Identification and Experimental Verification of Protective Antigens Against <Emphasis Type="Italic">Streptococcus suis</Emphasis> Serotype 2 Based on Genome Sequence Analysis

Streptococcus suis serotype 2 (S. suis 2) is an important zoonotic pathogen that can cause severe disease and even death in both humans and swine. No effective vaccine is clinically available. In this study, a reverse vaccinology method was first applied to identify protective antigens against S. suis 2. As a consequence, 153 genes encoding vaccine candidates were selected from the whole genome sequence by means of bioinformatics analysis, from which 10 genes were selected based on experimental evidences arising from the study of related bacteria such as Streptococcus pneumoniae, group B streptococcus, S. suis and so on. Of 10 target genes, 8 were successfully expressed in Escherichia coli Rosetta, and expressed proteins were purified and used as the immunogens for evaluating vaccine efficacy in a mouse infection model. The results have confirmed that RTX family exoprotein A (RfeA), epidermal surface antigen (ESA), immunoglobulin G (IgG)-binding protein (IBP), and suilysin (SLY) can induce a protective response of the vaccinated animals against S. suis 2, whereas RfeA, ESA, and IBP mainly induce humoral-mediated immunity, and SLY elicits a combined pattern of both humoral- and cellular-mediated immunity. Although immunoprotection of SLY against S. suis 2 was reported previously, RfeA, ESA, and IBP were explored first in this study.