Staphylococcal enterotoxins are prototype superantigens characterized by their ability to bind to major histocompatibility complex (MHC) class II molecules and subsequently activate a large fraction of T‐lymphocytes. The crystal structure of staphylococcal enterotoxin type A (SEA), a 27 kDa monomeric protein, was determined to 1.9 A resolution with an R‐factor of 19.9% by multiple isomorphous replacement. SEA is a two domain protein composed of a beta‐barrel and a beta‐grasp motif demonstrating the same general structure as staphylococcal enterotoxins SEB and TSST‐1. Unique for SEA, however, is a Zn2+ coordination site involved in MHC class II binding. Four amino acids including Ser1, His187, His225 and Asp227 were found to be involved in direct coordination of the metal ion. SEA is the first Zn2+ binding enterotoxin that has been structurally determined.

Toxic shock-like syndrome (TSLS) is characterized by hypotension or shock, fever, multiorgan system involvement, and a concurrent group A streptococcal infection. We analyzed 34 streptococcal strains isolated from patients with clinically well-documented TSLS for their pyrogenic toxin profiles and M-protein types. Although strains of nine different M types were represented in the sample, 74% of the isolates were of either M type 1 or 3. It was determined that 53% produced streptococcal pyrogenic exotoxin type A under in vitro growth conditions and that 85% contained the gene encoding this toxin. These values are in contrast to the published value of 15% for the incidence of this gene in a sample of general group A streptococcal isolates. As has been found with all group A streptococci examined to date, regardless of disease association, 100% of TSLS-associated isolates contained the gene encoding pyrogenic exotoxin type B. This toxin was detectably produced by 59% of isolates. The gene encoding pyrogenic toxin type C was found in only 21% of isolates. We conclude that the pyrogenic exotoxin type A gene is associated with group A streptococcal strains isolated from patients with TSLS and may play a causative role in this illness. However, other factors are also likely to be important, since not all strains from patients with TSLS contained the A toxin gene.

Group A streptococcal pyrogenic exotoxin (SPE) type C was partially purified by differential solubility in ethanol and acetate-buffered saline. Toxin prepared in this way consisted of protein and hyaluronic acid. After removal of hyaluronic acid, the toxin remained pyrogenic, enhanced susceptibility of rabbits to lethal endotoxin shock, was stable when treated with acid, base, or pepsin, but was inactivated by heat. Toxin further purified by thin-layer isoelectric focusing was pyrogenic and enhanced the susceptibility of rabbits to lethal endotoxin shock. Purified type C toxin appeared homogeneous when tested by Ouchterlony immunodiffusion and migrated as a single protein band in isoelectric focusing polyacrylamide gels (isoelectric point, 6.7) and sodium dodecyl sulfate-polyacrylamide gels (molecular weight, 13,200). The purified toxin was antigenically distinct from A and B SPE, and antisera raised against the purified toxin neutralized pyrogenic activity. The amino acid composition was determined.

Staphylococcal pyrogenic exotoxin (PE) type C enhanced the susceptibility of rabbits to lethal shock by endotoxin by as much as 50,000-fold. A graph of log PE type C dose used for pretreatment versus log 50% lethal dose of endotoxin gave a straight line with a slope of approximately-1. Rabbits that received PE type C alone showed fevers only, but those given both PE type C and endotoxin showed initial fever followed by hypothermia, labored breathing, diarrhea, evidence of vascular collapse, and finally death. When a PE type C dose of 3 ,ug/kg was used, pretreatment of the animals with PE for 2 h before giving the endotoxin was required to obtain maximal susceptibility. However, when 15 ,ug of PE type C per kg was utilized, the endotoxin could be given before, concurrently, or after PE type C. The capacity of PE type C to prepare rabbits for enhanced susceptibility to endotoxin was lost after 24 to 48 h. Animals could be protected from enhanced susceptibility to endotoxin by prior immunization with either PE type C or endotoxin. However, 30% of the rabbits which were immunized with PE type C failed to develop immunity, and after three injections of PE type C, these animals developed gram-negative bacteremia and succumbed. In addition, rabbits with diarrhea initially, possibly caused by Pasteurella infection, died less than 24 h after a single injection of PE type C.

No abstract