Autoantibodies to neutrophil extracellular traps represent a potential serological biomarker in rheumatoid arthritis.

Neutrophil extracellular traps (NETs) are networks of extracellular chromatin decorated with antimicrobial proteins, formed by neutrophils to entrap pathogens. NETs have been implicated in the generation of autoimmune reactions. Here, we investigate the reactivity of rheumatoid arthritis (RA) serum antibodies with NETs and explore whether anti-NET antibodies (ANETA) have a potential as biomarker in RA. To quantify ANETA, we developed an ELISA with NETs isolated from stimulated human neutrophils and verified the results by immunofluorescence staining of NETs. ANETA were detected in 22%-69% of RA sera. No significant differences were observed in the reactivity of RA sera with NETs originating from RA patients and healthy control neutrophils, nor with NETs induced by phorbol 12-myristate 13-acetate or the calcium ionophore A23187. ANETA were detected already at baseline in newly diagnosed RA patients and both increased and decreased levels were observed in samples with a median follow-up of 7 years. By ANETA ELISA, we showed that ANETA are also present in sera of patients with systemic lupus erythematosus (36%), Sjögren's syndrome (76%) and scleroderma (61%). In addition to antibodies to NETs, also the presence of NETs or NET fragments in RA sera was determined using a sandwich ELISA. Elevated levels of NETs or NET fragments were detected in 32% of the sera. To assess the potency of ANETA as a biomarker in RA, we compared ANETA positivity with other clinical features. The presence of ANETA was significantly higher in rheumatoid factor (RF)-positive patients, but did not correlate with anti-citrullinated protein antibodies (ACPA), nor with the presence of NET fragments in serum. In addition, no correlation was observed with age, gender, onset of the disease, disease activity and inflammatory markers. These findings suggest that ANETA may be an independent biomarker in RA and possibly also in other autoimmune diseases.