Enhanced tumor recognition and killing using the HSV-TK suicide gene

Genetically altered tumor cells expressing the Herpes Simplex Virus thymidine kinase gene (HSV-TK) is being used in the treatment of cancers based on the ability of the HSV-TK gene modified cells to kill nearby unmodified cells, termed as the bystander effect. Using this system, we have demonstrated in an i.p. therapeutic murine tumor model that the bystander effect was generated by a cytokine cascade post inoculation of HSV-TK gene modified tumor cells. TNF, IL-1, IL-6, IL-2, IFN-y and OM-CSF could be detected within the tumor by RT-PCR at various times post injection. Semi quantitative PCR showed an increase in TNF-a mRNA with time after treatment. We observed a hemorrhagic tumor necrosis and could demonstrate the influx of T cells and macrophages within the tumor post treatment. Based on these observations we proposed that the HSV-TK gene modified cells can alter the tumor microenvironment from inhibitory to one that is stimulatory. The present study investigated whether HSV-TK treatment could induce the expression of the costimulatory molecules 1CAM, B7-1 and B7-2 on lympbocytic infiltrates in vivo. In vivo, analysis for B7-1, B7-2 and ICAM expression by immunohistochemistry and RT-PCR revealed significant expression (p=0.031) on tumor infiltrating mononuclear cells from mice receiving HSV-TK treatment in comparison to mice not receiving the treatment. Furthermore, tumor infiltrating lymphocytes isolated from tumors after inoculation of the HSV-TK gene modified cells showed a significant increase in proliferation in response to syngeneic tumor cells. The results of this study suggest that the HSV-TK suicide gene can stimulate the release of cytokines within the tumor which induces the expression of costimulatory molecules and subsequent stimulation of tumor infiltrating lymphocytes. (Supported by CAGNO grant to AJM).