Influenza A (pHlNl) WT and mCherry viruses have been previously described . For infections, virus stocks were diluted in phosphate buffered saline (PBS) supplemented with 0.3% bovine albumin (BA) and 1 % PS (PBS/BA/PS). After viral infections, cells were maintained in DMEM supplemented with 0.3% BA, 1% PSG, and 1 |jtg/ml tosyl- sulfonyl phenylalanyl chloromethyl ketone (TPCK)-treated trypsin (Sigma) (Martinez-Sobrido and Garcia-Sastre, 2010). Virus titers were determined by standard plaque assay (plaque forming units (PFU)/ml) in MDCK cells (Nogales et al„ 2014b).