Fetal progenitors give rise to mature CD8⁺T cells that adopt different fates after infection. (A) Schematic of experimental design: CD8^–CD4^– progenitors from congenically marked gBT-I adult (CD45.2, Thy1.1) and neonatal (CD45.2, Thy1.2) mice were transferred by intrathymic injection into sublethally irradiated congenic recipient mice (CD45.1, Thy1.2) separately. Four weeks later, progenitor-derived splenic CD8⁺ T cells were sorted and adoptively co-transferred (1:1 ratio) into new congenic recipient mice (CD45.1, Thy1.2). These recipients were infected with 5 × 10³ LM-gB and serially bled to examine CD8⁺ T-cell responses. (B-C) Naïve phenotypes of progenitor-derived CD8⁺ T cells before adoptive cotransfer. (B) Representative plots displaying CD44 and CD122 expression by gBT-I progenitor-derived CD8⁺ T cells 4 weeks post–intrathymic transfer in blood. (C) Statistical analysis of the percentages of MP cells from gBT-I adult and fetal progenitor-derived cells. (D) Relative numbers of gBT-I adult (dashed) and fetal (solid) progenitor-derived donor CD8⁺ T cells postinfection and re-challenge (5 × 10⁴ WT LM-gB); arrows indicate days of infections. (E) Percentages of gBT-I adult and fetal progenitor-derived donor CD8⁺ T cells that are SLECs and MPECs. Data are representative of 2 experiments (n = 4-11 mice/group) (****P < .0001).