Binding of human immunoglobulin G to protein A in encapsulated Staphylococcus aureus

Abstract

Recent studies of the mechanism of resistance to phagocytosis in encapsulated Staphylococcus aureus have suggested that the capsule is readily penetrated by high-molecular-weight proteins such as antibodies and complement components. S. aureus strains contain a cell wall protein, protein A, that reacts with the Fc portion of immunoglobulins. The binding of immunoglobulin G (IgG) to encapsulated and unencapsulated S. aureus strains has been studied to assess the penetrability of the S. aureus capsule by IgG. Encapsulated S. aureus strains M and Smith diffuse bound large amounts of human IgG which were comparable to amounts bound by the unencapsulated strains Cowan I, M variant, and Smith compact. Trypsin treatment of bacteria reduced their ability to bind IgG. Bound IgG was not removed by extensive washing of bacteria with buffer. A non-protein A-containing, coagulase-negative, encapsulated staphylococcal strain did not bind IgG. These observations suggest that IgG is binding to cell wall protein A in encapsulated S. aureus. No differences in the rates of IgG binding by encapsulated and unencapsulated S. aureus strains were observed. It is concluded that the S. aureus capsule is freely permeable to IgG. This is of importance in considerations of the mechanisms of resistance to phagocytosis and antigen masking in encapsulated microorganisms.

Localization on encapsulated Cryptococcus neoformans of serum components opsonic for phagocytosis by macrophages and neutrophils

“…These results have led us to speculate that the capsule is permeable to potentially opsonic IgG in normal serum and that this IgG binds to sites beneath the capsular surface such that it cannot interact with the macrophage Fc receptors. Similar masking mechanisms have been proposed for the antiphagocytic action of the Staphylococcus aureus capsule (14,30,32,33). Ultrastructural studies showed binding of complement component C3 well beneath the S. aureus capsular surface at the cell wall (33).…”

“…Ultrastructural studies showed binding of complement component C3 well beneath the S. aureus capsular surface at the cell wall (33). Indirect evidence suggests that IgG from normal, nonimmune serum also binds to sites beneath the S. aureus capsular surface (14,30).…”

Distribution of capsular materials on the cell wall surface of strain Smith diffuse of Staphylococcus aureus

“…There are conflicting reports on the interaction of staphylococcal Ks and antibody (11,13,21,23). Our investigation of the K of S. aureus gives a new perspective to these discussions.…”

Synthesis and immunologic properties in mice of vaccines composed of Staphylococcus aureus type 5 and type 8 capsular polysaccharides conjugated to Pseudomonas aeruginosa exotoxin A

“…Further, outbreaks of bacteremia due to S. aureus, some caused by organisms resistant to methicillin, continue to occur (7,16). Curiously, the pathogenesis and mechanisms of protective immunity to S. aureus bacteremia remain obscure (29,37,45,48; Editorial, Lancet).…”