Mucosally Immunogenic Surface Exposed Proteins of Streptococcus Equi.

Streptococcus equi (Se) of Lancefield group C causes equine strangles, an acute highly contagious suppurative lymphadenitis of the head and neck. Isolates of Se exhibit remarkable genetic and antigenic uniformity. Vaccines prepared from whole cells or extracts do not confer a high level of protection against natural challenge, although stimulating strong serum opsonic responses. The objective of this study was to identify novel vaccine proteins using a combination of convalescent serum pool to probe an expression library and genomic sequence (www.sanger.ac.uk)torapidlyidentifyopenreadingframesinreactiveclones.Thirtyone ORFs were identified in inserts, which overlapped forming linkage groups located in 8 different regions of the Se genome. Ten of these ORFs encoded proteins with cell wall anchors as indicated by the presence of LPXTG or LXYC motifs, 7 encoded transmembranic proteins, and 4 encoded secreted proteins. With few exceptions, these proteins accounted for the immunoreactivity of the selected phages. Thus, surface exposed or secreted proteins of Se were many times more likely to stimulate antibody responses during infection than proteins located elsewhere. Moreover, genes for these exposed proteins tended to be distributed in loci; the most interesting were arranged in 2 operons consisting of four genes encoding proteins with the LPXTG motif together with genes for a sortase-like protein and a lipoprotein with homology to Sib38 of S. pyogenes (Linkage group 1). Another locus, Linkage group 2, contained 6 putative ORFs of which 3 encoded proteins with surface exposure, including an immunoreactive LPXTG containing protein with homology to the fibronectin-binding FNE protein of Se, an ABC transporter protein, and a lipoprotein with no known homology. Surface expression and reactivity of selected proteins was confirmed by antibody absorbtion and by ELISA with convalescent sera.