Naturally occurring mutations within 39 amino acids in the envelope glycoprotein of maedi-visna virus alter the neutralization phenotype.

Infectious molecular clones have been isolated from two maedi-visna virus (MVV) strains, one of which (KV1772kv72/67) is an antigenic escape mutant of the other (LV1-1KS1). To map the type-specific neutralization epitope, we constructed viruses containing chimeric envelope genes by using KV1772kv72/67 as a backbone and replacing various parts of the envelope gene with equivalent sequences from LV1-1KS1. The neutralization phenotype was found to map to a region in the envelope gene containing two deletions and four amino acid changes within 39 amino acids (positions 559 to 597 of Env). Serum obtained from a lamb infected with a chimeric virus, VR1, containing only the 39 amino acids from LV1-1KS1 in the KV1772kv72/67 backbone neutralized LV1-1KS1 but not KV1772kv72/67. The region in the envelope gene that we had thus shown to be involved in escape from neutralization was cloned into pGEX-3X expression vectors, and the resulting fusion peptides from both molecular clones were tested in immunoblots for reactivity with the KV1772kv72/67 and VR1 type-specific antisera. The type-specific KV1772kv72/67 antiserum reacted only with the fusion peptide from KV1772kv72/67 and not with that from LV1-1KS1, and the type-specific VR1 antiserum reacted only with the fusion peptide from LV1-1KS1 and not with that from KV1772kv72/67. Pepscan analysis showed that the region contained two linear epitopes, one of which was specific to each of the molecularly cloned viruses. This linear epitope was not bound by all type-specific neutralizing antisera, however, which indicates that it is not by itself the neutralization epitope but may be a part of it. These findings show that mutations within amino acids 559 to 597 in the envelope gene of MVV virus result in escape from neutralization. Furthermore, the region contains one or more parts of a discontinuous neutralization epitope.

Biological and genetic differences between lung- and brain-derived isolates of maedi-visna virus.

During the epidemic caused by maedi-visna virus (MVV) of sheep in Iceland, the pulmonary affection, maedi, was the predominant clinical manifestation. In some flocks, however, a central nervous system (CNS) affection, visna, was the main cause of morbidity and mortality. As there is only one breed of sheep in the country, host factors did apparently not play an important role in the different clinical manifestations. To obtain some information on possible viral genetic determinants of neurotropism and neurovirulence we studied both phenotypic and genotypic properties of two maedi-visna virus strains; a strain that was originally isolated from the brain of sheep with encephalitis (visna), and another strain isolated from the lungs of a sheep suffering from pneumonia (maedi). The brain isolate was found to grow faster in sheep choroid plexus cells than the lung isolate, whereas the growth rate in macrophages was similar for the maedi and visna virus strains. Intracerebral inoculation indicated that the visna virus isolate induced more severe brain lesions than the maedi isolate. In addition, a pathogenic molecular clone derived from a visna strain (KV1772kv72/67) was tested for growth in sheep choroid plexus cells and macrophages. The molecularly cloned virus retained the fast growth rate in choroid plexus cells. The nucleotide sequence of the env gene and the U3 of the LTR was determined for the maedi strain and compared to that of the visna strains. There was an 11.7% difference in deduced amino acid sequence in the Env protein and a 6% difference in the LTR. The molecular clone KV1772kv72/67 will be a useful reagent for characterization of viral determinants of cell tropism in vitro and possibly neurovirulence in vivo.