Staphylococcal enterotoxin H displays unique MHC class II-binding properties

Staphylococcal enterotoxin H (SEH) has been described as a superantigen by sequence homology with the SEE subfamily and briefly characterized for its in vivo activity. In this study, we demonstrate that SEH is a potent T cell mitogen and inducer of T cell cytotoxicity that possesses unique MHC class II-binding properties. The apparent affinity of SEH for MHC class II molec ules is the highest affinity ever measured for a staphylococcal enterotoxin (B-max1/2 similar to 0.5 nM for MHC class II expressed on Raji cells). An excess of SEA or SEA(F47A), which has reduced binding to the MHC class II a lpha-chain, is able to compete for binding of SEH to MHC class II, indicati ng an overlap in the binding sites at the MHC class LI beta-chain, The bind ing of SEH to MHC class II is like SEA, SED, and SEE dependent on the prese nce of zinc ions. However, SEH, in contrast to SEA, binds to the alanine-su bstituted DR1 molecule, beta H81A, believed to have impaired zinc-bridging capacity. Furthermore, alanine substitution of residues D167, D203, and D20 8 in SEH decreases the affinity for MHC class ZI as well as its in vitro po tency. Together, this indicates an MHC class II binding site on SEH with a different topology as compared with SEA. These unique binding properties wi ll be beneficial for SEH to overcome MHC class II isotype variability and p olymorphism as well as to allow an effective presentation on APCs also at l ow MHC class II surface expression.