ENHANCED SENSITIVITY TO NEUTRALIZING ANTIBODIES IN A VARIANT OF EQUINE INFECTIOUS-ANEMIA VIRUS IS LINKED TO AMINO-ACID SUBSTITUTIONS IN THESURFACE UNIT ENVELOPE GLYCOPROTEIN

Serial passage of the prototype (PR) cell-adapted Wyoming strain of eq uine infectious anemia virus (EIAV) in fetal donkey dermal (FDD) rathe r than fetal horse (designated fetal equine kidney [FEK]) cell culture s resulted in the generation of a variant virus strain which produced accelerated cytopathic effects in FDD cells and was 100- to 1,000-fold more sensitive to neutralizing antibodies than its parent. This neutr alization-sensitive variant was designated the FDD strain. Although th ere were differences in glycosylation between the PR and FDD strains, passage of the FDD virus in FEK cells did not reduce its sensitivity t o neutralizing antibody. Nucleotide sequencing of the region encoding the surface unit (SU) protein from the FDD strain revealed nine amino acid substitutions compared with the PR strain. Two of these substitut ions resulted in changes in the polarity of charge, four caused the in troduction of a charged residue, and three had no net change in charge . Nucleotide sequence analysis was extended to the region of the FDD v irus genome encoding the extracellular domain of the transmembrane env elope glycoprotein (TM). Unlike the situation with the FDD virus codin g region, there were minor variations in nucleotide sequence between i ndividual molecular clones containing this region of the TM gene. Alth ough each clone contained three nucleotide substitutions compared with the PR strain, only one of these was common to all, and this did not affect the amino acid content, Of the remaining two nucleotide substit utions, only one resulted in an amino acid change, and in each case, t his change appeared to be conservative. To determine if amino acid sub stitutions in the SU protein of FDD cell-grown viruses were responsibl e for the enhanced sensitivity to neutralizing antibodies, chimeric vi ruses were constructed by using an infectious molecular clone of EIAV. These chimeric viruses contained all of the amino acid substitutions found in the FDD virus strain and were significantly more sensitive to neutralizing antibodies than viruses from the parental (PR) molecular clone. These results demonstrated that sensitivity to neutralizing an tibodies in EIAV can be conferred by amino acid residues in the SU pro tein, However, such amino acid substitutions were not sufficient to en hance cytopathogenicity, as the chimeric viruses did not cause excessi ve degenerative effects in FDD cells, as was observed with the parenta l FDD virus strain.