Efficacious anti-CTLA-4 antibodies drive myeloid activation and TME reprograming through FcγR engagement and Type-I interferon signaling

Despite the clinical success of checkpoints inhibitors, a significant gap still exists in our understanding of their in vivo mechanism of action, thus limiting the rational development of next generation agents. Amongst these, anti-CTLA-4 antibodies were originally developed to block inhibitory signals into activated effector T cells. However, several recent studies have demonstrated that depletion of regulatory T cells expressing high levels of CTLA-4 is critical to anti-CTLA-4 anti-tumor activity. Whereas the mechanism of action remains controversial, the emerging data support clinical development of new antibodies with enhanced killing activity. Here, using single-cell RNA sequencing in in vivo and in vitro mouse models, we sought to dissect the impact of anti-CTLA-4 blocking, Treg depleting, and FcR engaging activity on the immune responses within tumors. We observed a rapid remodeling of the innate immune landscape as early as 24-hours post-treatment. Immune remodeling was driven mainly by FcγR-engagement, and not by Treg depletion or CTLA-4 blockade, and included reduction of suppressive macrophages and activation of type-I interferon signaling. Our findings indicate that FcγR engagement and innate immune remodeling are involved in successful anti-CTLA-4 treatment, supporting further development of optimized immunotherapy agents with these features. Dissecting the effect of blocking vs depleting anti-CTLA4 mAb on tumor immune microenvironment using single-cell RNA sequencing. Mice were inoculated with MCA-205 cells and treated with either Anti-CTLA-4 mIgG1 or Anti-CTLA-4 mIgG2a or untreated. Immune cells were harvested from tumors in 2 different time-points and underwent scRNAseq. We also utilized FOXP3-DTR, IFNAR KO and FCGR KO mice in different settings. Ex vivo incubation of iTregs with bone-marrow derived myeloid cells was performed.