Attenuation of peripheral T-cell immune aging phenotypes by T cell–specific deletion of p16^INK4a. (A) Representative flow analyses of CD4⁺ (top) and CD8⁺ (bottom) splenocytes showing the effect of T cell–specific p16^INK4a deletion on age-associated changes in memory and naive T-cell fractions. Memory and naive T cells are identified as described in Figure 1C. (B) Quantification of data from panel A showing the effect of T cell–specific p16^INK4a deletion on the ratio of memory versus naive T cells with aging in the CD4⁺ and CD8⁺ compartments; n = 5 mice per group. (C) Representative flow analyses showing the effect of aging and T cell–specific p16^INK4a deletion on homeostatic proliferation as measured by Ki67 expression in nonactivated (CD25⁻) CD4⁺ or CD8⁺ T cells from mice of indicated ages and genotypes. (D) Quantification of Ki67 expression as shown in panel C, n = 4 mice per group. Error bars indicate SEM; *P < .05, **P < .01, ***P < .001.