Toll-IL-1 Receptor Domain-Containing Adaptor Protein Is Critical for Early Lung Immune Responses against<i>Escherichia coli</i>Lipopolysaccharide and Viable<i>Escherichia coli</i>

Pulmonary bacterial diseases are a leading cause of mortality in the U.S. Innate immune response is vital for bacterial clearance from the lung, and TLRs play a critical role in this process. Toll-IL-1R domain-containing adaptor protein (TIRAP) is a key molecule in the TLR4 and 2 signaling. Despite its potential importance, the role of TIRAP-mediated signaling in lung responses has not been examined. Our goals were to determine the role of TIRAP-dependent signaling in the induction of lung innate immune responses against Escherichia coli LPS and viable E. coli, and in lung defense against E. coli in mice. LPS-induced neutrophil sequestration; NF-κB translocation; keratinocyte cell-derived chemokine, MIP-2, TNF-α, and IL-6 expression; histopathology; and VCAM-1 and ICAM-1 expression were abolished in the lungs of TIRAP−/− mice. A cell-permeable TIRAP blocking peptide attenuated LPS-induced lung responses. Furthermore, immune responses in the lungs of TIRAP−/− mice were attenuated against E. coli compared with TIRAP+/+ mice. TIRAP−/− mice also had early mortality, higher bacterial burden in the lungs, and more bacterial dissemination following E. coli inoculation. Moreover, we used human alveolar macrophages to examine the role of TIRAP signaling in the human system. The TIRAP blocking peptide abolished LPS-induced TNF-α, IL-6, and IL-8 expression in alveolar macrophages, whereas it attenuated E. coli-induced expression of these cytokines and chemokines. Taken together, this is the first study illustrating the crucial role of TIRAP in the generation of an effective early immune response against E. coli LPS and viable E. coli, and in lung defense against a bacterial pathogen.