In vitro studies on Borna virus. I. The use of cell cultures for the demonstration, titration and production of Borna virus.

Borna virus produces non-lytic infections in a wide spectrum of primary cell cultures and cell lines. The sensitivity and virus yields vary with the different cell systems. Accurate virus titrations can be performed in the RK 13 cell line by counting immunoflourescent microfoci between the 5th and 10th day after infection. Since the virus is not released from the cells and does not spread via the culture medium, the use of a semisolid overlay in unnecessary in virus titrations. The cell line most productive for Borna virus is the CV 1 line. The conditions for optimum virus production include a prolonged cultivation period of at least two weeks with regular changes of medium, and an incubation temperature of 35 degrees C. Harvest of the virus requires thorough disruption of the infected cells, preferably by ultrasonication, since Borna virus seems to be closely associated with cellular structures.