A conventional type I IFN bioassay was performed by incubating 96-well cell cultures for 20 hours with twofold dilutions of type I IFN as described above. The cells were infected with propagation-competent VSV (m.o.i. of 1 pfu/cell) and incubated until a cytopathic effect (CPE) was evident in mock-treated control cells. The cells were washed twice with PBS and stained for 1 h with 0.1% crystal violet in 10% formalin. The plates were washed with tap water to remove excess crystal violet and dried. The dye was dissolved by adding 100 ml of 70% ethanol to each well. The absorbance of crystal violet at 595 nm was determined with a microplate reader. The IFN titer was calculated as the reciprocal of the last IFN dilution causing 50% inhibition of virus-induced CPE (50 % reduction of absorbance at 595 nm compared to mock-infected cells) and was expressed as IFN units per volume. Alternatively, antiviral activity was calculated according to the following formula: Antiviral activity (%) = (OD595 +IFN -OD595 2IFN )6100/(OD595 Mock -OD595 2IFN ). OD595 +IFN and OD595 2IFN represent the absorbance of IFN-treated and non-treated cells following infection with VSV and staining with crystal violet, respectively. OD595 Mock denotes the absorbance of non-infected cells.