CTLA-4 blockade confers lymphocyte resistance to regulatory T-cells in advanced melanoma: Surrogate marker of efficacy of tremelimumab?

Purpose: Anti - CTL antigen-4 (CTLA-4) monoclonal antibody (mAb) has led to encouraging antitumor activity associated with immune-related adverse events in patients with heavily pretreated melanoma. However, mechanisms of action and surrogate immunologic markers of efficacy have not been reported thus far. Experimental Design: We monitored the immune responses of 10 melanoma patients included in a phase II clinical trial, which evaluated the efficacy of a second line of therapy of tremelimumab anti - CTLA-4 mAb in patients with metastatic melanoma.The frequency of lood leukocyte populations in association withTcell and regulatoryTcell (T_reg) functions were evaluated. Results: Prior to therapy, patients with advanced melanoma presented with a severe CD4 ⁺ and CD8⁺ Tcell lymphopenia associated with blunted T-cell proliferative capacities that could be assigned toT_reg. Tremelimumab rapidly restored the effector and memory CD4+ and CD8⁺ T-cell pool and TCR-dependent T-cell proliferation that became entirely resistant toT_reg-mediated suppression. Progression-free survival and overall survival was directly correlated with the acquisition of a biological response defined as the resistance of peripheral lymphocytes toT _reg-inhibitory effects (obtained in 7 of 10 patients). Conclusion: CTLA-4 blockade seems to be a valuable strategy to revive reactive memory Tcells anergized in the context of stage IV melanoma, and our work suggests that memory T-cell resistance toT_reg resulting from anti - CTLA-4 treatment might be a biological activity marker for tremelimumab in patients with melanoma.