Mice, Viruses, IL-15 Blocking, and NK Cell Depletion C57BL/6 mice were purchased from Charles River (Wilmington, MA) through the NCI program. Influenza A/HK-x31 (x31, H3N2) was generously donated by Dr. S. Mark Tompkins (University of Georgia, Athens, GA). Animals were infected intranasally (i.n.) with 10 3 PFU HKx31 diluted in 50 mL sterile PBS. IL-15 was blocked using 25 mg anti-IL-15 mAb (clone AIO3) (eBioscience, San Diego, CA) administered daily via intraperitoneal injection (i.p.) in 200 mL sterile PBS. IL-15 depletion was confirmed by reductions in frequencies of both NK and CD44 hi CD8 + T cell in antibody-treated animals compared to untreated animals 7 days after the initiation of mAb treatment. NK1.1+ cells were depleted via intravenous (i.v.) injections of 200 uL PBS containing 200 mg anti-NK1.1/mouse (clone PK136) every other day (UCSF monoclonal antibody core facility, San Francisco, CA), and depletion of NK cells was verified using an anti-NKp46 mAb (Clone 21A9.4, eBioscience) throughout the experiment and 2 days after the last injection of the NK1.1 depleting mAb.