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Journal of Virology, May 2001, p. 4584-4593, Vol. 75, No. 10
Retrovirus Center and Virology Section,
Department of Biomedicine, University of Pisa,
Pisa,1 and Department of
Biochemistry and Molecular Biology, University of Ferrara,
Ferrara,2 Italy
Received 8 November 2000/Accepted 21 February 2001
The broad resistance to antibody-mediated neutralization of
lentiviruses recently isolated from infected hosts is a poorly understood feature which might contribute to the ability of these viruses to persist and to the failure of experimental vaccines to
protect against virulent viruses. We studied the underlying molecular mechanisms by examining the evolution of a
neutralization-sensitive, tissue culture-adapted strain of feline
immunodeficiency virus upon reinoculation into specific-pathogen-free
cats. Reversion to broad neutralization resistance was observed in
seven of seven inoculated animals and, in individual hosts, started to
develop between less than 4 and more than 15 months from
infection. After comparison of the envelope sequences of the inoculum
virus, of an additional 4 neutralization-sensitive in vitro variants,
and of 14 ex vivo-derived variants (6 neutralization sensitive, 5 resistant, and 3 with intermediate phenotype), a Lys
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.10.4584-4593.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
During Readaptation In Vivo, a Tissue Culture-Adapted Strain of
Feline Immunodeficiency Virus Reverts to Broad Neutralization
Resistance at Different Times in Individual Hosts but through
Changes at the Same Position of the Surface Glycoprotein
Asn or
Glu change at position 481 in the V4 region of the surface
glycoprotein appeared as a key player in the reversion.
This conclusion was confirmed by mutagenesis of molecularly
cloned virus. Analysis of viral quasispecies and biological clones
showed that the intermediate phenotype was due to transient
coexistence of neutralization-sensitive and -resistant
variants. Since the amino acid position involved was the same in
four of four recent revertants, it is suggested that the number of
residues that control reversion to broad neutralization resistance in
FIV might be very limited. Amino acid 481 was found to be changed only
in one of three putative long-term revertants. These variants shared a
Ser
Asn change at position 557 in region V5, which probably
collaborated with other mutations in long-term maintenance of
neutralization resistance, as suggested by the study of mutagenized virus.
*
Corresponding author. Mailing address: Dipartimento di
Biomedicina, Università di Pisa, Via San Zeno 37, I-56127 Pisa,
Italy. Phone: 39 (050) 553 562. Fax: 39 (050) 559 455. E-mail:
bendinelli{at}biomed.unipi.it.
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