Defining a novel domain of staphylococcal toxic shock syndrome toxin-1 critical for major histocompatibility complex class II binding, superantigenicactivity, and lethality

Staphylococcal toxic shock syndrome toxin-1 (TSST-1) is implicated in the p athogenesis of superantigen-mediated shock. We previously identified TSST-1 residues G31/S32 to be important for major histocompatibility complex (MHC ) class II binding, as well as superantigenic and lethal activities. Howeve r, the site-directed TSST-1 mutant toxin, G31R, could still induce mitogene sis and low-level TNFalpha secretion, suggesting that additional MHC class II binding sites other than G31/S32 may exist. In the current study, a TSST -1-neutralizing monoclonal antibody, MAb5, was found to inhibit TSST-1 bind ing to human peripheral blood mononuclear cells, neutralize TSST-1-induced mitogenesis and cytokine secretion, and protect against TSST-1-induced leth ality in vivo. Epitope mapping revealed that MAb5 bound to TSST-1 residues 51-56 (T(51-56); (51)YYSPAF(56)). Peptide T(51-56) was synthesized and foun d to also inhibit TSST-1 binding to human monocytes as well as TSST-1-induc ed mitogenesis, cytokine secretion, and lethality in vivo. This T(51-56) ep itope, located within the beta 3/beta 4 loop, and the previously identified G31/S32 epitope, within the beta 1/beta 2 loop of TSST-1, are separated wi thin the primary sequence, but spatially juxtaposed to each other. Collecti vely, these findings suggest that a discontinuous epitope comprising of reg ions within both the beta 1/beta 2 and beta 3/beta 4 loops, are critical fo r MHC class II binding, and the consequent superantigenic and lethal activi ties of TSST-1.