Fibrinogen Stimulates Macrophage Chemokine Secretion Through Toll-Like Receptor 4

TLDR

Extravascular fibrin deposition, generated when plasma fibrinogen leaks out of vessels during inflammation, is an early and persistent hallmark of inflammatory responses. The study aimed to determine the physiological role of extravasated fibrinogen and discovered that it stimulates macrophage chemokine secretion. Fibrinogen induced robust secretion of MIP‑1α, MIP‑1β, MIP‑2, and MCP‑1 in mouse RAW264.7 and primary macrophages, as well as in human U937 and THP‑1 cells, a response that was heat‑labile, LPS‑independent, and required functional TLR4, suggesting extravascular fibrinogen promotes immune surveillance at inflammation sites.

Abstract

Abstract Extravascular fibrin deposition is an early and persistent hallmark of inflammatory responses. Fibrin is generated from plasma-derived fibrinogen, which escapes the vasculature in response to endothelial cell retraction at sites of inflammation. Our ongoing efforts to define the physiologic functions of extravasated fibrin(ogen) have led to the discovery, reported here, that fibrinogen stimulates macrophage chemokine secretion. Differential mRNA expression analysis and RNase protection assays revealed that macrophage inflammatory protein-1α (MIP-1α), MIP-1β, MIP-2, and monocyte chemoattractant protein-1 are fibrinogen inducible in the RAW264.7 mouse macrophage-like cell line, and ELISA confirmed that both RAW264.7 cells and primary murine thioglycolate-elicited peritoneal macrophages up-regulate the secretion of monocyte chemoattractant protein-1 >100-fold upon exposure to fibrinogen. Human U937 and THP-1 precursor-1 (THP-1) monocytic cell lines also secreted chemokines in response to fibrinogen, upon activation with IFN-γ and differentiation with vitamin D3, respectively. LPS contamination could not account for our observations, as fibrinogen-induced chemokine secretion was sensitive to heat denaturation and was unaffected by the pharmacologic LPS antagonist polymyxin B. Nevertheless, fibrinogen- and LPS-induced chemokine secretion both apparently required expression of functional Toll-like receptor 4, as each was diminished in macrophages derived from C3H/HeJ mice. Thus, innate responses to fibrinogen and bacterial endotoxin may converge at the evolutionarily conserved Toll-like recognition molecules. Our data suggest that extravascular fibrin(ogen) induces macrophage chemokine expression, thereby promoting immune surveillance at sites of inflammation.