Necroptotic cell binding of β₂ -glycoprotein I provides a potential autoantigenic stimulus in systemic lupus...

Systemic lupus erythematosus (SLE) is characterized by the development of autoantibodies against diverse self-antigens with damage to multiple organs. Immunization with the SLE autoantigen β₂ -glycoprotein I (β₂ GPI) and lipopolysaccharide (LPS), a known trigger of necroptosis, induces a murine model of SLE. We hypothesized that necroptotic cells, like apoptotic cells, provide a "scaffold" of cellular self-antigens, but, unlike apoptotic cells, necroptotic cells do so in a proinflammatory and immunogenic context. We demonstrate that β₂ GPI indeed binds to necroptotic cells and serves as a target for anti-β₂ GPI autoantibodies. We further demonstrate that necroptotic, but not apoptotic, cells promote antigenic presentation of β₂ GPI to CD4 T cells by dendritic cells. Finally, we show that β₂ GPI/LPS-immunized mice deficient in RIPK3 (receptor-interacting serine/threonine-protein kinase 3) or MLKL (mixed lineage kinase domain like), and consequently unable to undergo necroptosis, have reduced SLE autoantibody production and pathology. RIPK3^-/- mice had low levels of SLE autoantibodies and no renal pathology, while MLKL^-/- mice produced low levels of SLE autoantibodies initially, but later developed levels comparable with wild type (WT) mice and pathology intermediate to that of WT and RIPK3^-/- mice. Serum cytokine levels induced by LPS tended to be lower in RIPK3^-/- and MLKL^-/- mice than in WT mice, suggesting that impaired proinflammatory cytokine production may impact the initiation of autoantibody production in both strains. Our data suggest that self-antigen (i.e. β₂ GPI) presented in the context of necroptosis and proinflammatory signals may be sufficient to overcome immune tolerance and induce SLE.