Generation of peripheral B cells occurs via two spatially and temporally distinct pathways

Differential differentiation of B cell lymphopoiesis in lethal and non-lethal murine malaria models

B cells in protection against malaria and need of experiencing many episodes in humans to achieve a state of immunity is largely unknown. The cellular basis of such defects in terms of B cell generation, maturation and trafficking was studied by taking Plasmodium chabaudi, a non-lethal and Plasmodium berghei, a lethal murine model. A flow cytometry (FCF) based evaluation was used to study alterations in generation and maintenance of B cells in patients with Plasmodium falciparum malaria as well as in murine malaria models. A significant accumulation of mature B cells in bone marrow and immature B cells in circulation was a feature observed only in lethal malaria. At peak parasitaemia, both the models induce a significant decrease in T2 (transitional) B cells with expansion of T1B cells. Studies in patients with acute Pf malaria showed a significant expansion of memory B cells and TB cells with a concomitant decrease in naive2 B cells as compared with healthy controls. This study clearly demonstrates that acute malarial infection induces major disturbances in B cell development in lymphoid organs and trafficking in periphery.

Peripheral apoptosis and limited clonal deletion during physiologic murine B lymphocyte development

Self-reactive and polyreactive B cells generated during B cell development are silenced by either apoptosis, clonal deletion, receptor editing or anergy to avoid autoimmunity. The specific contribution of apoptosis to normal B cell development and self-tolerance is incompletely understood. Here, we quantify self-reactivity, polyreactivity and apoptosis during physiologic B lymphocyte development. Self-reactivity and polyreactivity are most abundant in early immature B cells and diminish significantly during maturation within the bone marrow. Minimal apoptosis still occurs at this site, however B cell receptors cloned from apoptotic B cells show comparable self-reactivity to that of viable cells. Apoptosis increases dramatically only following immature B cells leaving the bone marrow sinusoids, but above 90% of cloned apoptotic transitional B cells are not self-reactive/polyreactive. Our data suggests that an apoptosis-independent mechanism, such as receptor editing, removes most self-reactive B cells in the bone marrow. Mechanistically, lack of survival signaling rather than clonal deletion appears to be the underpinning cause of apoptosis in most transitional B cells in the periphery.

Integrin β1 regulates marginal zone B cell differentiation and PI3K signaling

Marginal zone (MZ) B cells represent innate-like B cells that mediate a fast immune response. The adhesion of MZ B cells to the marginal sinus of the spleen is governed by integrins. Here, we address the question of whether β1-integrin has additional functions by analyzing Itgb1fl/flCD21Cre mice in which the β1-integrin gene is deleted in mature B cells. We find that integrin β1-deficient mice have a defect in the differentiation of MZ B cells and plasma cells. We show that integrin β1-deficient transitional B cells, representing the precursors of MZ B cells, have enhanced B cell receptor (BCR) signaling, altered PI3K and Ras/ERK pathways, and an enhanced interaction of integrin-linked kinase (ILK) with the adaptor protein Grb2. Moreover, the MZ B cell defect of integrin β1-deficient mice could, at least in part, be restored by a pharmacological inhibition of the PI3K pathway. Thus, β1-integrin has an unexpected function in the differentiation and function of MZ B cells.

Bone marrow-derived naïve B lymphocytes improve heart function after myocardial infarction: a novel cardioprotective mechanism for empagliflozin

The role of adaptive immunity in myocardial recovery post myocardial infarction (MI), particularly the immune response by B lymphocytes, remains elusive. Bone marrow immune microenvironment in response to MI is remotely regulated by the hypothalamic pituitary adrenal (HPA) axis. We utilized the cardioprotective actions of SGLT2 inhibitor to identify and characterize bone marrow B cell subsets that respond to myocardial injury. Initially, we preformed ligation of left anterior descendant (LAD) coronary artery in male C57BL/6J mice to monitor the dynamic changes of immune cells across tissues. Mechanistic insights from mouse models demonstrated arrest of bone marrow B cell maturation and function 24 h post MI. A secondary MI model (twice MIs) in mice was established for the first time to evaluate the dosage-dependent cardioprotection of empagliflozin (EMPA). Single-cell RNA-Seq further demonstrated that EMPA restored bone marrow naïve B cell (B220⁺CD19⁺CD43^-IgM⁺IgD⁺) counts and function. Additionally, we recruited 14 acute MI patients with single LAD disease, and profiled B cells post percutaneous coronary intervention (PCI) (compared to 18 matched no-MI controls). We revealed a positive correlation of increased B cell counts with enhanced ejection fraction in MI patients with PCI while lymphopenia was associated with patients with heart failure. Mechanistically, MI triggers the release of glucocorticoids from neuroendocrine system, inducing NHE1-mediated autophagic death of bone marrow B cells while repressing B cell progenitor proliferation and differentiation. Infusion of B cells derived from bone marrow significantly improved cardiac function and diminished infarct size post MI. These findings provide new mechanistic insights into regulation of adaptive immune response post MI, and support targeting bone marrow B cell development for improved ventricular remodeling and reduced heart failure after MI.

Bruton's TK regulates myeloid cell recruitment during acute inflammation

BACKGROUND AND PURPOSE

Bruton's TK (BTK) is a non-receptor kinase best known for its role in B lymphocyte development that is critical for proliferation and survival of leukaemic cells in B-cell malignancies. However, BTK is expressed in myeloid cells, particularly neutrophils, monocytes and macrophages where its inhibition has been reported to cause anti-inflammatory properties.

EXPERIMENTAL APPROACH

We explored the role of BTK on migration of myeloid cells (neutrophils, monocytes and macrophages), in vitro using chemotaxis assays and in vivo using zymosan-induced peritonitis as model systems.

KEY RESULTS

Using the zymosan-induced peritonitis model of sterile inflammation, we demonstrated that acute inhibition of BTK prior to zymosan challenge reduced phosphorylation of BTK in circulating neutrophils and monocytes. Moreover, pharmacological inhibition of BTK with ibrutinib specifically inhibited neutrophil and Ly6Chi monocytes, but not Ly6Clo monocyte recruitment to the peritoneum. X-linked immunodeficient (XID) mice, which have a point mutation in the Btk gene, had reduced neutrophil and monocyte recruitment to the peritoneum following zymosan challenge. Pharmacological or genetic inhibition of BTK signalling substantially reduced human monocyte and murine macrophage chemotaxis, to a range of clinically relevant chemoattractants (C5a and CCL2). We also demonstrated that inhibition of BTK in tissue resident macrophages significantly decreases chemokine secretion by reducing NF-κB activity and Akt signalling.

CONCLUSION AND IMPLICATIONS

Our work has identified a new role of BTK in regulating myeloid cell recruitment via two mechanisms, reducing monocyte/macrophages' ability to undergo chemotaxis and reducing chemokine secretion, via reduced NF-κB and Akt activity in tissue resident macrophages.

NF-κB: At the Borders of Autoimmunity and Inflammation

The transcription factor NF-κB regulates multiple aspects of innate and adaptive immune functions and serves as a pivotal mediator of inflammatory response. In the first part of this review, we discuss the NF-κB inducers, signaling pathways, and regulators involved in immune homeostasis as well as detail the importance of post-translational regulation by ubiquitination in NF-κB function. We also indicate the stages of central and peripheral tolerance where NF-κB plays a fundamental role. With respect to central tolerance, we detail how NF-κB regulates medullary thymic epithelial cell (mTEC) development, homeostasis, and function. Moreover, we elaborate on its role in the migration of double-positive (DP) thymocytes from the thymic cortex to the medulla. With respect to peripheral tolerance, we outline how NF-κB contributes to the inactivation and destruction of autoreactive T and B lymphocytes as well as the differentiation of CD4⁺-T cell subsets that are implicated in immune tolerance. In the latter half of the review, we describe the contribution of NF-κB to the pathogenesis of autoimmunity and autoinflammation. The recent discovery of mutations involving components of the pathway has both deepened our understanding of autoimmune disease and informed new therapeutic approaches to treat these illnesses.

Meant to B: B cells as a therapeutic target in systemic lupus erythematosus

B cells have a prominent role in the pathogenesis of systemic lupus erythematosus (SLE). They are mediators of inflammation through the production of pathogenic antibodies that augment inflammation and cause direct tissue and cell damage. Multiple therapeutic agents targeting B cells have been successfully used in mouse models of SLE; however, these preclinical studies have led to approval of only one new agent to treat patients with SLE: belimumab, a monoclonal antibody targeting B cell-activating factor (BAFF). Integrating the experience acquired from previous clinical trials with the knowledge generated by new studies about mechanisms of B cell contributions to SLE in specific groups of patients is critical to the development of new treatment strategies that will help to improve outcomes in patients with SLE. In particular, a sharper focus on B cell differentiation to plasma cells is warranted.

Inhibition of Bruton's TK regulates macrophage NF-κB and NLRP3 inflammasome activation in metabolic inflammation

BACKGROUND AND PURPOSE

There are no medications currently available to treat metabolic inflammation. Bruton's tyrosine kinase (BTK) is highly expressed in monocytes and macrophages and regulates NF-κB and NLRP3 inflammasome activity; both propagate metabolic inflammation in diet-induced obesity.

EXPERIMENTAL APPROACH

Using an in vivo model of chronic inflammation, high-fat diet (HFD) feeding, in male C57BL/6J mice and in vitro assays in primary murine and human macrophages, we investigated if ibrutinib, an FDA approved BTK inhibitor, may represent a novel anti-inflammatory medication to treat metabolic inflammation.

KEY RESULTS

HFD-feeding was associated with increased BTK expression and activation, which was significantly correlated with monocyte/macrophage accumulation in the liver, adipose tissue, and kidney. Ibrutinib treatment to HFD-fed mice inhibited the activation of BTK and reduced monocyte/macrophage recruitment to the liver, adipose tissue, and kidney. Ibrutinib treatment to HFD-fed mice decreased the activation of NF-κB and the NLRP3 inflammasome. As a result, ibrutinib treated mice fed HFD had improved glycaemic control through restored signalling by the IRS-1/Akt/GSK-3β pathway, protecting mice against the development of hepatosteatosis and proteinuria. We show that BTK regulates NF-κB and the NLRP3 inflammasome specifically in primary murine and human macrophages, the in vivo cellular target of ibrutinib.

CONCLUSION AND IMPLICATIONS

We provide "proof of concept" evidence that BTK is a novel therapeutic target for the treatment of diet-induced metabolic inflammation and ibrutinib may be a candidate for drug repurposing as an anti-inflammatory agent for the treatment of metabolic inflammation in T2D and microvascular disease.

Transcriptional Control of Mature B Cell Fates

The mature naïve B cell repertoire consists of three well-defined populations: B1, B2 (follicular B, FOB), and marginal zone B (MZB) cells. FOB cells are the dominant mature B cell population in the secondary lymphoid organs and blood of both humans and mice. The driving forces behind mature B lineage selection have been linked to B cell receptor (BCR) signaling strength and environmental cues, but how these fate-determination factors are transcriptionally regulated remains poorly understood. We summarize emerging data on the role of transcription factors (TFs) - particularly the ETS and IRF families - in regulating MZB and FOB lineage selection. Indeed, genomic analyses have identified four major groups of target genes that are crucial for FOB differentiation, revealing previously unrecognized pathways that ultimately determine biological responses specific to this lineage.

Coexpression of CCR7 and CXCR4 During B Cell Development Controls CXCR4 Responsiveness and Bone Marrow Homing

The CXCL12-CXCR4 axis plays a key role in the retention of stem cells and progenitors in dedicated bone marrow niches. It is well-known that CXCR4 responsiveness in B lymphocytes decreases dramatically during the final stages of their development in the bone marrow. However, the molecular mechanism underlying this regulation and whether it plays a role in B-cell homeostasis remain unknown. In the present study, we show that the differentiation of pre-B cells into immature and mature B cells is accompanied by modifications to the relative expression of chemokine receptors, with a two-fold downregulation of CXCR4 and upregulation of CCR7. We demonstrate that expression of CCR7 in B cells is involved in the selective inactivation of CXCR4, and that mature B cells from CCR7-/- mice display higher responsiveness to CXCL12 and improved retention in the bone marrow. We also provide molecular evidence supporting a model in which upregulation of CCR7 favors the formation of CXCR4-CCR7 heteromers, wherein CXCR4 is selectively impaired in its ability to activate certain G-protein complexes. Collectively, our results demonstrate that CCR7 behaves as a novel selective endogenous allosteric modulator of CXCR4.

Effects of rhubarb on the expression of glucocorticoids receptor and regulation of cellular immunity in burn-induced septic rats

Background:

It is important to modulate the expression of glucocorticoids receptor (GR) in tress and maintain the immunity homeostasis in sepsis process. Rhubarb have been shown to have potential effects on anti-inflammatory and immune modulation. The present study was designed to investigate the effects of rhubarb on the expression of GR and cellular immunity in burn-induced septic rats.

Methods:

Sixty-six healthy male Sprague Dawley (SD) rats were randomized into sepsis group (n = 24), rhubarb group (n = 24), and control group (n = 18); each group were further randomized into 12, 24, and 72 h subgroups according to different time points. During onset of the sepsis model, the rats in the rhubarb group were infused with 50 mg/kg rhubarb powder dissolved into 1 mL saline through gastric tube, while sepsis and control groups were treated with saline. The binding activity of GR in liver cytosol and binding capacity of GR in peripheral blood leucocyte were analyzed by radiation ligands binding assay. The percentages of CD4⁺,CD8⁺,CD4⁺CD25⁺T cells, CD19⁺B cells as well as natural killer (NK) cells in the lymphocytes in peripheral blood were detected by flow cytometer. For assessing the differences among groups, one-way analysis of variance (ANOVA) with Scheffe multi-comparison techniques were employed. Comparisons between time-based measurements within each group were performed with ANOVA repeated measurement.

Results:

The binding activity of GR in liver cytosol and binding capacity of GR in peripheral blood leucocyte were significantly decreased in a time-dependent manner in sepsis group (t = 23.045, P < 0.01; t = 24.395, P < 0.05, respectively), which were increased in a time-dependent manner after rhubarb administration (t = 19.965, P < 0.05; t = 17.140, P < 0.05, respectively). Twelve hours after sepsis, the percentages of CD4⁺ T cells, CD4⁺/CD25⁺ T cell ratio, and CD19⁺ B cells in the peripheral blood were significantly increased in the sepsis group (t = −3.395, P < 0.01; t = 2.568, P < 0.05; t = 2.993, P < 0.05, vs. control mice, respectively). However, the percentage of NK cells in the peripheral blood were significantly decreased in the sepsis group (t = −2.022, P < 0.05, vs. control mice). Twelve hours after sepsis, the percentage of CD8⁺ T cells were significantly decreased in the peripheral blood in the sepsis group (t = −2.191, P < 0.05, vs. control mice) and were significantly increased in the rhubarb group (t = 2.953, P < 0.05, vs. sepsis mice). Seventy-two hours after sepsis, the ratio of CD4⁺/CD25⁺ T cell in peripheral blood were significantly increased in the sepsis group (t = 2.508, P < 0.05, vs. control mice) while were significantly decreased in the rhubarb group (t = 3.378, P < 0.05, vs. control mice). Furthermore, the percentages of CD19⁺ B cell in peripheral blood were significantly decreased at 72 h in the rhubarb group (t = 2.041, P < 0.05 vs. sepsis group).

Conclusions:

Rhubarb might play potential anti-inflammatory and immunomodulatory roles in the sepsis processes.

Active PI3K abrogates central tolerance in high-avidity autoreactive B cells

High-avidity autoreactive B cells are typically removed by central tolerance mechanisms in the bone marrow. Greaves et al. demonstrate that B cell–intrinsic expression of active PI3Kα prevents central tolerance and effectively promotes differentiation and activation of high-avidity autoreactive B cells in the periphery.

Autoreactive B cells that bind self-antigen with high avidity in the bone marrow undergo mechanisms of central tolerance that prevent their entry into the peripheral B cell population. These mechanisms are breached in many autoimmune patients, increasing their risk of B cell–mediated autoimmune diseases. Resolving the molecular pathways that can break central B cell tolerance could therefore provide avenues to diminish autoimmunity. Here, we show that B cell–intrinsic expression of a constitutively active form of PI3K-P110α by high-avidity autoreactive B cells of mice completely abrogates central B cell tolerance and further promotes these cells to escape from the bone marrow, differentiate in peripheral tissue, and undergo activation in response to self-antigen. Upon stimulation with T cell help factors, these B cells secrete antibodies in vitro but remain unable to secrete autoantibodies in vivo. Overall, our data demonstrate that activation of the PI3K pathway leads high-avidity autoreactive B cells to breach central, but not late, stages of peripheral tolerance.

The Mst1 Kinase Is Required for Follicular B Cell Homing and B-1 B Cell Development

The Mst1 and 2 cytosolic serine/threonine protein kinases are the mammalian orthologs of the Drosophila Hippo protein. Mst1 has been shown previously to participate in T-cell and B-cell trafficking and the migration of lymphocytes into secondary lymphoid organs in a cell intrinsic manner. We show here that the absence of Mst1 alone only modestly impacts B cell homing to lymph nodes. The absence of both Mst1 and 2 in hematopoietic cells results in relatively normal B cell development in the bone marrow and does not impact migration of immature B cells to the spleen. However, follicular B cells lacking both Mst1 and Mst2 mature in the splenic white pulp but are unable to recirculate to lymph nodes or to the bone marrow. These cells also cannot traffic efficiently to the splenic red pulp. The inability of late transitional and follicular B cells lacking Mst 1 and 2 to migrate to the red pulp explains their failure to differentiate into marginal zone B cell precursors and marginal zone B cells. Mst1 and Mst2 are therefore required for follicular B cells to acquire the ability to recirculate and also to migrate to the splenic red pulp in order to generate marginal zone B cells. In addition B-1 a B cell development is defective in the absence of Mst1.

Conditional Selection of B Cells in Mice With an Inducible B Cell Development

Developing B cells undergo defined maturation steps in the bone marrow and in the spleen. The timing and the factors that control these differentiation steps are not fully understood. By targeting the B cell-restricted mb-1 locus to generate an mb-1 allele that expresses a tamoxifen inducible Cre and another allele in which mb-1 expression can be controlled by Cre, we have established a mouse model with an inducible B cell compartment. With these mice, we studied in detail the kinetics of B cell development and the consequence of BCR activation at a defined B cell maturation stage. Contrary to expectations, transitional 1-B cells exposed to anti-IgM reagents in vivo did not die but instead developed into transitional 2 (T2)-B cells with upregulated Bcl-2 expression. We show, however, that these T2-B cells had an increased dependency on the B cell survival factor B cell activating factor when compared to non-stimulated B cells. Overall, our findings indicate that the inducible mb-1 mouse strain represents a useful model, which allows studying the signals that control the selection of B cells in greater detail.

No strict requirement for eosinophils for bone marrow plasma cell survival

Lasting antibody responses are maintained by long-lived plasma cells, which are thought to lodge in the BM in specialized survival niches. Eosinophils have been reported to function as a critical component of the BM survival niche where they are thought to provide pro-survival signals to nearby plasma cells. Recent study shows that many BM plasma cells are recently generated and chiefly short-lived cells, raising the possibility that rare plasma cell-eosinophil interactions are a rate-limiting step needed to establish lasting humoral immunity. To address these issues, we examined the impact of eosinophil depletion on short- and long-lived BM plasma cells in the context of antibody responses induced by both T-cell dependent and T-cell independent antigens. Surprisingly, our results failed to support a role for eosinophils in either plasma cell generation or survival. These studies included examination of plasma cell frequencies in mice lacking eosinophils either after antibody-mediated depletion, or due to mutation of the GATA1 locus.

The Autoimmune Risk Variant PTPN22 C1858T Alters B Cell Tolerance at Discrete Checkpoints and Differentially Shapes the Naive Repertoire

A common genetic variant in the gene encoding the protein tyrosine phosphatase nonreceptor type 22 (PTPN22 C1858T) has been linked to a wide range of autoimmune disorders. Although a B cell-intrinsic role in promoting disease has been reported, the mechanism(s) through which this variant functions to alter the preimmune B cell repertoire remains unknown. Using a series of polyclonal and transgenic self-reactive models harboring the analogous mutation in murine Ptpn22, we show evidence for enhanced BCR, B cell-activating factor receptor, and CD40 coreceptor programs, leading to broadly enhanced positive selection of B cells at two discrete checkpoints in the bone marrow and spleen. We further identified a bias for selection of B cells into the follicular mature versus marginal zone B cell compartment. Using a biomarker to track a self-reactive H chain in peripheral blood, we found evidence of similarly enhanced positive selection in human carriers of the PTPN22 C1858T variant. Our combined data support a model whereby the risk variant augments the BCR and coreceptor programs throughout B cell development, promoting enrichment of self-reactive specificities into the follicular mature compartment and thereby likely increasing the risk for seeding of autoimmune B cell responses.

Klhl6 Deficiency Impairs Transitional B Cell Survival and Differentiation

Klhl6 belongs to the KLHL gene family, which is composed of an N-terminal BTB-POZ domain and four to six Kelch motifs in tandem. Several of these proteins function as adaptors of the Cullin3 E3 ubiquitin ligase complex. In this article, we report that Klhl6 deficiency induces, as previously described, a 2-fold reduction in mature B cells. However, we find that this deficit is centered on the inability of transitional type 1 B cells to survive and to progress toward the transitional type 2 B cell stage, whereas cells that have passed this step generate normal germinal centers (GCs) upon a T-dependent immune challenge. Klhl6-deficient type 1 B cells showed a 2-fold overexpression of genes linked with cell proliferation, including most targets of the anaphase-promoting complex/cyclosome complex, a set of genes whose expression is precisely downmodulated upon culture of splenic transitional B cells in the presence of BAFF. These results thus suggest a delay in the differentiation process of Klhl6-deficient B cells between the immature and transitional stage. We further show, in the BL2 Burkitt's lymphoma cell line, that KLHL6 interacts with Cullin3, but also that it binds to HBXIP/Lamtor5, a protein involved in cell-cycle regulation and cytokinesis. Finally, we report that KLHL6, which is recurrently mutated in B cell lymphomas, is an off-target of the normal somatic hypermutation process taking place in GC B cells in both mice and humans, thus leaving open whether, despite the lack of impact of Klhl6 deficiency on GC B cell expansion, mutants could contribute to the oncogenic process.

GM-CSF and IL-4 Fusion Cytokine Induces B Cell-Dependent Hematopoietic Regeneration

Hematopoietic stem cells (HSCs) have the capacity to self-renew and differentiate into hematopoietic cells and have been utilized to replace diseased bone marrow for patients with cancers and blood disorders. Although remarkable progress has been made in developing new tools to manipulate HSCs for clinic use, there is still no effective method to expand HSCs in vivo for quick repopulation of hematopoietic cells following sublethal irradiation. We have recently described a novel synthetic cytokine that is derived from the fusion of granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin 4 (IL-4; named as GIFT4), and we have now discovered that GIFT4 fusokine promotes long-term hematopoietic regeneration in a B cell-dependent manner. We found that GIFT4 treatment triggered a robust expansion of endogenous bone marrow HSCs and multipotent progenitors in vivo. Delivery of GIFT4 protein together with B cells rescued lethally irradiated mice. Moreover, adoptive transfer of autologous or allogeneic GIFT4-treated B cells (GIFT4-B cells) enhanced long-term hematopoietic recovery in radiated mice and prevented the mice from irradiation-induced death. Our data suggest that GIFT4 as well as GIFT4-B cells could serve as means to augment HSC engraftment in the setting of bone marrow transplantation for patients with hematological malignancy.

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.

In old BALB/c mice, bone marrow pre-B cell and surrogate light chain reduction is associated with increased B cell reactivity to phosphorylcholine, but reduced T15 idiotype dominance

In young adult BALB/c mice, antibodies to phosphorylcholine (PC) bearing the T15 (TEPC 15) idiotype confer protection against pneumococcal infections. In old age, even though PC reactive B cells are often increased, the proportion of T15+ antibodies declines. We hypothesize that limited surrogate light chain (SLC) and compromise of the pre-B cell receptor checkpoint in old mice contribute to both reduced new B cell generation and changes in the anti-PC antibodies seen in old age. In old mice: 1) early pre-B cell loss is most pronounced at the preBCR checkpoint; however, the reduced pool of early pre-B cells continues to proliferate consistent with preBCR signaling; 2) increased PC reactivity is seen in bone marrow immature B cells; 3) deficient SLC promotes increased B cell PC reactivity and diminished T15 idiotype expression; and 4) as pre-B cell losses and reduced SLC become progressively more severe, increased T15 negative PC reactive B cells occur. These results associate a reduction in pre-B cells, imposed at the preBCR checkpoint, with increased reactivity to PC, but more limited expression of the protective T15 idiotype among PC reactive antibodies in old age.

Canonical NF-κB signaling is uniquely required for the long-term persistence of functional mature B cells

Although canonical NF-κB signaling is crucial to generate a normal mature B-cell compartment, its role in the persistence of resting mature B cells is controversial. To resolve this conflict, we ablated NF-κB essential modulator (NEMO) and IκB kinase 2 (IKK2), two essential mediators of the canonical pathway, either early on in B-cell development or specifically in mature B cells. Early ablation severely inhibited the generation of all mature B-cell subsets, but follicular B-cell numbers could be largely rescued by ectopic expression of B-cell lymphoma 2 (Bcl2), despite a persisting block at the transitional stage. Marginal zone (MZ) B and B1 cells were not rescued, indicating a possible role of canonical NF-κB signals beyond the control of cell survival in these subsets. When canonical NF-κB signaling was ablated specifically in mature B cells, the differentiation and/or persistence of MZ B cells was still abrogated, but follicular B-cell numbers were only mildly affected. However, the mutant cells exhibited increased turnover as well as functional deficiencies upon activation, suggesting that canonical NF-κB signals contribute to their long-term persistence and functional fitness.

Targeting Anti-Insulin B Cell Receptors Improves Receptor Editing in Type 1 Diabetes-Prone Mice

Autoreactive B lymphocytes that commonly arise in the developing repertoire can be salvaged by receptor editing, a central tolerance mechanism that alters BCR specificity through continued L chain rearrangement. It is unknown whether autoantigens with weak cross-linking potential, such as insulin, elicit receptor editing, or whether this process is dysregulated in related autoimmunity. To resolve these issues, we developed an editing-competent model in which anti-insulin Vκ125 was targeted to the Igκ locus and paired with anti-insulin VH125Tg. Physiologic, circulating insulin increased RAG-2 expression and was associated with BCR replacement that eliminated autoantigen recognition in a proportion of developing anti-insulin B lymphocytes. The proportion of anti-insulin B cells that underwent receptor editing was reduced in the type 1 diabetes-prone NOD strain relative to a nonautoimmune strain. Resistance to editing was associated with increased surface IgM expression on immature (but not transitional or mature) anti-insulin B cells in the NOD strain. The actions of mAb123 on central tolerance were also investigated, because selective targeting of insulin-occupied BCR by mAb123 eliminates anti-insulin B lymphocytes and prevents type 1 diabetes. Autoantigen targeting by mAb123 increased RAG-2 expression and dramatically enhanced BCR replacement in newly developed B lymphocytes. Administering F(ab')2123 induced IgM downregulation and reduced the frequency of anti-insulin B lymphocytes within the polyclonal repertoire of VH125Tg/NOD mice, suggesting enhanced central tolerance by direct BCR interaction. These findings indicate that weak or faulty checkpoints for central tolerance can be overcome by autoantigen-specific immunomodulatory therapy.

Inflammation rapidly reorganizes mouse bone marrow B cells and their environment in conjunction with early IgM responses

Mouse inflammation models cause accumulation of B cells in the bone marrow within 12 hours and prior to peak emergency granulopoiesis. Marrow B cells undergo spatial reorganization and are subjected to an altered cellular and secreted milieu.

Non-Hematopoietic and Hematopoietic SIRPα Signaling Differently Regulates Murine B Cell Maturation in Bone Marrow and Spleen

B lymphocyte development occurs in the bone marrow, while final differentiation and maturation can occur in both the bone marrow and the spleen. Here we provide evidence that signal regulatory protein α (SIRPα), an Ig-superfamily ITIM-receptor expressed by myeloid but not by lymphoid cells, is involved in regulating B cell maturation. Lack of SIRPα signaling in adult SIRPα-mutant mice resulted in a reduced maturation of B cells in the bone marrow, evident by reduced numbers of semi-mature IgD⁺IgM^hi follicular type-II (F-II) and mature IgD⁺IgM^lo follicular type-I (F-I) B cells, as well as reduced blood B cell numbers. In addition, lack of SIRPα signaling also impaired follicular B cell maturation in the spleen. Maturing BM or splenic B cells of SIRPα-mutant mice were found to express higher levels of the pro-apoptotic protein BIM and apoptosis was increased among these B cells. Bone marrow reconstitution experiments revealed that the B cell maturation defect in bone marrow and blood was due to lack of SIRPα signaling in non-hematopoietic cells, while hematopoietic SIRPα signaling was important for follicular B cell maturation in the spleen. Adding on to our previous findings of a stromal cell defect in SIRPα-mutant mice was the finding that gene expression of receptor activator of nuclear factor-ĸB ligand (RANKL) was significantly lower in cultured bone marrow stromal cells of SIRPα mutant mice. These data suggest a novel and opposite contribution of SIRPα signaling within non-hematopoietic and hematopoietic cells, respectively, to maintain B cell maturation and to prevent apoptosis in the bone marrow and spleen of adult mice.

Cell extrinsic alterations in splenic B cell maturation in Flt3-ligand knockout mice

B lymphopoiesis in bone marrow (BM) is critical for maintaining a diverse peripheral B cell pool to fight infection and establish lifelong immunity. The generation of immature B cells is reduced in Flt3-ligand (FL-/-) mice leading to deficiencies in splenic B cells. Here, we sought to understand the cellular basis of the spleen B cell deficiency in FL-/- mice. Significant reductions in transitional (TS) and follicular (FO) B cells were found in FL-/- mice, and increased frequencies, but not absolute numbers, of marginal zone (MZ) B cells. BAFF-R expression on splenic B cells and serum levels of B cell activating factor (BAFF) was comparable to wildtype (WT) mice. Mixed BM chimeras revealed that the reductions in TS and FO B cells were cell extrinsic. FL administration into FL-/- mice restored the deficiency in TS B cells and normalized the MZ compartment. Ki67 analysis revealed a significant decrease in the proliferative capacity of TS B cells in FL-/- mice. A Bcl2 transgene did not rescue TS cells in FL-/- mice, uncoupling FL-deficiency to Bcl2-dependent survival pathways. Upregulation of CD1d expression and adoptive transfer experiments suggested MZ skewing in FL-/- mice. These findings support an integral role for Flt3 signaling in peripheral B cell maturation.

Surrogate light chain is required for central and peripheral B-cell tolerance and inhibits anti-DNA antibody production by marginal zone B cells

Selection of the primary antibody repertoire takes place in pro-/pre-B cells, and subsequently in immature and transitional B cells. At the first checkpoint, μ heavy (μH) chains assemble with surrogate light (SL) chain into a precursor B-cell receptor. In mice lacking SL chain, μH chain selection is impaired, and serum autoantibody levels are elevated. However, whether the development of autoantibody-producing cells is due to an inability of the resultant B-cell receptors to induce central and/or peripheral B-cell tolerance or other factors is unknown. Here, we show that receptor editing is defective, and that a higher proportion of BM immature B cells are prone to undergoing apoptosis. Furthermore, transitional B cells are also more prone to undergoing apoptosis, with a stronger selection pressure to enter the follicular B-cell pool. Those that enter the marginal zone (MZ) B-cell pool escape selection and survive, possibly due to the B-lymphopenia and elevated levels of B-cell activating factor. Moreover, the MZ B cells are responsible for the elevated IgM anti-dsDNA antibody levels detected in these mice. Thus, the SL chain is required for central and peripheral B-cell tolerance and inhibits anti-DNA antibody production by MZ B cells.

Natural IgM is produced by CD5- plasma cells that occupy a distinct survival niche in bone marrow

Natural IgM is constitutively present in the serum, where it aids in the early control of viral and bacterial expansions. Natural IgM also plays a significant role in the prevention of autoimmune disease by promoting the clearance of cellular debris. Nevertheless, the origins of natural IgM have not been precisely defined. Previous studies focused on the role of CD5(+) B1 cells in the production of natural IgM, but we show in this article that a discrete population of CD5(-) IgM plasmablasts and plasma cells in the bone marrow (BM) produces the majority of serum IgM in resting mice. These Ab-secreting cells (ASC) originate from peritoneal cavity-resident cells, because transfer of peritoneal cells completely restores serum IgM and the specific compartment of BM ASC in Rag1-deficient mice. We show that BM natural IgM ASC arise from a fetal-lineage progenitor that is neither B1a nor B1b, and that this IgM ASC compartment contains a substantial fraction of long-lived plasma cells that do not occupy the IgG plasma cell survival niche in the BM; instead, they are supported by IL-5. In summary, we identified the primary source of natural IgM and showed that these ASC are maintained long-term in a unique survival niche within the BM.

Lasting antibody responses are mediated by a combination of newly formed and established bone marrow plasma cells drawn from clonally distinct precursors

Current models hold that serum Ab titers are maintained chiefly by long-lived bone marrow (BM) plasma cells (PCs). In this study, we characterize the role of subpopulations of BM PCs in long-term humoral responses to T cell-dependent Ag. Surprisingly, our results indicate that 40-50% of BM PCs are recently formed cells, defined, in part, by rapid steady-state turnover kinetics and secretion of low-affinity IgM Abs. Further, for months after immunization with a hapten-protein conjugate, newly formed Ag-induced, IgM-secreting BM PCs were detected in parallel with longer-lived IgG-secreting cells, suggesting ongoing and parallel input to the BM PC pool from two distinct pools of activated B cells. Consistent with this interpretation, IgM and IgG Abs secreted by cells within distinct PC subsets exhibited distinct L chain usage. We conclude that long-term Ab responses are maintained by a dynamic BM PC pool composed of both recently formed and long-lived PCs drawn from clonally disparate precursors.

The BiP cochaperone ERdj4 is required for B cell development and function

ERdj4 is a BiP cochaperone regulated by the unfolded protein response to facilitate degradation of unfolded and/or misfolded proteins in the endoplasmic reticulum. As the unfolded protein response plays a critical role in B cell maturation and antibody production, ERdj4 gene trap mice were generated to determine if this chaperone was required for B cell homeostasis. Homozygosity for the trapped allele resulted in hypomorphic expression of ERdj4 in bone marrow cells and abnormal development of hematopoietic lineages in the bone marrow. The number of myeloid cells was increased, while the number of erythroid and B lymphoid cells was reduced in ERdj4 gene trap mice compared to controls. An intrinsic B cell defect was identified that decreased survival of B cell precursors including large and small pre-B, and immature B cells. Consistent with impaired B lymphopoiesis, the number of mature follicular B cells was reduced in both the bone marrow and spleen of ERdj4 gene trap mice. Paradoxically, unchallenged ERdj4 gene trap mice showed non-specific hypergammaglobulinemia and gene trap B cells exhibited increased proliferation, survival and isotype switching in response to LPS stimulation. Although ERdj4 gene trap mice responded normally to T cell-independent antigen, they failed to mount a specific antibody response to T cell-dependent antigen in vivo. Collectively, these findings demonstrate that the chaperone activity of ERdj4 is required for survival of B cell progenitors and normal antibody production.

Rationale for B cell targeting in SLE

B cells are central pathogenic players in systemic lupus erythematosus and multiple other autoimmune diseases through antibody production as well as antibody independent function. At the same time, B cells are known to play important regulatory functions that may protect against autoimmune manifestations. Yet, the functional role of different B cell populations and their contribution to disease remain to be understood. The advent of agents that specifically target B cells, in particular anti-CD20 and ant-BLyS antibodies, have demonstrated the efficacy of this approach for the treatment of human autoimmunity. The analysis of patients treated with these and other B cell agents provides a unique opportunity to understand the correlates of clinical response and the significance of different B cell subsets. Here, we discuss this information and how it could be used to better understand SLE and improve the rational design of B cell-directed therapies in this disease.

B lymphocytes: development, tolerance, and their role in autoimmunity-focus on systemic lupus erythematosus

B lymphocytes are the effectors of humoral immunity, providing defense against pathogens through different functions including antibody production. B cells constitute approximately 15% of peripheral blood leukocytes and arise from hemopoietic stem cells in the bone marrow. It is here that their antigen receptors (surface immunoglobulin) are assembled. In the context of autoimmune diseases defined by B and/or T cell autoreactive that upon activation lead to chronic tissue inflammation and often irreversible structural and functional damage, B lymphocytes play an essential role by not only producing autoantibodies but also functioning as antigen-presenting cells (APC) and as a source of cytokines. In this paper, we describe B lymphocyte functions in autoimmunity and autoimmune diseases with a special focus on their abnormalities in systemic lupus erythematosus.

Uncoupling of natural IgE production and CD23 surface expression levels

CD23, the low affinity receptor for immunoglobulin E (IgE), has been proposed to play a critical role in the regulation of IgE production, based on altered IgE levels in CD23-deficient mice and transgenic mouse models, as well as in mouse strains with mutations in the CD23 gene, e.g. 129 substrains. Here, we have investigated a mouse line termed LxT1 that expresses reduced CD23 surface levels on B cells, and its influence on natural IgE production. Extensive phenotypic analysis showed that CD23 surface expression was reduced in LxT1 compared to the control, without affecting B cell development in general. This CD23(low) surface level in LxT1 mice is not as a result of reduced CD23 mRNA expression levels or intracellular accumulation, but linked to a recessive locus, a 129-derived region spanning 28 Mb on chromosome 8, which includes the CD23 gene. Sequence analysis confirmed five mutations within the CD23 coding region in LxT1 mice, the same as those present in New Zealand Black (NZB) and 129 mice. However, this CD23(low) phenotype was not observed in all 129 substrains despite carrying these same CD23 mutations in the coding region. Moreover, serum IgE levels in LxT1 mice are as low as those in the C57BL/6 (B6) strain, and much lower than those in 129 substrains. These data indicate that the CD23 surface level and serum IgE level are uncoupled and that neither is directly regulated by the mutations within the CD23 coding region. This study suggests that caution should be taken when interpreting the immunological data derived from mice with different genetic background, especially if the gene of interest is thought to influence CD23 surface expression or serum IgE level.

B cells as effectors and regulators of sex-biased arthritis

B cells have been implicated both with pathogenic as well as protective capabilities in induction and regulation of autoimmune diseases. Rheumatoid arthritis (RA) is an autoimmune disease that occurs more often in women than men. A significant role of B cells as antibody producing and antigen-presenting cells has been demonstrated in RA. Predisposition to RA is associated with the presence of certain HLA class II alleles that share sequences with DRB1*0401. To determine the role of HLA genes and B cells in vivo, we have generated transgenic mice carrying HLA genes, DRB1*0401 and DQ8, known to be associated with susceptibility to RA. Humanized mice can be induced to develop arthritis that mimics human disease in clinical, histopathological and sex bias. Effect of hormones on immune cells and their function has been described in humans and mice and has been suggested to be the major reason for female bias of autoimmune diseases. An immune response to an antigen requires presentation by HLA molecules thus suggesting a critical role of MHC in combination with sex hormones in susceptibility to develop rheumatoid arthritis. Based on our observations, we hypothesize that modulation of B cells by estrogen, presentation of modified antigens by DR4 and production of antigen-specific B cell modulating cytokines leads to autoreactivity in females. These data suggest that considering patient's sex may be crucial in selecting the optimal treatment strategy. Humanized mice expressing RA susceptible and resistant haplotype provide a means to investigate mechanism sex-bias of arthritis and future strategies for therapy.

Dynamics of the splenic innate-like CD19⁺CD45Rlo cell population from adult mice in homeostatic and activated conditions

In the adult spleen, CD19⁺CD45R(-/lo) (19⁺45R(lo)) lymphocytes of embryonic origin exist as a distinct population to that of the conventional B cell lineage. These cells display a plasmablast phenotype, and they spontaneously secrete IgG1 and IgA, whereas the bone marrow population of 19⁺45R(lo) cells contains B1 progenitors. In this study, we show that 19⁺45R(lo) cells are also present in Peyer's patches and in the spleen throughout the life span of wild-type mice, beginning at postnatal day 7. Although this population is heterogeneous, the surface phenotype of most of these cells distinguishes them from follicular, transitional, marginal zone, and B1 cells. In CBA/CaHN mice, few 19⁺45R(lo) cells were detected at postnatal day 7, and none was observed in the adult spleen. Splenic 19⁺45R(lo) cells exhibited homeostatic BrdU uptake in vivo and actively transcribed cell cycle genes. When transferred to immunodeficient RAG2⁻/⁻γchain⁻/⁻ recipient mice, 19⁺45R(lo) cells survived and differentiated into IgG1- and IgA-plasma cells. Moreover, in vitro stimulation of splenic 19⁺45R(lo) cells with LPS, CpG, BAFF/IL4, and CD40/IL4 induced cell proliferation, IgG1/IgA secretion and the release of IL-10, suggesting a potential immunoregulatory role for this subset of innate-like B cells.

Long-lived bone marrow plasma cells are induced early in response to T cell-independent or T cell-dependent antigens

The signals required to generate long-lived plasma cells remain unresolved. One widely cited model posits that long-lived plasma cells derive from germinal centers (GCs) in response to T cell-dependent (TD) Ags. Thus, T cell-independent (TI) Ags, which fail to sustain GCs, are considered ineffective at generating long-lived plasma cells. However, we show that long-lived hapten-specific plasma cells are readily induced without formation of GCs. Long-lived plasma cells developed in T cell-deficient mice after a single immunization with haptenated LPS, a widely used TI Ag. Long-lived plasma cells also formed in response to TD Ag when the GC response was experimentally prevented. These observations establish that long-lived plasma cells are induced in both TI and TD responses, and can arise independently of B cell maturation in GCs.

Chronic spinal cord injury impairs primary antibody responses but spares existing humoral immunity in mice

Spinal cord injury (SCI) results in immune depression. To better understand how injury inhibits humoral immunity, the effects of chronic thoracic SCI on B cell development and immune responses to thymus-independent type 2 and thymus-dependent Ags were determined. Mice received complete crush injury or control laminectomy at either thoracic level 3, which disrupts descending autonomic control of the spleen, or at thoracic level 9, which conserves most splenic sympathetic activity. Although mature B cell numbers were only mildly reduced, bone marrow B cell production was transiently but profoundly depressed immediately after injury. Despite the return of normal B cell production 4 wk after SCI, mice receiving thoracic level 3 injury showed a significant reduction in their ability to mount primary thymus-independent type 2 or thymus-dependent immune responses. The latter were marked by decreases in germinal center B cells as well as class-switched high-affinity Ab-secreting cells. Importantly, injury did not affect affinity maturation per se, pre-existing B cell memory, or secondary humoral immune responses. Taken together, these findings show that chronic high thoracic SCI impairs the ability to mount optimal Ab responses to new antigenic challenges, but spares previously established humoral immunity.

Lyn deficiency affects B-cell maturation as well as survival

Lyn, an Src-family protein tyrosine kinase expressed in B lymphocytes, contributes to initiation of BCR signaling and is also responsible for feedback inhibition of BCR signaling. Lyn-deficient mice have a decreased number of follicular B cells and also spontaneously develop a lupus-like autoimmunity. We used flow cytometric analysis, BrdU labeling and our mathematical models of B-cell population dynamics, to analyze how Lyn deficiency impacts B-cell maturation and survival. We found that Lyn-deficient transitional 1 (T1) cells develop normally, but T2 cells develop primarily from the T1 subset in the spleen and fail to also develop directly from BM immature B cells. Lyn-deficient T2 cells either mature to the follicular B-cell type at a close to normal rate, or die in this compartment rather than access the T3 anergic subset. The ≈ 40% of WT follicular cells that were short-lived exited primarily by joining the T3 anergic subset, whereas the ≈ 15% Lyn(-/-) follicular cells that were not long lived had a high death rate and died in this compartment rather than entering the T3 subset. We hypothesize that exaggerated BCR signaling resulting from weak interactions with self-antigens is largely responsible for these alterations in Lyn-deficient B cells.

BAFF-R expression correlates with positive selection of immature B cells

The interaction between BAFF and BAFF-R is crucial for the development of mature B cells. Here, we report that the expression of BAFF-R is first detectable on a fraction of mouse CD19(+) CD93(+) IgM(+) CD23(-) and human CD19(+) CD10(+) IgM(+) BM B cells. This BAFF-R(+) BM B-cell population shows higher levels of surface IgM expression and decreased RAG-2 transcripts than BAFF-R(-) immature B cells. When cultured, mouse BAFF-R(-), but not BAFF-R(+) immature B cells spontaneously undergo B-cell receptor editing. However, BAFF-R(+) immature B cells cultured in the presence of an anti-κ light chain antibody are induced to undergo receptor editing. This receptor editing correlates with down-modulation of surface BAFF-R expression and the up-regulation of RAG-2 at the RNA level. B-cell receptor (BCR) cross-linking on splenic T1 B cells results in down-modulation of the BAFF-R, and receptor editing and RAG-2 up-regulation in a minor fraction of B cells. BCR cross-linking on splenic T2/3 B cells results in partly down and partly up-modulation of BAFF-R expression and no evidence for receptor editing. Overall, our data indicate that BAFF-R expression is tightly regulated during B-cell development in mouse and human and its expression is correlated with positive selection.

Direct reduction of antigen receptor expression in polyclonal B cell populations developing in vivo results in light chain receptor editing

Secondary Ab V region gene segment rearrangement, termed receptor editing, is a major mechanism contributing to B lymphocyte self-tolerance. However, the parameters that determine whether a B cell undergoes editing are a current subject of debate. We tested the role that the level of BCR expression plays in the regulation of receptor editing in a polyclonal population of B cells differentiating in vivo. Expression of a short hairpin RNA for κ L chain RNA in B cells resulted in reduction in levels of this RNA and surface BCRs. Strikingly, fully mature and functional B cells that developed in vivo and efficiently expressed the short hairpin RNA predominantly expressed BCRs containing λ light chains. This shift in L chain repertoire was accompanied by inhibition of development, increased Rag gene expression, and increased λ V gene segment-cleavage events at the immature B cell stage. These data demonstrated that reducing the translation of BCRs that are members of the natural repertoire at the immature B cell stage is sufficient to promote editing.

B cells and autoimmunity

There is a growing appreciation for the role for B cells in autoimmune disorders in which inflammation is driven by T cells, in addition to the well-established role for B cells in autoimmune disorders characterized by pathogenic auto-antibodies. Current information on tolerance checkpoints in B cells, B cell depletion, BAFF blockade, regulatory B cells and clonal ignorance mediated by the SIAE/Siglec pathway will be reviewed.

Activation-induced cytidine deaminase mediates central tolerance in B cells

The Aicda gene product, activation-induced cytidine deaminase (AID), initiates somatic hypermutation, class-switch recombination, and gene conversion of Ig genes by the deamination of deoxycytidine, followed by error-prone mismatch- or base-excision DNA repair. These processes are crucial for the generation of genetically diverse, high affinity antibody and robust humoral immunity, but exact significant genetic damage and promote cell death. In mice, physiologically significant AID expression was thought to be restricted to antigen-activated, mature B cells in germinal centers. We now demonstrate that low levels of AID in bone marrow immature and transitional B cells suppress the development of autoreactivity. Aicda(-/-) mice exhibit significantly increased serum autoantibody and reduced capacity to purge autoreactive immature and transitional B cells. In vitro, AID deficient immature/transitional B cells are significantly more resistant to anti-IgM-induced apoptosis than their normal counterparts. Thus, early AID expression plays a fundamental and unanticipated role in purging self-reactive immature and transitional B cells during their maturation in the bone marrow.

CD40 signaling synergizes with TLR-2 in the BCR independent activation of resting B cells

Conventionally, signaling through BCR initiates sequence of events necessary for activation and differentiation of B cells. We report an alternative approach, independent of BCR, for stimulating resting B (RB) cells, by involving TLR-2 and CD40 - molecules crucial for innate and adaptive immunity. CD40 triggering of TLR-2 stimulated RB cells significantly augments their activation, proliferation and differentiation. It also substantially ameliorates the calcium flux, antigen uptake capacity and ability of B cells to activate T cells. The survival of RB cells was improved and it increases the number of cells expressing activation induced deaminase (AID), signifying class switch recombination (CSR). Further, we also observed increased activation rate and decreased threshold period required for optimum stimulation of RB cells. These results corroborate well with microarray gene expression data. This study provides novel insights into coordination between the molecules of innate and adaptive immunity in activating B cells, in a BCR independent manner. This strategy can be exploited to design vaccines to bolster B cell activation and antigen presenting efficiency, leading to faster and better immune response.

Analysis of the role of IL-21 in development of murine B cell progenitors in the bone marrow

IL-21 plays a key role in the late stage of B cell development, where it has been shown to induce growth and differentiation of mature B cells into Ig-secreting plasma cells. Because IL-21R has also been reported on bone marrow (BM) B cell progenitors, we investigated whether IL-21R influenced earlier stages of B cell development. IL-21R is functional as early as the pro-B cell stage, and the strength of receptor-mediated signaling increases as cells mature. The addition of IL-21 to B cell progenitors in cell culture resulted in the accelerated appearance of mature B cell markers and was associated with the induction of Aid, Blimp1, and germline transcripts. We also found that stimulation of both IL-21R and CD40 was sufficient to induce the maturation of early B cell progenitors into IgM- and IgG-secreting cells. Consistent with a role for IL-21 in promoting B cell differentiation, the number of B220(+)CD43(+)IgM(-) pro-B cells was increased, and the number of mature IgM(hi)IgD(hi) cells was decreased in BM of IL-21R-deficient mice. We also report in this paper that IL-21 is expressed by BM CD4(+) T cells. These results provide evidence that IL-21R is functional in B cell progenitors and indicate that IL-21 regulates B cell development.

B cell receptor editing in tolerance and autoimmunity

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

BAFF receptor signaling aids the differentiation of immature B cells into transitional B cells following tonic BCR signaling

BAFF is an important prosurvival cytokine for mature B cells. However, previous studies have shown that BAFFR is already expressed at the immature B cell stage, and that the prosurvival protein Bcl-2 does not completely complement the B cell defects resulting from the absence of BAFFR or BAFF. Thus, we hypothesized that BAFF also functions to aid the differentiation of nonautoreactive immature B cells into transitional B cells and to promote their positive selection. We found that BAFFR is expressed at higher levels on nonautoreactive than on autoreactive immature B cells and that its expression correlates with that of surface IgM and with tonic BCR signaling. Our data indicate that BAFFR signaling enhances the generation of transitional CD23(-) B cells in vitro by increasing cell survival. In vivo, however, BAFFR signaling is dispensable for the generation of CD23(-) transitional B cells in the bone marrow, but it is important for the development of transitional CD23(-) T1 B cells in the spleen. Additionally, we show that BAFF is essential for the differentiation of CD23(-) into CD23(+) transitional B cells both in vitro and in vivo through a mechanism distinct from that mediating cell survival, but requiring tonic BCR signaling. In summary, our data indicate that BAFFR and tonic BCR signals cooperate to enable nonautoreactive immature B cells to differentiate into transitional B cells and to be positively selected into the naive B cell repertoire.

G alpha q-containing G proteins regulate B cell selection and survival and are required to prevent B cell-dependent autoimmunity

Survival of mature B cells is regulated by B cell receptor and BAFFR-dependent signals. We show that B cells from mice lacking the G_αq subunit of trimeric G proteins (Gnaq^−/− mice) have an intrinsic survival advantage over normal B cells, even in the absence of BAFF. Gnaq^−/− B cells develop normally in the bone marrow but inappropriately survive peripheral tolerance checkpoints, leading to the accumulation of transitional, marginal zone, and follicular B cells, many of which are autoreactive. Gnaq^−/− chimeric mice rapidly develop arthritis as well as other manifestations of systemic autoimmune disease. Importantly, we demonstrate that the development of the autoreactive B cell compartment is the result of an intrinsic defect in Gnaq^−/− B cells, resulting in the aberrant activation of the prosurvival factor Akt. Together, these data show for the first time that signaling through trimeric G proteins is critically important for maintaining control of peripheral B cell tolerance induction and repressing autoimmunity.

The adaptor molecule Act1 regulates BAFF responsiveness and self-reactive B cell selection during transitional B cell maturation

The transitional stage is a key check-point for elimination of autoreactive B cells in the periphery. This selection process requires fine regulation of signals received through BCR and B cell activating factor (BAFF) receptor. We previously identified the adaptor molecule Act1 as a negative regulator of BAFF-mediated signaling. Deficiency of Act1 in mice results in peripheral B cell hyperplasia and development of autoimmunity. In this study, we demonstrate that Act1 plays a critical role in the regulation of transitional B cell survival and maturation. We found that the ratio of late-transitional (T2) to early-transitional (T1) cells was increased in spleens from Act1-deficient mice. Moreover, BAFF stimulation induced better T1 cell survival and promoted more efficient maturation of T1 cells into T2 cells ex vivo in the absence of Act1. BAFF stimulation induced higher levels of the anti-apoptotic Bcl-2 member Mc1-l in Act1-deficient T1 cells than in wild-type control cells, suggesting that Mcl-1 might be one of the key effector molecules for BAFF-mediated survival of the Act1-deficient transitional B cells. Importantly, costimulation with BAFF was able to rescue Act1-deficient T1 cells from BCR-induced apoptosis more effectively than Act1-sufficient T1 B cells. Finally, by using hen egg lysozyme double transgenic mice, we demonstrated that Act1 deficiency can promote the maturation of Ag-specific autoreactive B cells. Taken together, our results suggest that the transitional stage is a critical point of action of Act1 in the elimination of autoreactive B cells and in the regulation of peripheral B cell homeostasis.

S1P1 receptor directs the release of immature B cells from bone marrow into blood

S1P1 receptor expression is required for the egress of newly formed T cells from the thymus and exit of mature T and B cells from secondary lymphoid organs. In this study, we deleted the expression of the S1P1 receptor gene (S1pr1) in developing B cells in the bone marrow. Although B cell maturation within the bone marrow was largely normal in the B cell–specific S1pr1 knockout (B-S1pr1KO) mice, their newly generated immature B cells appeared in the blood at abnormally low numbers as compared with control mice. In the bone marrow of B-S1pr1KO mice, immature B cells in contact with the vascular compartment displayed increased apoptosis as compared with control mice. Forced expression of CD69, a negative regulator of S1P1 receptor expression, in developing bone marrow B cells also reduced the number of immature B cells in the blood. Attenuation of CXCR4 signaling, which is required for the proper retention of developing B cells in bone marrow, did not release immature B cells into the blood of B-S1pr1KO mice as effectively as in control mice. Our results indicate that the S1P1 receptor provides a signal necessary for the efficient transfer of newly generated immature B cells from the bone marrow to the blood.

A novel Rac-dependent checkpoint in B cell development controls entry into the splenic white pulp and cell survival

Rac1 and Rac2 GTPases transduce signals from multiple receptors leading to cell migration, adhesion, proliferation, and survival. In the absence of Rac1 and Rac2, B cell development is arrested at an IgD- transitional B cell stage that we term transitional type 0 (T0). We show that T0 cells cannot enter the white pulp of the spleen until they mature into the T1 and T2 stages, and that this entry into the white pulp requires integrin and chemokine receptor signaling and is required for cell survival. In the absence of Rac1 and Rac2, transitional B cells are unable to migrate in response to chemokines and cannot enter the splenic white pulp. We propose that loss of Rac1 and Rac2 causes arrest at the T0 stage at least in part because transitional B cells need to migrate into the white pulp to receive survival signals. Finally, we show that in the absence of Syk, a kinase that transduces B cell antigen receptor signals required for positive selection, development is arrested at the same T0 stage, with transitional B cells excluded from the white pulp. Thus, these studies identify a novel developmental checkpoint that coincides with B cell positive selection.