Anti-CTLA-4 immunotherapy does not deplete Foxp3 ^þ regulatory T cells (Tregs) in human cancers

Purpose: CTLA-4 was the first inhibitory immune checkpoint to be identified. Two mAbs, ipilimumab (IgG1) and tremelimumab (IgG2), which block the function of CTLA-4, have demonstrated durable clinical activity in a subset of patients with advanced solid malignancies by augmenting effector T-cell–mediated immune responses. Studies in mice suggest that anti-CTLA-4 mAbs may also selectively deplete intratumoral FOXP3 ^þ regulatory T cells via an Fc-dependent mechanism. However, it is unclear whether the depletion of FOXP3 ^þ cells occurs in patients with cancer treated with anti-CTLA-4 therapies. Experimental Design: Quantitative IHC was used to evaluate the densities of intratumoral CD4 ^þ , CD8 ^þ , and FOXP3 ^þ cells in stage-matched melanoma (n ¼ 19), prostate cancer n ¼ 17 and bladder cancer n ¼ 9 sam les treated with ipilimumab and in paired melanoma tumors (n ¼ 18) treated with tremelimumab. These findings were corroborated with multiparametric mass cytometry analysis of tumor-infiltrating cells from paired fresh melanoma tumors (n ¼ 5) treated with ipilimumab. Results: Both ipilimumab and tremelimumab increase infiltration of intratumoral CD4 ^þ and CD8 ^þ cells without significantly changing or depleting FOXP3 ^þ cells within the tumor microenvironment. Conclusions: Anti-CTLA-4 immunotherapy does not deplete FOXP3 ^þ cells in human tumors, which suggests that their efficacy could be enhanced by modifying the Fc portions of the mAbs to enhance Fc-mediated depletion of intratumoral regulatory T cells.