Potent T cell response to a class I‐binding 13‐mer viral epitope and the influence of HLA micropolymorphism in controlling epitope length

Abstract

The BZLF1 antigen of Epstein‐Barr virus includes three overlapping sequences of different lengths that conform to the binding motif of human leukocyte antigen (HLA) B*3501. These 9‐mer (⁵⁶LPQGQLTAY⁶⁴), 11‐mer (⁵⁴EPLPQGQLTAY⁶⁴), and 13‐mer (⁵²LPEPLPQGQLTAY⁶⁴) peptides all bound well to B*3501; however, the CTL response in individuals expressing this HLA allele was directed strongly and exclusively towards the 11‐mer peptide. In contrast, EBV‐exposed donors expressing HLA B*3503 showed no significant CTL response to these peptides because the single amino acid difference between B*3501 and B*3503 within the F pocket inhibited HLA binding by these peptides. The extraordinarily long 13‐mer peptide was the target for the CTL response in individuals expressing B*3508, which differs from B*3501 at a single position within the D pocket (B*3501, ¹⁵⁶Leucine; B*3508, ¹⁵⁶Arginine). This minor difference was shown to enhance binding of the 13‐mer peptide, presumably through a stabilizing interaction between the negatively charged glutamate at position 3 of the peptide and the positively charged arginine at HLA position 156. The 13‐mer epitope defined in this study represents the longest class I‐binding viral epitope identified to date as a minimal determinant. Furthermore, the potency of the response indicates that peptides of this length do not present a major structural barrier to CTL recognition.