Human triple negative breast cancer (TNBC) is an aggressive disease, associated with a high rate of recurrence and metastasis. Current therapeutics for TNBC are limited, highly toxic and show inconsistent efficacy due to a high degree of intra-tumoral and inter-tumoral heterogeneity. Oncolytic viruses (OVs) are an emerging treatment option for cancers. Several OVs are currently under investigation in preclinical and clinical settings. Here, we examine the oncolytic potential of two tanapoxvirus (TPV) recombinants expressing mouse monocyte chemoattractant protein (mMCP)-1 [also known as mCCL2] and mouse interleukin (mIL)-2, in human TNBC, in vitro and in vivo. Both wild-type (wt) TPV and TPV recombinants demonstrated efficient replicability in human TNBC cells and killed cancer cell efficiently in a dose-dependent manner in vitro. TPV/∆66R/mCCL2 and TPV/∆66R/mIL-2 expressing mCCL2 and mIL-2, respectively, suppressed the growth of MDA-MB-231 tumor xenografts in nude mice significantly, as compared to the mock-injected tumors. Histological analysis of tumors showed areas of viable tumor cells, necrotic foci and immune cell accumulation in virus-treated tumors. Moreover, TPV/∆66R/mIL-2-treated tumors showed a deep infiltration of mononuclear immune cells into the tumor capsule and focal cell death in tumors. In conclusion, TPV recombinants expressing mCCL2 and mIL-2 showed a significant therapeutic effect in MDA-MB-231 tumor xenografts, in nude mice through induction of potent antitumor immune responses. Considering the oncolytic potency of armed oncolytic TPV recombinants expressing mCCL2 and mIL-2 in an experimental nude mouse model, these viruses merit further investigation as alternative treatment options for human breast cancer.

Neuregulin (NRG), an epidermal growth factor is known to promote the growth of various cell types, including human melanoma cells through ErbB family of tyrosine kinases receptors. Tanapoxvirus (TPV) encoded protein TPV-15L, a functional mimic of NRG, also acts through ErbB receptors. Here, we show that the TPV-15L protein promotes melanoma proliferation. TPV recombinant generated by deleting the 15L gene (TPVΔ15L) showed replication ability similar to that of wild type TPV (wtTPV) in owl monkey kidney (OMK) cells, human lung fibroblast (WI-38) cells and human melanoma (SK-MEL-3) cells. Whereas, a TPV recombinant with both 15L and the thymidine kinase (TK) gene (66R) ablated (TPVΔ15LΔ66R) replicated less efficiently than TPVΔ15L and the parental virus. TPVΔ15L exhibited more robust tumor-regression in the melanoma-bearing nude mice than other TPV recombinants. Our results indicate that deletion of TPV-15L gene product which facilitates the growth of human melanoma cells can be an effective strategy to enhance the oncolytic potential of TPV for the treatment of melanoma.

Melanoma is the deadliest skin cancer with ever-increasing incidence. Despite the development in diagnostics and therapies, metastatic melanoma is still associated with significant morbidity and mortality. Oncolytic viruses (OVs) represent a class of novel therapeutic agents for cancer by possessing two closely related properties for tumor reduction: virus-induced lysis of tumor cells and induction of host anti-tumor immune responses. A variety of viruses, either in “natural” or in genetically modified forms, have exhibited a remarkable therapeutic efficacy in regressing melanoma in experimental and/or clinical studies. This review provides a comprehensive summary of the molecular and cellular mechanisms of action of these viruses, which involve manipulating and targeting the abnormalities of melanoma, and can be categorized as enhancing viral tropism, targeting the tumor microenvironment and increasing the innate and adaptive antitumor responses. Additionally, this review describes the “biomarkers” and deregulated pathways of melanoma that are responsible for melanoma initiation, progression and metastasis. Advances in understanding these abnormalities of melanoma have resulted in effective targeted and immuno-therapies, and could potentially be applied for engineering OVs with enhanced oncolytic activity in future.

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