Expression of C-reactive protein in the human respiratory tract

C-reactive protein (CRP) is a normal constituent of human sera synthesized by hepatocytes and induced by proinflammatory cytokines. The function of th is acute-phase reactant includes activation of complement and enhancement o f opsonophagocytosis. CRP binds to phosphorylcholine (ChoP), a constituent of eukaryotic membranes that is also found on the cell surface of major bac terial pathogens of the human respiratory tract, including Streptococcus pn eumoniae and Haemophilus influenzae. The presence of CRP on mucosal surface s and role in innate immunity in the human respiratory tract where ChoP-con taining organisms reside have not been previously studied. We have shown us ing a monoclonal antibody to CRP that CRP is present in inflamed (0.17 to 4 2 mug/ml) and uninflamed (<0.05 to 0.88 <mu>g/ml) secretions from the human respiratory tract in sufficient quantities for an antimicrobial effect. In addition, the CRP gene was expressed in epithelial cells of the human resp iratory tract using in situ hybridization on nasal polyps and reverse trans criptase PCR of pharyngeal cells in culture. The complement-dependent bacte ricidal activity of normal nasal airway surface fluid and sputum against Ch oP expressing H. influenzae was abolished when the secretions were pretreat ed to remove CRP. In summary, the results indicate that CRP is present in s ecretions of the human respiratory tract, that human respiratory epithelial cells are capable of CRP expression, and that this protein may contribute to bacterial clearance in the human respiratory tract.