Detection of type I IFN by reporter assays

To detect type I IFN in human coculture medium, 8 × 10⁴ HEK-Blue IFN cells (InvivoGen) in 100 μl were combined with 100 μl of coculture medium and incubated for 24 hours. Human embryonic kidney cell culture medium (100 μl) was combined with 100 μl of Quanti-Blue solution, and the production of type I IFN was quantified at optical density at 600 nm. To measure murine type I IFN production, 1 × 10⁵ L929-luc IFN-stimulated response element cells were stimulated for 4 hours with 100 μl of cell culture medium. The medium was removed, the cells were lysed in Bright-Glo luciferase substrate, and luciferase activity was determined with a Victor Light luminescence counter (PerkinElmer).