Identification of a phage-encoded Ig-binding protein from invasive Neisseria meningitidis.

Ig-binding proteins are employed by a variety of organisms to evade the immune system. To our knowledge, we now report for the first time that meningococcal strains from several capsular groups exhibit Ig-binding activity that is dependent on human serum factors. A protein mediating Ig binding was identified as T and B cell-stimulating protein B (TspB) by immunoprecipitation and by mass spectroscopic analysis of tryptic peptides. Recombinant TspB and derivatives verified Ig binding, with a preference for human IgG2 Fc, and localized the IgG-binding region to a highly conserved subdomain of TspB. Antiserum produced in mice against the conserved subdomain detected the presence of TspB on the cell surface by flow cytometry when bacteria were grown in the presence of human serum. By fluorescence microscopy, we observed formation of an extracellular matrix having characteristics of a biofilm containing TspB, human IgG, DNA, and large aggregates of bacteria. TspB is encoded by gene ORF6 in prophage DNA, which others have shown is associated with invasive meningococcal strains. Knocking out ORF6 genes eliminated IgG binding and formation of large bacterial aggregates in biofilm. Reintroduction of a wild-type ORF6 gene by phage transduction restored the phenotype. The results show that TspB mediated IgG binding and aggregate/biofilm formation triggered by factors in human serum. As has been observed for other Ig-binding proteins, the activities mediated by TspB may provide protection against immune responses, which is in accordance with the association of prophage DNA carrying ORF6 with invasive meningococcal strains.