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Human CMV (HCMV) infection provides an informative model of how long term human CD8+ T cell memory is maintained in the presence of Ag. To clarify the phenotypic identity of Ag-experienced human CD8+ T cells in vivo, we determined the expression of costimulation and chemokine receptors on Ag-specific CD8+ T cells by quantifying individual virus-specific clones in different cell populations using TCR clonotypic probing. In healthy HCMV carriers, expanded CD8+ clones specific for either HCMV tegument protein pp65 or immediate-early protein IE72 are found in both CD45RO high cells and the subpopulation of CD45RA high cells that lack the costimulatory molecule CD28. In contrast to previous suggested models of CD8+ T cell memory, we found that in healthy virus carriers highly purified CD28− CD45RA high CCR7− cells are not terminally differentiated, because following stimulation in vitro with …