Studies on the escape mutants of rabies virus which are resistant to neutralization by a highly conserved conformational epitope-specific monoclonal antibody #1-46-12

We investigated a virus-neutralizing conformational epitope of the rabies virus glycoprotein (G) that is recognized by an anti-G monoclonal antibody (mAb; #1-46-12) and shared by most of the laboratory strains of the virus. To investigate the epitope structure, we isolated escape mutants from the HEP-Flury virus (wild-type; wt) after repeated passages in culture in the presence of the mAb. Immunofluorescence studies indicated that the mutants could be classified into two groups; the Group I lacked the epitope, while Group II preserved the epitope. The latter was dominant under the passage conditions, since Group I disappeared during the continuous passages. G proteins showed different electrophoretic mobilities; G protein of Group I migrated at the same rate as wt G protein, while that of Group II migrated at a slower rate, which was shown to be due to acquisition of an additional oligosaccharide side chain. Nucleotide sequencing of the G gene strongly suggested that amino acid substitutions at Thr-36 by Pro and Ser-39 by Thr of the G protein are responsible for the escape mutations of Groups I and II, respectively. The latter is a unique mutation of the rabies virus that allows the G protein to be glycosylated additionally at Asn-37, a potential glycosylation site that is not glycosylated in the parent virus, in preserving the epitope-positive conformation. These results suggest that to keep the 1-46-12 epitope structure is of greater survival advantage for the virus to escape the neutralization than to destroy it, which could be achieved by acquiring an additional oligosaccharide chain at Asn-37.