Staphylococcus aureus carriers neutralize superantigens by antibodies specific for their colonizing strain: a potential explanation for their improved prognosis in severe sepsis.

Staphylococcus aureus is one of the most common causes of hospital-acquired infections. At the same time, 25% of healthy persons are symptom-free S. aureus carriers, and they have an increased risk of developing nosocomial S. aureus septicemia. Paradoxically, their prognosis is much better than that of noncarriers. We compared the antibody profiles for carriers and noncarriers toward S. aureus superantigens. In carriers, we found high titers of neutralizing antibodies specific for those superantigens that are expressed by their colonizing strain. The results show that carriage status confers strain-specific humoral immunity, which may contribute to protection during S. aureus septicemia.

egc-Encoded superantigens from Staphylococcus aureus are neutralized by human sera much less efficiently than are classical staphylococcal enterotoxins or toxic shock syndrome toxin.

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.