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Abstract

Toxic Shock Syndrome (TSS) is primarily caused by Toxic Shock Syndrome Toxin-1 (TSST-1), Staphylococcal Enterotoxin A (SEA), Staphylococcal Enterotoxin B (SEB), and Staphylococcal Enterotoxin C (SEC). These toxins belong to a family of pyrogenic toxin superantigens (PTAgs) produced by Staphylococcus aureus. These PTAgs share similar immunobiological properties, including the induction of massive release of cytokines and stimulation of T cell proliferation in a VP-specific manner. The crystal structures of most PTAgs are now known. They share a similar basic structure even though their primary sequences are different. Despite the structural and immunobiological similarities, no cross-reactivity between TSST-1 and other PTAgs has been demonstrated in serological assays and neutralization assays. Recently, our laboratory has developed a neutralizing murine anti-TSST-1 monoclonal antibody (MAb5) which demonstrated significant cross-reactivity with SEB in ELISA. Since previous studies have shown that MAb5 can neutralize TSST-l-induced superantigenic and lethal activities both in vitro and in vivo, the present study was undertaken to evaluate possible cross-neutralizing epitopes on TSST-1 and SEB using MAb5. Our experimental approach in addressing this question included: (1) assessment of the ability of MAb5 to cross-neutralize SEB-induced superantigenic activities in vitro and in vivo; (2) determination of the location of the crossreactive epitopes on TSST-1 and SEB by epitope peptide mapping; (3) evaluation of the importance of the cross-reactive epitope as a functional domain by the use of synthetic peptides. MAb5 was found to partially inhibit SEB-induced T cell mitogenesis (63%) and TNFa secretion (70%) in human blood mononuclear cells (HPBMC) in a dose dependent manner, while a control anti-TSST-1 monoclonal antibody, MAb6, had no effect. In order to locate the cross-reactive epitopes on TSST-1 and SEB, binding of MAb5 to a set of overlapping decapeptides homologous to TSST-1 (1-194) and SEB (1-239) were determined. MAb5 recognized a linear sequence of 5 amino acids ( " YSPAF5 6 ) on TSST- 1, as well as a linear sequence of 4 amino acids ( 8 5FGAN8 8 ) on SEB. These epitopes are structurally similar but are topologically located in different areas of the respective toxins. Furthermore, a 10 mer peptide corresponding to SEB (83-92), ⁸³ DWGANYYYQ ⁹² , was found to inhibit SEB-induced T cell mitogenesis (90%) and TNFa secretion (90%) from HPBMC in vitro, while an irrelevant, scrambled peptide had no effect. These data suggest that MAb5 recognizes structurally similar motifs of SEB and TSST-1, and contains neutralizing activity against these related PTAgs.