Hyper-replicative bovine leukemia virus by mutation of an envelope N-linked glycosylation site

Reverse genetics can be used in the bovine leukemia virus
(BLV) system to characterize mechanisms of viral persistence
and pathogenesis. The question addressed here pertains
to the role of glycans bound to the BLV envelope
glycoprotein (SU). A commonly accepted hypothesis is that
addition of carbohydrates to the SU protein potentially
creates a structure called « glycan shield » that confers
resistance to the virus against the host immune response.
On the other hand, glycosylation can also modulate attachment
of the virus to the cell membrane. To unravel the role
of SU glycosylation, three complementary strategies were
developed: pharmacological inhibition of different glycosylation
pathways, interference with glycan attachment and
site-directed mutagenesis of N-glycosylation sites in an
infectious BLV provirus. The different approaches show
that glycosylation is required for cell fusion, as expected.
Simultaneous mutation of all 8 potential N-glycosylation
sites destroys infectivity. Surprisingly, mutation of the
asparagine residue at position 230 creates a virus having an
increased capacity to form syncytia in vitro. Compared to
wild-type BLV, mutant N230 also replicates at accelerated
rates in vivo. Collectively, this data thus illustrates an example
of a N-glycosylation site that restricts viral replication,
contrasting with the hypothesis supported by glycan shield
model.