Induction of protective immune response against Streptococcus suis serotype 2 infection by the surface antigen HP0245

Streptococcus suis serotype 2 (SS2) is an emerging zoonotic pathogen. Prevention and control of the diseases caused by SS2 bacteria are hampered by the lack of effective vaccines. We report that an in vivo-induced protein HP0245 was located at the cell surface of SS2. The extracellular peptide of HP0245 was produced in Escherichia coli BL21 (DE3). Its immunogenicity was compared with SS2 bacterin. Like SS2 bacterin, protein HP0245(EC) formulated in aluminum hydroxide adjuvant provided 100% protection in mice challenged with a low dose (2 × LD(50) ) of SS2. However, 80% and 50% survival rates were observed in mice vaccinated with HP0245(EC) and SS2 bacterin, respectively, challenged with a high dose (5 × LD(50) ) of SS2. Immunization with HP0245(EC) induced significantly higher IgG2a titers compared with SS2 bacterin, which was more effective for opsonophagocytosis. No obvious histopathological change was found in the HP0245(EC) -vaccinated mice after challenge with the low dose of SS2, whereas a mild lesion was observed in the meninges of the mice vaccinated with SS2 bacterin. Homologous hp0245 genes with the highly conserved coding sequence of the extracellular peptide exist in all sequenced SS2 strains as well as most S. suis reference strains. Thus, HP0245 could be considered as a promising vaccine candidate for SS2.