Differential Incorporation of Cholesterol by Sindbis Virus Grown in Mammalian or Insect Cells

Cholesterol has been shown to be essential for the fusion of alphaviruses with artificial membranes (liposomes). Cholesterol has also been implicated as playing an essential and critical role in the processes of entry and egress of alphaviruses in living cells. Paradoxically, insects, the alternate host for alphaviruses, are cholesterol auxotrophs and contain very low levels of this sterol. To further evaluate the role of cholesterol in the life cycle of alphaviruses, the cholesterol levels of the alphavirus Sindbis produced from three different mosquito (Aedes albopictus) cell lines; one other insect cell line, Sf21 from Spodoptera frugiperda; and BHK (mammalian) cells were measured. Sindbis virus was grown in insect cells under normal culture conditions and in cells depleted of cholesterol by growth in serum delipidated by using Cab-O-sil, medium treated with methyl-beta-cyclodextrin, or serum-free medium. The levels of cholesterol incorporated into the membranes of the cells and into the virus produced from these cells were determined. Virus produced from these treated and untreated cells was compared to virus grown in BHK cells under standard conditions. The ability of insect cells to produce Sindbis virus after delipidation was found to be highly cell specific and not dependent on the level of cholesterol in the cell membrane. A very low level of cholesterol was required for the generation of wild-type levels of infectious Sindbis virus from delipidated cells. The data show that one role of the virus membrane is structural, providing the stability required for infectivity that may not be provided by the delipidated membranes in some cells. These data show that the amount of cholesterol in the host cell membrane in and of itself has no effect on the process of virus assembly or on the ability of virus to infect cells. Rather, these data suggest that the cholesterol dependence reported for infectivity and assembly of Sindbis virus is a reflection of differences in the insect cell lines used and the methods of delipidation.