Recognition of Hyaluronan Released in Sterile Injury Involves a Unique Receptor Complex Dependent on Toll-like Receptor 4, CD44, and MD-2

Abstract

Inflammation under sterile conditions is not well understood despite its importance in trauma and autoimmune disease. To investigate this process we established mouse models of sterile injury and explored the role of hyaluronan in mediating inflammation following injury. The response of cultured monocytes to hyaluronan was different than the response to lipopolysaccharide (LPS) despite both being dependent on Toll-like receptor 4 (TLR4). Cultured cells exposed to hyaluronan showed a pattern of gene induction that mimics the response seen in mouse skin after sterile injury with an increase in molecules such as transforming growth factor-␤2 and matrix metalloproteinase-13. These factors were not induced by LPS despite the mutual dependence of both hyaluronan and LPS on TLR4. Explanation for the unique response to hyaluronan was provided by observations that a lack of TLR4 or CD44 in mice diminished the response to sterile injury, and together with MD-2, was required for responsiveness to hyaluronan in vitro. Thus, a unique complex of TLR4, MD-2, and CD44 recognizes hyaluronan. Immunoprecipitation experiments confirmed the physical association of TLR4 and CD44. Taken together, our results define a previously unknown mechanism for initiation of sterile inflammation that involves recognition of released hyaluronan fragments as an endogenous signal of tissue injury.

Small hyaluronan oligosaccharides induce inflammation by engaging both toll-like-4 and CD44 receptors in human chondrocytes

“…It has also been reported that fragments of HA or HA at low molecular weight are able to interact with TLR-4 and CD44 receptors, thereby stimulating inflammation or increasing the inflammatory mechanism previously induced by other agents in different cell types [11][12][13][14][15][16]. Hence, the generation of lower molecular weight HA in pathologies may act as an endogenous danger signal, leading to the activation of both innate and acquired immunity.…”

Toll-like receptor signal transduction

“…Individual TLRs can recognize several structurally unrelated ligands. For example, TLR4 recognizes not only LPS but also taxol (Kawasaki et al, 2000), fusion (F) protein of respiratory syncytial virus (Kurt-Jones et al, 2000), extra domain A of fibronectin (Okamura et al, 2001), heat shock protein (HSP) 60 (Ohashi et al, 2000), HSP 70 (Habich et al, 2002), and hyaluronan (Taylor et al, 2004(Taylor et al, , 2007. It is important to understand how TLR4 recognizes these structurally unrelated ligands.…”

Involvement of the Toll-like receptor 4 pathway and use of TNF-α antagonists for treatment of the mucopolysaccharidoses

“…Results and Discussion TLR4 signaling may be induced either by LPS or by endogenous GAG fragments derived from the breakdown of extracellular matrix components (10,11) (Fig. 1A).…”

“…TLR4 signaling may be induced by two pathways (10,11). LPS represents the "classic" pathway, activating TLR4 signaling through interactions with the adapter protein, myeloid differentiation factor-88 (MyD88), LPS binding protein (LBP), CD14, and CXCR4 (12).…”