Variability and immunogenicity of caprine arthritis-encephalitis virus surface glycoprotein

Abstract

The complete surface glycoprotein (SU) nucleotide sequences of three French isolates of caprine arthritis-encephalitis virus (CAEV) were determined and compared with those of previously described isolates: three American isolates and one French isolate. Phylogenetic analyses revealed the existence of four distinct and roughly equidistant evolutionary CAEV subtypes. Four conserved and five variable domains were identified in the SU. The fine specificities of antibodies produced against these domains during natural infection were examined using a pepscan analysis. Nine immunogenic segments were delineated throughout the conserved and variable domains of SU, two of them corresponding to conserved immunodominant epitopes. Antigenic determinants which may be involved in the immunopathogenic process induced by CAEV were identified. These results also provide sensitive and specific antigen peptides for the serological detection and differentiation of CAEV and visna/maedi virus infections.

FIG. 1

Phylogenetic relationships of French CAEV isolates to prototype CAEV and MVV strains. The SU multiple sequence alignment was resampled by the bootstrap method (1,000 data sets); an unrooted tree generated by neighbor-joining analysis is shown. The horizontal and vertical orientations of branches are noninformative and for clarity only; branching patterns and branch lengths reflect phylogenetic distance relationships. The numbers on the nodes represent the percentage of bootstrap samples.

FIG. 2

Alignment of predicted SU precursor amino acid sequences of CAEV isolates. The sequences were aligned with a consensus sequence (Cons) consisting of the most frequent residue at a given position. Dashes in the sequence alignment represent deletions. Variable domains (V1 to V5) are delineated by overlines. ∗, conserved cysteine residue; –––, conserved potential N-linked glycosylation site.

FIG. 3

Relationships between immunogenicity and variability profiles of CAEV SU. Variable and conserved domains of the SU were established from all available sequences (those for three American and four French isolates). The variability profile is based on the frequency of amino acid substitutions relative to the consensus sequence and smoothed using a 10-amino-acid window size. The amino acid numbers on the x axis start with the mature SU amino-terminal Glu as residue 1 (29). The immunogenic sites identified in this study are delineated by hatched areas. Below the variability profile, the conserved regions are shown as white boxes and the variable regions are shown as black boxes. Stick with circle, conserved potential N-linked glycosylation site; arrowhead, conserved cysteine residue.

FIG. 4

Reactivity profiles of ELISA-positive immune sera to immunogenic CAEV SU synthetic peptides. Each graph represents ELISA reactivities to one peptide or to different peptides as indicated. Each bar in the different serum reactivity profiles represents the ELISA reactivity (absorbance value/cutoff [A₄₀₅/C.O.]) of one positive immune serum to an individual peptide. The number of positive immune sera to an individual immunodominant peptide or to combined immunoreactive peptides is indicated below each panel.