T lineage–specific deletion of p16^INK4a slows thymic involution and reduces an age-related DN1 to DN3 block. (A) Partial rescue of thymic involution with aging in Lck-Cre p16^L/L mice compared with Lck-Cre p16^+/+ mice (n = 5 per group). Representative images are shown of thymus and spleen from mice of indicated ages and genotypes. (B) T cell–specific deletion of p16^INK4a is associated with a partial but significant rescue of the age-associated decline of total thymocyte number (n = 5 per group). Total numbers of thymocytes and splenocytes from mice of indicated ages are shown. (C) The age-related arrest at the DN thymocyte stage was abolished by T lineage–specific deletion of p16^INK4a. (D) Quantification of Lin⁻CD4⁻CD8⁻ (DN) thymocyte fractions at indicated ages as shown in panel C; n = 5 mice per group. (E) T lineage–specific deletion of p16^INK4a attenuated the age-related DN1-DN3 block. Representative flow analysis of Lin⁻CD4⁻CD8⁻ DN1 (CD44⁺CD25⁻) and DN3 (CD44⁻CD25⁺) fractions in mice of indicated ages and genotypes is shown. (F) The ratio of the absolute number of DN1 to DN3 cells (DN1/DN3) as determined in panel E is shown at indicated ages. See also supplemental Figure 2; n = 5 mice per group. Error bars indicate SEM; *P < .05, **P < .01, ***P < .001.