Immunotherapy of tumor-bearing mice utilizing virus help.

Utilizing vaccinia virus (VV), a tumor-specific immunotherapy model was established in which a growing tumor regressed. C3H/HeN mice were primed with VV after low dose irradiation to generate amplified VV-reactive T cell activities. Then 4 weeks later, the mice were inoculated i.d. with syngeneic MH134 hepatoma cells, and 6 days after the tumor cell inoculation, live VV was injected into the tumor mass 3 times at 2-day intervals. Of 10 mice which had received VV priming and subsequent VV injection into the tumor mass, 8 exhibited complete tumor regression. On the contrary, mice which had received only intratumoral VV injection without VV priming failed to exhibit appreciable tumor regression. Mice whose tumor had completely regressed following the VV immunotherapy were shown to have acquired systemic antitumor immunity, which was confirmed by a challenge with syngeneic tumor cells after immunotherapy. In vitro analysis of these immune mice revealed that potent tumor-specific antibody responses were preferentially induced, but with no detectable antitumor cytotoxic T lymphocyte (CTL) responses. Such a potent tumor-specific immunity was not observed in mice which had received intratumoral VV injection in the absence of VV priming. Thus, the results clearly indicate that tumor regression was accompanied by the concurrent generation of a potent tumor-specific immunity, suggesting that cellular cooperation between VV-reactive T cells and tumor-specific effector cells might be functioning in this VV immunotherapy protocol. Therefore, the present model provides an effective maneuver for tumor-specific immunotherapy. This system is, in principle, applicable to the human situation.