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J Virol, April 1998, p. 2647-2654, Vol. 72, No. 4
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Generation of a Mutant Infectious Bursal Disease
Virus That Does Not Cause Bursal Lesions
Kun
Yao,1,2
Mark A.
Goodwin,3 and
Vikram N.
Vakharia1,2,*
Center for Agricultural Biotechnology,
University of Maryland Biotechnology Institute,1
and
VA-MD Regional College of Veterinary
Medicine,2 University of Maryland, College
Park, Maryland 20742, and
Georgia Poultry
Laboratory, Oakwood, Georgia 305663
Received 29 August 1997/Accepted 16 December 1997
A reverse genetics system for birnavirus, based on synthetic
transcripts of the infectious bursal disease virus (IBDV) genome, was
recently developed (E. Mundt and V. N. Vakharia, Proc. Natl. Acad.
Sci. USA 93:11131-11136, 1996). To study the function of the 17-kDa
nonstructural (NS) protein in viral growth and pathogenesis, we
constructed a cDNA clone of IBDV segment A in which the first and only
initiation codon (ATG) of NS protein was mutated to a stop codon (TAG).
Transfection of Vero cells with combined transcripts of either modified
or unmodified segment A, and with segment B, generated viable IBDV
progeny. When chicken embryo fibroblast cells infected with
transfectant viruses were analyzed by immunofluorescence assays using
NS-specific antiserum, the mutant virus did not yield a fluorescence
signal, indicating a lack of NS protein expression. Furthermore,
replication kinetics and cytotoxic effects of the mutant virus were
compared with those of the parental attenuated vaccine strain of IBDV
(D78) in vitro. The mutant virus grew to slightly lower titers than D78
virus and exhibited decreased cytotoxic and apoptotic effects in cell
culture. To evaluate the characteristics of the recovered viruses in
vivo, we inoculated 3-week-old chickens with D78 or mutant virus and
analyzed their bursa for histopathological lesions. The recovered D78
virus caused microscopic lesions and atrophy of the bursa, while the
mutant virus failed to induce any pathological lesions or clinical
signs of disease. In both instances, the virus was recovered from the
bursa, and the presence or absence of mutation in these viruses was
confirmed by nucleotide sequence analysis of NS gene. Although the
mutant virus exhibited a delay in replication in vivo, it induced
levels of IBDV neutralizing antibodies that were similar to those of
D78 virus. In addition, no reversion of mutation was detected in the
mutant virus recovered from inoculated chickens. These results
demonstrate that NS protein is dispensable for viral replication in
vitro and in vivo and that it plays an important role in viral
pathogenesis. Thus, generation of such NS protein-deficient virus will
facilitate the study of immunosuppression and aid in the development of
live-attenuated vaccines for IBDV.
*
Corresponding author. Mailing address: Center for
Agricultural Biotechnology, 6126 Plant Sciences Building, University of Maryland, College Park, MD 20742. Phone: (301) 405-4777. Fax: (301)
314-9075. E-mail: vakharia{at}umbi.umd.edu.
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