Analysis of the human cytomegalovirus pp65-directed T-cell response in healthy HLA-A2-positive individuals

Human cytomegalovirus (HCMV) is contained by T-lymphocyte responses focused towards the major tegument protein pp65. To systematically identify T-cell epitopes, we applied the following strategy: 441 overlapping 15mer peptides spanning the entire HCMV pp65 antigen in 1-aa steps were screened in enzyme-linked immunospot (ELispot) assays for interferon gamma (IFN-gamma) secretion by peripheral blood mononuclear cells (PBMCs) from nine healthy HCMV-seropositive subjects expressing human leukocyte antigen (HLA)-A2. This analysis confirmed a number of previously known epitopes and revealed several new ones. A total of 26 epitopes were identified, including 14 HLA-A2, four HLA-B7, -B35, -812 and -B44 restricted class I epitopes, six class II epitopes, and two epitopes of unknown restriction. Three novel HLA-A2 epitopes were confirmed using T2-cells, and one peptide for which only binding data had been published so far was verified. Two novel class II epitopes were confirmed by intracellular cytokine staining. Responses were usually oligoclonal against up to seven HLA-A2 epitopes, albeit with a few dominating epitopes. Clusters of overlapping epitopes (hot-spots) were identified. These and the newly identified T-cell epitopes may be of great value for epitope-based immunotherapeutic approaches, including peptide vaccines.