FIG. 4.

Absolute IL-2 levels in the supernatants of virus-infected Vero and MDCK cells as determined by ELISA. (a) Vero cells were infected with a genetically stable stop-start or an NS1-IgK signal peptide delNS1-IL-2 virus or with delNS1 virus as a control (MOI, ∼0.01), as indicated on the left, and incubated at 37°C for 12 to 16 h in the presence of 1 μg/ml trypsin to permit limited viral replication. Fetal calf serum (FCS) was added to a final concentration of 10% (vol/vol), and incubation was continued for another 24 h. IL-2 levels are given on a logarithmic scale as the means ± standard deviations from three independent experiments. (b) Vero cells or MDCK cells were infected with the indicated virus (MOI, ∼1) and incubated at 37°C for 2 h, followed by the addition of 10% (vol/vol) FCS to prevent viral replication. Supernatants were collected 24 h later and analyzed for IL-2.