Purification and antigenicity of an M-like protein of Streptococcus equi

A cell wall component of Streptococcus equi analogous to the M protein of group A streptococci has been identified and purified. A highly purified product has been obtained from cells by hot acid extraction, followed by acid precipitation, ammonium sulfate fractionation, and column chromatography. This product reacts with S. equi antiserum. The existence of this fraction in S. equi has been confirmed by the failure of trypsin-treated cells and their extracts to remove the long-chaining capacity of S. equi antiserum. The antigenicity of this M-like protein when incorporated in adjuvant has been assessed in rabbits and horses. In the rabbit, multiple doses of as low as 10 mug resulted in the production of serum capable of inducing long-chain formation and opsonizing and mouse protective antibody. Two doses of 100 mug elicited similar responses. The bactericidal capacity of this serum could be eliminated by absorption with whole S. equi cells or their extracts, but not by absorption with trypsin-treated cells or extracts therefrom. Horses vaccinated 15 months previously with a whole-cell killed vaccine and given 200 mug of the purified protein intramuscularly showed evidence of an anamnestic response to the antigen.