Figure 1. Growth characterization of recombinant M-F region exchange mutants.

(A) Schematic drawing of the highly variable region exchanged in recombinant viruses. The recombinant 5804PeH genome is drawn to scale. Black and white boxes represent open reading frames and UTRs, respectively. The genes are indicated by their respective abbreviation. (B) Cell-associated growth curves of the parental and two recombinant viruses in VerodogSLAMtag cells. Cells were infected with a multiplicity of infection (MOI) of 0.01, and samples were harvested daily for five days. Titers are expressed as 50% tissue culture infectious doses (TCID₅₀). Error bars indicate the standard deviation. (C) Syncytia formation in VerodogSLAMtag cells. Cells were infected with a MOI of 0.01 and overlaid with 0.5% methylcellulose. Photographs show a representative syncytia of one replicate taken 48 h post-infection using fluorescence excitation (top panels) and phase contrast (bottom panels) at 100× magnification. (D) Survival curve of ferrets infected with the different mutants. Animals were infected with 10⁵ TCID₅₀ intranasally. Death of an infected animal is indicated by a step down on the graph.