Therapeutic immune response induced by intratumoral expression of the fusogenic membrane protein of vesicular stomatitis virus and cytokines encoded by adenoviral vectors.

We assessed whether intratumoral expression of the fusogenic membrane protein of vesicular stomatitis virus (VSV-G), encoded by a replication-defective adenovirus vector (Ad.VSV-G), alone or in combination with local coexpression of cytokines induces tumor-specific immune responses in a syngeneic murine colon cancer model. We confirmed in vitro by dye colocalization that transduction of murine cells with Ad.VSV-G induces cell-cell fusion. In a bilateral syngeneic subcutaneous colon cancer model in C57BL/6 and BALB/c mice, we demonstrated that intratumoral injection of Ad.VSV-G leads to a significant growth reduction of the directly vector-treated tumor, but also of the contralateral not directly vector-treated tumor. When compared to monotherapy, the anti-neoplastic efficacy was significantly enhanced when intratumoral Ad.VSV-G administration was combined with adenovirus vectors encoding IL-2, IL-12, IL-18, IL-21, or GM-CSF. The anti-tumor effects of the first three cytokines in combination with VSV-G expression were somewhat greater than those of the latter two. However, the differences did not reach statistical significance. The combination therapy resulted also in a significantly enhanced survival when compared to monotherapy. In addition, we demonstrated that intratumoral expression of VSV-G in combination with the tested cytokines induced a strong tumor-specific cytotoxic T lymphocyte (CTL) response and infiltration of tumors with macrophages. The effects of the combination therapy were clearly greater than those of the monotherapy. Our experimental data indicate that intratumoral expression of VSV-G, particularly in combination with cytokines, is a promising novel tool for the development of in situ tumor vaccination approaches.