Changes occurring to CD8⁺ T cells with age and differentiation. (A) Representative example of CD28/CD27 staining using purified PBMCs gated on CD8⁺ T cells. (B) Data showing the correlation between highly differentiated CD28⁻CD27⁻ T cells, expressed as a percentage of CD8⁺ T cells, and age. (C) Graph showing the proliferation of CD28/CD27 subsets assessed by ³H thymidine incorporation on day 3 after stimulation with anti-CD3 and irradiated APCs. (D) Data showing the loss of telomere length with age within CD8⁺ T cells. (E) Graph showing telomerase activity in CD8⁺ CD28/CD27 subsets. Horizontal lines depict mean values. All P values were calculated using the Student t test.