NETs Are a Source of Citrullinated Autoantigens and Stimulate Inflammatory Responses in Rheumatoid Arthritis

Neutrophil NETs may contribute to pathogenesis of rheumatoid arthritis. Autoantigens Slip Through the NET Autoimmune diseases are caused when the body’s immune system attacks the very tissues it’s supposed to protect. Yet, what exactly induces this loss of tolerance to self remains murky. For some autoimmune diseases, autoantigens—cellular targets of the immune response—have been identified, although it remains unclear how these normally intracellular proteins are exposed to the immune response. One hypothesis as to how these proteins may be externalized is through the excretion of neutrophil extracellular traps (NETosis). NETosis is thought to be involved in neutrophil response to bacteria, but the secretion of self-antigens in the context of inflammatory stimuli may boost autoimmune response. Now, Khandpur et al. look at the role of NETosis in rheumatoid arthritis. Autoantibodies to citrullinated antigens (ACPAs) are thought to be pathogenic in rheumatoid arthritis. The authors observed increased NETosis in patients with rheumatoid arthritis compared with both healthy controls and patients with non-autoimmune osteoarthritis. Indeed, NETosis correlated with levels of ACPA, and ACPA actually altered the makeup of the proteins secreted by neutrophils. NETs from rheumatoid arthritis patients contained citrullinated proteins, and these NETs enhanced the inflammatory response in fibroblasts from inflamed joints. Thus, altered NETosis in rheumatoid arthritis patients may contribute to the pathogenesis of disease. The early events leading to the development of rheumatoid arthritis (RA) remain unclear, but formation of autoantibodies to citrullinated protein antigens (ACPAs) is considered a key pathogenic event. Neutrophils isolated from patients with various autoimmune diseases display enhanced neutrophil extracellular trap (NET) formation, a phenomenon that exposes autoantigens in the context of immunostimulatory molecules. We investigated whether aberrant NETosis occurs in RA, determined its triggers, and examined its deleterious inflammatory consequences. Enhanced NETosis was observed in circulating and RA synovial fluid neutrophils compared to neutrophils from healthy controls and from patients with osteoarthritis (OA). Further, netting neutrophils infiltrated RA synovial tissue, rheumatoid nodules, and skin. NETosis correlated with ACPA presence and levels and with systemic inflammatory markers. RA sera and immunoglobulin fractions from RA patients with high levels of ACPA and/or rheumatoid factor significantly enhanced NETosis, and the NETs induced by these autoantibodies displayed distinct protein content. Indeed, during NETosis, neutrophils externalized the citrullinated autoantigens implicated in RA pathogenesis, and anti–citrullinated vimentin antibodies potently induced NET formation. Moreover, the inflammatory cytokines interleukin-17A (IL-17A) and tumor necrosis factor–α (TNF-α) induced NETosis in RA neutrophils. In turn, NETs significantly augmented inflammatory responses in RA and OA synovial fibroblasts, including induction of IL-6, IL-8, chemokines, and adhesion molecules. These observations implicate accelerated NETosis in RA pathogenesis, through externalization of citrullinated autoantigens and immunostimulatory molecules that may promote aberrant adaptive and innate immune responses in the joint and in the periphery, and perpetuate pathogenic mechanisms in this disease.