A Human Fibrinogen Variant with Impaired StaphylococcusAureus Binding Function Promotes Host Survival during Septicemia in Mice

Staphylococcus aureus is a common gram-positive bacterium that is a causative agent for a number of life-threatening diseases. The emergence of antibiotic resistant strains of S. aureus (i.e., methicillin and vancomycin resistant) emphasizes a renewed need to better understand host-pathogen interactions for defining novel strategies to combat infection. The coagulation factor fibrinogen is a major target of S. aureus virulence factors as numerous fibrinogen-binding proteins are produced by the bacterium. Our previous studies, using Fibg^Δ5 mice that express a fibrinogen mutant lacking the binding motif for clumping factor A (ClfA) indicated that binding to this site is a major mechanism of pathogen virulence during acute septicemia. Specifically, in vivo studies showed that Fibγ^Δ5 mice have improved survival following S. aureus bacteremia suggesting that this interaction is critical for S. aureus virulence. To investigate a potential clinical translation of these findings, we utilized variants of fibrinogen γ', an alternatively sliced form of fibrinogen (comprising 8-15% of total plasma fibrinogen) that has structural similarities to fibrinogenγ^Δ5. Specifically, we analyzed human fibrinogen γ' heterodimer (hFib γ-γ') and recombinant fibrinogen γ' homodimer (rFib γ'-γ'). For bacteria under stationary phase growth conditions, ClfA mediated binding to immobilized fibrinogen whereas under exponential growth conditions binding to fibrinogen was largely independent of ClfA. Interestingly, hFib γ-γ' supported both ClfA dependent and independent binding to fibrinogen. However, studies performed with rFib γ'-γ' showed that this variant almost completely abolished ClfA dependent and independent binding to fibrinogen. Similar results were observed in analyses of fibrinogen-dependent 'clumping' in solution. Fibrinogen-dependent 'clumping' was fully dependent on ClfA in both stationary and exponentially growing S. aureus. Importantly, reconstitution of fibrinogen-deficient mice or WT mice with hFibγ' provided a significant prolongation in host survival relative to mice reconstituted with hFibγ indicating that hFibγ' confers protection alone or when present with other fibrinogen species. These findings provide the proof-of-concept that naturally occurring fibrinogen variants could offer significant therapeutic potential against infection and potentially other diseases.