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Abstract
We exposed chicken embryos at embryonation day 18 (ED18) to a classical virulent infectious
bursal disease virus (IBDV; cIBDV) and an attenuated strain of IBDV (aIBDV) and examined
the response of embryonic lymphoid cells to these viruses. Embryos responded much
more vigorously to cIBDV than to aIBDV. Following cIBDV exposure, embryonic thymus
and bursa showed cellular destruction, enhanced rate of apoptosis and presence of
viral proteins detectable by immunohistochemistry. At ED21, thymocytes from cIBDV-exposed
embryos were severely deficient (P<0.05) in responding to stimulation in vitro with
mitogens containing mouse anti-chicken CD28 mAb, PMA and ionomycin. Because purified
CD3(+) T cells were also refractory to the mitogens, the mitogenic inhibition of embryonic
thymocytes was not attributed to the presence of non-T cell suppressors. Cell suspensions
prepared from embryonic thymus and spleen had upregulated gene expression of IFN-gamma
and IL-6 cytokines and of chemokine IL-8. In sharp contrast to cIBDV, embryos exposed
to aIBDV had minimal detectable changes in the thymus and bursa, although the rate
of apoptosis was enhanced in the thymus. Viral antigen was not detectable in the bursa
until after hatch. Thymocytes from these embryos responded vigorously to the mitogens,
similar to the response of thymocytes from unexposed control embryos. In addition,
aIBDV induced a modest gene upregulation of IFN-gamma, IL-6 and IL-8 in thymus and
spleen. Relatively modest response of the embryo to aIBDV is significant because in
ovo vaccination with aIBDV-type viruses and several other non-pathogenic viruses result
in protective immunity that is well pronounced at hatch.