In poultry, the bursa of Fabricius functions as a gut-associated primary lymphoid tissue for the development of B-cells. In late embryonic development, a single wave of prebursal stem cells committed to the B-cell lineage colonize the bursa between embryonic days (ED) 10 - 14. These stem cells originate from precursors in the intra-embryonic mesenchyme and yolk sac and have completed H-chain and L-chain immunoglobulin gene rearrangements. The prebursal stem cells migrate into the nascent bursal follicles and proliferate to expand candidates for repertoire development. After 2-3 days of proliferation, around ED17, these cells undergo a differentiation event which initiates repertoire development by immunoglobulin gene conversion. At hatching, the structure and microenvironment of the bursal follicles change considerably from the embryonic period. During the embryonic period, a single epithelial layer and a basement membrane cover the bursal follicle. By hatching some of the developing B-cell pool migrate through the basement membrane and proliferate to form the outer cortex of the mature follicle. After development of the mature follicle, recent evidence indicates that developing B-cells are selected to migrate from the medulla to the cortex and continue to proliferate and undergo repertoire development. The bursa at hatch seeds the periphery with mature naïve B-cells up until bursal involution at sexual maturity. Naïve B-cells monitor for the presence of infection by surveillance of secondary lymphoid tissues. In response to infection, B-cells proliferate and then differentiate to memory B-cells or antibody-secreting plasma cells to prevent re-infection. Notably, the localization of the peripheral B-cell response differs from mammals as chickens have no lymph nodes, a splenic structure without separation of B-cell follicles and marginal zone B-cells and globular, encapsulated germinal centers. This raises the question whether final B-cell differentiation and formation of the antibody response follows mammalian rules or relies on specific conditions.

This Research Topic aims to feature recent advances in B-cell development in poultry, including updates on the mechanisms behind the various cell-fate decisions of bursal B-cells, such as proliferation, differentiation, apoptosis, migration, selection, repertoire development, and the export of B-cells the periphery after hatch. Also of special interest is the clarification of bursal microenvironmental factors that induce or modulate cell-fate decisions of bursal B-cells, including recent updates on the developmental biology of the bursa, which is essential for understanding the bursal microenvironment. This Research Topic also aims to expand our knowledge of B-cell biology in the periphery. In poultry, little is known about the biology of germinal centers or the differentiation events leading to the development of memory B-cells and long-lived plasma cells.

We welcome submissions that broaden our understanding of the mechanisms behind bursal B-cell fate decisions, the identity of the key factors in the bursal microenvironment involved in cell fate decisions, and the embryonic development of the bursa. This would also include extrabursal differentiation of B-cells in response to infection. We are interested in Original Research, Review, and Mini Review articles, focusing on, but not limited to, the following areas:

1). Embryonic development of the bursa of Fabricius
2). Phenotype analysis of bursal B-cells to identify different developmental subsets
3). Multi-omic analysis (transcriptomic, proteomic, and epigenomic) of developing B-cells in the bursa
4). Factors in the bursal microenvironment that guide B-cell developmental events (migration, proliferation, differentiation, and repertoire development)
5). Repertoire development by immunoglobulin gene conversion
6). The role of antigens derived from the cloacal lumen in bursal B-cell development
7). The different functions of the cortex and medulla of the bursal follicle
8). The role of bursa lympho-myeloid microenvironment in B-cell development
9). Extrabursal B-cell activation and differentiation in response to infection

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Frontiers in Immunology

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