Identification from a phage display library of peptides that bind to toxic shock syndrome toxin-1 and that inhibit its binding to major histocompatibility complex (MHC) class II molecules

Phage display technique is a powerful tool with which to identify novel binding sequences for antibody and receptor targets. Few studies, however, have used this technology to select affinity peptides for ligand molecules. Here, we screened a peptide phage library for binding to toxic shock syndrome toxin 1 (TSST-1) to examine whether peptide ligands for TSST-1 which mimic the structure of major histocompatibility complex (MHC) class II receptors could be identified. After three cycles of biopanning, four potent sequences reactive with TSST-1 were isolated (designated phages 2, 3, 8, and 11). Selected phage were found to react specifically with TSST-1 but not with other staphylococcal exotoxins. A synthetic peptide (pep3) corresponding to the most frequently identified sequence (phage3) was shown to inhibit binding of all four isolated phage to TSST-1, suggesting that they bind to a common site on TSST-1. Furthermore, pep3 was shown to compete with MHC class II molecules for binding to TSST-1 in a concentration-dependent manner. Comparison of their sequences with MHC class II molecules revealed that phage8 shared sequence homology with two regions of the beta chain of MHC class II molecules: amino acids 57-62, containing a residue (Tyr-60) involved in TSST-1 binding as suggested by X-ray crystallographic data of TSST-1-MHC class II complex; and amino acids 188-193, a region not previously known as a contact domain. These results suggest that the selected sequences recognized the MHC class II binding site on TSST-1. Thus, affinity selection for peptides binding to ligand molecules (e.g., TSST-1) rather than their cognate receptors (e.g., MHC class II) from a random phage display library represents a useful approach to understanding receptor-ligand interactions.