Analysis of antigen-specific T cells in naive and memory T-cell subsets. Tetramer complex recognizing CMV pp65 was used to assess the relative frequency CMV-specific T cells in various T-cell subsets. (A) Unsorted PBMCs were first assayed for CMV-specific T-cell frequency. (B) T cells were then sorted on the basis of CD62L and CD45RA expression and assessed for the frequency of tetramer-positive CD8⁺ T cells shortly after cell sorting (< 7 days) or (C) after in vitro expansion for 21 days.