CD137 enrichment is effective with memory and naive T-cell populations. (A) PBMCs from a CMV⁺/HLA-B7⁺ donor were used to compare the CD137 enrichment method versus the IFNγ-secretion assay (all plots, except the ex vivo staining, show groups after an additional 8 days of culture): (i) CMVpp65_(417-426)-pMHC multimer⁺ cells ex vivo; (ii) stimulation with peptide, no enrichment; (iii) stimulation with peptide and enrichment with IFNγ-secretion assay; (iv) stimulation with peptide and CD137 enrichment; (v) IFN⁻ group (flowthrough/depleted); (vi) CD137⁻ cells (flowthrough/depleted). (B) PBMCs from a healthy donor were stimulated with Melan-A_(26-35L) peptide directly ex vivo for 24 hours and then either cultured for an additional week in medium containing IL-7 and IL-15 (left), or enriched for CD137⁺ cells and cultured for one week (right). The enriched sample was then restimulated a second time (day 8 of culture) and assessed for (C) up-regulation of CD137 at 24 hours after peptide-specific stimulation (left, black line) or stimulation with irrelevant control peptide (filled). In parallel, the pattern of cytokine responses was determined by intracellular cytokine staining (right, 5-hour stimulation period, % responsive CD8⁺ cells noted).