Expression of activation marker (CD25) on CD8 T cells isolated from spleen of normal (□) and immunized (▪) DBA/2 mice. Animals were immunized three times with irradiated ESb 289 lymphoma cells. One week after the last challenge, spleen lymphocytes were isolated from immunized and normal (nonprimed) animals and double-stained for flow cytometry using antimouse MoAbs: FITC-conjugated anti-CD25 and PE-conjugated anti-CD8 (d0). Another part of lymphocytes (responders) was incubated for 6 days with the following stimulator cells (responder:stimulator cell ratio = 10:1): (1) SER⁺ liver macrophages (SER-pos. Mø), isolated from tumor-bearing DBA-2 mice 12 days after ADI using rosetting with unopsonized sheep erythrocytes; (2) 200 Gy irradiated ESb 289 cells (ESb); and (3) liver sinusoidal cells (SC) isolated from normal (non–tumor-bearing) animals. Lymphocytes were then stained using the same MoAbs as at day 0 followed by FACS analysis. Results were expressed as the percentage of CD25⁺ cells from the CD8 T-cell population.