Positive selection from newly formed to marginal zone B cells depends on the rate of clonal production, CD19, and btk

Notch2 controls developmental fate choices between germinal center and marginal zone B cells upon immunization

Sustained Notch2 signals induce trans-differentiation of Follicular B (FoB) cells into Marginal Zone B (MZB) cells in mice, but the physiology underlying this differentiation pathway is still elusive. Here, we demonstrate that most B cells receive a basal Notch signal, which is intensified in pre-MZB and MZB cells. Ablation or constitutive activation of Notch2 upon T-cell-dependent immunization reveals an interplay between antigen-induced activation and Notch2 signaling, in which FoB cells that turn off Notch2 signaling enter germinal centers (GC), while high Notch2 signaling leads to generation of MZB cells or to initiation of plasmablast differentiation. Notch2 signaling is dispensable for GC dynamics but appears to be re-induced in some centrocytes to govern expansion of IgG1+ GCB cells. Mathematical modelling suggests that antigen-activated FoB cells make a Notch2 dependent binary fate-decision to differentiate into either GCB or MZB cells. This bifurcation might serve as a mechanism to archive antigen-specific clones into functionally and spatially diverse B cell states to generate robust antibody and memory responses.

Noncanonical B Cells: Characteristics of Uncharacteristic B Cells

B lymphocytes were originally described as a cell type uniquely capable of secreting Abs. The importance of T cell help in Ab production was revealed soon afterward. Following these seminal findings, investigators made great strides in delineating steps in the conventional pathway that B cells follow to produce high-affinity Abs. These studies revealed generalized, or canonical, features of B cells that include their developmental origin and paths to maturation, activation, and differentiation into Ab-producing and memory cells. However, along the way, examples of nonconventional B cell populations with unique origins, age-dependent development, tissue localization, and effector functions have been revealed. In this brief review, features of B-1a, B-1b, marginal zone, regulatory, killer, NK-like, age-associated, and atypical B cells are discussed. Emerging work on these noncanonical B cells and functions, along with the study of their significance for human health and disease, represents an exciting frontier in B cell biology.

NONO regulates B-cell development and B-cell receptor signaling

The paraspeckle protein NONO is a multifunctional nuclear protein participating in the regulation of transcriptional regulation, mRNA splicing and DNA repair. However, whether NONO plays a role in lymphopoiesis is not known. In this study, we generated mice with global deletion of NONO and bone marrow (BM) chimeric mice in which NONO is deleted in all of mature B cells. We found that the global deletion of NONO in mice did not affect T-cell development but impaired early B-cell development in BM at pro- to pre-B-cell transition stage and B-cell maturation in the spleen. Studies of BM chimeric mice demonstrated that the impaired B-cell development in NONO-deficient mice is B-cell-intrinsic. NONO-deficient B cells displayed normal BCR-induced cell proliferation but increased BCR-induced cell apoptosis. Moreover, we found that NONO deficiency impaired BCR-induced activation of ERK, AKT, and NF-κB pathways in B cells, and altered BCR-induced gene expression profile. Thus, NONO plays a critical role in B-cell development and BCR-induced B-cell activation.

GPR174 signals via Gαs to control a CD86-containing gene expression program in B cells

GPR174 is abundantly expressed in B and T lymphocytes and has a role in restraining T cell responses, but the function of GPR174 in B cells is less clear. Here we report that upon in vitro culture B cells undergo a spontaneous GPR174-dependent activation process that is associated with marked changes in gene expression, including up-regulation of Cd86, Nr4a1, Ccr7, and phosphodiesterases. B cells lacking Gαs show a block in induction of the GPR174-dependent program. Spontaneous up-regulation of CD86 in cultured B cells is dependent on protein kinase A. Both GPR174- and Gαs-deficient B cells show enhanced survival in culture. In vivo, GPR174 contributes to NUR77 expression in follicular B cells and is needed for establishing a marginal zone compartment of normal size. Treatment of mice with lysophosphatidylserine (lysoPS), a GPR174 ligand, is sufficient to promote CD86 up-regulation by follicular B cells. These findings demonstrate that GPR174 can signal via Gαs to modulate B cell gene expression and show this can occur in vivo in response to lysoPS. Additionally, the findings illuminate a pathway that might be targeted to improve systems for the in vitro study of B cell responses.

A MZB Cell Activation Profile Present in the Lacrimal Glands of Sjögren's Syndrome-Susceptible C57BL/6.NOD-Aec1Aec2 Mice Defined by Global RNA Transcriptomic Analyses

The C57BL/6.NOD-Aec1Aec2 mouse has been extensively studied to define the underlying cellular and molecular basis for the onset and development of Sjögren's syndrome (SS), a human systemic autoimmune disease characterized clinically as the loss of normal lacrimal and salivary gland functions leading respectively to dry eye and dry mouth pathologies. While an overwhelming majority of SS studies in both humans and rodent models have long focused primarily on pathophysiological events and the potential role of T lymphocytes in these events, recent studies in our murine models have indicated that marginal zone B (MZB) lymphocytes are critical for both development and onset of SS disease. Although migration and function of MZB cells are difficult to study in vivo and in vitro, we have carried out ex vivo investigations that use temporal global RNA transcriptomic analyses to track early cellular and molecular events in these exocrine glands of C57BL/6.NOD-Aec1Aec2 mice. In the present report, genome-wide transcriptome analyses of lacrimal glands indicate that genes and gene-sets temporally upregulated during early onset of disease define the Notch2/NF-kβ14 and Type1 interferon signal transduction pathways, as well as identify chemokines, especially Cxcl13, and Rho-GTPases, including DOCK molecules, in the cellular migration of immune cells to the lacrimal glands. We discuss how the current results compare with our recently published salivary gland data obtained from similar studies carried out in our C57BL/6.NOD-Aec1Aec2 mice, pointing out both similarities and differences in the etiopathogeneses underlying the autoimmune response within the two glands. Overall, this study uses the power of transcriptomic analyses to identify temporal molecular bioprocesses activated during the preclinical covert pathogenic stage(s) of SS disease and how these findings may impact future intervention therapies as the disease within the two exocrine glands may not be identical.

Altered Phenotype and Enhanced Antibody-Producing Ability of Peripheral B Cells in Mice with Cd19-Driven Cre Expression

Given the importance of B lymphocytes in inflammation and immune defense against pathogens, mice transgenic for Cre under the control of Cd19 promoter (Cd19^Cre/+ mice) have been widely used to specifically investigate the role of loxP-flanked genes in B cell development/function. However, impacts of expression/insertion of the Cre transgene on the phenotype and function of B cells have not been carefully studied. Here, we show that the number of marginal zone B and B1a cells was selectively reduced in Cd19^Cre/+ mice, while B cell development in the bone marrow and total numbers of peripheral B cells were comparable between Cd19^Cre/+ and wild type C57BL/6 mice. Notably, humoral responses to both T cell-dependent and independent antigens were significantly increased in Cd19^Cre/+ mice. We speculate that these differences are mainly attributable to reduced surface CD19 levels caused by integration of the Cre-expressing cassette that inactivates one Cd19 allele. Moreover, our literature survey showed that expression of Cd19^Cre/+ alone may affect the development/progression of inflammatory and anti-infectious responses. Thus, our results have important implications for the design and interpretation of results on gene functions specifically targeted in B cells in the Cd19^Cre/+ mouse strain, for instance, in the context of (auto) inflammatory/infectious diseases.

Resting innate-like B cells leverage sustained Notch2/mTORC1 signaling to achieve rapid and mitosis-independent plasma cell differentiation

Little is known about how cells regulate and integrate distinct biosynthetic pathways governing differentiation and cell division. For B lineage cells it is widely accepted that activated cells must complete several rounds of mitosis before yielding antibody-secreting plasma cells. However, we report that marginal zone (MZ) B cells, innate-like naive B cells known to generate plasma cells rapidly in response to blood-borne bacteria, generate functional plasma cells despite cell-cycle arrest. Further, short-term Notch2 blockade in vivo reversed division-independent differentiation potential and decreased transcript abundance for numerous mTORC1- and Myc-regulated genes. Myc loss compromised plasma cell differentiation for MZ B cells, and reciprocally induced ectopic mTORC1 signaling in follicular B cells enabled division-independent differentiation and plasma cell-affiliated gene expression. We conclude that ongoing in situ Notch2/mTORC1 signaling in MZ B cells establishes a unique cellular state that enables rapid division-independent plasma cell differentiation.

Maternal B Cell-Intrinsic MyD88 Signaling Mediates LPS-Driven Intrauterine Fetal Death

Immunological networks balance tolerance towards paternal alloantigens during pregnancy with normal immune response to pathogens. Subclinical infections can impact this balance and lead to preterm birth or even intrauterine fetal death (IUFD). We recently showed that loss of maternal B cells renders murine fetuses susceptible to IUFD after LPS exposure. Since the signaling pathway involved in this B-cell mediated response remains unclear, we aimed to understand the participation of MyD88 in this response using B-cell-specific MyD88-deficient (BMyD88-/-) mice. B cells isolated from wild-type (WT), BMyD88-/-, CD19-/- and MyD88-/- dams on gestational day (gd) 10 responded differently to LPS concerning cytokine secretion. In vivo LPS challenge on gd 10 provoked IUFD in CD19-/- mothers with functional MyD88, while fetuses from BMyD88-/- and MyD88-/- mice were protected. These outcomes were associated with altered cytokine levels in the maternal serum and changes in CD4+ T-cell responses. Overall, the loss of MyD88 signaling in maternal B cells prevents the activation of cytokine release that leads to IUFD. Thus, while MyD88 signaling in maternal B cells protects the mother from infection, it ultimately kills the fetus. Understanding the cellular mechanisms underlying infection-driven pregnancy complications is the first step to designing powerful therapeutic strategies in the future.

Systematic memory B cell archiving and random display shape the human splenic marginal zone throughout life

Human memory B cells (MBCs) are generated and diversified in secondary lymphoid tissues throughout the organism. A paired immunoglobulin (Ig)-gene repertoire analysis of peripheral blood (PB) and splenic MBCs from infant, adult, and elderly humans revealed that throughout life, circulating MBCs are comprehensively archived in the spleen. Archive MBC clones are systematically preserved and uncoupled from class-switching. Clonality in the spleen increases steadily, but boosts at midlife, thereby outcompeting small clones. The splenic marginal zone (sMZ) represents a primed MBC compartment, generated from a stochastic exchange within the archive memory pool. This is supported by functional assays, showing that PB and splenic CD21⁺ MBCs acquire transient CD21^high expression upon NOTCH2-stimulation. Our study provides insight that the human MBC system in PB and spleen is composed of three interwoven compartments: the dynamic relationship of circulating, archive, and its subset of primed (sMZ) memory changes with age, thereby contributing to immune aging.

Pax5 regulates B cell immunity by promoting PI3K signaling via PTEN down-regulation

The transcription factor Pax5 controls B cell development, but its role in mature B cells is largely enigmatic. Here, we demonstrated that the loss of Pax5 by conditional mutagenesis in peripheral B lymphocytes led to the strong reduction of B-1a, marginal zone (MZ), and germinal center (GC) B cells as well as plasma cells. Follicular (FO) B cells tolerated the loss of Pax5 but had a shortened half-life. The Pax5-deficient FO B cells failed to proliferate upon B cell receptor or Toll-like receptor stimulation due to impaired PI3K-AKT signaling, which was caused by increased expression of PTEN, a negative regulator of the PI3K pathway. Pax5 restrained PTEN protein expression at the posttranscriptional level, likely involving Pten-targeting microRNAs. Additional PTEN loss in Pten,Pax5 double-mutant mice rescued FO B cell numbers and the development of MZ B cells but did not restore GC B cell formation. Hence, the posttranscriptional down-regulation of PTEN expression is an important function of Pax5 that facilitates the differentiation and survival of mature B cells, thereby promoting humoral immunity.

Marginal Zone Formation Requires ACKR3 Expression on B Cells

The marginal zone (MZ) contributes to the highly organized spleen microarchitecture. We show that expression of atypical chemokine receptor 3 (ACKR3) defines two equal-sized populations of mouse MZ B cells (MZBs). ACKR3 is required for development of a functional MZ and for positioning of MZBs. Deletion of ACKR3 on B cells distorts the MZ, and MZBs fail to deliver antigens to follicles, reducing humoral responses. Reconstitution of MZ-deficient CD19^ko mice shows that ACKR3^- MZBs can differentiate into ACKR3⁺ MZBs, but not vice versa. The lack of a MZ is rescued by adoptive transfer of ACKR3-sufficient, and less by ACKR3-deficient, follicular B cells (FoBs); hence, ACKR3 expression is crucial for establishment of the MZ. The inability of CD19^ko mice to respond to T-independent antigen is rescued when ACKR3-proficient, but not ACKR3-deficient, FoBs are transferred. Accordingly, ACKR3-deficient FoBs are able to reconstitute the MZ if the niche is pre-established by ACKR3-proficient MZBs.

New inducible mast cell-deficient mouse model (Mcpt5/Cma1DTR)

Mast cell-deficient mice are helpful for understanding the roles of mast cells in vivo. To date, a dozen mouse models for mast cell deficiency have been reported. However, mice with a specific depletion of all populations of mast cells have not been reported. We generated knock-in mice, termed Mcpt5/Cma1^DTR mice, expressing human diphtheria toxin A (DT) receptor under the endogenous promoter of Mcpt5 (also known as Cma1), which encodes mouse mast cell protease-5. Flow cytometry and histological analysis showed that intraperitoneal injection of DT induced almost complete depletion of mast cells in heterozygote Mcpt5/Cma1^DTR/+ mice. The deletion rates of mast cells in peritoneal cavity, mesentery, abdominal skin, ear skin, and glandular stomach were 99.9%, 100%, 98.7%, 97.7%, and 100%, respectively. Passive cutaneous anaphylaxis reaction also revealed mast cell deficiency in ear skin after DT treatment. Other than mast cells, a small percentage of marginal zone B cells in Mcpt5/Cma1^DTR/+ mice were killed by DT treatment. In conclusion, the Mcpt5/Cma1^DTR/+ mouse model is valuable for achieving conditional depletion of all populations of mast cells without inducing a marked reduction in other cells.

Marginal zone B cells: From housekeeping function to autoimmunity?

Marginal zone (MZ) B cells comprise a subset of innate-like B cells found predominantly in the spleen, but also in lymph nodes and blood. Their principal functions are participation in quick responses to blood-borne pathogens and secretion of natural antibodies. The latter is important for housekeeping functions such as clearance of apoptotic cell debris. MZ B cells have B cell receptors with low poly-/self-reactivity, but they are not pathogenic at steady state. However, if simultaneously stimulated with self-antigen and pathogen- and/or damage-associated molecular patterns (PAMPs/DAMPs), MZ B cells may participate in the initial steps towards breakage of immunological tolerance. This review summarizes what is known about the role of MZ B cells in autoimmunity, both in mouse models and human disease. We cover factors important for shaping the MZ B cell compartment, how the functional properties of MZ B cells may contribute to breaking tolerance, and how MZ B cells are being regulated.

Notch2-mediated plasticity between marginal zone and follicular B cells

Follicular B (FoB) and marginal zone B (MZB) cells are functionally and spatially distinct mature B cell populations in the spleen, originating from a Notch2-dependent fate decision after splenic influx of immature transitional B cells. In the B cell follicle, a Notch2-signal is provided by DLL-1-expressing fibroblasts. However, it is unclear whether FoB cells, which are in close contact with these DLL-1 expressing fibroblasts, can also differentiate to MZB cells if they receive a Notch2-signal. Here, we show induced Notch2IC-expression in FoB cells re-programs mature FoB cells into bona fide MZB cells as is evident from the surface phenotype, localization, immunological function and transcriptome of these cells. Furthermore, the lineage conversion from FoB to MZB cells occurs in immunocompetent wildtype mice. These findings demonstrate plasticity between mature FoB and MZB cells that can be driven by a singular signaling event, the activation of Notch2.

Early Covert Appearance of Marginal Zone B Cells in Salivary Glands of Sjögren's Syndrome-Susceptible Mice: Initiators of Subsequent Overt Clinical Disease

The C57BL/6.NOD-Aec1Aec2 mouse model has been extensively studied to define the underlying cellular and molecular bioprocesses critical in the onset of primary Sjögren’s Syndrome (pSS), a human systemic autoimmune disease characterized clinically as the loss of lacrimal and salivary gland functions leading to dry eye and dry mouth pathologies. This mouse model, together with several gene knockout mouse models of SS, has indicated that B lymphocytes, especially marginal zone B (MZB) cells, are necessary for development and onset of clinical manifestations despite the fact that destruction of the lacrimal and salivary gland cells involves a classical T cell-mediated autoimmune response. Because migrations and functions of MZB cells are difficult to study in vivo, we have carried out ex vivo investigations that use temporal global RNA transcriptomic analyses to profile autoimmunity as it develops within the salivary glands of C57BL/6.NOD-Aec1Aec2 mice. Temporal profiles indicate the appearance of Notch2-positive cells within the salivary glands of these SS-susceptible mice concomitant with the early-phase appearance of lymphocytic foci (LF). Data presented here identify cellular bioprocesses occurring during early immune cell migrations into the salivary glands and suggest MZB cells are recruited to the exocrine glands by the upregulated Cxcl13 chemokine where they recognize complement (C’)-decorated antigens via their sphingosine-1-phosphate (S1P) and B cell (BC) receptors. Based on known MZB cell behavior and mobility, we propose that MZB cells activated in the salivary glands migrate to splenic follicular zones to present antigens to follicular macrophages and dendritic cells that, in turn, promote a subsequent systemic cell-mediated and autoantibody-mediated autoimmune T cell response that targets exocrine gland cells and functions. Overall, this study uses the power of transcriptomic analyses to provide greater insight into several molecular events defining cellular bioprocesses underlying SS that can be modelled and more thoroughly studied at the cellular level.

Innate B Cells: the Archetype of Protective Immune Cells

The innate B cell (IBC) population is heterogeneous and involved in the primary immune response. IBC functions include a high ability to produce natural antibodies with IgM isotype, the elimination of apoptotic cells, and a capacity to be cognate help to T cells. Among IBC subsets, B-1 cells and marginal zone B cells are the main producers of IgM, act as rapid immune responders that may relocate to follicular lymphoid and differentiate to cytokine and antibody-secreting cells shortly after infection. IBCs functions are highly dependent on their localization site and the nature of their B cell receptor repertoire, suggesting a high plasticity range of different immune responses. In this review, we will describe the nature and functions of the different innate-like B cell subsets, first in mice and then in humans. Besides this, we will emphasize the strong ability of these cells to undertake different protective functions from the first line of defense against pathogens to the regulatory role of the broader immune response.

ADAM28: Another ambivalent protease in cancer

Emergence of novel therapeutic options in a perspective of personalized therapy of cancer relies on the discovery of precise molecular mechanisms involved in the switch from a localized tumor to invasive metastasis spread. Pro-tumor functions have been mostly ascribed to proteolytic enzymes from the metalloproteinase family including A Disintegrin And Metalloproteinases (ADAMs). Particularly, when expressed by cancer cells, ADAM28 protease supports cancer cell proliferation, survival and migration as well as metastatic progression. In sharp contrast, ADAM28 derived from the tumor microenvironment has shown to exert strong protective effects against deleterious metastasis dissemination. Indeed, depletion of host-derived ADAM28 (ADAM28 KO mice) accelerates colonization lung tissues, increases tumor foci implantation, and impairs T cell immune response. In this review, we outline specific ADAM28 functions when specifically expressed by carcinoma cells or by tumor microenvironment. Finally, we discuss about future research strategies that could be pursued to highlight new functions of this protease in cancer.

The regulation of DOCK family proteins on T and B cells

The dedicator of cytokinesis (DOCK) family proteins consist of 11 members, each of which contains 2 domains, DOCK homology region (DHR)-1 and DHR-2, and as guanine nucleotide exchange factors, they mediate activation of small GTPases. Both DOCK2 and DOCK8 deficiencies in humans can cause severe combined immunodeficiency, but they have different characteristics. DOCK8 defect mainly causes high IgE, allergic disease, refractory skin virus infection, and increased incidence of malignant tumor, whereas DOCK2 defect mainly causes early-onset, invasive infection with less atopy and increased IgE. However, the underlying molecular mechanisms causing the disease remain unclear. This paper discusses the role of DOCK family proteins in regulating B and T cells, including development, survival, migration, activation, immune tolerance, and immune functions. Moreover, related signal pathways or molecule mechanisms are also described in this review. A greater understanding of DOCK family proteins and their regulation of lymphocyte functions may facilitate the development of new therapeutics for immunodeficient patients and improve their prognosis.

Marginal zone SIGN-R1+ macrophages are essential for the maturation of germinal center B cells in the spleen

The mechanisms that regulate germinal center (GC) B cell responses in the spleen are not fully understood. Here we use a combination of pharmacologic and genetic approaches to delete SIGN-R1+ marginal zone (MZ) macrophages and reveal their specific contribution to the regulation of humoral immunity in the spleen. We find that while SIGN-R1+ macrophages were not essential for initial activation of B cells, they were required for maturation of the response and development of GC B cells. These defects could be corrected when follicular helper T (Tfh) cells were induced before macrophage ablation or when Tfh responses were enhanced. Moreover, we show that in the absence of SIGN-R1+ macrophages, DCIR2+ dendritic cells (DCs), which play a key role in priming Tfh responses, were unable to cluster to the interfollicular regions of the spleen and were instead displaced to the MZ. Restoring SIGN-R1+ macrophages to the spleen corrected positioning of DCIR2+ DCs and rescued the GC B cell response. Our study reveals a previously unappreciated role for SIGN-R1+ macrophages in regulation of the GC reaction and highlights the functional specification of macrophage subsets in the MZ compartment.

Homeostasis and regulation of autoreactive B cells

In contrast to the previous belief that autoreactive B cells are eliminated from the normal repertoire of B cells, many autoreactive B cells actually escape clonal deletion and develop into mature B cells. These autoreactive B cells in healthy individuals perform some beneficial functions in the host and are homeostatically regulated by regulatory T and B cells or other mechanisms to prevent autoimmune diseases. Autoreactive B-1 cells constitutively produce polyreactive natural antibodies for tissue homeostasis. Recently, autoreactive follicular B cells were reported to participate actively in the germinal center reaction. Furthermore, the selection and usefulness of autoreactive marginal zone (MZ) B cells found in autoimmune diseases are not well understood, although the repertoire of MZ B-cell receptors (BCRs) is presumed to be biased to detect bacterial antigens. In this review, we discuss the autoreactive B-cell populations among all three major B-cell subsets and their regulation in immune responses and diseases.

Visualization of T Cell Migration in the Spleen Reveals a Network of Perivascular Pathways that Guide Entry into T Zones

Lymphocyte homeostasis and immune surveillance require that T and B cells continuously recirculate between secondary lymphoid organs. Here, we used intravital microscopy to define lymphocyte trafficking routes within the spleen, an environment of open blood circulation and shear forces unlike other lymphoid organs. Upon release from arterioles into the red pulp sinuses, T cells latched onto perivascular stromal cells in a manner that was independent of the chemokine receptor CCR7 but sensitive to Gi protein-coupled receptor inhibitors. This latching sheltered T cells from blood flow and enabled unidirectional migration to the bridging channels and then to T zones, entry into which required CCR7. Inflammatory responses modified the chemotactic cues along the perivascular homing paths, leading to rapid block of entry. Our findings reveal a role for vascular structures in lymphocyte recirculation through the spleen, indicating the existence of separate entry and exit routes and that of a checkpoint located at the gate to the T zone.

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Perivascular pathways support T cell entry into splenic T zones, but not egress from them

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Attachment to the homing paths requires activation of GPCRs other than CCR7

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CCR7 mediates one-directional migration and entry into T zones

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Inflammation leads to modification of the homing paths and to rapid block of entry

Phosphoflow Protocol for Signaling Studies in Human and Murine B Cell Subpopulations

BCR signaling, involving phosphorylation of various downstream molecules, including kinases, lipases, and linkers, is crucial for B cell selection, survival, proliferation, and differentiation. Phosphoflow cytometry (phosphoflow) is a single-cell-based technique to measure phosphorylated intracellular proteins, providing a more quantitative read-out than Western blotting. Recent advances in phosphoflow basically allow simultaneous analysis of protein phosphorylation in B cell (sub)populations, without prior cell sorting. However, fixation and permeabilization procedures required for phosphoflow often affect cell surface epitopes or mAb conjugates, precluding the evaluation of the phosphorylation status of signaling proteins across different B cell subpopulations present in a single sample. In this study, we report a versatile phosphoflow protocol allowing extensive staining of B cell subpopulations in human peripheral blood or various anatomical compartments in the mouse, starting from freshly isolated or frozen cell suspensions. Both human and mouse B cell subpopulations showed different basal and BCR stimulation-induced phosphorylation levels of downstream signaling proteins. For example, peritoneal B-1 cells and splenic marginal zone B cells exhibited significantly increased basal (ex vivo) signaling and increased responsiveness to in vitro BCR stimulation compared with peritoneal B-2 cells and splenic follicular B cells, respectively. In addition, whereas stimulation with anti-IgM or anti-Igκ L chain Abs resulted in strong pCD79a and pPLCγ2 signals, IgD stimulation only induced CD79a but not pPLCγ2 phosphorylation. In summary, the protocol is user friendly and quantifies BCR-mediated phosphorylation with high sensitivity at the single-cell level, in combination with extensive staining to identify individual B cell development and differentiation stages.

Skin-Associated B Cells in Health and Inflammation

Traditionally, the skin was believed to be devoid of B cells, and studies of the skin immune system have largely focused on other types of leukocytes. Exciting recent data show that B cells localize to the healthy skin of humans and other mammalian species with likely homeostatic functions in host defense, regulation of microbial communities, and wound healing. Distinct skin-associated B cell subsets drive or suppress cutaneous inflammatory responses with important clinical implications. Localized functions of skin-associated B cell subsets during inflammation comprise Ab production, interactions with skin T cells, tertiary lymphoid tissue formation, and production of proinflammatory cytokines but also include immunosuppression by providing IL-10. In this review, we delve into the intriguing new roles of skin-associated B cells in homeostasis and inflammation.

Dedicator of cytokinesis protein 2 couples with lymphoid enhancer-binding factor 1 to regulate expression of CD21 and B-cell differentiation

BACKGROUND

B-cell receptor (BCR) signaling, combined with CD19 and CD21 signals, imparts specific control of B-cell responses. Dedicator of cytokinesis protein 2 (DOCK2) is critical for the migration and motility of lymphocytes. Although absence of DOCK2 leads to lymphopenia, little is known about the signaling mechanisms and physiologic functions of DOCK2 in B cells.

OBJECTIVE

We sought to determine the underlying molecular mechanism of how DOCK2 regulates BCR signaling and peripheral B-cell differentiation.

METHODS

In this study we used genetic models for DOCK2, Wiskott-Aldrich syndrome protein (WASP), and lymphoid enhancer-binding factor 1 deficiency to study their interplay in BCR signaling and B-cell differentiation.

RESULTS

We found that the absence of DOCK2 led to downregulation of proximal and distal BCR signaling molecules, including CD19, but upregulation of SH2-containing inositol 5 phosphatase 1, a negative signaling molecule. Interestingly, DOCK2 deficiency reduced CD19 and CD21 expression at the mRNA and/or protein levels and was associated with reduced numbers of marginal zone B cells. Additionally, loss of DOCK2 reduced activation of WASP and accelerated degradation of WASP, resulting into reduced actin accumulation and early activation of B cells. Mechanistically, the absence of DOCK2 upregulates the expression of lymphoid enhancer-binding factor 1. These differences were associated with altered humoral responses in the absence of DOCK2.

CONCLUSIONS

Overall, our study has provided a novel underlying molecular mechanism of how DOCK2 deficiency regulates surface expression of CD21, which leads to downregulation of CD19-mediated BCR signaling and marginal zone B-cell differentiation.

Transcription factors IRF8 and PU.1 are required for follicular B cell development and BCL6-driven germinal center responses

The IRF and Ets families of transcription factors regulate the expression of a range of genes involved in immune cell development and function. However, the understanding of the molecular mechanisms of each family member has been limited due to their redundancy and broad effects on multiple lineages of cells. Here, we report that double deletion of floxed Irf8 and Spi1 (encoding PU.1) by Mb1-Cre (designated DKO mice) in the B cell lineage resulted in severe defects in the development of follicular and germinal center (GC) B cells. Class-switch recombination and antibody affinity maturation were also compromised in DKO mice. RNA-seq (sequencing) and ChIP-seq analyses revealed distinct IRF8 and PU.1 target genes in follicular and activated B cells. DKO B cells had diminished expression of target genes vital for maintaining follicular B cell identity and GC development. Moreover, our findings reveal that expression of B-cell lymphoma protein 6 (BCL6), which is critical for development of germinal center B cells, is dependent on IRF8 and PU.1 in vivo, providing a mechanism for the critical role for IRF8 and PU.1 in the development of GC B cells.

B Cell-Mediated Antigen Presentation through MHC Class II Is Dispensable for Atherosclerosis Progression

Depletion of B cells attenuates plaque development and modulates T cell responses in mouse models of atherosclerosis, suggesting that Ag presentation by B cells may promote disease progression. Thus, we set out to determine the role of B cell-mediated MHC class II (MHC II) Ag presentation during atherosclerotic plaque development. We developed murine conditional MHC II deletion and expression systems under control of the B cell-restricted CD19 promoter in an experimental model of atherosclerosis. Mice lacking MHC II expression only on B cells exhibited systemic shifts in germinal center and marginal zone B cell populations, leading to a reduced Ab response compared with littermate control animals. However, all populations were present and normal cholesterol uptake was detected in the plasma following high-fat diet treatment. In a second model, in which conditional expression of MHC II is limited only to B cells, showed similar overall cellularity characteristics compared with mice with complete MHC II deficiency. High-fat diet feeding showed no major changes in atherosclerotic plaque size or plaque cellular content in either conditional deletion or conditional expression approaches, compared with control animals. By testing the necessity and sufficiency of MHC II on B cells in the progression of atherosclerosis, we determine that MHC II on B cells does not directly regulate lesion development in murine models.

The Prognostic Value of CD8+ and CD45RO+ T Cells Infiltration and Beclin1 Expression Levels for Early Postoperative Cholangitis of Biliary Atresia Patients after Kasai Operation

BACKGROUND

Postoperative cholangitis is a common but severe complication after Kasai portoenterostomy for biliary atresia (BA). This study aimed to identify its prognostic factors.

METHODS

Two sets of liver paraffin-embedded tissue samples were collected from BA patients who received Kasai portoenterostomy (n = 25 and n = 31, respectively). Patients were divided into non-cholangitis and cholangitis groups. The infiltration of CD4⁺, CD8⁺, CD45RO⁺, CD68⁺ cells and expression of Beclin1 were quantitatively evaluated in immunohistochemical analysis.

RESULTS

Cholangitis group had a significantly lower CD8⁺ T cell infiltration but a higher CD45RO⁺ cell infiltration, and a lower Beclin1 level than non-cholangitis group (all P < 0.01). Multivariate logistic regression analysis indicated that infiltration of CD8⁺ cells (odds ratio [OR], 0.112; 95% confidence interval [CI], 0.022-0.577) and CD45RO⁺ cells (OR, 3.88; 95% CI, 1.37-11.03), and Beclin1 level (OR, 0.088; 95% CI, 0.018-0.452) were independent influence factors for early postoperative cholangitis. Receiver operating characteristic (ROC) analysis showed that area under ROC curve (AUROC) values for CD8⁺ cells, CD45RO⁺ cells and Beclin1 were 0.857, 0.738 and 0.900, respectively.

CONCLUSION

Our findings demonstrated the CD8⁺ cells, CD45RO⁺ cells and Beclin1 level possessed the prognostic value for early postoperative cholangitis following Kasai operation, which may be helpful to develop new prevention and treatment strategies for postoperative cholangitis.

IKKα deficiency disrupts the development of marginal zone and follicular B cells

Only few genes have been confidently identified to be involved in the Follicular (FO) and Marginal Zone (MZ) B cell differentiation, migration, and retention in the periphery. Our group previously observed that IKKα kinase inactive mutant mice IKKα^K44A/K44A have significantly lower number of MZ B cells whereas FO B cell numbers appeared relatively normal. Because kinase dead IKKα can retain some of its biological functions that may interfere in revealing its actual role in the MZ and FO B cell differentiation. Therefore, in the current study, we genetically deleted IKKα from the pro-B cell lineage that revealed novel functions of IKKα in the MZ and FO B lymphocyte development. The loss of IKKα produces a significant decline in the percentage of immature B lymphocytes, mature marginal zone B cells, and follicular B cells along with a severe disruption of splenic architecture of marginal and follicular zones. IKKα deficiency affect the recirculation of mature B cells through bone marrow. A transplant of IKKα knockout fetal liver cells into Rag^-/- mice shows a significant reduction compared to control in the B cells recirculating through bone marrow. To reveal the genes important in the B cell migration, a high throughput gene expression analysis was performed on the IKKα deficient recirculating mature B cells (B220⁺IgM^hi). That revealed significant changes in the expression of genes involved in the B lymphocyte survival, homing and migration. And several among those genes identified belong to G protein family. Taken together, this study demonstrates that IKKα forms a vial axis controlling the genes involved in MZ and FO B cell differentiation and migration.

Microbial symbionts regulate the primary Ig repertoire

Symbiotic relationships help shape immune fitness. Chen et al. demonstrate that microbial symbionts influence host immunity by enriching frequencies of antibacterial specificities within the naive B cell receptor repertoire and that this may have consequences for mucosal and systemic immunity.

The ability of immunoglobulin (Ig) to recognize pathogens is critical for optimal immune fitness. Early events that shape preimmune Ig repertoires, expressed on IgM⁺ IgD⁺ B cells as B cell receptors (BCRs), are poorly defined. Here, we studied germ-free mice and conventionalized littermates to explore the hypothesis that symbiotic microbes help shape the preimmune Ig repertoire. Ig-binding assays showed that exposure to conventional microbial symbionts enriched frequencies of antibacterial IgM⁺ IgD⁺ B cells in intestine and spleen. This enrichment affected follicular B cells, involving a diverse set of Ig-variable region gene segments, and was T cell–independent. Functionally, enrichment of microbe reactivity primed basal levels of small intestinal T cell–independent, symbiont-reactive IgA and enhanced systemic IgG responses to bacterial immunization. These results demonstrate that microbial symbionts influence host immunity by enriching frequencies of antibacterial specificities within preimmune B cell repertoires and that this may have consequences for mucosal and systemic immunity.

Plasma Cell Differentiation Pathways in Systemic Lupus Erythematosus

Plasma cells (PCs) are responsible for the production of protective antibodies against infectious agents but they also produce pathogenic antibodies in autoimmune diseases, such as systemic lupus erythematosus (SLE). Traditionally, high affinity IgG autoantibodies are thought to arise through germinal center (GC) responses. However, class switching and somatic hypermutation can occur in extrafollicular (EF) locations, and this pathway has also been implicated in SLE. The pathway from which PCs originate may determine several characteristics, such as PC lifespan and sensitivity to therapeutics. Although both GC and EF responses have been implicated in SLE, we hypothesize that one of these pathways dominates in each individual patient and genetic risk factors may drive this predominance. While it will be important to distinguish polymorphisms that contribute to a GC-driven or EF B cell response to develop targeted treatments, the challenge will be not only to identify the differentiation pathway but the molecular mechanisms involved. In B cells, this task is complicated by the cross-talk between the B cell receptor, toll-like receptors (TLR), and cytokine signaling molecules, which contribute to both GC and EF responses. While risk variants that affect the function of dendritic cells and T follicular helper cells are likely to primarily influence GC responses, it will be important to discover whether some risk variants in the interferon and TLR pathways preferentially influence EF responses. Identifying the pathways of autoreactive PC differentiation in SLE may help us to understand patient heterogeneity and thereby guide precision therapy.

Dock8 regulates BCR signaling and activation of memory B cells via WASP and CD19

Dock8 deficiency leads to immunodeficiency, and the role of Dock8 in B-cell development and function has been revealed; however, the role of DocK8 on B-cell receptor (BCR) signaling and function of memory B cells remains elusive. In this study, we generated a Dock8 knockout mouse model and collected peripheral blood mononuclear cells from Dock8 patients to study the effect of Dock8 deficiency on the BCR signaling and activation of memory B cells with confocal microscopy and total internal reflection fluorescence microscopy. The activation of key, positive upstream BCR signaling molecules, pCD19 and phosphorylated Brutons tyrosine kinase (pBtk), is reduced. Interestingly, the total protein and activated levels of Wiskott-Aldrich syndrome protein (WASP) are decreased in Dock8-deficient mouse B cells. Our previous research has shown that WASP positively regulates cd19 transcription; furthermore, we found that Dock8 regulates cd19 transcription. What we found in Dock8 patients can be a phenotype copied from Dock8 mice. The early activation of memory B cells from Dock8 patients is disrupted with reduced BCR clustering, B-cell spreading, and signalosome recruitment into the degree of naïve B cells, as well as the transition from naïve B cells to unswitched memory B cells. Overall, our study provides a novel mechanism for Dock8 regulation of BCR signaling by regulating cd19 transcription, as well as the underlying mechanism of noncompetence of memory B cells in Dock8 patients.

Role of Bruton's tyrosine kinase in B cells and malignancies

Bruton’s tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. BTK was initially shown to be defective in the primary immunodeficiency X-linked agammaglobulinemia (XLA) and is essential both for B cell development and function of mature B cells. Shortly after its discovery, BTK was placed in the signal transduction pathway downstream of the B cell antigen receptor (BCR). More recently, small-molecule inhibitors of this kinase have shown excellent anti-tumor activity, first in animal models and subsequently in clinical studies. In particular, the orally administered irreversible BTK inhibitor ibrutinib is associated with high response rates in patients with relapsed/refractory chronic lymphocytic leukemia (CLL) and mantle-cell lymphoma (MCL), including patients with high-risk genetic lesions. Because ibrutinib is generally well tolerated and shows durable single-agent efficacy, it was rapidly approved for first-line treatment of patients with CLL in 2016. To date, evidence is accumulating for efficacy of ibrutinib in various other B cell malignancies. BTK inhibition has molecular effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and highlight the importance of BTK inhibition in cancer therapy.

Fcµ Receptor Promotes the Survival and Activation of Marginal Zone B Cells and Protects Mice against Bacterial Sepsis

The marginal zone B cells (MZB) are located at the interface between the circulation and lymphoid tissue and as a gatekeeper play important roles in both innate and adaptive immune responses. We have previously found that MZB are significantly reduced in mice deficient in the IgM Fc receptor (FcμR) but how FcμR regulates the development and function of MZB remains unknown. In this study, we found that both marginal zone precursor (MZP) and MZB were decreased in FcμR^−/− mice. The reduction of MZP and MZB was not due to impaired proliferation of these cells but rather due to their increased death. Further analysis revealed that FcμR^−/− MZB had reduced tonic BCR signal, as evidenced by their decreased levels of phosphorylated SYK and AKT relative to WT MZB. MZB in FcμR^−/− mice responded poorly to LPS in vivo when compared with MZB in WT mice. Consistent with the reduced proportion of MZB and their impaired response to LPS, antibody production against the type 1 T-independent Ag, NP-LPS, was significantly reduced in FcμR^−/− mice. Moreover, FcμR^−/− mice were highly susceptible to Citrobacter rodentium-induced sepsis. These results reveal a critical role for FcμR in the survival and activation of MZB and in protection against acute bacterial infection.

Bruton's Tyrosine Kinase, a Component of B Cell Signaling Pathways, Has Multiple Roles in the Pathogenesis of Lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by the loss of adaptive immune tolerance to nucleic acid-containing antigens. The resulting autoantibodies form immune complexes that promote inflammation and tissue damage. Defining the signals that drive pathogenic autoantibody production is an important step in the development of more targeted therapeutic approaches for lupus, which is currently treated primarily with non-specific immunosuppression. Here, we review the contribution of Bruton’s tyrosine kinase (Btk), a component of B and myeloid cell signaling pathways, to disease in murine lupus models. Both gain- and loss-of-function genetic studies have revealed that Btk plays multiple roles in the production of autoantibodies. These include promoting the activation, plasma cell differentiation, and class switching of autoreactive B cells. Small molecule inhibitors of Btk are effective at reducing autoantibody levels, B cell activation, and kidney damage in several lupus models. These studies suggest that Btk may promote end-organ damage both by facilitating the production of autoantibodies and by mediating the inflammatory response of myeloid cells to these immune complexes. While Btk has not been associated with SLE in GWAS studies, SLE B cells display signaling defects in components both upstream and downstream of Btk consistent with enhanced activation of Btk signaling pathways. Taken together, these observations indicate that limiting Btk activity is critical for maintaining B cell tolerance and preventing the development of autoimmune disease. Btk inhibitors, generally well-tolerated and approved to treat B cell malignancy, may thus be a useful therapeutic approach for SLE.

Altered Marginal Zone B Cell Selection in the Absence of IκBNS

Marginal zone (MZ) B cells reside in the splenic MZ and play important roles in T cell-independent humoral immune responses against blood-borne pathogens. IκBNS-deficient bumble mice exhibit a severe reduction in the MZ B compartment but regain an MZ B population with age and, thus, represent a valuable model to examine the biology of MZ B cells. In this article, we characterized the MZ B cell defect in further detail and investigated the nature of the B cells that appear in the MZ of aged bumble mice. Flow cytometry analysis of the splenic transitional B cell subsets demonstrated that MZ B cell development was blocked at the transitional-1 to transitional-2-MZ precursor stage in the absence of functional IκBNS. Immunohistochemical analysis of spleen sections from wild-type and bumble mice revealed no alteration in the cellular MZ microenvironment, and analysis of bone marrow chimeras indicated that the MZ B cell development defect in bumble mice was B cell intrinsic. Further, we demonstrate that the B cells that repopulate the MZ in aged bumble mice were distinct from age-matched wild-type MZ B cells. Specifically, the expression of surface markers characteristic for MZ B cells was altered and the L chain Igλ⁺ repertoire was reduced in bumble mice. Finally, plasma cell differentiation of sorted LPS-stimulated MZ B cells was impaired, and aged bumble mice were unable to respond to NP-Ficoll immunization. These results demonstrate that IκBNS is required for an intact MZ B cell compartment in C57BL/6 mice.

NLR Nod1 signaling promotes survival of BCR-engaged mature B cells through up-regulated Nod1 as a positive outcome

The microenvironment, including microbial products, plays a role in mature B cell survival. Hayakawa et al. show that B cell antigen receptor ligand–mediated Nod1 up-regulation in vivo in B cell development leads to preferential mature B cell survival as a competitive survival, increasing the Nod1⁺ B cell pool with age.

Although B cell development requires expression of the B cell antigen receptor (BCR), it remains unclear whether engagement of self-antigen provides a positive impact for most B cells. Here, we show that BCR engagement by self-ligand during development in vivo results in up-regulation of the Nod-like receptor member Nod1, which recognizes the products of intestinal commensal bacteria. In anti-thymocyte/Thy-1 autoreactive BCR knock-in mice lacking self–Thy-1 ligand, immunoglobulin light chain editing occurred, generating B cells with up-regulated Nod1, including follicular and marginal zone B cells with natural autoreactivity. This BCR editing with increased Nod1 resulted in preferential survival. In normal adult mice, most mature B cells are enriched for Nod1 up-regulated cells, and signaling through Nod1 promotes competitive survival of mature B cells. These findings demonstrate a role for microbial products in promoting survival of mature B cells through up-regulated Nod1, providing a positive effect of BCR engagement on development of most B cells.

CD19 regulates ADAM28-mediated Notch2 cleavage to control the differentiation of marginal zone precursors to MZ B cells

As the first line of defence, marginal zone (MZ) B cells play principal roles in clearing blood‐borne pathogens during infection and are over‐primed in autoimmune diseases. However, the basic mechanisms underlying MZ B‐cell development are still unclear. We found here that CD19 deficiency blocked the differentiation of marginal zone precursors (MZP) to MZ B cells, whereas CD19 expression in CD19‐deficient MZP rescues MZ B‐cell generation. Furthermore, CD19 regulates Notch2 cleavage by up‐regulating ADAM28 expression in MZP. Finally, we found that CD19 suppressed Foxo1 expression to promote ADAM28 expression in MZP. These results suggest that CD19 controls the differentiation of MZP to MZ B cells by regulating ADAM28‐mediated Notch2 cleavage. Thus, we demonstrated the basic mechanisms underlying the differentiation of MZP to MZ B cells.

Altered B cell signalling in autoimmunity

Recent work has provided new insights into how altered B cell-intrinsic signals - through the B cell receptor (BCR) and key co-receptors - function together to promote the pathogenesis of autoimmunity. These combined signals affect B cells at two distinct stages: first, in the selection of the naive repertoire; and second, during extrafollicular or germinal centre activation responses. Thus, dysregulated signalling can lead to both an altered naive BCR repertoire and the generation of autoantibody-producing B cells. Strikingly, high-affinity autoantibodies predate and predict disease in several autoimmune disorders, including type 1 diabetes and systemic lupus erythematosus. This Review summarizes how, rather than being a downstream consequence of autoreactive T cell activation, dysregulated B cell signalling can function as a primary driver of many human autoimmune diseases.

A generalized quantitative antibody homeostasis model: regulation of B-cell development by BCR saturation and novel insights into bone marrow function

In a pair of articles, we present a generalized quantitative model for the homeostatic function of clonal humoral immune system. In this first paper, we describe the cycles of B-cell expansion and differentiation driven by B-cell receptor engagement. The fate of a B cell is determined by the signals it receives via its antigen receptor at any point of its lifetime. We express BCR engagement as a function of apparent affinity and free antigen concentration, using the range of 10⁻¹⁴–10⁻³ M for both factors. We assume that for keeping their BCR responsive, B cells must maintain partial BCR saturation, which is a narrow region defined by [Ag]≈K_D. To remain in this region, B cells respond to changes in [Ag] by proliferation or apoptosis and modulate K_D by changing BCR structure. We apply this framework to various niches of B-cell development such as the bone marrow, blood, lymphoid follicles and germinal centers. We propose that clustered B cells in the bone marrow and in follicles present antigen to surrounding B cells by exposing antigen captured on complement and Fc receptors. The model suggests that antigen-dependent selection in the bone marrow results in (1) effector BI cells, which develop in blood as a consequence of the inexhaustible nature of soluble antigens, (2) memory cells that survive in antigen rich niches, identified as marginal zone B cells. Finally, the model implies that memory B cells could derive survival signals from abundant non-cognate antigens.

Transitional B cells commit to marginal zone B cell fate by Taok3-mediated surface expression of ADAM10

Notch2 and B cell antigen receptor (BCR) signaling determine whether transitional B cells become marginal zone B (MZB) or follicular B (FoB) cells in the spleen, but it is unknown how these pathways are related. We generated Taok3^-/- mice, lacking the serine/threonine kinase Taok3, and found cell-intrinsic defects in the development of MZB but not FoB cells. Type 1 transitional (T1) B cells required Taok3 to rapidly respond to ligation by the Notch ligand Delta-like 1. BCR ligation by endogenous or exogenous ligands induced the surface expression of the metalloproteinase ADAM10 on T1 B cells in a Taok3-dependent manner. T1 B cells expressing surface ADAM10 were committed to becoming MZB cells in vivo, whereas T1 B cells lacking expression of ADAM10 were not. Thus, during positive selection in the spleen, BCR signaling causes immature T1 B cells to become receptive to Notch ligands via Taok3-mediated surface expression of ADAM10.

Role of Natural IgM Autoantibodies (IgM-NAA) and IgM Anti-Leukocyte Antibodies (IgM-ALA) in Regulating Inflammation

Natural IgM autoantibodies (IgM-NAA) are rapidly produced to inhibit pathogens and abrogate inflammation mediated by invading microorganisms and host neoantigens. IgM-NAA achieve this difficult task by being polyreactive with low binding affinity but with high avidity, characteristics that allow these antibodies to bind antigenic determinants shared by pathogens and neoantigens. Hence the same clones of natural IgM can bind and mask host neoantigens as well as inhibit microorganisms. In addition, IgM-NAA regulate the inflammatory response via mechanisms involving binding of IgM to apoptotic cells to enhance their removal and binding of IgM to live leukocytes to regulate their function. Secondly, we review how natural IgM prevents autoimmune disorders arising from pathogenic IgG autoantibodies as well as by autoreactive B and T cells that have escaped tolerance mechanisms. Thirdly, using IgM knockout mice, we show that regulatory B and T cells require IgM to effectively regulate inflammation mediated by innate, adaptive and autoimmune mechanisms. It is therefore not surprising why the host positively selects such autoreactive B1 cells that generate protective IgM-NAA, which are also evolutionarily conserved. Fourthly, we show that IgM anti-leukocyte autoantibodies (IgM-ALA) levels and their repertoire can vary in normal humans and disease states and this variation may partly explain the observed differences in the inflammatory response after infection, ischemic injury or after a transplant. Finally we also show how protective IgM-NAA can be rendered pathogenic under non-physiological conditions. IgM-NAA have therapeutic potential. Polyclonal IgM infusions can be used to abrogate ongoing inflammation. Additionally, inflammation arising after ischemic kidney injury, e.g., during high-risk elective cardiac surgery or after allograft transplantation, can be prevented by pre-emptively infusing polyclonal IgM, or DC pretreated ex vivo with IgM, or by increasing in vivo IgM with a vaccine approach. Cell therapy with IgM pretreated cells, is appealing as less IgM will be required.

Pro-B cells propagated in stromal cell-free cultures reconstitute functional B-cell compartments in immunodeficient mice

Up to now long-term in vitro growth of pro-B cells was thought to require stromal cells. However, here we show that fetal liver (FL) and bone marrow (BM) derived pro-B cells can be propagated long-term in stromal cell-free cultures supplemented with IL-7, stem cell factor and FLT3 ligand. Within a week, most cells expressed surface CD19, CD79A, λ5, and VpreB antigens and had rearranged immunoglobulin D-J heavy chain genes. Both FL and BM pro-B cells reconstituted the B-cell compartments of immuno-incompetent Rag2-deficient mice, with FL pro-B cells generating follicular, marginal zone (MZB) and B1a B cells, and BM pro-B cells giving rise mainly to MZB cells. Reconstituted Rag2-deficient mice generated significant levels of IgM and IgG antibodies to a type II T-independent antigen; mice reconstituted with FL pro-B cells generated surprisingly high IgG₁ titers. Finally, we show for the first time that mice reconstituted with mixtures of pro-B and pro-T cells propagated in stromal cell-free in vitro cultures mounted a T-cell-dependent antibody response. This novel stromal cell-free culture system facilitates our understanding of B-cell development and might be applied clinically.

Mst1 positively regulates B-cell receptor signaling via CD19 transcriptional levels

As a key regulator of hippo signaling pathway, Mst kinases are emerging as one of the key signaling molecules that influence cell proliferation, organ size, cell migration, and cell polarity. In B lymphocytes, Mst1 deficiency causes the developmental defect of marginal zone (MZ) B cells, but how Mst1 regulates B-cell receptor (BCR) activation and differentiation remains elusive. Using genetically manipulated mouse models and total internal reflection fluorescence microscopy, we have demonstrated that Mst1 positively regulates BCR signaling via modulating CD19 transcriptional levels. Consistent with this, Mst1-deficient mice exhibited reduced BCR signaling, which is concurrent with defective BCR clustering and B-cell spreading on stimulatory lipid bilayers. The disruption of CD19-mediated Btk signaling by Mst1 deficiency leads to the severe defect in the differentiation of MZ and germinal center B cells. Mechanistic analysis showed that Mst1 upregulates the messenger RNA level of CD19 via regulating the transcriptional factor TEAD2 that directly binds to the consensus motif in the 3' untranslated region of cd19. Overall, our results reveal a new function of Mst1 in B cells and the mechanism by which Mst1 regulates the activation and differentiation of peripheral B cells.

Endothelial Plasmalemma Vesicle-Associated Protein Regulates the Homeostasis of Splenic Immature B Cells and B-1 B Cells

Plasmalemma vesicle-associated protein (Plvap) is an endothelial protein with roles in endothelial diaphragm formation and maintenance of basal vascular permeability. At the same time, Plvap has roles in immunity by facilitating leukocyte diapedesis at inflammatory sites and controlling peripheral lymph node morphogenesis and the entry of soluble Ags into lymph node conduits. Based on its postulated role in diapedesis, we have investigated the role of Plvap in hematopoiesis and show that deletion of Plvap results in a dramatic decrease of IgM⁺IgD^lo B cells in both the spleen and the peritoneal cavity. Tissue-specific deletion of Plvap demonstrates that the defect is B cell extrinsic, because B cell and pan-hematopoietic Plvap deletion has no effect on IgM⁺IgD^lo B cell numbers. Endothelial-specific deletion of Plvap in the embryo or at adult stage recapitulates the full Plvap knockout phenotype, whereas endothelial-specific reconstitution of Plvap under the Chd5 promoter rescues the IgM⁺IgD^lo B cell phenotype. Taken together, these results show that Plvap expression in endothelial cells is important in the maintenance of IgM⁺ B cells in the spleen and peritoneal cavity.

Role of Natural Autoantibodies and Natural IgM Anti-Leucocyte Autoantibodies in Health and Disease

We review how polyreactive natural IgM autoantibodies (IgM-NAA) protect the host from invading micro-organisms and host neo-antigens that are constantly being produced by oxidation mechanisms and cell apoptosis. Second, we discuss how IgM-NAA and IgM anti-leukocyte antibodies (IgM-ALA) inhibits autoimmune inflammation by anti-idiotypic mechanisms, enhancing removal of apoptotic cells, masking neo-antigens, and regulating the function of dendritic cells (DC) and effector cells. Third, we review how natural IgM prevents autoimmune disorders arising from pathogenic IgG autoantibodies, triggered by genetic mechanisms (e.g., SLE) or micro-organisms, as well as by autoreactive B and T cells that have escaped tolerance mechanisms. Studies in IgM knockout mice have clearly demonstrated that regulatory B and T cells require IgM to effectively regulate inflammation mediated by innate, adaptive, and autoimmune mechanisms. It is, therefore, not surprising why the host positively selects such autoreactive B1 cells that generate IgM-NAA, which are also evolutionarily conserved. Fourth, we show that IgM-ALA levels and their repertoire can vary in normal humans and disease states and this variation may partly explain the observed differences in the inflammatory response after infection, ischemic injury, or after a transplant. We also show how protective IgM-NAA can be rendered pathogenic under non-physiological conditions. We also review IgG-NAA that are more abundant than IgM-NAA in plasma. However, we need to understand if the (Fab)(2) region of IgG-NAA has physiological relevance in non-disease states, as in plasma, their functional activity is blocked by IgM-NAA having anti-idiotypic activity. Some IgG-NAA are produced by B2 cells that have escaped tolerance mechanisms and we show how such pathogenic IgG-NAA are regulated to prevent autoimmune disease. The Fc region of IgG-NAA can influence inflammation and B cell function in vivo by binding to activating and inhibitory FcγR. IgM-NAA has therapeutic potential. Polyclonal IgM infusions can be used to abrogate on-going inflammation. Additionally, inflammation arising after ischemic kidney injury, e.g., during high-risk elective cardiac surgery or after allograft transplantation, can be prevented by pre-emptively infusing polyclonal IgM or DC pretreated ex vivo with IgM or by increasing in vivo IgM with a vaccine approach. Cell therapy is appealing as less IgM will be required.

Positive selection of B10 cells is determined by BCR specificity and signaling strength

B10 cells, a regulatory B cell subset, negatively regulate immune responses in an IL-10-dependent manner. However, the mechanism of B10 cell development is unclear. We found that B10 cells mainly identified self-antigens. TgVH3B4 transgenic mice, whose VH was derived from an actin-reactive natural antibody, exhibit elevated numbers of actin-binding B10 cells. Immunization of TgVH3B4 mice with actin induced elevated B10 cell numbers in an antigen-specific manner, indicating positive selection of B10 cells by self-antigens. Furthermore, higher BCR signaling strength facilitated B10 cell development. We also observed that actin-reactive IgG levels were unchanged in TgVH3B4 mice after immunization with actin in contrast to the elevated OVA-reactive IgG level after immunization with OVA, indicating that B10 cells acted in an antigen-specific manner to inhibit the immune response. Our data demonstrate for the first time that B10 cells are positively selected by self-reactivity and that higher BCR signaling strength promotes B10 cell development.

Reduced Number of Transitional and Naive B Cells in Addition to Decreased BAFF Levels in Response to the T Cell Independent Immunogen Pneumovax®23

Protective immunity against T cell independent (TI) antigens such as Streptococcus pneumoniae is characterized by antibody production of B cells induced by the combined activation of T cell independent type 1 and type 2 antigens in the absence of direct T cell help. In mice, the main players in TI immune responses have been well defined as marginal zone (MZ) B cells and B-1 cells. However, the existence of human equivalents to these B cell subsets and the nature of the human B cell compartment involved in the immune reaction remain elusive. We therefore analyzed the effect of a TI antigen on the B cell compartment through immunization of healthy individuals with the pneumococcal polysaccharide (PnPS)-based vaccine Pneumovax^®23, and subsequent characterization of B cell subpopulations. Our data demonstrates a transient decrease of transitional and naïve B cells, with a concomitant increase of IgA⁺ but not IgM⁺ or IgG⁺ memory B cells and a predominant generation of PnPS-specific IgA⁺ producing plasma cells. No alterations could be detected in T cells, or proposed human B-1 and MZ B cell equivalents. Consistent with the idea of a TI immune response, antigen-specific memory responses could not be observed. Finally, BAFF, which is supposed to drive class switching to IgA, was unexpectedly found to be decreased in serum in response to Pneumovax^®23. Our results demonstrate that a characteristic TI response induced by Pneumovax^®23 is associated with distinct phenotypical and functional changes within the B cell compartment. Those modulations occur in the absence of any modulations of T cells and without the development of a specific memory response.

Receptor editing and genetic variability in human autoreactive B cells

Lang et al. show in a humanized mouse model that human B cells undergo central tolerance via a combination of receptor editing and clonal deletion.

The mechanisms by which B cells undergo tolerance, such as receptor editing, clonal deletion, and anergy, have been established in mice. However, corroborating these mechanisms in humans remains challenging. To study how autoreactive human B cells undergo tolerance, we developed a novel humanized mouse model. Mice expressing an anti–human Igκ membrane protein to serve as a ubiquitous neo self-antigen (Ag) were transplanted with a human immune system. By following the fate of self-reactive human κ⁺ B cells relative to nonautoreactive λ⁺ cells, we show that tolerance of human B cells occurs at the first site of self-Ag encounter, the bone marrow, via a combination of receptor editing and clonal deletion. Moreover, the amount of available self-Ag and the genetics of the cord blood donor dictate the levels of central tolerance and autoreactive B cells in the periphery. Thus, this model can be useful for studying specific mechanisms of human B cell tolerance and to reveal differences in the extent of this process among human populations.