Binding of the host complement regulator, factor H (FH), by some pathogenic microbes constitutes an important virulence mechanism, whereby complement is broken down to help microbes survive in the host. Although it has been hypothesized for the past two decades that GBS type III binds FH via sialic acid present on its capsule, neither the binding of FH to GBS has been demonstrated nor the mechanism of interaction identified. We observed that FH bound to both wild-type and capsule or sialic acid-deficient GBS that were used as negative controls. Wild-type and acapsular GBS were incubated with serum or pure FH degraded almost 90 of C3b, suggesting that the GBS-bound FH maintained cofactor activity. In addition, dot-blot analysis showed approximately 5-10 of C5 and C9 formation, as compared to an Escherichia coli control, suggesting breakdown at the C3b level. Protease treatment of the bacteria completely abolished binding of FH. Using overlay assays and mass spectroscopic analysis, we identified the FH receptor as the streptococcal histidine triad (SHT) surface protein. The ability of binding FH to SHT was further confirmed by using recombinant SHT. This report describes the identification of the SHT as an FH-binding protein on the surface of GBS type III, revealing a novel mechanism by which the bacterium acquires FH to evade complement opsonization.
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