Both TN and MP CD8⁺T cells from neonatal gBT-I mice preferentially become short-lived effectors after infection. (A) Schematic of experimental design: TN and MP cells were sorted from congenically marked gBT-I adult (CD45.2, Thy1.2) and neonatal (CD45.2, Thy1.1) mice and adoptively cotransferred (1:1 ratio) into adult recipient mice (CD45.1, Thy1.2). These recipients were infected with 5 × 10³ colony-forming unit (CFU) LM-gB and serially bled to examine primary CD8⁺ T-cell responses. Recipients were also challenged at 28 dpi with 5 × 10⁴ WT LM-gB to assess the memory recall response. Arrows indicate days of infections. Relative numbers and ratios of TN (B) and MP (C) donor cells from different aged mice at various times after infection. Percentages of MP (D) and TN (E) donor cells from different aged mice that display a short-lived effector cell (SLEC) or memory precursor effector cell (MPEC) phenotype at the peak of the primary response (7 dpi). Data are representative of 2 experiments (n = 9-12 mice/group). Significance was assessed by Student t test (****P < .0001). DPI, days postinfection.