Effects of long terminal repeat sequence variation on equine infectious anemia virus replication in vitro and in vivo

The long terminal repeat (LTR) is report to be one of the most variable por tions of the equine infectious anemia virus (EIAV) genome. To date, however no information is available on the effects of observed sequence variations on viral replication properties, despite a widespread assumption of the bi ological importance of EIAV LTR variation. EIAV LTR sequence variability is confined mostly to a small position of the enhancer within the U3 segment of the LTR. Analysis of published EIAV LTR sequences revealed six different types of LTR based on the pattern of putative transcription factor motifs within the variable region of the enhancer. To test directly the significan ce of LTR variation, the in vitro and in vivo replication properties of two variant LTR species were investigated using two isogenic viruses, EIAV(19- 2) and EIAV(19-2-6A), differing only within the enhancer region. The result s of these studies demonstrated that the two variants replicated with simil ar kinetics and to equal levels in cultured equine fibroblasts or in equine macrophage, the natural target cell of EIAV, even after prolonged serial p assage in the latter cell type. Furthermore, EIAV(19-2) and EIAV(19-2-6A) v ariants demonstrated similar replication levels in experimentally infected ponies. However, ponies infected with EIAV(19-2-6A) exhibits a rapid switch in the prevalent LTR type, such that by 112 days postinfection, no origina l-LTR-type viruses were evident. This specific and rapid shift in LTR quasi species indicates an in vivo selection that is not reflected in simple in v itro replication rates, suggesting undefined selection pressures in vivo th at drive LTR variation during persistent EIAV infection. (C) 1999 Academic Press.