Amino acid mutations in the env gp90 protein that modify N-linked glycosylation of the Chinese EIAV vaccine strain enhance resistance to neutralizing antibodies.

The Chinese EIAV vaccine is an attenuated live virus vaccine obtained by serial passage of a virulent horse isolate (EIAVL) in donkeys (EIAVD) and, subsequently, in donkey cells in vitro. In this study, we compare the env gene of the original horse virulent virus (EIAVL) with attenuated strains serially passaged in donkey MDM (EIAVDLV) and donkey dermal cells (EIAVFDDV). Genetic comparisons among parental and attenuated strains found that vaccine strains contained amino acid substitutions/deletions in gp90 that resulted in a loss of three potential N-linked glycosylation sites, designated g5, g9, and g10. To investigate the biological significance of these changes, reverse-mutated viruses were constructed in the backbone of the EIAVFDDV infectious molecular clone (pLGFD3). The resulting virus stocks were characterized for replication efficiency in donkey dermal cells and donkey MDM, and were tested for sensitivity to neutralization using sera from two ponies experimentally infected with EIAVFDDV. Results clearly show that these mutations generated by site-directed mutagenesis resulted in cloned viruses with enhanced resistance to serum neutralizing antibodies that were also able to recognize parental viruses. This study indicates that these mutations played an important role in the attenuation of the EIAV vaccine strains.