Staphylococcal Enterotoxin D Is a Promiscuous Superantigen Offering Multiple Modes of Interactions With the MHC Class II Receptors

Dimerization of MHC class II molecules on the cell surface of human THP-1 monocytic cell line is a requirement for staphylococcal superantigen (SAG)-induced cytokine gene expression. The capacities of various SAG to induce this response are governed by their modes of interaction with MHC class II molecules. Staphylococcal enterotoxin A (SEA), with its two binding sites, dimerizes MHC class II molecules and subsequently induces cytokine gene expression in THP-1 cells. Here, we demonstrate that staphylococcal enterotoxin D (SED) and staphylococcal enterotoxin E (SEE) induce, similarly, IL-1β and TNF-α gene expression in these cells. Using mutated toxins that lost their binding site with the MHC class II α- or β-chain, we demonstrate that this response is also mediated by the dimerization of MHC class II molecules through two binding sites. Furthermore, SED forms Zn2+-dependent homodimers that allow multiple modes of MHC class II clustering, including ligation of α-chains (α/α), β-chains (β/β), or the α- and β-chains of two different class II molecules. The β/β interaction following Zn2+-dependent SED/SED homodimer formation seems to be mediated by the appearance of a novel binding site on SED that interacts with histidine 81 of the MHC class II β-chain. The different modes of SED interactions also influence SED-induced T cell activation where simultaneous ligation of the α- and β-chains is essential for optimal response. These various modes of SED binding may be used to preserve bivalency regardless of variability in the MHC class II α/β/peptide complexes.