A single specific amino acid residue in peptide antigens is sufficient to activate memory CTL: Potential role of cross-reactive peptides in memory T cell maintenance

In the present study, we examined the structural requirements of peptide Ag s for productive interactions with the TCR of CTL. For this purpose, we use d as a model a previously identified immunodominant epitope that represents the target of EBV-specific HLA-A11-restricted CTL responses. By the use of peptides having minimal sequence homology with the wild-type epitope, we d emonstrated that it is possible to selectively expand and reactivate memory CTL precursors without triggering the lytic mechanisms of wild-type specif ic effecters. In fact, stimulation of PBL from EBV-seropositive donors by p olyalanine analogues, sharing only the putative TCR contact residue with th e natural epitope, exclusively induced clonal expansion and reactivation of EBV-specific memory CTL precursors. Interestingly, these polyalanine pepti des failed to trigger the cytotoxic function of CTLs specific for the wild- type viral epitope, This clearly indicates that reactivation of memory CTL precursors and triggering of the cytotoxic function have different requirem ents. The same phenomenon was observed using as stimulators naturally occur ring peptides carrying the appropriate TCR contact residue. These data stro ngly suggest that cross-reactive peptides may play an important role in the expansion and reactivation of CTL clones from the memory T cell pool, and may be involved in long-term maintenance of T cell memory.