Influenza virus strains; swine influenza virus (SwIV; Sw/OH/ 07, H1N1), avian influenza virus (AvIV; Ch/NY/95, H7N2) and human influenza virus (HuIV; Hu/OH/06, H1N1) were propagated in 10-day-old embryonated chicken eggs (AvIV was kindly provided by Dr. S.M. Goyal, Department of Veterinary Population Medicine, University of Minnesota, Saint Paul, MN). Progenitor epithelial cells were infected with SwIV, AvIV and HuIV at a multiplicity of infection (MOI) of 1. After adsorption for 1 h, the cells were washed and fresh medium was added to the cells. At intervals after infection, released viruses in culture supernatants were titrated in MDCK cells [58] .