Streptococcal mitogenic exotoxin Z, a novel acidic superantigenic toxin produced by a T1 strain of Streptococcus pyogenes

Abstract

Streptococcus pyogenes T1 was previously found to produce an acidic mitogenic exotoxin, designated Aβ, antigenically distinct from erythrogenic toxin type A (ETA) of strains T1 and NY5. Following chemical analysis and biological characterization, we have renamed this toxin streptococcal mitogenic exotoxin Z (SMEZ). Physicochemical separation of SMEZ from ETA was successfully performed on a hydrophobic chromatograph. The isoelectric point was pH 5.3, and the molecular size was estimated to be 28 kDa. These values were similar to those of ETA, but the amino acid composition and the NH2-terminal sequence of SMEZ were distinct from those of any mitogenic exotoxins hitherto described. Its mitogenic activity was found to be more potent than that of ETA in rabbit lymphocyte cultures. A specific antiserum raised against SMEZ did not cross- react with ETA, ETB, or ETC in the neutralization tests of mitogenic and erythrogenic activities. Its superantigenic nature was evident from the reverse transcriptase PCR findings of the T- cell receptor Vβ profiles of rabbit lymphocytes stimulated in vitro. The Vβ 8 subfamily was unique to SMEZ, while the Vβ 2 and 6 subfamilies were found to be common among lymphocytes stimulated with ETA, ETB, ETC, or SMEZ. The results from this study provide an additional example of the diversity that exists among mitogenic or superantigenic exotoxins of streptococcal origin.