Toll-like receptors (TLRs) have been implicated in the regulation of host responses to microbial Ags. This study characterizes the role of TLR4 in the innate immune response to intrapulmonary administration of Haemophilus influenzae in the mouse. Two different strains of mice efficiently cleared aerosolized H. influenzae concurrent with a brisk elaboration of IL-1β, IL-6, TNF-α, macrophage-inflammatory protein (MIP)-1α, and MIP-2 in bronchoalveolar lavage and a corresponding mobilization of intrapulmonary neutrophils. Congenic strains of mice deficient in TLR4 demonstrated a substantial delay in clearance of H. influenzae with diminished IL-1β, IL-6, TNF-α, MIP-1α, and MIP-2 in bronchoalveolar lavage and a notable absence of intrapulmonary neutrophils. In TLR4-expressing animals, but not TLR4-deficient animals, TNF-α and MIP-1α expression was up-regulated in epithelial cells of the conducting airway in response to H. influenzae which was preceded by an apparent activation of the NF-κB pathway in these cells based on the findings of decreased overall IκB and an increase in its phosphorylated form. This study demonstrates a critical role of TLR4 in mediating an effective innate immune response to H. influenzae in the lung. This suggests that the airway epithelia might contribute to sensing of H. influenzae infection and signaling the innate immune response.

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Innate immunity serves as a first line defense in vertebrate organisms by providing an initial barrier to microorganisms and triggering antigen-specific responses. Antimicrobial peptides are thought to be effectors of innate immunity through their antibiotic activity and direct killing of microorganisms. Evidence to support this hypothesis in vertebrates is indirect, based on expression profiles and in vitro assays using purified peptides. Here we investigated the function of antimicrobial peptides in vivo using mice deficient in an antimicrobial peptide, mouse ␤- defensin-1 (mBD-1). We find that loss of mBD-1 results in delayed clearance of Haemophilus influenzae from lung. These data demonstrate directly that antimicrobial peptides of vertebrates provide an initial block to bacteria at epithelial surfaces.