Staphylococcal enterotoxin K: A potential new target for immunotherapy against Staphylococcus aureus

Abstract

Staphylococcus aureus is a major human pathogen with an extraordinary arsenal of virulence factors. The staphylococcal enterotoxins (SEs) are a family of secreted toxins with superantigenic properties. Neutralization of these superantigens has been shown to be protective in animal models of sepsis and toxic shock. SEK is a newly described superantigen that is one of the most prevalent SEs in clinical isolates, and is the only one to be associated with the community-acquired MRSA strain, USA300. However, studies into the expression and biological significance of this toxin are lacking. Using murine monoclonal antibodies, we developed a capture ELISA for the specific and sensitive measurement of SEK in biological fluids. Using this ELISA, we observed detectable levels of SEK secretion by all clinical isolates that encode sek. Analysis of these measurements from 36 isolates representing various MRSA and MSSA strains revealed significantly higher secretion of SEK by MRSA strains that co-encode SEB. However, in the setting of infection, we demonstrate that there is comparable accumulation of SEK in abscesses regardless of whether the infecting strain exhibits low or high in-vitro secretion of SEK. We then test the capacity of these mAbs to inhibit the superantigenic effects of SEK, and to protect mice from SEK intoxication and S. aureus septicemia. In-vitro, we demonstrate that these mAbs inhibit SEK-induced proliferation of human PBMCs in a dose-dependent manner. Using BALB/c mice, we then show that intraperitoneal administration of SEK results in lethality within 24 hours, and that pretreatment with a combination of mAb-4G3 (IgG2b) and mAb-5G3 (IgG1) protects mice from SEK-induced lethality. In the setting of sepsis, treatment with this combination of mAbs also protects mice from challenge with a blood-borne isolate of CA-MRSA strain USA300. Furthermore, septic mice that received mAb treatment in conjunction with vancomycin exhibit less morbidity than mice that received vancomycin alone. Taken together, our work demonstrates that SEK is commonly present in the setting of infection, and may contribute more to S. aureus pathogenesis than previously appreciated. Moreover, the monoclonal antibodies described in these studies have the potential to be further developed into diagnostic and therapeutic modalities.