In addition, clarified supernatants were concentrated, and the pellets were resuspended in sterile water. VLPs were sedimented by ultracentrifugation through cesium chloride (CsCl) (0.4 g/ml) at 116,0009g for 18 h at 4°C as described earlier by others [1] . CsCl was removed by dialysis against PBS, and VLPs were concentrated using an Amicon Ultrafilter.