Monoclonal antibodies directed against the T-cell activation molecule CD137 (interleukin-A or 4-1BB) block human lymphocyte-mediated suppression of tumor xenografts in severe combined immunodeficient mice

A monoclonal antibody specific for the human analog of the murine T-cell activation molecule 4-1BB was generated and is shown here to react selectively with activated human CD4⁺ and CD8⁺ T lymphocytes. Treatment of these T cells in a one-way mixed lymphocyte culture with the anti-h4-1BB antibody enhanced the cell proliferation of the allostimulated lymphocytes. Previous studies in the mouse have shown that treatment of tumor-bearing mice with antibodies to 4-1BB augments antitumor immunity that is mediated by both CD4⁺ and CD8⁺ T cells. The authors consider the possibility that a similar approach may be efficacious for human cancer immunotherapy. This question was addressed by evaluating the effect of an anti-h4-1BB monoclonal antibody on human lymphocyte-mediated suppression of a human tumor xenograft in SCID mice. Mice treated with a control antibody and co-injected with the tumor and peripheral blood lymphocytes exhibited a lymphocyte dose-dependent suppression of tumor growth. In mice treated with the anti-h4-1BB antibody, the lymphocyte, mediated tumor suppression was completely eliminated and tumors grew progressively (as was observed in mice inoculated with tumors without lymphocytes). This monoclonal antibody specific for anti-h4-1BB, which augments the proliferation of allostimulated cells in vitro, blocks T- cell anti-tumor activity in vivo. These results suggest that although 4-1BB plays a role in the human peripheral blood lymphocyte-mediated suppression of tumor growth, antibodies to this molecule on human cells fail to stimulate anti-tumor activity, as was observed in tumor-bearing mice treated with an antibody to murine 4-1BB.