LAUSR

Myeloid cells represent one of the most abundant immune cell types in solid tumors and are generally associated with poor clinical outcome. We previously identified the orphan C-type lectin receptors (CLRs) CLEC-1 as over-expressed in situation of established immune tolerance and reported that CLEC-1 expression by dendritic cells (DCs) and macrophages is enhanced by TGFβ and tempers downstream T cells responses. Recent literature demonstrates that tumors could hijack physiological mechanisms of CLRs that normally restrain immune cell-mediated tissue damage to suppress myeloid cell activation and promote tumor immune evasion. Here we report that CLEC-1 is highly expressed by myeloid cells purified from human tumor microenvironment (e.g. ovarian cancer) and significantly more expressed by in vitro generated M2 macrophages compared to monocytes or M1 macrophages. RNAseq public data indicates that Clec1a is highly expressed in XCR1-expressing DC including CD8α+ and CD103+ DCs from lymphoid and peripheral tissues respectively. As these DCs are specialized in cross-presentation of dying/dead cell associated antigens, we evaluated whether CLEC-1 could be a receptor of damaged cells. We found that both human and mouse CLEC-1 fusion protein, but not irrelevant CLRs or negative controls, binds specifically to late apoptotic and secondary necrotic healthy or tumor cells induced by chemotherapy, radiation (UV, X-ray) or culture stress conditions. No binding was observed with viable, primary necrotic or with early apoptotic cells suggesting that CLEC-1 ligand(s) (CLEC-1L) correspond(s) to DAMP modified during the cell death process. Using a CLEC-1 reporter NFAT-lacZ BWZ.36 cell line, we show that this interaction is functional. Importantly, we observed in vivo that dendritic cells from Clec1a deficient mice more efficiently cross-present antigen (OVA-loaded dead cells) to CD8+ T cells (OT-1) and that CLEC-1 deficient mice, but not wild-type, eradicate MC38 colorectal tumors in combination with cytotoxic and immunogenic chemotherapy (eg. cyclophosphamide). Treatment of wild-type mice with a Fc-CLEC-1 fusion protein also synergized with chemotherapy and induced complete response in 37% of mice versus 0% in chemotherapy group. We then generated, screened and identified different anti-human Clec-1 antagonist monoclonal antibodies (mAbs) with the capacity to block the CLEC-1/CLEC-1L interaction. We discovered that antagonist CLEC-1 mAbs, but not non-antagonist CLEC-1 control mAbs, increase the phagocytosis of CLEC-1L-positive human tumor cells by human TGFβ-polarized DCs but also of CLEC-1L-positive opsonized tumor cells (eg: Raji cells opsonized with anti-CD20 Rituximab) by human macrophages. Altogether, these data indicate that tumor cells inhibit myeloid cells phagocytosis through CLEC-1 and that antagonists of the CLEC-1/CLEC-1L novel myeloid checkpoint pathway constitute a novel cancer immunotherapy approach synergistic with chemotherapy. Citation Format: Vanessa Gauttier, Marion Drouin, Javier Saenz, Berangere Evrard, Caroline Mary, Geraldine Teppaz, Ariane Desalle, Virginie Thepenier, Emmanuelle Wilhelm, Nicolas Poirier, Elise Chiffoleau. CLEC-1 is a novel myeloid immune checkpoint for cancer immunotherapy controlling damaged and tumor cells phagocytosis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3259.