Interaction of human factor H with PspC of Streptococcus pneumoniae

Streptococcus pneumoniae has acquired virulence factors such as the polysaccharide capsule and various surface proteins, which prevent opsonization mediated by the complement system. PspC is one of the multi-functional pneumococcal surface proteins capable of eliciting an antibody response in mice. Our study further explores the role of pneumococcal surface proteins in resistance to complement mediated opsonophagocytosis by providing evidence that PspC binds human Factor H (FH), a regulatory protein of the alternative complement pathway. The present study was carried out to map the binding regions on PspC and FH, and to assess the functional activity of FH upon binding to PspC.

FH binding to D39 and other pneumococcal strains was observed by flow cytometry. A series of FH truncated and deletion mutants and PspC mutants were used to localize binding regions within these molecules. The functional activity of FH upon binding to PspC was measured by a haemolysis assay.

FH binding to D39 and not to TRE108 (PspC-) cells was demonstrated by flow cytometry. Pneumococcal isolates of 14 different strains varied in their ability to bind FH. The binding region of FH within PspC to the first 225 amino acids of the alpha-helical domain was localized. The corresponding binding site for PspC is located within the SCR 6-10 region of FH. Haemolysis of rabbit red blood cells was inhibited by FH even in the presence of PspC.

FH binding is specific to PspC on the pneumococcal cell surface. The binding region on PspC mapped to the non-conserved N-terminal region of the alpha-helical domain. The binding site on FH to PspC is different from the active site that functions in degradation of C3b. A haemolysis assay provided evidence that the functional activity of FH was maintained upon binding to PspC. Thus, binding of FH to PspC might be an important mechanism by which S. pneumoniae resist complement activation and opsonophagocytosis.