Interaction of human complement with Sbi, a staphylococcal immunoglobulin-binding protein: indications of a novel mechanism of complement evasion by Staphylococcus aureus.

Staphylococcal immunoglobulin-binding protein, Sbi, is a 436-residue protein produced by many strains of Staphylococcus aureus. It was previously characterized as being cell surface-associated and having binding capacity for human IgG and beta(2)-glycoprotein I. Here we show using small angle x-ray scattering that the proposed extracellular region of Sbi (Sbi-E) is an elongated molecule consisting of four globular domains, two immunoglobulin-binding domains (I and II) and two novel domains (III and IV). We further show that together domains III and IV (Sbi-III-IV), as well as domain IV on its own (Sbi-IV), bind complement component C3 via contacts involving both the C3dg fragment and the C3a anaphylatoxin domain. Preincubation of human serum with either Sbi-E or Sbi-III-IV is inhibitory to all complement pathways, whereas domain IV specifically inhibits the alternative pathway. Monitoring C3 activation in serum incubated with Sbi fragments reveals that Sbi-E and Sbi-III-IV both activate the alternative pathway, leading to consumption of C3. By contrast, inhibition of this pathway by Sbi-IV does not involve C3 consumption. The observation that Sbi-E activates the alternative pathway is counterintuitive to intact Sbi being cell wall-associated, as recruiting complement to the surface of S. aureus would be deleterious to the bacterium. Upon re-examination of this issue, we found that Sbi was not associated with the cell wall fraction, but rather was found in the growth medium, consistent with it being an excreted protein. As such, our data suggest that Sbi helps mediate bacterial evasion of complement via a novel mechanism, namely futile fluid-phase consumption.

S. aureus IgG-binding proteins SpA and Sbi: host specificity and mechanisms of immune complex formation.

The evasion of the host immune response is central to the pathogenicity of Staphylococcus aureus, and is facilitated by the ability of the cell wall-associated protein A (SpA) to bind immunoglobulin G Fc fragments, thereby impeding phacocytosis and classical pathway complement fixation. SpA also acts as a B-cell superantigen through interactions with the heavy-chain variable part of Fab fragments, and sequesters immunoglobulins by forming large insoluble immune complexes with human IgG. Here we show that the formation of insoluble immune complexes is mediated by the binding of (VH3+) Fab fragments in addition to Fc, and that SpA forms soluble complexes with IgG Fc fragments. We compared these results with those for Sbi, a second staphylococcal immunoglobulin-binding protein, and note that this protein requires only the Fc fragment for precipitation with human IgG. Homology models of immunoglobulin-binding domains of SpA and Sbi in complex with Fc reveal the molecular basis of the Fab-independent formation of insoluble complexes by Sbi. Finally, we compared the sequences of the spa and sbi genes from human strains to those infecting a range of animal hosts to determine whether Sbi and SpA have acquired specificity for host IgG. We note remarkable sequence conservation within the IgG-binding domains of these genes, consistent with a lack of host specificity. The Fab-independent binding of IgG by Sbi could have significant clinical implications. The use of SpA in immunoadsorption therapy can cause severe side-effects, thought to be mediated by Fc gamma R recognition and complement fixation. The formation of insoluble immune complexes with Sbi occurs only via Fc binding and free Fc regions are unlikely to be available for Fc gamma R recognition and complement fixation.