Semliki Forest virus (SFV), an enveloped alphavirus, infects cells by endocytosis followed by low pH-triggered fusion of the virus and endocytic vesicle membranes. Progeny virus is released by budding from the cell plasma membrane. In vitro SFV fusion with artificial liposomes is triggered by low pH and is dependent on the presence of cholesterol and sphingolipid in the target liposome membrane. In tissue culture, both SFV fusion and virus exit are strongly cholesterol-dependent when assayed in cholesterol-depleted insect cells. We here describe the preparation of insect cells that while not containing detectable amounts of cholesterol, have adapted to sterol-depleted conditions, resulting in a more permissive phenotype for SFV infection. Although still less efficient at supporting SFV infection than control cholesterol-containing cells, the adapted cells show a 45-fold increase in primary infection by SFV, increased release of progeny virus, and enhanced virus growth kinetics compared to nonadapted cholesterol-depleted cells. The adapted cells are also about 35-fold more permissive for low pH-induced fusion of SFV with the plasma membrane, suggesting that adaptation correlates with a change in the cell membrane.
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