CTLA-4 was the first inhibitory immune checkpoint to be identified. Two monoclonal antibodies, 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 monoclonal antibodies may also selectively deplete intratumoral FOXP3+ regulatory T cells via an Fc-dependent mechanism. However, it is unclear whether the depletion of FOXP3+ cells occurs in cancer patients treated with anti-CTLA-4 therapies.