Abstract

Resistance to immune checkpoint blockade (ICB) remains a major obstacle in cancer immunotherapy. We rationally engineered a second-generation recombinant Modified Vaccinia virus Ankara (MQ833) by deleting viral immune-evasion genes (E3L, E5R, WR199) and incorporating Flt3L, OX40L, and matrix-anchored IL12. Intratumoral MQ833 elicited robust tumor regressions across multiple models, including ICB-resistant and MHC-I-deficient tumors. Single-cell RNA sequencing revealed extensive remodeling of the tumor microenvironment, characterized by neutrophil and monocyte recruitment and activation, M2 macrophage depletion, M1 polarization, and effector T-cell differentiation and proliferation. Conditional Ifnar1 knockout mice demonstrated that MQ833 efficacy requires type I interferon signaling in neutrophils, macrophages/monocytes, and T cells. Moreover, Nos2 deficiency impaired therapeutic efficacy, confirming iNOS-positive myeloid cells as key effectors. Together, these findings show that MQ833 activates innate and adaptive IFN cross-talk to reprogram myeloid and T cells, defining a rationally designed viral immunotherapy capable of overcoming ICB resistance.