not-yet-known not-yet-known not-yet-known unknown Oncolytic virotherapy (OVT) aims to disrupt the tumor microenvironment and provide a unique therapeutic approach against solid tumors. Herpes Simplex Virus Type-1 (HSV-1) has shown strong promise for treating various solid tumors and has been approved to treat melanoma and glioma in human patients. Previously, we reported the generation of an engineered HSV-1 vaccine strain VC2, which has shown exceptional promise as an oncolytic and immunotherapeutic virus. In the present work, we engineered VC2 to constitutively express the murine Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF) gene inserted in place of HSV-1 Glycoprotein C (gC). We tested the efficacy of VC2-GMCSF for its ability to generate anti-tumor response in the 4T1 stage four metastatic breast cancer mouse model. GM-CSF expression enhanced VC2 viral replication and infectious virus production. Tumors formed after 7 days of engraftment in the mammary fat-pad of Balb/CJ mice were treated by injecting ~10 6 plaque forming units (PFU) once. Intra-tumor treatment did not appreciably reduce average primary tumor sizes. However, metastatic foci were significantly reduced in mice lungs treated with VC2-GMCSF compared to VC2 or mock treatment. VC2-GMCSF intratumoral treatment induced a stronger intratumor T cell infiltration but not an increased cytotoxic activity. A significant T cell infiltration was observed in the metastatic areas in VC2-GMCSF treated animals, which was associated with reduced pro-tumor marker PDL1 and VEGF gene expression. These results show that constitutive expression of GM-CSF enhanced the overall efficacy of VC2 for oncolytic virotherapy. VC2-GMCSF holds promise as an oncolytic and immunotherapeutic virotherapy for breast and other cancers.