The ontogeny and cloning of B cells in the bursa of Fabricius

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.

Development and characterization of a CRISPR/Cas9-mediated RAG1 knockout chicken model lacking mature B and T cells

Although birds have been used historically as a model animal for immunological research, resulting in remarkable achievements, immune cell development in birds themselves has yet to be fully elucidated. In this study, we firstly generated an immunodeficient chicken model using a CRISPR/Cas9-mediated recombination activating gene 1 (RAG1) knockout, to investigate avian-specific immune cell development. Unlike previously reported immunoglobulin (Ig) heavy chain knockout chickens, the proportion and development of B cells in both RAG1^+/- and RAG1^-/- embryos were significantly impaired during B cell proliferation (embryonic day 16 to 18). Our findings indicate that, this is likely due to disordered B cell receptor (BCR)-mediated signaling and interaction of CXC motif chemokine receptor (CXCR4) with CXCL12, resulting from disrupted Ig V(D)J recombination at the embryonic stage. Histological analysis after hatching showed that, unlike wild-type (WT) and RAG1^+/- chickens, lymphatic organs in 3-week old RAG1^-/- chickens were severely damaged. Furthermore, relative to WT chickens, RAG1^+/- and RAG1^-/- birds had reduced serum Igs, fewer mature CD4⁺ and CD8⁺ T lymphocytes. Furthermore, BCR-mediated B cell activation in RAG1^+/- chickens was insufficient, leading to decreased expression of the activation-induced deaminase (AID) gene, which is important for Ig gene conversion. Overall, this immunodeficient chicken model underlines the pivotal role of RAG1 in immature B cell development, Ig gene conversion during embryonic stages, and demonstrates the dose-dependent regulatory role of RAG1 during immune cell development. This model will provide ongoing insights for understanding chicken immune system development and applied in the fields of immunology and biomedical science.

Full-Length Transcriptome: A Reliable Alternative for Single-Cell RNA-Seq Analysis in the Spleen of Teleost Without Reference Genome

Fish is considered as a supreme model for clarifying the evolution and regulatory mechanism of vertebrate immunity. However, the knowledge of distinct immune cell populations in fish is still limited, and further development of techniques advancing the identification of fish immune cell populations and their functions are required. Single cell RNA-seq (scRNA-seq) has provided a new approach for effective in-depth identification and characterization of cell subpopulations. Current approaches for scRNA-seq data analysis usually rely on comparison with a reference genome and hence are not suited for samples without any reference genome, which is currently very common in fish research. Here, we present an alternative, i.e. scRNA-seq data analysis with a full-length transcriptome as a reference, and evaluate this approach on samples from Epinephelus coioides-a teleost without any published genome. We show that it reconstructs well most of the present transcripts in the scRNA-seq data achieving a sensitivity equivalent to approaches relying on genome alignments of related species. Based on cell heterogeneity and known markers, we characterized four cell types: T cells, B cells, monocytes/macrophages (Mo/MΦ) and NCC (non-specific cytotoxic cells). Further analysis indicated the presence of two subsets of Mo/MΦ including M1 and M2 type, as well as four subsets in B cells, i.e. mature B cells, immature B cells, pre B cells and early-pre B cells. Our research will provide new clues for understanding biological characteristics, development and function of immune cell populations of teleost. Furthermore, our approach provides a reliable alternative for scRNA-seq data analysis in teleost for which no reference genome is currently available.

Effects of Sunset Yellow FCF on Immune System Organs During Different Chicken Embryonic Periods

Introduction

Sunset Yellow FCF (SY), used frequently in ready-made foods, cosmetics, and the pharmaceutical industry, may cause many health problems. This study is intended to evaluate the morphological and cellular effects of SY on the embryonic chicken immune system throughout incubation.

Material and Methods

Babcock white leghorn eggs were randomly divided into four groups. Besides a control group, there were three treatment groups which received a single injection of 200, 1,000, or 2,000 ng of SY into the air sac just before incubation. The eggs were opened on the 10^th, 13^th, 16^th, and 21^st days of incubation. Samples of the bursa of Fabricius, thymus, and spleen were taken from embryos. Serial sections of 5 μm thickness were stained with histological methods and routine histological procedures were performed.

Results

An increase in the spleen volume was determined as the hatching time of the chicks approached. The highest eosinophil ratio was found in the SY_1,000 and SY_2,000 groups (P < 0.05), where the most significant change was developmental retardation in the thymus. In the bursa of Fabricius, there was less lymphocyte accumulation and eosinophilic cell infiltration with increasing doses.

Conclusion

It was concluded that in ovo administered SY has undesired effects on embryonic development of the bursa of Fabricius, spleen, and thymus, and on spleen volume.

In and Out of the Bursa-The Role of CXCR4 in Chicken B Cell Development

In contrast to mammals, early B cell differentiation and diversification of the antibody repertoire in chickens do not take place in the bone marrow but in a specialized gut associated lymphoid tissue (GALT), the bursa of Fabricius. During embryonic development, B cell precursors migrate to the bursa anlage, where they proliferate and diversify their B cell receptor repertoire. Around hatch these diversified B cells start to emigrate from the bursa of Fabricius and populate peripheral lymphoid organs, but very little is known how the migratory processes are regulated. As CXCL12 (syn. SDF-1) and CXCR4 were shown to be essential for the control of B cell migration during the development of lymphoid tissues in mammals, we analyzed expression and function of this chemokine/chemokine-receptor pair in the chicken bursa. We found a strong variation of mRNA abundance of CXCL12 and CXCR4 in different stages of bursa development, with high abundance of CXCL12 mRNA in the bursa anlage at embryonic day 10 (ED10). In situ hybridization demonstrated disseminated CXCL12 expression in the early bursa anlage, which condensed in the developing follicles and was mainly restricted to the follicle cortex post-hatch. Flow cytometric analysis detected CXCR4 protein already on early B cell stages, increasing during bursal development. Post-hatch, a subpopulation with the hallmarks of emigrating B cells became detectable, which had lower CXCR4 expression, suggesting that downregulation of CXCR4 is necessary to leave the CXCL12-high bursal environment. In vivo blockade of CXCR4 using AMD3100 at the time of B cell precursor immigration strongly inhibited follicle development, demonstrating that CXCL12 attracts pre-bursal B cells into the bursal anlage. Altogether, we show that CXCL12 and its receptor CXCR4 are important for both populating the bursa with B cells and emigration of mature B cells into the periphery post hatch, and that CXCR4 function in primary B cell organs is conserved between mammals and birds.

Clonal anergy of CD117+chB6+ B cell progenitors induced by avian leukosis virus subgroup J is associated with immunological tolerance

BACKGROUND

The pathogenesis of immunological tolerance caused by avian leukosis virus subgroup J (ALV-J), an oncogenic retrovirus, is largely unknown.

RESULTS

In this study, the development, differentiation, and immunological capability of B cells and their progenitors infected with ALV-J were studied both morphologically and functionally by using a model of ALV-J congenital infection. Compared with posthatch infection, congenital infection of ALV-J resulted in severe immunological tolerance, which was identified as the absence of detectable specific antivirus antibodies. In congenitally infected chickens, immune organs, particularly the bursa of Fabricius, were poorly developed. Moreover, IgM-and IgG-positive cells and total immunoglobulin levels were significantly decreased in these chickens. Large numbers of bursa follicles with no differentiation into cortex and medulla indicated that B cell development was arrested at the early stage. Flow cytometry analysis further confirmed that ALV-J blocked the differentiation of CD117+chB6+ B cell progenitors in the bursa of Fabricius. Furthermore, both the humoral immunity and the immunological capability of B cells and their progenitors were significantly suppressed, as assessed by (a) the antibody titres against sheep red blood cells and the Marek's disease virus attenuated serotype 1 vaccine; (b) the proliferative response of B cells against thymus-independent antigen lipopolysaccharide (LPS) in the spleen germinal centres; and (c) the capacities for proliferation, differentiation and immunoglobulin gene class-switch recombination of B cell progenitors in response to LPS and interleukin-4(IL-4) in vitro.

CONCLUSIONS

These findings suggested that the anergy of B cells in congenitally infected chickens is caused by the developmental arrest and dysfunction of B cell progenitors, which is an important factor for the immunological tolerance induced by ALV-J.

2D DIGE analysis of the bursa of Fabricius reveals characteristic proteome profiles for different stages of chicken B-cell development

Antibody producing B-cells are an essential component of the immune system. In contrast to human and mice where B-cells develop in the bone marrow, chicken B-cells develop in defined stages in the bursa of Fabricius, a gut associated lymphoid tissue. In order to gain a better understanding of critical biological processes like immigration of B-cell precursors into the bursa anlage, their differentiation and final emigration from the bursa we analyzed the proteome dynamics of this organ during embryonic and posthatch development. Samples were taken from four representative developmental stages (embryonic day (ED) 10, ED18, day 2, and day 28) and compared in an extensive 2D DIGE approach comprising six biological replicates per time point. Cluster analysis and PCA demonstrated high reliability and reproducibility of the obtained data set and revealed distinctive proteome profiles for the selected time points, which precisely reflect the differentiation processes. One hundred fifty three protein spots with significantly different intensities were identified by MS. We detected alterations in the abundance of several proteins assigned to retinoic acid metabolism (e.g. retinal-binding protein 5) and the actin-cytoskeleton (e.g. vinculin and gelsolin). By immunohistochemistry, desmin was identified as stromal cell protein associated with the maturation of B-cell follicles. Strongest protein expression difference (10.8-fold) was observed for chloride intracellular channel 2. This protein was thus far not associated with B-cell biology but our data suggest an important function in bursa B-cell development.

Blocked B cell differentiation and emigration support the early growth of Myc-induced lymphomas

Avian leukosis virus induces lymphoma in chickens after proviral integration within the c-Myc gene, and subsequent expansion of Myc-overexpressing lymphocytes within transformed bursal follicles. The clonal expansion of these follicles allowed us to examine how Myc influences cell differentiation, growth, and apoptosis in lymphoid progenitors soon after the onset of Myc overexpression. Immunohistochemical analysis of developmental markers established that Myc overexpression consistently blocks lymphocyte differentiation at a late embryonic stage. Myc-transformed follicles also grow much more rapidly than normal follicles. This rapid growth is not mediated by suppression of apoptosis, as normal and Myc-transformed follicles showed similar rates of cell death by TUNEL immunohistochemical analysis of cells undergoing DNA degradation. Measurements of DNA synthesis and mitotic index showed modest effects of Myc to increase lymphocyte proliferation, as normal lymphocytes already divide rapidly. The major mechanism mediating rapid growth of transformed follicles instead involved failure of myc-overexpressing lymphocytes to emigrate from transformed follicles, while normal lymphocytes actively emigrate after hatching, as measured by BrdU pulse-chase labeling and immunohistochemical measurements. This failure to undergo the normal program of differentiation and subsequent bursal retention of lymphocytes accounts for most of the growth of transformed follicles, while Myc-induced proliferation makes a smaller contribution.

Perinatal deletion of B cells expressing surface Ig molecules that lack V(D)J-encoded determinants in the bursa of Fabricius is not due to intrafollicular competition

During embryonic development, the avian bursa of Fabricius selects B cell precursors that have undergone productive V(D)J recombination for expansion in oligoclonal follicles. During this expansion, Ig diversity is generated by gene conversion. We have used retroviral gene transfer in vivo to introduce surface Ig molecules that lack V(D)J-encoded determinants into B cell precursors. This truncated mu heavy chain supports both B cell expansion within embryo bursal lymphoid follicles and gene conversion. We show that individual follicles can be colonized exclusively by cells expressing the truncated mu chain and lacking endogenous surface IgM, ruling out a requirement for V(D)J-encoded determinants in the establishment of bursal lymphoid follicles. In striking contrast to their normal development in the embryo, bursal cells expressing the truncated mu-chain exhibit reduced rates of cell division and increased levels of apoptosis after hatching. The level of apoptosis in individual follicles reflects the proportion of cells within the follicle that express the truncated mu-chain. In particular, high levels of apoptosis are associated with follicles containing exclusively cells expressing the truncated micro receptor. Thus, apoptotic elimination of such cells is not due to competition within the follicle by cells expressing endogenous surface IgM receptors. This provides the first direct demonstration that the regulation of B cell development in the avian bursa after hatching differs fundamentally from that seen in the embryo. The requirement for intact IgM expression when the bursa is exposed to exogenous Ag implicates a role for Ag in avian B cell development after hatching.

Progesterone receptor in chicken bursa of Fabricius and thymus: evidence for expression in B-lymphocytes

In the present work constitutive progesterone receptor (PR) expression in the chicken bursa of Fabricius was detected in the stromal, smooth muscle and follicular medullary cells and smooth muscle cells of blood vessels. PR expression was increased during sexual maturation and after estrogen treatment. Bursal medullary PR-positive cells were further characterized to be B-lymphocytes by flow cytometric analysis. In addition, estrogen induced expression of PR in the bursal FAE-cells (follicle-associated epithelial cells). In the thymus PR was expressed constitutively in the connective tissue cells of the capsule and interfollicular septa, in a few medullary cells and in vascular smooth muscle. The PR-positive medullary cells consisted of epithelial cells, large polygonal cells resembling macrophages and plasma cells. T-lymphocytes were PR-negative. Estrogen up-regulated PR expression in the thymus. Immunoblotting studies revealed that both isoforms of PR, i.e. PR-A and PR-B, were expressed in the bursa of Fabricius and thymus with PR-B dominance. These results suggest that the chicken primary lymphoid organs bursa and thymus are under regulation of estrogen and progesterone. Expression of PR in B-lymphocytes, macrophages and plasma cells in the chicken is documented for the first time and suggests evidence for direct action of progesterone on immune responses.

Immunohistochemical evidence that gonads and gonad-associated tissues are sites for enrichment with immunoglobulin-containing cells in adult chickens

Immunohistochemical staining of tissue sections prepared from gonads and gonad-associated tissues obtained from adult chickens was performed in order to assess the possibility that these tissues may be sites of enrichment with IgM-containing cells in various B lineages. Evidence is presented which suggests that IgM-containing B lineage cells are present in 1) the ovarian stroma and subcapsular areas of the ovary and 2) the interstitium and subcapsular areas of the epididymis of the testes. These represent new sites reported for B lineage cells in adult chickens. Some questions relevant to the physiologic, ontogenetic, and phylogenetic implications of these observations relative to vertebrate hematolymphopoietic processes are included.

Creation of immunoglobulin diversity by intrachromosomal gene conversion

Not all vertebrates create an immunoglobulin repertoire through the recombination of individual members of variable (V), diversity (D) and joining (J) gene segment families. In chickens, for example, a diverse set of immunoglobulins is created by intrachromosomal gene conversion of the single variable gene segments of the immunoglobulin heavy and light chain genes. Recent evidence from other species such as the rabbit suggests that gene conversion may be a more widespread mechanism for the creation of immunologic diversity than previously supposed.

A cytogenetic approach for detecting the selective toxicity of drugs in avian embryonic B and T lymphocytes

The developing immune system of late stage embryos and neonates may be particularly susceptible to the toxicity of drugs and environmental contaminants due to high rates of cell proliferation and ongoing processes of differentiation. We have developed a cytogenetic assay to study the mechanisms of the selective targeting of cyclophosphamide (CP) to B lymphocytes compared to T lymphocytes in chicken embryos at days 18-19 of incubation. 5-Bromo-2'-deoxyuridine (BrdU; 3 mg/200 microliters PBS; two doses; 3-h interval) was pipetted onto the inner shell membrane in order to label DNA of replicating lymphoid cells. CP (1.25-40 mg/kg) was injected 1 h after the initial BrdU dose, and the embryos were exposed to colcemid (10 micrograms/100 microliters H2O) at hour 17. Three hours later, the bursa and thymus were removed, and the lymphocytes were swollen in hypotonic solution, fixed, and processed through a fluorescence-plus-Giemsa technique to differentiate sister chromatids. Based on reductions in mitotic indices, B cells were approximately 213 times more susceptible than T cells to the cytotoxicity of CP. Because the mitotic indices of B and T cells were comparable (21.3 +/- 3.7%, vs. 25.5 +/- 6.9%), the differential toxicity cannot be ascribed to greater numbers of B cells being in mitosis. CP induced a dose-related increase in the sister-chromatid exchange frequency in B cells of up to 10.4-fold above controls, representing one of the most sensitive vertebrate systems for detecting the genotoxicity of CP. The average generation time was slowed from 9.8 +/- 0.3 h in control B cells to 19.4 +/- 0.9 h in embryos exposed to 10 mg CP/kg. Furthermore, an analysis of control SCE data from 56 embryos indicated that there was a significant overdispersion of B cells exhibiting relatively high SCE frequencies compared to a Poisson distribution. Our data indicate that the chicken embryo in the late developmental stage is a good model for detecting the presence and selective toxicity of drugs and environmental toxins in differentiating B and T lymphocytes in vivo.

The "adjuvant effect" of the polymorphic B-G antigens of the chicken major histocompatibility complex analyzed using purified molecules incorporated in liposomes

The polymorphic B-G region of the chicken major histocompatibility complex has previously been shown to mediate an "adjuvant effect" on the humoral response to other erythrocyte alloantigens. We demonstrate here that B-G molecules purified with monoclonal antibodies exert this adjuvant effect on the production of alloantibodies to chicken class I (B-F) molecules, when the two are in the same liposome. The adjuvant effect may in part be mediated by antibodies, since the antibody response to B-G molecules occurs much faster than the response to B-F molecules, and conditions in which antibodies to B-G are present increase the speed of the response to B-F molecules. We also found that the presence of B-G molecules in separate liposomes results in a lack of response to B-F molecules. In the light of this and other data, we consider the possible roles for the polymorphic B-G molecules, particularly for the generation of B cell diversity, in the immune systems of birds and other animals.

Selective expression of RAG-2 in chicken B cells undergoing immunoglobulin gene conversion

Chickens create their immunoglobulin (Ig) repertoires during B cell development in the bursa of Fabricius by intrachromosomal gene conversion. Recent evidence has suggested that Ig gene conversion may involve cis-acting DNA elements related to those involved in V(D)J recombination. Therefore, we have examined the potential role of the V(D)J recombination activating genes, RAG-1 and RAG-2, in regulating chicken Ig gene conversion. In contrast to the coexpression of RAG-1 and RAG-2 observed in mammalian B cells that undergo V(D)J recombination, chicken B cells isolated from the bursa of Fabricius express high levels of the RAG-2 mRNA but do not express RAG-1 mRNA. The developmental and phenotypic characteristics of the bursal lymphocytes and chicken B cell lines that express RAG-2 mRNA demonstrate that selective RAG-2 expression occurs specifically in B cells undergoing Ig diversification by gene conversion. These data suggest that RAG-2 plays a fundamental role in Ig-specific gene conversion.

TCDD inhibits the support of B-cell development by the bursa of Fabricius

The toxic effects produced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its congeners include inhibition of lymphoid development. We have previously found an inhibition of B-cell development in the bursa of Fabricius of chick embryos treated with TCDD congeners in ovo. In the present study, the bursae of ten-day-old chick embryos were removed and cultured on filter paper for 24 hr in media with or without TCDD or 3,3',4,4'-tetrachlorazoxybenzene (TCAOB). Following culture, the bursae were transplanted onto the chorioallantoic membrane (CAM) of ten-day-old eggs of the same strain or of a strain expressing a different B-cell surface alloantigen. After 5 days on the CAM the number of B-cells was determined or the grafts were sectioned for subsequent immunohistochemistry. Results were as follows: 1) A lower number of lymphoid cells (dose dependent) was observed in the TCDD-treated transplants amounting to 40-50% of that in the controls at 10(-9) M TCDD. Higher concentrations of TCDD compromised survival of the grafts. A single concentration of TCAOB (10(-8) M) was tested, resulting in a lymphoid cell number of 60% of that of the controls. 2) The bursal epithelium showed relatively normal development even in cases where B-cell development was affected. 3) Lymphoid cells in the grafted bursae originated from the embryo of the host egg. These findings suggest that the TCDD congeners had a direct effect on the bursa of Fabricius, leading to an inhibition of lymphoid development. It is likely that the microenvironment is affected by these compounds, thus resulting in a decrease in the attraction of stem cells and/or in the capacity to induce proliferation of the colonizing cells.

Cross-linking of the surface immunoglobulin on lymphocytes from the bursa of Fabricius results in second messenger generation

The bursa of Fabricius represents the major site of B lymphocyte development in avian species. Although the vast majority of bursal lymphocytes express cell surface immunoglobulin (sIg), it is generally considered that the bursa does not represent a significant site of antigen-induced B cell maturation to Ig secretion. However, the question as to whether antigen, either exogenous or self, can induce positive or negative selection of bursal lymphocytes in such a way as to modify the peripheral B cell repertoire remains open. Clearly, such intrabursal selection would require that bursal lymphocyte sIg have the molecular machinery to transduce signals into the cell as a consequence of its interaction with antigen. In this report we demonstrate that exposure of bursal lymphocytes to antibodies directed against sIg induced a rapid increase in cytosolic free calcium ion concentrations [Ca2+]i. Furthermore, such antibodies also induced a rapid increase in intracellular phosphatidic acid concentrations followed by a rise in intracellular phosphatidylinositol. Increased [Ca2+]i, phosphatidic acid and phosphatidylinositol levels required the cross-linking of sIg and were not induced by antibodies to other bursal cell surface antigens. Thus, cross-linking of the sIg on bursal lymphocytes results in second messenger generation, demonstrating that bursal sIg is a functional signal transduction element.

Somatic diversification of the chicken immunoglobulin light-chain gene

The bursa of Fabricius provides a unique organ for the study of lineage-specific development in a multicellular organism. Unlike mammalian B cells, B cells in the chicken develop in a single wave of differentiation, beginning with the commitment of progenitor cells to the B cell lineage between days 10 and 15 of embryogenesis. By day 18 of embryogenesis, all lymphoid progenitor cells capable of differentiation along the B cell lineage have migrated to the bursa of Fabricius. Following migration to the bursa, these lymphoid progenitors enter exponential growth and begin to populate each of the 10(4) bursal follicles. Between day 18 of embryogenesis and 2-4 weeks of age, B cells undergo a stage of bursal-dependent differentiation. By the end of this period, chickens are able to mount primary immune responses against virtually all antigens. In addition, by this time sufficient numbers of B cells have migrated from the bursa to peripheral lymphoid organs so that the B cell immune system can be maintained even if the bird is bursectomized. Bursectomy of chicks after 4 weeks of age has no long-term effects on the development and maintenance of the B cell immune system in adult birds. Because of the central nature of the surface Ig molecule to B cell development in mammals, the chicken IgL gene locus has been intensively studied during avian B cell development. The chicken IgL locus is a particular interest because it has only one V region capable of rearrangement. Rearrangement of the IgL gene is not dependent on the bursal environment. B cell progenitors rearrange their IgL gene between days 10-15 of embryogenesis, prior to migration to the bursa. IgL gene rearrangement occurs by a deletional mechanism in which a precise joining of the IgL recombination signal sequences leads to a circular episomal element. During this deletion it appears that single nonrandom bases are added to both the V and J coding segments. Subsequent V-J joining occurs at random. Most progenitor B cells appear to rearrange only a single IgL allele. The high frequency of in-frame alleles observed in avian B cell lines appears to result from the selective amplification of cells with productive IgL rearrangements during bursal development between days 12 and 18 of embryogenesis. To create an immunological repertoire, chickens must diversify the coding sequence of this single functional V gene segment during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Bu-2, a novel avian cell surface antigen on B cells and a population of non-lymphoid cells, is expressed homogeneously in germinal centers

A monoclonal antibody (mAb), Hy30, was generated against bursal cells. Under reducing conditions, it immunoprecipitates a cell surface antigen with an apparent molecular weight of 66 kD that is distinct from immunoglobulin and MHC class II. The antigen recognized by Hy30 is found on greater than 95% of all bursal cells, on a population of thymus resident cells, and on blood, spleen, and marrow mononuclear cells. Immunohistological analysis of tissue sections revealed that the majority of the staining by Hy30 occurs in the B cell areas of the spleen and thymus. Flow cytometric analysis demonstrated that the antigen recognized by Hy30 is found on both immunoglobulin positive (Ig+) and negative (Ig-) cells. Analysis of a chicken rendered B cell deficient by treatment with cyclophosphamide at hatch confirmed this observation. In these birds, the medullary dendritic cells of the bursa and Ig- cells of the "B-cell areas" of the spleen and thymus expressed the Hy30 Ag. Analysis of several avian tumor lines did not suggest an obvious lineage for these Hy30+ Ig- cells. However, analysis of germinal centers suggested that these cells may be tissue macrophages or dendritic cells. This is consistent with previously reported histology of these "B-cell areas". Flow cytometric analysis of the antigen recognized by Hy30 demonstrates that it is distinct from the Bu-1 B-cell surface antigen and we designate it as Bu-2.

Selection for B cells with productive IgL gene rearrangements occurs in the bursa of Fabricius during chicken embryonic development

The vast majority of immunoglobulin-expressing mature chicken B lymphocytes contain one functionally rearranged and one unrearranged allele of the immunoglobulin light chain (IgL) gene. Therefore, nearly all IgL V-J rearrangements present in mature chickens are in-frame. In contrast, the Ig genes of mature mammalian B cells contain a high proportion of out-of-frame V-J joints. To investigate the basis for this difference, gene rearrangement at the chicken IgL locus was characterized during embryonic development and in mature B-cell lines. Joining of the single functional variable (VL) segment with the single joining (JL) segment occurs in cells in multiple tissues during a transient period of chicken embryogenesis. Only one-third of the V-J joints cloned from days 10-12 of development are in-frame. An increasing proportion of in-frame V-J joints is observed within the bursa of Fabricius at successively later stages of development. Our data suggest that the bursa of Fabricius serves during embryonic development as a site of selective amplification of cells that have undergone productive V-J joining, such that nearly all V-J joints present in postembryonic B cells are in-frame. The high frequency of rearranged alleles joined in-frame that is found in posthatching bursal cells and mature B-cell lines appears to result from a low frequency with which cells undergo IgL rearrangement at both alleles, rather than from an increase in the precision of V-J joining in avian species.

Tolerance to class I major histocompatibility complex antigens in chicken B cell chimeras. Effect of B cell depletion on transferability of tolerance

B cells from bursa of Fabricius of newly hatched chickens are able to reconstitute the B cell compartment of chemically bursectomized chickens. The resulting B cell chimerism can be detected with monoclonal antibodies against donor B cell alloantigen. Chimeric chickens accept donor-type skin grafts and are unresponsive to donor major histocompatibility complex (MHC) antigens in graft-vs.-host splenomegaly assay and mixed lymphocyte reaction. To study the capability of B cells to induce tolerance to selected MHC antigens, we transplanted class I or total MHC-incompatible bursa cells into cyclophosphamide-treated recipients. The recipients of class I or total MHC-incompatible bursa cells were equally tolerant of donor-MHC antigens. To further analyze the mechanisms of tolerance to class I antigens vs. total MHC, spleen cells from tolerant chickens were transferred to irradiated, histocompatible secondary hosts. The secondary recipients were also unresponsive to bursa cell donor-strain MHC antigens. However, if the chimeric B cells were depleted before the spleen cell transfer, the transfer of tolerance to total MHC was severely inhibited. Instead, most recipients of B cell-depleted spleen cells tolerant of class I antigens were still tolerant of bursa cell donor MHC. Our results indicate differences in the transferability of tolerance to class I antigens vs. entire MHC, although in primary recipients of bursa cells the tolerance is similar. These data suggest that a mechanism that is not dependent on the presence of donor cell chimerism contributes to the maintenance of tolerance to donor class I antigens. The transfer of tolerance to total MHC disparity requires the presence of chimeric cells indicating that donor alloantigen expression is needed for induction of tolerance in the secondary hosts.

Effects of the TCDD congeners 3,3',4,4'-tetrachlorobiphenyl and 3,3',4,4'-tetrachloroazoxybenzene on lymphoid development in the bursa of Fabricius of the chick embryo

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and its congeners, such as 3,3',4,4'-tetrachlorobiphenyl (TCB) and 3,3',4,4'-tetrachloroazoxybenzene (TCAOB), act on targets in the immune system, probably by interacting with the Ah-receptor, causing a characteristic pattern of effects typified by inhibition of lymphoid development in the thymus. There are, however, also reports of effects on B cells. Birds have a well-defined site of B-lymphocyte development, the bursa of Fabricius, analogous to the thymus for T cells. Thus, we wanted to determine whether this organ is suitable for studying effects on B-cell development. TCB and TCAOB were administered by injection into the air sacs of White Leghorn eggs on Day 13 of incubation. Effects observed on Day 19 were as follows: (1) There was a reduction in bursal dry weight but not in body weight in the treated groups. (2) The number of lymphoid cells in the bursae decreased in a dose-dependent manner, and bursae from high-dose groups (300 and 30 micrograms/kg egg of TCB and TCAOB, respectively) were almost completely devoid of lymphoid cells. For the reduction of lymphoid cell number, the ED50 for TCB and TCAOB was approximately 45 and 1.4 micrograms/kg egg, respectively. (3) Histological sections showed that embryos from treated egges contained fewer bursal follicles and that follicles contained fewer lymphoid cells compared with controls. (4) Aryl hydrocarbon hydroxylase activity was 30 and 50 times that of the control at high doses of TCB and TCAOB (estimated ED50: 200-300 and 4 micrograms/kg egg, respectively). These findings suggest that lymphoid development in the bursa of Fabricius of the chick embryo is inhibited by TCDD congeners.

The effect of alterations in myc gene expression on B cell development in the bursa of Fabricius

Infection of 18-day embryonic bursal lymphocytes with a v-myc-containing retrovirus leads directly to a polyclonal proliferation of surface immunoglobulin-positive (slg+) cells in the bursa of Fabricius detected four weeks after hatching. These v-myc-expressing bursal cells repopulate the follicles of chemically ablated bursae more efficiently than total normal 18-day embryonic bursal cells. In contrast, comparable normal bursal cells lose the ability to repopulate follicles by four weeks. Bursal lymphocytes expressing either a retroviral v-myc or a c-myc gene deregulated by adjacent retroviral integration retain the ability of embryonic bursal lymphocytes to diversify their immunoglobulin light chain genes. These results suggest that retroviral deregulation of myc expression during avian B cell development induces outgrowth of a population of cells with the cardinal phenotypic characteristics of bursal stem cells.

Characterization of the estrogen-sensitive cells expressing progesterone receptor in the bursa of Fabricius

Cells expressing the progesterone receptor (PR) in the bursa of Fabricius (BF) were studied with immunohistochemistry at light-microscopic level, with immunoelectron microscopy (immuno-EM) and with non-specific esterase histochemistry. The antibody (IgG-RB) directed to the B component of the chick oviduct progesterone receptor was shown by immunoblotting to be specific for the PR and to recognize the PR also in the bursa. Two cell types in the BF contain the PR: stromal cells in the interfollicular-subepithelial area and smooth muscle cells lining the BF. The PR was localized in the nuclei of these cells. The bursal epithelium and the cells inside the follicles were not stained for PR. Electron microscopically the immunoreaction precipitate was localized on condensed heterochromatin and on dispersed euchromatin. The cells expressing the PR resembled electron microscopically fibroblasts. Their cytoplasm was rich in rough endoplasmic reticulum indicating active protein synthesis. By non-specific esterase histochemistry we showed that the PR-containing cells were not macrophages, which are morphologically indistinguishable from stromal cells. In the bursae of young untreated chicks the PR was not seen, but was inducible by estradiol treatment and was spontaneously expressed after the onset of sexual maturation. It is concluded that both the stromal fibroblasts and the smooth muscle cells in the BF are estrogen and progesterone sensitive. The expression of PR after the onset of sexual maturation indicates that the BF is directly affected by sexual maturation-associated factors. We suggest that estrogen and progesterone participate in tissue remodelling during bursal involution via the stromal cells and may affect bursal functions via the smooth muscle cells.

Structure and function of the immune system

The major function of the immune system is to protect the host from environmental agents such as microbes or chemicals, thereby preserving the integrity of the body. This is done by the recognition of self and response to non-self. The immune response has been artificially divided into innate immunity (resistance) and specific immunity. Specific immunity is further divided into humoral immunity, the one involved with antibody, and cellular immunity, which is orchestrated by T cells. It is essential to understand that although these divisions have helped in understanding and analyzing the immune response, the system functions as a single unit rather than as a separate entity. In this paper, a simplified analysis of specific immunity will be given. However, the importance of nonspecific immunity, especially as it pertains to its role in preventing exposure of environmental substances, should not be forgotten.

Somatic diversification of the chicken immunoglobulin light chain gene is limited to the rearranged variable gene segment

Previous studies have shown that the chicken lambda immunoglobulin light chain gene undergoes a single rearrangement that results in functional VJ joining of the unique variable (V lambda 1) and joining (J lambda) coding regions. The immunologic repertoire of lambda genes is created through extensive sequence diversification within the rearranged locus during B cell development in the bursa of Fabricius. This sequence diversification was detected only at the rearranged V lambda 1 segment and not within the 5' leader sequence, the J lambda segment, or the unrearranged V lambda 1 segment. The selective diversification of the rearranged V lambda 1 segment was associated with unique DNAase I-hypersensitive sites on the rearranged allele. While probes for V lambda 1 sequences detect multiple homologous V lambda segments, probes for both the 5' leader and J lambda segments fail to detect homologous sequences. Taken together, these results suggest that a highly selective process, possibly gene conversion, operates during B cell ontogeny to generate diversity within the lambda gene.

Immunoglobulin-bearing B cells reconstitute and maintain the murine Ly-1 B cell lineage

Previous reconstitution studies in irradiated mice distinguished Ly-1 B from other B cells. The predominant B cell populations in spleen and lymph node are readily reconstituted from progenitors present in adult bone marrow. Ly-1 B cells, in contrast, are reconstituted from cells in peritoneum rather than bone marrow. Further studies, presented here, demonstrate that Ly-1 B populations are reconstituted from peritoneal cells that already express both surface immunoglobulin (IgM) and Ly-1, that is, Ly-1 B. Such cells reconstitute and maintain the Ly-1 B population in irradiated recipients for at least 6 months.

Counting components of the chicken's B cell system

At the risk of representing a chicken as a hybrid between a hummingbird and an ostrich, we can summarize the preceding sections and order-of-magnitude estimates as follows. Chicken B lymphocytes are derived from less than 10(5) lymphoid precursor cells, which either have already rearranged their Ig genes before they colonize the embryonic bursa, or (more probably) rapidly give rise to cells with rearranged genes within the bursa's 10(4) follicles. Since the bird's functional germline Ig V genes are few in number (less than or equal to 10?), most rearrangements have similar outcomes. The B cells proliferate rapidly in the bursa, in an antigen-independent manner, undergoing somatic modifications of their Ig V genes at a high rate (probably at least once in every 10(3) cell divisions). In the young chick, B cells are produced in the bursa at a rate of 10(7) to 10(8) per d; many of these die but the rest contribute to formation of the adult bird's B cell pool of about 10(10) lymphocytes, with a repertoire of at least 10(6) different antibody specificities. the bird's B cells are entirely self-renewing, in the sense that none are derived from Ig-negative precursors at any time after hatching.