CD4⁺T cell proliferation. CD4⁺ T cells from prevaccination (time 0), post-6 GPS vaccinations (time 6), and post-12 GPS vaccinations (time 12) from patient 4 were incubated with indicated peptides at 20 µg/mL (“20”) or 50 µg/mL (“50”) for 5 days, and 1 µCi [³H]-thymidine was added to the cultures for 20 hours. The cell proliferation was determined by [³H]-thymidine incorporation. Data are mean ± SD from quadruplicate cultures. Negative controls were also used (incubation with no peptide present and with irrelevant peptides [B2A2 long fragment of BCR-ABL]). After 6 vaccinations, cell proliferation increased sixfold to 331, eightfold to 427, 11-fold to 122A1, and 13-fold to 122A (P = .008) There was no significant dose dependency of the peptides, and the CD4⁺ T-cell response was sustained through the period of vaccination, although the degree varied among the individual peptides.