Mechanisms that regulate inflammation and repair after acute lung injury are incompletely understood. The extracellular matrix glycosaminoglycan hyaluronan is produced after tissue injury and impaired clearance results in unremitting inflammation. Here we report that hyaluronan degradation products require MyD88 and both Toll-like receptor (TLR)4 and TLR2 in vitro and in vivo to initiate inflammatory responses in acute lung injury. Hyaluronan fragments isolated from serum of individuals with acute lung injury stimulated macrophage chemokine production in a TLR4- and TLR2-dependent manner. Myd88(-/-) and Tlr4(-/-)Tlr2(-/-) mice showed impaired transepithelial migration of inflammatory cells but decreased survival and enhanced epithelial cell apoptosis after lung injury. Lung epithelial cell-specific overexpression of high-molecular-mass hyaluronan was protective against acute lung injury. Furthermore, epithelial cell-surface hyaluronan was protective against apoptosis, in part, through TLR-dependent basal activation of NF-kappaB. Hyaluronan-TLR2 and hyaluronan-TLR4 interactions provide signals that initiate inflammatory responses, maintain epithelial cell integrity and promote recovery from acute lung injury.

We recently found that low-molecular-weight hyaluronan was induced by cyclic stretch in lung fibroblasts and accumulated in lungs from animals with ventilator-induced lung injury. The low-molecularweight hyaluronan produced by stretch increased interleukin-8 production in epithelial cells, and was accompanied by an upregulation of hyaluronan synthase-3 mRNA. We hypothesized that lowmolecular-weight hyaluronan induced by high VT was dependent on hyaluronan synthase 3, and was associated with ventilator-induced lung injury. Effects of high VT ventilation in C57BL/6 wild-type and hyaluronan synthase-3 knockout mice were compared. Significantly increased neutrophil infiltration, macrophage inflammatory protein-2 production, and lung microvascular leak were found in wild-type animals ventilated with high VT. These reactions were significantly reduced in hyaluronan synthase-3 knockout mice, except the capillary leak. Wild-type mice ventilated with high VT were found to have increased low-molecular-weight hyaluronan in lung tissues and concomitant increased expression of hyaluronan synthase-3 mRNA, neither of which was found in hyaluronan synthase-3 knockout mice. We conclude that high VT induced lowmolecular-weight hyaluronan production is dependent on de novo synthesis through hyaluronan synthase 3, and plays a role in the inflammatory response of ventilator-induced lung injury.Keywords: hyaluronic acid; knockout mice; mechanical ventilation; tidal volumeThe management of acute lung injury and acute respiratory distress syndrome (ARDS) requires the use of positive-pressure ventilation to provide adequate oxygenation. When ARDS develops, the lungs are affected nonhomogeneously, which leads to areas with different compliance. As the low compliant areas increase, the uneven distribution delivered by traditional or even smaller tidal volumes (Vt) will result in overdistension of those normal, compliant areas. A clinical trial by the ARDS Network has documented that mechanical ventilation with a smaller Vt (6 m/kg) decreased the mortality rate in patients with ARDS (1), suggesting the potential role of conventional Vt in lung injury. Ventilator-induced lung injury (VILI) has been studied in different animal models with high Vt ventilation (2-4). It has been characterized by neutrophil sequestration, increased vascular permeability, and elevated levels of chemoattractant cytokines, particularly macrophage inflammatory protein 2 (MIP-2), the (Received in original form May 19, 2004; accepted in final form March 18, 2005) Supported by the National Institutes of Health grants HL03920, 2T32HL07874, and funds from the Texas A&M University System Health Science Center to A.P.S. rodent equivalent for human interleukin 8 (IL-8) (5-10). Furthermore, MIP-2 receptor knockout mice have been shown to have less VILI than wild-type mice (9). However, the mechanisms of VILI during large Vt ventilation are not fully understood.Several studies of fetal lung cells have documented that cyclic stretch led to increased extracellular matrix sy...