Induction of potent antitumor natural killer cell activity by herpes simplex virus-thymidine kinase and ganciclovir therapy in an orthotopic mouse model of prostate cancer

Adenovirus-mediated transduction of the herpes simplex virus-thymidine kinase (HSV-tk) gene followed by ganciclovir is suspected to induce immune- mediated, systemic antitumor activities in the RM-1 mouse prostate cancer model (S. J. Hall et al., Int. J. Cancer, 70: 183-187, 1997). Although numerous investigators have also implied a role for the immune system in both local and systemic effects resulting from HSV-tk treatment, the candidate effector cell(s) mediating these activities are unknown. Fresh lymphocytes harvested from treated tumors (tumor-infiltrating lymphocytes) generated significant in vitro lytic activity against the parental cell line, RM-1, and an unrelated prostate cancer cell line. In vitro antibody and complement depletion of CD3+ T cells and natural killer (NK) cells from tumor- infiltrating lymphocytes indicated that NK cells were the dominant mediator of the observed tumor cell lysis. Concurrently, no cytotoxic T-cell activity was ascertained within splenocytes of treated mice. In vivo depletion of NK cells resulted in a 20% reduction in growth suppression within the primary tumor and complete abrogation of the inhibition of preestablished lung metastases. Depletion of T cells had no effect on either response. Here, we identify the presence of NK cells within adenovirus/HSV-tk- and ganciclovir- treated tumors, which serve to mediate both local and systemic antitumor activities in this model, and lay the mechanistic groundwork for further improvements in this gene therapy strategy.