The chicken B-cell receptor complex and its role in avian B-cell development

Summary: The bursa of Fabricius is critical to normal B-lymphocyte development in birds. During embryonic life, B-cell precursors migrate to the bursal rudiment and those which have undergone productive V(D)J recombination colonize lymphoid follicles and undergo immunoglobulin V gene diversification by gene conversion. The chicken surface IgM complex appears structurally and functionally equivalent to its mammalian counterpart, with homologs to CD79a and CD79b. Expression of a truncated Ig chain is sufficient to drive the early stages of B-cell development in the embryo bursa. Bursal cells expressing the truncated receptor complex proliferate in bursal follicles, and those which contain V gene rearrangements undergo V gene diversification by gene conversion. The bursa is a gut-associated organ and antigen is focused to bursal lymphoid follicles after hatch. While expression of the truncated chain is sufficient to support B-cell development in the embryo, B cells expressing this receptor are rapidly eliminated after hatch. We suggest the possibility that B-cell development in the bursa after hatch is driven by encounter with antigen leading to redistribution of B cells within the lymphoid follicle, B-cell proliferation and V gene repertoire development by gene conversion.     

B-cell precursors in early chicken embryos.

The ontogeny of B-cell precursors in chicken embryos from day 3 of incubation onwards has been studied. Purified antibodies to chicken Ig L, gamma, mu, alpha chains were used in a sensitive indirect immunofluorescence assayed on fixed cell smears and wax-embedded tissue sections; the location and morphology of immunoglobulin positive (Ig+) cells were determined either in phase contrast or after histological staining. Lymphoid cells containing small amounts of cytoplasmic immunoglobulin were found in 3 day and older embryonic yolk sac, 11 and 12 day blood, 11, 12 and 13 day bursal mesenchyme. cIg+ large basophilic cells were first seen in 14 day bursal follicles. It is concluded that cells enter the embryonic bursa at different developmental stages: some appear to be uncommitted stem cells, whilst others have already commenced B-cell maturation in an extra-bursal site.     

Allergic contact dermatitis in the B-cell deficient chicken.

Allergic contact dermatitis to oxazolone was induced in three chickens rendered B-cell deficient by combined chemical bursectomy with testosterone and cyclophosphamide.     

Humoral immune responses characteristic of testosterone-propionate-treated chickens.

White Leghorn chickens treated with testosterone-propionate on the 3rd day of embryonation were immunized with a mixture of sheep red blood cells, Brucella abortus and Salmonella pullorum at various ages, and the resulting agglutinins were titrated. The production of IgM antibody against sheep red blood cells was not affected significantly by testosterone-propionate. On the contrary, immune responses against the bacterial antigens were strongly suppressed by the same treatment. Production of IgG antibodies was strongly suppressed by the same treatment. There was little correlation between the production of IgM antibody against sheep red blood cells and the presence of bursal follicles. Immune responses against bacterial antigens correlated with the presence of the follicles. Production of IgG antibodies also correlated with the maintenance of bursal lymphoid structure.     

Early and late B-cell development in the mouse.

A common principle in B-cell development is the stringent selection of cells expressing appropriate antibody V regions as surface receptors. Cells failing to do so appear destined to rapid death. These life-death decisions are mediated by signals whose nature is not yet understood but whose generation involves immunoglobulin receptor complexes on B cells and B-cell progenitors.     

The surrogate light chain in B-cell development

The proteins encoded by the V_prB and λ_s genes associate with each other to form a light (L) chain-like structure, the surrogate L chain. It can form Ig-like complexes with three partners - the classical heavy (H) chain, the D_HJ_HC_μ-protein, or the newly discovered p55 chain; these are expressed on the surface of pre-B cells at different stages of development. Here, Friz Melchers and colleagues review the structures of the V_preB and λ_s genes in mouse and their relatives in humans, describe their pattern of expression, and speculate on their possible evolution and functions.     

B-cell development in the amphibian Xenopus

Summary: The amphibian Xenopus and mammals have similar organization and usage of their immunoglobulin gene loci with combinatorial joining of V, D and J elements. The differences in B‐cell development between mammals and this amphibian are due to major differences in developmental kinetics, cell number and lymphoid organ architecture. Unlike mammals, the immune system of Xenopus develops early under pressure to develop quickly and to produce a heterogeneous repertoire before lymphocyte numbers reach 5,000, thereby imposing a limitation on clonal amplification. In addition, it is submitted to metamorphosis. Thus, during the early antigen-independent period, several features of B‐cell development related to immune diversification are under strict genetically preprogramed control: 1) D reading frames contribute complementary determining region 3 with features that occur in mammals by somatic selection, 2) the temporal stepwise utilization of VH genes in Xenopus occur in families probably because of structural DNA features rather than their position in the locus. Larval and adult immune responses differ in heterogeneity. Larval rearrangements lack N diversity. During the course of immune responses, somatic mutants are generated at the same rate as in other vertebrates but are not optimally selected, probably due to the simpler organization of the lymphoid organs, with neither lymph nodes nor germinal centers resulting in poor affinity maturation. Switch from IgM to other isotypes is mediated by loop-excision deletion of the IgM constant region gene via switch regions which, unlike their mammalian counterpart, are A-T rich and reveal conserved microsites for the breakpoints.     

The role of microRNAs in B-cell development and function

MicroRNA (miRNA)-mediated gene silencing at the translational level has led to novel discoveries for numerous biological processes. Recently, there has been increasing evidence to indicate that miRNAs are involved in normal immune functions and inflammation. In this review, we focus on recent advances that have elucidated the role of miRNAs in B-cell development, differentiation, apoptosis and function. While the regulatory mechanisms of miRNAs in controlling and maintaining B-cell fate remain largely uncharacterized, further studies on miRNAs and their targets will increase our understanding of B-cell development and function. Such studies may be able to provide new therapeutic strategies for treating autoimmune diseases.     

Macrophage antimicrobial functions in a chicken MHC chromosome dosage model.

Macrophages respond to certain inflammatory signals with a marked increase in respiratory burst and the production of reactive oxygen intermediates; these metabolites play an essential role in the destruction of invading microorganisms. In this study, macrophage antibacterial inflammatory responses were compared among chickens having two (disomic), three (trisomic), or four (tetrasomic) copies of the major histocompatibility complex (MHC)-encoding microchromosome (B15 haplotype). Phorbol myristate acetate (PMA)-stimulated superoxide anion (O2-) production by cross-linked dextran (Sephadex)-elicited peritoneal macrophages was measured at early (4 h), intermediate (24 h), and late (42 h) stages of the inflammatory response using ferrocytochrome c reduction. Significantly elevated O2- production was observed for trisomic versus disomic macrophages during both early and intermediate stages of the inflammatory response. Late in the response, tetrasomic macrophages produced a significantly higher level of O2- than disomic cells. When PMA was used to trigger hydrogen peroxide (H2O2) production, no significant genotype difference was found for any stage of the inflammatory response. Phagocytosis of heat-killed Salmonella enteritidis by macrophages differed among the three genotypes: trisomic macrophages were superior to disomic cells during early inflammation, no genotypic difference was observed at the intermediate stage, and disomic cells had greater phagocytic capacity than aneuploid macrophages late in the response. Likewise, when S. enteritidis was cultured with macrophages to induce oxygen intermediate secretion, H2O2 production followed a kinetic pattern among the genotypes similar to that observed for bacterial phagocytosis. Endogenous superoxide dismutase (SOD), catalase, and glutathione peroxidase (GP) activities were determined for the macrophages during intermediate and late inflammatory stages. Tetrasomic macrophages had reduced SOD activity at the late stage, no significant difference was observed in catalase activity among genotypes at either time point, and trisomic macrophages had enhanced GP activity compared to disomic cells at both time points. These results indicate that differences in MHC gene dosage are associated with differences in chicken macrophage activation for the acquisition of selected antibacterial functions.     

The development of fibrocartilage in the elastic tendon of the chicken wing

The elastic tendon of the chicken wing has five morphologically distinct regions. One of these regions is a distally located fibrocartilage from which fibrous connections extend to the capsule of the distal radius. In adult birds, this region shows the characteristics of a tendon-compressed fibrocartilage, with an accumulation of proteoglycans between thick collagen bundles arranged in a basket-weave formation. Here we study the development of this fibrocartilage in order to of compare it with other tendon fibrocartilages and try to identify the factors involved in fibrocartilage differentiation. This fibrocartilage initially developed by cell enlargement and accumulation of vimentin, with simultaneous deposition of proteoglycans in the extracellular matrix and an increase in the amount and thickness of collagen bundles. Elastic fibers were minor components associated with the collagen bundles. Cells could be classified into two main types. One was typically fibrocartilaginous and the other was fibroblast-like, the latter occurring in close association with the collagen bundles. These results establish the steps in the development of the elastic tendon fibrocartilage and provide a basis for future studies.     

Protein kinase C family functions in B-cell activation

Members of the protein kinase C (PKC) family play important but distinct roles in B-cell activation, as demonstrated by emerging genetic and biochemical studies. PKCbeta is indispensable for B-cell antigen receptor (BCR)-induced NF-kappaB activation and B-cell survival. Recent evidence indicates that PKCbeta might regulate inhibitor of kappaB kinase (IKK) and NF-kappaB activation through interaction with the CARMA1/Bcl10/MALT signaling complex in BCR microdomains. By contrast, the novel PKC isoform PKCdelta is specifically required to maintain the tolerance of self-reactive B cells.     

Purine metabolism and B-lymphocyte development in the chicken bursa of fabricius.

The activity of three enzymes involved in the salvage pathway of purine nucleosides--purine nucleoside phosphorylase (PNP), xanthine dehydrogenase (XDH), and hypoxanthine-guanine phosphoribosyl transferase (HGPRT)--was investigated in cellular fractions of the chicken bursa of Fabricius differentially enriched in epithelial cells or lymphocytes. Markedly increasing levels of PNP and XDH were observed along with the enrichment in epithelial cells together with a slight, though significant, decrease in HGPRT activity. By contrast, a dramatic fall in PNP and XDH activities was detected along with the enrichment in lymphocytes together with a slight, though significant, increase in HGPRT activity. This sharply different distribution of the three enzymes, all sharing hypoxanthine as a substrate, clearly indicates that lymphocytes preferentially channel hypoxanthine into the salvage and interconversion pathways, phosphorylating it to IMP, while epithelial cells rapidly catabolize such a purine base to uric acid. Moreover, epithelial cells, unlike lymphocytes, are able to retain high intracellular levels of both hypoxanthine and inosine. These results support the possibility that epithelial cells contribute to the normal development of bursal lymphocytes by supplying such actively proliferating cells with purine rings and at the same time by preventing them from accumulating potentially toxic high levels of purine nucleotides being able to rapidly eliminate excess hypoxanthine as uric acid from the bursa environment into the bloodstream.     

Haemolysis in chicken serum: II. Ontogenetic development

Development of total complement (C) and C1 activity was followed in Line 96 chickens from day 13 of embryonic life to 40 days post-hatching. Both activities were demonstrable on day 13, and levels rose slowly in the late prehatching period. At hatching, on day 21, there was a sharp rise in both activities; both titres were roughly five times those of day 19 embryos. Further increases were seen to about day 10, followed by a levelling off (perhaps even a drop in the case of C1) for about 10 days. On about the twenty-first day the titration curve rose again. The source of the C detected in the embryo and young chicken is unknown. The pattern is consistent with transfer from the egg, but it might also reflect synthesis by the developing animal.     

Early B-cell development in chickens, sheep and rabbits.

Studies of the immune system of various species have revealed that antibody repertoire can be generated in many different ways. This review underlines some general principles for comparing the different processes which represent the basic framework of these systems.     

Growing up on the streets: why B-cell development differs from T-cell development.

B-cell development differs significantly from T-cell development in that negative selection of autoreactive B cells can occur in the same microenvironment in which productive immune responses begin. Here, Sarah Townsend and colleagues discuss how this 'growing up on the streets' might provide a mechanism that fills holes in the B-cell repertoire, much as major histocompatibility complex polymorphism fills holes in the T-cell repertoire.     

B-cell antigen-receptor signalling in lymphocyte development

Signalling through the B-cell antigen receptor (BCR) is required throughout B-cell development and peripheral maturation. Targeted disruption of BCR components or downstream effectors indicates that specific signalling mechanisms are preferentially required for central B-cell development, peripheral maturation and repertoire selection. Additionally, the avidity and the context in which antigen is encountered determine both cell fate and differentiation in the periphery. Although the signalling and receptor components required at each stage have been largely elucidated, the molecular mechanisms through which specific signalling are evoked at each stage are still obscure. In particular, it is not known how the pre-BCR initiates the signals required for normal development or how immature B cells regulate the signalling pathways that determine cell fate. In this review, we will summarize the recent studies that have defined the molecules required for B-cell development and maturation as well as the theories on how signals may be regulated at each stage.