Immunogenic <i>Streptococcus equi</i> cell surface proteins identified by ORFeome phage display

This work utilized an ORFeome phage display platform to systematically identify antigenic epitopes produced by <i>Streptococcus equi</i> subspecies equi (<i>S. equi</i>), an important equine pathogen and the causative agent of horses strangles. Three major <i>S. equi</i> 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 <i>S. equi</i> genomes, in contrast to their diversity in <i>S. equi</i> subsp. zooepidemicus. With the well-characterized <i>S. equi</i> 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 <i>S. equi</i> immunogenic proteins and provides peptide regions that could serve as vaccine candidates against <i>S. equi</i> or as diagnostic markers to specifically identify <i>S. equi</i> infections.