(A) Neonatal and adult C57BL/6 mice were infected with rLCMV/INDG i.c. or were left uninfected. On days 7 and 50, NP396-specific CD8⁺ T cells in the spleen (day 7) and blood (day 50) were enumerated by flow cytometry using MHC class I tetramers (H-2D^bNP396). Numbers indicate percentages of NP396-specific CD8⁺ cells within the CD8⁺ T cell compartment (mean ± SD of 3–4 mice). (B) Neonatal (rectangles) or adult (triangles) C57BL/6 mice were infected with rLCMV/INDG i.c. or were left uninfected (circles). Mice were sacrificed 7 and 50 days later, and NP396-specific CTL activity was determined after in vitro restimulation of splenocytes in the presence (filled symbols) or absence (open symbols) of rIL-2. Shown are mean ± SEM of 3–4 mice per group. Cultures with rIL-2 were not significantly different from cultures without rIL-2 (not shown). (C) Mice were challenged with ARM i.v. 50 days after infection as described above. The frequency of NP396-specific CD8⁺ T cells in blood was measured using MHC class I tetramers. Values differing significantly from those of ARM-challenged mice without prior rLCMV/INDG infection (circles) are indicated. (D) rLCMV/INDG carrier mice (50 days old) and naive adult control mice were challenged with ARM i.v. Seven days later mice were sacrificed, and the NP396-specific primary ex vivo CTL activity of splenocytes was measured. Shown are mean ± SEM of 3 mice. Specific lytic activity of naive control splenocytes was < 3% (not shown). Most panels are representative of 2 similar experiments. *P < 0.05; **P < 0.01.