Jake Harper
B and T cells are a specific type of white blood cell called lymphocytes. Lymphocytes fight harmful invaders and abnormal cells, such as cancer cells. In chickens, B cells develop in a primary lymphoid organ unique to birds, the bursa of Fabricius. T cells, on the other hand, develop in the thymus, spleen, and lymph nodes. This occurs when haematopoietic stem cells enter the thymic anlages and become competent B and T cells. After development, B cells travel to important parts of the lymphatic system, including the spleen and lymph nodes. T cells protect the body by destroying pathogens and sending signals that help coordinate the immune system's response to threats.
| Characteristics | Values |
|---|---|
| Where B cells develop in chickens | Bursa of Fabricius, a primary lymphoid organ unique to birds |
| When the bursa develops | Around embryonic day (E) 4-5 |
| When the bursa is colonized by pre-bursal cells | Between E8 and E14 |
| When B cells undergo rapid expansion | After migration and colonization of bursal follicles |
| When bursal cells reach maximum size | 8-10 weeks after hatching |
| B cell stages in chickens | Pre-bursal, bursal and post-bursal |
| B cell stages in mammals | Pro-B cells, pre-B cells and immature B cells |
| Large B cells | More proliferative and differentiated than small B cells |
| Ratio of small to large B cells | Reverses as the chicken grows |
| Pre-bursal B cells | Originate from dorsal mesenchyme, embryonic spleen, and bone marrow |
| Antibodies in birds | Low diversity from gene rearrangements of Ig heavy and light chains |
| Antibodies in mammals | Vast repertoire of antibodies due to many combinations of V, D and J gene segments |
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What You'll Learn
B cells in the bursa of Fabricius
The bursa of Fabricius is a primary lymphoid organ unique to birds, which acts as the central site for B-cell development. It is an epithelial and lymphoid organ that develops from the epithelial rudiment of the cloaca around embryonic day (E) 4–5. The bursa is named after Hieronymus Fabricius, who described it in 1621, and it is found only in birds.
During embryonic development, between embryonic day (E) 8 and E14, pre-bursal B cells from the dorsal mesenchyme, embryonic spleen, and bone marrow colonize the bursa of Fabricius. These pre-bursal cells then migrate into bursal follicles, where they undergo rapid expansion and proliferation. The bursal cells continuously divide, reaching their maximum size at 8–10 weeks of age, before gradually undergoing atrophy.
The bursa of Fabricius plays a pivotal role in humoral immunity, with its removal resulting in impaired antibody responses. Research has also revealed the importance of the bursa in orchestrating the V-gene repertoire and antibody diversity. The bursa's epithelial and secretory dendritic cells contribute to the microenvironment of the developing bursal follicle, influencing B-cell differentiation.
In summary, the bursa of Fabricius is essential for B-cell development in chickens, providing a unique model to study early B-cell differentiation. The bursa's role in antibody production and B-cell maturation makes it a critical organ for the chicken's immune system.
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Pre-bursal B cells
Chicken B cell development occurs in a primary lymphoid organ unique to birds, the bursa of Fabricius. The bursa is the primary site of B cell lymphopoiesis and is responsible for the early stages of B cell differentiation.
During the pre-bursal phase, hematopoietic precursor cells, which derive from the intra-embryonic mesenchyme in para-aortic foci, commit to the B cell lineage and spread via the blood to various lymphoid organs like the spleen and bone marrow. The bursa develops from the epithelial rudiment of the cloaca around embryonic day (E) 4-5 and is colonized by pre-bursal cells from the hematopoiesis site between E8 and E14. Pre-bursal B cells from the dorsal mesenchyme, embryonic spleen, and bone marrow colonize the bursa of Fabricius during embryonic development.
In the second migration step, the earliest bursal B cells migrate from the bursal mesenchyme into developing follicle buds. Formation of these follicle buds is triggered by immigrating precursors of bursal secretory dendritic cells (BSDCs), which are of hematopoietic origin and induce a so-called dendro-epithelial tissue. In the third migration step, B cells migrate to the follicle border, and in the fourth migration step, a small fraction of B cells emigrates mainly from the follicular cortex of the bursa to the peripheral lymphoid organs.
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B cell differentiation
Following the migration and colonization of the cells into bursal follicles while immunoglobulin (Ig) gene rearranged, the cells undergo rapid expansion. The bursal cells continuously divide, reaching maximum size at 8–10 weeks of age, and then gradually go through atrophy. Unlike mammals, in which the developmental stages of B cells are divided into pro-B cells, pre-B cells and immature B cells, chickens have three distinct B cell stages: pre-bursal, bursal and post-bursal B cells.
In chickens, B cell precursors migrate to the bursa and start to express the chB6 antigen, a highly glycosylated type I transmembrane protein. Subsequently, both bursal and peripheral chicken B cells can be identified by their expression of chB6. Before migration, immature cells commit to the B cell lineage by rearranging their Ig genes.
Chicken B cells undergo discrete developmental changes in surface carbohydrate structure that appear to play a role in directing lymphocyte migration during embryogenesis. For example, cells expressing sialyl Lewis x begin appearing in the bursa anlage between embryonic days 10 and 12. These cells appear to form the nascent bursal follicles and are induced to proliferate. As cells mature further, they undergo another phenotypic change and switch from expressing high levels of Lewis x to become Lewis x-low. Expression of sialyl Lewis(x) and Lewis(x) defines distinct stages of chicken B cell maturation.
Recent transcriptional analysis has revealed that the MAPK, Wnt, Notch and JAK-STAT signaling pathways are essential in B cell development and that those pathway-related genes are differentially expressed in bursal B cells at various stages of B cell development. While B cell development in the bursa is relatively well understood, very little is known about post-bursal B cell differentiation into plasma and memory cells.
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T cell development
During embryonic development, the thymus is colonized by hematopoietic stem cells, which differentiate into T cell precursors. These precursors then undergo a complex process of differentiation and maturation to become functional T cells. This process involves the expression of specific genes and proteins, as well as the development of the T cell receptor (TCR) complex, which is essential for antigen recognition.
One unique feature of avian T cell development is the presence of a new lineage of cytoplasmic CD3+ lymphoid cells, known as TCR0 cells. These cells lack the typical TCR1 and TCR2 receptors found in mammals but instead express a unique receptor isotype, TCR3, which is generated exclusively in the thymus.
Another interesting aspect of T cell development in chickens is the role of early feeding of hydrated nutritional supplements. Research has shown that providing chicks with these supplements affects the development of their immune system, including the function of T cells. This is reflected in the increased weight of the thymus, improved resistance to diseases, and an earlier appearance of certain antibodies.
Furthermore, studies have utilized monoclonal antibodies specific for chicken T cell surface antigens to gain a deeper understanding of T cell development in birds. By analyzing the differentiation pathways, functional processes, and molecules associated with T cells, researchers have gained insights into the unique aspects of avian T cell development and function.
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B cell maturation
Pre-bursal Stage
The bursa of Fabricius develops from the epithelial rudiment of the cloaca around embryonic day (E) 4–5. During this early stage, pre-bursal cells migrate from the dorsal mesenchyme, embryonic spleen, and bone marrow to colonize the bursa between E8 and E14. These pre-bursal stem cells undergo rapid proliferation and divide continuously, reaching their maximum size at 8–10 weeks of age.
Bursal Stage
In the bursa, the pre-bursal stem cells differentiate into small and large B cells. Large B cells have a higher capacity for proliferation and differentiation compared to small B cells. The ratio of small to large B cells reverses as the chicken grows, indicating a change in the status and stage of B cell development. Gene conversion within the bursa enables the development of antibodies with diverse recognition abilities.
Post-bursal Stage
As the chicken reaches sexual maturity, the bursa involutes, ceasing the production of new B cells. This results in a dramatic switch in the composition of the peripheral B cell pool. B cells mature further and migrate to seed other organs of the immune system, such as the spleen. B cells in the spleen exhibit phenotypic changes, switching from expressing high levels of Lewis x to becoming Lewis x-low.
B Cell Function
B cells, or bursa-derived cells, are essential for the adaptive immune response in chickens. They play a crucial role in antibody production and humoral immunity. Mature B cells often require T cells to help produce antibodies, but a subset of mature B cells, known as thymus-independent B cells, can respond to antigens without T cell assistance. B cells acquire the ability to distinguish between self and non-self-antigens and form memories of previous antigen encounters, enabling a more rapid response during subsequent interactions.
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Frequently asked questions
Chicken B cells go to the Bursa of Fabricius, a primary lymphoid organ unique to birds, where they undergo rapid expansion and complete the immune system.
The bursa of Fabricius is the primary site of B cell lymphopoiesis. Chicken B cells undergo discrete developmental changes in surface carbohydrate structure, which directs lymphocyte migration during embryogenesis.
After leaving the Bursa of Fabricius, B cells travel to other parts of the chicken's lymphatic system, including the spleen and lymph nodes.
