TLR4 Activity Is Required in the Resolution of Pulmonary Inflammation and Fibrosis after Acute and Chronic Lung Injury

Pulmonary fibrosis is an inflammation-driven lung disease with a poor prognosis and no cure. Here we report that basal toll-like receptor 4 (TLR4) activity is critical for the resolution of acute and chronic inflammation and pulmonary fibrosis in mouse models of lung injury. We found that genetic or pharmacologic inhibition of TLR4 exacerbates bleomycin-induced pulmonary inflammation, fibrosis, dysfunction, and animal death through promoting formation of an immunosuppressive tissue microenvironment and attenuating autophagy-associated degradation of collagen and cell death in the fibrotic lung tissues. In contrast, pharmacologic activation of TLR4 resulted in a quick resolution of acute inflammation, reversed the established pulmonary fibrosis, improved lung function, and rescued mice from death. Similarly, blocking TLR4 impaired the resolution of silica-induced chronic inflammation and fibrosis. Importantly, altering autophagic activity could reverse the TLR4-regulated lung inflammation, fibrosis, dysfunction, and animal death. Rapamycin, an autophagy activator, reversed the effects of TLR4 antagonism. In contrast, inhibition of autophagy by 3-methyladenine reversed the proresolving and antifibrotic roles of TIR4 agonists and increased animal death. These results not only highlight a pivotal role for TLR4-mediated basal immunity, particularly autophagic activity, in the proresolution of inflammation and fibrosis after chemical-induced lung injury but also provide proof for the concept for activating TLR4 signaling, particularly TLR4-mediated autophagy, as a novel therapeutic strategy against chronic fibroproliferative diseases that are unresponsive to current therapy. (Am J Pathol 2012, 180: 275-292; DOI: 10.1016/j.ajpath.2011.09.019)