ST-LOUIS ENCEPHALITIS-VIRUS ESTABLISHES A PRODUCTIVE, CYTOPATHIC AND PERSISTENT INFECTION OF SF9 CELLS

The Sf9 cell line, commonly used for gene expression by recombinant ba culovirus, has been productively infected by St Louis encephalitis (SL E) virus. a flavivirus. SLE viral infection produced a c.p.e. in the S f9 cells characterized by giant cells and the presence of 10-fold fewe r cells in the infected cultures after the first week of infection com pared with uninoculated control cultures. Infected Sf9 cells expressed SLE viral antigens, and intracellular virus particles were observed b y electron microscopy. Titres of cell-associated SLE virus rose slight ly over an 8 week period, whereas titres of cell-free virus remained s table, suggesting that SLE virus establishes a productive and persiste nt infection of Sf9 cells. The SLE virus produced by the Sf9 cells cou ld be neutralized by SLE virus-immune mouse ascitic fluid, and no evid ence of escape mutants was detected. Sf9 cells persistently infected w ith SLE virus could be superinfected with a recombinant baculovirus an d expressed recombinant antigen. The successful infection of Sf9 cells by SLE virus represents the first report of production of c.p.e. by S LE virus in insect cells under routine cell culture conditions and of the infection of Sf9 cells by a human pathogen.