CD4 and infection: WT and Ikzf3–/– mice were intranasally infected with a sublethal dose of IAV strain A/PR/8/34 (H1N1, termed PR8), and nucleoprotein-specific (NP-specific) CD4+ T cells of the draining mediastinal lymph node (mLN) and lungs were assessed at 8 days after infection (Figure 2A).