Identification and Characterization of Novel Superantigens from Streptococcus pyogenes

Abstract

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 β 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 Vβ2.1- and Vβ4-expressing T cells. This might reflect a specific role for this subset of Vβs in the immune defense of gram-positive bacteria.

HLA Transgenic Mice Provide Evidence for a Direct and Dominant Role of HLA Class II Variation in Modulating the Severity of Streptococcal Sepsis

“…Regular mice are generally not a model of choice for STSS because they respond poorly to SAgs, particularly to the streptococcal SAgs (18 -20). To elicit appropriate responses in these mice, investigators have resorted to using doses of SAgs that are much higher than physiologic (43,44), have augmented SAg effects by coadministering LPS (19,45), or treated the mice with D-galactosamine to prolong the biologic half-life of cytokines by delaying their hepatic clearance (18). Whereas these models provided valuable information, these manipulations may not represent actual events of the human disease.…”

Complete genome sequence of an M1 strain of Streptococcus pyogenes

“…3). The products of several of these genes have now been characterized and indeed shown to be among the class of superantigens (48). Genes encoding previously proposed virulence activities that had not been identified or cloned before the start of this sequencing project have now been located such as NADase, hyaluronidase, streptolysin S, amylase, phosphatase, and proteinase.…”

The Three-dimensional Structure of a Superantigen-like Protein, SET3, from a Pathogenicity Island of the Staphylococcus aureus Genome

“…The completion of the S. pyogenes genome sequence (16) revealed six new superantigens for which structures and functions have been characterized (17). Although it is unsurprising that the staphylococcal genome would also encode a new group of superantigen-like proteins, the structural and functional variation in this family leaves many questions as to their activity.…”