The capsule of Streptococcus equi ssp. zooepidemicus is a target for attenuation in vaccine development.

Streptococcus equi ssp. zooepidemicus (SEZ) is an important pathogen associated with a wide range of diseases in many mammalian species. The development of novel effective vaccines would be beneficial to control SEZ infection. In the present study, the importance of the SEZ capsule was examined using a newly constructed capsule-deficient mutant ΔhasB strain. Transmission electron microscopy confirmed a decrease in the abundance of extracellular capsular polysaccharide on the mutant SEZ. Compared to the parental wild-type SEZ, the ΔhasB mutant was highly attenuated in mice and provided 100% protection against lethal challenge when administered as a live vaccine. Real-time PCR analysis showed a marked increased in the levels of IL-4 and IFN-γ mRNA in immunized mice. The role that the capsule plays in SEZ pathogenicity was also explored with respect to the mechanistic design of an attenuated vaccine target. The capsule could resist complement C3 deposition on the surface of SEZ cells and aid in preventing complement-mediated opsonization and phagocytosis by cultured macrophages. These results suggest that the capsule of SEZ plays an important role in pathogenicity and may serve as a target for attenuation in vaccine development.