Uptake of environmental antigens by the bursa of Fabricius

Impact of early posthatch feeding on the immune system and selected hematological, biochemical, and hormonal parameters in broiler chickens

Under commercial conditions, chicks hatch within a 24 to 48 h window, a period known as the hatching window. Subsequently, they undergo various treatments before finally being transported to the broiler farm. These procedures may delay the chicks' access to food and water, sometimes receiving them as late as 72 h after hatching. Previous studies have indicated that fasting during this initial period is detrimental, leading to impaired body growth, compromised immune system response, and hindered muscle development. The objective of this study was to assess the impact of early posthatch feeding on immune system organs and selected hematological, biochemical, and hormonal parameters. The experiment utilized Ross 308 broiler eggs incubated under typical commercial hatchery conditions. The experimental group's eggs were hatched in HatchCare hatchers (HC) with immediate access to feed and water, while the control group's eggs were hatched under standard conditions (ST). Thirty chickens from each group were assessed on the 1st (D1), 7th (D7), 21st (D21), and 35th (D35) day after hatching. On D1, the HC group exhibited lower hemoglobin, hematocrit, and total serum protein values, suggesting that early access to water prevents initial dehydration in newborn chicks. Conversely, the ST group showed a stress reaction on D1 due to feed deprivation, leading to an almost 2-fold higher serum corticosterone concentration compared to the HC group. However, this increase did not result in a significant change in the heterophil/lymphocyte ratio. Furthermore, the HC group displayed an increase in triglyceride concentration and a decrease in HDL concentration on D1. On D7, the HC group exhibited an increased relative weight of the bursa and a higher CD4+ cell number in the cecal tonsil (CT), indicating a more rapid development of these organs resulting from early stimulation of the gastrointestinal tract. However, early feeding did not influence the numbers of Bu-1+, CD4+, and CD8+ cells or the germinal center (GC) areas in the spleen. In conclusion, early feeding contributes to the welfare of newborn chicks by reducing dehydration and stress levels and stimulating the development of gut-associated lymphoid tissue.

In ovo feeding of probiotic lactobacilli differentially alters expression of genes involved in the development and immunological maturation of bursa of Fabricius in pre-hatched chicks

Compelling evidence indicates that immunological maturation of the gut-associated lymphoid tissues, including the bursa of Fabricius, is dependent upon antigenic stimulation post-hatch. In view of these data, the present study investigated the impact of exposing the immune system of chick embryos to antigenic stimuli, via in ovo delivery of poultry-specific lactobacilli, on the expression of genes associated with early bursal development and maturation. Broiler line embryonated eggs were inoculated with 106 and 107 colony-forming units (CFUs) of an individual or a mixture of Lactobacillus species, including L. crispatus (C25), L. animalis (P38), L. acidophilus (P42), and L. reuteri (P43), at embryonic day 18 (ED18). The bursa of Fabricius was collected from pre-hatched chicks (ED20) to measure the expression levels of various immune system genes. The results revealed that L. acidophilus and the mixture of Lactobacillus species at the dose of 106 CFU consistently elicited higher expression of genes responsible for B cell development, differentiation, and survival (B cell activating factor (BAFF), BAFF-receptor (BAFF-R)), and antibody production (interleukin (IL)-10) and diversification (TGF-β). Similar expression patterns were also noted in T helper (Th) cell-associated cytokine genes, including Th1-type cytokines (interferon (IFN)-γ and IL-12p40), Th2-type cytokines (IL-4 and IL-13) and Th17 cytokine (IL-17). Overall, these results suggest that the supplementation of poultry-specific lactobacilli to chick embryos might be beneficial for accelerating the development and immunological maturation of the bursa of Fabricius. However, further studies are required to determine if the changes in gene expression are associated with the developmental trajectory and phenotypes of bursal cells.

Distribution and developmental changes of IL-21 immunopositive cells in the bursa of Fabricius of Jinhu silky chicken

Bursa of Fabricius (BOF) is a unique immune organ of birds. It is the place where lymphocytes develop, differentiate and mature. Young chicken BOF is susceptible to infection and damage, and even atrophy, causing immune suppression, and bringing huge economic losses to chicken production. Therefore, studying the regulatory mechanism of chicken bursa development is of great practical significance for disease prevention and diagnosis. Jinhu silky chicken (JSC) is a local excellent breed in the Fujian Province of China and with strong disease resistance. However, studies on the disease resistance of JSC are scarce. This study aimed to provide a theoretical basis for reproduction and disease control of JSC. Developmental features of the structure and the IL-21-positive cell (IL-21 PC) distribution on the BOF in JSC were measured from 7 to 300 days of age. Bursas of chicken (n = 36) were taken at 7, 35, 70, 150, 240, and 300 days of age for preparation of paraffin sections and stained with hematoxylin-eosin (HE) and immunohistochemistry. The microstructure of JSC's BOF was similar to that of other poultry. The cortical-medullary boundary of the bursa nodule was not obvious at 7 days of age, but it was evident after 35 days of age. Before 70 days of age, IL-21 positive cells (PC) were scattered on the BOF. At 150 days of age, the number of IL-21 PC in the bursa were the highest and the nuclei were clear. The level of IL-21 PC gradually decreased with age. The BOF degenerated and disappeared in 300-day-old JSC. The histological structure of the BOF was similar to that of other poultry. IL-21 PC were widespread in the BOF at different ages, but the numbers were different.

Sensitivity of broiler performance, organ weights and plasma constituents to amino acid supplementation and reused litter exposure using ideal protein-formulated rations

Effects of amino acid supplementation to ideal protein (IP) formulated rations were investigated on growth performance, plasma metabolites and organ weights of broilers placed on 100% recycled (reused) litter. Day-old Ross308 male broilers were raised on either clean or reused litter and fed for three weeks on one of five isoenergetic diets, where an IP-based control diet (C) was compared with diets containing threonine (T) or arginine (A) at 25% above requirements, or with 1% supplemented glutamine (G), or with each amino acid added (TAG). Litter and diet treatments did not strongly interact on outcomes. Reused litter placement resulted in greater weight gain, smaller feed conversion ratio and heavier bursal weights (P < 0.05) compared to clean litter placement. Relative to C and T birds, TAG birds reduced weight gain and feed intake (P < 0.05). Plasma uric acid levels in G birds were greater than in C, T and A birds (P < 0.001). Collectively, since the outcomes of placement on reused litter increased performance and the control diet was IP formulated, the absence of increased growth performance in response to amino acid supplementation would be consistent with amino acids tested being excess to requirements.

Diversification of immunoglobulin genes by gene conversion in the domestic chicken (Gallus gallus domesticus)

Sustainable modern poultry production depends on effective protection against infectious diseases and a diverse range of antibodies is key for an effective immune response. In the domestic chicken, somatic gene conversion is the dominant process in which the antibody immunoglobulin genes are diversified. Affinity maturation by somatic hypermutation (SHM) also occurs, but the relative contribution of gene conversion versus somatic hypermutation to immunoglobulin (Ig) gene diversity is poorly understood. In this study, we use high throughput long-read sequencing to study immunoglobulin diversity in multiple immune-associated tissues in Rhode Island Red chickens. To better understand the impact of genetic diversification in the chicken, a novel gene conversion identification software was developed (BrepConvert). In this study, BrepConvert enabled the identification of over 1 million gene conversion events. Mapping the occurrence of putative somatic gene conversion (SGC) events throughout the variable gene region revealed repetitive and highly restricted patterns of genetic insertions in both the antibody heavy and light chains. These patterns coincided with the locations of genetic variability in available pseudogenes and align with antigen binding sites, predominately the complementary determining regions (CDRs). We found biased usage of pseudogenes during gene conversion, as well as immunoglobulin heavy chain diversity gene (IGHD) preferences during V(D)J gene rearrangement, suggesting that antibody diversification in chickens is more focused than the genetic potential for diversity would suggest.

Integrated Transcriptome and Histone Modification Analysis Reveals NDV Infection Under Heat Stress Affects Bursa Development and Proliferation in Susceptible Chicken Line

Two environmental factors, Newcastle disease and heat stress, are concurrently negatively impacting poultry worldwide and warrant greater attention into developing genetic resistance within chickens. Using two genetically distinct and highly inbred layer lines, Fayoumi and Leghorn, we explored how different genetic backgrounds affect the bursal response to a treatment of simultaneous Newcastle disease virus (NDV) infection at 6 days postinfection (dpi) while under chronic heat stress. The bursa is a primary lymphoid organ within birds and is crucial for the development of B cells. We performed RNA-seq and ChIP-seq targeting histone modifications on bursa tissue. Differential gene expression revealed that Leghorn, compared to Fayoumi, had significant down-regulation in genes involved in cell proliferation, cell cycle, and cell division. Interestingly, we also found greater differences in histone modification levels in response to treatment in Leghorns than Fayoumis, and biological processes enriched in associated target genes of H3K27ac and H3K4me1 were similarly associated with cell cycle and receptor signaling of lymphocytes. Lastly, we found candidate variants between the two genetic lines within exons of differentially expressed genes and regulatory elements with differential histone modification enrichment between the lines, which provides a strong foundation for understanding the effects of genetic variation on NDV resistance under heat stress. This study provides further understanding of the cellular mechanisms affected by NDV infection under heat stress in chicken bursa and identified potential genes and regulatory regions that may be targets for developing genetic resistance within chickens.

Assessment of the role of intracloacal inoculation of live infectious bursal disease vaccine in breaking through maternally derived antibodies

Infectious bursal disease (IBD) remains a potential worldwide threat to the poultry industry despite several vaccination approaches. Because maternally derived antibodies (MDA) constitute a critical problem for IBD vaccination, we examined the efficiency of the intracloacal vaccination approach in breaking through MDA. Experiment 1 determined the ability of the vaccinal strain to multiply in the bursa of Fabricius (BF) in chicks with a high level of MDA. Using real-time polymerase chain reaction, we quantified the strain in the bursae of vaccinated and non-vaccinated chicks. Experiment 2 was performed on three groups of chicks with high levels of MDA: group 1, non-vaccinated non-challenged; group 2, non-vaccinated challenged; and group 3, vaccinated challenged. Seroconversion to IBDV was measured using enzyme-linked immunosorbent assay. Groups 2 and 3 were challenged by vvIBDV at 25 days of age. Experiment 3 studied the effect of early IBD vaccinal strain multiplication on the immune response of vaccinated and non-vaccinated chicks to other vaccines. In experiment 1, the vaccinal strain showed progressive multiplication and reached the detectable titre in BF at 12 h post-vaccination despite high MDA titre. Experiment 2 showed that chicks in group 3 had significant seroconversion against IBDV. After challenge, group 3 showed significant improvements in several measured parameters compared with group 2. Moreover, results of experiment 3 proved that early multiplication of the vaccinal strain in the BF has no significant effect on the immune system or immune response to other vaccines. These results proved the promising success of this IBD vaccination approach.RESEARCH HIGHLIGHTS IBD vaccinal strain succeeded in multiplying in BF after intracloacal inoculation.Vaccinated chicks showed significant seroconversion of IBDV antibody titres.Vaccinated chicks showed a significant protection level against vvIBDV.Early IBD vaccination did not affect the immune response to other vaccines.

Antigen Sampling CSF1R-Expressing Epithelial Cells Are the Functional Equivalents of Mammalian M Cells in the Avian Follicle-Associated Epithelium

The follicle-associated epithelium (FAE) is a specialized structure that samples luminal antigens and transports them into mucosa-associated lymphoid tissues (MALT). In mammals, transcytosis of antigens across the gut epithelium is performed by a subset of FAE cells known as M cells. Here we show that colony-stimulating factor 1 receptor (CSF1R) is expressed by a subset of cells in the avian bursa of Fabricius FAE. Expression was initially detected using a CSF1R-reporter transgene that also label subsets of bursal macrophages. Immunohistochemical detection using a specific monoclonal antibody confirmed abundant expression of CSF1R on the basolateral membrane of FAE cells. CSF1R-transgene expressing bursal FAE cells were enriched for expression of markers previously reported as putative M cell markers, including annexin A10 and CD44. They were further distinguished from a population of CSF1R-transgene negative epithelial cells within FAE by high apical F-actin expression and differential staining with the lectins jacalin, PHA-L and SNA. Bursal FAE cells that express the CSF1R-reporter transgene were responsible for the bulk of FAE transcytosis of labeled microparticles in the size range 0.02-0.1 μm. Unlike mammalian M cells, they did not readily take up larger bacterial sized microparticles (0.5 μm). Their role in uptake of bacteria was tested using Salmonella, which can enter via M cells in mammals. Labeled Salmonella enterica serovar Typhimurium entered bursal tissue via the FAE. Entry was partially dependent upon Type III secretion system-1. However, the majority of invading bacteria were localized to CSF1R-negative FAE cells and in resident phagocytes that express the phosphatidylserine receptor TIM4. CSF1R-expressing FAE cells in infected follicles showed evidence of cell death and shedding into the bursal lumen. In mammals, CSF1R expression in the gut is restricted to macrophages which only indirectly control M cell differentiation. The novel expression of CSF1R in birds suggests that these functional equivalents to mammalian M cells may have different ontological origins and their development and function are likely to be regulated by different growth factors.

Changes in bursal B cells in chicken during embryonic development and early life after hatching

The bursa of Fabricius, the primary lymphoid organ for B cell development found only in birds, offers novel approaches to study B cell differentiation at various developmental stages. Here, we explored the changes and mechanism involved in the developmental stages of bursal B cells. The bursal B cells rapidly increased in the late embryonic stage and around hatching, which coincided with changes in specific cell surface markers. Moreover, the cells in the bursa were divided by size into small (low forward- and side-scatter) or large (high forward- and side-scatter) via flow cytometry. It is intriguing that the proportion of small and large B cells was reversed during this period. Because little is known about this phenomenon, we hypothesized that size-based B cell population could be used as an indicator to distinguish their status and stage during B cell development in chicken. The results demonstrated that large B cells are actively proliferating cells than small B cells. Additionally, large B cells showed higher mRNA expression of both proliferation- and differentiation-associated genes compared to small B cells. Taken together, these data show that large bursal B cells are the main source of proliferation and differentiation during B cell development in chickens.

Identification of the gene product recognized by monoclonal antibody GIIF3

The chicken as a research model has a disadvantage compared with the mouse and the human because of the low number of available antibodies against gene products of interest. The goal of this study was to identify the antigen recognized by monoclonal antibody (mAb) GIIF3, which is a 42 kDa protein that appears in follicle-associated epithelium of the guinea hen as well as in different muscle types during chicken embryonic development. The 42 kDa protein, immunoprecipitated from chicken gizzard protein lysates, was evaluated by mass spectrometry. Mass spectrometry analysis revealed peptides specific for the chicken β- or γ-actin isoforms. The mAb GIIF3 can be used as a new research tool for smooth muscle cell and bursa of Fabricius developmental studies.

Local glucocorticoid production in lymphoid organs of mice and birds: Functions in lymphocyte development

Circulating glucocorticoids (GCs) are powerful regulators of immunity. Stress-induced GC secretion by the adrenal glands initially enhances and later suppresses the immune response. GC targets include lymphocytes of the adaptive immune system, which are well known for their sensitivity to GCs. Less appreciated, however, is that GCs are locally produced in lymphoid organs, such as the thymus, where GCs play a critical role in selection of the T cell antigen receptor (TCR) repertoire. Here, we review the roles of systemic and locally-produced GCs in T lymphocyte development, which has been studied primarily in laboratory mice. By antagonizing TCR signaling in developing T cells, thymus-derived GCs promote selection of T cells with stronger TCR signaling. This results in increased T cell-mediated immune responses to a range of antigens. We then compare local and systemic GC patterns in mice to those in several bird species. Taken together, these studies suggest that a combination of adrenal and lymphoid GC production might function to adaptively regulate lymphocyte development and selection, and thus antigen-specific immune reactivity, to optimize survival under different environmental conditions. Future studies should examine how lymphoid GC patterns vary across other vertebrates, how GCs function in B lymphocyte development in the bone marrow, spleen, and the avian bursa of Fabricius, and whether GCs adaptively program immunity in free-living animals.

The role of the bursa of Fabricius in the immune response to vaccinal antigens and the development of immune tolerance in chicks (Gallus domesticus) vaccinated at a very young age

Vaccination is recognized to be the most cost-effective means of preventing, controlling, and even eradicating infectious diseases. Conventional poultry are vaccinated through various routes including eye/nose drops, drinking water, vent brush, or injection. Efficient vaccination is an essential part of any good poultry management.The bursa of Fabricius is intimately connected to the cloaca and the intestinal system. It is well-known as a primary lymphoid organ in the chicken and a major channel through which environmental antigens stimulate the immune system. In this study we tested whether direct instillation of various viral vaccines and antigens into the cloaca (per bursam), could stimulate higher antibody titers and generate improved protection. Despite the very rapid absorption of the vaccines or antigens from the cloaca to the lumen of the Bursa of Fabricius, per bursam inoculation failed to generate a satisfactory immune response. In contrast conventional administration of live or inactivated commercial vaccines led to an acceptable level of seroconversion and protection against challenge.An interesting finding in this study was the fact that administration of a single priming dose of antigenic material at age 1 or 5 days, did not improve the response to a second administration at 14 days of age as expected. Instead, in most cases there was a reduced serum antibody response suggesting the induction of tolerance. This was true for all routes of administration (intramuscular, per ocular and per bursam) and for all formulations of vaccine.The current study reveals: 1) no advantage for direct application of live or inactivated vaccines or antigens into the bursa of Fabricius compared to common routes of vaccination, 2) that apparent desensitization or tolerance effects have important implications for poultry management, since in many countries, vaccination of day old chicks is compulsory or a well-accepted part of flock vaccination.According to our results, early vaccination can in fact reduce or inhibit a secondary immune response to subsequent vaccination and increase susceptibility to disease agents.

New insight into the origin of IgG-bearing cells in the bursa of Fabricius

The bursa of Fabricius is a primary lymphoid organ for B-cell development and gut-associated lymphoid tissue. After hatching, IgG-containing cells with reticular branches are found in the medulla of bursal follicles on frozen sections stained with anti-Cγ antibody, and IgM(+)IgG(+) B cells are detected in single-cell suspension of the bursa. IgG-containing cells in the medulla do not biosynthesize IgG and are composed of aggregated maternal IgG and environmental antigens. Then, those cells in the medulla are acknowledged as follicular dendritic cells retaining immune complexes. Also, it is presumed that IgM(+)IgG(+) B cells are generated by the attachment of immune complexes to IgM(+) bursal B cells because IgM(+)IgG(+) B cells are induced by antigen-dependent attachment of maternal IgG. Therefore, it is reasonable to suppose that immune complexes exert further B-cell differentiation in the medulla.

The origin of IgG-containing cells in the bursa of Fabricius

The bursa of Fabricius of the chicken is known as a primary lymphoid organ for B-cell development. Morphologically, the origin of IgG-containing cells in the bursa has not been clear until now, because abundant maternal IgG (MIgG) is transported to the chick embryo and distributed to the bursal tissue around hatching. Thus, it has been difficult to find out whether these cells themselves biosynthesize IgG or if they acquire MIgG via attachment to their surface. Our present study employing in situ hybridization clarified that IgG-containing cells in the medulla of bursal follicles did not biosynthesize IgG. To study the role of MIgG in the development of those IgG-containing cells, MIgG-free chicks were established from surgically bursectomized hen (SBx-hen). We found that, on the one hand, deprivation of MIgG from chicks completely inhibited the development of IgG-containing cells in the medulla after hatching. On the other hand, administration of MIgG to MIgG-free chicks recovered the emergence of those cells. In addition, we observed that those cells did not bear a B-cell marker and possessed dendrites with aggregated IgG. These results demonstrate that IgG-containing cells in the medulla are reticular cells that capture aggregated MIgG. Moreover, we show that the isolation of the bursa from environmental stimuli by bursal duct ligation (BDL) suppressed the development of IgG-containing cells after hatching. Thus, it is implied that environmental stimulations play a key role in MIgG aggregations and dendritic distributions of aggregated MIgG in the medulla after hatching.

Unique features and distribution of the chicken CD83+ cell

The central importance of dendritic cells (DC) in both innate and acquired immunity is well recognized in the mammalian immune system. By contrast DC have yet to be characterized in avian species despite the fact that avian species such as the chicken have a well-developed immune system. CD83 has proven to be an excellent marker for DC in human and murine immune systems. In this study we identify chicken CD83 (chCD83) as the avian equivalent of the human and murine DC marker CD83. We demonstrate for the first time that unlike human and murine CD83, chCD83 is uniquely expressed in the B cell areas of secondary lymphoid organs and in organs with no human or murine equivalent such as the bursa and Harderian gland. Furthermore through multicolor immunofluorescence, we identify chCD83(+) populations that have unique attributes akin to both DC and follicular DC. These attributes include colocalization with B cell microenvironments, MHC class II expression, dendritic morphology, and distribution throughout peripheral and lymphoid tissues.

Diversified bursal medullary B cells survive and expand independently after depletion following neonatal infectious bursal disease virus infection

The primary immunoglobulin repertoire of chickens is generated not by gene rearrangement but by a subsequent process of gene conversion in proliferating immature B cells within the follicles of a specialized gut-associated lymphoid organ, the bursa of Fabricius. Neonatal infection with infectious bursal disease virus can eliminate almost the entire bursal B-cell compartment. Thereafter, two types of follicle reappear. Larger follicles, with rapidly proliferating B cells and normal structure, are correlated with partial recovery of antibody response. Smaller follicles, lacking distinct cortex and medulla, appear unable to produce antigen-responsive B cells. To understand the genesis of the two types of follicle, we analysed their VL sequences and activation-induced deaminase mRNA levels. The results provide a model of bursal repopulation in which surviving bursal stem cells generate new follicles with normal morphology and function, while surviving medullary B cells continue to proliferate slowly, under the influence of stromal cells, giving rise to the smaller follicles. The latter remain fixed in a stage of development incapable of further gene diversification.

Antibodies, immunoglobulin genes and the bursa of Fabricius in chicken B cell development

The bursa of Fabricius is critical for the normal development of B lymphocytes in birds. It is productively colonized during embryonic life by a limited number of B cell precursors that have undergone the immunoglobulin gene rearrangements required for expression of cell surface immunoglobulin. Immunoglobulin gene rearrangement occurs in the absence of terminal deoxynucleotidyl transferase and generates minimal antibody diversity. In addition, observations that immunoglobulin heavy and light chain variable gene rearrangement occur at the same time and that allelic exclusion of immunoglobulin expression is regulated at the level of variable region gene rearrangement provide a striking contrast to rodent and primate models of immunoglobulin gene assembly. Following productive colonization of the bursa, developing B cells undergo rapid proliferation and the immunoglobulin V region genes that generate the specificity of the B cell surface immunoglobulin receptor undergo diversification. Immunoglobulin diversity in birds is generated by somatic gene conversion events in which sequences derived from upstream families of pseudogenes replace homologous sequences in unique and functionally rearranged immunoglobulin heavy and light chain variable region genes. This mechanism is distinct from and much more efficient than mechanisms of antibody diversification seen in rodents and primates. While the bursal microenvironment is not required for immunoglobulin gene rearrangement and expression, it is essential for the generation of antibody diversity by gene conversion. Following hatch, gut derived antigens are taken up by the bursa. While bursal development prior to hatch occurs in the absence of exogenous antigen, chicken B cell development after hatch may therefore be influenced by the presence of environmental antigen. This review focuses on the differences between B cell development in the chicken as compared to rodent and primate models.

Cell surface immunoglobulin regulated checkpoints in chicken B cell development

The bursa of Fabricius is critical for the normal development of B lymphocytes in avian species. Productive colonization of bursal follicles by B cell precursors requires surface immunoglobulin expression. We have shown using retroviral gene transfer that expression of chimeric receptors containing the extracellular and transmembrane domains of murine CD8alpha and CD8beta fused to the cytoplasmic domains of chicken Igalpha and Igbeta can support productive bursal colonization in the chicken embryo in bursal cells lacking the expression of endogenous sIgM. We show here that chimeric receptor expression does not support continued bursal cell development after hatch. However intrabursal administration of anti-CD8 antibodies that ligate the CD8alpha:Igalpha chimeric receptor results in maintained numbers of bursal cells that express the chimeric receptor in the absence of endogenous sIgM. These results support a model in which sIgM receptor expression is required for productive bursal colonization in the chick embryo but sIgM receptor ligation is required to support later B cell development after hatch.

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.

Developmentally programmed expression of AID in chicken B cells

In mice, activation induced deaminase, AID, is expressed only in germinal center B cells. It is required for the initiation of somatic hypermutation and class switch recombination. In chickens and most mammals immunoglobulin gene rearrangement generates limited diversity and the primary immunoglobulin repertoire depends on subsequent somatic hypermutation or gene conversion. Immunoglobulin gene conversion in chickens starts in the embryonic bursa, before antigen exposure. The demonstrated requirement for AID for gene conversion in the bursal lymphoma cell line, DT40, implies developmental regulation of AID expression. To test this prediction, we examined the timing and location of AID mRNA expression. An abrupt increase in AID mRNA coincided with the onset of extensive Ig gene conversion in the bursa. Expression was also detected at earlier stages, implying either that expression of AID is not the only controlling factor for gene conversion, or that gene conversion can precede the formation of bursal follicles.

The avian B-cell receptor complex: distinct roles of Igalpha and Igbeta in B-cell development

The bursa of Fabricius has evolved in birds as a gut-associated site of B-cell lymphopoiesis that is segregated from the development of other hematopoietic lineages. Despite differences in the developmental progression of chicken as compared to murine B-cell lymphopoiesis, cell-surface immunoglobulin (sIg) expression has been conserved in birds as an essential checkpoint in B-cell development. B-cell precursors that express an sIg complex that includes the evolutionarily conserved Igalpha/beta heterodimer colonize lymphoid follicles in the bursa, whereas B-cell precursors that fail to express sIg due to non-productive V(D)J recombination are eliminated. Productive retroviral gene transfer has allowed us to introduce chimeric receptor constructs into developing B-cell precursors in vivo. Chimeric proteins comprising the extracellular and transmembrane regions of murine CD8alpha fused to the cytoplasmic domain of chicken Igalpha efficiently supported B-cell development in precursors that lacked endogenous sIg expression. By contrast, expression of an equivalent chimeric receptor containing the cytoplasmic domain of Igbeta actively inhibited B-cell development. Consequently, the cytoplasmic domains of Igalpha and Igbeta play functionally distinct roles in chicken B-cell development.

Effect of environmental antigens on the Ig diversification and the selection of productive V-J joints in the bursa

In chickens, a single set of unique functional segments of both Ig H and L chain genes is rearranged during early embryogenesis to generate a pool of B cell progenitors that will be diversified in the bursa by gene conversion, forming the preimmune repertoire. After hatching, bursal cells are exposed to environmental Ags in the bursal lumen. We prepared B cells from each single bursal follicle and used PCR-directed Ig L chain gene analysis to study the differentiation of B cells and the effect of antigenic stimulation from the bursal lumen on the neonatal chicken B cell repertoire formation. Selective amplification of B cell clones with a productive V-J joint was observed during the late embryonic stage, possibly by the interaction with ligands expressed on the bursal stroma and further accelerated in the neonatal chicken. Administration of the artificial Ags into the bursal lumen before the isolation of bursa by bursal duct ligation in the embryo caused a significant increase in lymphocytes with a productive V-J joint in the neonatal chicken bursa compared with the isolated bursa. Intra- and interclonal diversity of a complementarity-determining region measured by an evolutionary distance increased during bursal development. Clonal diversification did not require stimulation by artificial Ags from the bursal lumen. Thus, the preimmune repertoire in the bursa is generated by gene conversion during Ag-independent B cell proliferation, and antigenic stimulation from the bursal epithelium to bursal B cells plays roles in the selection of clones with a productive V-J joint.

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.

Immunity, vaccination and the avian intestinal tract

Defence of the intestinal mucosal surface from enteric pathogens is initially mediated by secretory IgA (SIgA). As oral immunization of non-replicating antigen induces minimal SIgA antibody titers, novel immunization strategies which selectively induce mucosal immune responses in mammals are now being assessed in chickens. The strategies reviewed include the route of antigen delivery, the incorporation of antigenic components in delivery vehicles, the inclusion of immunomodulators in the vaccine formula or in the diet, and manipulation of intestinal microflora. The differences in anatomical organization and immunological mechanisms between birds and mammals must be considered when manipulating avian intestinal immunity with the latest immunotechnologies developed for mammals. Our knowledge of the function and functioning of the avian mucosal system is discussed. Progress in our understanding of this system, the location of precursor IgA B cells and antigen sampling by these sites is not as advanced as knowledge of the mammalian system, highlighting the need for ongoing research into the avian application of novel vaccination strategies.

Development of B cells expressing surface immunoglobulin molecules that lack V(D)J-encoded determinants in the avian embryo bursa of fabricius

Immunoglobulin gene rearrangement in avian B cell precursors generates surface Ig receptors of limited diversity. It has been proposed that specificities encoded by these receptors play a critical role in B lineage development by recognizing endogenous ligands within the bursa of Fabricius. To address this issue directly we have introduced a truncated surface IgM, lacking variable region domains, into developing B precursors by retroviral gene transfer in vivo. Cells expressing this truncated receptor lack endogenous surface IgM, and the low level of endogenous Ig rearrangements that have occurred within this population of cells has not been selected for having a productive reading frame. Such cells proliferate rapidly within bursal epithelial buds of normal morphology. In addition, despite reduced levels of endogenous light chain rearrangement, those light chain rearrangements that have occurred have undergone variable region diversification by gene conversion. Therefore, although surface expression of an Ig receptor is required for bursal colonization and the induction of gene conversion, the specificity encoded by the prediversified receptor is irrelevant and, consequently, there is no obligate ligand for V(D)J-encoded determinants of prediversified avian cell surface IgM receptor.

B cell emigration directly from the cortex of lymphoid follicles in the bursa of Fabricius

In the peripheral blood (PBL) of juvenile chickens three populations of B cells have previously been distinguished based on life-span and origin of cells within each population. In this report we show that the largest PBL B cell subset, population 1 B cells, which are short-lived cells corresponding to about 60% of PBL B cells and the vast majority of bursal emigrants, exit from the bursa directly from the follicular cortex. This conclusion is based on the specific labeling of rapidly dividing cortical lymphocytes with bromodeoxyuridine, followed by their detection in the periphery prior to the appearance of bromodeoxyuridine labeled cells in the bursa medulla. Furthermore, the rate of emigration of cortical lymphocytes, 1.00 +/- 0.1% of PBL B cells per hour, is indistinguishable from the emigration rate of B cells from the bursa as a whole. The anatomical organization of the bursa has evolved to focus gut-derived antigens from the bursal lumen into the lymphoid follicles. The emigration of cortical bursal cells is discussed in relation to the exposure of bursal lymphocytes to extrinsic antigen.

Role of environmental antigens in B cell proliferation in the bursa of Fabricius at neonatal stage

Effect of environment antigens on S phase entry rate of bursa cells was studied by utilizing in vivo incorporation of bromodeoxyuridine (BrdUrd). S phase entry rate of bursa cells in chick embryo was determined to be 6.4%/h, this rate increased to 10.0%/h at neonatal stage and decreased to 6.4%/h 6 weeks after hatching. Evidence is presented that isolation of the bursa from the gut before hatching suppressed S phase entry rate of bursa cells after hatching, whereas S phase entry rates of thymocytes were not affected. On tissue sections, a decrease of BrdUrd+ cells was observed in medullary areas of the isolated bursa. It is proposed that in situ stimulation of bursa cells by environmental antigens to the bursa accelerates S phase entry rates, and that medullary areas are the major sites for antigen-driven B cell proliferation after hatching.

Bursa-dependent subpopulations of peripheral B lymphocytes in chicken blood

By selective labeling of juvenile chicken bursal cells with colloidal fluorescein isothiocyanate in situ, the emigration rate of bursal lymphocytes to the periphery was estimated at approximately 0.84% and 0.96% of the peripheral blood lymphocyte (PBL) and splenic B cell pool per hour, respectively. Emigrant bursal cells were found primarily in blood and spleen, with very small numbers migrating to thymus, bone marrow, and gut-associated lymphoid tissues. Emigrant bursal cells expressed high levels of both major histocompatibility complex class II antigen and the Ov alloantigen, a phenotype found on a population comprising approximately 4% of bursal cells from which the bursal emigrants may be derived. Surgical bursectomy at 3 weeks of age revealed that peripheral blood B cells could be divided into three distinct populations. Specifically, 60% of the peripheral blood B cells were short lived with a half-life of about 30 h in the blood. These cells accounted for the great majority of emigrants from the bursa to the peripheral blood. Approximately 35% of PBL B cells had a half-life of 12 days following bursectomy and comprised cells which did not divide in the periphery. Consequently, we propose that physiological differences between this population and the majority of bursal emigrants are established intrabursally. The remaining PBL B cells, whose relative proportion increases with age from about 5% of PBL B cells at 2-3 weeks of age, are short lived and are being continually produced from (a) post-bursal site(s) of B cell production.

Abnormalities associated with the bursal microenvironment in spontaneous dysgammaglobulinemia of UCD line 140 chickens

UCD line 140 chickens have been previously reported to develop a syndrome of spontaneous 7s immunoglobulin deficiency and the presence of autoantibodies. Earlier studies demonstrated that these inbred birds have normal peripheral blood T and B cell numbers; they also respond normally to allogeneic stimulation. Although the 7s immunodeficiency does not manifest itself until several months of age, line 140 birds have a premature degeneration of bursa. Because of the recent development of monoclonal reagents specific for bursal elements, including surface epithelium, basement membrane associated epithelium, follicle associated epithelium, and lymphoid subpopulations, we have examined line 140 and control birds for the expression of bursal epithelial cell antigens. Line 140 birds, in contrast to control chickens, have a dramatic early alteration in the expression of an epithelial cell marker in the bursa, thymus, and intestine. Moreover, to further address this issue, we transplanted bursa from 10-day embryos onto the chorioallantoic membrane, a privileged site. Bursae from control birds became abnormal when transplanted onto line 140 CAM; they remained normal when transplanted among several control chicken lines. In contrast, line 140 bursa remained abnormal independent of the transplant procedure. Due to the marked bursal abnormality observed specific to the dysgammaglobulinemia chicken line, we propose that the microenvironmental features of line 140 bursa may predispose these birds to the development of humoral immunodeficiency and autoantibodies.

Nurse cells of the bursa of Fabricius: do they exist?

Cell-cell interactions in B lymphocyte development have so far been incompletely characterized, mostly due to lack of a special organ for B cell maturation in the mammalian species. Certain well-known lymphostromal interactions in the thymus have raised the question whether similar interactions with nurse cells would also operate in the development of B cells. We have tested this hypothesis in the chicken bursa of Fabricius, an organ specific for the B cell maturation. To identify possible nurse cells, with viable lymphocytes enclosed, the cells in the bursa of Fabricius were dispersed with collagenase and trypsin. Light and electron microscopic examination of bursa cell suspensions showed four types of aggregates, identified by low magnification light microscopy as potential nurse cell-like complexes. Electron microscopy revealed that all aggregates consisted of epithelial cells, and complexes of epithelial cells with lymphocytes enclosed were not observed. These findings indicate that interactions similar to those seen in the avian and mammalian thymus between epithelial nurse cells and T lymphocytes are not a part of the avian B cell differentiation process.

Reappraisal of histogenesis in the bursal lymphoid follicle of the chicken

The development of the bursal follicle and the appearance of the follicle-associated epithelial (FAE) cell and the reticuloepithelial (REp) cell were studied. The stages of development of the bursal follicle were observed by light and electron microscopy; an anticytokeratin monoclonal antibody was also used. At the beginning of follicle development, a mesenchymal cell cluster is observed in the tunica propria; the cluster becomes wedged in a niche of the surface epithelium, and gradually it is completely surrounded by the epithelium itself, which closes under the clump of mesenchymal cells. The epithelial cells lying upon the mesenchymal clump become necrotic, and a number of mesenchymal cells bulge out, forming the FAE cells. The epithelial cells that have closed under the mesenchymal nodule become stratified and form the REp cells; they become star-shaped because the medullary-lymphoid cells grow between them. Finally, the cortex is formed, possibly as a result of the migration of medullary cells before they peripheralize. It is concluded that FAE cells are not specialized epithelial cells, as they do not react to an anticytokeratin monoclonal antibody; on the contrary, they are formed by mesenchymal stemcells that bulge into the lumen and change their character after moving into the epithelium. The REp cells appear in the follicular primordium shortly after the bursal follicle begins to develop; the pronounced reactivity of the REp cells to an anticytokeratin monoclonal antibody supports the hypothesis of their epithelial origin.

Effects of local and systemic immunization on serum antibody titres in splenectomized chickens

The role of spleen in local (per anum) and systemic (intravenous = i.v.) immunization was studied by splenectomizing chickens and immunizing them twice with sheep red blood cells (SRBC) and Brucella abortus. The number of germinal centers in spleen and the splenic weight were also recorded. Splenectomy had no effect on the serum titres evoked by the per anum immunization but it affected the anti-SRBC and, to a lesser extent, anti-Brucella titres in serum after i.v. immunization. The effect of splenectomy was less obvious both in the secondary response and when the chickens were younger at operation and the time between splenectomy and immunization was increased. The i.v. immunizations induced germinal centre formation in the spleen whereas immunization per anum did not significantly do so. Also the splenic weight in the i.v. immunized chickens was higher than that in the per anum immunized birds. The present study shows that the spleen is the major site of antibody production against i.v. administered antigens in chickens. Antibody production to antigens applied per anum occurs mainly in other lymphatic organs, most probably in the gut-associated lymphoid tissues.

Effects of antigen and antigen concentration on serum antibody titres in chickens primed per bursam

A novel method for intrabursal immunization was applied to study the priming effect in 2-day-old and 4-week-old chickens. This immunization method separates the bursa of Fabricius totally from the rest of the gut-associated lymphoid tissue (GALT), and closely mimics the natural way antigens come in contact with the bursal epithelium. Sheep red blood cells (SRBC) and Brucella abortus organisms in 4 different concentrations were used as antigens. In 4-week-old birds, a suppressive effect of per bursam priming on the secondary anti-SRBC titres was recorded, an observation not reported earlier. Priming had no effect on the secondary anti-SRBC titres in 2-day-old chickens. In both age groups, a positive priming effect on anti-Brucella titres was observed. Antigen concentrations too low to induce primary immune responses in serum were still capable of causing the priming effect. A certain minimum concentration, however, was required also for the priming effect to occur. The observed suppressive effect of priming on the secondary anti-SRBC titres suggests that bursa exerts inhibitory immuno-regulatory functions when handling certain antigens. Important factors are the age of the chickens and the antigen concentration used, which influence both the primary and the secondary immune responses.

Immunoglobulin diversification in bursal duct-ligated chickens

The role of external antigen contact on immunoglobulin (Ig) diversification occurring in chicken bursal cells was evaluated. The entry of environmental antigens into the lumen of the bursa of Fabricius was prevented by ligating the bursal duct prior to hatching (BDL: bursal duct ligation). We used two-dimensional gel electrophoresis to compare the heterogeneity of Ig molecules from bursa cells of normal and BDL chickens. We have found that Ig diversity obtained from BDL chickens' bursae in two-dimensional gel analysis was similar to that of control birds. Furthermore, by using two monoclonal anti-idiotype antibodies to study intrabursal Ig diversification we have shown that frequencies of the Cld-1 and Cld-2 idiotypes were also unaltered following bursal ligation. We conclude that primary B cell diversification in the bursa is independent of the external antigen flow from the bursal lumen.

Emigration of B cells from chicken bursa of Fabricius

The extent of emigration of cells from the bursa of Fabricius to the periphery was estimated. Per anum application of fluorescein isothiocyanate to label bursal cells in situ was used. Migrant cells can be visualized on frozen sections or cell suspensions of peripheral organs by their fluorescence. The data show that at 2-3 weeks after hatching about 5% of bursal cells leave the bursa per day. Since the bursal cells divide rapidly, this indicates that the vast majority (95%) of bursal cells die in situ. Cells that leave the bursa are surface IgM positive and go first to peripheral blood and later into B cell areas of spleen, thymus and cecal tonsils. The results are also discussed on the basis of their implication for the generation of antibody diversity in the chicken bursa.

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.