Target structures of the CD8(+)-T-cell response to human cytomegalovirus: the 72-kilodalton major immediate-early protein revisited

Cell-mediated immunity plays an essential role in the control of infection with the human cytomegalovirus (HCMV). However, only a few CD8(+)-T-cell ep itopes are known, with the majority being contained in the pp65 phosphoprot ein, which is believed to dominate the CD8(+)-T-cell response to HCMV. Here , we have readdressed the issue of CD8(+) T cells specific for the 72-kDa m ajor immediate-early protein (IE-1), which is nonstructural but is found ve ry early and throughout the replicative cycle. Using a novel flow-cytometri c assay, we were able to identify CD8(+)-T-cell epitopes (by IE-1 peptide-s pecific induction of cytokine synthesis) and simultaneously measure the fre quency of cells directed against them. For this purpose, 81 pentadecamer pe ptides covering the complete 491-amino-acid sequence of IE-1 were tested on peripheral blood mononuclear cells of anti-HCMV immunoglobulin G-seroposit ive donors. At least 10 new epitopes were identified, and the fine specific ity and presenting HLA molecule of the first of them was determined. The fr equencies of CD8(+) T cells directed against IE-1 were similar to those dir ected against pp65 in donors tested with known pp65-derived peptides. Impor tantly, additional testing of a corresponding set of peptides covering the complete sequence of pp65 on 10 of these donors identified individuals whos e CD8(+) T cells recognized IE-1 but not pp65 and vice versa, clearly illus trating that either protein may be a major target. In summary, our results suggest that IE-1 is far more important as a CD8(+)-T-cell target than curr ent opinion suggests.