Immunogenic Streptococcus equi cell surface proteins identified by ORFeome phage display.

Equine strangles caused by Streptococcus equi subspecies equi (S. equi) remains a significant cause of morbidity and mortality in horses, and there is a need for improved diagnostic and vaccination strategies for addressing this pathogen. ORFeome phage display is a platform that allows for rapid screening for potential antigenic epitopes by construction of phage-displayed peptide libraries. In this study, an S. equi ORFeome library was used to screen serum from a panel of 17 horses with known exposure to S. equi to identify antigenic bacterial proteins. From this screen, three major S. equi proteins were identified: a novel proline-rich repeat domain protein, a serine peptidase, and the M-like protein SeM. These three proteins are predicted to be expressed on the surface of the bacterial cell by the presence of N- and C-terminal signals. The proline-rich repeat protein and serine peptidase were confirmed to be immunogenic by enzyme-linked immunoassay (ELISA) using the recombinant full-length proteins against sera from horses with strangles, horses infected with the related pathogen S. equi subsp. zooepidemicus, and healthy control horses. Due to the native IgG binding activity of SeM, ELISA against the full-length protein was not conducted, but the specificity of the antibody response against the recovered ORFeome clones was confirmed and an antigenic region identified. Both the proline-rich repeat protein and serine peptidase were found to be highly conserved in global S. equi genomes, indicating these proteins may be useful as vaccine candidates against S. equi or as diagnostic markers to specifically identify S. equi infections in horses. IMPORTANCE: This work utilized an ORFeome phage display platform to systematically identify antigenic epitopes produced by Streptococcus equi subspecies equi (S. equi), an important equine pathogen and the causative agent of horses strangles. Three major S. equi surface proteins were identified: a novel proline-rich repeat domain protein, a serine peptidase, and the M-like protein SeM. The proline-rich repeat protein and serine peptidase were confirmed to be immunogenic in horses with strangles, and their sequences were shown to be conserved in global S. equi genomes, in contrast to their diversity in S. equi subsp. zooepidemicus. With the well-characterized S. equi immunogenic protein SeM, this paper identified an immunogenic region outside of the reported critical IgG-binding region. This work provides novel insights to the understanding of the S. equi immunogenic proteins and provides peptide regions that could serve as vaccine candidates against S. equi or as diagnostic markers to specifically identify S. equi infections.