Identification and Evaluation of Conserved Subunit Vaccine Candidates Conferring Cross-Serotype Protection Against...

Streptococcus suis is a major zoonotic pathogen with substantial serotype diversity and remains a serious threat to global swine production and public health. Accordingly, vaccines capable of inducing cross-serotype protection are urgently needed. Here, we integrated comparative genomics, bioinformatics, and reverse vaccinology to identify five conserved candidate antigens from four clinically important S. suis serotypes (2, 7, 8, and 9), designated rP1-rP5. The previously reported antigen PstB was included as a reference control (rP6), as it has demonstrated protective efficacy against serotypes 2, 7, and 9. All six recombinant proteins were successfully expressed and purified, and in silico analyses supported their predicted antigenic features and immunogenic potential. Following three immunizations in mice, all candidates elicited robust antigen-specific IgG responses. In opsonophagocytic assays, antisera against rP1, rP3, rP4, and rP6 displayed functional activity against serotypes 2, 7, 8, and 9. In a mouse infection model, vaccination with rP3, rP4, and rP6 reduced bacterial burdens in blood and multiple organs and conferred partial protection against all four serotypes, with rP4 producing a more pronounced reduction in tissue bacterial burdens than rP3 and rP6. Overall, rP4 and rP6 exhibited the most consistent cross-serotype protection in both bacterial burden and survival analyses, whereas rP3 showed protective efficacy, particularly against serotype 2, and may serve as a useful component of a multivalent formulation. Collectively, these findings support the rational design of broadly protective, multicomponent subunit vaccines against S. suis.