To evaluate IDO enzyme activity during influenza infection B6 mice were infected with sub-lethal doses (50 PFU/mouse, i/n) of influenza A/PR/8/34 (PR8) virus and levels of the tryptophan catabolite kynurenine (Kyn) released by cells expressing functional IDO enzyme activity were assessed in lungs and msLNs by performing HPLC analysis on tissue homogenates. Here, IDO1 is linked to influenza.