Bursectomy of chicken embryos at 60 hours of incubation leads to an oligoclonal B cell compartment and restricted Ig diversity

Chickens that have been surgically bursectomized at 60 h of embryonic development usually generate Ig producing B cells; however, the bursectomized chickens are incapable of specific antibody responses, even after repeated immunization. In the present work, we analyzed the molecular basis of this immunodeficiency. In the bursectomized chickens, DNA sequencing revealed a repertoire of Ig L and H chains with a low number of different V-J and V-D-J joints, indicating an oligoclonal B cell compartment. In addition, the L and H chains belonging to each B cell clone had similar gene conversion events in the V region. In situ hybridization to Harderian gland tissue sections showed, that B cells of the bursectomized chickens were, however, capable of terminal plasma cell maturation. Thus, in chickens that were lacking the bursal microenvironment, 1) only a few B cell precursors differentiated into mature Ig-producing B cells, 2) low rate of gene conversion resulted in restricted Ig diversity. Regarding the chicken B cell differentiation, the present data support a model that the induction of B cell differentiation is a bursa-independent event, whereas the bursa of Fabricius has a crucial role in the amplification and diversification of the embryonic B cell repertoire.