Bursal Secretory Dendritic Cells Research Articles

Effect of Intermediate Plus Vaccine and vvIBDV on Bursa Secretory Cells and Their Glycoprotein Production.

There are two types of secretory cells in the chicken bursa of Fabricius (BF): (a) interfollicular epithelial cells (IFE), and (b) bursal secretory dendritic cells (BSDC) in the medulla of bursal follicles. Both cells produce secretory granules, and the cells are highly susceptible to IBDV vaccination and infection. Before and during embryonic follicular bud formation, an electron-dense, scarlet-acid fuchsin positive substance emerges in the bursal lumen, the role of which is unknown. In IFE cells, IBDV infection may induce rapid granular discharge, and in several cells, peculiar granule formation, which suggests that the glycosylation of protein is injured in the Golgi complex. In control birds, the discharged BSDC granules appear in membrane-bound and subsequently solubilized, fine-flocculated forms. The solubilized, fine-flocculated substance is Movat-positive and can be a component of the medullary microenvironment, which prevents the medullary B lymphocytes from nascent apoptosis. Vaccination interferes with the solubilization of the membrane-bound substance, resulting in: (i) aggregation of a secreted substance around the BSDC, and (ii) solid lumps in the depleted medulla. The non-solubilized substance is possibly not "available" for B lymphocytes, resulting in apoptosis and immunosuppression. In IBDV infection, one part of the Movat-positive Mals fuse together to form a medullary, gp-containing "cyst". The other part of Mals migrate into the cortex, recruiting granulocytes and initiating inflammation. During recovery the Movat-positive substance appears as solid, extracellular lumps between the cells of FAE and Mals. Possibly the Mals and Movat-positive extracellular lumps glide into the bursal lumen via FAE to eliminate cell detritus from the medulla.

Glycoprotein Production by Bursal Secretory Dendritic Cells in Normal, Vaccinated, and Infectious Bursal Disease Virus (IBDV)-Infected Chickens.

The aim of this study is to follow the gp production in IBDV-vaccinated and challenged birds. The progress of IBDV infection was monitored using anti-VP2 immunocytochemistry, light and transmission electron microscopy. In the medulla of the bursal follicle, the Movat pentachrome staining discovered an extracellular glycoprotein (gp) produced by bursal secretory dendritic cells (BSDCs). The secretory granules of BSDCs either discharge resulting in extracellular gp or fuse together forming intracellular corpuscles. The double fate of granules suggests a dual function of BSDCs: (a.) For the discharged granules, gp contributes to the medullary microenvironment (ME). (b.) The intracellular corpuscles may be the sign of BSDC transformation to a macrophage-like cell (Mal). Infectious bursal disease virus (IBDV) infection accelerates the BSDC transformation to Mal. The decreased number of BSDCs is feedback for the precursor cells of BSDCs lodging in the cortico-medullary epithelial arches (CMEA), where they proliferate. Opening the CMEA, the precursor cells enter the medulla, and differentiate to immature BSDCs. The virus uptake in the corpuscles prevents the granular discharge resulting in the absence of gp and alteration in ME. In vaccine-take birds, the mitotic rate of BSDC precursor cells cannot restore the precursor pool; therefore, in the case of IBDV challenge, the number of newly formed BSDCs is too low for outbreak of clinical disease. The BSDCs, as a primary target of IBDV, may contribute to the long-lasting immunosuppressive status of IBDV-infected chickens.

The bursal secretory dendritic cell (BSDC) and the enigmatic chB6+ macrophage-like cell (Mal)

The bursal secretory dendritic cell (BSDC) was discovered more than 40 yr ago. It is a highly polarized, granulated cell, locating in the medulla of bursal follicle. The cytoplasmic granules either discharge or fuse together forming large, irregular-shaped, dense bodies. Formation of the dense bodies could be the first sign of BSDC transformation to macrophage-like cell (Mal) which is the result of terminal maturation of BSDC. The BSDC is non-phagocytic, unlike Mal. The discharged substance may be attached to the cell membrane (membrane-bound form) and after detaching, appears as a flocculated substance in the extracellular space of medulla. Movat pentachrome staining shows, that this substance is a glycoprotein (gp), which may be contributed to the microenvironment of the medulla. Medullary lymphocytes are floating in the gp. Precursors of the BSDC locate in the corticomedullary epithelial arches, which operate under the effect of Notch/Serrate signaling. The Notch signaling determines the fate of lymphoblast-like precursor cells and inhibits the appearance of immunoglobulin heavy chain. In the arches, the precursor cells proliferate and entering the medulla differentiate. The dense bodies pack the virus particles, which prevents the granular discharge, resulting in disappearance of extracellular gp, but gp emerges inside the virus containing Mal. In infected birds, the Mal contains either apoptotic cells or virus particles. If vaccination or infectious bursal disease virus (IBDV) infection use up the BSDC precursors, the recovery of follicle is critical.

Coronavirus infection retards the development of the cortico-medullary capillary network in the bursa of Fabricius of chicken.

Coronavirus infection delays the development of the cortico-medullary (CM) capillary network which results in retarded development of bursal follicles. The smaller size of the medulla in the coronavirus-infected birds may lead to a lower number of B lymphocytes and bursal secretory dendritic cells, which negatively affects the reactivity and efficacy of the immune system. Contrary to the wild-type infectious bronchitis virus (IBV) strain, infection induced by H120 vaccine virus exerts only a moderate influence on caveolin-1 expression of the CM capillary web and on follicular development compared to the untreated controls.

Expression of caveolin-1 in the interfollicular but not the follicle-associated epithelial cells in the bursa of fabricius of chickens.

The surface epithelium of the bursa of Fabricius consists of interfollicular (IFE) and follicle-associated epithelium (FAE). The IFE comprises (i) cylindrical-shaped secretory cells (SC) and (ii) cuboidal basal cells (BCs). The FAE provides histological and two-way functional connections between the bursal lumen and medulla of the follicle. We used a carbon solution and anti-caveolin-1 (Cav-1) to study the endocytic activity of FAE. Carbon particles entered the intercellular space of FAE, but the carbon particles were not internalized by the FAE cells. Cav-1 was not detectable in the FAE cells or the medulla of the bursal follicle. The absence of Cav-1 indicates that no caveolin-mediated endocytosis occurs in the FAE cells, B cells, bursal secretory dendritic cells (BSDC), or reticular epithelial cells. Surprisingly, a significant number of Cav-1 positive cells can be found among the SC, which are designated SC II. Cav-1 negative cell are called SC I, and they produce mucin for lubricating the bursal lumen and duct. Occasionally, BCs also express Cav-1, which suggests that BC is a precursor of a SC. Transmission electron microscopy confirmed the existence of type I and II SC. The SC II are highly polarized and have an extensive trans-Golgi network that is rich in different granules and vesicles. Western blot analysis of bursa lysates revealed a 21-23 kDa compound (caveolin) and Filipin fluorescence histochemistry provided evidence for intracellular cholesterol. High amount of cholesterol in the feces shows the cholesterol efflux from SC II. The presence of Cav-1 and cholesterol in SC II indicates, that the bursa is a complex organ in addition to possessing immunological function contributes to the cholesterol homeostasis in the chickens.

Ontogeny of ramified CD45 cells in chicken embryo and their contribution to bursal secretory dendritic cells.

Embryonic tissues contain highly ramified stellate-shaped cells expressing CD45 and MHC II antigens but their origin and immunophenotype are unknown. Using staged avian embryos and cell-type-specific antibodies, we establish a detailed spatiotemporal ontogeny of cells that express CD45, the earliest marker of hematopoietic stem cells in the chick. CD45 immunostaining marks three distinct embryonic cell populations: round, ramified and amoeboid cells. The round and ramified CD45+ cells appear first in yolk-sac blood islands before the onset of circulation. A subpopulation of round cells co-expresses the thrombocyte-specific CD51/CD61 antigen. Amoeboid cells express macrophage-specific antigens and frequently occur in regions of apoptosis. Ramified cells are distributed uniformly in the embryonic mesenchyme, colonize lymphoid and non-lymphoid organs and later express MHC II. To study the origin of CD45+ cells, 2-day-old chick embryos were ablated from the yolk sac before the establishment of circulation and incubated for 2-5days. Large numbers of CD45+MHC II+ ramified cells differentiated in the yolk sac. Yolk-sac chimeras were generated by grafting embryos into GFP-expressing de-embryonated yolk sacs. GFP/CD45 co-expressing ramified and amoeboid cells colonized all organ primordia in the donor embryo. We also recombined GFP+ yolk sac with the bursa of Fabricius and found ramified GFP+CD45+ cells in the bursa where they differentiated into dendritic cells. Thus, CD45 cells are first present in the yolk sac during primitive hematopoiesis and then migrate from the extra-embryonic yolk sac to give rise to cells throughout all organ primordia, including dendritic cells in the bursa of Fabricius.

Avian dendritic cells: Phenotype and ontogeny in lymphoid organs

Dendritic cells (DC) are critically important accessory cells in the innate and adaptive immune systems. Avian DCs were originally identified in primary and secondary lymphoid organs by their typical morphology, displaying long cell processes with cytoplasmic granules. Several subtypes are known. Bursal secretory dendritic cells (BSDC) are elongated cells which express vimentin intermediate filaments, MHC II molecules, macrophage colony-stimulating factor 1 receptor (CSF1R), and produce 74.3+ secretory granules. Avian follicular dendritic cells (FDC) highly resemble BSDC, express the CD83, 74.3 and CSF1R molecules, and present antigen in germinal centers. Thymic dendritic cells (TDC), which express 74.3 and CD83, are concentrated in thymic medulla while interdigitating DC are found in T cell-rich areas of secondary lymphoid organs. Avian Langerhans cells are a specialized 74.3−/MHC II+ cell population found in stratified squamous epithelium and are capable of differentiating into 74.3+ migratory DCs. During organogenesis hematopoietic precursors of DC colonize the developing lymphoid organ primordia prior to immigration of lymphoid precursor cells. This review summarizes our current understanding of the ontogeny, cytoarchitecture, and immunophenotype of avian DC, and offers an antibody panel for the in vitro and in vivo identification of these heterogeneous cell types.

Retrospection to discovery of bursal function and recognition of avian dendritic cells; past and present

In 1954 the discovery of bursal function was one of the major contributions to the formation of the T and B cell concept of immunology. In 1978 the avian dendritic cells; bursal secretory dendritic cell (BSDC) and follicular dendritic cell (FDC) in the cecal tonsil were recognized. In 1982 the interdigitating dendritic cell was described in the periarteriolar lymphatic sheath (PALS) of the spleen. This paper is a retrospection of the stories of the discovery of bursal function and recognition of avian dendritic cells and includes the markers which can be used for monitoring and characterizing avian dendritic cells.

Locally applied testosterone is a novel method to influence the development of the avian bursa of Fabricius

Bursa of Fabricius is a primary lymphoid organ in the birds and its microenvironment is responsible for the B cell maturation. The epithelial anlage develops through epithelial–mesenchymal interaction into which –as a third party – hemopoietic cells immigrate. Chemical bursectomy can be made by dipping or injecting eggs by testosterone solution. Both methods of treatments compromise the development of the whole embryo. Heparin beads of 70–150 µm in diameter were soaked in 5% testosterone solution and implanted into the tail bud of 4 day old chicken embryos before the bursal anlage began to develop at embryonic day 5. The locally applied testosterone inhibited the differentiation of the bursal secretory dendritic cell (BSDC) precursors, which were the inducer of the follicular epithelial bud formation. Testosterone treatment did not blocked the entering of B cells and other hemopoietic cells into the bursal mesenchyme, but in the absence of BSDC precursor cells the dendroepithelial microenvironment is not established, subsequently B cell precursors are lodged only in the bursal mesenchyme. This method is suitable for locally introducing other regulatory molecules, involved in bursal development.

Impact of heterophil granulocyte depletion caused by 5-fluorouracil on infectious bursal disease virus infection in specific pathogen free chickens

The purpose of this study was to investigate the influence of the cytostatic drug, 5-fluorouracil (5-FU), which causes depletion of heterophil granulocytes, on clinical symptoms and histological lesions during the progress of infectious bursal disease virus (IBDV) infection in chickens. The aim was to disclose the mechanism behind the clinical disease symptoms. Three groups of specific pathogen free chickens were used for the experiment. Chickens in groups 1 and 3 were pretreated with 5-FU, while chickens in group 2 were treated with a placebo. After 5 days, the chickens in groups 2 and 3 were inoculated with the classical IBDV strain F52/70. Bursae of Fabricius were sampled at fixed intervals, and the progress of the infection was monitored by various histological techniques and reverse transcriptase-polymerase chain reaction (RT-PCR). We found correlation between histological observations and RT-PCR results. In the 5-FU pretreated chickens, IBDV caused only mild clinical symptoms, even though histological alterations similar to alterations caused by IBDV were still observed. The 5-FU pretreatment resulted in severe heterophil granulocyte depletion by days 2 and 3 after infection (post inoculation) and increased numbers of bursal secretory dendritic cells in the medulla of the follicles. IBDV infection seemed to induce fusion of secretory dendritic cells, resulting in formation of multinucleated giant cells, loaded with apoptotic B cells and virus particles associated with granules of bursal secretory dendritic cells. Our results indicate that the heterophil granulocytes together with the bursal secretory dendritic cells contribute to the outbreak and/or progress of clinical symptoms.

In ovo vitelline duct ligation results in transient changes of bursal microenvironments

The avian bursa of Fabricius has a direct connection to the cloaca via the bursal duct. Using the bursal duct ligation technique, it has been clearly shown that the B cells of the bursal follicles develop under the influence of cloacal antigens. These antigens have been suggested to be present on the bursal secretory dendritic cells in immunoglobulin G (IgG)-containing complexes. We studied the effect of maternal (yolk) antigens on the early development of B cells and the appearance of IgG-containing complexes of the bursal dendritic cells with a novel embryo manipulation technique, in ovo vitelline duct ligation. This operation blocked the direct (intestinal) transport of yolk substances into the intestine, but left the vitelline circulation intact. Vitelline duct ligation performed on embryonic day 17 resulted in serious but transient bursal underdevelopment during the first week of life: (1) IgG and the follicular dendritic cell marker 74.3 were not detectable on the bursal secretory dendritic cells, in spite of a normal serum IgG level and free communication with the cloacal lumen; (2) the number of B cells in the follicles was greatly reduced and they showed an altered phenotype, resembling that of the prebursal B cells. The intracloacal administration of different proteins effectively restored the bursal phenotype. These data suggest that maternal antigens indirectly help the maturation of bursal secretory dendritic cells and concomitantly that of B cells during the first week of life.

Our current understanding of humoral immunity of poultry

With the inception of the modem era of avian immunology in the mid-1950s following the discovery of the role of the bursa of Fabricius (BF), steady and persistent research has been conducted by scientists. Findings that have advanced our knowledge of humoral immunity (HI) include the distinct role of B cells in antibody production, explanation of antibody diversity by gene rearrangement and conversion, identification of a bursal secretory dendritic cell and dendritic cells of the spleen, characterization of cytokine factors, and mapping of MHC class II genes. Numerous experiments have been conducted to evaluate the HI responses of poultry through antibody titer determination and quantitation of immunoglobulin concentrations in the humors. Antibody studies have included the genetics of HI regarding both quantity and persistence of responses as influenced by multiple genes and MHC haplotypes. In the past decade, emergence of monoclonal antibody reagents to identify cell surface markers on B cells and accessory cells has allowed for discrimination of HI functions of cells by immunohistochemistry, flow cytometry, and other techniques. The signaling mechanisms of avian B cells are becoming better understood. Continued work in avian immunity has led to more advances in identification, characterization, and gene sequencing of important cytokines. Additional work is required to identify and sequence genes for many more cytokines that have direct effects on B cells. A brief history of research published on avian B cells is presented with an examination of some of the important research findings reported in recent years.

Origin of the bursal secretory dendritic cell.

The origin of vimentin-positive secretory dendritic cells of the bursa of Fabricius was studied by chick-quail chimera, parabiosis and immunohistochemistry using species-specific monoclonal antibodies. Quail bursal primordia of different ages were transferred to coelomic cavity of 3-day-old chicken embryos and further incubated for 18 days. In transplanted quail bursas the secretory dendritic cells of chicken and quail origin were detected by double staining of vimentin plus 74.3 and vimentin plus QCPN monoclonal antibodies, respectively. In bursal primordia of 5- and 6-day-old quail embryos both dendritic cells and B cells were of host, i.e. chicken origin. Mixed dendritic cell population of quail and chick origin emerged in chimeric birds of 6.5 days of age. In quail embryos transplanted at 7 and 8 days of age both dendritic cells and B cells were mixed i.e. of chicken and quail origin. Bursal secretory dendritic cells and medullary epithelial cells create "dendro-epithelial tissue" to receive pre-B cells. Colonization of dendro-epithelial tissue by pre-B cells initiates at day 7, thus the colonization of bursal anlage by blood-borne cells is a two-step process; entering of dendritic cells at day 6.5 is followed by that of B cells at day 7 and afterwards. It is discussed that bursal secretory dendritic cells and their product are key elements of bursal function therefore the mammalian bursa equivalent organ might be represented by a cell, which is analogous with the bursal secretory dendritic cell.

Diverse expression of the K-1 antigen by cortico-medullary and reticular epithelial cells of the bursa of Fabricius in chicken and guinea fowl

The immunocytochemical study of the K-1 monoclonal antibody indicates that the epithelial components of the bursa of Fabricius of the chicken and guinea fowl express the K-1 positive molecule. During embryogenesis, the K-1 antigen expression appears together with the bud-formation. As the number of B cells increases in the developing follicle, the K-1 expression gradually diminishes in the medullary reticular epithelial cells and completely ceases by hatching, which suggests that the molecule is developmentally regulated. After hatching, the expression of the molecule is restricted to the sealing off zone of the lymphoepithelial or medullary region of the follicle: i.e. to the cortico-medullary (CM) epithelial cells and the follicle associated epithelium (FAE) supporting cells in guinea fowl and to the latter ones in the chicken. The expression of the K-1 antigen by these epithelial components may support their structural identity. After hatching, the K-1 molecule is restricted to the CM epithelial cells and/or FAE supporting cells, which suggests that the function of the embryonic epithelial bud is taken over by the CM epithelial cells. The K-1 positive CM epithelial cells form arches, which encompass blast-like cells. The possible relationship of the CM epithelial cells and blast-like cells, which may represent the precursors of bursal secretory dendritic cells is discussed.

Development of the follicle-associated epithelium and the secretory dendritic cell in the bursa of fabricius of the guinea fowl (Numida meleagris) studied by novel monoclonal antibodies

Two stromal elements, follicle-associated epithelium and secretory dendritic cells of the bursa of Fabricius were studied by light microscopy and two novel MAbs, that were produced against splenic cell suspensions of guinea fowls. Both antigens recognized by these MAbs, designated GIIF3 and NIC2, are localized in the cytoplasm of the stromal cells, and their molecular weights are 50 and 30 kD, respectively. During embryogenesis the GIIF3 and NIC2 cells emerge in the mesenchyme of the folds before follicle formation. The GIIF3 and the NIC2-positive cells accumulate under the surface epithelium of the plicae and migrate into the epithelium, that precedes the bud-formation. From the bud, the GIIF3-positive cells migrate up to the luminal surface, and they transform to distinct, highly polarized follicle-associated epithelial cells. Single GIIF3-positive cells are also present in the interfollicular epithelium. The NIC2 MAb recognized mesenchymal cells harbor in the lymphoepithelial compartment of the folliculus, and they elaborate cytoplasmic granules. Around Day 20 of embryogenesis large amount of NIC2-positive substance appear extracellularly in the medulla and around it. This period well correlates with the starting up of the bursal functions; clonal expansion of B cells, and generation of immune repertoire. After hatching the NIC2 stainability diminishes, and it is restricted to the medullary bursal secretory dendritic cells. The NIC2-positive, possibly elderly bursal secretory dendritic cells, are capable for migration into the follicle-associated epithelium. In eight-day old birds some cells of the follicle-associated epithelium reveals temporary NIC2 positivity, that may prove the transport of the follicle-associated epithelial cells into luminal direction. By 12 weeks of age the presence of NIC2-positive substance in the intercellular space of the FAE, rather than in the cells of FAE may indicate the termination of the transport of secretory substance. In conclusion, two types of mesenchymal cells enter the surface epithelium of the bursal folds. The GIIF3-positive cells appear on the luminal surface of the follicles and occupy the place of the follicle-associated epithelial cells. The NIC2-positive cells become secretory in nature and differentiate to bursal secretory dendritic cells. The follicle formation possibly, requires the joint presence of both GIIF3 and NIC2 cells in the epithelium.

B7 11:45 Effect of infectious bursal disease virus (IBDV) on the bursal secretory dendritic cell

B7 11:45 Effect of infectious bursal disease virus (IBDV) on the bursal secretory dendritic cell

Distribution of bursal secretory dendritic cells in the chicken.

The bursa of Fabricius provided the microenvironment for B-cell differentiation. Continuous contact between lymphoid cells and antigen in the bursa further suggested that antigenic material has an important influence on the maintenance and development of B cells in the bursa. In addition, a dendritic cell, the bursal secretory dendritic cell (BSDC), has been identified in the medulla. The hypothesis that, in the bursal follicles, the contact between the lymphoid cells and the antigen may be mediated by dendritic cells prompted us to identify a bursal dendritic cell that becomes activated after contact with the antigen. A polyclonal antiserum to S-100 protein was used to identify bursal dendritic cells because S-100 protein, a calcium-binding protein, has been shown to be a marker for the identification of chicken dendritic cells following recent contact with antigen. At every age investigated, S-100-positive cells showed a location and shape identical to those described for BSDCs. Positive cells were found within and under the follicle-associated epithelial cells (FAE), indicating that these cells were strategically placed where they would encounter the antigen. In addition, positive cells were found arranged along the corticomedullary junction, which is a regenerative zone for the BSDC. After 10 weeks of age, the number of positive cells dramatically decreased, suggesting that the endocytic activity of the FAE may become impaired as the bursa regresses. The polyclonal antiserum to S-100 protein identified the BSDCs in the bursal follicles. Positive cells may be BSDCs that have undergone a functional activation after contact with the antigen. These cells may have a role as antigen-presenting cells in the bursal follicles. Hence, these cells may be involved in the events that lead to B-cell differentiation.

Distribution of bursal secretory dendritic cells in the chicken

Distribution of bursal secretory dendritic cells in the chicken

Dendritic cells in the bursal follicles and germinal centers of the chicken's caecal tonsil express vimentin but not desmin.

Immunohistochemical studies with anti-vimentin and anti-desmin monoclonal antibodies were designed to determine the origin of bursal secretory dendritic cells (SDC) and follicular dendritic cells. The binding sites of anti-vimentin, anti-desmin, and anti-chicken-IgG specific monoclonal antibodies were visualized with a biotinylated anti-mouse-IgG, ABC Elite kit, and 4-chloronaphthol. Cells were double stained (anti-vimentin and rabbit anti-chicken-IgG Fc) to determine if the vimentin positive cells possessed surface IgG. Vimentin positive cells were observed in the cortex and medulla of the bursa and germinal center and lymphoepithelial compartment of the caecal tonsil. The mesenchymal reticular cell, the basic supporting cell of the germinal center, was stained prominently by anti-vimentin and anti-desmin. Both antibodies stained the bursal cortex but only anti-vimentin bound the bursal secretory dendritic cell of the medulla. In addition to being vimentin positive and desmin negative, the bursal secretory dendritic cell possessed and the follicular dendritic cell appeared to possess IgG on their surfaces. In all the observations, B-cells were vimentin negative. These studies suggest that follicular dendritic cells and mesenchymal reticular cells in the caecal tonsil's germinal centers may be functionally different cell populations while the bursal secretory dendritic cell and follicular dendritic cell of the caecal tonsil may have a common origin.

Embryogenesis of the Bursa of Fabricius: Stem Cell, Microenvironment, and Receptor-Paracrine Pathways

The bursa of Fabricius is an ideal model system to answer the plethora of questions related to the origin of B cells and microenvironmental issues leading to the education of the stem cell. Prior to the 1960s, lymphocytes were thought to be derived from epithelial or mesenchymal cells. Later work demonstrated the bloodborne nature of the stem cell contributing to B cells. Stem cells entered the bursa of quail and chicken between 7 and 11 and 7.5 and 14 d of embryogenesis, respectively. Interspecific chimeric studies, quail and chick, emphasized the intraembryonic origin and sites of the stem cell.The bloodborne and stromal cells that contribute to the microenvironment of the bursa orchestrate the events leading to B cell differentiation. The separation of the endodermal and mesodermal components of the bursa revealed a singular role for the endoderm in the genesis of the bursa but did not exclude a role for the mesoderm. A dark mesenchymal cell was shown to play a role in bud formation. This cell gave rise to the bursal secretory dendritic cell (BSDC), unique in its membrane association with IgG. A receptor-paracrine thesis has been proposed to explain the interaction between in-frame B cells and Ig-positive BSCD in the expansion of in-frame B cells and the subsequent development of the B cell repertoire. Cell adhesion molecules have been integrated into this thesis.