FIG 2.

Replication efficiency of serially passaged viruses in MDCK and non-IFN-responsive MDCK cells. (A) Replication kinetics of wt and ΔNS1 viruses in MDCK-V2 (gray lines) and MDCK cells (black lines). Cell cultures were infected with the initial virus stock (solid lines) or ΔNS1 virus (dotted lines) at an MOI of 0.001. At the indicated times after infection, samples were obtained from the culture supernatant, and the viral titers were determined by plaque assay in MDCK-V2 cells. (B) The efficiency of replication of the various viruses of the M and V passage series were evaluated by low-multiplicity infections in MDCK-V2 and MDCK cells after 10 serial passages. Cultures were infected with either the initial virus stock (P0), one of the M1 to M6 or V1 to V6 viruses, or ΔNS1 virus at a multiplicity of 0.001 PFU/cell. At various times after infection, samples were collected and titers were determined on MDCK-V2 cells. Data are ratios of maximal titers of infections in MDCK-V2 to those in MDCK cells, as shown in panel A (averages and standard deviations for 3 replicates). *, P < 0.05. (C) Absolute titers of a representative kinetics experiment. (D) The efficiency of replication of some viruses of the M and V passage series was evaluated by using low-multiplicity infections in either MDCK-V5 and MDCK cells after 10 serial passages. Cultures were infected with either the initial virus stock (P0) or the M1, M6, V1, V6, or ΔNS1 virus at a multiplicity of 0.001 PFU/cell. At various times after infection, samples were collected and titers were determined on MDCK-V2 cells. Data are ratios of maximal titers of infections in MDCK-V2 to those in MDCK cells, as shown in panel A (averages and standard deviations for 3 replicates). *, P < 0.05.