Identification of a precursor to phosphatidyl choline-specific B-1 cells suggesting that B-1 cells differentiate from splenic conventional B cells in vivo: cyclosporin A blocks differentiation to B-1

Mitochondrial Permeability Transition in Stem Cells, Development, and Disease

The mitochondrial permeability transition (mPT) is a process that permits rapid exchange of small molecules across the inner mitochondrial membrane (IMM) and thus plays a vital role in mitochondrial function and cellular signaling. Formation of the pore that mediates this flux is well-documented in injury and disease but its regulation has also emerged as critical to the fate of stem cells during embryonic development. The precise molecular composition of the mPTP has been enigmatic, with far more genetic studies eliminating molecular candidates than confirming them. Rigorous studies in the recent decade have implicated central involvement of the F₁F_o ATP synthase, or complex V of the electron transport chain, and continue to confirm a regulatory role for Cyclophilin D (CypD), encoded by Ppif, in modulating the sensitivity of the pore to opening. A host of endogenous molecules have been shown to trigger flux characteristic of mPT, including positive regulators such as calcium ions, reactive oxygen species, inorganic phosphate, and fatty acids. Conductance of the pore has been described as low or high, and reversibility of pore opening appears to correspond with the relative abundance of negative regulators of mPT such as adenine nucleotides, hydrogen ion, and divalent cations that compete for calcium-binding sites in the mPTP. Current models suggest that distinct pores could be responsible for differing reversibility and conductance depending upon cellular context. Indeed, irreversible propagation of mPT inevitably leads to collapse of transmembrane potential, arrest of ATP synthesis, mitochondrial swelling, and cell death. Future studies should clarify ambiguities in mPTP structure and reveal new roles for mPT in dictating specialized cellular functions beyond cell survival that are tied to mitochondrial fitness including stem cell self-renewal and fate. The focus of this review is to describe contemporary models of the mPTP and highlight how pore activity impacts stem cells and development.

The mitochondrial permeability transition pore: an evolving concept critical for cell life and death

In this review, we summarize current knowledge of perhaps one of the most intriguing phenomena in cell biology: the mitochondrial permeability transition pore (mPTP). This phenomenon, which was initially observed as a sudden loss of inner mitochondrial membrane impermeability caused by excessive calcium, has been studied for almost 50 years, and still no definitive answer has been provided regarding its mechanisms. From its initial consideration as an in vitro artifact to the current notion that the mPTP is a phenomenon with physiological and pathological implications, a long road has been travelled. We here summarize the role of mitochondria in cytosolic calcium control and the evolving concepts regarding the mitochondrial permeability transition (mPT) and the mPTP. We show how the evolving mPTP models and mechanisms, which involve many proposed mitochondrial protein components, have arisen from methodological advances and more complex biological models. We describe how scientific progress and methodological advances have allowed milestone discoveries on mPTP regulation and composition and its recognition as a valid target for drug development and a critical component of mitochondrial biology.

Fc Receptor-Like 6 (FCRL6) Discloses Progenitor B Cell Heterogeneity That Correlates With Pre-BCR Dependent and Independent Pathways of Natural Antibody Selection

B-1a cells produce "natural" antibodies (Abs) to neutralize pathogens and clear neo self-antigens, but the fundamental selection mechanisms that shape their polyreactive repertoires are poorly understood. Here, we identified a B cell progenitor subset defined by Fc receptor-like 6 (FCRL6) expression, harboring innate-like defense, migration, and differentiation properties conducive for natural Ab generation. Compared to FCRL6- pro B cells, the repressed mitotic, DNA damage repair, and signaling activity of FCRL6+ progenitors, yielded VH repertoires with biased distal Ighv segment accessibility, constrained diversity, and hydrophobic and charged CDR-H3 sequences. Beyond nascent autoreactivity, VH11 productivity, which predominates phosphatidylcholine-specific B-1a B cell receptors (BCRs), was higher for FCRL6+ cells as was pre-BCR formation, which was required for Myc induction and VH11, but not VH12, B-1a development. Thus, FCRL6 revealed unexpected heterogeneity in the developmental origins, regulation, and selection of natural Abs at the pre-BCR checkpoint with implications for autoimmunity and lymphoproliferative disorders.

Cell lines generated from a chronic lymphocytic leukemia mouse model exhibit constitutive Btk and Akt signaling

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature CD5⁺ B cells in blood. Spontaneous apoptosis of CLL cells in vitro has hampered in-depth investigation of CLL pathogenesis. Here we describe the generation of three monoclonal mouse cell lines, EMC2, EMC4 and EMC6, from the IgH.TEμ CLL mouse model based on sporadic expression of SV40 large T antigen. The cell lines exhibit a stable CD5⁺CD43⁺IgM⁺CD19⁺ CLL phenotype in culture and can be adoptively transferred into Rag1^−/− mice. RNA-seq analysis revealed only minor differences between the cell lines and their primary tumors and suggested that NF-κB and mTOR signaling pathways were involved in cell line outgrowth. In vitro survival and proliferation was dependent on constitutive phosphorylation of Bruton's tyrosine kinase (Btk) at Y551/Y223, and Akt(S473). Treatment of the cell lines with small molecule inhibitors specific for Btk (ibrutinib) or PI3K (idelalisib), which is upstream of Akt, resulted in reduced viability, proliferation and fibronectin-dependent cell adhesion. Treatment of cell line-engrafted Rag1^−/− mice with ibrutinib was associated with transient lymphocytosis, reduced splenomegaly and increased overall survival. Thus, by generating stable cell lines we established a novel platform for in vitro and in vivo investigation of CLL signal transduction and treatment modalities.

Development or disease: duality of the mitochondrial permeability transition pore

Mitochondria is not only a dynamic organelle that produces ATP, but is also an important contributor to cell functions in both development and cell death processes. These paradoxical functions of mitochondria are partially regulated by the mitochondrial permeability transition pore (mPTP), a high-conductance channel that can induce loss of mitochondrial membrane potential, impairment of cellular calcium homeostasis, oxidative stress, and a decrease in ATP production upon pathological activation. Interestingly, despite their different etiologies, several neurodegenerative diseases and heart ischemic injuries share mitochondrial dysfunction as a common element. Generally, mitochondrial impairment is triggered by calcium deregulation that could lead to mPTP opening and cell death. Several studies have shown that opening of the mPTP not only induces mitochondrial damage and cell death, but is also a physiological mechanism involved in different cellular functions. The mPTP participates in regular calcium-release mechanisms that are required for proper metabolic regulation; it is hypothesized that the transient opening of this structure could be the principal mediator of cardiac and brain development. The mPTP also plays a role in protecting against different brain and cardiac disorders in the elderly population. Therefore, the aim of this work was to discuss different studies that show this controversial characteristic of the mPTP; although mPTP is normally associated with several pathological events, new critical findings suggest its importance in mitochondrial function and cell development.

Essential role for the transcription factor Bhlhe41 in regulating the development, self-renewal and BCR repertoire of B-1a cells

Innate-like B-1a cells provide a first line of defense against pathogens, yet little is known about their transcriptional control. Here we identified an essential role of the transcription factor Bhlhe41, with a lesser contribution of Bhlhe40, in controlling late stages of B-1a cell differentiation. Bhlhe41^–/–Bhlhe40^–/– B-1a cells were severely reduced as compared to their wild-type counterparts. Mutant B-1a cells exhibited an abnormal cell-surface phenotype and altered B-cell receptor (BCR) repertoire exemplified by loss of the phosphatidylcholine-specific V_H12/Vκ4 BCR. Expression of a pre-rearranged V_H12/Vκ4 BCR failed to rescue the mutant phenotype and revealed enhanced proliferation accompanied with increased cell death. Bhlhe41 directly repressed the expression of cell cycle regulators and inhibitors of BCR signaling, while enabling pro-survival cytokine signaling. Thus, Bhlhe41 controls the development, BCR repertoire and self-renewal of B-1a cells.

Permeability transition pore-mediated mitochondrial superoxide flashes regulate cortical neural progenitor differentiation

In the process of neurogenesis, neural progenitor cells (NPCs) cease dividing and differentiate into postmitotic neurons that grow dendrites and an axon, become excitable, and establish synapses with other neurons. Mitochondrial biogenesis and aerobic metabolism provide energy substrates required to support the differentiation, growth and synaptic activity of neurons. Mitochondria may also serve signaling functions and, in this regard, it was recently reported that mitochondria can generate rapid bursts of superoxide (superoxide flashes), the frequency of which changes in response to environmental conditions and signals including oxygen levels and Ca²⁺ fluxes. Here we show that the frequency of mitochondrial superoxide flashes increases as embryonic cerebral cortical neurons differentiate from NPCs, and provide evidence that the superoxide flashes serve a signaling function that is critical for the differentiation process. The superoxide flashes are mediated by mitochondrial permeability transition pore (mPTP) opening, and pharmacological inhibition of the mPTP suppresses neuronal differentiation. Moreover, superoxide flashes and neuronal differentiation are inhibited by scavenging of mitochondrial superoxide. Conversely, manipulations that increase superoxide flash frequency accelerate neuronal differentiation. Our findings reveal a regulatory role for mitochondrial superoxide flashes, mediated by mPTP opening, in neuronal differentiation.

Autoantigen can promote progression to a more aggressive TCL1 leukemia by selecting variants with enhanced B-cell receptor signaling

(Auto)antigen engagement by the B-cell receptor (BCR) and possibly the sites where this occurs influence the outcome of chronic lymphocytic leukemia (CLL). To test if selection for autoreactivity leads to increased aggressiveness and if this selection plays out equally in primary and secondary tissues, we used T-cell leukemia (TCL)1 cells reactive with the autoantigen phosphatidylcholine (PtC). After repeated transfers of splenic lymphocytes from a single mouse with oligoclonal PtC-reactive cells, outgrowth of cells expressing a single IGHV-D-J rearrangement and superior PtC-binding and disease virulence occurred. In secondary tissues, increased PtC-binding correlated with enhanced BCR signaling and cell proliferation, whereas reduced signaling and division of cells from the same clone was documented in cells residing in the bone marrow, blood, and peritoneum, even though cells from the last site had highest surface membrane IgM density. Gene-expression analyses revealed reciprocal changes of genes involved in BCR-, CD40-, and PI3K-signaling between splenic and peritoneal cells. Our results suggest autoantigen-stimulated BCR signaling in secondary tissues promotes selection, expansion, and disease progression by activating pro-oncogenic signaling pathways, and that--outside secondary lymphoid tissues--clonal evolution is retarded by diminished BCR-signaling. This transferrable, antigenic-specific murine B-cell clone (TCL1-192) provides a platform to study the types and sites of antigen-BCR interactions and genetic alterations that result and may have relevance to patients.

Killer B lymphocytes: the evidence and the potential

Immune regulation plays a critical role in controlling potentially dangerous inflammation and maintaining health. The Fas ligand/Fas receptor axis has been studied extensively as a mechanism of killing T cells and other cells during infections, autoimmunity, and cancer. FasL expression has been primarily attributed to activated T cells and NK cells. Evidence has emerged that B lymphocytes can express FasL and other death-inducing ligands, and can mediate cell death under many circumstances. Among B cell subsets, the expression of both Fas ligand and IL-10 is highest on the CD5(+) B cell population, suggesting that CD5(+) B cells may have a specialized regulatory function. The relevance of killer B cells to normal immune regulation, disease pathogenesis, and inflammation is discussed.

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.

The persistent elimination of B cells responding to blood group A carbohydrates by synthetic group A carbohydrates and B-1 cell differentiation blockade: novel concept in preventing antibody-mediated rejection in ABO-incompatible transplantation

We demonstrated a novel strategy for specific and persistent inhibition of antibody (Ab) production against blood group A or B carbohydrate determinants necessary for successful ABO-incompatible transplantation. Similar to human blood group O or B individuals, mice have naturally occurring Abs against human blood group A carbohydrates in their sera. B cells with receptors for A carbohydrates in mice belonging to the CD5+CD11b+B-1a subset have phenotypic properties similar to those of human B cells. These cells could be temporarily eliminated by injecting synthetic A carbohydrates (GalNAcα1–3, Fucα1–2Gal) conjugated to bovine serum albumin (A-BSA) and anti-BSA Abs. In mice that received the injection of A-BSA/anti-BSA Abs, the serum levels of anti-A IgM were reduced, but immunization with human A erythrocytes resulted in increased serum levels of anti-A Abs. When combined with cyclosporin A (CsA) treatment, which blocks B-1a cell differentiation, and treatment with A-BSA/anti-BSA Abs, the serum levels of anti-A Abs were persistently undetectable in the mice even after the immunization. B cells with receptors for A carbohydrates were markedly reduced in these mice. These results are consistent with the hypotheses that treatment with A-BSA/anti-BSA Abs temporarily depletes B cells responding to A determinants, and CsA treatment prevents the replenishment of these 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.

EBV latent membrane protein 2A induces autoreactive B cell activation and TLR hypersensitivity

EBV is associated with systemic lupus erythematosus (SLE), but how it might contribute to the etiology is not clear. Since EBV-encoded latent membrane protein 2A (LMP2A) interferes with normal B cell differentiation and function, we sought to determine its effect on B cell tolerance. Mice transgenic for both LMP2A and the Ig transgene 2-12H specific for the ribonucleoprotein Smith (Sm), a target of the immune system in SLE, develop a spontaneous anti-Sm response. LMP2A allows anti-Sm B cells to overcome the regulatory checkpoint at the early preplasma cell stage by a self-Ag-dependent mechanism. LMP2A induces a heightened sensitivity to TLR ligand stimulation, resulting in increased proliferation or Ab-secreting cell differentiation or both. Thus, we propose a model whereby LMP2A induces hypersensitivity to TLR stimulation, leading to activation of anti-Sm B cells through the BCR/TLR pathway. These data further implicate TLRs in the etiology of SLE and suggest a mechanistic link between EBV infection and SLE.

Peritoneal B-2 cells comprise a distinct B-2 cell population with B-1b-like characteristics

B-1 and B-2 cells are lymphocyte populations that differ in development, surface marker expression, tissue localization, and function. Though mainly found in the spleen, lymph nodes, and circulation of mice, small numbers of B-2 cells are found in the peritoneal cavity, a site predominantly populated by B-1 cells. Here, we characterized peritoneal B-2 cells, and determined their relationship to B-1 cells. We found that peritoneal B-2 cells appear to be intermediate between splenic B-2 and peritoneal B-1 cells in terms of surface marker expression of B220, CD80, and CD43, expression of several marker genes, and in vitro viability and IgM secretion. Adoptive transfer of peritoneal B-2 cells into severe combined immunodeficiency mice resulted in the acquisition of a phenotype reminiscent of B-1b cells, as shown by up-regulation of Mac-1 and CD43, and down-regulation of CD23. Moreover, adoptively transferred peritoneal B-2 cells recapitulated B-1 cell function by producing natural IgM in recipient mice. These data suggest that peritoneal B-2 cells express some characteristics of B-1b cells and that this similarity increases with additional time in the peritoneal cavity.

Proteolytic activity of IgG antibodies from blood of acquired immunodeficiency syndrome patients

Proteolytic activity of polyclonal IgG antibodies (Abs) from the blood of AIDS patients was analyzed for the first time. These Abs were shown to display higher activity in hydrolysis of beta-casein than in hydrolysis of human immunodeficiency virus (HIV)-1 reverse transcriptase (RT) or human serum albumin (HSA). Several abzymatic criteria were applied and it was shown that RT, HSA, and beta-casein hydrolyzing activities are an intrinsic property of polyclonal Abs from AIDS patients. Casein-hydrolyzing Abs were detected in the blood serum for 95% of AIDS patients, and it was shown that they possess serine protease-like catalytic activity. The substrate specificities of polyclonal Ab proteases and typical human proteases are different. Depending on the patient, the IgGs exhibit various pH optima of proteolytic activity. The products of casein hydrolysis by Ab proteases were different from those in the case of trypsin, chymotrypsin, and proteinase K.

Calcineurin Inhibitors Block B-1 Cell Differentiation: The Relevance to Immunosuppressive Treatment in ABO-Incompatible Transplantation

Naturally occurring antibodies (Abs) against human blood group A/B-carbohydrate determinants are a major barrier to ABO-incompatible organ transplantation. We previously described that B cells recognizing the blood group A antigens (Ag) belong to a CD5+ B-1a cell subset that exists in blood group O human peripheral blood as well as in mice Recent evidence suggests that B-1a cells are selected by a T-independent type 2 (TI-2) Ag with repetitive arrays of epitopes which can promote BCR cross-linking. In support of the induced-differentiation model, CD5 can be induced on spleen B-2 cells in vitro after stimulation via surface IgM receptors, an induction that is blocked by cyclosporine (CsA). We examined whether mouse peritoneal cavity CD5+ B-1a cells could be reduced by CsA/tacrolimus. For 2 weeks, daily intraperitoneal (i.p.) injection of various doses of CsA/tacrolimus decreased the percentage of PerC CD5+ B-1a cells and increased B-2 cells in a dose-dependent fashion. However, this treatment had no impact on anti-A Abs, suggesting that Ab-producing cells may be resistant to calcineurin inhibitors. We speculate that specific elimination of the anti-A Ab-producing cells with subsequent CsA/tacrolimus therapy might induce lasting inhibition of anti-A Ab production in ABO-incompatible organ transplantation.

Regulation of B-cell development by antibody specificity

Although autoantigen-induced negative selection plays an important role in shaping the mature B-cell repertoire, studies in recent years have suggested that differentiation into any of the three mature B-cell subsets (marginal zone B cells, follicular B cells and B-1 B cells) is not a passive product of autoreactive B-cell elimination. Instead, evidence suggests that entry into a mature subset involves active B-cell receptor signaling and self-antigen-mediated positive selection.

Morphological characterization of mouse B-1 cells

At least three B cell subsets, B-1a, B-1b and B-2 are present circulating peripherally in the mouse. In these animals, B-1 cells constitute a minor fraction of B cells in spleen and are absent in lymph nodes although they represent the main B cell population in peritoneal and pleural cavities. Currently these cells are identified by a surface phenotypic repertoire; they express Mac-1, IgM(high), and B220(low). B-1a cells express CDS. The aim of this work emerged from the fact that the morphology of B-1 cells is not fully characterized. Here we identified B-1 cells using colloidal gold immunocytochemical assays and purified B-1 cells from supernatants of adherent peritoneal cell cultures by a magnetic bead technique. These techniques lead us to demonstrate that, in mice, either B-1a or B-1b cells have a unique morphology distinct from that of B-2 cells.

Mechanisms of Peritoneal B-1a Cells Accumulation Induced by Murine Lupus Susceptibility LocusSle2

The abundance of B-1a cells found in the peritoneal cavity of mice is under genetic control. The lupus-prone mouse New Zealand Black and New Zealand White (NZB x NZW)F(1) and its derivative NZM2410 are among the strains with the highest numbers of peritoneal B1-a cells. We have previously identified an NZM2410 genetic locus, Sle2, which is associated with the production of large numbers of B-1a cells. In this paper, we examined the mechanisms responsible for this phenotype by comparing congenic C57BL/6 mice with or without Sle2. Fetal livers generated more B-1a cells in B6.Sle2 mice, providing them with a greater starting number of B-1a cells early in life. Sle2-expressing B1-a cells proliferated significantly more in vivo than their B6 counterparts, and reciprocal adoptive transfers showed that this phenotype is intrinsic to Sle2 peritoneal B cells. The rate of apoptosis detected was significantly lower in B6.Sle2 peritoneal cavity B-1a cells than in B6, with or without exogenous B cell receptor cross-linking. Increased proliferation and decreased apoptosis did not affect Sle2 peritoneal B-2 cells. In addition, a significant number of peritoneal cavity B-1a cells were recovered in lethally irradiated B6.Sle2 mice reconstituted with B6.Igh(a) bone marrow, showing radiation resistance in Sle2 B-1a cells or its precursors. Finally, B6.Sle2 adult bone marrow and spleen were a significant source of peritoneal B-1a cells when transferred into B6.Rag2(-/-) mice. This suggests that peritoneal B-1a cells are replenished throughout the animal life span in B6.Sle2 mice. These results show that Sle2 regulates the size of the B-1a cell compartment at multiple developmental checkpoints.

Regulation of BCR signal transduction in B-1 cells requires the expression of the Src family kinase Lck

Although found predominantly in the peritoneal and pleural cavities, B-1 cells are also present in other peripheral tissues such as spleen and lung. While similar in surface phenotypes, such as CD5, all B-1 cells are not equivalent in their response to stimuli. Here, we report that the src family kinase Lck is required to confer the BCR hyporesponsiveness typical of CD5+ B-1 cells and appears involved in the maintenance of their unique function. Splenic B-1 cells express CD5 but not Lck and are not hyporesponsive; however, within the peritoneum, these B-1 cells are induced to express Lck and acquire a hyporesponsive phenotype. Peritoneal B-1 cells from lck-deficient mice, while CD5+, no longer exhibit attenuated BCR signaling. Interestingly, lck-null mice exhibited increased natural antibody levels characteristic of B-1 cells. Taken together, these results demonstrate a key role for Lck in modulating the signaling and cellular fate of B-1 B cells.

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.

Role of different B-cell subsets in the specific and polyclonal immune response to T-independent antigens type 2

Role of different B-cell subsets in the immune response to T-independent antigen type 2 (TI-2) was studied. BALB/c and C57BL/6 mice were immunized by polyvinylpyrrolidon (PVP), and the numbers of antibody- and Ig-forming cells (AFC and IFC, respectively) were determined by ELISPOT method. The number of cells producing non-specific Ig (nIFC) was calculated as the difference between the number of IFC and AFC; the number of nIFC induced by PVP was calculated as the difference between the number of nIFC in immune and control splenocytes. Immunization by PVP induced not only the AFC appearance, but also the increase in the number of the antigen-induced nIFC. The treatment of splenocytes by anti-CD5 antibodies and guinea pig complement reduced the increase in the numbers of newly formed AFC and nIFC to approximately 40% of control level. It means that CD5+ cells play an important role not only in the specific, but also in polyclonal immune response to non-self TI-2. To be sure that the decrease of AFC and nIFC numbers is due to depletion for CD5+ B-cells, but not CD5+ T-cells, splenocytes were separated to B-1 and B-2 subsets, and the numbers of AFC, IFC and nIFC were determined in each B-cell subpopulation separately. The overwhelming majority of newly formed AFC and nIFC was detected in B-1 subset. The numbers of AFC and nIFC in B-1 compartment was approximately 10-fold greater than in B-2 cells. A close parallelism between AFC and nIFC formation was observed. It is concluded that specific and polyclonal immune response to non-self TI-2-PVP-depends mainly on CD5+ B-1 subset.

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.

Peritoneal CD5+ B-1 cells have signaling properties similar to tolerant B cells

CD5(+) B (or B-1) cells are the normal precursors of B cell chronic lymphocytic leukemia. They differ from conventional B (B-2) cells with respect to their phenotype and mitogenic responses and are often secretors of the natural polyreactive antibodies in the serum. The origin of B-1 cells remains controversial, and the relationship between B-1 cells and autoreactive B cells is unclear. Here, we compare the signaling pathways that are activated by the engagement of the B cell antigen receptor (BCR) in B-1 and B-2 cells. Stimulation of the BCR leads to the induced activation of the three major classes of mitogen-activated protein kinases (MAPKs), ERK, JNK, and p38 MAPK, as well as the Akt kinase and the transcription factors nuclear factor of activated T cells (NF-AT) and NF-kappaB in B-2 cells. In contrast, B-1 cells have constitutive activation of ERK and NF-AT but exhibit delayed JNK and lack p38 MAPK and NF-kappaB induction upon BCR cross-linking. The lack of NF-kappaB activation in B-1 cells may be due to a lack of Akt activation in these cells. Furthermore, our study using specific inhibitors reveals that the extended survival of B-1 cells in culture is not due to the constitutive activation of ERK; nor is it due to Akt signaling or Bcl-x(L) up-regulation, since these are not induced in B-1 cells. The current findings of altered MAPK and NF-AT activation and lack of NF-kappaB induction in B-1 cells indicate that these cells have signaling properties similar to tolerant B cells that are chronically exposed to self-antigens. Indeed, BCR stimulation of B-1 cells does not lead to their full activation as indicated by their lack of maximal up-regulation of specific markers such as CD25, CD69, and CD86.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

Intact active bone transplantation synergizes with anti-CD40 ligand therapy to induce B cell tolerance

Blockade of T cell costimulatory pathways can result in the prolongation of allograft survival through the suppression of Th1 responses; however, late allograft rejection is usually accompanied by an emerging allograft-specific humoral response. We have recently determined that intact active bone (IAB) fragments transplanted under the kidney capsule can synergize with transient anti-CD40 ligand (CD40L) treatment to induce robust donor-specific allograft tolerance and suppress the alloantibody response. In this study, we take advantage of the ability of galactosyltransferase-deficient knockout (GT-Ko) mice to respond to the carbohydrate epitope, galactose-alpha1,3-galactose (Gal), to investigate whether IAB plus transient anti-CD40L therapy directly tolerize B cell responses. GT-Ko mice tolerized to Gal-expressing C3H hearts and IAB plus transient anti-CD40L therapy were challenged with pig kidney membranes that express high levels of Gal. The anti-Gal IgM and IgG responses were significantly suppressed in IAB-tolerant mice compared with controls, while the non-Gal anti-pig Ab responses were comparable. The anti-pig T cell cytokine response (IFN-gamma and IL-4) was comparable in IAB-tolerant and control mice. The tolerant state for the anti-Gal IgM response could be reversed with repeated immunization, whereas the tolerant state for the IgG response was robust and resisted repeated immunization. These observations provide an important proof-of-concept that adjunct therapies can synergize with anti-CD40L Abs to tolerize B cell responses independent of their effects on T cells. This model, which does not require mixed chimerism, provides a unique opportunity for investigating the mechanism of peripheral tolerance in a clinically relevant population of carbohydrate-specific B cells.

B-1a B cells that link the innate and adaptive immune responses are lacking in the absence of the spleen

Splenectomized individuals are prone to overwhelming infections with encapsulated bacteria and splenectomy of mice increases susceptibility to streptococcal infections, yet the exact mechanism by which the spleen protects against such infections is unknown. Using congenitally asplenic mice as a model, we show that the spleen is essential for the generation of B-1a cells, a B cell population that cooperates with the innate immune system to control early bacterial and viral growth. Splenectomy of wild-type mice further demonstrated that the spleen is also important for the survival of B-1a cells. Transfer experiments demonstrate that lack of these cells, as opposed to the absence of the spleen per se, is associated with an inability to mount a rapid immune response against streptococcal polysaccharides. Thus, absence of the spleen and the associated increased susceptibility to streptococcal infections is correlated with lack of B-1a B cells. These findings reveal a hitherto unknown role of the spleen in generating and maintaining the B-1a B cell pool.

Age-dependent increase of peritoneal B-1b B cells in SCID mice

The impact of increasing age upon immunoglobulin production and B-lymphocyte generation in "leaky" severe combined immune-defective (SCID) mice was examined by enzyme-linked immunosorbent assay and flow cytometry. By 1 year of age, the mice had normal numbers of B cells in their peritoneal cavity, while their spleen had very few immunoglobulin M-positive (IgM+) cells. The majority of B cells expressed the CD11b marker characteristic of the B-1b subset. B-1a (CD5+) cells were present at a lower frequency and B-2 cells were absent. The frequency of mice producing detectable immunoglobulin increased with age, and isotype diversity within individual mice was variable. IgM production was most frequently observed followed by IgG3 and IgG2a, then IgG1, and finally IgA. The selective persistence of the B-1 B-cell subset in the peritoneal cavity of aging SCID mice is a natural model for the study of those genetic and environmental influences that determine lymphocyte longevity.

The ontogeny of B cells in the thymus of normal, CD3 epsilon knockout (KO), RAG-2 KO and IL-7 transgenic mice

The ontogeny of thymic B cells was determined by three-color flow cytometry and the presence or absence of B cell progenitors confirmed by cell culture experiments. In the thymus of young normal mice, CD117(+), B220(low) pro- and pre-B cells are present but disappear with age. B220(low), CD5(+), B-1 B cells are present in the thymus of older animals following the appearance of similar cells in the peritoneal cavity and blood. In CD3 epsilon gene-deleted mice, the phenotypic progression and number of thymic B cells remains unaltered, showing that blocking T cell development does not automatically result in an increase of thymic B lymphopoiesis. Pro-B cells in RAG-2 knockout mice are found in the fetal and neonatal blood, spleen and thymus, but with increasing age are only found in the bone marrow. B lymphopoiesis in adult IL-7 transgenic mice is dramatically altered with CD117(+) pro- and pre-B cells present in spleen, lymph node and blood. In the thymus of adult IL-7 transgenic mice, the fraction of CD117(+) thymic B cells is significantly increased. These results show that in the steady state, the phenotype of thymic B cells is critically dependent on both mouse age and the phenotype of circulating B cells.

Splenic and peritoneal B-1 cells differ in terms of transcriptional and proliferative features that separate peritoneal B-1 from splenic B-2 cells

B-1 cells constitute a distinct B cell subset with characteristic phenotypic and functional features. B-1 cells are highly represented among peritoneal lymphocytes; substantial numbers of B-1 cells are also located within splenic tissue. Here a number of differences in transcription factor and gene expression were identified that separate peritoneal B-1 and splenic B-2 cells, and then splenic B-1 cells obtained from immunoglobulin transgenic mice were tested for these parameters. Splenic B-1 cells resembled splenic B-2 cells rather than peritoneal B-1 cells in terms of nuclear expression of DNA-binding STAT3, CREB, and PU.1, with respect to transcriptional activation of IL-10, and in the failure to enter cell cycle in response to PMA. Splenic B-1 cells (B-1S) appear to constitute a unique population of B-1 cells, which, while sharing with peritoneal B-1 cells (B-1P) certain phenotypic features, differ from them in transcription factor and gene expression and in signaling for cell cycle progression.

A VH12 transgenic mouse exhibits defects in pre-B cell development and is unable to make IgM+ B cells

V(H)12 B cells undergo stringent selection at multiple checkpoints to favor development of B-1 cells that bind phosphatidylcholine. Selection begins with the V(H) third complementarity-determining region (CDR3) at the pre-B cell stage, in which most V(H)12 pre-B cells are selectively eliminated, enriching for those with V(H)CDR3s of 10 aa and a fourth position Gly (designated 10/G4). To understand this selection, we compared B cell differentiation in mice of two V(H)12 transgenic lines, one with the favored 10/G4 V(H)CDR3 and one with a non-10/G4 V(H)CDR3 of 8 aa and no Gly (8/G0). Both H chains drive B cell differentiation to the small pre-BII cell stage, and induce allelic exclusion and L chain gene rearrangement. However, unlike 10/G4 pre-B cells, 8/G0 pre-B cells are deficient in cell division and unable to differentiate to B cells. We suggest that this is due to poor 8/G0 pre-B cell receptor expression and to an inability to form an 8/G0 B cell receptor. Our findings also suggest that V(H)12 H chains have evolved such that association with surrogate and conventional L chains is most efficient with a 10/G4 CDR3. Thus, selection for phosphatidylcholine-binding B-1 cells is most likely the underlying evolutionary basis for the loss of non-10/G4 pre-B cells.

Anti-Sm B cell differentiation in Ig transgenic MRL/Mp-lpr/lpr mice: altered differentiation and an accelerated response

To determine the regulation of B cells specific for the ribonucleoprotein Sm, a target of the immune system in human and mouse lupus, we have generated mice carrying an anti-Sm H chain transgene (2-12H). Anti-Sm B cells in nonautoimmune 2-12H-transgenic (Tg) mice are functional, but, in the absence of immunization, circulating anti-Sm Ab levels are not different from those of non-Tg mice. In this report, we compare the regulation of anti-Sm B cells in nonautoimmune and autoimmune MRL/Mp-lpr/lpr (MRL/lpr) and bcl-2-22-Tg mice. Activation markers are elevated on splenic and peritoneal anti-Sm B cells of both nonautoimmune and autoimmune genetic backgrounds indicating Ag encounter. Although tolerance to Sm is maintained in 2-12H/bcl-2-22-Tg mice, it is lost in 2-12H-Tg MRL/lpr mice, as the transgene accelerates and increases the prevalence of the anti-Sm response. The 2-12H-Tg MRL/lpr mice have transitional anti-Sm B cells in the spleen similar to nonautoimmune mice. However, in contrast to nonautoimmune mice, there are few if any peritoneal anti-Sm B-1 cells. These data suggest that a defect in B-1 differentiation may be a factor in the loss of tolerance to Sm and provide insight into the low prevalence of the anti-Sm response in lupus.

B1 cells: similarities and differences with other B cell subsets

Since their discovery, B1 B cells' origins and developmental pathways have eluded characterization. In the past year, focus on B1 B cells has shifted dramatically from developmental to functional aspects of these cells. Most advances have been made in describing the physiological activities of B1 cells, including their migration, activation by antigen and role in both autoimmunity and malignancy.

Lupus-specific antiribonucleoprotein B cell tolerance in nonautoimmune mice is maintained by differentiation to B-1 and governed by B cell receptor signaling thresholds

Systemic lupus erythematosus is an autoimmune disease characterized by the presence of autoantibodies. One of the unique targets of the immune system in systemic lupus erythematosus is Sm, a ribonucleoprotein present in all cells. To understand the regulation of B cells specific to the Sm Ag in normal mice, we have generated an Ig H chain transgenic mouse (2-12H Tg). 2-12H Tg mice produce B cells specific for the Sm that remain tolerant due to ignorance. We demonstrate here that anti-Sm B cells of 2-12H Tg mice can differentiate into Sm-specific peritoneal B-1 cells that remain tolerant. Differentiation to B-1 and tolerance are governed by the strength of B cell receptor signaling, since manipulations of the B cell receptor coreceptors CD19 and CD22 affect anti-Sm B cell differentiation and autoantibody production. These results suggest a differentiation scheme in which peripheral ignorance to Sm is maintained in mice by the differentiation of anti-Sm B cells to B-1 cells that have increased activation thresholds.

Cutting edge commentary: origins of B-1 cells

The origin of B-1a cells, a minority population of B cells that express CD5, are abundant in coelomic cavities, and often produce autoantibodies, has been the subject of study for many years. Accumulating evidence demonstrates that the hypothesis that only B cells arising in fetal or neonatal tissues have the potential to become B-1a cells cannot be true. Rather, B cell receptor-mediated signaling initiated by ligation of autoantigen has now been shown to be required for induction of the B-1a phenotype. Furthermore, cells with a functional B-1a phenotype can be induced from adult precursors by appropriate Ag. At the same time, microenvironment-specific events may determine the likelihood that a given B cell, either adult or fetal derived, enters this pathway. CD5 expression and possibly localization to the peritoneum appear to provide some protection to autoreactive cells otherwise slated for elimination.

CD5-positive and CD5-negative human B cells converge to an indistinguishable population on signalling through B-cell receptors and CD40

Whether CD5 on B cells marks a subset functionally distinct from the conventional CD5 negative (CD5neg) adult population or is more an indicator of activation, remains contentious. Here we have investigated whether CD5 positive (CD5pos) and CD5neg B cells can be distinguished in terms of their response to surrogate signals aimed to model, in vitro, T-cell dependent (TD) and T-independent (TI) encounters with antigen in vivo: the predominantly CD5pos B-cell population found in cord blood, CD5 B cells positively selected from tonsils and their CD5neg counterparts, were compared. Neonatal B cells displayed a near-identical phenotype to that of adult CD5pos B cells, being characterized by uniform immunoglobulin M (IgM), immunoglobulin D (IgD), CD23 and CD44 coexpression. When cultured with anti-IgM maintained at high density on CD32-tranfected mouse L cells to model TI responses or on CD40 ligand (CD40L)-bearing L cells (with or without captured anti-IgM) to model TD encounters, DNA synthesis was stimulated to a similar extent in all three populations. Focusing on CD5 and CD23, we found that - although the signals delivered promoted distinct profiles of expression - under each condition of activation, the phenotypes that emerged for adult CD5pos and CD5neg B cells were remarkably similar. Neonatal B cells displayed a greater diminution in CD5 expression than adult CD5pos B cells following CD40 signals but otherwise the two populations again behaved similarly. The inclusion of interleukin-4 (IL-4) to cultures where cells were costimulated via surface (s)IgM and CD40 resulted in a complete loss of CD5 expression and a corresponding hyperexpression of CD23, irrespective of the population studied. The near-identical response of CD5pos and CD5neg B cells to surrogate TD or TI signals in vitro and their convergence to indistinguishable phenotypes is wholly supportive of CD5 being a fluctuating marker of activation rather than it delineating functionally distinct subsets.

The level of expression of mu heavy chain modifies the composition of peripheral B cell subpopulations

The B cell receptor (BCR) has a decisive role in transducing signals required for the development of B cells and their survival in the periphery. However, the processes that initiate these signals remain unclear and concepts of constitutive and ligand-dependent signaling have been proposed. Using a mu-transgenic mouse model, we have analyzed the impact of high surface IgM expression on the composition of the splenic B cell population. kappa-deficient mice homozygous for the H3-mu transgene have B cells with a higher BCR surface density than H3 heterozygous mice. This higher BCR expression is associated with an increase in the percentage and the total number of splenic B cells. In addition, an important proportion of CD23(-)CD21(+) marginal zone (MZ) B cells can be observed in H3 homozygous mice. However, these modifications operate in the absence of impairment of the positive selection process of the H3-mu/lambda1 combination over the H3-mu/lambda2 + 3 ones. These results suggest that (i) a constitutive BCR signaling directly correlated with BCR surface density is responsible for the efficient B cell colonization of the periphery with an accumulation of B cells in the MZ and (ii) a ligand-dependent BCR signal is responsible for the clonotype composition of the mature B cell repertoire.