The extracellular adherence protein from Staphylococcus aureus inhibits the classical and lectin pathways of complement by blocking formation of the C3 proconvertase

J Immunol. 2014 Dec 15;193(12):6161-6171. doi: 10.4049/jimmunol.1401600. Epub 2014 Nov 7.


The pathogenic bacterium Staphylococcus aureus actively evades many aspects of human innate immunity by expressing a series of small inhibitory proteins. A number of these proteins inhibit the complement system, which labels bacteria for phagocytosis and generates inflammatory chemoattractants. Although the majority of staphylococcal complement inhibitors act on the alternative pathway to block the amplification loop, only a few proteins act on the initial recognition cascades that constitute the classical pathway (CP) and lectin pathway (LP). We screened a collection of recombinant, secreted staphylococcal proteins to determine whether S. aureus produces other molecules that inhibit the CP and/or LP. Using this approach, we identified the extracellular adherence protein (Eap) as a potent, specific inhibitor of both the CP and LP. We found that Eap blocked CP/LP-dependent activation of C3, but not C4, and that Eap likewise inhibited deposition of C3b on the surface of S. aureus cells. In turn, this significantly diminished the extent of S. aureus opsonophagocytosis and killing by neutrophils. This combination of functional properties suggested that Eap acts specifically at the level of the CP/LP C3 convertase (C4b2a). Indeed, we demonstrated a direct, nanomolar-affinity interaction of Eap with C4b. Eap binding to C4b inhibited binding of both full-length C2 and its C2b fragment, which indicated that Eap disrupts formation of the CP/LP C3 proconvertase (C4b2). As a whole, our results demonstrate that S. aureus inhibits two initiation routes of complement by expression of the Eap protein, and thereby define a novel mechanism of immune evasion.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry
  • Bacterial Proteins / immunology*
  • Bacterial Proteins / metabolism*
  • Binding Sites
  • Complement C2 / immunology
  • Complement C2 / metabolism
  • Complement C3 Convertase, Alternative Pathway / antagonists & inhibitors*
  • Complement C3b / immunology
  • Complement C3b / metabolism
  • Complement C4b / immunology
  • Complement C4b / metabolism
  • Complement Pathway, Classical / immunology*
  • Complement Pathway, Mannose-Binding Lectin / immunology*
  • Cytotoxicity, Immunologic
  • Humans
  • Models, Immunological
  • Neutrophils / immunology
  • Phagocytosis / immunology
  • Protein Binding
  • Protein Interaction Domains and Motifs
  • RNA-Binding Proteins / chemistry
  • RNA-Binding Proteins / immunology*
  • RNA-Binding Proteins / metabolism*
  • Staphylococcal Infections / immunology
  • Staphylococcal Infections / metabolism
  • Staphylococcus aureus / immunology*
  • Staphylococcus aureus / metabolism*


  • Bacterial Proteins
  • Complement C2
  • Eap-N protein, Staphylococcus aureus
  • RNA-Binding Proteins
  • Complement C3b
  • Complement C4b
  • Complement C3 Convertase, Alternative Pathway