The differential regulation of HSV entry mechanisms

Glycoprotein D is a critical component of the machinery that HSV requires to enter from an extracellular state or spread from cell-to-cell. Several lines of experimentation were conducted on cells that lacked functional gD receptors, or on viruses that carried mutations in gD, to determine whether and to what extent these distinct entry processes can be uncoupled with respect to gD function. The experiments herein clarify that while gD is essential for both entry processes, the role gD plays in cell-cell spread differs from its role in entry of free virus. gD triggers fusion upon association with host receptors in both processes but receptors that enable cell-cell spread do not necessarily enable entry of free virus. Moreover, regions of the protein that are required for extracellular virus to enter susceptible cells are not required for cell-cell spread. Similarly, point mutations in gD that enhance cell-cell spread on resistant cells do not enhance entry of free virus. Finally, viruses that carry mutations in factors other than gD, enhance cell-cell spread without affecting entry of free virus yet still require gD even when a functional gD-receptor is absent. The critical roles gD effects in the entry processes are more easily compensated while the virus remains in the infected cell than when it becomes extracellular since entry from the extracellular space is much more sensitive to modifications in viral or cellular factors than is cell-cell spread.