In vivo dynamics of equine infectious anemia viruses emerging during febrile episodes: insertions/duplications at the principal neutralizing domain.

Equine infectious anemia virus (EIAV) is a good model for studying mechanisms generating escaped retrovirus variants. We previously sequenced the entire gp90-encoding region of 22 cDNA clones obtained from five antigenically distinct isolates (F1V to F5V) recovered during febrile episodes in horse 493 experimentally infected with the Japanese virulent EIAV strain V70. The results showed that the mutations occurred in the principal neutralizing domain (PND) by insertions/duplications. In this study, we further characterized the PND of virus isolates sequentially recovered during 22 febrile episodes in seven horses newly infected with V70 or one of the V70-derived variants. Sequencing of 70 cDNA clones derived from the 22 episodes confirmed the generation of various new viral quasispecies with insertions/duplications in the PND. Although the insertion/duplication sequences in a total of 92 cDNA clones were extensively heterogeneous, we hypothesized that all the insertions/duplications occurred during reverse transcription from viral genomic RNA to minus strand DNA. The insertion/duplication regions were derived from a part of the PND sequence, which consisted of five small units. These small units, some with various substitutions and/or deletions, were also generated, especially in regions with insertions/duplications. Of particular note was that all these virus variants, except for two cDNA variants, were generated by essentially four different duplication pathways. Thus, these results extend the significance of insertions/duplications in the PND to the novel generation of EIAV in vivo during febrile episodes.