Development Of M-like Protein Subunit Vaccine Against Streptococcus Equi Subsp. Zooepidemicus Infection

Streptococcus equi subsp. zooepidemicus causes disease in several animal species, including bovine mastitis, uterine infections and abortion of elderly horses, arthritis and septicimia of pigs. Its virulence factors include non-antigenic hyaluronic acid capsule, hyaluronidase. streptolysin O, streptokinase, IgG Fc receptor proteins, peptideglycan and M-like protein. M-like protein, an a -helical coiled-coil molecule attached to the cell membrane and extending out the capsule, elicits very strong B and T cell responses resulting in protective immunity mediated by opsonic antibodies. However, vaccines based on acid or mutanolysin extracted M-like protein mainly focused rather on the prevention of strangles of foals than on that of swine streptococcosis. To develope a highly efficient and safe M-like protein subunit vaccine against S. zooepidemicus infection of swine is expected.Hot-acid extracted M-like protein of S. zooepidemicus strain ATCC35246, the recombinant M-like protein and the whole inactive ATCC35246 cells were immunized mice respectively with ISA206 adjuvant to select the optimal antigen. The challenge test showed that hot-acid extracted M-like protein had quite strong antigenicity and the potential value to be developed as subunit vaccine.Subunit vaccines often induce weak immune response and protection when administered alone. In order to improve the efficacy of M-like protein subunit vaccine, three adjuvants, aluminium hydroxide, bacterial CpG and ISA206 were tested. They were injected into mice together with hot-acid extracted M-like protein separately. The results showed that ISA206 adjuvant could induce short and long term immunity without obviously adverse reactions, and its immune enhancement was higher than that of the other two. It suggested that ISA206 adjuvant could be used as an immunoadjuvant for the prevention of swine streptococcosis.In addition, adhesion role of M-like protein from S. zooepidemicus strain ATCC35246 to HEp-2 cells was evaluated by the adhesion and adhesive inhibition experiments. Evidence presented in the study indicated that M-like protein was a component of the adhesion mechanism of S. zooepidemicus strain ATCC35246, andthe binding site might reside near the N terminus. The inability of hot-acid extracted and recombinant M-like protein to completely inhibit streptococcal adhesion was probably due to alternative adhesions.