Ipilimumab-dependent cell-mediated cytotoxicity of regulatory T cells ex vivo by nonclassical monocytes in melanoma patients

Significance mAbs directed to inhibitory immune receptors represent a very promising class of immunotherapeutics. This study suggests a potential mechanism of action of ipilimumab (a fully human anti–cytotoxic T lymphocyte–associated antigen 4), by which FcγRIIIA (CD16)-expressing nonclassical monocytes kill regulatory T cells ex vivo via antibody-dependent cell-mediated cytotoxicity. Notably, at baseline, responder patients display significantly higher peripheral frequencies of nonclassical monocytes and a selective enrichment in tumor-infiltrating CD68 ⁺CD16 ⁺ macrophages compared with nonresponder patients. If further confirmed, these findings may contribute to the generation of predictive biomarker panels, antibody design, and the development of rational combination therapies to promote antitumor immunity.

Enhancing immune responses with immune-modulatory monoclonal antibodies directed to inhibitory immune receptors is a promising modality in cancer therapy. Clinical efficacy has been demonstrated with antibodies blocking inhibitory immune checkpoints such as cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) or PD-1/PD-L1. Treatment with ipilimumab, a fully human CTLA-4–specific mAb, showed durable clinical efficacy in metastatic melanoma; its mechanism of action is, however, only partially understood. This is a study of 29 patients with advanced cutaneous melanoma treated with ipilimumab. We analyzed peripheral blood mononuclear cells (PBMCs) and matched melanoma metastases from 15 patients responding and 14 not responding to ipilimumab by multicolor flow cytometry, antibody-dependent cell-mediated cytotoxicity (ADCC) assay, and immunohistochemistry. PBMCs and matched tumor biopsies were collected 24 h before (i.e., baseline) and up to 4 wk after ipilimumab. Our findings show, to our knowledge for the first time, that ipilimumab can engage ex vivo FcγRIIIA (CD16)-expressing, nonclassical monocytes resulting in ADCC-mediated lysis of regulatory T cells (Tregs). In contrast, classical CD14 ⁺⁺CD16 ⁻ monocytes are unable to do so. Moreover, we show that patients responding to ipilimumab display significantly higher baseline peripheral frequencies of nonclassical monocytes compared with nonresponder patients. In the tumor microenvironment, responders have higher CD68 ⁺/CD163 ⁺ macrophage ratios at baseline and show decreased Treg infiltration after treatment. Together, our results suggest that anti–CTLA-4 therapy may target Tregs in vivo. Larger translational studies are, however, warranted to substantiate this mechanism of action of ipilimumab in patients.