Materials and Methods Strains, cultivation and purification of virions. African green monkey kidney (Vero) cells (CCL-81; ATCC, Mannasas, VA) and human foreskin fibroblasts (HFT) (S1) cells were grown in alpha-minimum essential medium supplemented with 10% fetal calf serum (Invitrogen, Carlsbad, CA) and passaged as described in Desai et al. (S2). Virus stocks were prepared in Vero cells as described (S2). HFT cells were infected with KOS virus at an MOI of 10 PFU/cell. 48