A defect in the response to secondary influenza virus challenge in WASP^— animals. Eight to 9 weeks after primary intraperitoneal inoculation with influenza (PR8), animals received a secondary intranasal inoculation with strain X31. Splenic T cells were harvested 8 to 9 days later. (A) Percentage of splenic CD8⁺ T cells specific for the influenza NP antigen. Data are pooled from 3 separate experiments (WT: n = 10; WASP^—: n = 9). (B) Cytokine production by stimulated splenic T cells. Spleen cells were cultured in the presence of Brefeldin A and NP peptide and then analyzed by flow cytometry as described in “Materials and methods.” The percentage of CD8⁺ cells positive for TNF-α and IFN-γ are shown (WT: n = 4; WASP^—:n = 3). Error bar equals standard error.