Abstract A101: Single-cell analysis reveals the pivotal role of the innate immune compartment in aCTLA-4 antitumor response

Immunotherapies have revolutionized and continue to revolutionize cancer patient care; however, much of our understanding of the mechanism of action of these therapies is currently limited. The accumulation of regulatory T cells (Tregs) in the tumor hampers antitumor activity and correlates with bad prognosis in several human cancers. Anti-CTLA-4 monoclonal antibodies (mAbs) have been extensively studied, and their activity depends both on the blockade of the CTLA-4 coinhibitory molecule, as well as the intratumoral depletion of Tregs, increasing effector cell abundance, and favoring tumor rejection. Fc-gamma receptor (FcgR) coengagement has been proven to be important for the action of aCTLA-4, in addition to its Treg-depleting activity via antibody-dependent cellular cytotoxicity (ADCC); however, it is unclear what are the cellular changes involved. We therefore wished to dissect the differences between aCTLA-4 mIgG1, an antibody that only blocks the CTLA-4 receptor, and aCTLA-4 mIgG2a, which has a dual activity of both blocking the receptor and depleting Tregs. To address this, we performed single-cell RNA-seq of infiltrating leukocytes from tumors in mice treated with aCTLA-4 mIgG1, mIgG2a, or left untreated (UT). This high-resolution comparison revealed unique cellular profiles generated by each treatment. While the blocking-only mIgG1 was similar to UT, mIgG2a with ADCC effector function demonstrated major changes. Tumors in mIgG2a treated mice showed an immediate decline of immune suppressive macrophages, and the emergence of proinflammatory monocytes, as well as NK cells and naive CD8 T cells. In addition, mIgG2a-treated mice displayed a gradual increase of CD4 T-cell subsets throughout treatment. Finally, a vast tissue repair signature was observed in later time points of aCTLA-4 mIgG2a treatment, comprising MRC1+ macrophages, neutrophils, and myeloid-derived suppressor cells (MDSCs), while CD8 T-cell abundance declined. In summary, our findings provide an in-depth view of the differences in mechanisms of action between an aCTLA-4 blocking mAb and an optimized blocking-depleting mAb, underscoring how FcgRs coengagement leads to enhanced antitumor response via the innate immune compartment. Citation Format: Ido Yofe, Adam Jelinski, Isabelle Solomon, Tomer Landsberger, Marc Robert de Massy, Karl Peggs, Sergio Quezada, Ido Amit. Single-cell analysis reveals the pivotal role of the innate immune compartment in aCTLA-4 antitumor response [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A101.