Vesicular stomatitis virus (VSV) is a novel, anti-cancer therapy that selectively targets cancer cells with defective antiviral responses; however, not all malignant cells are sensitive to the oncolytic effects of VSV. Herein, we explore the mechanistic determinants of mutant M protein VSV (M51R-VSV) susceptibility in malignant melanoma cells.
Cell viability after VSV infection was measured by the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) viability assay in a panel of melanoma cell lines. VSV infectability, viral protein synthesis and viral progeny production were quantified by flow cytometry, 35S-methionine electrophoresis, and viral plaque assays, respectively. Interferon (IFN) responsiveness was determined using MTS assay after β-IFN pre-treatment. Xenografts were established in athymic nude mice and treated with intratumoral M51R-VSV.
Cell viability after M51R-VSV infection at a multiplicity of infection (MOI) of 10 pfu/mL, 48 hours post-infection) ranged between 0±1 and 59±9% (mean ± standard deviation). Sensitive cell lines supported VSV infection, viral protein synthesis, and viral progeny production. In addition, when pre-treated with β-IFN, sensitive cells became resistant to M51R-VSV, suggesting that IFN-mediated antiviral signaling is defective in these cells. In contrast, resistant melanoma cells do not support VSV infection, viral protein synthesis, or viral replication, indicating that anti-viral defenses remain intact. In a murine xenograft model, intratumoral M51R-VSV treatment decreased tumor growth relative to controls after 26 days in SK-Mel 5 (−21±19% vs. 2100±770%, p<0.0001) and SK-Mel 3 (2000±810% vs 7000±3000%, p=0.008) established tumors.
M51R-VSV is a viable, anti-cancer therapy, but susceptibility varies among melanomas. Future work will exploit specific mechanisms of resistance to expand the therapeutic efficacy of M51R-VSV.