Antitumor efficacy of CD137 ligation is maximized by the use of a CD137 single-chain Fv-expressing whole-cell tumor vaccine compared with CD137-specific monoclonal antibody infusion

Tumor-destructive immune responses can be generated by engaging CD137 (4-1BB) via infusing a monoclonal antibody specific for CD137 or vaccinating with a single-chain Fv (scFv) CD137-expressing whole-cell tumor vaccine. We assessed whether such a vaccine can induce tumor rejection in the neu-transgenic (neu-Tg) mouse breast cancer model and compared the antitumor efficacy of vaccination with the infusion of a CD137-specific antibody. Mammary carcinoma cells (MMC) from a neu-Tg mouse were transfected to stably express surface scFv derived from the anti-CD137 rat hybridoma 1D8 or 3H3. The anti-CD137 scFv-expressing cells were rejected when transplanted into neu-Tg mice by a mechanism that involved both CD4⁺ and CD8⁺ T cells, and vaccination with such cells delayed the outgrowth of MMC cells transplanted 3 days previously. T cells from neu-Tg mice that had been vaccinated proliferated and produced IFN-γ when stimulated by MMC but not by antigen-negative variant breast cancer cells that did not express the neu tumor antigen. In addition, antibodies binding to the MMC but not to antigen-negative variant cells were detected in sera from some but not all of the immunized mice. Complete regression of s.c. transplanted MMC tumors was observed in mice repeatedly immunized against MMC-1D8 starting on the day the MMC cells were transplanted. In contrast, repeated administration of either of two different anti-CD137 monoclonal antibodies did not induce complete tumor regression, although tumor growth was delayed.