Neutrophil extracellular traps: features of their formation in rheumatoid arthritis and osteoarthritis

Abstract

The similarity of clinical manifestations and some pathogenetic mechanisms in rheumatoid arthritis (RA) and osteoarthritis (OA) is of particular interest in studying the features of extracellular neutrophil traps (NETs) formation in these musculoskeletal diseases. Our objective was to evaluate circulating neutrophil extracellular trap formation related to autoimmune inflammation in RA and reactive inflammation in OA.39 RA patients comprised the main group; 35 OA patients, control group; 33 healthy individuals were included into the reference group. Circulating neutrophils were isolated by the one-step centrifugation in double layer of iohexol density gradient. NETs were induced by phorbol-12-myristate-13–acetate (PMA) being visualized by fluorescence microscopy.The RA disease activity at the inclusion period should not exceed 2.6 DAS28 points. ОA patients were in clinical remission at the inclusion timepoint. In 17 RA patients, an increase in DAS28 exceeded 3.2 points; in 25 OA patients, an exacerbation was diagnosed during the study. The mean percentage of spontaneous and induced NETs in reference group was 3.8% (2.6-5.0) and 12.2% (9.0-15.4), respectively. Appropriate values for the patients with inactive RA were 5.9% (5.6-6.2) and 26% (23.9-28.1); 16.6% (16.1-17.1) and 38.0% (36.6-39.4) in active RA cases, respectively; 5.4% (5.2-5.6) and 20.3% (18.3-22.3) in OA without synovitis; 13.1% (12.6-18.1) and 28.3% (6. 4-30.2) in OA with synovitis, respectively. Spontaneous increase of NETs during RA activation was 181.4%; induced increase in NETs was 46.2%; and in cases of OA exacerbation these values were 142.6% and 39.4%, respectively. In RA patients, the rates of spontaneous NETs formation were 3.9 times higher than for induced values. Appropriate index was 3.6 times higher among the OA patients. The increase in spontaneous -to-induced NETs formation was more pronounced in active RA than in OA with synovitis. The main difference in NETs composition for active RA patients (in 88% of cases) and OA synovitis (in 50% of cases) is related to the contents of citrulline epitopes (p = 0.03).Transition of RA from the remission state to active inflammation, as well as OA exacerbation are accompanied by a significant increase in NETs, especially, spontaneous formation. The degree of increase in RA was higher than in OA, thus, probably, indicating higher involvement of neutrophils in generation of NETs during autoimmune inflammation rather than in reactive inflammation. A sufficient role of citrullinated epitopes in NETs demonstrates their influence upon induction and maintenance of autoimmune response to RA-specific autoantigens.

Keywords

• rheumatoid arthritis
• osteoarthritis
• neutrophils
• nets
• netosis
• autoimmune inflammation
• reactive inflammation