Infectious bursal disease virus (IBDV) causes lymphocytolysis and immunosuppression in infected poultry. The IBDV genome encodes a
polyprotein VP243 that is post-translationally cleaved by the VP4
protease into the two structural
proteins pVP2 and VP3. The objective of the present study was to determine if IBDV
polyprotein induced suppression of bursal B lymphocyte growth and their capacity for proliferation. Bursal B cells were examined both for chickens infected with IBDV and for chickens orally inoculated with
a DNA construct expressing IBDV VP243
polyprotein. Bursae were collected at 0, 12, 24 and 48 hours after inoculation. Proliferation of bursal B cells (purified AvBu1(+) cells) in response to
concanavalin A mitogenic stimulation was significantly suppressed by
infection at 1 day old with either the classical STC or variant E strains of IBDV.
Oral administration of
DNA constructs expressing the IBDV VP243
polyprotein from either the classical STC or variant E strains in the pCR3.1 vector resulted in persistent, moderate levels of construct in the bursa until at least 48 hours after inoculation. The VP243
DNA construct similarly induced suppression of proliferation for bursal lymphocytes independently of the
virus infection. Expression of VP243
polyprotein in transiently transfected DT40 B lymphocyte culture also suppressed cell growth and proliferative responses to
mitogen stimulation.
Polyprotein expression did not affect cell viability and suppression of proliferation probably occurred by means of cell cycle arrest. The expression of the mature
viral proteins VP2, VP4 or VP3 did not change the rate of cell proliferation or response of B cell cultures to
mitogen. The results suggested that IBDV
polyprotein is a mediator of immunosuppression.