In-situ apoptosis in chickens infected with infectious bursal disease virus.

Groups of specific pathogen-free chickens aged 3 weeks were inoculated with one of three strains of infectious bursal disease virus (IBDV), namely: (1) a classical virulent strain, IM-IBDV; (2) an antigenic variant-E strain, VE-IBDV; and (3) an attenuated vaccine strain, B2-IBDV. The bursa of Fabricius and thymus were examined 1 to 5 days after inoculation for histopathological lesions, IBDV antigen, and in-situ apoptosis. Infection of chickens with IM-, VE-, or B2-IBDV resulted in the appearance of large numbers of apoptotic bursal lymphocytes, both in IBDV antigen-positive and antigen-negative bursal follicles. B2-IBDV induced a slower increase in the IBDV antigen-positive cells and apoptotic cells than did IM- and VE-IBDV. The results suggested that virulent strains and vaccine strains of IBDV enhance endogenous apoptosis of bursal lymphocytes that are free of detectable IBDV antigen. Infection of chickens with IM-IBDV, but not with VE- or B2-IBDV, caused a significant increase in apoptotic cells in the thymic cortical lymphocytes. The apoptotic lymphocytes in the thymus were negative for IBDV antigen. This result indicated that histopathological lesions in the thymus of IM-IBDV-infected chickens may be due to enhanced endogenous apoptosis of thymic lymphocytes without IBDV antigen production in the cells.