Host Prothrombin and Fibrinogen Support S. Aureus Virulence In the Context of Acute Bacteremia

Abstract 821

Staphylococcus aureus is a common and re-emerging gram-positive bacteria that is a leading cause of both community and hospital-acquired infections. Multiple bacteria-derived products appear to drive pathogenicity, but some of the most potent virulence factors identified are those that engage host hemostatic system components. To directly investigate the hypothesis that host prothrombin and key thrombin substrates significantly contribute to S. aureus virulence in the context of acute bacteremia, we evaluated the survival profile of mice with either genetically-imposed deficiencies or functional alterations in prothrombin and thrombin targets following intravenous infection with S. aureus. Mice expressing 10% of normal levels of prothrombin (termed fII^flox/- mice) were found to have a major survival advantage following intravenous infection with ∼2.5 × 10⁸ colony forming units (cfu) of wild-type Newman-strain S. aureus. This difference in survival did not appear to be tied to platelet function as mice that were either pharmacologically depleted of platelets or deficient in the platelet integrin receptor subunit α_IIb did not display a survival advantage when similarly challenged with intravenous S. aureus. Fibrinogen was found to be at least one thrombin substrate critical to supporting S. aureus virulence, as the genetic elimination of fibrinogen resulted in dramatically prolonged survival following acute S. aureus bacteremia relative to fibrinogen-sufficient animals. One mechanism driving this prolongation in survival was linked to S. aureus binding of host fibrinogen via the bacterial fibrinogen receptor, clumping factor A (ClfA). Mice expressing a mutant form of fibrinogen that retains normal clotting function but lacks the binding motif for ClfA located on the fibrinogen g chain (termed Fibγ^Δ5) exhibited a profound survival advantage to wild-type animals over a wide bacterial dose range (e.g., LD₅₀ to an LD₁₀₀ for control animals). Additional studies revealed that the survival advantage in Fibγ^Δ5 mice correlated with reduced bacterial burdens in the hearts and kidneys, diminished histological evidence of cardiac damage, and significantly lower levels of plasma markers of cardiac damage (i.e., lactose dehydrogenase and cardiac troponin I) relative to infected control mice. To determine whether the observed survival advantages for fII^flox/- and Fibγ^Δ5 mice were strictly linked to ClfA, identical survival studies were performed with ClfA-deficient S. aureus. Consistent with the concept that ClfA is a potent virulence factor, mice of each genotype survived longer after infection with ClfA-deficient S. aureus than with the same dose of ClfA-positive bacteria. However, the survival advantage observed in Fibγ^Δ5 mice over wild-type animals was largely, but not entirely eliminated when challenged with i.v. ClfA-deficient S. aureus. The fact that Fibγ^Δ5 mice continued to display a modest, but significant, survival advantage relative to control animals when infected with bacteria lacking ClfA suggests that the loss of C-terminal residues of the gamma chain provides an additional, but still unidentified, benefit to the host. These data indicate that the host prothrombin and fibrinogen as well as the bacterial fibrinogen receptor ClfA are, cooperatively, major determinants of S. aureus virulence in the context of bacteremia, and that strategies designed to disrupt S. aureus engagement of these proteins may limit disease progression and improve host survival in S. aureus bacteremia/sepsis.