All gamma-herpesviruses encode at least one homolog of the cellular enzyme formyl-glycineamide-phosphoribosyl-amidotransferase. Murid herpesvirus-4 (MuHV-4) does so via its envelope glycoproteins gp70 and gH/gL. We show that the vector can infect mouse, rat and human glioma cell lines and primary cultures obtained from human glioblastoma in vitro. We analysed here how gH-gL-directed neutralization works. The results show, however, that none of these soluble forms are able to block viral infectivity. In this way we could establish fibroblast clones that maintained latent MuHV-4 episomes without detectable lytic replication. Using a BoHV-4 bacterial artificial chromosome clone, we produced a strain deleted for ORF73 and a revertant strain.
This implies that GAGs normally displace gp150 to allow GAG-independent cell binding. Thus, they regulate viral tropism both positively and negatively depending upon the presence or absence of their receptor. Since neither gp70 nor gp150 matches the expected profile of a first contact glycoprotein, our understanding of MuHV-4 GAG interactions must be incomplete. Here we relate the seemingly disconnected gp70 and gp150 GAG interactions by showing that the MuHV-4 gH/gL also binds to GAGs. gH/gL-blocking and gp70-blocking antibodies individually had little effect on cell binding, but together were strongly inhibitory. Thus, there was redundancy in GAG binding between gp70 and gH/gL. Gp150-deficient MuHV-4 largely resisted blocks to gp70 and gH/gL binding, consistent with its GAG independence.
The failure of wild-type MuHV-4 to do the same argues that gp150 is normally engaged only down-stream of gp70 or gH/gL. MuHV-4 GAG dependence is consequently two-fold: gp70 or gH/gL binding provides virions with a vital first foothold, and gp150 is then engaged to reveal GAG-independent binding.