Characterization and Protective Immunogenicity of the SzM Protein of Streptococcus zooepidemicus NC78 from a Clonal Outbreak of Equine Respiratory Disease

Abstract

ABSTRACTStreptococcus zooepidemicusof Lancefield group C is a highly variable tonsillar and mucosal commensal that usually is associated with opportunistic infections of the respiratory tract of vertebrate hosts. More-virulent clones have caused epizootics of severe respiratory disease in dogs and horses. The virulence factors of these strains are poorly understood. The antiphagocytic protein SeM is a major virulence factor and protective antigen ofStreptococcus equi, a clonal biovar of an ancestralS. zooepidemicusstrain. Although the genome ofS. zooepidemicusstrain H70, an equine isolate, contains a partial homolog (szm) ofsem, expression of the gene has not been documented. We have identified and characterized SzM from an encapsulatedS. zooepidemicusstrain from an epizootic of equine respiratory disease in New Caledonia. The SzM protein of strain NC78 (SzMNC78) has a predicted predominantly alpha-helical fibrillar structure with an LPSTG cell surface anchor motif and resistance to hot acid. A putative binding site for plasminogen is present in the B repeat region, the sequence of which shares homology with repeats of the plasminogen binding proteins of human group C and G streptococci. Equine plasminogen is activated in a dose-dependent manner by recombinant SzMNC78. Only 23.20 and 25.46% DNA homology is shared with SeM proteins ofS. equistrains CF32 and 4047, respectively, and homology ranges from 19.60 to 54.70% for SzM proteins of otherS. zooepidemicusstrains. As expected, SzMNC78reacted with convalescent-phase sera from horses with respiratory disease associated with strains ofS. zooepidemicus. SzMNC78resembles SeM in binding equine fibrinogen and eliciting strong protective antibody responses in mice. Sera of vaccinated mice opsonizedS. zooepidemicusstrains NC78 and W60, the SzM protein of which shared partial amino acid homology with SzMNC78. We conclude that SzM is a protective antigen of NC78; it was strongly reactive with serum antibodies from horses during recovery fromS. zooepidemicus-associated respiratory disease.