TGF-β Inhibition Improves Oncolytic Herpes Viroimmunotherapy in Murine Models of Rhabdomyosarcoma

Oncolytic viruses are an emerging class of cancer therapeutics that couple cytotoxicity with the induction of an anti-tumor immune response. Host-virus interactions are complex and modulated by a tumor microenvironment whose immunosuppressive activities can limit the effectiveness of cancer immunotherapies. In an effort to improve this aspect of oncolytic virotherapy, we combined the oncolytic herpes virus HSV1716 with the transforming growth factor beta receptor 1 (TGF-βR1) inhibitor A8301 to treat syngeneic models of murine rhabdomyosarcoma. Mice that received HSV1716 or A8301 alone showed little to no benefit in efficacy and survival over controls. Conversely, mice given combination therapy exhibited tumor stabilization throughout the treatment regimen, which was reflected in significantly prolonged survival times including some complete responses. In vitro cell viability and virus replication assays showed that the rhabdomyosarcoma cell lines were generally insensitive to HSV1716 and A8301. Likewise, in vivo virus replication assays showed that HSV1716 titers moderately decreased in the presence of A8301. The enhanced efficacy instead appears to be dependent on the generation of an improved anti-tumor T cell response as determined by its loss in athymic nude mice and following in vivo depletion of either CD4+ or CD8+ cells. These data suggest TGF-β inhibition can augment the immunotherapeutic efficacy of oncolytic herpes virotherapy.