The HSV-TK–specific CD8⁺T cells are boosted after reinfusion of HyTK-positive donor T cells. Samples of PBMCs were obtained from UPN 1574 before and at various time points after the first and second infusion of HyTK-modified donor T cells. Aliquots of these cells were incubated for 6 hours with medium alone or with all 6 candidate 15-mer peptides that were identified to induce IFN-γ production in samples of PBMCs obtained from this patient. After 6 hours of stimulation, cells were permeabilized, stained with anti–IFN-γ and anti-CD8 mAbs, and examined by flow cytometry. Values indicate the percentage of CD8⁺ T cells producing IFN-γ.