Comparison of proliferating (BrdU⁺) CD4⁺ T cells and CD8⁺ T cells from various tissues of uninfected adults and neonatal rhesus macaques. (A) CD4⁺ T cells. (B) CD8⁺ T cells. Black bars represent means of 13 normal neonatal macaques (3-21 days old), and white bars represents 10 normal adult macaques (> 4 years old). Note that neonates have far more proliferating T cells than adults, especially in the intestine and spleen. Bars represent mean percentages of BrdU⁺ lymphocytes gated through CD4 and/or CD8 ± SEM. Significant differences between adults and neonates are indicated by asterisks (**P < .01, ***P < .001) using a Mann Whitney U test.