In adult rhesus macaques with predominant memory T-cell phenotype, CD8 T cells divide more extensively than CD4 T cells in response to in vitro anti-CD3 stimulation. FCM histogram overlays represent CD4 (dotted line) and CD8 (solid line) T cell division profiles detected by dilution of the CFSE label. Splenocytes were cultured for 96 hours with immobilized anti-CD3 mAb alone or in the presence of soluble costimulatory anti-CD28 mAb. Cells were stained with CD4 and CD8β mAbs, and gates were set on CD4⁺ or CD8⁺ populations as indicated. Each of the distinct CFSE peaks represents cells that underwent a discrete number of divisions; height of individual peaks represents the relative numbers of cells that underwent that number of divisions. Of note, cell division mostly depended on CD28 costimulation in the neonate (1 day old) but not in the adult (6 years old) animal. Also, there was a higher number of cell divisions in CD8 cells compared with CD4 cells in the adult animal, irrespective of the presence of CD28 costimulation. These results are representative of 6 neonate and 8 adult (from 5 to 23 years old) animals analyzed in 3 independent experiments.