CXCL4 drives fibrosis by promoting several key cellular and molecular processes.

Fibrosis is a major cause of mortality worldwide, characterized by myofibroblast activation and excessive extracellular matrix deposition. Systemic sclerosis is a prototypic fibrotic disease in which CXCL4 is increased and strongly correlates with skin and lung fibrosis. Here we aim to elucidate the role of CXCL4 in fibrosis development. CXCL4 levels are increased in multiple inflammatory and fibrotic mouse models, and, using CXCL4-deficient mice, we demonstrate the essential role of CXCL4 in promoting fibrotic events in the skin, lungs, and heart. Overexpressing human CXCL4 in mice aggravates, whereas blocking CXCL4 reduces, bleomycin-induced fibrosis. Single-cell ligand-receptor analysis predicts CXCL4 to affect endothelial cells and fibroblasts. In vitro, we confirm that CXCL4 directly induces myofibroblast differentiation and collagen synthesis in different precursor cells, including endothelial cells, by stimulating endothelial-to-mesenchymal transition. Our findings identify a pivotal role of CXCL4 in fibrosis, further substantiating the potential role of neutralizing CXCL4 as a therapeutic strategy.

CXCL4 suppresses tolerogenic immune signature of monocyte-derived dendritic cells.

RNA sequencing and DNA methylomic profiling were performed after differentiating monocytes for 6 days into moDCs with/without CXCL4 presence. We show that CXCL4 downregulates genes associated with tolerogenicity in DCs including C1Q. Expression profiles of C1Q genes were negatively correlated with their DNA methylation profiles and with immunogenic genes.

Tie2 signaling cooperates with TNF to promote the pro-inflammatory activation of human macrophages independently of macrophage functional phenotype.

Angiopoietin (Ang) -1 and -2 and their receptor Tie2 play critical roles in regulating angiogenic processes during development, homeostasis, tumorigenesis, inflammation and tissue repair. Tie2 signaling is best characterized in endothelial cells, but a subset of human and murine circulating monocytes/macrophages essential to solid tumor formation express Tie2 and display immunosuppressive properties consistent with M2 macrophage polarization. However, we have recently shown that Tie2 is strongly activated in pro-inflammatory macrophages present in rheumatoid arthritis patient synovial tissue. Here we examined the relationship between Tie2 expression and function during human macrophage polarization. Tie2 expression was observed under all polarization conditions, but was highest in IFN-γ and IL-10 -differentiated macrophages. While TNF enhanced expression of a common restricted set of genes involved in angiogenesis and inflammation in GM-CSF, IFN-γ and IL-10 -differentiated macrophages, expression of multiple chemokines and cytokines, including CXCL3, CXCL5, CXCL8, IL6, and IL12B was further augmented in the presence of Ang-1 and Ang-2, via Tie2 activation of JAK/STAT signaling. Conditioned medium from macrophages stimulated with Ang-1 or Ang-2 in combination with TNF, sustained monocyte recruitment. Our findings suggest a general role for Tie2 in cooperatively promoting the inflammatory activation of macrophages, independently of polarization conditions.