Phagocytosis of necroptotic cells optimizes type 1 conventional dendritic cells for induction of a cytotoxic T-cell response
Whether a T-cell response to dead cells arises depends on whether they harbor recognizable  antigens, but also how dead cell debris impacts conventional dendritic cells (cDCs). In all tissues, cell debris is continuously phagocytosed by cDCs that can migrate to lymph nodes (LNs) and initiate T-cell responses, depending on their maturation state. The cDC1 lineage excels at antigen cross-presentation, which is required for induction of the cytotoxic CD8+ T-lymphocyte (CTL) response. At steady state, cDC1s undergo homeostatic maturation, which leads to T-cell non-responsiveness. This cDC1 state has recently been defined by ex vivo transcriptomics as resulting from phagocytosis of apoptotic cell debris. Necroptotic (tumor) cell death has been described as more immunogenic than apoptotic cell death, but its impact on cDC maturation has not been defined. In this study, we use an in vitro model to compare side-by-side the impact of well-defined apoptotic versus necroptotic tumor cell debris on the CTL response induced by cDCs, as well as on phenotype and function of both cDC1s and cDC2s. We confirm that cDC2s are less efficient than cDC1s in dead cell phagocytosis and find that they minimally respond to it in terms of gene expression. Apoptotic cell debris is more efficiently phagocytosed by cDC1s than necroptotic cell debris and induces a cDC1 cell state in vitro that has an evident transcriptomic relationship to the homeostatic maturation state defined ex vivo, thus validating our approach. We identify necroptosis as more potent than apoptosis in inducing a CTL response and attribute this to the ability of necroptotic cell debris to induce a specific, transcriptomically defined maturation state in cDC1s, characterized by cytokine and phosphoinositide signaling, cytoskeletal and metabolic activity and functional differentiation. We suggest that this cDC1 signature may be used to diagnose immunogenicity of necroptosis ex vivo. Overall design: Bulk RNA sequencing of murine primary splenic cDC1 and cDC2 co-cultured for 6 h with medium from wildtype TC-1 cells, or medium from apoptotic TC-1-iBimS cells, or medium from necroptotic TC-1-iRIPK3 cells.