FIG. 2.

Comparison of the multistep growth kinetics of the F cleavage site mutations in the presence or absence of trypsin. Monolayer cultures of Vero cells were infected at an input MOI of 0.01 PFU per cell with the indicated HMPVΔSH/G virus. Three conditions were evaluated: the multicycle growth and plaque assay were both performed in the presence of trypsin (A); multicycle growth was in the absence of trypsin, and the plaque assay was performed with trypsin (B); and multicycle growth and the plaque assay were both performed in the absence of trypsin (C). Supernatant aliquots (0.5 ml out of a total medium volume of 2 ml per well) were taken on the indicated days postinfection and replaced by an equivalent volume of fresh medium with or without trypsin as appropriate. The samples were flash frozen and analyzed later in parallel by plaque assay. Each time point was represented by two wells, and each virus titration was done in duplicate. Means are shown. The standard errors were calculated but the bars are not shown, because the errors were very small and the bars were obscured by the symbols. w/o, without.