Formation of disulphide-linked mu 2 omega 2 tetramers in pre-B cells by the 18K omega-immunoglobulin light chain

The Role of the Pre-B Cell Receptor in B Cell Development, Repertoire Selection, and Tolerance

Around four decades ago, it had been observed that there were cell lines as well as cells in the fetal liver that expressed antibody μ heavy (μH) chains in the apparent absence of bona fide light chains. It was thus possible that these cells expressed another molecule(s), that assembled with μH chains. The ensuing studies led to the discovery of the pre-B cell receptor (pre-BCR), which is assembled from Ig μH and surrogate light (SL) chains, together with the signaling molecules Igα and β. It is expressed on a fraction of pro-B (pre-BI) cells and most large pre-B(II) cells, and has been implicated in IgH chain allelic exclusion and down-regulation of the recombination machinery, assessment of the expressed μH chains and shaping the IgH repertoire, transition from the pro-B to pre-B stage, pre-B cell expansion, and cessation.

Murine Bone Marrow Niches from Hematopoietic Stem Cells to B Cells

After birth, the development of hematopoietic cells occurs in the bone marrow. Hematopoietic differentiation is finely tuned by cell-intrinsic mechanisms and lineage-specific transcription factors. However, it is now clear that the bone marrow microenvironment plays an essential role in the maintenance of hematopoietic stem cells (HSC) and their differentiation into more mature lineages. Mesenchymal and endothelial cells contribute to a protective microenvironment called hematopoietic niches that secrete specific factors and establish a direct contact with developing hematopoietic cells. A number of recent studies have addressed in mouse models the specific molecular events that are involved in the cellular crosstalk between hematopoietic subsets and their niches. This has led to the concept that hematopoietic differentiation and commitment towards a given hematopoietic pathway is a dynamic process controlled at least partially by the bone marrow microenvironment. In this review, we discuss the evolving view of murine hematopoietic–stromal cell crosstalk that is involved in HSC maintenance and commitment towards B cell differentiation.

The Sel1L-Hrd1 Endoplasmic Reticulum-Associated Degradation Complex Manages a Key Checkpoint in B Cell Development

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a principal mechanism that targets ER-associated proteins for cytosolic proteasomal degradation. Here, our data demonstrate a critical role for the Sel1L-Hrd1 complex, the most conserved branch of ERAD, in early B cell development. Loss of Sel1L-Hrd1 ERAD in B cell precursors leads to a severe developmental block at the transition from large to small pre-B cells. Mechanistically, we show that Sel1L-Hrd1 ERAD selectively recognizes and targets the pre-B cell receptor (pre-BCR) for proteasomal degradation in a BiP-dependent manner. The pre-BCR complex accumulates both intracellularly and at the cell surface in Sel1L-deficient pre-B cells, leading to persistent pre-BCR signaling and pre-B cell proliferation. This study thus implicates ERAD mediated by Sel1L-Hrd1 as a key regulator of B cell development and reveals the molecular mechanism underpinning the transient nature of pre-BCR signaling.

Dimerization-dependent folding underlies assembly control of the clonotypic αβT cell receptor chains

In eukaryotic cells, secretory pathway proteins must pass stringent quality control checkpoints before exiting the endoplasmic reticulum (ER). Acquisition of native structure is generally considered to be the most important prerequisite for ER exit. However, structurally detailed protein folding studies in the ER are few. Furthermore, aberrant ER quality control decisions are associated with a large and increasing number of human diseases, highlighting the need for more detailed studies on the molecular determinants that result in proteins being either secreted or retained. Here we used the clonotypic αβ chains of the T cell receptor (TCR) as a model to analyze lumenal determinants of ER quality control with a particular emphasis on how proper assembly of oligomeric proteins can be monitored in the ER. A combination of in vitro and in vivo approaches allowed us to provide a detailed model for αβTCR assembly control in the cell. We found that folding of the TCR α chain constant domain Cα is dependent on αβ heterodimerization. Furthermore, our data show that some variable regions associated with either chain can remain incompletely folded until chain pairing occurs. Together, these data argue for template-assisted folding at more than one point in the TCR α/β assembly process, which allows specific recognition of unassembled clonotypic chains by the ER chaperone machinery and, therefore, reliable quality control of this important immune receptor. Additionally, it highlights an unreported possible limitation in the α and β chain combinations that comprise the T cell repertoire.

Principles and engineering of antibody folding and assembly

Antibodies are uniquely suited to serve essential roles in the human immune defense as they combine several specific functions in one hetero-oligomeric protein. Their constant regions activate effector functions and their variable domains provide a stable framework that allows incorporation of highly diverse loop sequences. The combination of non-germline DNA recombination and mutation together with heavy and light chain assembly allows developing variable regions that specifically recognize essentially any antigen they may encounter. However, this diversity also requires tailor-made mechanisms to guarantee that folding and association of antibodies is carefully this diversity also requires tailor-made mechanisms to guarantee that folding and association of antibodies is carefully controlled before the protein is secreted from a plasma cell. Accordingly, the generic immunoglobulin fold β-barrel structure of antibody domains has been fine-tuned during evolution to fit the different requirements. Work over the past decades has identified important aspects of the folding and assembly of antibody domains and chains revealing domain specific variations of a general scheme. The most striking is the folding of an intrinsically disordered antibody domain in the context of its partner domain as the basis for antibody assembly and its control on the molecular level in the cell. These insights have not only allowed a better understanding of the antibody folding process but also provide a wealth of opportunities for rational optimization of antibody molecules. In this review, we summarize current concepts of antibody folding and assembly and discuss how they can be utilized to engineer antibodies with improved performance for different applications. This article is part of a Special Issue entitled: Recent advances in the molecular engineering of antibodies.

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.

Insight into the basis of autonomous immunoreceptor activation

Expression of the pre-T cell receptor (pTCR) by immature thymocytes is crucial for T cell development. The pTCR comprises an invariant pre-Tα chain that pairs with a newly rearranged TCRβ chain and CD3 signaling components. Despite its similarity to the mature αβTCR, which binds to specific peptide-loaded major histocompatibility molecules, the pTCR functions in a ligand-independent manner. Precisely how pTCR functions autonomously has remained a source of intense debate. Recently, the structure of the extracellular domain of the pTCR has been determined, providing insight into the mechanism of pTCR autonomous signaling. In this review, we reflect on the current understanding of pTCR function and draw comparisons to the mechanisms employed by the mature αβTCR and the related pre-B cell receptor.

Allelic exclusion of immunoglobulin genes: models and mechanisms

The allelic exclusion of immunoglobulin (Ig) genes is one of the most evolutionarily conserved features of the adaptive immune system and underlies the monospecificity of B cells. While much has been learned about how Ig allelic exclusion is established during B-cell development, the relevance of monospecificity to B-cell function remains enigmatic. Here, we review the theoretical models that have been proposed to explain the establishment of Ig allelic exclusion and focus on the molecular mechanisms utilized by developing B cells to ensure the monoallelic expression of Ig kappa and Ig lambda light chain genes. We also discuss the physiological consequences of Ig allelic exclusion and speculate on the importance of monospecificity of B cells for immune recognition.

An unfolded CH1 domain controls the assembly and secretion of IgG antibodies

A prerequisite for antibody secretion and function is their assembly into a defined quaternary structure, composed of two heavy and two light chains for IgG. Unassembled heavy chains are actively retained in the endoplasmic reticulum (ER). Here, we show that the C(H)1 domain of the heavy chain is intrinsically disordered in vitro, which sets it apart from other antibody domains. It folds only upon interaction with the light-chain C(L) domain. Structure formation proceeds via a trapped intermediate and can be accelerated by the ER-specific peptidyl-prolyl isomerase cyclophilin B. The molecular chaperone BiP recognizes incompletely folded states of the C(H)1 domain and competes for binding to the C(L) domain. In vivo experiments demonstrate that requirements identified for folding the C(H)1 domain in vitro, including association with a folded C(L) domain and isomerization of a conserved proline residue, are essential for antibody assembly and secretion in the cell.

A unique role for the lambda5 nonimmunoglobulin tail in early B lymphocyte development

Precursor BCR (pre-BCR) signaling governs proliferation and differentiation of pre-B cells during B lymphocyte development. However, it is controversial as to which parts of the pre-BCR, which is composed of Igmu H chain, surrogate L chain (SLC), and Igalpha-Igbeta, are important for signal initiation. Here, we show in transgenic mice that the N-terminal non-Ig-like (unique) tail of the surrogate L chain component lambda5 is critical for enhancing pre-BCR-induced proliferation signals. Pre-BCRs with a mutated lambda5 unique tail are still transported to the cell surface, but they deliver only basal signals that trigger survival and differentiation of pre-B cells. Further, we demonstrate that the positively charged residues of the lambda5 unique tail, which are required for pre-BCR self-oligomerization, can also mediate binding to stroma cell-associated self-Ags, such as heparan sulfate. These findings establish the lambda5 unique tail as a pre-BCR-specific autoreactive signaling motif that could increase the size of the primary Ab repertoire by selectively expanding pre-B cells with functional Igmu H chains.

Structural insight into pre-B cell receptor function

The pre-B cell receptor (pre-BCR) serves as a checkpoint in B cell development. In the 2.7 angstrom structure of a human pre-BCR Fab-like fragment, consisting of an antibody heavy chain (HC) paired with the surrogate light chain, the "unique regions" of VpreB and lambda5 replace the complementarity-determining region 3 (CDR3) loop of an antibody light chain and appear to "probe" the HC CDR3, potentially influencing the selection of the antibody repertoire. Biochemical analysis indicates that the pre-BCR is impaired in its ability to recognize antigen, which, together with electron microscopic visualization of a pre-BCR dimer, suggests ligand-independent oligomerization as the likely signaling mechanism.

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.

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.

Only VpreB1, but not VpreB2, is expressed at levels which allow normal development of B cells

The surrogate light chain (SLC) consists of the polypeptides lambda5 and, in the mouse, either VpreB1 or VpreB2. SLC associates with BILL-Cadherin and other glycoproteins to form the pro-B cell receptor (pro-BCR) at the pre-BI cell stage, and with the immunoglobulin mu heavy chain to form the pre-BCR at the pre-BII cell stage. The function of the pro-BCR, if any, is unknown, whereas the pre-BCR is crucial for proliferative expansion of pre-BII cells. To shed light on the functional properties of VpreB1 and VpreB2 in vivo, mice with either one or two VpreB1, or one or two VpreB2, alleles have been investigated. We show that B cell development in mice with two VpreB1 alleles is indistinguishable from that of normal mice. In contrast, mice with two VpreB2 alleles show an approximately 1.6-fold increase in pre-BI and a 35% decrease in pre-BII cell numbers, while mice with only one VpreB2 allele show a reduction in B cell development manifested in a 2-fold enrichment in pre-BI cells and a 75% reduction in pre-BII cells. However, such a gene dosage effect is not observed for VpreB1. Our results suggest that the difference between VpreB1- and VpreB2-deficient mice is due to lower VpreB2 protein expression, thus limiting the formation of pre-BCRs and thereby the number of large, cycling pre-BII cells.

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: selector of fitting immunoglobulin heavy chains for the B-cell repertoire

In this Opinion article, I address the role of the pre-B-cell receptor (pre-BCR) in the development of antigen-specific B cells in terms of immunoglobulin heavy chain (IgH) variable-region repertoire selection, precursor B-cell differentiation and proliferation, and IgH allelic exclusion. Comparisons with the role of the pre-T-cell receptor (pre-TCR) in T-cell development raise provocative questions. Why do B- and T-cell lineages both use a surrogate chain - the surrogate light chain and the pre-TCR alpha-chain, respectively - as a step to develop their repertoires of antigen-recognizing cells? What are the functions of the pre-BCR and pre-TCR in lymphocyte differentiation and antigen-receptor allelic exclusion? This article, together with the accompanying article by Harald von Boehmer, hopes to answer some of these questions.

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.

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.

Selection of Ig mu heavy chains by complementarity-determining region 3 length and amino acid composition

Although it is generally accepted that Ig heavy chains (HC) are selected at the pre-B cell receptor (pre-BCR) checkpoint, the characteristics of a functional HC and the role of pre-BCR assembly in their selection have remained elusive. We determined the characteristics of HCs that successfully passed the pre-BCR checkpoint by examining transcripts harboring V(H)81X and J(H)4 gene segments from J(H)(+/-) and lambda5(-/-)mice. V(H)81X-J(H)4-HC transcripts isolated from cells before or in the absence of pre-BCR assembly had no distinguishing complementarity-determining region 3 traits. In contrast, transcripts isolated subsequent to passage through the pre-BCR checkpoint had distinctive complementarity-determining regions 3 of nine amino acids in length (49%) and a histidine at position 1 (73%). Hence, our data define specific structural requirements for a functional HC, which is instrumental in shaping the diverse B cell repertoire.

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.

The transient expression of pre-B cell receptors governs B cell development

Only a subpopulation of relatively large pre-B cells express pre-B cell receptors (preBCR) that can be seen with very sensitive immunofluorescence methods. Inefficient assembly of the multicomponent preBCR coupled with their ligand-induced endocytosis may account for the remarkably low in vivo levels of preBCR expression. Signaling initiated via the preBCR promotes cellular proliferation and RAG-1 and RAG-2 downregulation to interrupt the immunoglobulin V(D)J gene rearrangement process. Silencing of the surrogate light chain genes, VpreB and lambda5, then terminates preBCR expression to permit cell cycle exit, recombinase gene upregulation, and VJ(L) rearrangement by small pre-B cells destined to become B cells.

Detection of functional V(H)81X heavy chains in adult mice with an assessment of complementarity-determining region 3 diversity and capacity to form pre-B cell receptor

Recent reports have indicated that up to 50% of all H chain proteins formed cannot associate with the surrogate L chain complex and therefore fail to form a pre-B cell receptor (pBCR), which is required for allelic exclusion and, in most cases, verifies that the H chain can assemble with the L chain to form an Ab molecule. Certain V(H) genes, such as V(H)81X, appear to be particularly prone to encoding for nonpairing (dysfunctional) H chains. It has been suggested that sequence variability at complementarity-determining region 3, especially when increased by the enzyme TdT, often precludes the ability of V(H)81X-using H chains to form pBCR. To determine whether a motif exists that accounts for the ability of H chains to pair with surrogate L chain complex/L chain, we have bred a mouse line in which H chain recombination can only occur on one allele, allowing us to compile a pool of H chains capable of forming Ab molecules in the absence of dysfunctional H chains. Somewhat unexpectedly, we have found V(H)81X H chains capable of Ab formation and cell surface expression in the presence of TdT. Scrutiny of these H chains has revealed that, although highly prone to encode for dysfunctional H chains, sequence variability is not severely limited among functional V(H)81X H chains. We also demonstrate that surface Ig expression is highly indicative of the capacity of a H chain to form pBCR.

Differential surrogate light chain expression governs B-cell differentiation

Surrogate light chain expression during B lineage differentiation was examined by using indicator fluorochrome-filled liposomes in an enhanced immunofluorescence assay. Pro-B cells bearing surrogate light chain components were found in mice, but not in humans. A limited subpopulation of relatively large pre-B cells in both species expressed pre-B cell receptors. These cells had reduced expression of the recombinase activating genes, RAG-1 and RAG-2. Their receptor-negative pre-B cell progeny were relatively small, expressed RAG-1 and RAG-2, and exhibited selective down-regulation of VpreB and λ5expression. Comparative analysis of the 2 pre-B cell subpopulations indicated that loss of the pre-B cell receptors from surrogate light chain gene silencing was linked with exit from the cell cycle and light chain gene rearrangement to achieve B-cell differentiation.

Surrogate light chain-mediated interaction of a soluble pre-B cell receptor with adherent cell lines

Signals initiated by the precursor B cell receptor (pre-BCR) are critical for B cell progenitors to mature into precursor B cells. The pre-BCR consists of a homodimer of microH chains, the covalently associated surrogate L (SL) chain composed of VpreB and lambda5, and the transmembrane signal molecules Ig(alpha) and Igbeta. One way to explain how maturation signals are initiated in late progenitor B cells is that the pre-BCR is transported to the cell surface and interacts from there with a ligand on stroma cells. To address this hypothesis, we first produced soluble Fab-like pre-BCR and BCR fragments, as well as SL chain, in baculovirus-infected insect cells. Flow cytometry revealed that, in contrast to Fab-like BCR fragments, the soluble pre-BCR binds to the surface of stroma and several other adherent cell lines, but not to B and T lymphoid suspension cells. The specific binding of the soluble pre-BCR to stroma cells is saturable, sensitive to trypsin digestion, and not dependent on bivalent cations. The binding of pre-BCR seems to be independent of the H chain of IgM (microH chain), because SL chain alone was able to interact with stroma cells. Finally, soluble pre-BCR specifically precipitated a 135-kDa protein from ST2 cells. These findings not only demonstrate for the first time the capacity of a pre-BCR to specifically bind to a structure on the surface of adherent cells, but also suggest that the pre-BCR interacts via its SL chain with a putative ligand on stroma cells.

The initiation of B cell clonal expansion occurs independently of pre-B cell receptor formation

Current models of B cell development posit that clonal expansion occurs as a direct result of Ig H chain expression. To test this hypothesis, we isolated a population of early B cells in which H chain recombination is initiated and assessed V(H)DJ(H) rearrangements in both cycling and noncycling cells. We found that actively dividing cells within this population are enriched for H chain rearrangements that are productive when compared with their counterparts in G(0)/G(1), apparently supporting a role for H chain expression in initiating early B cell division; entrance into the cell cycle was accompanied by V(H) gene-dependent H chain selection. However, we also identified a phenotypically identical population of actively cycling early B cells in the absence of H chain expression in recombination activating gene knockout mice. In addition, actively cycling early B cells could be detected in pre-B cell receptor (pBCR)-negative lambda5 knockout mice, but we found no evidence for V(H)-dependent H chain selection in this population. Given these results, we suggest that the initiation of clonal expansion, at this early stage in B cell development, occurs independently of H chain expression. Although the cycling cell pool is enriched for pBCR-positive cells in mice expressing surrogate L chain, pBCR formation is not required for the initiation of cell division.

Conventional and surrogate light chains differentially regulate Ig mu and Dmu heavy chain maturation and surface expression

Positive selection of precursor (pre-) B cells by Ig membrane mu H chains (mum HC) and counterselection mediated by the truncated HC Dmu depend on the ability of each HC to form a pre-B cell receptor (pre-BCR) signaling complex with the surrogate L chain (SLC) components lambda5 and Vpre-B. To better understand how pre-BCR signaling output is determined by its Ig components and the SLC, we investigated the regulation of pre-BCR surface expression and HC secretory maturation in a new nonlymphoid system. We took this approach as a means to distinguish B-lineage-specific effects from pre-BCR-intrinsic properties that may influence these aspects of pre-BCR homeostasis necessary for signaling. As in pre-B cells, the SLC in nonlymphoid cells supported only a limited degree of mum HC maturation and low pre-BCR surface expression levels compared with conventional LCs, indicating that this was due to an intrinsic property of the SLC. We identified the non-Ig region of lambda5 as harboring the restrictive activity responsible for this phenotype. This property of lambda5 was also evident with Dmu, but the overall SLC- and L chain-dependent requirements for Dmu maturation and surface expression were markedly different from those for mum. Surprisingly, Dmu was modified in an unusual manner that was only dependent on Vpre-B. These results establish a novel function of lambda5 in limiting surface pre-BCR levels and reveal biochemical properties of Ig molecules that may underlie the diverse consequences of pre-BCR signaling in vivo by different HCs.

Immunoglobulin heavy chain expression shapes the B cell receptor repertoire in human B cell development

Developing B cells must pass a series of checkpoints that are regulated by membrane-bound Ig(mu) through the Igalpha-Igbeta signal transducers. To determine how Ig(mu) expression affects B cell development and Ab selection in humans we analyzed Ig gene rearrangements in pro-B cells from two patients who are unable to produce Ig(mu) proteins. We find that Ig(mu) expression does not affect V(H), D, or J(H) segment usage and is not required for human Igkappa and Iglambda recombination or expression. However, the heavy and light chains found in pro-B cells differed from those in peripheral B cells in that they showed unusually long CDR3s. In addition, the Igkappa repertoire in Ig(mu)-deficient pro-B cells was skewed to downstream Jkappas and upstream Vkappas, consistent with persistent secondary V(D)J rearrangements. Thus, Ig(mu) expression is not required for secondary V(D)J recombination in pro-B cells. However, B cell receptor expression shapes the Ab repertoire in humans and is essential for selection against Ab's with long CDR3s.

Loss of precursor B cell expansion but not allelic exclusion in VpreB1/VpreB2 double-deficient mice

The pre-B cell receptor consists of immunoglobulin (Ig) mu heavy chains and surrogate light chain, i.e., the VpreB and lambda5 proteins. To analyze the role of the two VpreB proteins, mice lacking the VpreB1 and VpreB2 genes were generated. VpreB1(-/-) VpreB2(-/-) mice were impaired in their B cell development at the transition from pre-BI to large pre-BII cells. Pre-BII cells did not expand by proliferation, consequently 40-fold less small pre-BII and immature B cells were found in bone marrow, and the generation of immature and mature conventional B cells in spleen appeared reduced. In addition, only low numbers of B-1a cells were detected in the peritoneum. Surprisingly, Ig heavy chain allelic exclusion was still active, apparently ruling out a signaling role of a VpreB1/VpreB2-containing receptor in this process.

Induction of pre-B cell proliferation after de novo synthesis of the pre-B cell receptor

The assembly of a pre-B cell receptor (pre-BCR) composed of an Ig mu heavy chain (mu H-chain), the surrogate light (SL) chain, and the Ig alpha/beta dimer is critical for late pro-B cells to advance to the pre-B cell stage. By using a transgenic mouse model, in which mu H-chain synthesis is solely driven by a tetracycline-controlled transactivator, we show that de novo synthesis of mu H-chain in transgenic pro-B cells not only induces differentiation but also proliferation. This positive effect of mu H-chain synthesis on proliferation requires the presence of SL chain and costimulatory signals provided by stromal cells or IL-7. We conclude that pre-BCR signaling induces clonal expansion of early pre-B cells.

Induction of pre-B cell proliferation after de novo synthesis of the pre-B cell receptor

The assembly of a pre-B cell receptor (pre-BCR) composed of an Ig mu heavy chain (mu H-chain), the surrogate light (SL) chain, and the Ig alpha/beta dimer is critical for late pro-B cells to advance to the pre-B cell stage. By using a transgenic mouse model, in which mu H-chain synthesis is solely driven by a tetracycline-controlled transactivator, we show that de novo synthesis of mu H-chain in transgenic pro-B cells not only induces differentiation but also proliferation. This positive effect of mu H-chain synthesis on proliferation requires the presence of SL chain and costimulatory signals provided by stromal cells or IL-7. We conclude that pre-BCR signaling induces clonal expansion of early pre-B cells.

Antibody regulation of B cell development

Antibodies on the surface of B lymphocytes trigger adaptive immune responses and control a series of antigen-independent checkpoints during B cell development. These physiologic processes are regulated by a complex of membrane immunoglobulin and two signal transducing proteins known as Ig alpha and Ig beta. Here we focus on the role of antibodies in governing the maturation of B cells from early antigen-independent through the final antigen-dependent stages.

Transgenic human lambda 5 rescues the murine lambda 5 nullizygous phenotype

The human lambda 5 (hu lambda 5) gene is the structural homologue of the murine lambda 5 (m lambda 5) gene and is transcriptionally active in pro-B and pre-B lymphocytes. The lambda 5 and VpreB polypeptides together with the Ig mu H chain and the signal-transducing subunits, Ig alpha and Ig beta, comprise the pre-B cell receptor. To further investigate the pro-B/pre-B-specific transcription regulation of hu lambda 5 in an in vivo model, we generated mouse lines that contain a 28-kb genomic fragment encompassing the entire hu lambda 5 gene. High levels of expression of the transgenic hu lambda 5 gene were detected in bone marrow pro-B and pre-B cells at the mRNA and protein levels, suggesting that the 28-kb transgene fragment contains all the transcriptional elements necessary for the stage-specific B progenitor expression of hu lambda 5. Flow cytometric and immunoprecipitation analyses of bone marrow cells and Abelson murine leukemia virus-transformed pre-B cell lines revealed the hu lambda 5 polypeptide on the cell surface and in association with mouse Ig mu and mouse VpreB. Finally, we found that the hu lambda 5 transgene is able to rescue the pre-B lymphocyte block when bred onto the m lambda 5-/- background. Therefore, we conclude that the hu lambda 5 polypeptide can biochemically and functionally substitute for m lambda 5 in vivo in pre-B lymphocyte differentiation and proliferation. These studies on the mouse and human pre-B cell receptor provide a model system to investigate some of the molecular requirements necessary for B cell development.

Functional and Molecular Analysis of Hematopoietic Progenitors Derived From the Aorta-Gonad-Mesonephros Region of the Mouse Embryo

Herein, we show that CD34, c-kit double-positive (CD34+c-kit+) cells from the aorta-gonad-mesonephros (AGM) region of the developing mouse are multipotent in vitro and can undergo both B-lymphoid and multimyeloid differentiation. Molecular analysis of individual CD34+c-kit+ cells by single-cell reverse transcriptase–polymerase chain reaction (RT-PCR) shows coactivation of erythroid (β-globin) and myeloid (myeloperoxidase [MPO]) but not lymphoid-affiliated (CD3, Thy-1, and λ5) genes. Additionally, most cells coexpress the stem cell–associated transcriptional regulators AML-1, PU.1, GATA-2 and Lmo2, as well as the granulocyte colony-stimulating factor receptor (G-CSF-R). These results show that the CD34+c-kit+ population from the AGM represents a highly enriched source of multipotent hematopoietic cells, and suggest that limited coactivation of distinct lineage-affiliated genes is an early event in the generation of hematopoietic stem and progenitor cells during ontogeny.

Functional and Molecular Analysis of Hematopoietic Progenitors Derived From the Aorta-Gonad-Mesonephros Region of the Mouse Embryo

Herein, we show that CD34, c-kit double-positive (CD34+c-kit+) cells from the aorta-gonad-mesonephros (AGM) region of the developing mouse are multipotent in vitro and can undergo both B-lymphoid and multimyeloid differentiation. Molecular analysis of individual CD34+c-kit+ cells by single-cell reverse transcriptase–polymerase chain reaction (RT-PCR) shows coactivation of erythroid (β-globin) and myeloid (myeloperoxidase [MPO]) but not lymphoid-affiliated (CD3, Thy-1, and λ5) genes. Additionally, most cells coexpress the stem cell–associated transcriptional regulators AML-1, PU.1, GATA-2 and Lmo2, as well as the granulocyte colony-stimulating factor receptor (G-CSF-R). These results show that the CD34+c-kit+ population from the AGM represents a highly enriched source of multipotent hematopoietic cells, and suggest that limited coactivation of distinct lineage-affiliated genes is an early event in the generation of hematopoietic stem and progenitor cells during ontogeny.

Extended duration of DH-JH rearrangement in immunoglobulin heavy chain transgenic mice: implications for regulation of allelic exclusion

Here we show that suppression of VH-DJH rearrangement in mice bearing a mu heavy (H) chain transgene (mu-tg mice) is associated with an extended period of DH-JH rearrangement, the first step of Immunoglobulin H chain gene rearrangement. Whereas DH-JH rearrangement is normally initiated and completed at the pro-B cell stage, in mu-tg mice it continues beyond this stage and occurs most frequently at the small (late) pre-B stage. Despite ongoing DH-JH rearrangement in late pre-B cells of mu-tg mice, VH-DJH rearrangement is not detectable in these cells. We infer that the lack of VH-DJH rearrangement primarily reflects tg-induced acceleration of B cell differentiation past the stage at which rearrangement of VH elements is permissible. In support of this inference, we find that the normal representation of early B lineage subsets is markedly altered in mu-tg mice. We suggest that the effect of a productive VH-DJH rearrangement at an endogenous H chain allele may be similar to that of a mu-tg; i.e., cells that make a productive VH-DJH rearrangement on the first attempt rapidly progress to a developmental stage that precludes VH-DJH rearrangement at the other allele (allelic exclusion).

Novel mechanisms control the folding and assembly of lambda5/14.1 and VpreB to produce an intact surrogate light chain

Surrogate light chain, which escorts the mu heavy chain to the cell surface, is a critical component of the pre-B cell receptor complex. The two proteins that comprise the surrogate light chain, VpreB and lambda5/14.1, contain both unique regions and Ig-like domains. The unique regions have been postulated to function in the assembly of the surrogate light chain. However, by using transient transfection of COS7 cells, we show that deletion of the unique regions of both proteins did not inhibit the assembly of surrogate light chain. Instead, in vivo folding studies showed that the unique region of lambda5/14.1 acts as an intramolecular chaperone by preventing the folding of this protein when it is expressed in the absence of its partner, VpreB. The Ig domains of both lambda5/14.1 and VpreB are atypical. The one in VpreB lacks one of the canonical beta strands whereas the one in lambda5/14.1 has an extra beta strand. Deletion of the extra beta strand in lambda5/14.1 completely abrogated the formation of the surrogate light chain, demonstrating that complementation of the incomplete Ig domain in VpreB by the extra beta strand in lambda5/14.1 was necessary and sufficient for the folding and assembly of these proteins. Our studies reveal two novel mechanisms for regulating surrogate light chain formation: (i) the presence of an intramolecular chaperone that prevents folding of the unassembled subunit but that remains part of the mature assembled protein, and (ii) splitting an Ig domain between two proteins to control their folding and assembly.

Partial block in B lymphocyte development at the transition into the pre-B cell receptor stage in Vpre-B1-deficient mice

The surrogate light chain (SL) is composed of two polypeptides, Vpre-B and lambda5. In large pre-BII cells the SL chain associates with Ig mu heavy chain (muH) to form the pre-B cell receptor (pre-BCR). In mice there are two Vpre-B genes which are 98% identical within the coding regions. The two genes are co-expressed at the RNA level and encode functional proteins that can assemble with lambda5. However, it is not known whether both gene products serve the same function in vivo. Here we have established mice that lack the Vpre-B1 gene (VpreB1(-/-)), but still express the Vpre-B2 gene, both as RNA and protein. In Vpre-B1(-/-) mice, the bone marrow cellularity and the percentage of B220+ cells is normal. However, among the B220+ cells, the percentage of pre-BI cells is increased, and the percentage of pre-BII and immature B cells is slightly decreased, suggesting that the lack of Vpre-B1 causes a partial block at the transition from pre-BI to pre-BII cells, i.e. into the pre-BCR stage. The number of cells that produce a functional pre-BCR is thus lower, but the cells that reach this stage are normal as they can be expanded by proliferation and then differentiate into more mature cells. The spleens of Vpre-B1 homozygous mutant mice show normal numbers of B and T lymphocytes. Moreover, the Ig loci are allelicly excluded and the homozygous mutant mice respond with normal levels of antigen-specific antibodies to T-dependent antigens. These results demonstrate that VpreB2 alone is capable of supporting B lymphocyte development in the bone marrow and can give rise to immuno-competent cells in the periphery.

Antigen receptor engagement turns off the V(D)J recombination machinery in human tonsil B cells

The germinal center (GC) is an anatomic compartment found in peripheral lymphoid organs, wherein B cells undergo clonal expansion, somatic mutation, switch recombination, and reactivate immunoglobulin gene V(D)J recombination. As a result of somatic mutation, some GC B cells develop higher affinity antibodies, whereas others suffer mutations that decrease affinity, and still others may become self-reactive. It has been proposed that secondary V(D)J rearrangements in GCs might rescue B cells whose receptors are damaged by somatic mutations. Here we present evidence that mature human tonsil B cells coexpress conventional light chains and recombination associated genes, and that they extinguish recombination activating gene and terminal deoxynucleotidyl transferase expression when their receptors are cross-linked. Thus, the response of the recombinase to receptor engagement in peripheral B cells is the opposite of the response in developing B cells to the same stimulus. These observations suggest that receptor revision is a mechanism for receptor diversification that is turned off when antigen receptors are cross-linked by the cognate antigen.

Pre-B Cell Receptor-Mediated Selection of Pre-B Cells Synthesizing Functional μ Heavy Chains

Ig gene rearrangements could generate VH-D-JH joining sequences that interfere with the correct folding of a μ-chain, and thus, its capability to pair with IgL chains. Surrogate light (SL) chain might be the ideal molecule to test the capacity of a μ-chain to pair with a L chain early in development, in that only pre-B cells that assemble a membrane μ-SL complex would be permitted to expand and further differentiate. We have previously identified two SL chain nonpairing VH81X-μ-chains with distinct VH-D-JH joining regions. Here, we show that one of these VH81X-μ-chains does not rescue B cell development in JH knock-out mice, because flow cytometric analysis of bone marrow cells from VH81X-μ transgenic JH knock-out mice revealed normal numbers of pro-B cells, but essentially no pre-B and surface IgM+ B cells. Immunoprecipitation analysis of transfected pre-B and hybridoma lines revealed that the same μ-chain fails to pair not only with SL chain but also with four distinct κ L chains. These findings demonstrate that early pre-B cells are selected for maturation on the basis of the structure of a μ-chain, in particular its VH-D-JH joining or CDR3 sequence, and that one mechanism for this selection is the capacity of a μ-chain to assemble with SL chain. Therefore, we propose a new function of SL chain in early B cell development: SL chain is part of a quality control mechanism that tests a μ-chain for its ability to pair with conventional L chains.