Abstract 6365: TRIM21 is a novel endogenous partner of the inhibitory myeloid checkpoint CLEC-1 involved in tumor antigen cross-presentation

Abstract Myeloid cells represent the most abundant immune component of the tumor microenvironment, where they often assume immunosuppressive roles. Therefore, understanding the mechanisms regulating the immunosuppressive activity of myeloid cells is of major interest in oncology. We have identified CLEC-1, a member of the C-type lectin receptor (CLR) family, as a myeloid cell-driven brake for anti-tumor T cell response in different murine models, including mesothelioma, hepatocellular and colorectal carcinomas. However, the molecular mechanism underlying the immunomodulatory role of CLEC-1 remained to be fully elucidated. Here, using both murine and human cells, we identified and characterized a novel endogenous ligand for CLEC-1, shedding new light on CLEC-1 driven inhibitory myeloid checkpoint activity. First, using single cell RNA sequencing, we confirmed that CLEC-1 is expressed by human myeloid cells, especially by conventional type-one dendritic cells and by tumor-associated macrophages. Using an Fc-fusion human CLEC-1 protein (huCLEC-1) as a bait for potential endogenous ligands, we observed both in human and murine cells that huCLEC-1 binds specifically to stressed or dying cells following programmed necrosis, for instance after X-ray or chemotherapy treatments. CLEC-1 interaction with its ligand expressed by necrotic cells was able to activate the NFAT promoter in a CLEC-1-CD3zeta NFAT reporter cell line in vitro; furthermore, CLEC-1 ligand targeting via Fc-CLEC-1 treatment in vivo extended the overall survival of chemotherapy-treated mc38-colorectal cancer bearing mice, altogether demonstrating the functional relevance of CLEC-1 interaction with its ligand. Immunoprecipitation of proteins bound to huCLEC-1 subsequently enabled the identification of TRIM21, an intra-cellular Fc receptor and E3 ubiquitin ligase as a novel interaction partner for CLEC-1. Direct interaction between recombinant TRIM21 and CLEC-1 human proteins was subsequently confirmed by ELISA and Biacore affinity analysis. TRIM21 was indeed detected at the cell surface of necrotic cells following genotoxic treatments in vitro in several human cancer cell lines and, interestingly, high TRIM21 expression was predictive of worse overall survival in patients with hepatocellular carcinoma, pancreatic cancer, or glioma. Mechanistically, we demonstrated that CLEC-1 genetic deletion in mice enhances the capacity of dendritic cells to cross-present tumor antigens, a process known to be regulated by TRIM21 through its E3 ubiquitin ligase activity. Antagonist anti-CLEC-1 or anti-TRIM21 antibodies are therefore being evaluated to further confirm the involvement of the newly identified CLEC-1 interaction with TRIM21 in the regulation of CLEC-1’s function as an inhibitory myeloid checkpoint. Altogether our data highlight the CLEC-1/TRIM21 axis as a new target for cancer immunotherapy. Citation Format: Irene Baccelli, Vanessa Gauttier, Marion Drouin, Caroline Mary, Isabelle Girault, Emmanuelle Wilhelm, Sabrina Pengam, Géraldine Teppaz, Julien Taurelle, Ariane Desselle, Virginie Thepenier, Nicolas Poirier, Elise Chiffoleau. TRIM21 is a novel endogenous partner of the inhibitory myeloid checkpoint CLEC-1 involved in tumor antigen cross-presentation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6365.