Visna/maedi virus Env protein expressed by a vaccinia virus recombinant induces cell-to-cell fusion in cells of different origins in the apparent absence of Env cleavage: role of glycosylation and of proteoglycans

The in vivo productive infection by the ovine Visna/maedi lentivirus (VISNA) is restricted to cells of the monocyte/macrophage lineage. The basis for this restriction is not understood. Although the VISNA envelope (Env) glycoprotein is the main target for virus neutralization, studies on the role of this protein in virus infection are limited. A vaccinia virus recombinant (VV-env-MV) containing the entire VISNA env sequence was generated and shown to produce in infected cells a protein of about 165 kDa (referred to as gp150). During VV-env-MV infection, expression of env caused extensive cell-to-cell fusion in cell lines of different origins. Pulse-chase and Western blot analyses revealed that gp150 is not cleaved in VV-env-MV infected cells. The glycoprotein gp150 formed oligomers held by disulfide bonding. Cell-to-cell fusion was prevented in the presence of the inhibitor of glycosilation, tunicamycin, but it was markedly enhanced by an inhibitor of proteoglycan synthesis, β-D-xyloside. These findings showed that the receptor for VISNA Env is widely distributed within cells, that fusion-from-within of cells can occur in the apparent absence of proteolytic cleavage of gp150, and that fusion require a glycosylated Env but not the addition of proteoglycan chains at the cell surface. This recombinant virus could have utility as a potential vaccine against VISNA.