Introduction. Oncolytic viruses (OV), a novel class of anti-cancer therapies, selectively replicate in cancer cells and activate the immune response. The unique OV-targeted destruction of malignant cells sets OV-based therapies apart from the non-specific conventional chemo-, radio-, and even targeted antibody-based therapies. However, the sensitivity of cancer cell lines to rVSV and, hence, the success of virotherapy, may differ depending on cancer type and the strength of the anti-viral response. At the same time, the oncolytic potency of rVSV decreases as a result of the insertion of larger genes (such as interleukin-12, IL12, and granulocyte macrophage colony-stimulating factor, GMCSF, used to potently activate the dampened immune response) into the viral genome as compared to green fluorescent protein (GFP). Therefore, the combination of rVSV and mRNA-delivered immunostimulatory factors may increase the efficiency of anti-tumor therapy.
Methods. rVSV-mIL12-mGMCSF cytotoxicity was assessed in vitro on three murine cancer cell lines: B16-F10, LL/2, and CT26 24 hours post-infection by flow cytometry. In vivo experiments were performed on syngeneic models via intratumoral injection of rVSV. Quantitative PCR (qPCR) analysis was performed to assess the mRNA expression levels 12 hours post-infection in vitro.
Results. Based on the obtained data, Lewis lung carcinoma (LL/2) cells were more susceptible to oncolysis compared to colon carcinoma (CT26) or melanoma (B16-F10). Detection of expression levels of certain antiviral and pro-apoptotic genes in response to the rVSV infection by qPCR showed higher levels of IFNβ and p53 and lower levels of IFIT, RIG-I, and N-cadherin in LL/2 cells. Subsequently, C57BL/6 and BALB/c mice, infused subcutaneously with B16-F10 and LL/2 cells, respectively, were injected intratumorally with rVSV-mIL12-mGMCSF to assess the synergistic effect of rVSV and immunostimulatory factors. The in vivo study demonstrated tumor growth inhibition (TGII) of over 50% for both cancer models. Finally, we assessed the combined effect of two platforms: mRNA-LNP-based delivery of mIL-12 and mGMCSF and rVSV-mediated oncolysis, leading to TGII of 95% in animals with colon cancer.
Conclusions. The unique combination therapy approach significantly improved therapy outcome. These promising results are giving hope that this novel approach will also be successful for melanoma and lung carcinoma models in the near future.