Clonally diverse CTL response to a dominant viral epitope recognizes potential epitope variants

RNA viruses undergo rapid sequence variation as the result of error-prone R NA replication mechanisms. When viable mutations arise in RNA regions encod ing B or T cell epitopes, mutant viruses that can evade immune detection ma y be selected. In the carefully studied CTL response to the Gag p11C(C-M) e pitope in SIVmac-infected Mamu-A*01(+) rhesus monkeys, it has been shown th at CTL recognition of that epitope can occur even in the face of accruing m utations. To explore the underlying mechanism for this breadth of recogniti on, we have constructed Mamu-A*01 tetramers which discriminate T cells spec ific for epitope variants. Using these reagents we have defined discrete su bsets of p11C(C-M)-specific T cells that cross-react with cells presenting variant peptides. We have found that individual Mamu-A*01(+) monkeys differ functionally in their ability to recognize epitope variants despite consis tently strong recognition of the p11C(C-M) epitope. This functional differe nce is accounted for by the relative number of variant-specific T cells and by differences in the functionally relevant TCR repertoire of the infected monkeys. We have also found that monkeys immunized with DNA vaccine constr ucts encoding only the wild-type epitope sequence develop p11C(C-M)-specifi c CTL cross-reactive with variant peptides. Thus, cross-reactive CTL do not merely arise secondary to the emergence and immune presentation of viral C TL escape mutants but rather arise de novo following priming with a dominan t epitope peptide sequence. Taken together, our results support the concept that the CTL response to a dominant viral epitope, although highly focused , can be clonally diverse and recognize potential epitope variants.