Activated and "Memory" Phenotype of Circulating T Lymphocytes in Intrauterine Life

We examined the expression of T cell markers in the peripheral blood of five immunologically normal human fetuses at 18-20 weeks of gestational age. The distribution of T cells expressing CD1, CD3, CD4, CD8, CD56, and the α/β and γ/δ receptors for antigen was comparable to that of newborns and normal adults, except for the absence of γ/δ cells expressing the δTCS-1 epitope. The Vβ repertoire, as evaluated by two-color flow cytometry using mAbs to specific Vβ families, was also comparable to that of adult samples. A significant fraction (8.9 to 16.4%) of fetal CD3+ T cells expressed the α chain of IL-2R (CD25) in the absence of HLA-DR; this suggests that antigenic stimuli trigger, during intrauterine life, an unusual pathway of T cell activation. Consistent with this, 7 to 27% of fetal T cells were found to express the CD45R0 marker of "memory" cells.