| Streptococcus equi is a highly host adapted clonal pathogen. A novel gene cluster of S. equi designated Linkage Group I is comprised of 6 genes, 5 of which encode for surface associated proteins, SzPSe, Se73.9, Se51.9, Se44.2 and Se46.8. The sixth gene Se30.0 encodes for a novel sortase. Sequence analysis suggests that Se73.9 and Se51.9 are encoded by Operon 1 whereas Se44.2 and Se46.8 are encoded by Operon 2. A similar organization of genes is also present in the commensal S. zooepidemicus but with considerable differences in the sequences of SzPSe, Se44.2 and Se46.8. Both convalescent sera and nasal washes recognize SzPSe, Se44.2 and Se46.8. Se73.9 and Se51.9 react weakly with serum antibodies although Se51.9 induces variable mucosal antibody responses in convalescent horses. Binding studies have demonstrated that SzPSe and Se46.8 bind to equine fibrinogen and Se46.8 also has a weak affinity for fibronectin. Se44.2, Se73.9 and Se51.9 aggregated equine platelets. SzPSe, Se73.9 and Se51.9 are important tonsil binding proteins with specificities for crypts, collagen rich propria, and epithelium respectively. Competitive infection experiments in horses with wild type S. equi CF32 and insertional mutants of S. equi Se73.9 and Se44.2 indicated that Se73.9 is not essential for colonization of lymph nodes though its disruption reduces virulence. Preliminary data suggest an important role for Se44.2 in the early stages of S. equi pathogenesis. Vaccines consisting of pools of proteins administered subcutaneously have confirmed the immunogenicity of Linkage Group I proteins and have identified proteins uniquely recognized during S. equi infection. However, susceptibility of these vaccinates to comingling challenge suggests that the pools of proteins lacked the correct combination of critical antigens or that an alternate presentation to stimulate mucosal or cell-mediated responses is required. Evidence was obtained that Se44.2 is among the first of S. equi proteins to be recognized by the host immune system following infection. Linkage Group I encodes for important surface proteins of S. equi that contribute to the complex pathogenesis of strangles as adhesins and in induction of host immune response. |
