HERPESVIRUS ENTRY MEDIATOR HVEM MEDIATES CELL-CELL SPREAD IN BHK(TK-)CELL CLONES

95-19 and U(S)11cl19.3 are BHK(TK-)-derived cell lines that are highly resistant to postattachment entry of herpes simplex virus type 1 (HSV -1) and HSV-2 but not to later steps in single-step replication. The r esistance properties of these two cell types are not identical. U(S)11 cl19.3 cells are fully susceptible to pseudorabies virus (PRV), as sho wn by single-step growth experiments, whereas 95-19 cells are resistan t to entry of free PRV but not to entry by cell-cell spread. We have t ested the ability of HVEM to overcome the block to infection in both c ell lines following transient and stable transfection. HVEM was able t o mediate entry of free HSV-1 into both cell lines, as shown by an inc rease in the number of beta-galactosidase-expressing cells in cultures transiently transfected with an HVEM expression plasmid and infected with lacZ-expressing HSV-1. In stably transfected 95-19 cells, HVEM en hanced infection by free HSV-1, as shown by an increase in the number of infectious centers obtained following infection. In both cell types , HVEM strongly enhanced entry of HSV-1 and HSV-2 by cell-cell spread, suggesting that HVEM can function as an entry mediator both in entry of free virus and in entry by cell-cell spread.