CTLA4 and melanoma: However, other work in the B16/BL6 murine melanoma model by Simpson et al. reported a previously undescribed mechanism of action for the anti-CTLA-4 mAb, which involved an antibody-dependent cellular cytotoxicity (ADCC)-mediated depletion of intra-tumoral Treg cells by FcRIV-expressing macrophages.