CD14 facilitates invasive respiratory tract infection by Streptococcus pneumoniae

Rationale: CD14 is a pattern recognition receptor that can interact with a variety of bacterial ligands. During gram-negative infection, CD14 plays an important role in the induction of a protective immune response by virtue of its capacity to recognize lipopolysaccharide in the bacterial cell wall. Knowledge of the contribution of CD14 to host defense against gram-positive infections is limited. Objectives: To study the role of CD14 in gram-positive bacterial pneumonia. Methods: CD14 knockout (KO) and normal wild-type (WT) mice were intranasally infected with Streptococcus pneumoniae. Measurements and Main Results: CD14 KO mice demonstrated a strongly reduced lethality, which was accompanied by a more than 10-fold lower bacterial load in lung homogenates but not in bronchoalveolar lavage fluid at 48 hours after infection. Strikingly, CD14 KO mice failed to develop positive blood cultures, whereas WT mice had positive blood cultures from 24 hours onward and eventually invariably had evidence of systemic infection. Lung inflammation was attenuated in CD14 KO mice at 48 hours after infection, as evaluated by histopathology and cytokine and chemokine levels. Intrapulmonary delivery of recombinant soluble CD14 to CD14 KO mice rendered them equally susceptible to S. pneumoniae as WT mice, resulting in enhanced bacterial growth in lung homogenates and bacteremia, indicating that the presence of soluble CD14 in the bronchoalveolar compartment is sufficient to cause invasive pneumococcal disease. Conclusion: These data suggest that S. pneumoniae uses (soluble) CD14 present in the bronchoalveolar space to cause invasive respiratory tract infection.