, MQ833 represents a promising therapeutic approach for ICB-resistant tumors by mobilizing and activating neutrophils and macrophages, addressing a common mechanism of tumor resistance to ICB.