Background: Vesicular stomatitis virus (VSV) is a novel, anti-cancer therapy that targets cancer cells selectively with defective antiviral responses; however, not all malignant cells are sensitive to the oncolytic effects of VSV. Herein, we have explored the mechanistic determinants of mutant M protein VSV (M51R-VSV) susceptibility in malignant melanoma cells.
Methods: 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, (35)S-methionine electrophoresis, and viral plaque assays, respectively. Interferon (IFN) responsiveness was determined using MTS assay after β-IFN pretreatment. Xenografts were established in athymic nude mice and treated with intratumoral M51R-VSV.
Results: Cell viability after M51R-VSV infection at a multiplicity of infection of 10 pfu/mL, 48 hours postinfection) 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 pretreated 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 antiviral 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. 2,100 ± 770%; P < .0001) and in SK-Mel 3 (2,000 ± 810% vs. 7,000 ± 3,000%; P = .008) established tumors.
Conclusion: 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.
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