Immunoglobulin diversification in embryonic chicken bursae and in individual bursal follicles

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Abstract
Previous studies have shown that the same immunoglobulin (Ig) Vλ gene (Vλ 1) is rearranged in all chicken B cells, and that extensive sequence diversification of this gene occurs during B cell development in the bursa of Fabricius. We used two‐dimensional gel electrophoresis to compare the heterogeneity of Ig λ light chains produced by B cells at different stages of bursal development. Somatically diversified light chains were observed in Ig molecules produced by bursal cells as early as 15 days of embryonic incubation. The two principal species of light chain observed probably represent glycosylated and nonglycosylated forms of λ chain encoded by alleles of a single λ gene. Extensive diversification was observed during late embryogenesis. We also studied λ light chain diversity in cyclophosphamide‐treated birds repopulated with normal bursal cells. In these birds, individual bursal follicles are repopulated by single B cell precursors. Follicular cells derived from single B cell precursors were able to produce a spectrum of light chains almost as diverse as that of the total bursal cell population. We used two monoclonal anti‐idiotype antibodies to study idiotype expression in individual normal or reconstituted follicles. About 30% of follicles contained 0.1% to 5% of lymphocytes which reacted with one or both of the antibodies. The results indicate that within individual bursal follicles bursa stem cells undergo Ig hyperdiversification.