IL-4 exposure before but not during LCMV infection regulates the CD8⁺ T cell response. (A–D) WT BALB/cByJ (CD45.1) CD8⁺ T cells were transferred into WT BALB/cByJ (CD45.2) or IL-4 KO BALB/cJ recipients (CD45.2), which were then infected with LCMV and analyzed 7 d after infection. The numbers of total splenic donor CD8⁺ T cells (A) and donor NP_118–126/L^d tetramer–binding CD8⁺ T cells (B) were calculated for each recipient group. (C and D) Expression levels of Eomes (C) and T-bet (D) on antigen-specific CD8⁺ T cells in the indicated recipient animals. MFI, mean fluorescence intensity. (E and F) Naive (CD44^lo CXCR3^lo) and memory phenotype (CD44^hi CXCR3^hi) CD8⁺ T cells were sorted from WT BALB/cByJ (CD45.1) and IL-4Rα KO BALB/cByJ (CD45.2) mice, and equal numbers of phenotype-matched WT and IL-4Rα KO cells were cotransferred into WT BALB/cByJ recipients (CD45.1/2). 1 d after adoptive transfer, recipients were infected with LCMV. Total numbers of splenic NP_118–126/L^d tetramer–binding CD8⁺ T cells (E) or the frequency of donor NP_118–126/L^d tetramer–positive (Tet⁺) cells (F) within the total tetramer-positive population was calculated for each recipient. Error bars indicate mean ± SD. (A–D) P > 0.05 (not significant [ns]) for all comparisons, using unpaired Student’s t test. (E and F) P-values are represented as *, P < 0.05 by paired Student’s t test. (A–D) Data (n = 5 for WT, n = 5 for IL-4 KO recipients) are representative of two experiments. (E and F) Data are combined from two separate experiments (n = 6 for naive phenotype recipients, n = 5 for memory phenotype recipients total).