FIG. 5.
Passaging of ncp BVDV-infected cells in the presence of rbo IFN-α is unable to eliminate the virus. MDBK cells in 25-cm2 tissue culture flasks were infected with the SuwaNcp BVDV strain as described in Materials and Methods. At 24 h postinfection, the virus titer in the supernatant was analyzed (P1′), and the medium was replaced with fresh medium without (▴) or with 1 (⧫), 10 (□), or 100 (○) ng/ml rbo IFN-αI.1. Thereafter, cells were passaged twice a week for 10 times as indicated (P1 to P10) with or without IFN as described for the first passage. For the last passage (P11), all samples were incubated in medium without IFN. After every passage, the BVDV titer in the supernatant (A) and the amount of intracellular viral RNA (B) were analyzed as described for Fig. 4 (representative experiments are shown). In order to exclude the selection of IFN-resistant cells, parallel cultures of MDBK cells at passages 1, 7, and 10 were infected with VSV at an MOI of 0.01, and the amount of VSV in the supernatant was titrated 24 h postinfection (C). TCID50, 50% tissue culture infective dose.