Central function

The bursa of Fabricius functions as a central lymphoid organ, required for development of the antigen-specific B cell repertoire. It is necessary for the differentiation of prebursal stem cells into bursal stem cells present in the bursa until the 5th week after hatching. These cells are capable of reconstituting the bursal morphology and specific antibody production when transferred into birds with the bursal lymphoid structures destroyed with cyclophosphamide in the newly hatched period. They comprise a clearly defined stage in the stepwise maturation of the avian antibody-producing cells.

The bursa of Fabricius is necessary for the development of the postbursal B cell population, including the mature antigen-specific B cells and the self-renewing postbursal stem cells. Neither of these populations is capable of homing, in contrast to the

Figure 2 The bursa of Fabricius is necessary for the development of the antigen-specific B cell repertoire. Prebursal stem cells with germ line immunoglobulin genes rearranged are found in the intraembryonic mesenchyme from embryonic (Embr.) day 7 onwards. After arrival in the bursa of Fabricius they become bursal stem cells undergoing extensive, high rate gene conversion, which leads to diversification of the germ line repertoire; these cells need the bursal microenvironment for further maturation. Postbursal cells, present both in the bursa and periphery, include mature antigen-specific B cells and self-renewing postbursal stem cells, capable of further maturation without bursal influence.

Figure 2 The bursa of Fabricius is necessary for the development of the antigen-specific B cell repertoire. Prebursal stem cells with germ line immunoglobulin genes rearranged are found in the intraembryonic mesenchyme from embryonic (Embr.) day 7 onwards. After arrival in the bursa of Fabricius they become bursal stem cells undergoing extensive, high rate gene conversion, which leads to diversification of the germ line repertoire; these cells need the bursal microenvironment for further maturation. Postbursal cells, present both in the bursa and periphery, include mature antigen-specific B cells and self-renewing postbursal stem cells, capable of further maturation without bursal influence.

prebursal or bursal stem cells, to the cyclophospham-ide-treated bursal reticulum, even though they are capable of reconstituting antibody production. In other words, the postbursal cells have passed the stage of bursal development once and for all.

The bursa of Fabricius is necessary for the diversification of the antibody repertoire by providing a site for extensive, high rate gene conversion, which occurs between only one functional V gene (VHi or Vu) and a group of V pseudogenes. The bursal microenvironment selects and promotes for further differentiation bursal stem cells undergoing productive gene conversions, whereas all the others (>90%), probably with unproductive conversion events, die within the bursa. Only minor gene conversion events and somatic point mutations may occur outside the bursa. Molecular events in avian B cell development, as related to the central function of the bursa of Fabricius, are summarized in Figure 2.

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