High frequencies of antigen-specific CD4⁺ T cells in memory compartments. (A) Libraries were prepared from naive (T_N), central memory (T_CM), and effector memory (T_EM) CD4⁺ T cells and screened using the recall antigens TT, CMV, and PPD (each at 5 µg/ml). After 3 d, proliferation was measured after 16-h pulse with [³H]thymidine. Dotted lines represent the cut-off value. Numbers indicate of positive cultures over the total cultures tested. Results are representative of six independent experiments performed with six different donors. (B) Frequencies of TT-, CMV-, and PPD-specific T cells in naive and memory compartments in six independent experiments performed with six different donors. Results are presented as precursor frequency per million CD4⁺ T cells. (C and D) Libraries were prepared from naive and memory T cells from four different donors and screened with KLH. Shown are cpm values from a representative experiment performed with a single donor and is representative of four independent experiments performed with four different donors (C) and precursor frequencies for the four different donors (D). Dotted line represents the cut-off value.