Cutting edge commentary: origins of B-1 cells

The layered development of mouse B and T Cells

Traditional models of lymphopoiesis present B and T cell development as a linear process that initiates in the fetus and continues after birth in the bone marrow and thymus, respectively. However, this view of lymphocyte development is not in accord with reports, dating back several decades, indicating that the types of lymphocytes generated before and after birth differ. In this regard, selected γδ T cells, and those that utilize the Vγ3 receptor in particular, and innate-like B-1 B cells preferentially arise during fetal blood cell development. This review synthesizes data from multiple laboratories, with an emphasis on our own work using mouse models, demonstrating that innate and conventional B and T cells emerge in hematopoietic stem cell independent and dependent waves of development that are differentially regulated. This layering of lymphocyte development has implications for understanding the composition of the adult immune system and may provide insights into the origin of various lymphocytic leukemias.

CXCL13-CXCR5 axis: Regulation in inflammatory diseases and cancer

Chemokine C-X-C motif ligand 13 (CXCL13), originally identified as a B-cell chemokine, plays an important role in the immune system. The interaction between CXCL13 and its receptor, the G-protein coupled receptor (GPCR) CXCR5, builds a signaling network that regulates not only normal organisms but also the development of many diseases. However, the precise action mechanism remains unclear. In this review, we discussed the functional mechanisms of the CXCL13-CXCR5 axis under normal conditions, with special focus on its association with diseases. For certain refractory diseases, we emphasize the diagnostic and therapeutic role of CXCL13-CXCR5 axis.

Usp9X Is Required for Lymphocyte Activation and Homeostasis through Its Control of ZAP70 Ubiquitination and PKCβ Kinase Activity

To achieve a durable adaptive immune response, lymphocytes must undergo clonal expansion and induce a survival program that enables the persistence of Ag-experienced cells and the development of memory. During the priming phase of this response, CD4(+)T lymphocytes either remain tolerized or undergo clonal expansion. In this article, we show that Usp9X functions as a positive regulatory switch during T lymphocyte priming through removal of inhibitory monoubiquitination from ZAP70. In the absence of Usp9X, an increased amount of ZAP70 localized to early endosomes consistent with the role of monoubiquitin in endocytic sorting. Usp9X becomes competent to deubiquitinate ZAP70 through TCR-dependent phosphorylation and enhancement of its catalytic activity and association with the LAT signalosome. In B lymphocytes, Usp9X is required for the induction of PKCβ kinase activity after BCR-dependent activation. Accordingly, inUsp9Xknockout B cells, there was a significant reduction in phospho-CARMA1 levels that resulted in reduced CARMA1/Bcl-10/MALT-1 complex formation and NF-κB-dependent cell survival. The pleiotropic effect of Usp9X during Ag-receptor signaling highlights its importance for the development of an effective and durable adaptive immune response.

Human B-1 cells take the stage

B-1 cells play critical roles in defending against microbial invasion and in housekeeping removal of cellular debris. B-1 cells secrete natural antibody and manifest functions that influence T cell expansion and differentiation and in these and other ways differ from conventional B-2 cells. B-1 cells were originally studied in mice where they are easily distinguished from B-2 cells, but their identity in the human system remained poorly defined for many years. Recently, functional criteria for human B-1 cells were established on the basis of murine findings, and reverse engineering resulted in identification of the phenotypic profile, CD20(+)CD27(+)CD43(+)CD70(-), for B-1 cells found in both umbilical cord blood and adult peripheral blood. Human B-1 cells may contribute to multiple disease states through production of autoantibody and stimulation/modulation of T cell activity. Human B-1 cells could be a rich source of antibodies useful in treating diseases present in elderly populations where natural antibody protection may have eroded. Manipulation of human B-1 cell numbers and/or activity may be a new avenue for altering T cell function and treating immune dyscrasias.

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.

A CD25⁻ positive population of activated B1 cells expresses LIFR and responds to LIF

B1 B cells defend against infectious microorganisms by spontaneous secretion of broadly reactive “natural” immunoglobulin that appears in the absence of immunization. Among many distinguishing characteristics, B1 B cells display evidence of activation that includes phosphorylated STAT3. In order to identify the origin of pSTAT3 we examined interleukin-2 receptor (IL-2R) expression on B1 cells. We found that some (about 1/5) B1a cells express the IL-2R α chain, CD25. Although lacking CD122 and unresponsive to IL-2, B1a cells marked by CD25 express increased levels of activated signaling intermediates, interruption of which results in diminished CD25. Further, CD25⁺ B1a cells contain most of the pSTAT3 found in the B1a population as a whole. Moreover, CD25⁺ B1a cells express leukemia inhibitory factor receptor (LIFR), and respond to LIF by upregulating pSTAT3. Together, these results define a new subset of B1a cells that is marked by activation-dependent CD25 expression, expresses substantial amounts of activated STAT3, and contains a functional LIFR.

Human induced pluripotent stem cells are capable of B-cell lymphopoiesis

Induced pluripotent stem (iPS) cells offer a unique potential for understanding the molecular basis of disease and development. Here we have generated several human iPS cell lines, and we describe their pluripotent phenotype and ability to differentiate into erythroid cells, monocytes, and endothelial cells. More significantly, however, when these iPS cells were differentiated under conditions that promote lympho-hematopoiesis from human embryonic stem cells, we observed the formation of pre-B cells. These cells were CD45(+)CD19(+)CD10(+) and were positive for transcripts Pax5, IL7αR, λ-like, and VpreB receptor. Although they were negative for surface IgM and CD5 expression, iPS-derived CD45(+)CD19(+) cells also exhibited multiple genomic D-J(H) rearrangements, which supports a pre-B-cell identity. We therefore have been able to demonstrate, for the first time, that human iPS cells are able to undergo hematopoiesis that contributes to the B-cell lymphoid lineage.

CD22 expression mediates the regulatory functions of peritoneal B-1a cells during the remission phase of contact hypersensitivity reactions

Although contact hypersensitivity (CHS) has been considered a prototype of T cell-mediated immune reactions, recently a significant contribution of regulatory B cell subsets in the suppression of CHS has been demonstrated. CD22, one of the sialic acid-binding immunoglobulin-like lectins, is a B cell-specific molecule that negatively regulates BCR signaling. To clarify the roles of B cells in CHS, CHS in CD22(-/-) mice was investigated. CD22(-/-) mice showed delayed recovery from CHS reactions compared with that of wild-type mice. Transfer of wild-type peritoneal B-1a cells reversed the prolonged CHS reaction seen in CD22(-/-) mice, and this was blocked by the simultaneous injection with IL-10 receptor Ab. Although CD22(-/-) peritoneal B-1a cells were capable of producing IL-10 at wild-type levels, i.p. injection of differentially labeled wild-type/CD22(-/-) B cells demonstrated that a smaller number of CD22(-/-) B cells resided in lymphoid organs 5 d after CHS elicitation, suggesting a defect in survival or retention in activated CD22(-/-) peritoneal B-1 cells. Thus, our study reveals a regulatory role for peritoneal B-1a cells in CHS. Two distinct regulatory B cell subsets cooperatively inhibit CHS responses. Although splenic CD1d(hi)CD5(+) B cells have a crucial role in suppressing the acute exacerbating phase of CHS, peritoneal B-1a cells are likely to suppress the late remission phase as "regulatory B cells." CD22 deficiency results in disturbed CHS remission by impaired retention or survival of peritoneal B-1a cells that migrate into lymphoid organs.

Old mice retain bone marrow B1 progenitors, but lose B2 precursors, and exhibit altered immature B cell phenotype and light chain usage

The bone marrow of old adult mice ( approximately 2 years old) has reduced B lymphopoiesis; however, whether the B1 pathway in adult bone marrow is also compromised in senescence is not known. Herein, we show that phenotypic (IgM(-)Lin(-)CD93(+)[AA4.1(+)] CD19(+)B220(low/-)) B1 progenitors are retained in old bone marrow even as B2 B cell precursors are reduced. Moreover, B1 progenitors from both young adult and old mice generated new B cells in vitro enriched for CD43 expression, likely due to their activation, and exhibited increased lambda light chain usage and diminished levels of kappa light chain expression. B1 progenitors were shown to have lower surrogate light chain (lambda5) protein levels than did B2 pro-B cells in young mice and these levels decreased in both B1 and B2 precursor pools in old age. These results indicate that the B1 B cell pathway persists during old age in contrast to the B2 pathway. Moreover, B1 B cell progenitors generated new B cells in the adult bone marrow that have distinct surface phenotype and light chain usage. This is associated with decreased surrogate light chain expression, a characteristic held in common by B1 progenitors as well as B2 precursors in old mice.

Current status of liver transplantation across ABO blood-type barrier

Outcomes of ABO-blood type incompatible liver transplantation have recently improved owing to various treatments. The typical clinical manifestations of antibody mediated rejection (AMR) are hepatic necrosis and intrahepatic biliary complication (IHBC). The prognosis of AMR is poor. AMR is the result of circulatory disturbance which is caused by injury to the endothelium due to an antibody-antigen-complement reaction. Diffuse C4d staining in the portal capillaries and periportal areas in severe AMR. Since natural antibodies against A/B carbohydrate determinants are likely to develop as a result of exposure to environmental bacteria that express similar determinants, the B-1 lineage has been speculated to be the major population of B-cell types responding to A/B determinants. Calcineurin inhibitors block B-1 cell differentiation. Rituximab can be used to deplete both cells that are producing IgM antibodies and those that have already differentiated into B-1 cells. Mycophenolate mofetil is required to inhibit the production of IgG subclass of antibodies. The outcome is now similar to that of blood-type-matched transplantation. However, there are still issues to be solved in order to further improve the outcome via a decrease of infection.

Fetal B-cell lymphopoiesis and the emergence of B-1-cell potential

Most B cells in peripheral lymphoid tissues are produced in adult bone marrow and are referred to as B-2 cells. A minor B-cell population, known as the B-1-cell population, that is mainly involved in innate immune responses has been identified in mice. In contrast to B-2 cells, B-1-cell progenitors are produced most efficiently during fetal life. This Review focuses on the emergence of B-1-cell potential during embryogenesis, summarizes recent advances in the delineation of a fetal B-1-cell-specified progenitor, and discusses the possibility that distinct fetal and adult B-cell developmental programmes might be operative in humans.

B cell conducts the lymphocyte orchestra

The interest for B cells has recently been revived. They normally play a role in the development, the regulation, as well as the activation of lymphoid architecture: they regulate dendritic cells and T-cell subsets function through cytokine production. Receptor editing is also essential in B cells and aids in preventing autoimmunity. Both abnormalities in the distribution of B-cell subsets and clinical benefit response to B-cell depletion in autoimmune states illustrate their importance. A new area has thus been reached, whereby B lymphocytes return as a significant contributor to autoimmune disorders.

Fc receptor homolog 3 is a novel immunoregulatory marker of marginal zone and B1 B cells

Two members of the recently identified FcR homolog (FcRH) family in mice demonstrate preferential B cell expression. One of these, FcRH3, encodes a type I transmembrane protein with five extracellular Ig domains and a cytoplasmic tail with a consensus ITIM and a noncanonical ITAM. Analysis of full-length cDNAs from five different mouse strains defines two FcRH3 alleles. A panel of FcRH3-specific mAbs was generated to define its expression pattern and functional potential on B lineage cells. Although poorly detected on the majority of bone marrow or peripheral blood cells, FcRH3 was readily identified on splenic marginal zone (MZ) and MZ precursor B cells, but not on the bulk of newly formed B cells, follicular B cells, germinal center B cells, and plasma cells. In the peritoneal cavity, FcRH3 was found on B1 cells, and not on the majority of B2 cells. Consistent with its possession of an ITIM and ITAM-like sequence, FcRH3 was tyrosine phosphorylated following pervanadate treatment, and its coligation with the BCR inhibited calcium mobilization. These results suggest FcRH3 is a novel immunoregulatory marker of MZ and B1 B lineage cells.

Btk regulates multiple stages in the development and survival of B-1 cells

B-1 cells are important players in the first line of defense against pathogens. According to current models for the origin of B-1 cells, they either represent a separate lineage from conventional B-2 cells or differentiate from conventional B-2 cells via an intermediate, B-1(int), in response to positive selection by antigen. Here we show that Btk, a Tec family kinase that mediates B cell antigen receptor (BCR) signaling, is required at multiple stages of B-1 cell development. VH12 anti-phosphatidylcholine (PtC) IgH transgenic mice provide a model for the induced differentiation of B-1 cells. This transgene selects for PtC-reactive cells and induces them to adopt a B-1 phenotype. Both processes have been shown to depend on Btk. To determine whether this is secondary to a requirement for Btk in the development of mature B-2 cells, we crossed VH12 transgenic mice to mice expressing low levels of Btk. B-2 cell development occurs normally in Btk(lo) mice despite reduced responsiveness to BCR crosslinking. Analysis of VH12.Btk(lo) mice reveals that Btk regulates the B-1(int) to B-1 transition and/or the survival of splenic B-1 cells, in part via a mechanism independent of its role in BCR signaling. We also show that Btk mediates the survival of, and expression of IL-10 by, those B-1 cells that do develop and migrate to the peritoneum. Multiple roles for Btk in B-1 cell development and maintenance may explain the particular sensitivity of this population to mutations in components of Btk signaling pathways.

Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

The individual contribution of Igalpha and Igbeta for BCR-triggered fates is unclear. Prior evidence supports conflicting ideas concerning unique as well as redundant functions for these proteins in the context of BCR/pre-BCR signaling. Part of this ambiguity may reflect the recent appreciation that Igalpha and Igbeta participate in both Ag-independent (tonic) and Ag-dependent signaling. The present study undertook defining the individual requirement for Igalpha and Igbeta under conditions where only ligand-independent tonic signaling was operative. In this regard, we have constructed chimeric proteins containing one or two copies of the cytoplasmic domains of either Igalpha or Igbeta and Igalpha/Igbeta heterodimers with targeted Tyr-->Phe modifications. The ability of these proteins to act as surrogate receptors and trigger early bone marrow and peripheral B cell maturation was tested in RAG2(-/-) primary pro-B cell lines and in gene transfer experiments in the muMT mouse model. We considered that the threshold for a functional activity mediated by the pre-BCR/BCR might only be reached when two functional copies of the Igalpha/Igbeta ITAM domain are expressed together, and therefore the specificity conferred by these proteins can only be observed in these conditions. We found that the ligand-independent tonic signal is sufficient to drive development into mature follicular B cells and both Igalpha and Igbeta chains supported formation of this population. In contrast, neither marginal zone nor B1 mature B cell subsets develop from bone marrow precursors under conditions where only tonic signals are generated.

The mosaic of B-cell subsets (with special emphasis on primary Sjögren's syndrome)

Major breakthroughs have occurred with classification of B-cells into populations and subpopulations. With respect to their expression of CD5, they comprise the B1 and B2 populations, with the former further divided into B1a and B1b subpopulations. The oncologic process starts from transitional type 1 (T1) and T2 immature B-cells, through marginal zone or germinal center B-cells, ending up with memory B-cells and plasma cells (PCs). They may also be categorized based on their functional commitment with polarized B effector (Be)1 and Be2, with B-activating factor of the tumor-necrosis factor-producing B-cells, and with short-lived and long-lived PCs. Such a seemingly homogeneous family of cells has thus turned out to be a genuine mosaic of B-lymphocyte subsets.

Murine isolated lymphoid follicles contain follicular B lymphocytes with a mucosal phenotype

Isolated lymphoid follicles (ILFs) are organized intestinal lymphoid structures whose formation can be induced by luminal stimuli. ILFs have been demonstrated to act as inductive sites for the generation of immune responses directed toward luminal stimuli; however, the phenotype of the immune response initiated within ILFs has largely been uninvestigated. To gain a better understanding of the immune responses initiated within ILFs, we examined phenotypic and functional aspects of the largest cellular component of the murine ILF lymphocyte population, B lymphocytes. We observed that murine ILF B lymphocytes are composed of a relatively homogenous population of follicular B-2 B lymphocytes. Consistent with their proximity to multiple stimuli, ILF B lymphocytes displayed a more activated phenotype compared with their counterparts in the spleen and Peyer's patch (PP). ILF B lymphocytes also expressed higher levels of immunomodulatory B7 and CD28 family members B7X and programmed death-1 compared with their counterparts in the spleen and PP. ILF B lymphocytes preferentially differentiate into IgA-producing plasma cells and produce more IL-4 and IL-10 and less interferon-gamma compared with their counterparts in the spleen. Immunoglobulin repertoire analysis from individual ILFs demonstrated that ILFs contain a polyclonal population of B lymphocytes. These findings indicate that murine ILFs contain a polyclonal population of follicular B-2 B lymphocytes with a phenotype similar to PP B lymphocytes and that, in unchallenged animals, ILFs promote immune responses with a homeostatic phenotype.

B-1 B lymphocytes require Blimp-1 for immunoglobulin secretion

B-1 B cells produce circulating natural antibodies that provide “innate-like” protection against bacterial and viral pathogens. They also provide adaptive responses to blood and air-borne pathogens. B lymphocyte–induced maturation protein 1 (Blimp-1) is a transcriptional repressor that is required for the formation of B-2–derived antibody-secreting plasma cells. In this study, we used mice lacking Blimp-1 in the B cell lineage to show that Blimp-1 is not necessary for the formation or self-renewal of B-1 B cells but that Blimp-1 is required for normal immunoglobulin (Ig) secretion by B-1 cells. B-1 cells lacking Blimp-1 do not repress Pax5 mRNA and do not induce X-box binding protein 1, and μ secreted mRNA normally, showing that B-1 and B-2 cells both use a common pathway for Ig secretion. Blimp-1–deficient B-1 B cells are also defective in providing early protection against influenza infection.

Phenotypically distinct B cell development pathways map to the three B cell lineages in the mouse

A recent article by Montecino-Rodriguez et al. [Montecino-Rodriguez, E., Leathers, H. & Dorshkind, K. (2006) Nat. Immunol.7, 293-301] has distinguished the early progenitors for B-1 cells, which principally develop in neonates, from early progenitors for B-2 cells, which principally develop in adult bone marrow. Here we introduce syndecan-1 (CD138) and MHC class II (I-A) as markers of early B cell development [Hardy, R. R., Carmack, C. E., Shinton, S. A., Kemp, J. D. & Hayakawa, K. (1991) J. Exp. Med. 173, 1213-1225; Hardy fractions B-D] and show that the expression of these markers distinguishes the predominant B cell development pathway in neonates from the corresponding predominant pathway in adults (both progenitors are present but differently represented in each case). We show that pre-B cells (Hardy fraction D) in the predominant adult pathway express high levels of CD138 and intermediate levels of I-A, whereas the corresponding pre-B cells in the pathway that predominates in neonates do not express either of these markers. As expected, because most of the pre-B cells in adults express CD138, we find that sorted CD138+ adult pre-B cells differentiate to IgM+ B cells in vitro. Sorted CD138- pre-B cells from neonates, the majority subset at this age, also mature to IgM+ cells (without passing through a CD138+ stage). Importantly, our studies here confirm the differential representation of adult and neonatal progenitor populations and further demonstrate that CD138 expression subdivides the adult CD19+, B220-6B2-/low population shown to contain B-1 progenitors in a way consistent with the predominance of B-1b progenitors in adults. Thus, CD138 expression provides a key route to distinguishing early B cell development pathway for what now are clearly three B cell lineages.

A shared gene-expression signature in innate-like lymphocytes

Innate and adaptive immunities are the two major arms of the immune system, which rely on distinct cell types. These cells can be distinguished not only by the source of diversity for non-self recognition, of germline or somatic origin, but also by their localization and the pattern and rates of response after encounter of antigenic triggers. In addition, subsets of lymphocytes exist whose receptors require rearrangement but result in semi-invariant structures with a high degree of self-specificity. We hypothesized that these innate-like lymphocytes might share a common gene transcription signature that relates them to classic members of the innate immune system. This relationship was first observed in agonist-induced CD8alphaalpha T cells in fetal/neonatal thymus. We then asked whether this notion could be extended to other innate-like lymphocytes, by comparison of gene expression profiles of innate-like lymphocytes and closely paired adaptive system counterparts (NKT versus CD4T, CD8alphaalphaT versus CD8alphabetaT, and B1 versus B2). A statistically significant 'innate signature' indeed was distilled. Particularly intriguing was the high representation of interferon-inducible guanosine triphophatases crucial for resistance against intracellular pathogens and of small G proteins involved in intracellular vacuole maturation and trafficking. Overall, this combined expression pattern can be designated as an innate signature among lymphocytes.

B lymphocytes on the front line of autoimmunity

The paradigm that B cell response to self antigens (Ag) is promoted by antibodies (Ab) has become unsatisfactory. Studies over the last decade have indeed revealed that B cells serve extraordinarily diverse functions within the immune system other than Ab production. They normally play a role in the development in the regulation, as well as the activation of lymphoid architecture, regulating dentritic cells and T cell subsets function through cytokine production. Receptor editing is also essential in B cells and aids in preventing autoimmunity. Both abnormalities in the distribution of B cells subsets and clinical benefit response to B cell depletion in autoimmune states illustrate their importance. Transgenic animal models have demonstrated that sensitivity of B cells to Ag receptor cross-linking correlates to autoimmunity: negative signaling by CD5 and CD22 in maintaining tolerance through recruitment of phosphatase has thus been documented. In short, a new area has been reached, whereby B lymphocytes return as a significant contributor to autoimmune disorders.

Dual isotype expressing B cells [kappa(+)/lambda(+)] arise during the ontogeny of B cells in the bone marrow of normal nontransgenic mice

Central to the clonal selection theory is the tenet that a single B cell expresses a single receptor with a single specificity. Previously, based on our work in anti-phosphocholine transgenic mouse models, we suggested that B cells escaped clonal deletion by coexpression of more than one receptor on their cell surface. We argued that "receptor dilution" was necessary when: (i) the expressed immunoglobulin receptor is essential for immune protection against pathogens and (ii) this protective receptor is autoreactive and would be clonally deleted, leaving a hole in the B cell repertoire. Here, we demonstrate that dual isotype expressing B cells arise during the normal ontogeny of B cells in the bone marrow and populate both the spleen and peritoneal cavity of nontransgenic mice. Furthermore, single cell analysis of the expressed immunoglobulin light chains suggests that receptor editing may play a role in the generation of a significant fraction of dual isotype expressing B cells.

Origins of peripheral B cells in IL-7 receptor-deficient mice

The interleukin 7 (IL-7) signaling pathway is critical for early lymphoid differentiation. We found dramatic perturbations in fetal liver B cell development and confirmed a complete absence of developing B cells in the adult bone marrow in mice lacking the IL-7 receptor alpha (IL-7Ralpha) gene. We show that peripheral B-2 and B-1 cell populations are deficient in IL-7Ralpha-/- mice. B-2 follicular cell and peritoneal B-1 cell percentages are reduced, while B-2 marginal zone cell percentages are increased. A comparison of bone marrow and splenic populations at different ages revealed that the splenic B cell populations seen in adult IL-7Ralpha-/- mice first appear during neonatal development. We have measured N-nucleotide addition at the joints of V(D)J rearrangements in splenic B cells and have used it as a somatic marker to define and separate bone marrow-derived B cells from fetal liver-derived B cells. B cells isolated from the bone marrow and spleen of adult and neonatal IL-7Ralpha-deficient mice harbor high levels of N-nucleotide additions similar to those found in equivalent wild-type B cell populations. We conclude that the majority of splenic B cells in IL-7Ralpha-deficient mice originate from the bone marrow and not the fetal liver.

Staphylococcal Protein A Deletes B-1a and Marginal Zone B Lymphocytes Expressing Human Immunoglobulins: An Immune Evasion Mechanism

Protein A (SpA) of Staphylococcus aureus is endowed with the capacity to interact with the H chain variable region (V(H)) of human Abs and to target >40% of B lymphocytes. To investigate whether this property represents a virulence factor and to determine the in vivo consequences of the confrontation of SpA with B lymphocytes, we used transgenic mice expressing fully human Abs. We found that administration of soluble SpA reduces B-1a lymphocytes of the peritoneal cavity and marginal zone B lymphocytes of the spleen, resulting in a markedly deficient type 2 humoral response. Single-cell PCR analysis and sequencing of the Ab V(H) gene repertoire revealed a significant reduction of V(H)3+ marginal zone B cells. Since the two B lymphocyte subsets targeted are involved in innate immune functions, our data suggest that crippling of humoral immunity by S. aureus represents an immune evasion mechanism that may aggravate recurrent infections.

Immune receptor signaling, aging, and autoimmunity

With advancing age, the immune system undergoes changes that predispose to autoimmune reactivity. Aging reduces the efficiency of physical barriers, decreasing protection against invasive pathogens, and exposing previously hidden antigens in the body's own tissues. Self-antigens acquire alterations that increase their immunogenicity. In addition, the ability of innate immunity to eliminate infectious agents deteriorates, resulting in inappropriate persistence of immune stimulation and antigen levels exceeding the threshold for the activation of B or T cells. B cell turnover is reduced and numbers of naïve T cells decline to the advantage of increasing numbers of memory T cells. In parallel, the loss of co-stimulatory T cell molecules may increase reactivity of T cells, and render them less susceptible to downregulation. Since optimal immune reactivity requires a tight balance of transduction pathways in both T and B lymphocytes, and because these pathways are altered in systemic autoimmune diseases, we would like to propose that, with age, alterations of the immune receptor signaling machinery underlie the higher incidence of autoimmune phenomena in the elderly. Consistently, aging is associated with alterations in several components of the signaling complex in B cells, memory and naïve T cells, and a reduced activation of several lipid rafts-associated proteins. Because the coincidence of autoimmune disease with other ailments increases the burden of disease and limits therapeutic options in the aged, further investigation of these pathways in the elderly represents a challenge that will need to be addressed in order to devise effective preventive and therapeutic interventions.

Analysis of RAG expression by peripheral blood CD5+ and CD5- B cells of patients with childhood systemic lupus erythematosus

BACKGROUND

The assembly of immunoglobulin genes during B cell development in the bone marrow is dependent on the expression of recombination activating genes (RAG) 1 and 2. Recently, RAG expression in peripheral blood IgD+ B cells outside the bone marrow has been demonstrated and is associated with the development of autoimmune diseases.

OBJECTIVE

To investigate RAG expression in the CD5+ or CD5- IgD+ B cell compartment in childhood systemic lupus erythematosus (SLE).

METHODS

Using a combination of flow cytometric cell sorting and reverse transcriptase polymerase chain reaction analysis of cDNA libraries generated from individual cells, the expression of RAG, VpreB, and CD154 mRNA by individual peripheral blood B cells of three paediatric SLE patients was examined in detail.

RESULTS

While only one patient had a significantly increased frequency of RAG+ B cells in the CD5- B cell population, all patients showed higher frequencies of RAG+ B cells in the CD5+IgD+ B cell population. The frequency of RAG+ IgD+CD5+/- B cells was reduced during intravenous cyclophosphamide treatment. In healthy age matched children, RAG expressing IgD+ B cells were hardly detectable. Coexpression of RAG and VpreB or CD154 mRNA could only be found in SLE B cells.

CONCLUSIONS

RAG expression in peripheral blood B cells of SLE patients is particularly increased in the IgD+CD5+ B cell population. CD5+ and CD5- B cells in SLE have the potential to undergo receptor revision leading to the generation of high affinity pathogenic autoantibodies.

B Lymphocytes Are Required for Development and Treatment of Autoimmune Diseases

Recent studies have revealed that B cells serve extraordinarily diverse functions within the immune system in addition to antibody production. These functions contribute to autoimmunity. They initiate the development of lymphoid architecture and regulate dendritic and T-cell function through cytokine production. Receptor editing is also essential to prevent autoimmunity. Both abnormalities in the distribution of B-cell subsets and the benefits of ablative B-cell therapy of autoimmune states confirm their importance. Results from transgenic models have demonstrated that the sensitivity of B cells to antigen receptor cross-linking correlates to autoimmunity, with particular reference to negative signaling by CD5 and CD22. These mechanisms maintain tolerance by recruiting src-homology 2 domain-containing protein tyrosine phosphatase-1. These findings open new prospects for immunotherapy of autoimmune diseases.

B-lymphocytes, innate immunity, and autoimmunity

Having evolved to generate a huge Ag-specific repertoire and to mount T cell-dependent responses and long-term memory, the B lymphocyte is a central player in the adaptive branch of immune defense. However, accumulating evidence indicates that B-1 cells of the peritoneal cavity and marginal zone (MZ) B cells of the spleen also can play innate-like immune functions. Their anatomical locations allow frequent Ag encounter. Secreting essentially germline-encoded, polyreactive Abs, and responding rapidly and vigorously to stimulation, these two B cell subsets have evolved to impart potentially protective responses. With their additional capacities to secrete factors that can directly mediate microbial destruction and to express Toll-like receptors (TLR), B cells provide an important link between the innate and adaptive branches of the immune system. Currently, the relevance of these innate-like B cells to the pathogenesis of autoimmune disease is the focus of investigation. In experimental models of autoimmunity, the sequestration of autoreactive B cells in the MZ has been proposed to be essential for the maintenance of self-tolerance. The low activation threshold of MZ B cells makes them particularly reactive to high loads and/or altered self-Ags, potentially exacerbating autoimmune disease. Their expansion in autoimmune models and their association with autoantibody secretion indicate that they may participate in tissue damage. The demonstration that B cell depletion therapies may represent a highly beneficial therapeutic goal in autoimmune disorders suggests that specific elimination of B-1 and MZ B cells may represent a more efficient immunointervention strategy in systemic autoimmunity.

Identification and characterization of circulating human transitional B cells

Murine B-cell development begins in bone marrow and results in the generation of immature transitional B cells that transit to the spleen to complete their maturation. It remains unclear whether the same developmental pathway takes place in humans. Using markers characteristic of human bone marrow immature B cells, we have identified a population of circulating human B cells with a phenotype most similar to mouse transitional type I (T1) B cells, although these human counterparts express CD5. These cells die rapidly in culture, and B-cell activation factor member of the tumor necrosis factor (TNF) family (BAFF) does not effect their survival regardless of B-cell receptor (BCR) stimulation. In contrast, bone marrow stromal cells or interleukin-4 (IL-4) significantly enhanced their survival. In the presence of T-cell signals provided by IL-4 or CD40 ligation, BCR stimulation can induce progression into cell cycle. Interestingly, circulating B cells that phenotypically and functionally resemble murine T2 B cells are found in cord blood and adult peripheral blood, suggesting that B-cell maturation may not be restricted to the spleen. Notably, increased proportions of T1 B cells were found in blood of patients with systemic lupus erythematosus (SLE), although bone marrow production and selection appeared to be normal.

Development of B cells producing natural autoantibodies to thymocytes and senescent erythrocytes

Natural antibodies produced by CD5+ B-1 B cells include those with specificity for senescent erythrocytes (anti-BrMRBC, anti-PtC) and for thymocytes (anti-thymocyte autoantibody, ATA). Here we describe work from our laboratories studying two prototypic examples, V(H)11Vkappa9-encoded anti-BrMRBC and V(H)3609Vkappa21c-encoded ATA. Using V(H)11-mu transgenic mice, we discovered that certain natural autoantibodies utilize V(H) genes that are selected against in bone marrow B cell development, but not fetal liver, effectively restricting their generation to fetal/neonatal life. Studies with ATA-mu transgenic mice demonstrated a critical requirement for self antigen in the accumulation of B cells with this specificity and for the production of high levels of serum ATA. Finally, analysis of B cell development in ATA-mu kappa transgenic mice revealed two distinct responses by B cells to expression of this B cell receptor (BCR): most developing B cells in spleen of adult mice were blocked at an immature stage and only escaped apoptosis by editing their BCR to eliminate the ATA specificity; nevertheless, high levels of serum ATA were observed, indicating that some B cells differentiated to antibody-forming cells without altering their specificity. Thus, our studies reveal mechanisms for restricting the generation of B cells producing natural autoantibodies, demonstrate a key positive selection step in their development, and show that most developing B cells in adult mice bearing such specificities fail to reach a mature stage.

The development of early and mature B?cells is impaired in mice deficient for the Ets-1 transcription factor

The Ets-1 transcription factor is essential for normal development of the natural killer and T cell lineages; however, its role in B cell development remains poorly understood. To address this issue, we used gene targeting to inactivate Ets-1 in mice (Ets-1(-/-)). We show here that the development of B cell precursors, particularly steps requiring pre-B cell receptor function, is defective in Ets-1(-/-) mice. Peripheral B cell subsets were analyzed in RAG2-deficient mice reconstituted with Ets-1(-/-) fetal liver cells. In such Ets-1(-/-) chimeric mice, B cell precursors develop into IgM/IgD-bearing cells, but B-1a cells as well as transitional-2 and marginal zone B cell subsets of the spleen are absent. In response to B cell receptor stimulation, Ets-1(-/-) splenic B cells fail to express the CD69 and CD25 activation markers. Furthermore, despite activation of ERK and JNK signaling pathways, Ets-1-deficient B cells do not proliferate and die following BCR engagement. These findings demonstrate that the effect of Ets-1 inactivation is not restricted to the terminal B cell differentiation stage, but also affects the development and function of earlier B cell subsets.

Alternative routes to maturity: branch points and pathways for generating follicular and marginal zone B cells

Positive and negative selection of developing B cells is critical for generating a functional non-pathogenic B-cell repertoire. Newly formed B cells in the bone marrow or peripheral lymphoid system can be eliminated by one of several negative selection mechanisms or recruited through a poorly understood positive selection mechanism. In this review, we focus on the growing literature on the relevance of immature (transitional) peripheral B cells to the area of B-cell positive selection, with an emphasis on the notion that transitional B cells can be subdivided into several functionally distinct subpopulations. In this discussion, we consider the nature of these transitional B-cell subsets and their relevance to selection events that influence whether developing B cells eventually give rise to follicular versus marginal zone B cells. In addition, we attempt to initiate a resolution of current controversies surrounding transitional B-cell subsets and offer an alternative model of peripheral B-cell maturation and the follicular versus marginal zone decision.

Positive selection and lineage commitment during peripheral B-lymphocyte development

Although it is appreciated that the antigen receptor on B cells is required for peripheral B-lymphocyte development and survival, it has been unclear whether this receptor interacts with self-antigens during development or if it signals constitutively in an antigen-independent fashion. The analysis of mutant mice in which antigen receptor signaling in B cells is either attenuated or enhanced has revealed the existence of a follicular versus marginal zone B-lymphocyte cell-fate decision. These analyses indicate that weak antigen receptor-derived signals favor marginal zone B-cell generation, and relatively strong signals favor the development of mature follicular B cells. Even stronger signals derived from the antigen receptor favor the generation of B1 B cells. This signal strength model for B-cell development supports the notion that self-antigens of varying affinity may mediate positive selection and lineage commitment. Direct evidence supporting such a view has been obtained from the analysis of antigen receptor knockin mice. Specific antigen receptors guide B cells to develop into specific lineages. Although Notch-2, nuclear factor-kappaBp50, and other genes are essential for marginal zone B-cell development, instructive signals delivered by the antigen receptor represent the primary force driving positive selection and lineage commitment in B lymphocytes.

Polyreactive antigen-binding B (PAB-) cells are widely distributed and the PAB population consists of both B-1+ and B-1- phenotypes

B cells that make polyreactive antibodies (PAB+ cells) express polyreactive Ig receptors on their surface and can bind a variety of different antigens. The present study shows that PAB+ cells are widely distributed, are present in varying numbers in different lymphoid organs and that their phenotype varies depending on the organs from which they are isolated. Up to 10 times more cells in PAB+ enriched populations bind antigens as compared to PAB- populations. Comparison of PAB+ with B-1+ cells showed that a high percentage of PAB+ cells are B-1+, but that many PAB+ cells do not express B-1 cell surface markers and, in fact, are B-1-. It is concluded that the B cell population consists of PAB+/B-1+, PAB+/B-1-, PAB-/B-1+, and PAB-/B-1- cells. The presence of PAB+ cells in the thymus points to the possibility that PAB+ cells may carry endogenous host antigens from peripheral tissues to the thymus where they may contribute to immunological tolerance.

Maintenance of peritoneal B-1a lymphocytes in the absence of the spleen

Positive selection by autoantigens is believed to play an important role in the generation/maintenance of B-1a cells. Recently, it has been described that splenectomy results in the loss of an already established B-1a cell pool. To elucidate whether the spleen influences the peritoneal B-1a repertoire, we have analyzed the consequences of splenectomy in the recently established IgL-transgenic L2 mouse model. L2 mice are characterized by a severe block of B-2 development and predominance of B-1a cells, which exhibit a pronounced IgH oligoclonality, presumably due to positive selection by autoantigens. In this study, we show that, in striking contrast to splenectomized normal mice, L2 mice exhibit unchanged frequencies of peritoneal B-1a cells. The IgH repertoire of these B-1a cells, however, was severely perturbed in that the previously described predominant B-1a H chains were no longer present. The repertoire changes were partial since phosphatidylcholine-specific B-1a cells were present in similar numbers before and after splenectomy. Thus, splenic Ags appear to act as "survival factors" for major subsets of peritoneal B cells. The loss of B-1a cells in the absence of such factors is compensated by repertoire changes among B-1a cells in B cell lymphopenic L2 but not normal mice.

Specific in vivo deletion of B-cell subpopulations expressing human immunoglobulins by the B-cell superantigen protein L

Some pathogens have evolved to produce proteins, called B-cell superantigens, that can interact with human immunoglobulin variable regions, independently of the combining site, and activate B lymphocytes that express the target immunoglobulins. However, the in vivo consequences of these interactions on human B-cell numbers and function are largely unknown. Using transgenic mice expressing fully human immunoglobulins, we studied the consequences of in vivo exposure of protein L of Peptostreptococcus magnus with human immunoglobulins. In the mature pool of B cells, protein L exposure resulted in a specific reduction of splenic marginal-zone B cells and peritoneal B-1 cells. Splenic B cells exhibited a skewed light-chain repertoire consistent with the capacity of protein L to bind specific kappa gene products. Remarkably, these two B-cell subsets are implicated in innate B-cell immunity, allowing rapid clearance of pathogens. Thus, the present study reveals a novel mechanism that may be used by some infectious agents to subvert a first line of the host's immune defense.

IL-9-Induced Expansion of B-1b Cells Restores Numbers but Not Function of B-1 Lymphocytes in xid Mice

Mice expressing the X-linked immunodeficiency (xid) mutation lack functional Bruton's tyrosine kinase and were shown to be specifically deficient in peritoneal B-1 lymphocytes. We have previously shown that IL-9, a cytokine produced by TH2 lymphocytes, promotes B-1 cell expansion in vivo. To determine whether IL-9 overexpression might compensate the xid mutation for B-1 lymphocyte development, we crossed xid mice with IL-9-transgenic mice. In this model, IL-9 restored normal numbers of mature peritoneal B-1 cells that all belonged to the CD5(-) B-1b subset. Despite this normal B-1 lymphocyte number, IL-9 failed to restore classical functions of B-1 cells, namely, the production of natural IgM Abs, the T15 Id Ab response to phosphorylcholine immunization, and the antipolysaccharide humoral response against Streptococcus pneumoniae. By using bromelain-treated RBC, we showed that the antigenic repertoire of these IL-9-induced B-1b lymphocytes was different from the repertoire of classical CD5(+) B-1a cells, indicating that the lack of B-1 function by B-1b cells is associated with distinct Ag specificities. Taken together, our data show that B-1b cell development can restore the peritoneal B-1 population in xid mice but that these B-1b cells are functionally distinct from CD5(+) B-1a lymphocytes.

The selection of marginal zone B cells differs from that of B-1a cells

Transgenic (Tg) L2 mice expressing high levels of the lambda2 (315) L chain contain only B cell populations involved in the first line of defense, i.e., B-1 and marginal zone (MZ) B cells. The strongly oligoclonal IgH chain repertoire of Tg B-1a cells in such mice was attributed to strong positive selection by autoantigens. In this study, we show that the MZ B cells of L2 mice correspond very closely to MZ B cells of normal mice, as revealed by surface marker expression and gene expression profiling. We demonstrate that the IgH chain repertoire of these Tg MZ B cells is extremely heterogeneous. This is in sharp contrast to the oligoclonality found in B-1a cells of the same mice, which was attributed to strong positive selection mediated by autoantigens. Therefore, the strong positive selection of the IgH chain repertoire in L2 mice is B-1a specific. Thus, our data demonstrate that despite common functional properties, MZ B and B-1a cells exhibit striking differences in their selection and/or maintenance requirements.

Reduced dosage of Bruton's tyrosine kinase uncouples B cell hyperresponsiveness from autoimmunity in lyn-/- mice

The development of autoimmunity is correlated with heightened sensitivity of B cells to B cell Ag receptor (BCR) cross-linking. BCR signals are down-regulated by Lyn, which phosphorylates inhibitory receptors. lyn(-/-) mice have reduced BCR signaling thresholds and develop autoantibodies, glomerulonephritis, splenomegaly due to myeloid hyperplasia, and increased B-1 cell numbers. Bruton's tyrosine kinase (Btk), a critical component of BCR signaling pathways, is required for autoantibody production in lyn(-/-) mice. It is unclear whether Btk mediates autoimmunity at the level of BCR signal transduction or B cell development, given that lyn(-/-)Btk(-/-) mice have a severe reduction in conventional B and B-1 cell numbers. To address this issue, we crossed a transgene expressing a low dosage of Btk (Btk(low)) in B cells to lyn(-/-)Btk(-/-) mice. Conventional B cell populations were restored to levels similar to those in lyn(-/-) mice. These cells were as hypersensitive to BCR cross-linking as lyn(-/-) B cells as measured by proliferation, Ca(2+) flux, and activation of extracellular signal-regulated kinase and Akt. However, lyn(-/-)Btk(low) mice did not produce anti-ssDNA, anti-dsDNA, anti-histone, or anti-histone/DNA IgM or IgG. They also lacked B-1 cells and did not exhibit splenomegaly. Thus, B cell hyperresponsiveness is insufficient for autoimmunity in lyn(-/-) mice. These studies implicate B-1 and/or myeloid cells as key contributors to the lyn(-/-) autoimmune phenotype.

Ly49E expression points toward overlapping, but distinct, natural killer (NK) cell differentiation kinetics and potential of fetal versus adult lymphoid progenitors

Using a new antibody, we found previously that contrary to adult natural killer (NK) cells, fetal NK cells have a unique phenotype, as they exclusively express Ly49E. This can be explained by an intrinsic different NK differentiation potential of fetal versus adult lymphoid progenitors, by immaturity of fetal NK cells or by instability of Ly49E expression. Here, we show that adult progenitor cells were still capable of differentiating into Ly49E-expressing NK cells but at a much lower frequency. Surprisingly, Ly49E expression in vitro did not require stromal cells. Kinetic analysis in vivo showed that Ly49E was expressed early, together with CD94/NKG2 and Ly49G2, followed by Ly49C, and finally Ly49D. Transfer of sorted Ly49E-positive fetal NK cells showed stable Ly49E expression, and later, part of these cells up-regulated other Ly49 members. These data indicate that although there are intrinsic differences, there is no strict fetal and adult wave of NK cell differentiation.

Role of the chemokine stromal cell-derived factor 1 in autoantibody production and nephritis in murine lupus

In normal mice, stromal cell-derived factor 1 (SDF-1/CXCL12) promotes the migration, proliferation, and survival of peritoneal B1a (PerB1a) lymphocytes. Because these cells express a self-reactive repertoire and are expanded in New Zealand Black/New Zealand White (NZB/W) mice, we tested their response to SDF-1 in such mice. PerB1a lymphocytes from NZB/W mice were exceedingly sensitive to SDF-1. This greater sensitivity was due to the NZB genetic background, it was not observed for other B lymphocyte subpopulations, and it was modulated by IL-10. SDF-1 was produced constitutively in the peritoneal cavity and in the spleen. It was also produced by podocytes in the glomeruli of NZB/W mice with nephritis. The administration of antagonists of either SDF-1 or IL-10 early in life prevented the development of autoantibodies, nephritis, and death in NZB/W mice. Initiation of anti-SDF-1 mAb treatment later in life, in mice with established nephritis, inhibited autoantibody production, abolished proteinuria and Ig deposition, and reversed morphological changes in the kidneys. This treatment also counteracted B1a lymphocyte expansion and T lymphocyte activation. Therefore, PerB1a lymphocytes are abnormally sensitive to the combined action of SDF-1 and IL-10 in NZB/W mice, and SDF-1 is key in the development of autoimmunity in this murine model of lupus.

Antibody repertoire development in fetal and neonatal piglets. VI. B cell lymphogenesis occurs at multiple sites with differences in the frequency of in-frame rearrangements

B cell lymphogenesis in mammals occurs in various tissues during development but it is generally accepted that it operates by the same mechanism in all tissues. We show that in swine, the frequency of in-frame (IF) VDJ rearrangements differs among yolk sac, fetal liver, spleen, early thymus, bone marrow, and late thymus. All VDJ rearrangements recovered and analyzed on the 20th day of gestation (DG20) from the yolk sac were 100% IF. Those recovered at DG30 in the fetal liver were >90% IF, and this predominance of cells with apparently a single IF rearrangement continued in all organs until approximately DG45, which corresponds to the time when lymphopoiesis begins in the bone marrow. Thereafter, the proportion of IF rearrangements drops to approximately 71%, i.e., the value predicted whether VDJ rearrangement is random and both chromosomes were involved. Unlike other tissues, VDJs recovered from thymus after DG50 display a pattern suggesting no selection for IF rearrangements. Regardless of differences in the proportion of IF rearrangements, we observed no significant age- or tissue-dependent changes in CDR3 diversity, N region additions, or other characteristics of fetal VDJs during ontogeny. These findings indicate there are multiple sites of B cell lymphogenesis in fetal piglets and differences in the frequency of productive VDJ rearrangements at various sites. We propose the latter to result from differential selection or a developmentally dependent change in the intrinsic mechanism of VDJ rearrangement.

Positive selection of anti-thy-1 autoreactive B-1 cells and natural serum autoantibody production independent from bone marrow B cell development

A natural serum autoantibody specific for the Thy-1 glycoprotein (anti-Thy-1 autoantibody [ATA]) is produced by B-1 cells that are positively selected by self-antigen. Here, using ATA micro kappa transgenic mice we show that cells with this B cell receptor are negatively selected during bone marrow (BM) development. In a Thy-1 null environment, BM ATA B cells progress to a normal follicular stage in spleen. However, in a self-antigen-positive environment, development is arrested at an immature stage in the spleen, concomitant with induction of CD5. Such cells are tolerant and short-lived, different from B-1. Nonetheless, ATA-positive selection was evident by self-antigen-dependent high serum ATA production, comprising approximately 90% of serum immunoglobulin M in ATA micro kappa mice. Splenectomy did not eliminate ATA production and transfer of tolerant splenic B cells did not induce it. These findings demonstrate that B-1 positive selection, resulting in the production of natural serum ATA, arises independently from the major pathway of BM B cell development and selection.

CD19 function in early and late B cell development: I. Maintenance of follicular and marginal zone B cells requires CD19-dependent survival signals

Loss of membrane-bound Ig results in the rapid onset of apoptosis in recirculating B cells. This observation implies that a competent B cell receptor (BCR) is not only required for Ag-dependent differentiation, but also for continued survival in the peripheral immune system. Expression of the B cell coreceptor, CD19, is likewise essential for key B cell differentiative events including the formation of B-1, germinal center, and marginal zone (MZ) B cells. In this study, we report that CD19 also exerts a role before Ag encounter by promoting the survival of naive recirculating B cells. This aspect of CD19 signaling was first suggested by the analysis of mixed bone marrow chimeras, wherein CD19-/- B cells fail to effectively compete with wild-type B cells to reconstitute the peripheral B cell compartment. Consistent with this observation, Bromodeoxyuridine- and CFSE-labeling studies reveal a shorter in vivo life span for CD19-/- B cells vs their wild-type counterparts. Moreover, we find that CD19 is necessary for propagation of BCR-induced survival signals and thus may contribute to homeostatic mechanisms of tonic signaling. To determine whether provision of a constitutive survival signal could compensate for the loss of CD19 in vivo, Bcl-2-transgenic mice were bred onto the CD19-/- background. Here, we observe an increase in follicular B cell numbers and selective recovery of the MZ B cell compartment. Together these findings suggest that maintenance of the follicular and MZ B cell compartments require CD19-dependent survival signals.

Human CD5 promotes B-cell survival through stimulation of autocrine IL-10 production

CD5 is a negative regulator of B-cell receptor (BCR) signaling that is up-regulated after BCR stimulation and likely contributes to B-cell tolerance in vivo. However, CD5 is constitutively expressed on the B-1 subset of B cells. Contrary to CD5(-) B-2 B cells, B-1 B cells are long-lived because of autocrine interleukin-10 (IL-10) production through unknown mechanisms. We demonstrate herein a direct relationship between CD5 expression and IL-10 production. Human peripheral blood CD5(+) B cells produce more IL-10 than CD5(-) B cells after BCR activation. Introducing CD5 into CD5(-) B cells induces the production of IL-10 by activating its promoter and the synthesis of its mRNA. The cytoplasmic domain of CD5 is sufficient for this process. CD5 also protects normal human B cells from apoptosis after BCR stimulation while reducing the BCR-induced Ca(2+) response. We conclude that CD5 supports the survival of B cells by stimulating IL-10 production and by concurrently exerting negative feedback on BCR-induced signaling events that can promote cell death.

Defective CD19-dependent signaling in B-1a and B-1b B lymphocyte subpopulations

Peritoneal and pleural cavities in mice and humans contain a unique population of B-lymphocytes called B-1 cells that are defective in B cell antigen receptor (BCR) signaling but have an increased propensity to produce autoantibodies. Several molecules such as Btk, Vav, and CD19 known to be important for BCR signaling have been shown to be critical for the development of B-1 cells from undefined precursors. Here we demonstrate that B-1 cell unresponsiveness to BCR cross-linking is in part due to defective signaling through CD19, a molecule known to modulate signaling thresholds in B cells. The defective CD19 signaling is manifested in reduced synergy between mIgM and CD19 to stimulate calcium mobilization in B-1 cells. BCR induced tyrosine phosphorylation of CD19 was transient in B-1 cells while it was prolonged in splenic B-2 cells. In both B-1 and B-2 cells BCR cross-linking induced a modest increase of CD19 associated Lyn, a Src family protein tyrosine kinase (PTK) thought to be important for CD19 phosphorylation. However, the tyrosine phosphorylated CD19 in B-1 cells binds less phosphatidylinositol 3-kinase (PI3-K) compared to B-2 cells. Most interestingly, we find that Vav-1 and Vav-2, proteins thought to be critical for CD19 signal transduction, are severely reduced in B-1 cells resulting in a complete absence of any CD19 associated Vav. Also we showed that both B-1a and B-1b B cells failed to proliferate in response to BCR cross-linking which in part appears to be due to defects in CD19 mediated amplification of BCR induced calcium mobilization.