**22. Combing oncolytic virus treatment with immune check point blockade**

Immune check point blockade therapy (ICB) is extensively in cancer treatment, and long-term clinical outcomes are promising. Clinical responses are associated with pre-existing antitumor immune responses, such as an increased number of TILs, a high mutation load, and the formation of a diverse neoantigen repertoire [150, 151]. Combination therapy utilizing ICB and oncolytic viruses are appealing because the oncolytic virus can drive recruitment of TILs into immune-deficient tumors and prompt the production of soluble tumor antigens, danger signals, and pro-inflammatory cytokines, which can improve T cell recruitment and boost immune cell activation. Viral infection also raises the expression of CTLA4, PDL1, and other immunological checkpoint molecules, which would normally inhibit T cell activation (and so antitumor immunity), but also makes tumors more susceptible to ICB (**Figure 7**) [152, 153]. Preclinical research with a B16–F10 melanoma indicated that localized injection of tumors with oncolytic Newcastle disease virus caused infiltration of tumor-specific CD4+ T cells and CD8+ T cells into both the injected tumor and distant tumors, as well as improved tumor susceptibility to systemic CTLA4 inhibition 18. An oncolytic virus Maraba demonstrated therapeutic potential as a neoadjuvant in a preclinical model of triple-negative breast cancer and sensitized previously refractory tumors to ICB [154]. Several additional oncolytic viruses, including B18R-deficient vaccinia virus and vesicular stomatitis virus expressing a library of melanoma antigens (VSV- ASMEL), also shown substantial (P 0.05) therapeutic effect when used in tandem with ICB [155, 156]. Administration of T- VEC intratumorally, followed by anti-CTLA4 antibody (ipilimumab) treatment via intravenous injection, demonstrated an object response rate of 50%, with 44% of patients showing robust responses lasting more than 6 months in a phase Ib clinical trial. Also, no dose limiting toxicities were observed in the patients [157]. Additionally, a recent study reported that treatment with oncolytic poxvirus CF33-hNIS-ΔF14.5 modulates tumor microenvironment in TNBC model, and increases the response of tumor cells towards anti-PD-L1 antibody. Tumor microenvironment is one of the central plays in tumor growth, metastasis and

#### *Drug Repurposing - Molecular Aspects and Therapeutic Applications*

**Figure 7.** *Represents the role of oncolytic virus in immunotherapy.*

development of resistance. Further *in vivo* and *in vitro* analysis revealed that infection with the virus stimulated expression of PD-L1 in TNBC cells. Also, exposure of mice model of TNBC to oncolytic poxvirus CF33-hNIS-ΔF14.5 enhanced infiltration of CD8+ T cells and increased expression of proinflammatory cytokines IFNγ and IL-6 by tumor cells. Combinational treatment with oncolytic poxvirus CF33 hNIS-ΔF14.5 and anti-PD-L1 antibody augmented TME modulation and induced 50% tumor regression in mice models. Administration of these as single agents failed to inhibit tumor growth. Besides, it was also observed that the recovered mice with combinational treatment did not develop tumor after re-challenge with the same cancer cells suggesting that they developed immunity against those cancer cells [158, 159].

Taken together, studies demonstrate that oncolytic virus treatment positively induces tumor immune microenvironment modulation in triple-negative breast cancer model making them responsive to the immune checkpoint inhibitors and hence warrants further studies to determine the clinical applicability of this combination approach.
