Phagocytosis Escape by a Staphylococcus aureus Protein That Connects Complement and Coagulation Proteins at the Bacterial Surface

Abstract

Upon contact with human plasma, bacteria are rapidly recognized by the complement system that labels their surface for uptake and clearance by phagocytic cells. Staphylococcus aureus secretes the 16 kD Extracellular fibrinogen binding protein (Efb) that binds two different plasma proteins using separate domains: the Efb N-terminus binds to fibrinogen, while the C-terminus binds complement C3. In this study, we show that Efb blocks phagocytosis of S. aureus by human neutrophils. In vitro, we demonstrate that Efb blocks phagocytosis in plasma and in human whole blood. Using a mouse peritonitis model we show that Efb effectively blocks phagocytosis in vivo, either as a purified protein or when produced endogenously by S. aureus. Mutational analysis revealed that Efb requires both its fibrinogen and complement binding residues for phagocytic escape. Using confocal and transmission electron microscopy we show that Efb attracts fibrinogen to the surface of complement-labeled S. aureus generating a ‘capsule’-like shield. This thick layer of fibrinogen shields both surface-bound C3b and antibodies from recognition by phagocytic receptors. This information is critical for future vaccination attempts, since opsonizing antibodies may not function in the presence of Efb. Altogether we discover that Efb from S. aureus uniquely escapes phagocytosis by forming a bridge between a complement and coagulation protein.

Staphylococcal manipulation of host immune responses

“…Ecb associates with both fH and C3b to facilitate the complement inhibitory attributes of factor H 45 . Efb also binds fibrinogen and prevents fibrinogen interaction with αMβ2, an integrin on neutrophils that activates proinflammatory responses, as well as fibrinogen-mediated platelet activation 46,47 .Efb and Ecb inhibitory activities have been observed for human as well as mouse convertases and fibrinogen. In the mouse intravenous challenge model, the S. aureus Δ efb Δ ecb mutant displayed reduced time-to-death and increased survival as well as diminished abscess formation in organ tissues 48 .…”

Evasion and interactions of the humoral innate immune response in pathogen invasion, autoimmune disease, and cancer

“…Two secreted homeostasis factors activate prothrombin without enzymatic cleavage and prevent clot formation [15, 73, 78, 79]. As another important survival mechanism, S. aureus forms a pseudo-capsule of fibrin and fibrinogen [73, 79].…”

“…Extracellular Binding Protein complexes with fibrinogen and surface bound C3b to create a shield. This shield hides immune factors, such as Ab and C3b, that are bound to the bacterial surface preventing phagocytosis [15]. Other S. aureus surface proteins associate directly with multiple members of the coagulation pathway.…”

The Staphylococcus aureus polysaccharide capsule and Efb-dependent fibrinogen shield act in concert to protect against phagocytosis

“…This Extracellular fibrinogen binding protein (Efb) is a 16 kDa protein that binds to complement C3b on bacteria and simultaneously attracts fibrinogen to the surface. In doing so, Efb covers bacteria with a thick layer of fibrinogen that potently prevents recognition of surface-associated antibodies and C3b by phagocytic cells (Ko et al 2013). …”