Calu-3 cells as novel efficient cell culture system for Borna disease virus 1 (BoDV-1)

Borna disease virus 1 (BoDV-1) is the causative agent of zoonotic infections resulting in severe encephalitis with a high case-fatality rate in humans. To search for an efficient in-vitro cell culture system for the generation of BoDV-1 virus stocks, we compared different human cell lines (A549, Caco-2, Calu-3, oligodendroglioma cells, SK-N-AS) and Vero cells with respect to BoDV-1 infection kinetics. While all cell lines included in the study supported the release of BoDV-1 RNA and its intracellular accumulation at the end of the experiment, viral RNA was detected in the supernatant of Calu-3 cells as early as six days post infection. In addition, two weeks post infection RNA loads remained highest in the supernatant of this cell line, which also yielded highest BoDV-1 RNA copies per cell. Using the ratio of BoDV-1 RNA in the supernatant and the cell pellet as a surrogate marker, Calu-3 cells released BoDV-1 RNA most efficiently. Notably, favipiravir (T-705), a drug with a described cell-type dependent inhibitory activity against BoDV-1 in vitro, which has been used as an off-label therapy in some cases of human BoDV-1 infection, did not reduce BoDV-1 RNA shedding into the supernatant of permanently infected Calu-3 cells and had only a limited effect in a de-novo infection model. In conclusion, our results underline the broad viral tropism of BoDV-1 in vitro and first describe a high release efficiency for BoDV-1 RNA in Calu-3 cells.