The S2 gene of equine infectious anemia virus is a highly conserved determinant of viral replication and virulence properties in experimentally infected ponies

Equine infectious anemia virus (EIAV) is genetically one of the simplest le ntiviruses in that the viral genome encodes only three accessory genes, tat , rev, and S2. Although serological analyses demonstrate the expression of the S2 protein in persistently infected horses, the role of this viral gene remains undefined. We recently reported that the S2 gene is not essential for EIAV replication in primary equine macrophages, as EIAV mutants lacking the S2 gene replicate to levels similar to those of the parental virus (F. Li, B. A. Puffer, and R. C. Montelaro, J. Virol. 72:8344-8348, 1998). We n ow describe in vivo studies that examine the evolution and role of the S2 g ene in ponies experimentally infected with EIAV. The results of these studi es reveal for the first time that the S2 gene is highly conserved during pe rsistent infection and that deletion of the S2 gene reduces viral virulence and virus replication levels compared to those of the parental virus conta ining a functional S2 gene. These data indicate that the EIAV S2 gene is in fact an important determinant of viral replication and pathogenic properti es in vivo, despite the evident lack of S2 influence on viral replication l evels in vitro. Thus, these observations suggest in vivo functions of EIAV S2 that are not adequately reflected in simple infections of cultured cells , including natural target macrophages.