Felty's syndrome autoantibodies bind to deiminated histones and neutrophil extracellular chromatin traps

Objective To test the hypothesis that autoantigen modifications by peptidylarginine deiminase type 4 (PAD‐4) increase immunoreactivity. Methods We assembled sera from patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Felty's syndrome (FS), and antineutrophil cytoplasmic antibody–associated vasculitides (AAVs), as well as sera from control subjects without autoimmune diseases. The sera were tested for binding to activated neutrophils, deiminated histones, and neutrophil extracellular chromatin traps (NETs). IgG binding to lipopolysaccharide‐activated neutrophils was assessed with confocal microscopy, and binding to in vitro–deiminated histones was measured using enzyme‐linked immunosorbent assay (ELISA) and Western blotting. In addition, we quantitated histone deimination in freshly isolated neutrophils from the blood of patients and control subjects. Results Increased IgG reactivity with activated neutrophils, particularly binding to NETs, was paralleled by preferential binding to deiminated histones over nondeiminated histones by ELISA in a majority of sera from FS patients but only in a minority of sera from SLE and RA patients. Immunoblotting revealed autoantibody preference for deiminated histones H3, H4, and H2A in most FS patients and in a subset of SLE and RA patients. In patients with AAVs, serum IgG preferentially bound nondeiminated histones over deiminated histones. Increased levels of deiminated histones were detected in neutrophils from RA patients. Conclusion Circulating autoantibodies in FS are preferentially directed against PAD‐4–deiminated histones and bind to activated neutrophils and NETs. Thus, increased reactivity with modified autoantigens in FS implies a direct contribution of neutrophil activation and the production of NET‐associated nuclear autoantigens in the initiation or progression of FS.