Two distinct mechanisms of augmented antitumor activity by modulation of immunostimulatory/inhibitory signals

Purpose: Blockade of CTL-associated antigen-4 (CTLA-4), an inhibitory immunomodulatory molecule on T cells, has been shown to enhance T-cell responses and induce tumor rejection, and a number of clinical trials with anti-CTLA-4 blocking monoclonal antibody (mAb) are under way. However, accumulating evidence indicates that anti-CTLA-4 mAb increases the number of CD4⁺CD25 ⁺ Foxp3⁺ regulatory T cells (Treg) and that anti-CTLA4 mAb alone is often insufficient to reject established tumors in mice and humans. Thus, finding maneuvers to control Tregs and other immunosuppressive mechanisms remains a critical challenge. Experimental Design: The potential to enhance antitumor immune responses by combining anti-CTLA-4 mAb with anti-glucocorticoid-induced tumor necrosis factor receptor family related gene (GITR) mAb, a costimulatory molecule that abrogates directly/indirectly Treg-mediated immune suppression or anti-CD25 mAb that depletes Tregs was analyzed with two tumor models, CT26 (a murine colon carcinoma cell line) and CMS5a (a murine fibrosarcoma cell line). Results: Anti-CTLA-4/anti-GITR mAb combination treatment exhibited far stronger antitumor effects compared with either antibody alone. This strong antitumor effect was attributed to (a) increased numbers of CD8⁺ T cells infiltrating tumor sites in anti-CTLA-4 mAb-treated mice and (b) increased cytokine secretion and Treg resistance of tumor-specific CD8⁺ T cells with strongly upregulated CD25 expression in anti-GITR mAb-treated mice, indicating distinct quantitative/qualitative changes induced by modulating CTLA-4 and GITR signaling. Conclusions: This study shows that combined treatment with different immune modulators can augment antitumor immune responses and provides justification for exploring anti-CTLA-4/anti-GITR mAb combination treatment in the clinic.