Identification and characterization of novel superantigens from Streptococcus pyogenes.

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 beta 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 Vbeta2.1- and Vbeta4-expressing T cells. This might reflect a specific role for this subset of Vbetas in the immune defense of gram-positive bacteria.

The streptococcal superantigen SMEZ exhibits wide allelic variation, mosaic structure, and significant antigenic variation.

The frequencies of the newly identified streptococcal superantigen genes smez, spe-g, and spe-h were determined in a panel of 103 clinical isolates collected between 1976 and 1998 at various locations throughout New Zealand. smez and spe-g were found in every group A Streptococcus (GAS) isolate, suggesting a chromosomal location. The spe-h gene was found in only 24% of the GAS isolates and is probably located on a mobile DNA element. The smez gene displays extensive allelic variation and appears to be in linkage equilibrium with the M/emm type. 22 novel smez alleles were identified from 21 different M/emm types in addition to the already reported alleles smez and smez-2 with sequence identities between 94. 5 and 99.9%. Three alleles are nonfunctional due to a single base pair deletion. The remaining 21 alleles encode distinct SMEZ variants. The mosaic structure of the smez gene suggests that this polymorphism has arisen from homologous recombination events rather than random point mutation. The recently resolved SMEZ-2 crystal structure shows that the polymorphic residues are mainly surface exposed and scattered over the entire protein. The allelic variation did not affect either Vbeta specificity or potency, but did result in significant antigenic differences. Neutralizing antibody responses of individual human sera against different SMEZ variants varied significantly. 98% of sera completely neutralized SMEZ-1, but only 85% neutralized SMEZ-2, a very potent variant that has not yet been found in any New Zealand isolate. SMEZ-specific Vbeta8 activity was found in culture supernatants of 66% of the GAS isolates, indicating a potential base for the development of a SMEZ targeting vaccine.

Antibasement membrane antibody disease without clinical evidence of renal disease.

A 20-yr-old man is described with the rare presentation of antibasement membrane antibody (ABMA) disease confirmed on renal biopsy, but with normal renal function and urinary sediment. This distinct subgroup of ABMA appears to have an excellent prognosis. The relevant literature is reviewed and summarized.