The pre-B-cell receptor

B cells and atherosclerosis: A HIV perspective

Atherosclerosis remains a leading cause of cardiovascular disease (CVD) globally, with the complex interplay of inflammation and lipid metabolism at its core. Recent evidence suggests a role of B cells in the pathogenesis of atherosclerosis; however, this relationship remains poorly understood, particularly in the context of HIV. We review the multifaceted functions of B cells in atherosclerosis, with a specific focus on HIV. Unique to atherosclerosis is the pivotal role of natural antibodies, particularly those targeting oxidized epitopes abundant in modified lipoproteins and cellular debris. B cells can exert control over cellular immune responses within atherosclerotic arteries through antigen presentation, chemokine production, cytokine production, and cell-cell interactions, actively participating in local and systemic immune responses. We explore how HIV, characterized by chronic immune activation and dysregulation, influences B cells in the context of atherosclerosis, potentially exacerbating CVD risk in persons with HIV. By examining the proatherogenic and antiatherogenic properties of B cells, we aim to deepen our understanding of how B cells influence atherosclerotic plaque development, especially within the framework of HIV. This research provides a foundation for novel B cell-targeted interventions, with the potential to mitigate inflammation-driven cardiovascular events, offering new perspectives on CVD risk management in PLWH.

Spliceosome component PHD finger 5A is essential for early B lymphopoiesis

The spliceosome, a multi-megadalton ribonucleoprotein complex, is essential for pre-mRNA splicing in the nucleus and ensuring genomic stability. Its precise and dynamic assembly is pivotal for its function. Spliceosome malfunctions can lead to developmental abnormalities and potentially contribute to tumorigenesis. The specific role of the spliceosome in B cell development is poorly understood. Here, we reveal that the spliceosomal U2 snRNP component PHD finger protein 5A (Phf5a) is vital for early B cell development. Loss of Phf5a results in pronounced defects in B cell development, causing an arrest at the transition from pre-pro-B to early pro-B cell stage in the bone marrow of mutant mice. Phf5a-deficient B cells exhibit impaired immunoglobulin heavy (IgH) chain expression due to defective V-to-DJ gene rearrangement. Mechanistically, our findings suggest that Phf5a facilitates IgH gene rearrangement by regulating the activity of recombination-activating gene endonuclease and influencing chromatin interactions at the Igh locus.

B Lymphocytes in Alzheimer's Disease-A Comprehensive Review

Alzheimer's disease (AD) represents the most common type of neurodegenerative dementia and is characterized by extracellular amyloid-β (Aβ) deposition, pathologic intracellular tau protein tangles, and neuronal loss. Increasing evidence has been accumulating over the past years, supporting a pivotal role of inflammation in the pathogenesis of AD. Microglia, monocytes, astrocytes, and neurons have been shown to play a major role in AD-associated inflammation. However recent studies showed that the role of both T and B lymphocytes may be important. In particular, B lymphocytes are the cornerstone of humoral immunity, they constitute a heterogenous population of immune cells, being their mature subsets significantly impacted by the inflammatory milieu. The role of B lymphocytes on AD pathogenesis is gaining interest for several reasons. Indeed, the majority of elderly people develop the process of "inflammaging", which is characterized by increased blood levels of proinflammatory molecules associated with an elevated susceptibility to chronic diseases. Epitope-specific alteration pattern of naturally occurring antibodies targeting the amino-terminus and the mid-domain of Aβ in both plasma and cerebrospinal fluid has been described in AD patients. Moreover, a possible therapeutic role of B lymphocytes depletion was recently demonstrated in murine AD models. Interestingly, active immunization against Aβ and tau, one of the main therapeutic strategies under investigation, depend on B lymphocytes. Finally. several molecules being tested in AD clinical trials can modify the homeostasis of B cells. This review summarizes the evidence supporting the role of B lymphocytes in AD from the pathogenesis to the possible therapeutic implications.

The order of immunoglobulin light chain κ and λ usage in primary and secondary lymphoid tissues of germ-free and conventional piglets

In pigs (Sus scrofa), the initial immunoglobulin rearrangement of the κ light chain is replaced by λ before the heavy chains rearrange, and the light chains may rearrange even later. This study investigates whether these developmental differences are reflected in the usage of IGK and IGL genes. We found large differences between peripheral B cells and those developing in the bone marrow, and between B cells in germ-free piglets and conventional pigs. During early B cell development in the bone marrow, more 3' V and 5' J gene segments for both light chains are used. However, in the peripheral naive repertoire, more 5' IGLV and 3' IGLJ genes are used. A similar shift toward the use of more 5' IGKV and 3' IGKJ genes is observed later after antigen exposure in conventional pigs. The expression profile showed that most λ⁺ B cells are generated earlier, while κ⁺ B cells develop from late precursors that already contain the λ rearrangement. The initial λ rearrangement is retained in both λ⁺ and κ⁺ B lymphocytes, and multiple λ transcripts can be found in individual cells. The overall pool of the IGLV repertoire is therefore much larger and more diversified than for IGKV. The κ repertoire is further restricted to the preferential use of only two major IGKV genes, reflecting the limitation for only two consecutive rearrangements. Tracing of silenced λ transcripts in κ⁺ B cells further confirmed the unconventional mechanism of differential rearrangements in pigs. Our results underline the diversity of the immune system among mammals.

Comparative Aspects of Immunoglobulin Gene Rearrangement Arrays in Different Species

Studies in humans and mice indicate the critical role of the surrogate light chain in the selection of the productive immunoglobulin repertoire during B cell development. However, subsequent studies using mutant mice have also demonstrated that alternative pathways are allowed. Our recent investigation has shown that some species, such as pig, physiologically use preferential rearrangement of authentic light chains, and become independent of surrogate light chains. Here we summarize the findings from swine and compare them with results in other species. In both groups, allelic and isotypic exclusions remain intact, so the different processes do not alter the paradigm of B-cell monospecificity. Both groups also retained some other essential processes, such as segregated and sequential rearrangement of heavy and light chain loci, preferential rearrangement of light chain kappa before lambda, and functional κ-deleting element recombination. On the other hand, the respective order of heavy and light chains rearrangement may vary, and rearrangement of the light chain kappa and lambda on different chromosomes may occur independently. Studies have also confirmed that the surrogate light chain is not required for the selection of the productive repertoire of heavy chains and can be substituted by authentic light chains. These findings are important for understanding evolutional approaches, redundancy and efficiency of B-cell generation, dependencies on other regulatory factors, and strategies for constructing therapeutic antibodies in unrelated species. The results may also be important for explaining interspecies differences in the proportional use of light chains and for the understanding of divergences in rearrangement processes. Therefore, the division into two groups may not be definitive and there may be more groups of intermediate species.

Consequences of the different order of immunoglobulin gene rearrangements in swine

Swine use a reverse order of immunoglobulin chain rearrangement compared to humans and mice, and this altered and modified order should have measurable consequences. Here we perform new and defining experiments with developing and mature B cells, characterizing the B cell populations that do not exist in other species. First, we have finally confirmed that light chains κ and λ are rearranged and expressed on the surface before any heavy chain rearrangements using western-blot. And second, we have analyzed a pool of mature B cells on the single-cell level to demonstrate that many κ⁺ mature B cells carry λ transcripts. According to these findings, we believe that there may be more groups of mammals; one of which uses a pre-BCR-driven developmental pathway for B cell generation (like mice and humans), the second group uses a pre-BCR-independent one (like swine), and some may be even intermediate.

Immunoglobulin light chain κ precedes λ rearrangement in swine but a majority of λ+ B cells are generated earlier

Developmental pathways for B cell lymphogenesis are sufficiently known only in mice and humans. However, both of these species rearrange immunoglobulin heavy chains (IgH) before light chains (IgL) while IgL precedes IgH rearrangement in swine. We demonstrate here that this reversed order of rearrangements have some concealed consequences: (1) we confirmed that although IgLκ rearrangement is initial, most IgLλ⁺ B cells are generated earlier and before IgH rearrangements, while most IgLκ⁺ B cells later and after IgH rearrangements, (2) the second IgLκ rearrangement can occur after IgLλ rearrangement, (3) early formed B cells bear only single in-frame IgH rearrangements, (4) many IgLκ⁺ B cells carry IgLλ rearrangements that can be productive and occurring on both alleles in one cell, and (5) although VpreB and λ5 genes are present in swine, they are preferentially expressed in non-B cells. In summary, our findings reveal that swine use an alternative B cell developmental pathway as compared to mice and humans.

Dimensionality reduction for visualizing single-cell data using UMAP

Advances in single-cell technologies have enabled high-resolution dissection of tissue composition. Several tools for dimensionality reduction are available to analyze the large number of parameters generated in single-cell studies. Recently, a nonlinear dimensionality-reduction technique, uniform manifold approximation and projection (UMAP), was developed for the analysis of any type of high-dimensional data. Here we apply it to biological data, using three well-characterized mass cytometry and single-cell RNA sequencing datasets. Comparing the performance of UMAP with five other tools, we find that UMAP provides the fastest run times, highest reproducibility and the most meaningful organization of cell clusters. The work highlights the use of UMAP for improved visualization and interpretation of single-cell data.

B cells in multiple sclerosis therapy-A comprehensive review

For decades, B cells were ignored in multiple sclerosis (MS) pathogenesis, and the disease was always regarded as a T cell-mediated disorder. Recent evidence shows that there is an antigen-driven B-cell response in the central nervous system of patients with MS, and memory B cells/plasma cells are detectable in MS lesions. The striking efficacy of B cell-depleting therapies in reducing the inflammatory activity of the disease highlights that B cells may play more pathogenetic roles than expected. B cells express several unique characteristic markers on their surface, for example, CD19, CD20 molecules, that provide selective targets for monoclonal antibodies. In this respect, several B cell-targeted therapies emerged, including anti-CD20 antibodies (rituximab, ocrelizumab, and ofatumumab), anti-CD19 antibody (inebilizumab), and agents targeting the BAFF/APRIL signaling pathway (atacicept, belimumab, and LY2127399). In this review, we discuss, in detail, the immunobiology of B cells and their protective and destructive roles in MS pathogenesis. In the second part, we list the completed and ongoing clinical trials investigating the safety and efficacy of B cell-related monoclonal antibodies in MS.

Inflammatory immune cells may impair the preBCR checkpoint, reduce new B cell production, and alter the antibody repertoire in old age

Aging impairs development of new B cells and diminishes the expression of protective antibodies. Reduced numbers of B cell precursors generally occur in old (~2 yrs.) mice. At the pro-B to pre-B cell transition, the pre-B cell receptor (preBCR) checkpoint directs pre-B cell expansion and selection of the pre-B cell immunoglobulin (Ig) μ heavy chain variable region repertoire. The preBCR is comprised of Ig μ heavy chain + surrogate light chains (SLC; λ5/VpreB). In old B cell precursors, SLC is decreased and fewer pre-B cells form the preBCR. In pro-B cells, SLC is complexed with cadherin 17 to form a "pro-B cell receptor" whose signaling is postulated to increase apoptotic sensitivity. We propose that inflammation in old mice, in part mediated by the age-associated B cells (ABC), promotes apoptosis among pro-B cells, particularly those relatively high in SLC. The remaining pro-B cells, with lower SLC, now generate pre-B cells with limited capacity to form the preBCR. Ig μ heavy chains vary in their capacity to associate with SLC and form the preBCR. We speculate that limited SLC restricts formation of the preBCR to a subset of Ig μ heavy chains. This likely impacts the composition of the antibody repertoire among B cells.

The BTG2-PRMT1 module limits pre-B cell expansion by regulating the CDK4-Cyclin-D3 complex

Developing pre-B cells in the bone marrow alternate between proliferation and differentiation phases. We found that protein arginine methyl transferase 1 (PRMT1) and B cell translocation gene 2 (BTG2) are critical components of the pre-B cell differentiation program. The BTG2-PRMT1 module induced a cell-cycle arrest of pre-B cells that was accompanied by re-expression of Rag1 and Rag2 and the onset of immunoglobulin light chain gene rearrangements. We found that PRMT1 methylated cyclin-dependent kinase 4 (CDK4), thereby preventing the formation of a CDK4-Cyclin-D3 complex and cell cycle progression. Moreover, BTG2 in concert with PRMT1 efficiently blocked the proliferation of BCR-ABL1-transformed pre-B cells in vitro and in vivo. Our results identify a key molecular mechanism by which the BTG2-PRMT1 module regulates pre-B cell differentiation and inhibits pre-B cell leukemogenesis.

Age-associated B cells (ABC) inhibit B lymphopoiesis and alter antibody repertoires in old age

With old age (∼2y old), mice show substantial differences in B cell composition within the lymphoid tissues. In particular, a novel subset of IgM⁺ CD21/35^lo/- CD23^- mature B cells, the age-associated B cells or ABC, increases numerically and proportionately. This occurs at the expense of other B cell subsets, including B2 follicular B cells in spleen and recirculating primary B cells in bone marrow. Our studies suggest that ABC have a distinctive antibody repertoire, as evidenced by relatively high reactivity to the self-antigens phosphorylcholine (PC) and malondialdehyde (MDA). While PC and MDA are found on apoptotic cells and oxidized lipoproteins, antibodies to these antigens are also cross-reactive with epitopes on bacterial species. In old mice, ABC express TNFα and are pro-inflammatory. ABC can inhibit growth and/or survival in pro-B cells as well as common lymphoid progenitors (CLP). In particular, ABC cause apoptosis in pro-B cells with relatively high levels of the surrogate light chain (SLC) and, consequently, promote an "SLC low" pathway of B cell differentiation in old mice. SLC together with μ heavy chain comprises the pre-B cell receptor (preBCR) critical for pre-B cell expansion and selection of the μ heavy chain Vh repertoire. The low level of SLC likely impairs normal preBCR driven proliferation and alters μ heavy chain Vh selection thereby affecting the antibody specificities of new B cells. In this manner, ABC may contribute to both qualitative and quantitative disruptions of normal B lymphopoiesis in old age.

Identification of Stage-Specific Surface Markers in Early B Cell Development Provides Novel Tools for Identification of Progenitor Populations

Whereas the characterization of B lymphoid progenitors has been facilitated by the identification of lineage- and stage-specific surface markers, the continued identification of differentially expressed proteins increases our capacity to explore normal and malignant B cell development. To identify novel surface markers with stage-specific expression patterns, we explored the reactivity of CD19(+) B cell progenitor cells to Abs targeted to 176 surface proteins. Markers with stage-specific expression were identified using a transgenic reporter gene system subdividing the B cell progenitors into four surface IgM(-) stages. This approach affirmed the utility of known stage-specific markers, as well as identifying additional proteins that selectively marked defined stages of B cell development. Among the stage-specific markers were the cell adhesion proteins CD49E, CD11A, and CD54 that are highly expressed selectively on the most immature progenitors. This work identifies a set of novel stage-specific surface markers that can be used as a complement to the classical staining protocols to explore B lymphocyte development.

B Lymphoblastic Leukemia With a Novel t(11;15) (q23;q15) and Unique Burkittoid Morphologic and Immunophenotypic Findings in a 9-Year-Old Boy

B lymphoblastic leukemia is a B progenitor cell neoplasm with a range of immature immunophenotypes and several associated cytogenetic lesions. In contrast, Burkitt leukemia/lymphoma is a mature B lymphocyte neoplasm with a characteristic germinal center immunophenotype and MYC rearrangement. With modern immunophenotyping and cytogenetic methods, the distinction between these 2 entities is seldom ambiguous. Herein, we report a case of a 9-year-old white boy with circulating leukemic cells that demonstrate morphologic overlap between Burkitt leukemia and B lymphoblastic leukemia. Flow cytometry and immunohistochemical stains demonstrated expression of sets of markers with overlap between immature and mature immunophenotypes. While the leukemic cells tested positive for terminal deoxynucleotidyl transferase (TdT), they expressed CD20, BCL6 (in a subset), and lambda-restricted surface light chain. Molecular studies confirmed a true clonal light chain rearrangement, whereas fluorescent in situ hybridization (FISH) results were negative for MYC rearrangement. Metaphase cytogenetics identified a novel gene rearrangement, t(11;15)(q23;q15), that does not involve the MLL gene. This unique cytogenetic abnormality involves the loss of INO80, an adenosine triphosphatase (ATPase) with DNA binding ability. This cytogenetic abnormality may represent a unique feature of this overlap entity of B lymphoblastic lymphoma that expresses markers of maturity and demonstrates Burkitt-like morphology.

The ageing human B cell repertoire: a failure of selection?

B cells undergo a number of different developmental stages, from initial formation of their B cell receptor (BCR) genes to differentiation into antibody-secreting plasma cells. Because the BCR is vital in these differentiation steps, autoreactive and exogenous antigen binding to the BCR exert critical selection pressures to shape the B cell repertoire. Older people are more prone to infectious disease, less able to respond well to vaccination and more likely to have autoreactive antibodies. Here we review evidence of changes in B cell repertoires in older people, which may be a reflection of age-related changes in B cell selection processes.

Transcription factor networks in B-cell differentiation link development to acute lymphoid leukemia

B-lymphocyte development in the bone marrow is controlled by the coordinated action of transcription factors creating regulatory networks ensuring activation of the B-lymphoid program and silencing of alternative cell fates. This process is tightly connected to malignant transformation because B-lineage acute lymphoblastic leukemia cells display a pronounced block in differentiation resulting in the expansion of immature progenitor cells. Over the last few years, high-resolution analysis of genetic changes in leukemia has revealed that several key regulators of normal B-cell development, including IKZF1, TCF3, EBF1, and PAX5, are genetically altered in a large portion of the human B-lineage acute leukemias. This opens the possibility of directly linking the disrupted development as well as aberrant gene expression patterns in leukemic cells to molecular functions of defined transcription factors in normal cell differentiation. This review article focuses on the roles of transcription factors in early B-cell development and their involvement in the formation of human leukemia.

Self-Restrained B Cells Arise following Membrane IgE Expression

Among immunoglobulins (Igs), IgE can powerfully contribute to antimicrobial immunity and severe allergy despite its low abundance. IgE protein and gene structure resemble other Ig classes, making it unclear what constrains its production to thousand-fold lower levels. Whether class-switched B cell receptors (BCRs) differentially control B cell fate is debated, and study of the membrane (m)IgE class is hampered by its elusive in vivo expression. Here, we demonstrate a self-controlled mIgE⁺ B cell stage. Primary or transfected mIgE⁺ cells relocate the BCRs into spontaneously internalized lipid rafts, lose mobility to chemokines, and change morphology. We suggest that combined proapoptotic mechanisms possibly involving Hax1 prevent mIgE⁺ memory lymphocyte accumulation. By uncoupling in vivo IgE switching from cytokine and antigen stimuli, we show that these features are independent from B cell stimulation and instead result from mIgE expression per se. Consequently, few cells survive IgE class switching, which might ensure minimal long-term IgE memory upon differentiation into plasma cells.

Unique Features of Fish Immune Repertoires: Particularities of Adaptive Immunity Within the Largest Group of Vertebrates

Fishes (i.e., teleost fishes) are the largest group of vertebrates. Although their immune system is based on the fundamental receptors, pathways, and cell types found in all groups of vertebrates, fishes show a diversity of particular features that challenge some classical concepts of immunology. In this chapter, we discuss the particularities of fish immune repertoires from a comparative perspective. We examine how allelic exclusion can be achieved when multiple Ig loci are present, how isotypic diversity and functional specificity impact clonal complexity, how loss of the MHC class II molecules affects the cooperation between T and B cells, and how deep sequencing technologies bring new insights about somatic hypermutation in the absence of germinal centers. The unique coexistence of two distinct B-cell lineages respectively specialized in systemic and mucosal responses is also discussed. Finally, we try to show that the diverse adaptations of immune repertoires in teleosts can help in understanding how somatic adaptive mechanisms of immunity evolved in parallel in different lineages across vertebrates.

ZNF423: Transcriptional modulation in development and cancer

Krüppel-like zinc finger proteins are versatile players in biology that have been implicated in mammalian development and disease. Among these proteins, ZNF423 and its mouse ortholog Zfp423 were initially implicated in midline patterning of the central nervous system but have emerged as critical transcriptional modulators in cancer. Epigenetically uncurbed ZNF423 interferes with lymphopoiesis by sequestration of the essential early B-cell factor 1 (EBF1) causing B-cell maturation arrest, a hallmark of acute lymphoblastic leukemia. Conversely, its presence in neuroblastoma, a primitive neuroectodermal tumor of childhood, allows retinoic acid-induced differentiation and is associated with a favorable outcome of neuroblastoma patients. Such opposing effects may be explained by the cellular context, but also by the multifunctionality of ZNF423 that is mediated by 30 zinc fingers forming various functional domains. This review summarizes current knowledge of ZNF423, focusing on its role in development and cancer.

The Eμ enhancer region influences H chain expression and B cell fate without impacting IgVH repertoire and immune response in vivo

The IgH intronic enhancer region Eμ is a combination of both a 220-bp core enhancer element and two 310-350-bp flanking scaffold/matrix attachment regions named MARsEμ. In the mouse, deletion of the core-enhancer Eμ element mainly affects VDJ recombination with minor effects on class switch recombination. We carried out endogenous deletion of the full-length Eμ region (core plus MARsEμ) in the mouse genome to study VH gene repertoire and IgH expression in developing B-lineage cells. Despite a severe defect in VDJ recombination with partial blockade at the pro-B cell stage, Eμ deletion (core or full length) did not affect VH gene usage. Deletion of this regulatory region induced both a decrease of pre-B cell and newly formed B cell compartments and a strong orientation toward the marginal zone B cell subset. Because Igμ H chain expression was decreased in Eμ-deficient pre-B cells, we propose that modification of B cell homeostasis in deficient animals was caused by "weak" pre-B cell and BCR expression. Besides imbalances in B cell compartments, Ag-specific Ab responses were not impaired in animals carrying the Eμ deletion. In addition to its role in VDJ recombination, our study points out that the full-length Eμ region does not influence VH segment usage but ensures efficient Igμ-chain expression required for strong signaling through pre-B cells and newly formed BCRs and thus participates in B cell inflow and fate.

Physiological level production of antigen-specific human immunoglobulin in cloned transchromosomic cattle

Therapeutic human polyclonal antibodies (hpAbs) derived from pooled plasma from human donors are Food and Drug Administration approved biologics used in the treatment of a variety of human diseases. Powered by the natural diversity of immune response, hpAbs are effective in treating diseases caused by complex or quickly-evolving antigens such as viruses. We previously showed that transchromosomic (Tc) cattle carrying a human artificial chromosome (HAC) comprising the entire unrearranged human immunoglobulin heavy-chain (hIGH) and kappa-chain (hIGK) germline loci (named as κHAC) are capable of producing functional hpAbs when both of the bovine immunoglobulin mu heavy-chains, bIGHM and bIGHML1, are homozygously inactivated (double knockouts or DKO). However, B lymphocyte development in these Tc cattle is compromised, and the overall production of hpAbs is low. Here, we report the construction of an improved HAC, designated as cKSL-HACΔ, by incorporating all of the human immunoglobulin germline loci into the HAC. Furthermore, for avoiding the possible human-bovine interspecies incompatibility between the human immunoglobulin mu chain protein (hIgM) and bovine transmembrane α and β immunoglobulins (bIgα and bIgβ) in the pre-B cell receptor (pre-BCR) complex, we partially replaced (bovinized) the hIgM constant domain with the counterpart of bovine IgM (bIgM) that is involved in the interaction between bIgM and bIgα/Igβ; human IgM bovinization would also improve the functionality of hIgM in supporting B cell activation and proliferation. We also report the successful production of DKO Tc cattle carrying the cKSL-HACΔ (cKSL-HACΔ/DKO), the dramatic improvement of B cell development in these cattle and the high level production of hpAbs (as measured for the human IgG isotype) in the plasma. We further demonstrate that, upon immunization by tumor immunogens, high titer tumor immunogen-specific human IgG (hIgG) can be produced from such Tc cattle.

Role of the Igh intronic enhancer Eμ in clonal selection at the pre-B to immature B cell transition

We previously described a checkpoint for allelic exclusion that occurs at the pre-B cell to immature B cell transition and is dependent upon the IgH intronic enhancer, Eμ. We now provide evidence that the breach in allelic exclusion associated with Eμ deletion results from decreased Igμ levels that make it difficult for emerging BCRs to reach the signaling threshold required for positive selection into the immature B cell compartment. We show that this compartment is smaller in mice carrying an Eμ-deficient, but functional, IgH allele (VHΔ(a)). Pre-B cells in such mice produce ≈ 50% wild-type levels of Igμ (mRNA and protein), and this is associated with diminished signals, as measured by phosphorylation of pre-BCR/BCR downstream signaling proteins. Providing Eμ-deficient mice with a preassembled VL gene led not only to a larger immature B cell compartment but also to a decrease in "double-producers," suggesting that H chain/L chain combinations with superior signaling properties can overcome the signaling defect associated with low Igμ-chain and can eliminate the selective advantage of "double-producers" that achieve higher Igμ-chain levels through expression of a second IgH allele. Finally, we found that "double-producers" in Eμ-deficient mice include a subpopulation with autoreactive BCRs. We infer that BCRs with IgH chain from the Eμ-deficient allele are ignored during negative selection owing to their comparatively low density. In summary, these studies show that Eμ's effect on IgH levels at the pre-B cell to immature B cell transition strongly influences allelic exclusion, the breadth of the mature BCR repertoire, and the emergence of autoimmune B cells.

Runx1 is essential at two stages of early murine B-cell development

The t(12;21) chromosomal translocation, targeting the gene encoding the RUNX1 transcription factor, is observed in 25% of pediatric acute lymphoblastic leukemia (ALL) and is an initiating event in the disease. To elucidate the mechanism by which RUNX1 disruption initiates leukemogenesis, we investigated its normal role in murine B-cell development. This study revealed 2 critical functions of Runx1: (1) to promote survival and development of progenitors specified to the B-cell lineage, a function that can be substituted by ectopic Bcl2 expression, and (2) to enable the developmental transition through the pre-B stage triggered by the pre-B-cell antigen receptor (pre-BCR). Gene expression analysis and genomewide Runx1 occupancy studies support the hypothesis that Runx1 reinforces the transcription factor network governing early B-cell survival and development and specifically regulates genes encoding members of the Lyn kinase subfamily (key integrators of interleukin-7 and pre-BCR signaling) and the stage-specific transcription factors SpiB and Aiolos (critical downstream effectors of pre-BCR signaling). Interrogation of expression databases of 257 ALL samples demonstrated the specific down-regulation of the SPIB and IKZF3 genes (the latter encoding AIOLOS) in t(12;21) ALL, providing novel insight into the mechanism by which the translocation blocks B-cell development and promotes leukemia.

Immunoglobulin light chains in medaka (Oryzias latipes)

The gene segments encoding antibodies have been studied in many capacities and represent some of the best-characterized gene families in traditional animal disease models (mice and humans). To date, multiple immunoglobulin light chain (IgL) isotypes have been found in vertebrates and it is unclear as to which isotypes might be more primordial in nature. Sequence data emerging from an array of fish genome projects is a valuable resource for discerning complex multigene assemblages in this critical branch point of vertebrate phylogeny. Herein, we have analyzed the genomic organization of medaka (Oryzias latipes) IgL gene segments based on recently released genome data. The medaka IgL locus located on chromosome 11 contains at least three clusters of IgL gene segments comprised of multiple gene assemblages of the kappa light chain isotype. These data suggest that medaka IgL gene segments may undergo both intra- and inter-cluster rearrangements as a means to generate additional diversity. Alignments of expressed sequence tags to concordant gene segments which revealed each of the three IgL clusters are expressed. Collectively, these data provide a genomic framework for IgL genes in medaka and indicate that Ig diversity in this species is achieved from at least three distinct chromosomal regions.

A VpreB3 homologue in a marsupial, the gray short-tailed opossum, Monodelphis domestica

A VpreB surrogate light (SL) chain was identified for the first time in a marsupial, the opossum Monodelphis domestica. Comparing the opossum VpreB to homologues from eutherian (placental mammals) and avian species supported the marsupial gene being VpreB3. VpreB3 is a protein that is not known to traffic to the cell surface as part of the pre-B cell receptor. Rather, VpreB3 associates with nascent immunoglobulin chains in the endoplasmic reticulum. Homologues of other known SL chains VpreB1, VpreB2, and λ5, which are found in eutherian mammals, were not found in the opossum genome, nor have they been identified in the genomes of nonmammals. VpreB3 likely evolved from earlier gene duplication, independent of that which generated VpreB1 and VpreB2 in eutherians. The apparent absence of VpreB1, VpreB2, and λ5 in marsupials suggests that an extracellular pre-B cell receptor containing SL chains, as it has been defined in humans and mice, may be unique to eutherian mammals. In contrast, the conservation of VpreB3 in marsupials and its presence in nonmammals is consistent with previous hypotheses that it is playing a more primordial role in B cell development.

Chromatin topology and the regulation of antigen receptor assembly

During an organism's ontogeny and in the adult, each B and T lymphocyte generates a unique antigen receptor, thereby creating the organism's ability to respond to a vast number of different antigens. The antigen receptor loci are organized into distinct regions that contain multiple variable (V), diversity (D), and/or joining (J) and constant (C) coding elements that are scattered across large genomic regions. In this review, we discuss the epigenetic modifications that take place in the different antigen receptor loci, the chromatin structure adopted by the antigen receptor loci to allow recombination of elements separated by large genomic distances, and the relationship between epigenetics and chromatin structure and how they relate to the generation of antigen receptor diversity.

Therapeutic targeting of B cells for rheumatic autoimmune diseases

Autoreactive B cells are characterized by their ability to secrete autoantibodies directed against self-peptides. During the last decade, it has become increasingly apparent that B lymphocytes not only produce autoantibodies but also exert important regulatory roles independent of their function as antibody-producing cells. This is especially relevant in the context of autoimmunity, because autoreactive B cells have been shown to possess the ability to activate pathogenic T cells, to produce pro-inflammatory cytokines, and to promote the formation of tertiary lymphoid tissue in target organs. The production of monoclonal antibodies against B-cell-surface molecules has facilitated the characterization of several distinct B lymphocyte subsets. These cell-surface molecules have not only served as useful cell differentiation markers but have also helped to unravel the important biological functions of these cells. Some of these molecules, all of which are expressed on the cell surface, have proven to be effective therapeutic targets. In both animal models and in clinical assays, the efficient elimination of B lymphocytes has been shown to be useful in the treatment of rheumatoid arthritis and other autoimmune diseases. The treatment of most rheumatic autoimmune diseases relies mainly on the use of cytotoxic immunosuppressants and corticosteroids. Although this has resulted in improved disease survival, patients may nonetheless suffer severe adverse events and, in some cases, their relapse rate remains high. The increasing need for safer and more effective drugs along with burgeoning new insights into the pathogenesis of these disorders has fueled interest in biological agents; clinical trials involving the B-cell depletion agent rituximab have been especially promising. This article reviews the current knowledge of B-cell biology and pathogenesis as well as the modern therapeutic approaches for rheumatic autoimmune diseases focusing in particular on the targeting of B-cell-specific surface molecules and on the blocking of B-cell activation and survival.

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.

Sin1-mTORC2 suppresses rag and il7r gene expression through Akt2 in B cells

Mammalian target of rapamycin (mTOR) is an important mediator of phosphoinositol-3-kinase (PI3K) signaling. PI3K signaling regulates B cell development, homeostasis, and immune responses. However, the function and molecular mechanism of mTOR-mediated PI3K signaling in B cells has not been fully elucidated. Here we show that Sin1, an essential component of mTOR complex 2 (mTORC2), regulates B cell development. Sin1 deficiency results in increased IL-7 receptor (il7r) and RAG recombinase (rag1 and rag2) gene expression, leading to enhanced pro-B cell survival and augmented V(D)J recombinase activity. We further show that Akt2 specifically mediates the Sin1-mTORC2 dependent suppression of il7r and rag gene expression in B cells by regulating FoxO1 phosphorylation. Finally, we demonstrate that the mTOR inhibitor rapamycin induces rag expression and promotes V(D)J recombination in B cells. Our study reveals that the Sin1/mTORC2-Akt2 signaling axis is a key regulator of FoxO1 transcriptional activity in B cells.

The pre-B-cell receptor associated protein VpreB3 is a useful diagnostic marker for identifying c-MYC translocated lymphomas

BACKGROUND

During B-cell development, precursor B cells transiently express the pre-B-cell receptor composed of μ heavy chain complexed with VpreB and λ5 surrogate light chain polypeptides. Recent profiling studies unexpectedly revealed abundant transcripts of one member of the VpreB family, VpreB3, in a subset of mature B cells and Burkitt lymphoma.

DESIGN AND METHODS

Here we used a novel antibody to investigate the normal expression pattern of VpreB3 protein in human hematopoietic and lymphoid tissues, and to determine whether VpreB3 could serve as a useful diagnostic biomarker for select B-cell lymphomas.

RESULTS

We found that VpreB3 protein is normally expressed by precursor B cells in bone marrow and by a subset of normal germinal center B cells in secondary lymphoid organs. Among lymphoid malignancies, we found an association between VpreB3 expression and B-cell tumors with c-MYC abnormalities. VpreB3 was highly expressed in all cases of Burkitt lymphoma, whether of endemic or sporadic origin (44/44 cases, 100%), all cases of B-cell lymphoma, unclassifiable, with features intermediate between diffuse large B-cell lymphoma and Burkitt lymphoma (5/5 cases, 100%), and the majority of diffuse large B-cell lymphomas harboring a c-MYC translocation (15/18 cases, 83%). The expression of VpreB3 in diffuse large B-cell lymphomas without a c-MYC translocation was associated with c-MYC polysomy in 25/75 cases (33%) but only rarely observed in diffuse large B-cell lymphomas lacking a c-MYC abnormality (9/98 cases, 9%).

CONCLUSIONS

We conclude that for B-cell tumors with features suggesting a possible c-MYC translocation, such as intermediate to large cell size and high proliferation rate, the presence of VpreB3 should prompt subsequent confirmatory genetic testing, whereas the absence of VpreB3 is virtually always associated with wild-type c-MYC alleles.

The development of mature B lymphocytes requires the combined function of CD19 and the p110δ subunit of PI3K

Mice lacking either CD19 or p110δ have reduced numbers of marginal zone and B1 B cells but normal numbers of naïve B2 cells which occupy the follicles of the lymphoid organs. We show here that mice lacking both CD19 and p110δ have normal B cell development in the bone marrow but have a significant reduction in the number of naïve B2 cells in the bone marrow, spleen and lymph nodes. These p110δ/CD19 double mutant B cells show a survival defect and reduced responsiveness to the pro-survival cytokine BAFF despite normal NFκB2/p100 processing and elevated expression of Bcl-2. Although the combined loss of p110δ and CD19 did not increase switching to Ig-lambda in immature B cells, mature B lymphocytes from the lymph nodes of p110δ/CD19 double mutant mice express highly elevated levels of mRNA encoding RAG-1 and RAG-2, which confirms the existing synergy between CD19 and p110δ-mediated signaling.

B cell signaling and fate decision

Antigen receptors on the surface of B lymphocytes trigger adaptive immune responses after encountering their cognate antigens but also control a series of antigen-independent checkpoints during B cell development. These physiological processes are regulated by the expression and function of cell surface receptors, intracellular signaling molecules, and transcription factors. The function of these proteins can be altered by a dynamic array of post-translational modifications, using two interconnected mechanisms. These modifications can directly induce an altered conformational state in the protein target of the modification itself. In addition, they can create new binding sites for other protein partners, thereby contributing to where and when such multiple protein assemblies are activated within cells. As a new type of post-transcriptional regulator, microRNAs have emerged to influence the development and function of B cells by affecting the expression of target mRNAs.

Gene expression profiling of minimally differentiated acute myeloid leukemia: M0 is a distinct entity subdivided by RUNX1 mutation status

Minimally differentiated acute myeloid leukemia (AML-M0) is defined by immature morphology and expression of early hematologic markers. By gene expression profiling (GEP) and subsequent unsupervised analysis of 35 AML-M0 samples and 253 previously reported AML cases, we demonstrate that AML-M0 cases express a unique signature that is largely separated from other molecular subtypes. Hematologic transcription regulators such as CEBPA, CEBPD, and ETV6, and the differentiation associated gene MPO appeared strongly down-regulated, in line with the primitive state of this leukemia. AML-M0 frequently carries loss-of-function RUNX1 mutation. Unsupervised analyses revealed a subdivision between AML-M0 cases with and without RUNX1 mutations. RUNX1 mutant AML-M0 samples showed a distinct up-regulation of B cell–related genes such as members of the B-cell receptor complex, transcription regulators RUNX3, ETS2, IRF8, or PRDM1, and major histocompatibility complex class II genes. Importantly, prediction with high accuracy of the AML-M0 subtype and prediction of patients carrying RUNX1 mutation within this subtype were possible based on the expression level of only a few transcripts. We propose that RUNX1 mutations in this AML subgroup cause lineage infidelity, leading to aberrant coexpression of myeloid and B-lymphoid genes. Furthermore, our results imply that AML-M0, although originally determined by morphology, constitutes a leukemia subgroup.

Cis-regulatory elements and epigenetic changes control genomic rearrangements of the IgH locus

Immunoglobulin variable region exons are assembled from discontinuous variable (V), diversity (D), and joining (J) segments by the process of V(D)J recombination. V(D)J rearrangements of the immunoglobulin heavy chain (IgH) locus are tightly controlled in a tissue-specific, ordered and allele-specific manner by regulating accessibility of V, D, and J segments to the recombination activating gene proteins which are the specific components of the V(D)J recombinase. In this review we discuss recent advances and established models brought forward to explain the mechanisms underlying accessibility control of V(D)J recombination, including research on germline transcripts, spatial organization, and chromatin modifications of the immunoglobulin heavy chain (IgH) locus. Furthermore, we review the functions of well-described and potential new cis-regulatory elements with regard to processes such as V(D)J recombination, allelic exclusion, and IgH class switch recombination.

Bos taurus genome sequence reveals the assortment of immunoglobulin and surrogate light chain genes in domestic cattle

BACKGROUND

The assortment of cattle immunoglobulin and surrogate light chain genes has been extracted from the version 3.1 of Bos taurus genome sequence as a part of an international effort to sequence and annotate the bovine genome.

RESULTS

63 variable lambda chain and 22 variable kappa chain genes were identified and phylogenetically assigned to 8 and 4 subgroups, respectively. The specified phylogenetic relationships are compatible with the established ruminant light chain variable gene families or subgroups. Because of gaps and uncertainties in the assembled genome sequence, the number of genes might change in the future versions of the genome sequence. In addition, three bovine surrogate light chain genes were identified. The corresponding cDNAs were cloned and the expression of the surrogate light chain genes was demonstrated from fetal material.

CONCLUSION

The bovine kappa gene locus is compact and simple which may reflect the preferential use of the lambda chain in cattle. The relative orientation of variable and joining genes in both loci are consistent with a deletion mechanism in VJ joining. The orientation of some variable genes cannot be determined from the data available. The number of functional variable genes is moderate when compared to man or mouse. Thus, post-recombinatorial mechanisms might contribute to the generation of the bovine pre-immune antibody repertoire. The heavy chains probably contribute more to recombinational immunoglobulin repertoire diversity than the light chains but the heavy chain locus could not be annotated from the version 3.1 of Bos taurus genome.

The epigenetic profile of Ig genes is dynamically regulated during B cell differentiation and is modulated by pre-B cell receptor signaling

Ag receptor loci poised for V(D)J rearrangement undergo germline transcription (GT) of unrearranged genes, and the accessible gene segments are associated with posttranslational modifications (PTM) on histones. In this study, we performed a comprehensive analysis of the dynamic changes of four PTM throughout B and T cell differentiation in freshly isolated ex vivo cells. Methylation of lysines 4 and 79 of histone H3, and acetylation of H3, demonstrated stage and lineage specificity, and were most pronounced at the J segments of loci poised for, or undergoing, rearrangement, except for dimethylation of H3K4, which was more equally distributed on V, D, and J genes. Focusing on the IgL loci, we demonstrated there are no active PTM in the absence of pre-BCR signaling. The kappa locus GT and PTM on Jkappa genes are rapidly induced following pre-BCR signaling in large pre-B cells. In contrast, the lambda locus shows greatly delayed onset of GT and PTM, which do not reach high levels until the immature B cell compartment, the stage at which receptor editing is initiated. Analysis of MiEkappa(-/-) mice shows that this enhancer plays a key role in inducing not only GT, but PTM. Using an inducible pre-B cell line, we demonstrate that active PTM on Jkappa genes occur after GT is initiated, indicating that histone PTM do not make the Jkappa region accessible, but conversely, GT may play a role in adding PTM. Our data indicate that the epigenetic profile of IgL genes is dramatically modulated by pre-BCR signaling and B cell differentiation status.

A role for the IgH intronic enhancer E mu in enforcing allelic exclusion

The intronic enhancer (Eμ) of the immunoglobulin heavy chain (IgH) locus is critical for V region gene assembly. To determine Eμ's subsequent functions, we created an Igh allele with assembled V_H gene but with Eμ removed. In mice homozygous for this Eμ-deficient allele, B cell development was normal and indistinguishable from that of mice with the same V_H knockin and Eμ intact. In mice heterozygous for the Eμ-deficient allele, however, allelic exclusion was severely compromised. Surprisingly, this was not a result of reduced suppression of V-DJ assembly on the second allele. Rather, the striking breakdown in allelic exclusion took place at the pre-B to immature B cell transition. These findings reveal both an important role for Eμ in influencing the fate of newly arising B cells and a second checkpoint for allelic exclusion.

Camelid immunoglobulins and nanobody technology

It is well established that all camelids have unique antibodies circulating in their blood. Unlike antibodies from other species, these special antibodies are devoid of light chains and are composed of a heavy-chain homodimer. These so-called heavy-chain antibodies (HCAbs) are expressed after a V-D-J rearrangement and require dedicated constant gamma-genes. An immune response is raised in these so-called heavy-chain antibodies following classical immunization protocols. These HCAbs are easily purified from serum, and the antigen-binding fragment interacts with parts of the target that are less antigenic to conventional antibodies. Since the antigen-binding site of the dromedary HCAb is comprised in one single domain, referred to as variable domain of heavy chain of HCAb (VHH) or nanobody (Nb), we designed a strategy to clone the Nb repertoire of an immunized dromedary and to select the Nbs with specificity for our target antigens. The monoclonal Nbs are well produced in bacteria, are very stable and highly soluble, and bind their cognate antigen with high affinity and specificity. We have successfully developed recombinant Nbs for research purposes, as probe in biosensors, to diagnose infections, and to treat diseases like cancer or trypanosomosis.

Interferon regulatory factor 4 and 8 in B-cell development

Interferon regulatory factor 4 (IRF4) and 8 are members of the interferon regulatory factor family of transcription factors and have been shown to be essential for the development and function of T cells, macrophages and dendritic cells. A series of recent studies have further demonstrated critical functions for IRF4 and 8 at several stages of B-cell development including pre-B-cell development, receptor editing, germinal center reaction and plasma cell generation. Collectively, these new studies provide molecular insights into the function of IRF4 and 8 and underscore a requirement for IRF4 and 8 throughout B-cell development. This review focuses on the recent advances on the roles of IRF4 and 8 in B-cell development.