Staphylococcus aureus exploits cathelicidin antimicrobial peptides produced during early pneumonia to promote staphylokinase-dependent fibrinolysis.

The increasing prevalence of Staphylococcus aureus strains isolated from hospital- and community-acquired respiratory tract infections is an important public health concern worldwide. The majority of S. aureus strains produce staphylokinase, a plasminogen activator capable of inactivating neutrophil alpha-defensins and of impairing phagocytosis via opsonin degradation. Cathelicidin antimicrobial peptides are present at sites of infection before the release of neutrophil alpha-defensins. Therefore, we hypothesized that staphylokinase interacts with cathelicidin during the early pathogenesis of S. aureus airway infection. In a mouse intranasal infection model, cathelicidin was strongly up-regulated in the airways during the development of staphylococcal pneumonia. In vitro, cathelicidin bound directly to staphylokinase and augmented staphylokinase-dependent plasminogen activation and fibrinolysis at concentrations consistent with those detected in the airways during infection. These data suggest that staphylokinase production may be a novel virulence mechanism by which S. aureus exploits cathelicidin to promote fibrinolysis, leading to enhanced bacterial dissemination and invasive infection.