Changes to peptide structure, not concentration, contribute to expansion of the lowest avidity cytotoxic T lymphocytes

The efficient in vitro expansion of antigen‐specific CD8 + cytotoxic T lymphocytes (CTL) for use in adoptive immunotherapy represents an important clinical goal. Furthermore, the avidity of expanded CTL populations often correlates closely with clinical outcome. In our study, high‐avidity CTL lines could be expanded ex vivo from an antigen‐primed animal using low peptide concentration, and intermediate peptide concentrations favored the generation of lower avidity CTL. Further increases in peptide concentration during culture inhibited the expansion of all peptide‐specific CD8 + cells. In contrast, a single amino acid variant peptide efficiently generated functional CTL populations at high or low peptide concentration, which responded to wild‐type epitope with the lowest average avidity seen in this study. We propose that for some peptides, the efficient generation of low‐avidity CTL responses will be favored by stimulation with altered peptide rather than high concentrations of wild‐type epitope. In addition, some variant peptides designed to have improved binding to major histocompatibility complex class I may reduce rather than enhance the functional avidity for the wild‐type peptide of ex vivo‐expanded CTL. These observations are relevant to in vitro expansion of CTL for immunotherapy and strategies to elicit regulatory or therapeutic immunity to neo‐self‐antigen when central tolerance has eliminated high‐avidity, cognate T cells.