Treatment of murine tumors with recombinant Myxoma viruses
Oncolytic viruses are designed to be novel cancer treatments that not only specifically target cancer cells, but can act as vectors to express proteins of interest within the tumor bed. Myxoma virus (MYXV), a rabbit-specific poxvirus, was evaluated as an oncolytic viral treatment for cancer in immunocompetent mouse models of cancer. In addition, a recombinant MYXV that expresses the murine gene for interleukin 15 (IL-15) was also used, in order to determine if increased expression of IL-15 within the tumor bed could improve MYXV treatment by recruiting additional anti-tumor immune cells to the tumor. In vitro experiments evaluated MYXV infectivity in murine melanoma, glioma, and lymphoma cells using a recombinant MYXV that expresses the fluorescent protein tdTomato (MYXV:Tomato). These studies indicated that B16-F10 melanoma cells were fully-permissive to MYXV infection, while GL261 glioma cells were partially permissive, and EL4 cells were non-permissive. All stages of virus morphology were observed in the permissive cells and there was evidence of cell-to-cell spread of MYXV. Other studies have shown that the susceptibility of human cancer cell lines to MYXV infection depends on the presence of phosphorylated Akt, which was present in the B16-F10 melanoma and GL261 glioma cell lines. Cancer cell death occurred following infection with MYXV and was likely due to cell lysis and necrosis as apoptosis was not evident at 48 hours post infection. Initial in vivo studies using immunocompetent mice bearing intracranial melanoma or lymphoma tumors demonstrated that a single inoculation of intratumoral (IT) MYXV:Tomato was safe but did not produce any significant increase in survival, while studies using intracranial GL261 were inconclusive. However, other studies have shown that MYXV treatment of melanoma not located in the brain can increase survival if multiple injections are given. Therefore, weekly IT injections of MYXV or MYXV expressing the gene for murine IL-15 (MYXV:IL15) were administered in an immunocompetent mouse model of subcutaneous melanoma. The injections were safe and MYXV:IL15 did not cause any detectable adverse clinical signs or any significant increase in viral clearance. A statistically significant increase in survival time was obtained in mice given repeated IT injections of MYXV or MYXV:IL15 as compared to controls. The survival time in tumor-bearing mice treated with MYXV expressing IL-15 was not significantly different than mice treated with MYXV. In addition, MYXV-driven expression of IL-15 was evaluated to determine if it would recruit natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) to the tumor. An extensive pathologic study of the mouse model including histopathology and immunohistochemistry (IHC) was performed at various time points to determine the levels of immune cell infiltration in the tumor and the effects of intratumoral virus inoculation. The number of lymphocytes and neutrophils observed histologically to be infiltrating into the tumors as well as into the tissue surrounding the tumors was significantly increased in mice treated with live MYXV or MYXV:IL15 versus PBS treated control mice. However, IHC revealed that a statistically significant increase in the number of infiltrating cytotoxic lymphocytes (NK cells or CTLs) was not present in MYXV:IL15 treated mice versus control mice, although increased levels of IL-15 were detected within the tumors. Serum neutralization assay revealed that some of the mice treated with MYXV, MYXV:IL15, or UV-MYXV, developed an antibody response to MYXV. The presence of an antibody response did not correlate with survival time or the amount of leukocyte response observed by histology or IHC. In conclusion, treatment with weekly IT injections of MYXV or MYXV:IL15 significantly prolonged survival in mice bearing B16-F10 tumors, which correlated with an increase in the number of neutrophils and lymphocytes infiltrating in and around the tumors, but MYXV:IL15 did not increase survival over MYXV alone.
cancer, interleukin-15, Melanoma, myxoma virus, oncolytic virus, neutrophils, C57BL/6 mouse, B16-F10
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign