Upregulation of antitumor immunity by IL-12 gene-transfected AK-5 tumor cells in vivo.

We have earlier demonstrated a significant role for IL-12 in the regression of a rat histiocytic tumor, AK-5. In order to analyze further the antitumor immunity induced by interleukin (IL)-12, we have established IL-12-secreting tumor cell clones by gene transfection. Significant enhancement in the lytic potential of splenocytes by the culture supernatants containing IL-12 demonstrated retention of biological activity by the tumor-cell-derived cytokine. Athymic nude mice transplanted subcutaneously with tumor cells engineered to secret IL-12 showed a significant reduction in tumor size, with enhanced antibody-dependent cellular cytotoxicity. Analysis of the serum samples from animals injected with the IL-12 gene-transfected AK-5 cells on different days revealed a significant increase in circulatory IL-12, interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha and antitumor antibodies, all of which contributed to the reduction in tumor mass. The enhanced proliferative capacity of splenocytes from these animals indicated the presence of highly activated immune cells in vivo. Similarly, intraperitoneal transplantation of IL-12 gene-transfected tumor cells in syngeneic Wistar rats induced a significant increase in cellular cytotoxicity, with a concomitant reduction in circulatory IL-12 (p40) protein. Administration of antibodies to IL-12 and IFN-gamma reduced the expression of the costimulatory molecules B7.1 and B7.2 and the cytolytic effectors granzyme B and Fas-L, suggesting their involvement in IFN-gamma-dependent antitumor immune response induced by IL-12. The present study thus demonstrates that IL-12 gene therapy could be among the promising approaches for an effective cancer therapy.