Cell and viruses.

Human embryonic kidney 293T (293T) and Madin-Darby canine kidney (MDCK) cells were grown in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin (100 units/ml)–streptomycin (100 μg/ml)–2 mM l-glutamine (P-S-G) at 37°C in air enriched with 5% CO₂. Influenza A/Puerto Rico/8/1934 (PR8) H1N1 virus (26) was grown in MDCK cells as previously described (27, 28). Viral titers were determined by an immunofocus assay and are presented as fluorescence-forming units (FFU) per milliliter (27, 28).