Hypoxia and TLR9 activation drive CXCL4 production in systemic sclerosis plasmacytoid dendritic cells via mtROS and HIF-2α

Objective. SSc is a complex disease characterized by vascular abnormalities and inflammation culminating in hypoxia and excessive fibrosis. Previously, we identified chemokine (C-X-C motif) ligand 4 (CXCL4) as a novel predictive biomarker in SSc. Although CXCL4 is well-studied, the mechanisms driving its production are unclear. The aim of this study was to elucidate the mechanisms leading to CXCL4 production.Methods. Plasmacytoid dendritic cells (pDCs) from 97 healthy controls and 70 SSc patients were cultured in the presence of hypoxia or atmospheric oxygen level and/or stimulated with several toll-like receptor (TLR) agonists. Further, pro-inflammatory cytokine production, CXCL4, hypoxia-inducible factor (HIF) -1 alpha and HIF-2 alpha gene and protein expression were assessed using ELISA, Luminex, qPCR, FACS and western blot assays.Results. CXCL4 release was potentiated only when pDCs were simultaneously exposed to hypoxia and TLR9 agonist (P < 0.0001). Here, we demonstrated that CXCL4 production is dependent on the overproduction of mitochondrial reactive oxygen species (mtROS) (P = 0.0079) leading to stabilization of HIF-2 alpha (P = 0.029). In addition, we show that hypoxia is fundamental for CXCL4 production by umbilical cord CD34 derived pDCs.Conclusion. TLR-mediated activation of immune cells in the presence of hypoxia underpins the pathogenic production of CXCL4 in SSc. Blocking either mtROS or HIF-2 alpha pathways may therapeutically attenuate the contribution of CXCL4 to SSc and other inflammatory diseases driven by CXCL4.