egc -Encoded Superantigens from Staphylococcus aureus Are Neutralized by Human Sera Much Less Efficiently than Are Classical Staphylococcal Enterotoxins or Toxic Shock Syndrome Toxin

Abstract

PCR was employed to determine the presence of all known superantigen genes (sea, seq, and tst) and of the exotoxin-like gene cluster (set) in 40 Staphylococcus aureus isolates from blood cultures and throat swabs; 28 isolates harbored superantigen genes, five on average, and this strictly correlated with their ability to stimulate T-cell proliferation. In contrast, the set gene cluster was detected in every S. aureus strain, suggesting a nonredundant function for these genes which is different from T-cell activation. No more than 10% of normal human serum samples inhibited the T-cell stimulation elicited by egc-encoded enterotoxins (staphylococcal enterotoxins G, I, M, N, and O), whereas between 32 and 86% neutralized the classical superantigens. Similarly, intravenous human immunoglobulin G preparations inhibited egc-encoded superantigens with 10-to 100-fold-reduced potency compared with the classical enterotoxins. Thus, there are surprisingly large gaps in the capacity of human serum samples to neutralize S. aureus superantigens.Staphylococcus aureus persists as a commensal microorganism in 10 to 30% of the population, but the organism is also a common cause of food poisoning and infections of different severity such as skin abscesses and wound infections, osteomyelitis, endocarditis, pneumonia, toxic shock syndrome, and staphylococcal scarlet fever (21). S. aureus is one of the most frequent causes of hospital-acquired infections, and the emergence and spread of multiresistant strains give rise to concern. The pathogenicity of S. aureus is multifactorial, and the versatility of this organism is underscored by recent clinical studies (4,14,16,31,37,38).Superantigens activate large subpopulations of T lymphocytes by directly cross-linking certain T-cell receptor V␤ domains with conserved structures on major histocompatibility complex class II molecules (32). They belong to the most potent T-cell mitogens known and can induce massive systemic cytokine release, leading to the symptoms of toxic shock syndrome (22). Among the virulence factors of S. aureus are the staphylococcal enterotoxins, the causative agents of food poisoning. They also act as superantigens. Whole-genome sequencing of several S. aureus clinical isolates has revealed that all 17 known staphylococcal enterotoxins (staphylococcal enterotoxins A to E and G to Q and toxic shock syndrome toxin 1) are encoded on mobile genetic elements together with other virulence factors (3,18,40). For example, the recently described enterotoxin gene cluster egc, which contains the five superantigen genes seg, sei, sem, sen, and seo, as well as two pseudogenes is located on the genomic island SaPI3 (18).egc is special in that it functions as an operon and its genes are transcribed into a single polycistronic mRNA (13). In addition, a large cluster of up to 11 genes with sequence homology to superantigens has been discovered on the genomic island SaPI2; they have been termed staphylococcal exotoxinlike genes, or set (3,10,18,39). For an overview of the organization ...

Immune Cell Activation by Enterotoxin Gene Cluster (egc)-Encoded and Non-egc Superantigens from Staphylococcus aureus

“…Despite this, they are not a prominent cause of toxic shock syndrome (4,11,25) but their presence appears to be associated mainly with symptom-free carriage and inversely correlated with the severity of S. aureus infection (12,26). Astonishingly, neutralizing anti-egc serum Abs are very uncommon in healthy individuals (18). In search of an explanation for these counterintuitive observations, we proposed two hypotheses: 1) egc and non-egc SAgs have unique intrinsic properties and drive the immune system into different directions and 2) egc and non-egc SAgs are released by S. aureus under different conditions, which shape the immune response to them.…”

“…Egc SAgs are by far the most prevalent staphylococcal SAgs (18,24). Despite this, they are not a prominent cause of toxic shock syndrome (4,11,25) but their presence appears to be associated mainly with symptom-free carriage and inversely correlated with the severity of S. aureus infection (12,26).…”

“…Human PBMC (10 5 /well) were stimulated with recombinant SAgs titrated over a broad concentration range (0.1 pg/ml to 1 g/ml) as described previously (18,19). A control protein (S. aureus anti-factor RsbW), which was overexpressed and purified in parallel to the recombinant SAgs, did not induce T cell activation.…”

“…We used four clinical S. aureus isolates from a strain collection from hospitals in Northeast Germany that were investigated previously (18). Their SAg gene repertoire was determined by multiplex-PCR (pSA17: seb, selp; pSA20: sec, sell, selp; aSA2: seg, sei, selm, seln, selo, selu; aSA4: seg, sei, selm, seln, selo) as described before (10).…”

“…Abs against non-egc SAgs (e.g., TSST-1, SEA, SEB, SEC) are common in the healthy population (15)(16)(17)(18). In S. aureus carriers, these Abs are highly specific for the SAgs of the colonizing strain and very effectively neutralize their SAg effects (19).…”