Modification of the carboxyl terminal group affects replacement set analysis of a cytotoxic T cell epitope

Abstract

An Epstein-Barr virus-specific cytotoxic T cell (CTL) clone was previously shown to recognize the epitope FLRGRAYGL. The ability of substituted peptides to sensitize target cells for lysis by this clone was investigated using two peptide replacement sets, one synthesized with free carboxyl termini and another made with a carboxyl terminal beta-alanine-diketopiperazine (beta A-DKP) group. Curiously, the effect of certain substitutions differed for the two peptide sets. For example, NH2-FLRGRAYGL-beta A-DKP was 15-fold more active than NH2-FLRGRAYGI-beta A-DKR, but NH2-FLRGRAY-GL-COOH had the same activity as NH2-FLRGRAYGI-COOH. Evidence presented here illustrates that the activity of beta A-DKP peptide preparations was entirely due to contaminating peptides with free-COOH termini. The amount of active-COOH contaminants within a -beta A-DKP preparation thus appeared to vary depending on the substitution, resulting in some anomalous results from replacement set analysis using -beta A-DKP peptides.