Hemoglobin drives inflammation and initiates antigen spread and nephritis in lupus

Hemoglobin (Hb) has well-documented inflammatory effects and is normally efficiently scavenged; clearance mechanisms can be overwhelmed during conditions of erythrocyte lysis, a condition that may occur in systemic lupus erythematosus. Whether Hb is preferentially inflammatory in lupus and additionally induces autoreactivity against prominent autoantigens was assessed. Peripheral blood mononuclear cells derived from SLE patients secreted higher levels of lupus-associated inflammatory cytokines when incubated with Hb, effects negated by haptoglobin. Hb (more particularly, ferric Hb) triggered the preferential release of lupus-associated cytokines from splenocytes, B cells, CD4 T cells, CD8 T cells and plasmacytoid dendritic cells isolated from aging NZM2410 mice, and also had mitogenic effects on B cells. Ferric Hb activated multiple signaling pathways which were differentially responsible for the generation of specific cytokines; inflammatory signaling also appeared to be cell-context dependent. Pull-downs, followed by mass spectrometry, revealed interactions of Hb with several lupus-associated autoantigens; co-incubation of ferric Hb with apoptotic blebs (structures which contain packaged autoantigens, believed to trigger lupus autoreactivity) revealed synergies (in terms of cytokine release and autoantibody production in vitro) that were also restricted to the lupus genotype. Infusion of ferric Hb into NZM2410 mice led to enhanced release of lupus-associated cytokines, the generation of a spectrum of autoantibodies, and enhanced-onset glomerulosclerosis. Given that the biased recognition of ferric Hb in a lupus milieu, in concert with lupus-associated autoantigens, elicits the generation of inflammatory cytokines from multiple immune cell types and stimulates the generation of potentially pathogenic autoantibodies, neutralization of Hb could have beneficial effects.