The pre-B cell receptor and its role in proliferation and Ig heavy chain allelic exclusion

Contribution of Immunoglobulin Enhancers to B Cell Nuclear Organization

B cells undergo genetic rearrangements at immunoglobulin gene (Ig) loci during B cell maturation. First V(D)J recombination occurs during early B cell stages followed by class switch recombination (CSR) and somatic hypermutation (SHM) which occur during mature B cell stages. Given that RAG1/2 induces DNA double strand breaks (DSBs) during V(D)J recombination and AID (Activation-Induced Deaminase) leads to DNA modifications (mutations during SHM or DNA DSBs during CSR), it is mandatory that IgH rearrangements be tightly regulated to avoid any mutations or translocations within oncogenes. Ig loci contain various cis-regulatory elements that are involved in germline transcription, chromatin modifications or RAG/AID recruitment. Ig cis-regulatory elements are increasingly recognized as being involved in nuclear positioning, heterochromatin addressing and chromosome loop regulation. In this review, we examined multiple data showing the critical interest of studying Ig gene regulation at the whole nucleus scale. In this context, we highlighted the essential function of Ig gene regulatory elements that now have to be considered as nuclear organizers in B lymphocytes.

Conditional antibody expression to avoid central B cell deletion in humanized HIV-1 vaccine mouse models

HIV-1 vaccine development aims to elicit broadly neutralizing antibodies (bnAbs) against diverse viral strains. In some HIV-1-infected individuals, bnAbs evolved from precursor antibodies through affinity maturation. To induce bnAbs, a vaccine must mediate a similar antibody maturation process. One way to test a vaccine is to immunize mouse models that express human bnAb precursors and assess whether the vaccine can convert precursor antibodies into bnAbs. A major problem with such mouse models is that bnAb expression often hinders B cell development. Such developmental blocks may be attributed to the unusual properties of bnAb variable regions, such as poly-reactivity and long antigen-binding loops, which are usually under negative selection during primary B cell development. To address this problem, we devised a method to circumvent such B cell developmental blocks by expressing bnAbs conditionally in mature B cells. We validated this method by expressing the unmutated common ancestor (UCA) of the human VRC26 bnAb in transgenic mice. Constitutive expression of the VRC26UCA led to developmental arrest of B cell progenitors in bone marrow; poly-reactivity of the VRC26UCA and poor pairing of the VRC26UCA heavy chain with the mouse surrogate light chain may contribute to this phenotype. The conditional expression strategy bypassed the impediment to VRC26UCA B cell development, enabling the expression of VRC26UCA in mature B cells. This approach should be generally applicable for expressing other bnAbs that are under negative selection during B cell development.

Signalling circuits that direct early B-cell development

In mammals, the B-cell lineage arises from pluripotent progenitors in the bone marrow. During their development, B-cells undergo lineage specification and commitment, followed by expansion and selection. These processes are mediated by regulated changes in gene expression programmes, rearrangements of immunoglobulin (Ig) genes, and well-timed rounds of proliferation and apoptosis. Many of these processes are initiated by environmental factors including cytokines, chemokines, and cell-cell contacts. Developing B-cells process these environmental cues into stage-specific functions via signalling pathways including the PI3K, MAPK, or JAK-STAT pathway. The cytokines FLT3-Ligand and c-Kit-Ligand are important for the early expansion of the B-cell precursors at different developmental stages and conditions. Interleukin 7 is essential for commitment to the B-cell lineage and for orchestrating the Ig recombination machinery. After rearrangement of the immunoglobulin heavy chain, proliferation and apoptosis, and thus selection, are mediated by the clonal pre-B-cell receptor, and, following light chain rearrangement, by the B-cell receptor.

Molecular Regulation of Differentiation in Early B-Lymphocyte Development

B-lymphocyte differentiation is one of the best understood developmental pathways in the hematopoietic system. Our understanding of the developmental trajectories linking the multipotent hematopoietic stem cell to the mature functional B-lymphocyte is extensive as a result of efforts to identify and prospectively isolate progenitors at defined maturation stages. The identification of defined progenitor compartments has been instrumental for the resolution of the molecular features that defines given developmental stages as well as for our understanding of the mechanisms that drive the progressive maturation process. Over the last years it has become increasingly clear that the regulatory networks that control normal B-cell differentiation are targeted by mutations in human B-lineage malignancies. This generates a most interesting link between development and disease that can be explored to improve diagnosis and treatment protocols in lymphoid malignancies. The aim of this review is to provide an overview of our current understanding of molecular regulation in normal and malignant B-cell development.

Activation-Induced Cytidine Deaminase Expression in Human B Cell Precursors Is Essential for Central B Cell Tolerance

Activation-induced cytidine deaminase (AID), the enzyme-mediating class-switch recombination (CSR) and somatic hypermutation (SHM) of immunoglobulin genes, is essential for the removal of developing autoreactive B cells. How AID mediates central B cell tolerance remains unknown. We report that AID enzymes were produced in a discrete population of immature B cells that expressed recombination-activating gene 2 (RAG2), suggesting that they undergo secondary recombination to edit autoreactive antibodies. However, most AID+ immature B cells lacked anti-apoptotic MCL-1 and were deleted by apoptosis. AID inhibition using lentiviral-encoded short hairpin (sh)RNA in B cells developing in humanized mice resulted in a failure to remove autoreactive clones. Hence, B cell intrinsic AID expression mediates central B cell tolerance potentially through its RAG-coupled genotoxic activity in self-reactive immature B cells.

BTK inhibitors in chronic lymphocytic leukemia: a glimpse to the future

The treatment of chronic lymphocytic leukemia (CLL) with inhibitors targeting B cell receptor signaling and other survival mechanisms holds great promise. Especially the early clinical success of Ibrutinib, an irreversible inhibitor of Bruton's tyrosine kinase (BTK), has received widespread attention. In this review we will focus on the fundamental and clinical aspects of BTK inhibitors in CLL, with emphasis on Ibrutinib as the best studied of this class of drugs. Furthermore, we summarize recent laboratory as well as clinical findings relating to the first cases of Ibrutinib resistance. Finally, we address combination strategies with Ibrutinib, and attempt to extrapolate its current status to the near future in the clinic.

Recirculating bone marrow B cells in C57BL/6 mice are more tolerant of highly hydrophobic and highly charged CDR-H3s than those in BALB/c mice

To test whether mechanisms controlling the range of diversity of the developing antibody repertoire in C57BL/6 mice (IgH(b)) operate similarly to those identified in BALB/c mice (IgH(a)), we compared the sequences of VH 7183-containing H-chain transcripts from sorted adult bone marrow C57BL/6 B-cell subsets with those previously obtained from BALB/c mice. Patterns of VDJ gene segment utilization and CDR-H3 amino acid composition, charge, and average length in C57BL/6 pro-B cells were similar, although not identical, to BALB/c pro-B cells. However, C57BL/6 mature, recirculating B cells failed to demonstrate the reduction in the use of VH81X and the narrowing in the range of variance of CDR-H3 hydrophobicity that characterizes B-cell maturation in BALB/c mice. To further test the ability of the C57BL/6 strain to discard B cells expressing highly charged CDR-H3s, we introduced a mutant IgH(a) DH allele that forces use of arginine, asparagine, and histidine. Unlike BALB/c mice, C57BL/6 mice congenic for the charged DH maintained normal numbers of mature, recirculating B cells that were enriched for charged CDR-H3s. Together these findings indicate that the mature C57BL/6 B-cell pool permits expression of immunoglobulins with antigen-binding sites that are typically discarded during late-stage bone marrow B-cell development in BALB/c mice.

The pre-B cell receptor; selecting for or against autoreactivity

Antibodies represent a crucial component of humoral immunity as protection against invading pathogens, to which they bind and thereby trigger mechanisms that lead to the disposal of the pathogen. Antibodies are assembled from Ig heavy chains (HCs) and light chains (LCs) and are found in both a secreted and a membrane-bound form, termed B cell receptors (BCRs), where the latter allows the 'right' B cell to respond upon recognition of its cognate antigen. The antibody repertoire is almost unlimited because of a process in which germ line V(D)J gene segments, encoding the variable (antigen-binding) region of the antibody HCs and LCs, are recombined. As this process is random, it is apparent that it results in a vast variety of antibodies, those that recognize foreign but also those that recognize self- (auto-) antigens. Control mechanisms are, therefore, in place to ensure that as few autoreactive B cells as possible are allowed to proceed in development. This counter-selection takes place through various mechanisms and at several stages as the cells develop from pre-B cells to antibody-secreting plasma cells. At the first major checkpoint, at the pre-BI to pre-BII cell transition, antibody HCs assemble with the invariant surrogate LC (SLC) forming a pre-BCR. Herein, we will discuss the role of the pre-BCR in the selection at this stage, how a dysfunctional pre-BCR affects selection and its effects on later stages, and whether the pre-BCR selects for or against autoreactivity.

Selection of individual VH genes occurs at the pro-B to pre-B cell transition

B cells are subjected to selection at multiple checkpoints during their development. The selection of Ab H chains is difficult to study because of the large diversity of the CDR3. To study the selection of individual Ab H chain V region genes (V(H)), we performed CDR3 spectratyping of ∼ 75-300 rearrangements per individual V(H) in C57BL6/J mice. We measured the fraction of rearrangements that were in-frame in B cell DNA. We demonstrate that individual V(H)s have different fractions of in-frame rearrangements (IF fractions) ranging from 10 to 90% and that these IF fractions are reproducible in different mice. For most V(H)s, the IF fraction in pro-B cells approximated 33% and then shifted to the nearly final (mature) B cell value by the cycling pre-B cell stage. The frequency of high in-frame (IF) V(H) usage increased in cycling pre-B cells compared with that in pro-B cells, whereas this did not occur for low IF V(H)s. The IF fraction did not shift as much in BCR-expressing B cells and was minimally affected by L chain usage for most V(H). High IF clan II/III V(H)s share more positively charged CDR2 sequences, whereas high IF clan I J558 CDR2 sequences are diverse. These data indicate that individual V(H)s are subjected to differential selection, that V(H) IF fraction is mainly established through pre-BCR-mediated selection, that it may operate differently in clan I versus II/III V(H)s, and that it has a lasting influence on the Ab repertoire.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

Id1 has a physiological role in regulating early B lymphopoiesis

Basic helix-loop-helix E proteins play critical roles in B-cell development by stimulating B cell-specific gene expression and immunoglobulin gene rearrangement. The function of E proteins can be effectively suppressed by their naturally occurring inhibitors, Id1 to 4. Ectopic expression of Id1 has been shown to block B-cell development at the early pro-B cell stage. However, whether Id1 plays a physiological role in controlling B lymphopoiesis was not known. Although Id1-deficient mice do not exhibit significant abnormalities in steady-state B lymphopoiesis, we detected more robust B-cell engraftment in transplant recipients of Id1-deficient bone marrow compared to those of wild-type donor cells. In culture, Id1 ablation dramatically enhances B-lineage cell production without any marked effects on myeloid differentiation. Consistently, Id1 expression was found in pro-B but not pre-B cells as measured by enhanced green fluorescent protein (EGFP) fluorescence and by quantitative reverse transcription-PCR. Although loss of Id1 did not alter the number of B-cell colonies generated from whole bone marrow or the proliferation rate of developing B cells, B-cell colonies were detectable at a much earlier time point and the size of the colonies were larger. Therefore, we infer that Id1-deficient progenitors possess higher potential to differentiate to the pre-B cell stage when a proliferative burst occurs. Taken together, we present evidence to suggest that Id1 plays a physiological role in restraining the developmental progression, which may be important for proper B-cell differentiation in the bone marrow.

Transcription of productive and nonproductive VDJ-recombined alleles after IgH allelic exclusion

The process of allelic exclusion ensures that each B cell expresses a B-cell receptor encoded by only one of its Ig heavy (IgH) and light (IgL) chain alleles. Although its precise mechanism is unknown, recruitment of the nonfunctional IgH allele to centromeric heterochromatin correlates with the establishment of allelic exclusion. Similarly, recruitment in activated splenic B cells correlates with cell division. In the latter, the recruited IgH allele was reported to be transcriptionally silent. However, it is not known whether monoallelic recruitment during establishment of allelic exclusion correlates with transcriptional silencing. To investigate this, we assessed the transcriptional status of both IgH alleles in single primary cells over the course of B-cell development, using RNA fluorescence in situ hybridization. Before allelic exclusion both alleles are transcribed. Thereafter, in pre-BII and subsequent developmental stages both functional and nonfunctional VDJ- and DJ-transcription is observed. Thus, after the establishment of IgH allelic exclusion, monoallelic recruitment to heterochromatin does not silence VDJ- or DJ-transcription, but serves another purpose.

Transcriptome analysis in primary B lymphoid precursors following induction of the pre-B cell receptor

Pre-BCR signals are part of a checkpoint where early precursor (pre-) B cells with a pairing Ig muH chain (muHC) are clonally expanded before they differentiate into IgL-rearranging, resting pre-B cells. A pre-BCR consists of two muHCs, two surrogate L chains and the signal transducer Igalpha/Igbeta. The molecular circuits by which the pre-BCR controls proliferation and differentiation of pre-B cells are poorly characterized. Therefore, we identified the differential transcriptome by genome-wide expression profiling in progenitor (pro-) B cells from a Rag2-deficient mouse, in which the expression of a transgenic muHC and thus a pre-BCR as well as pre-BCR-mediated clonal expansion can be controlled by tetracycline (muHC-inducible mouse). This analysis revealed that pre-BCR signals upregulate components of the BCR signalosome, open the IgL chain (LC) locus and induce the krüppel-like transcription factor KLF2, a key regulator of quiescence and lymphocyte migration. Hence, pre-BCR signals establish the molecular network for BCR signaling even before the production of an IgLC and induce the expression of KLF2, a candidate for controlling clonal expansion and migration of functional pre-B cells.

Generation and characterization of stromal cell independent IL-7 dependent B cell lines

In-vitro B cell cultures have played a significant role in the study of B cell development. Their utility in developmental and biochemical studies, however, has been limited by the challenges associated with obtaining and maintaining adequate cell numbers of pure and/or rare populations. Although B cell lines allow for circumvention of some of these issues, they have traditionally been generated via viral infection or genetic transformation and are thus less representative of in-vivo cells. In order to avoid such alterations in cell state, we have designed a procedure for the creation of B cell lines directly from murine bone marrow. In this study, we describe the generation and characterization of these IL-7 dependent cell lines. Our lines, established from both wild type and mutant mice, do not require stromal cell support for generation or maintenance. In addition, clones survive repeated freeze/thaw cycles and, in the presence of IL-7, can be kept in culture indefinitely. Phenotypically, our lines resemble pro/pre-B cells and exhibit IL-7 and preBCR signaling profiles that mimic ex-vivo B cells. These lines promise to be useful in the study of the signaling pathways that regulate B cell development.

IgG1 B cell receptor signaling is inhibited by CD22 and promotes the development of B cells whose survival is less dependent on Ig alpha/beta

We describe a mouse strain in which B cell development relies either on the expression of membrane-bound immunoglobulin (Ig) gamma1 or mu heavy chains. Progenitor cells expressing gamma1 chains from the beginning generate a peripheral B cell compartment of normal size with all subsets, but a partial block is seen at the pro- to pre-B cell transition. Accordingly, gamma1-driven B cell development is disfavored in competition with developing B cells expressing a wild-type (WT) IgH locus. However, the mutant B cells display a long half-life and accumulate in the mature B cell compartment, and even though partial truncation of the Ig alpha cytoplasmic tail compromises their development, it does not affect their maintenance, as it does in WT cells. IgG1-expressing B cells showed an enhanced Ca(2+) response upon B cell receptor cross-linking, which was not due to a lack of inhibition by CD22. The enhanced Ca(2+) response was also observed in mature B cells that had been switched from IgM to IgG1 expression in vivo. Collectively, these results suggest that the gamma1 chain can exert a unique signaling function that can partially replace that of the Ig alpha/beta heterodimer in B cell maintenance and may contribute to memory B cell physiology.

The pre-B-cell receptor

The pre-B-cell receptor (pre-BCR) is composed of two immunoglobulin mu heavy chains and two surrogate light chains, which associate with the signaling molecules Igalpha and Igbeta (Igalpha/beta). The production of a functional pre-BCR is the first checkpoint in the current model of B-cell development. The pre-BCR mediates signals resulting in heavy chain allelic exclusion, down-regulation of the recombination machinery, developmental progression, V(H) repertoire selection, proliferation and down-regulation of the surrogate light chain genes. Recent studies suggest that some of these processes could take place at an earlier stage in B-cell development than previously thought, and might not result from signals mediated by the pre-BCR.

Fate decisions regulating bone marrow and peripheral B lymphocyte development

In adult mammals, bone marrow pluripotent hematopoietic stem cells generate B lymphoid-specified progeny that progress through a series of well-characterized stages before generating B-cell receptor expressing B lymphocytes. These functionally immature B lymphocytes then migrate to the spleen wherein they differentiate through transitional stages into follicular or marginal zone B lymphocytes capable of responding to T-dependent and -independent antigens, respectively. During the terminal stages of B lymphocyte development in the bone marrow, as well as immediately following egress into the peripheral compartments, B lymphocytes are counterselected to eliminate B lymphocytes with potentially dangerous self-reactivity. These developmental and selection events in the bone marrow and periphery are dependent on the integration of intrinsic genetic programs with extrinsic microenvironmental signals that drive progenitors toward increasing B lineage commitment and maturation. This chapter provides a comprehensive overview of the various stages of primary and secondary B lymphocyte development with an emphasis on the selection processes that affect decisions at critical checkpoints. Our intent is to stress the concept that at many steps in the developmental process leading to a mature immunocompetent B lymphocyte, B lineage cells are integrating multiple and different signaling inputs that are translated into specific and appropriate cell fate decisions.

Mechanism and control of V(D)J recombination at the immunoglobulin heavy chain locus

V(D)J recombination assembles antigen receptor variable region genes from component germline variable (V), diversity (D), and joining (J) gene segments. For B cells, such rearrangements lead to the production of immunoglobulin (Ig) proteins composed of heavy and light chains. V(D)J is tightly controlled at the Ig heavy chain locus (IgH) at several different levels, including cell-type specificity, intra- and interlocus ordering, and allelic exclusion. Such controls are mediated at the level of gene segment accessibility to V(D)J recombinase activity. Although much has been learned, many long-standing questions regarding the regulation of IgH locus rearrangements remain to be elucidated. In this review, we summarize advances that have been made in understanding how V(D)J recombination at the IgH locus is controlled and discuss important areas for future investigation.

Powered by pairing: The surrogate light chain amplifies immunoglobulin heavy chain signaling and pre-selects the antibody repertoire

Selective expansion of functional pre-B cells is accomplished by the assembly of a signaling-competent pre-B cell receptor (pre-BCR) consisting of immunoglobulin mu heavy chains (muHC), surrogate light chains (SLC) and Igalpha/Igbeta. Here, we review recent data showing that muHCs, in the absence of SLC, deliver autonomous differentiation signals. However, enhanced signaling necessary for pre-B cell expansion requires cross-linking of pre-BCRs via the non-immunoglobulin tail of SLC's subunit lambda5. We also discuss how SLC's ability to modulate the strength of pre-BCR signals is controlled by a muHC's idiotype and its affinity to the chaperone BiP. In this model, BiP in concert with SLC functions as a pre-selector of the antibody repertoire.

Similarity and divergence in the development and expression of the mouse and human antibody repertoires

Over the past twenty years diverse groups in Northeast Asia, Western Europe, and North America have competed to map, sequence, and characterize the immunoglobulin loci of mouse and man. Now that this work is near completion, it has become evident that the human and mouse germline repertoires share broad similarities in gene composition, organization, and other general principles. In spite of these similarities, the repertoires expressed by adult mice and humans are distinct and differ from each other in detail. In both species the mechanisms used to create repertoire diversity appear designed to generate a random range of antigen binding sites. However, a detailed analysis reveals significant constraints in the sequence and amino acid composition of the third complementarity region of the H chain (CDR-H3), which lies at the center of the antigen binding site. The mechanisms used to regulate the composition of the repertoire, their significance to the development and maintenance of immune competence, and the contribution of violation of normal repertoire boundaries to the development of diseases of immune function remain foci of ongoing investigation.

The pre-B-cell receptor induces silencing of VpreB and lambda5 transcription

The pre-B-cell receptor (pre-BCR), composed of Ig heavy and surrogate light chain (SLC), signals pre-BII-cell proliferative expansion. We have investigated whether the pre-BCR also signals downregulation of the SLC genes (VpreB and lambda5), thereby limiting this expansion. We demonstrate that, as BM cells progress from the pre-BI to large pre-BII-cell stage, there is a shift from bi- to mono-allelic lambda5 transcription, while the second allele is silenced in small pre-BII cells. A VpreB1-promoter-driven transgene shows the same pattern, therefore suggesting that VpreB1 is similarly regulated and thereby defines the promoter as a target for transcriptional silencing. Analyses of pre-BCR-deficient mice show a temporal delay in lambda5 downregulation, thereby demonstrating that the pre-BCR is essential for monoallelic silencing at the large pre-BII-cell stage. Our data also suggest that SLP-65 is one of the signaling components important for this process. Furthermore, the VpreB1/lambda5 alleles undergo dynamic changes with respect to nuclear positioning and heterochromatin association, thereby providing a possible mechanism for their transcriptional silencing.

The pre-B cell receptor in B cell development: recent advances, persistent questions and conserved mechanisms

B cell development is a process tightly regulated by the orchestrated signaling of cytokine receptors, the pre-B cell receptor (BCR) and the B cell receptor (BCR). It commences with common lymphoid progenitors (CLP) up-regulating the expression of B cell-related genes and committing to the B cell lineage. Cytokine signaling (IL-7, stem cell factor, FLT3-L) is essential at this stage of development as it suppresses cell death, sustains proliferation and facilitates heavy chain rearrangements. As a result of heavy chain recombination, the pre-BCR is expressed, which then becomes the primary determiner of survival, cell cycle entry and allelic exclusion. In this review, we discuss the mechanisms of B cell lineage commitment and describe the signaling pathways that are initiated by the pre-BCR. Finally, we compare pre-BCR and pre-TCR structure, signal transduction and function, drawing parallels between early pre-B and pre-T cell development.

Development of the expressed Ig CDR-H3 repertoire is marked by focusing of constraints in length, amino acid use, and charge that are first established in early B cell progenitors

To gain insight into the mechanisms that regulate the development of the H chain CDR3 (CDR-H3), we used the scheme of Hardy to sort mouse bone marrow B lineage cells into progenitor, immature, and mature B cell fractions, and then performed sequence analysis on V(H)7183-containing Cmu transcripts. The essential architecture of the CDR-H3 repertoire observed in the mature B cell fraction F was already established in the early pre-B cell fraction C. These architectural features include V(H) gene segment use preference, D(H) family usage, J(H) rank order, predicted structures of the CDR-H3 base and loop, and the amino acid composition and average hydrophobicity of the CDR-H3 loop. With development, the repertoire was focused by eliminating outliers to what appears to be a preferred repertoire in terms of length, amino acid composition, and average hydrophobicity. Unlike humans, the average length of CDR-H3 increased during development. The majority of this increase came from enhanced preservation of J(H) sequence. This was associated with an increase in the prevalence of tyrosine. With an accompanying increase in glycine, a shift in hydrophobicity was observed in the CDR-H3 loop from near neutral in fraction C (-0.08 +/- 0.03) to mild hydrophilic in fraction F (-0.17 +/- 0.02). Fundamental constraints on the sequence and structure of CDR-H3 are thus established before surface IgM expression.

Both the pre-BCR and the IL-7Ralpha are essential for expansion at the pre-BII cell stage in vivo

During B cell development, proliferative expansion takes place after expression of the pre-BCR. At this pre-BII cell stage, the IL-7Ralpha is also expressed. Some in vitro studies suggest that pre-BCR-dependent expansion relies on the IL-7Ralpha, and others that it does not. It has also been suggested that the pre-BCR mediates down-regulation of the IL-7Ralpha. However, the in vivo relationship between the pre-BCR and the IL-7Ralpha has not been previously examined. Here, we have investigated this by establishing mice lacking both receptors. Our results show that in the absence of the IL-7Ralpha, the pre-BII population is reduced, as previously seen in mice lacking the pre-BCR, demonstrating that the IL-7Ralpha is important at this stage. A deficiency in both receptors results in a further reduction of the pre-BII cell population. We conclude that both the IL-7Ralpha and the pre-BCR are required for optimal pre-BII cell expansion. Furthermore, IL-7Ralpha expression levels are normal in pre-BCR-deficient mice, suggesting that the pre-BCR does not mediate its down-regulation. As a consequence of the absence of both receptors, the peripheral B cell pool is severely depleted, resulting in atypical splenic B cell structures and reduced serum Ig levels.

The Unique Region of Surrogate Light Chain Component λ5 Is a Heavy Chain-Specific Regulator of Precursor B Cell Receptor Signaling

Signals transduced by precursor-BCRs (pre-BCRs) composed of Ig mu heavy chains (HCs) and the surrogate L chain components lambda5 and VpreB are critical for B cell development. A conserved unique region (UR) of lambda5 was shown to activate pre-BCR complexes in transformed cells and to engage putative ligands, but its contribution to pre-B cell development is not known. It is also not clear why the lambda-like sequences in lambda5 are used to select HCs that will associate mainly with kappa L chains. In this study, we show that, in transformed and primary mouse B cell progenitors, receptors containing full-length HCs and lacking the lambda5UR were expressed at higher surface levels, but exhibited reduced activity compared with normal pre-BCRs in supporting developmental changes that accompany the progenitor to pre-B cell transition in primary cell culture systems and in the bone marrow in vivo. In contrast, deletion of the lambda5UR did not change net signaling output by the Dmu-pre-BCR, a developmentally defective receptor that exhibited impaired activity in the primary cell culture system. Moreover, the lambda-like sequences in lambda5 were more accommodating than kappa in supporting surface expression and signaling by the different HCs. These results show that the lambda5UR is important, although not essential, for surrogate L chain-dependent receptor signaling in primary cells, and furthermore may help allow discrimination of signaling competency between normal and Dmu-pre-BCRs. That the lambda-like portion of lambda5 in the absence of the UR was nondiscriminatory suggests that the lambda5UR focuses pre-BCR-dependent selection on the HC V region.

Activation of NF-κB promotes the transition of large, CD43+ pre-B cells to small, CD43− pre-B cells

The regulation of the transcription factor nuclear factor-kappaB (NF-kappaB) during B-cell development was examined using cells isolated from the bone marrow of transgenic mice expressing a kappaB luciferase reporter gene. The results indicate that the highest level of NF-kappaB activity is present in cells expressing the pre-B-cell receptor. Furthermore, cross-linking of Igbeta on CD43(+) pre-B cells is able to activate NF-kappaB in recombination-activating gene 1-deficient mice, preceding their further differentiation into CD43(-) pre-B cells. Expression of a dominant negative form of IkappaBalpha using a transgenic approach or by retroviral infection leads to a reduction in the number of CD43(+) pre-B cells. These data therefore indicate that activation of NF-kappaB in CD43(+) pre-B cells, as a result of signaling by the pre-B-cell receptor, facilitates the continued development of large, CD43(+) pre-B cells into small CD43(-) pre-B cells.

Regulation of immunoglobulin heavy-chain gene rearrangements

Regulated assembly of antigen receptor gene segments to produce functional genes is a hallmark of B- and T-lymphocyte development. The immunoglobulin heavy-chain (IgH) and T-cell receptor beta-chain genes rearrange first in B and T lineages, respectively. Both loci require two recombination events to assemble functional genes; D-to-J recombination occurs first followed by V-to-DJ recombination. Despite similarities in overall rearrangement patterns, each locus has unique regulatory features. Here, we review the characteristics of IgH gene rearrangements such as developmental timing, deletion versus inversion, DH gene segment utilization, ordered recombination of VH gene segments, and feedback inhibition of rearrangement in pre-B cells. We summarize chromatin structural features of the locus before and during recombination and, wherever possible, incorporate these into working hypotheses for understanding regulation of IgH gene recombination. The picture emerges that the IgH locus is activated in discrete, independently regulated domains. A domain encompassing DH and JH gene segments is activated first, within which recombination is initiated. VH genes are activated subsequently and, in part, by interleukin-7. These observations lead to a model for feedback inhibition of IgH rearrangements.

Mechanisms of selection mediated by interleukin-7, the preBCR, and hemokinin-1 during B-cell development

Many of the stromal-derived signals and factors that regulate B lymphopoiesis have been identified. We review recent evidence from our laboratory that shows that there are at least three phases during B-cell development when cells direct their own maturation, independent of stromal cells. Following the expression of the preB-cell receptor (preBCR), cells acquire the ability to proliferate in low levels of interleukin-7 (IL-7), which acts as a self-selecting mechanism to expand cells that have successfully expressed a preBCR in environments that are non-permissive to preBCR- cells. Second, the preBCR is required for a contact-mediated event between B-cell progenitors. Disruption at this stage prevents the further maturation of progenitors to the lipopolysaccharide (LPS)-responsive stage. Finally, the transition from IL-7 receptor to mature antigen receptor-based signaling is enhanced by a novel member of the tachykinin family, hemokinin-1. This series of maturation, survival, and differentiation signals is generated by B-lineage cells as they progress through developmental checkpoints on the way to becoming functionally mature cells.

CD44 is dispensable for B lymphopoiesis

Several studies have implicated a role for the cell surface glycoprotein CD44 in lymphocyte differentiation, adhesion to the matrix, homing, activation and apoptosis. However, the requirement of CD44 in B cell maturation remains elusive. To address this point, we analyzed the generation and activation of B lymphocytes in CD44-deficient mice. Unexpectedly, both maturation and autoreconstitution of early bone marrow B cell progenitors after sublethal irradiation as well as frequencies of splenic B cell subsets are indistinguishable in wild-type and CD44-deficient mice. Furthermore, splenic CD44-deficient B lymphocytes show a normal activation pattern after stimulation with LPS, anti-IgM/IL-4 or anti-CD40/IL-4. These results show for the first time that CD44 is clearly dispensable for development, reconstitution and activation of B lymphocytes.

Surface mu heavy chain signals down-regulation of the V(D)J-recombinase machinery in the absence of surrogate light chain components

Early B cell development is characterized by stepwise, ordered rearrangement of the immunoglobulin (Ig) heavy (HC) and light (LC) chain genes. Only one of the two alleles of these genes is used to produce a receptor, a phenomenon referred to as allelic exclusion. It has been suggested that pre–B cell receptor (pre-BCR) signals are responsible for down-regulation of the VDJ_H-recombinase machinery (Rag1, Rag2, and terminal deoxynucleotidyl transferase [TdT]), thereby preventing further rearrangement on the second HC allele. Using a mouse model, we show that expression of an inducible μHC transgene in Rag2^−/− pro–B cells induces down-regulation of the following: (a) TdT protein, (b) a transgenic green fluorescent protein reporter reflecting endogenous Rag2 expression, and (c) Rag1 primary transcripts. Similar effects were also observed in the absence of surrogate LC (SLC) components, but not in the absence of the signaling subunit Ig-α. Furthermore, in wild-type mice and in mice lacking either λ5, VpreB1/2, or the entire SLC, the TdT protein is down-regulated in μHC⁺LC⁻ pre–B cells. Surprisingly, μHC without LC is expressed on the surface of pro–/pre–B cells from λ5^−/−, VpreB1^−/−VpreB2^−/−, and SLC^−/− mice. Thus, SLC or LC is not required for μHC cell surface expression and signaling in these cells. Therefore, these findings offer an explanation for the occurrence of HC allelic exclusion in mice lacking SLC components.

Three-dimensional modeling of a pre-B-cell receptor

Signals delivered by the immunoglobulin (Ig)-like pre-B-cell receptor (pre-BCR) are critical for efficient maturation of early precursor B (pre-B) cells. A pre-BCR contains two immunoglobulin mu-heavy chains (muHC), two surrogate light chains (SLC) consisting of the non-covalently associated polypeptides, VpreB and lambda5, and the heterodimeric signaling transducer Igalpha/beta. Although, it is generally accepted that signals initiated from the pre-BCR are required for efficient expansion and differentiation of pre-B cells, the three-dimensional structure of this receptor has not yet been determined by either NMR or X-ray spectroscopy. Therefore, we used indirect computer-assisted molecular modeling techniques to predict for the first time three-dimensional coordinates of the pre-BCR, the conformation of the SLC components, VpreB and lambda5, and the position and flexibility of the so-called non-Ig-like unique tails at the C-terminus of VpreB and the N-terminus of lambda5. Structure prediction revealed that these unique tails of VpreB and lambda5 protrude from the SLC at the position where the CDR3 of a conventional IgLchain would be located. Thus, the unique tails are accessible for ligand binding, which supports the recent finding that the lambda5 unique tail is required for pre-BCR/stroma cell interaction. Further, the non-covalent interaction of the extra beta-strand of lambda5 (beta8) with VpreB is predicted to result in a stabilization of the tertiary structure of VpreB. In summary, three-dimensional computer modeling suggests that the structure of a pre-BCR resembles that of a conventional B-cell receptor (BCR) and that the lambda5 unique tail could be a major binding site for pre-BCR ligands.

Binding of Free Immunoglobulin Light Chains to VpreB3 Inhibits Their Maturation and Secretion in Chicken B Cells

The VpreB3 gene product was first characterized as an immunoglobulin (Ig) mu heavy chain-binding protein in mouse precursor B (pre-B) cells. Although its function is unknown, it has been proposed to participate in the assembly and transport of the pre-B cell receptor. We have identified a VpreB3 orthologous gene in chicken that is located close to the immunoglobulin light chain (LC) gene cluster and specifically expressed in the bursa of Fabricius. By overexpressing VpreB3 in the DT40 IgM(+) immature chicken B cell line, we have characterized VpreB3 as an endoplasmic reticulum-resident glycoprotein that binds preferentially to free IgLC. However, binding to IgHC is observed in IgLC-deficient DT40 cells. Interaction of VpreB3 with free IgLC is partly covalent and induces retention of free IgLC in the endoplasmic reticulum, preventing their secretion without affecting IgM surface expression. Our results demonstrate that this evolutionarily conserved molecule may play a role in the regulation of the maturation and secretion of free IgLC in B cells. We discuss possible implications in the regulation of the immune response.

The roles of transcription factors in B lymphocyte commitment, development, and transformation

Studies of normal blood cell development and malignant transformation of hematopoietic cells have shown that the correctly regulated expression of stage- and lineage-specific genes is a key issue in hematopoiesis. Experiments in transgenic mice have defined a number of transcription factors such as SCL/Tal, core-binding factor/acute myeloid leukemia, and c-myb, all crucial for the establishment of definitive hematopoiesis and development of all blood cell lineages. Other regulators such as IKAROS, E47/E2A, early B cell factor, Sox-4, and B cell-specific activator protein (Pax-5) appear crucial, more or less selectively, for B lymphopoiesis, allowing for detailed analysis of the development of this lineage. In addition, several of these transcription factors are found translocated in human tumors, often resulting in aberrant gene expression or production of modified proteins. This article concerns the role of transcription factors in B lymphoid development with special focus on lineage initiation and commitment events but also to some extent on the roles of transcription factors in human B lymphoid malignancies.

Unraveling V(D)J recombination; insights into gene regulation

V(D)J recombination assembles antigen receptor genes from component gene segments. We review findings that have shaped our current understanding of this remarkable mechanism, with a focus on two major reports--the first detailed comparison of germline and rearranged antigen receptor loci and the discovery of the recombination activating gene-1.

A feedback mechanism regulates monoallelic odorant receptor expression

In olfactory neurons, expression of a single odorant receptor (OR) from a repertoire of >1000 genes is required for odor coding and axonal targeting. Here, we demonstrate a role for OR protein as an essential regulator in the establishment of monoallelic OR expression. An OR-promoter-driven reporter expresses in a receptor-like pattern but, unlike a native OR, is coexpressed with an additional OR allele. Expression of a functional OR from the identical promoter eliminates expression of other OR alleles. The presence of an untranslatable OR coding sequence in the mRNA is insufficient to exclude expression of a second OR. Together, these data identify the OR protein as a critical element in a feedback pathway that regulates OR selection.

Cutting edge: signaling and cell surface expression of a mu H chain in the absence of lambda 5: a paradigm revisited

Pre-B cell receptor (pre-BCR) signals are essential for pro-B cells to mature efficiently into pre-B cells. The pre-BCR is an Ig-like transmembrane complex that is assembled from two mu H chains (mu HC) and two surrogate L chains consisting of the non-covalently associated polypeptides VpreB and lambda5. In lambda5(-/-) mice, pro-B cell maturation is impaired, but not completely blocked, implying that a mu HC induces differentiation signals in the absence of lambda5. Using a mouse model, in which transgenic mu HC expression can be controlled by tetracycline, we show that in the absence of lambda5, the transgenic mu HC promotes in vivo differentiation of pro-B cells, induces IL-7-dependent cell growth, and is expressed on the surface of pre-B cells. Our findings not only show that an incomplete pre-BCR can initiate signals, but also challenge the paradigm that an IgHC must associate with an IgLC or a SLC to gain transport and signaling competency.

Identification of a pre-BCR lacking surrogate light chain

SLP-65^−/− pre-B cells show a high proliferation rate in vitro. We have shown previously that λ5 expression and consequently a conventional pre-B cell receptor (pre-BCR) are essential for this proliferation. Here, we show that pre-B cells express a novel receptor complex that contains a μ heavy chain (μHC) but lacks any surrogate (SL) or conventional light chain (LC). This SL-deficient pre-BCR (SL⁻pre-BCR) requires Ig-α for expression on the cell surface. Anti-μ treatment of pre-B cells expressing the SL⁻pre-BCR induces tyrosine phosphorylation of substrate proteins and a strong calcium (Ca²⁺) release. Further, the expression of the SL⁻pre-BCR is associated with a high differentiation rate toward κLC-positive cells. Given that B cell development is only partially blocked and allelic exclusion is unaffected in SL-deficient mice, we propose that the SL⁻pre-BCR is involved in these processes and therefore shares important functions with the conventional pre-BCR.

Immunoglobulin mu heavy chains do not mediate tyrosine phosphorylation of Ig alpha from the ER-cis-Golgi

Signals delivered by Ig receptors guide the development of functional B lymphocytes. For example, clonal expansion of early mu heavy chain ( mu HC)-positive pre-B cells requires the assembly of a signal-competent pre-B cell receptor complex (pre-BCR) consisting of a mu HC, a surrogate L chain, and the signal dimer Ig alpha beta. However, only a small fraction of the pre-BCR is transported to the cell surface, suggesting that pre-BCR signaling initiates already from an intracellular compartment, e.g., the endoplasmic reticulum (ER). The finding that differentiation of pre-B cells and allelic exclusion at the IgH locus take place in surrogate L chain-deficient mice further supports the presence of a mu HC-mediated intracellular signal pathway. To determine whether a signal-competent Ig complex can already be assembled in the ER, we analyzed the consequence of pervanadate on tyrosine phosphorylation of Ig alpha in J558L plasmacytoma and 38B9 pre-B cells transfected with either a transport-competent IgL chain-pairing or an ER-retained nonpairing micro HC. Flow cytometry, combined Western blot-immunoprecipitation-kinase assays, and confocal microscopy revealed that both the nonpairing and pairing mu HC assembled with the Ig alpha beta dimer; however, in contrast to a pairing mu HC, the nonpairing mu HC was retained in the ER-cis-Golgi compartment, and neither colocalized with the src kinase lyn nor induced tyrosine phosphorylation of Ig alpha after pervanadate treatment of cells. On the basis of these findings, we propose that a signal-competent Ig complex consisting of mu HC, Ig alpha beta, and associated kinases is assembled in a post-ER compartment, thereby supporting the idea that a pre-BCR must be transported to the cell surface to initiate pre-BCR signaling.

Interaction of murine precursor B cell receptor with stroma cells is controlled by the unique tail of lambda 5 and stroma cell-associated heparan sulfate

Efficient clonal expansion of early precursor B (pre-B) cells requires signals delivered by an Ig-like integral membrane complex, the so-called pre-B cell receptor (pre-BCR). A pre-BCR consists of two membrane micro H chains, two covalently associated surrogate L chains, and the heterodimeric signaling transducer Igalphabeta. In contrast to a conventional Ig L chain, the surrogate L chain is a heterodimer composed of the invariant polypeptides VpreB and lambda5. Although it is still unclear how pre-BCR signals are initiated, two recent findings support a ligand-dependent initiation of pre-BCR signals: 1) a pre-BCR/galectin-1 interaction is required to induce phosphorylation of Igalphabeta in a human precursor B line, and 2) soluble murine as well as human pre-BCR molecules bind to stroma and other adherent cells. In this study, we show that efficient binding of a soluble murine pre-BCR to stroma cells requires the non-Ig-like unique tail of lambda5. Surprisingly however, a murine pre-BCR, in contrast to its human counterpart, does not interact with galectin-1, as revealed by lactose blocking, RNA interference, and immunoprecipitation assays. Finally, the binding of a murine pre-BCR to stroma cells can be blocked either with heparin or by pretreatment of stroma cells with heparitinase or a sulfation inhibitor. Hence, efficient binding of a murine pre-BCR to stroma cells requires the unique tail of lambda5 and stroma cell-associated heparan sulfate. These findings not only identified heparan sulfate as potential pre-BCR ligands, but will also facilitate the development of appropriate animal models to determine whether a pre-BCR/heparan sulfate interaction is involved in early B cell maturation.

Unexpected requirement for ZAP-70 in pre-B cell development and allelic exclusion

ZAP-70, a member of the Syk family of tyrosine kinases, has been reported to be expressed exclusively in T and NK cells. We show here that it is expressed throughout B cell development and that it plays a role in the transition of pro-B to pre-B cells in the bone marrow, a checkpoint controlled by signals from the pre-B cell receptor (pre-BCR), which monitors for successful rearrangement of immunoglobulin heavy chain genes. Whereas mice deficient in Syk show a partial block at this step, mice mutant in both Syk and ZAP-70 show a complete block at the pro-B cell stage and a failure of heavy chain allelic exclusion, hallmarks of defective pre-BCR signaling.