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Immunological response mounted by Aboriginal Australians living in the Northern Territory of Australia against Streptococcus pyogenes serum opacity factor

Department of Biological Sciences, University of Wollongong, Wollongong, NSW 2522, Australia¹
Menzies School of Health Research, Darwin, NT, Australia²
Department of Microbiology, GBF-National Centre for Biotechnology, Braunschweig, Germany³
Queensland Institute of Medical Research, Brisbane, Qld, Australia⁴
Cooperative Research Centre for Aboriginal and Tropical Health, Darwin, NT, Australia⁵

Author for correspondence: Mark J. Walker. Tel: +61 2 4221 3439. Fax: +61 2 4221 4135. e-mail: mwalker{at}uow.edu.au

Streptococcus pyogenes (Group A streptococcus) interacts with host fibronectin via a number of distinct surface components. The streptococcal serum opacity factor (SOF) is a cell-surface protein of S. pyogenes which causes opalescence of human serum and mediates bacterial binding to fibronectin. In this study, hexahistidyl-tagged fusion proteins encompassing full-length SOF, and domains of SOF encompassing opacity factor activity and fibronectin-binding regions, were used in the characterization of the Aboriginal immune response to SOF. Anti-SOF serum IgG responses were found to be significantly higher (P<0·0001) in Aboriginal adults and children when compared to a non-Aboriginal adult group. The Aboriginal immune response against the fibronectin-binding region of SOF was significantly reduced when compared to the response against the whole SOF protein and N-terminal domains examined in this study (P<0·001). This pattern of immune response was also observed in rabbits immunized with recombinant SOF. Comparison of the deduced amino acid sequence of SOF from a number of common Australian isolates with other SOF sequences revealed that the N-terminus of SOF exhibits sequence similarity values ranging from 42·9% to 96·5%. The C-terminus containing the fibronectin-binding domain and membrane-spanning regions was more highly conserved, exhibiting sequence similarity values ranging from 84·6% to 100% within the fibronectin-binding repeats. These data suggest that the immune response against SOF is directed toward the variable N-terminus of the SOF protein. Phylogenetic analysis indicated that the sof genes of S. pyogenes do not exhibit geographical variation.

Keywords: SfbII, fibronectin-binding protein, virulence types, phylogeny

Abbreviations: GAS, group A streptococcus; HRP, horseradish peroxidase; SOF, serum opacity factor; VT, virulence type

The GenBank accession numbers for the sequences reported in this paper are AF367011 (sof VT3.2), AF367012 (sof VT3.1), AF367013 (sof VT2.2), AF367014 (sof VT21), AF367015 (sof VT37.1) and AF367016 (sof 13).

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