CO:03:3 | Contribution of mitochondrial DNA to the onset and maintenance of inflammation in Systemic Sclerosis patients

Martina Orlandi¹, Giorgia Sinigaglia², Amelia Spinella¹, Isabella Martusciello², Marco De Pinto¹, Giorgia Roveta¹, Luca Keller¹, Marcello Pinti², Dilia Giuggioli¹ | ¹Dipartimento di scienze mediche e chirurgiche dei bambini e degli adulti, Università di Modena e Reggio Emilia, Italy; ²Università degli Studi di Modena e Reggio Emilia, Italy

Background. Systemic sclerosis (SSc) is a chronic inflammatory disease of the connective tissue characterized by microvascular damage, skin fibrosis and immune system defects. A possible role in the progression of the disease could be played by circulating molecules able to maintain chronic inflammation. Circulating free mitochondrial DNA (cf-mtDNA) triggers inflammation by acting as a Damage-Associated Molecular Pattern (DAMP) and promoting activation of innate immune cells. Cf-mtDNA is sensed by immune cells via TLR9, inducing the expression of IL-6, TNFlalfa, and IFNalfa while cytosolic inflammasome component NLRP3 senses ox-mtDNA and promotes the maturation of IL-1beta and IL-18, indicating the involvement of proinflammatory pathways. The aim of the study was to shed light on the contribution of mtDNA to the onset and maintenance of the chronic proinflammatory state observed in SSc. Specifically, we sought to determine whether, and to what extent, the release of mtDNA in the bloodstream contributes to the inflammatory status observed in patients with SSc.

Materials and Methods. We enrolled 70 patients with SSc (mean age 60 14; 63 F, 7 M) along with 67 age- and sex-matched healthy controls. Total DNA was extracted from plasma samples, and a droplet digital PCR assay was used to quantify cf-mtDNA. Monocytes were isolated from a subset of patients and treated with mtDNA alone or in combination with LPS, to evaluate its capability to trigger inflammation.

Results. cf-mtDNA levels were lower in SSc patients than controls (70 vs 67; p<0.01) (Fig1A). The differences were preserved after stratification according to sex; a positive correlation was observed with age only in female patients (Fig 1B). However, cf-mtDNA alone was able to induce an increase in maximal mitochondrial respiration in SSc patients while no effect has been observed in healthy controls, suggesting that it could exert a pro-inflammatory activity that could contribute to the maintenance of chronic inflammation in SSc (Fig 2 A-B).

Conclusions. Circulating mtDNA has been demonstrated to enhance inflammation in several chronic inflammatory diseases, including cardiovascular diseases. Nevertheless, its role has never been explored in SSc, despite a deep involvement of cardiac complications in disease prognosis. Thus, this study will explore for the first time a basic mechanism that could link inflammation, chronic activation of innate immune cells and mtDNA, providing a possible marker for evaluating the natural course of the disease.