Plasma extracellular DNA and neutrophilic leukocyte activity in patients with rheumatoid arthritis

Neutrophilic leukocytes play a key role for the joint damage in development of rheumatoid arthritis (RA). The specific death mode of these cells (netosis) may be an important reason of increase of cell-free DNA (cfDNA) in peripheral blood of the RA patients. Of great interest would be studies of alleged relationships between the of blood cfDNA contents being able of playing the role of an auto-antigen participating in the initiation of autoimmune reactions, and indices of neutrophil activation in this immunopathological disorder. The aim of the present study was to determine the levels of cfDNA in blood plasma of patients with RA depending on the clinical course of the disease, and to evaluate possible relationships between this index and activation of neutrophilic leukocytes. The study was conducted on 28 conditionally healthy donors and 63 patients with RA from the Rheumatology Department at the Clinic of Immunopathology (Novosibirsk). The level of cfDNA was determined using PicoGreen fluorescent dye. Neutrophils from the peripheral blood of donors and patients with rheumatoid arthritis were isolated in a Ficoll-Urografin density gradient. Neutrophilic leukocytes accounted for more than 98% of the fraction of isolated cells, and their viability was 99%. A portion of freshly isolated neutrophils was stimulated by phorbol myristate acetate. Concentration of myeloperoxidase in blood plasma of donors and patients with RA was determined using the Human MPO ELISA kit. It has been shown that the increased concentration of extracellular DNA in blood plasma of RA patients correlates with an higher degree of disease activity, and this parameter may serve as a relatively independent indicator of the disease intensity. A correlation was found between the level of cfDNA and common biochemical markers used to assess the activity of disease, i.e., DAS-28 and C-reactive protein levels in serum (p < 0.05). Decrease of cfDNA concentrations is detected during treatment of the RA patients. This is due to the expected prognosis, i.e., a decreased manifestation of the disease, which also means correct administration of therapy. A relationship was found between the level of cfDNA and blood myeloperoxidase concentration in RA patients. The data obtained during the study suggest a possible connection between increased concentration of extracellular DNA, and activation of neutrophilic leukocytes in rheumatoid arthritis, with increased netosis in the affected joints.