Immune responses of lymphocyte subsets to K562 and K562DR1 stimulation. Fresh PBMC or purified CD4⁺ or CD8⁺ cells (A) or cells from a NK-cell line (B) were cocultured with irradiated K562 or K562DR1 cells for 5 days. Eighteen hours after ³H-pulsing, the proliferative response was determined by ³H-incorporation. Responder cells were plated in triplicate at 5 × 10⁴ per well with 5 × 10³/well stimulator cells/well. Donor-1 was HLA-DR1⁺ (HLA-DR1/1, DQ5/5), donor-2 HLA-DR2⁺ (HLA-DR2/2, DQ6/6).