Unique evolution characteristics of the envelope protein of EIAV _LN40, a virulent strain of equine infectious anemia virus

The Chinese equine infectious anemia virus (EIAV) virulent strain EIAV _LN40 is derived from a naturally occurring virus by continuously passing in horses for 16 generations. Its genome sequence is 23% different from that of the American strains or the Japanese strains, and the variation of envelope gp90 surface unit (SU) is as high as 41%. In this study, evolutions of the EIAV_LN40 gp90 gene in four infected horses were analyzed. Results showed that new quasispecies arose in the early stage of infection in all EIAV_LN40-infected horses. These quasispecies belonged to branches different from EIAV_LN40 in a phylogenetic tree. In contrast, the gp90 sequences of viruses isolated after disease onset remained in the same phylogenetic branch as EIAV_LN40, with some having exactly the same sequences. The glycosylation sites 191NSSN and 237NNTW in the V3 and V4 region present or absent simultaneously in most of the predicted amino acid sequences. Changes in the glycosylation sites within V3, V4, and V5 regions are usually associated with the disease status. Glycosylation sites (191NSSN, 237NNTW, and 280NDTS) within these three regions were present in EIAV_LN40 and most of the quasispecies isolated after, but not before disease onset. These unique evolutionary characteristics of SU have not been reported for EIAV and other lentiviruses. Our results provide a reference for a further understanding of the mechanism underlying the persistent infection and escape from immune surveillance of EIAV.