Mitochondrial DNA copy number in monocytes and peripheral blood in patients with systemic sclerosis

Abstract

Innate immune cells are important participants in inflammatory and fibrotic processes in systemic scleroderma (SSc). The pathogenesis of SSc involves immune cells, primarily macrophages, whose disorders are based on mitochondrial cell dysfunction. Mitochondrial DNA (mtDNA) copy number is used as a surrogate marker of mitochondrial cell dysfunction. The aim of the study was to evaluate the number of mtDNA copies in CD14+ monocytes and in all cell populations circulating in the blood in patients with SSc compared to healthy controls.The study included 25 patients with SSc (22 women and 3 men, median age 49 (43-57) years and disease duration 4.6 (1.0-9.6) years) and 25 people without autoimmune diseases or chronic inflammatory diseases matched by age and gender. The majority of patients (80%) had a limited form of SSc. All study participants did not receive antirheumatic therapy. DNA was isolated from CD14+ monocytes and whole blood. Absolute mtDNA copy number was measured using digital PCR. The number of mtDNA copies per cell used for analysis was calculated as the ratio of mtDNA and nDNA copies.It was found that in patients with SSc, the number of mtDNA copies in CD14+ monocytes was higher (108 (60-162) vs 72 (59-79), p = 0.01), and the indicator of all cell populations circulating in the blood did not differ in compared with the control group (109 (72-171) and 128 (85-227), p = 0.17). A negative relationship was found between the number of mtDNA copies and the duration of the disease, and a positive relationship with LPS-stimulated IL-6 secretion by cultured CD14+ monocytes.The study results suggest that increase of mtDNA copy number in CD14+ monocytes is a possible mechanism to maintain the reduced function of defective mitochondria in monocytes from patients with SSc associated with the development and progression of SSc.