Induction of apoptosis in vitro by the 17-kDa nonstructural protein of infectious bursal disease virus: Possible role in viral pathogenesis

Infectious bursal disease virus (IBDV) causes severe immunodeficiency in yo ung chickens by destroying the precursors of antibody-producing B cells in the bursa of Fabricius. It has been shown that IBDV infection induces apopt osis in chicken embryo and tissue culture cells. We previously reported tha t an IBDV mutant lacking the expression of 17-kDa nonstructural (NS) protei n exhibited decreased apoptotic effects in cell culture as compared to the parental IBDV, suggesting that the NS protein may be involved in induction of apoptosis. Here, we report that the NS protein of IBDV alone is capable of inducing apoptosis in cell culture. Transfection of chicken B-lymphocyte cell line (RP9) and chicken embryo fibroblast calls with a plasmid DNA, co ntaining the NS protein gene under the control of the immediate-early promo ter-enhancer region of human cytomegalovirus, induced programmed cell death in both cell lines. Apoptosis changes, such as chromatin condensation, DNA fragmentation, and the appearance of apoptotic nuclear bodies, were observ ed in cell cultures 48-h posttransfection, As reported earlier, the mutant IBDV grew to lower titers with slower replication kinetics and lower cytopa thogenicity when compared to that of the parental virus. Here, we demonstra te that the mutant virus is closely associated with cells and its yield fro m the supernatant was approximately 30-fold lower than the wild-type due to increased cell association, indicating a deficiency in lysis of virus-infe cted cells. Taken together, our results indicate that the NS protein of IBD V is highly cytotoxic, which brings about the release of the viral progeny from cells, and thus play an important role in viral pathogenesis. (C) 2001 Academic Press.