B Cell Development in Birds

Abstract

Avian B cell development is dependent on a specialized organ, the bursa of Fabricius. The embryonic bursa is colonized by limited numbers of B cell precursors that subsequently develop within the bursa and emigrate to generate a population of peripheral B cells with a highly diverse repertoire of specificities. Rearrangement of immunoglobulin genes is initiated prior to migration into the bursal mesenchyme and productive colonization of bursal follicles by B cell precursors typically requires their expression of cell surface immunoglobulin. Each bursal follicle is colonized by a small number of B cell precursors that proliferate rapidly and diversify their immunoglobulin genes by somatic gene conversion. Starting just prior to hatch, mature B cells emigrate from the bursa to the periphery, where they form the peripheral B cell repertoire. After hatch, cells of the bursal epithelium, which separates the lymphoid compartment of the bursa from the gut lumen, develop into a follicle-associated epithelium that transports material from the gut lumen into the lymphoid tissue. Intrabursal exposure of developing B cells to gut-derived antigen modifies the frequency and/or developmental status of emigrating B cells, suggesting a role for the bursa in priming the peripheral B cell compartment. Mature B cells in the periphery of avian species appear to behave in a manner analogous to mammalian B cells. Thus they respond to challenge with T cell–dependent and T cell–independent antigens with the production of antibody, the generation of memory, and the switching of B cells from IgM to IgG and/or IgA secretion.