[en] Viruses have co-evolved with their host to ensure efficient replication and transmission without inducing excessive pathogenicity that would indirectly impair their persistence. This is exemplified by the bovine leukemia virus (BLV) system in which lymphoproliferative disorders develop in ruminants after latency periods of several years. In principle, the equilibrium reached between the virus and its host could be disrupted by emergence of more pathogenic strains. Intriguingly but fortunately, such a hyperpathogenic BLV strain was never observed in the field nor designed in vitro. In this study, we aimed at understanding the role of envelope N-linked glycosylation with the hypothesis that this posttranslational modification could either favor BLV infection by allowing viral entry or allow immune escape by using glycans as a shield. Using reverse genetics of an infectious molecular provirus, we have identified a N-linked envelope glycosylation site (N230) that limits viral replication and pathogenicity. Indeed, mutation N230E unexpectedly leads to enhanced fusogenicity and protein stability. Occurrence of this mutation may thus represent a potential threat associated with emergence of hyperpathogenic BLV strains and possibly of new variants of the related primate T-lymphotropic viruses.
Infection by retroviruses requires the interaction of the viral envelope protein (SU) with a membrane-associated receptor allowing fusion and release of the viral genomic RNA into the cell. Here, we have shown that N-linked glycosylation of the Bovine Leukemia Virus (BLV) SU protein is, as expected, essential for cell infection in vitro. Consistently, mutation of all glycosylation sites of a BLV provirus destroys infectivity in vivo. However, single mutations do not significantly modify replication in vivo. Instead, a particular mutation at SU codon 230 increases replication and accelerates pathogenesis. This unexpected observation has important consequences in terms of disease control and managing. Occurrence of this codon substitution may indeed represent a potential threat associated with emergence of hyperpathogenic BLV strains.