Binding of natural variants of staphylococcal superantigens SEG and SEI to TCR and MHC class II molecule

SEG and SEI are staphylococcal superantigens (SAgs) identified recently that belong to the egc operon and whose genes are in tandem orientation. Only a few allelic variants of SEG and SEI have been reported. Here we analyzed four Staphylococcus aureus strains with genotypic variation in both SAgs. However, both SAgs retain key residues in their putative TCR and MHC binding sites and, accordingly, their superantigenic properties. Thus, SEI significantly stimulates mouse T-cells bearing Vβ3, 5 and 13, while SEG stimulates Vβ7 and 9 in the draining node when inoculated in the footpad. As another member of the SEB subfamily, SEG also stimulates mouse Vβ8.1+2. However, the increase in Vβ8.1+2 T-cells observed at day 2 after inoculation reverts to normal values at day 4, whereas it remains high at day 4 following inoculation with SEC3 or SSA. T-cell stimulation assays in the mouse and analysis of the putative Vβ8.2 binding site on SEG, which includes three non-conserved residues, suggest a possibly unique interaction between Vβ8.2 and SEG. We also analyzed biochemical and biophysical characteristics of SEI and SEG binding to their cognate human β chains by surface plasmon resonance, and binding to the HLA-DR1 MHC class II molecule by gel filtration. SEI binds human Vβ5.2 and Vβ1 with apparent KD's of 23 and 118 μM, respectively; SEG binds Vβ13.6 with a KD of 5 μM. As suggested by sequence homology, SEI requires Zn2+ for strong binding to DR1, which goes undetected in the presence of EDTA. SEG and SEI have characteristics such as co-expression, different interaction with MHC class II and stimulation of completely different subsets of human and mouse T-cells, which indicate complementary superantigenic activity and suggest an important advantage to staphylococcal strains in producing them both.