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J. Exp. Med.,
Volume 189, Number 1, January 4, 1999 89-102
By
From the Department of Molecular Medicine, School of Medicine, University of Auckland, 92019 Auckland, New Zealand
Three novel streptococcal superantigen genes (spe-g, spe-h, and spe-j) were identified from the
Streptococcus pyogenes M1 genomic database at the University of Oklahoma. A fourth novel gene
(smez-2) was isolated from the S. pyogenes strain 2035, based on sequence homology to the
streptococcal mitogenic exotoxin z (smez) gene. SMEZ-2, SPE-G, and SPE-J are most closely
related to SMEZ and streptococcal pyrogenic exotoxin (SPE)-C, whereas SPE-H is most similar to the staphylococcal toxins than to any other streptococcal toxin. Recombinant (r)SMEZ,
rSMEZ-2, rSPE-G, and rSPE-H were mitogenic for human peripheral blood lymphocytes with half-maximal responses between 0.02 and 50 pg/ml (rSMEZ-2 and rSPE-H, respectively).
SMEZ-2 is the most potent superantigen (SAg) discovered thus far. All toxins, except rSPE-G,
were active on murine T cells, but with reduced potency. Binding to a human B-lymphoblastoid line was shown to be zinc dependent with high binding affinity of 15-65 nM. Evidence
from modeled protein structures and competitive binding experiments suggest that high affinity binding of each toxin is to the major histocompatibility complex class II 
chain. Competition
for binding between toxins was varied and revealed overlapping but discrete binding to subsets
of class II molecules in the hierarchical order (SMEZ, SPE-C) > SMEZ-2 > SPE-H > SPE-G.
The most common targets for the novel SAgs were human V2.1- and V4-expressing T
cells. This might reflect a specific role for this subset of Vs in the immune defense of gram-positive bacteria.
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