Abstract

Bacterial superantigen Staphylococcus aureus enterotoxin C2 (SEC2) is a very potent activator of T cells. Previous crystal studies on SEC2 showed that residue Tyr26 which is located near the T cell receptor (TCR) binding sites could potentially participate in T cell activating. In SEC1, a superantigen highly homologous to SEC2, the residue Val26 played a critical role in stimulating certain Vβ expansion. Moreover, residues 20, 22 and 26 in SEC1 determined its serologic cross-reaction. In order to investigate the potential roles of Tyr26 in SEC2, two SEC2 mutants SEC2(Y26V) and SEC2(Y26A) were constructed by site-directed mutagenesis. The activities and toxicities of the two mutants were determined both in vivo and in vitro. Results showed that both SEC2(Y26V) and SEC2(Y26A) remained potent immune stimulating activities compared with native SEC2. SEC2(Y26V) had a decreased pyrogenic effect on rabbit model. Both of the two mutants had significantly decreased binding affinities to anti-SEC2 IgG. Our result indicated that Tyr26 is a critical site for the serological characters of SEC2 rather than for its superantigen activity. This may also provide an insight in constructing a novel mutant to substitute native SEC2 in cancer immunotherapy avoiding being neutralized by anti-SEC2 IgG in vivo.

Key words: Staphylococcal enterotoxin C2, Site-directed mutagenesis, superantigen,serological, property.