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Journal of Virology, May 2001, p. 4570-4583, Vol. 75, No. 10
Department of Molecular Genetics and
Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh,
Pennsylvania 15261,1 and Department of
Veterinary Science, Gluck Equine Research Center, University of
Kentucky, Lexington, Kentucky 405462
Received 20 November 2000/Accepted 16 February 2001
A primary mechanism of lentivirus persistence is the ability of
these viruses to evolve in response to biological and immunological selective pressures with a remarkable array of genetic and antigenic variations that constitute a perpetual natural experiment in genetic engineering. A widely accepted paradigm of lentivirus evolution is that
the rate of genetic variation is correlated directly with the levels of
virus replication: the greater the viral replication, the more
opportunities that exist for genetic modifications and selection of
viral variants. To test this hypothesis directly, we examined the
patterns of equine infectious anemia virus (EIAV) envelope variation
during a 2.5-year period in experimentally infected ponies that
differed markedly in clinical progression and in steady-state levels of
viral replication as indicated by plasma virus genomic RNA assays. The
results of these comprehensive studies revealed for the first time
similar extents of envelope gp90 variation in persistently infected
ponies regardless of the number of disease cycles (one to six) and
viremia during chronic disease. The extent of envelope variation was
also independent of the apparent steady-state levels of virus
replication during long-term asymptomatic infection, varying from
undetectable to 105 genomic RNA copies per ml of plasma. In
addition, the data confirmed the evolution of distinct virus
populations (genomic quasispecies) associated with sequential febrile
episodes during acute and chronic EIA and demonstrated for the first
time ongoing envelope variation during long-term asymptomatic
infections. Finally, comparison of the rates of evolution of the
previously defined EIAV gp90 variable domains demonstrated distinct
differences in the rates of nucleotide and amino acid sequence
variation, presumably reflecting differences in the ability of
different envelope domains to respond to immune or other biological
selection pressures. Thus, these data suggest that EIAV variation can
be associated predominantly with ongoing low levels of virus
replication and selection in target tissues, even in the absence of
substantial levels of plasma viremia, and that envelope variation
continues during all stages of persistent infection as the virus
successfully avoids clearance by host defense mechanisms.
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.10.4570-4583.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Equine Infectious Anemia Virus Genomic Evolution in
Progressor and Nonprogressor Ponies

*
Corresponding author. Mailing address: Department of
Molecular Genetics and Biochemistry, University of Pittsburgh School of
Medicine, Pittsburgh, PA 15261. Phone: (412) 648-8869. Fax: (412)
383-8859. E-mail: rmont{at}pitt.edu.
Present address: UMR 754 INRA/Université Claude Bernard/
Ecole Nationale Vétérinaire de Lyon, Laboratoire
d'Immunologie et de Biologie Pulmonaire, Hôpital Louis Pradel,
69003 Lyon, France.
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