Downregulated IRF8 in Monocytes and Macrophages of Patients with Systemic Sclerosis May Aggravate the Fibrotic Phenotype

Monocytes and macrophages may be involved in the pathogenesis of systemic sclerosis (SSc); however, the etiology and regulation of monocyte and macrophage function in SSc remain unknown. IRF8 is a transcriptional regulator that is essential for the differentiation and function of monocytes and macrophages and thus may be involved in the regulation of macrophage phenotypes in SSc fibrosis. In this study, we measured IRF8 levels in circulating monocytes of 26 patients with SSc (diffuse cutaneous SSc, n = 11; limited cutaneous SSc, n = 15) and 14 healthy controls. IRF8 levels were significantly suppressed in monocytes of patients with diffuse cutaneous SSc and correlated negatively with the modified Rodnan total skin thickness score. Next, we assessed expression levels of cell surface markers, cytokine profiles, and components of extracellular matrix in IRF8-silenced monocyte-derived macrophages. IRF8-silenced monocyte-derived macrophages displayed an M2 phenotype and significantly upregulated mRNA and protein levels of profibrotic factors and extracellular matrix components. Finally, we assessed skin fibrosis in myeloid cell–specific IRF8 conditional knockout (Irf8flox/flox; Lyz2Cre/+) mice and found upregulated mRNA levels of extracellular matrix components and increased bleomycin-induced skin fibrosis. In conclusion, altered IRF8 regulation in monocytes and macrophages may be involved in SSc pathogenesis.