Distribution of SP CD4⁺, CD8⁺ and DP T cells in various tissues and their phenotype from normal uninfected neonates of 2 age groups. (A) Mean percentages of SP and DP CD4⁺/CD8⁺ T-cell populations of normal uninfected neonates are shown. Each bar represents the mean plus or minus SEM percentage of T-cell subsets in newborn to 3-day-old (group A) versus 12- to 21-day-old (group B) neonates. Note that DP T cells increase in tissues, particularly the intestine with age. (B) Representative flow cytograms of SP CD4⁺, CD8⁺, and DP T cells showing the percentages of naive (CD28⁺CD95⁻), central memory (CD28⁺CD95⁺), and effector memory (CD28⁻CD95⁺) T-cell populations in each polygonal gate as well as other naive/memory markers (CD45RA, CCR7), L-selectin (CD62L), and the early activation marker (CD69) in jejunum LPLs in each quadrant from a normal uninfected 21-day-old neonate examined by 9-color flow cytometry. Note, that in contrast to SP T cells, essentially all DP cells in the jejunum have a “transitional memory” (CD28⁺CD95⁺CCR7⁺) phenotype. Plots were generated by gating first through lymphocytes using forward scatter (FSC) and side scatter (SSC) parameters and then through CD3⁺ T cells. (C) Mean frequencies of naive and memory cell populations are shown for different tissues from neonates of different ages. Each bar represents mean plus or minus SEM. * indicates significant differences between group A and B neonates for the specified T-cell subsets.