Heterogeneity of central memory CD8⁺ T cells revealed by cloning with cytokines. Central memory CD8⁺ T cells (CD45RA–, CCR7⁺) were cloned by single cell deposition in the presence of a cytokine cocktail or irradiated PBMCs and PHA plus exogenous IL-2. Following expansion for 3 weeks, the clones were analyzed for the expression of CD45RA and CCR7. (Ai-v) Five representative clones generated by cytokine stimulation. Cloning efficiency was approximately 20%. (vi) Phenotype of one representative clone generated by stimulation with PHA and feeder cells (cloning efficiency > 50%). (B) Frequency of cytokine and PHA-derived clones of 4 independent experiments.