Defect in the generation of light-chain diversity in bursectomized chickens.

The avian bursa of Fabricius has been regarded as a central organ for B-cell development, but there is controversy about the existence of other sites for differentiation of B cells. We have recently shown that chickens surgically bursectomized as early embryos, before the bursal primordium appears, can produce cytoplasmic, surface and serum immunoglobulins of IgM, IgG and IgA classes but are unable to generate specific antibodies in response to antigen. We have therefore examined the structure and diversity of immunoglobulins of bursectomized chickens. Analysis of serum IgG revealed normal gamma-heavy chains but altered light chains with more basic and less diverse isoelectric points than normal. These light chains may represent germ-line specificities not diversified by somatic mutations. Thus the bursa of Fabricius appears not to be necessary for the production of immunoglobulin molecules as such but to function in the creation and expansion of the antibody repertoire, possibly by providing a microenvironment for somatic mutations.