FIG. 2.

Cell-free versus cell-to-cell transmission of HIV-1 is modulated by BST-2. In the top half of the figure, the unrestricted transmission of virions by cell-free virions is shown. This situation applies to viruses expressing Vpu and/or viruses replicating in BST-2–negative cells. In the absence of Vpu, virions produced from BST-2–expressing cells remain tethered to the surface of the donor cell (bottom half of the figure). These particles are fusion competent and fully infectious (Klimkait and others 1990) and can fuse with an adjacent target cell to establish a cell-to-cell virus transmission.