B-cell anergy: from transgenic models to naturally occurring anergic B cells?

Circulating CD21low B cells in common variable immunodeficiency resemble tissue homing, innate-like B cells

The homeostasis of circulating B cell subsets in the peripheral blood of healthy adults is well regulated, but in disease it can be severely disturbed. Thus, a subgroup of patients with common variable immunodeficiency (CVID) presents with an extraordinary expansion of an unusual B cell population characterized by the low expression of CD21. CD21(low) B cells are polyclonal, unmutated IgM(+)IgD(+) B cells but carry a highly distinct gene expression profile which differs from conventional naïve B cells. Interestingly, while clearly not representing a memory population, they do share several features with the recently defined memory-like tissue, Fc receptor-like 4 positive B cell population in the tonsils of healthy donors. CD21(low) B cells show signs of previous activation and proliferation in vivo, while exhibiting defective calcium signaling and poor proliferation in response to B cell receptor stimulation. CD21(low) B cells express decreased amounts of homeostatic but increased levels of inflammatory chemokine receptors. This might explain their preferential homing to peripheral tissues like the bronchoalveolar space of CVID or the synovium of rheumatoid arthritis patients. Therefore, as a result of the close resemblance to the gene expression profile, phenotype, function and preferential tissue homing of murine B1 B cells, we suggest that CD21(low) B cells represent a human innate-like B cell population.

B cell antigen receptor endocytosis and antigen presentation to T cells require Vav and dynamin

Antigen binding to the B cell antigen receptor (BCR) initiates an array of signaling events. These include endocytosis of ligand-receptor complexes via clathrin-coated pits, trafficking of the internalized ligand to lysosomes, degradation of the associated proteins to peptides, and peptide presentation on nascent major histocompatibility complex class II to T cells. The signal transduction events supporting BCR internalization are not well understood. We have identified a pathway supporting BCR internalization that includes the Vav1 and/or Vav3 isoforms and the GTPase dynamin. Vav1 and -3 are not required for B cell development and maturation, nor for a variety of BCR-induced signaling events nor for BCR signaling leading to major histocompatibility complex class II and CD80 expression, but Vav1 and/or -3 are absolutely required for BCR endocytosis and BCR-induced Rac-GTP loading. This is the first demonstration of a link between Vav and Rac in BCR internalization leading to antigen presentation to T cells.

Inositol 1,4,5-trisphosphate 3-kinase B is a negative regulator of BCR signaling that controls B cell selection and tolerance induction

Inositol 1,4,5-trisphosphate 3-kinase B (or Itpkb) converts inositol 1,4,5-trisphosphate to inositol 1,3,4,5-tetrakisphosphate upon Ag receptor activation and controls the fate and function of lymphocytes. To determine the role of Itpkb in B cell tolerance, Itpkb(-/-) mice were crossed to transgenic mice that express a BCR specific for hen egg lysozyme (IgHEL). B cells from Itpkb(-/-) IgHEL mice possess an anergic phenotype, hypoproliferate in response to cognate Ag, and yet they exhibit enhanced Ag-induced calcium signaling. In IgHEL transgenic mice that also express soluble HEL, lack of Itpkb converts anergy induction to deletion. These data establish Itpkb as a negative regulator of BCR signaling that controls the fate of developing B cells and tolerance induction.

Endocytic sequestration of the B cell antigen receptor and toll-like receptor 9 in anergic cells

In autoimmune prone murine strains, sequential engagement of the B cell antigen receptor (BCR) on the cell surface and toll-like receptors (TLRs) in late endosomes is necessary and sufficient for secretion of autoantibodies. However, ubiquitous nucleoprotein self-antigens fail to elicit productive TLR activation, and break self-tolerance in anergic DNA-reactive B cells. The mechanisms limiting TLR activation in these cells are largely unknown. Here, we demonstrate that in anergic 3H9/Vkappa8 and Ars/A1 B cells the normal endocytic transit of both the ligated BCR and TLR9 into late endosomes is abrogated. The BCR and TLR9 arrest together just outside late endosomes, indicating that they enter this compartment along a single, regulated endocytic route. Access to late endosomes could be restored by reversing anergy through several methods, including conferring genetic susceptibility to autoimmunity, complementing proximal BCR signaling or by preventing BCR binding to self-antigen. Downstream of the BCR, JNK, which is activated in naive but not anergic B cells, regulated entry into late endosomes. Restoration of BCR and TLR9 endocytic trafficking rescued TLR9 activation by BCR-captured ligands. These results indicate that B cell anergy is reinforced by the exclusion of both TLRs and their BCR captured ligands from subcellular environments necessary for TLR activation.

Mesenchymal stem cells and their use as cell replacement therapy and disease modelling tool

Mesenchymal stem cells (MSCs) from adult somatic tissues may differentiate in vitro and in vivo into multiple mesodermal tissues including bone, cartilage, adipose tissue, tendon, ligament or even muscle. MSCs preferentially home to damaged tissues where they exert their therapeutic potential. A striking feature of the MSCs is their low inherent immunogenicity as they induce little, if any, proliferation of allogeneic lymphocytes and antigen-presenting cells. Instead, MSCs appear to be immunosuppressive in vitro. Their multi-lineage differentiation potential coupled to their immuno-privileged properties is being exploited worldwide for both autologous and allo-geneic cell replacement strategies. Here, we introduce the readers to the biology of MSCs and the mechanisms underlying immune tolerance. We then outline potential cell replacement strategies and clinical applications based on the MSCs immunological properties. Ongoing clinical trials for graft-versus-host-disease, haematopoietic recovery after co-transplantation of MSCs along with haematopoietic stem cells and tissue repair are discussed. Finally, we review the emerging area based on the use of MSCs as a target cell subset for either spontaneous or induced neoplastic transformation and, for modelling non-haematological mesenchymal cancers such as sarcomas.

CD25+ T(reg) specifically suppress auto-Ab generation against pancreatic tissue autoantigens

To study B-cell tolerance against non-lymphoid tissue autoantigens, we generated transgenic rat insulin promoter (RIP)-OVA/hen egg lysozyme (HEL) mice expressing the model antigens, OVA and HEL, in pancreatic islets. Their vaccination with OVA or HEL induced far less auto-Ab titers compared with non-transgenic controls. Depletion of CD25(+) cells during immunization completely restored auto-Ab production, but did not affect antibodies against a foreign control antigen. Depletion at later time-points was not effective. OVA-specific CD25(+) FoxP3(+) T(reg) were more frequent in the autoantigen-draining pancreatic LN than in other secondary lymphatics of RIP-OVA/HEL mice. Consistently, B cells were suppressed in that LN and also in the spleen, which is known to concentrate circulating antigen, such as the antigens used for vaccination. Suppression involved preventing expansion of autoreactive B cells in response to autoantigen, reducing antibody production per B-cell and isotype changes. These findings demonstrate that CD25(+) T(reg) suppress auto-Ab production against non-lymphoid tissue antigens in an antigen-specific manner.

Cancer-expanded myeloid-derived suppressor cells induce anergy of NK cells through membrane-bound TGF-beta 1

NK cells, the important effector of innate immunity, play critical roles in the antitumor immunity. Myeloid-derived suppressor cells (MDSC), a population of CD11b(+)Gr-1(+) myeloid cells expanded dramatically during tumor progression, can inhibit T cells and dendritic cells, contributing to tumor immune escape. However, regulation of NK cell innate function by MDSC in tumor-bearing host needs to be investigated. In this study, we found that the function of NK cells from liver and spleen was impaired significantly in all tumor-bearing models, indicating the impairment of hepatic NK cell function by tumor is a universal phenomenon. Then we prepared the orthotopic liver cancer-bearing mice as tumor model to investigate how hepatic NK cells are impaired. We show that down-regulation of NK cell function is inversely correlated with the marked increase of MDSC in liver and spleen. MDSC inhibit cytotoxicity, NKG2D expression, and IFN-gamma production of NK cells both in vitro and in vivo. After incubation with MDSC, NK cells could not be activated to produce IFN-gamma. Furthermore, membrane-bound TGF-beta1 on MDSC is responsible for MDSC-mediated suppression of NK cells. The impaired function of hepatic NK cells in orthotopic liver cancer-bearing mice could be restored by depletion of MDSC, but not regulatory T cells. Therefore, cancer-expanded MDSC can induce anergy of NK cells via membrane-bound TGF-beta1. MDSC, but not regulatory T cells, are main negative regulator of hepatic NK cell function in tumor-bearing host. Our study provides new mechanistic explanations for tumor immune escape.

B cells and immunological tolerance

Work from multiple groups continues to provide additional evidence for the powerful and highly diverse roles, both protective and pathogenic, that B cells play in autoimmune diseases. Similarly, it has become abundantly clear that antibody-independent functions may account for the opposing influences that B cells exercise over other arms of the immune response and ultimately over autoimmunity itself. Finally, it is becoming apparent that the clinical impact of B-cell depletion therapy may be, to a large extent, determined by the functional balance between different B-cell subsets that may be generated by this therapeutic intervention. In this review, we postulate that our perspective of B-cell tolerance and our experimental approach to its understanding are fundamentally changed by this view of B cells. Accordingly, we first discuss current knowledge of B-cell tolerance conventionally defined as the censoring of autoantibody-producing B cells (with an emphasis on human B cells). Therefore, we discuss a different model that contemplates B cells not only as targets of tolerance but also as mediators of tolerance. This model is based on the notion that the onset of clinical autoimmune disease may require a B-cell gain-of-pathogenic function (or a B-cell loss-of-regulatory-function) and that accordingly, disease remission may depend on the restoration of the physiological balance between B-cell pathogenic and protective functions.

Functional anergy in a subpopulation of naive B cells from healthy humans that express autoreactive immunoglobulin receptors

Self-reactive B cells not controlled by receptor editing or clonal deletion may become anergic. We report that fully mature human B cells negative for surface IgM and retaining only IgD are autoreactive and functionally attenuated (referred to as naive IgD⁺IgM⁻ B cells [B_ND]). These B_ND cells typically make up 2.5% of B cells in the peripheral blood, have antibody variable region genes in germline (unmutated) configuration, and, by all current measures, are fully mature. Analysis of 95 recombinant antibodies expressed from the variable genes of single B_ND cells demonstrated that they are predominantly autoreactive, binding to HEp-2 cell antigens and DNA. Upon B cell receptor cross-linkage, B_ND cells have a reduced capacity to mobilize intracellular calcium or phosphorylate tyrosines, demonstrating that they are anergic. However, intense stimulation causes B_ND cells to fully respond, suggesting that these cells could be the precursors of autoantibody secreting plasma cells in autoimmune diseases such as systemic lupus erythematosus or rheumatoid arthritis. This is the first identification of a distinct mature human B cell subset that is naturally autoreactive and controlled by the tolerizing mechanism of functional anergy.

Role of B cells in systemic lupus erythematosus and rheumatoid arthritis

B cell tolerance to many self-proteins is actively maintained by either purging self-reactive B receptors through clonal deletion and receptor editing, or by functional silencing known as anergy. However, these processes are clearly incomplete as B cell driven autoimmune diseases still occur. The significance of B cells in two such diseases, rheumatoid arthritis and systemic lupus erythematosus, is highlighted by the ameliorative effects of B cell depletion. It remains to be determined, however, whether the key role of the B cell in autoimmune disease is autoantibody production or another antibody-independent function.

Regulation of B-cell entry into the cell cycle

B cells are induced to enter the cell cycle by stimuli including ligation of the B-cell receptor (BCR) complex and Toll-like receptor (TLR) agonists. This review discusses the contribution of several molecules, which act at distinct steps in B-cell activation. The adapter molecule Bam32 (B-lymphocyte adapter of 32 kDa) helps promote BCR-induced cell cycle entry, while the secondary messenger superoxide has the opposite effect. Bam32 and superoxide may fine tune BCR-induced activation by competing for the same limited resources, namely Rac1 and the plasma membrane phospholipid PI(3,4)P(2). The co-receptor CD22 can inhibit BCR-induced proliferation by binding to novel CD22 ligands. Finally, regulators of B-cell survival and death also play roles in B-cell transit through the cell cycle. Caspase 6 negatively regulates CD40- and TLR-dependent G(1) entry, while acting later in the cell cycle to promote S-phase entry. Caspase 6 deficiency predisposes B cells to differentiate rather than proliferate after stimulation. Bim, a pro-apoptotic Bcl-2 family member, exerts a positive regulatory effect on cell cycle entry, which is opposed by Bcl-2. New insights into what regulates B-cell transit through the cell cycle may lead to thoughtful design of highly selective drugs that target pathogenic B cells.

Maintenance of tolerance by regulation of anti-myeloperoxidase B cells

Anti-neutrophil cytoplasmic autoantibodies directed toward myeloperoxidase or proteinase 3 are detected in sera of patients with small vessel vasculitis and participate in the pathogenesis of this disease. Autoantibodies develop when self-reactive B cells escape the regulation that ensures self-tolerance. In this study, regulation of anti-myeloperoxidase B cells was examined in mice that express an anti-myeloperoxidase Vkappa1C-Jkappa5 light-chain transgene, which confers anti-myeloperoxidase specificity when combined with a variety of heavy chains. Vkappa1C-Jkappa5 transgenic mice have splenic anti-myeloperoxidase B cells but do not produce circulating anti-myeloperoxidase antibodies. Two groups of transgenic mice that differed by their relative dosage of the transgene were compared; high-copy mice had a mean relative transgene dosage of 1.92 compared with 1.02 in the low-copy mice. These mice exhibited a 90 and 60% decrease in mature follicular B cells, respectively. High-copy mice were characterized by a large population of anti-myeloperoxidase B cells, a preponderance of B-1 cells, and an increased percentage of apoptotic myeloperoxidase-binding B cells. Low-copy mice had similar changes in B cell phenotype with the exception of an expanded marginal zone population. B cells from low-copy mice but not high-copy mice produced anti-myeloperoxidase antibodies after stimulation with lipopolysaccharide. These results indicate that tolerance to myeloperoxidase is maintained by central and peripheral deletion and that some myeloperoxidase-binding B cells are positively selected into the marginal zone and B-1 B cell subsets. A defect in these regulatory pathways could result in autoimmune disease.

Tolerance of NK cells encountering their viral ligand during development

During development, T and B cells encountering their cognate ligands via antigen-specific receptors are deleted or rendered anergic. Like T and B cells, natural killer (NK) cells express certain receptors, such as Ly49H, associated with immunoreceptor tyrosine-based activation motif-bearing adaptor proteins that transmit activating signals through Syk family kinases. Ly49H binds with high affinity to a mouse cytomegalovirus (MCMV)-encoded glycoprotein, m157, but does not recognize self-antigens. For comparison with the behavior of immature T and B cells exposed to foreign antigens, we addressed the fate of Ly49H(+) NK cells that encountered their viral ligand during development by retroviral transduction of bone marrow stem cells with m157. In chimeric mice expressing m157, we observed a reduction in Ly49H(+) NK cells in multiple tissues and less Ly49H on the cell surface. NK cells exposed to m157 during development appeared less mature, produced less interferon gamma when stimulated through Ly49H, and were unable to kill m157-bearing target cells. After MCMV infection, these NK cells were severely impaired in their ability to proliferate. Thus, if immature NK cells encounter ligands for their activating receptors, regulatory mechanisms exist to keep these cells in an unresponsive state.

Constitutive activation of distinct BCR-signaling pathways in a subset of CLL patients: a molecular signature of anergy

Stimulation through the B-cell antigen receptor (BCR) is believed to be involved in the natural history of chronic lymphocytic leukemia (CLL). Some cases respond to the in vitro cross-linking of surface immunoglobulin (sIg) with effective activation. In contrast, the remaining cases do not respond to such stimulation, thereby resembling B cells anergized after antigen encounter in vivo. However the biochemical differences between the 2 groups are ill defined, and in humans the term B-cell anergy lacks a molecular definition. We examined the expression and activation of key molecules involved in signaling pathways originating from the BCR, and we report that a proportion of CLL patients (a) expresses constitutively phosphorylated extracellular signal-regulated kinase (ERK)1/2 in the absence of AKT activation; (b) displays constitutive phosphorylation of MEK1/2 and increased nuclear factor of activated T cells (NF-AT) transactivation; and (c) is characterized by cellular unresponsiveness to sIg ligation. This molecular profile recapitulates the signaling pattern of anergic murine B cells. Our data indicate that constitutive activation of mitogen activated protein (MAP) kinase signaling pathway along with NF-AT transactivation in the absence of AKT activation may also represent the molecular signature of anergic human B lymphocytes. CLL cases with this signature may be taken as a human model of anergic B cells aberrantly expanded.

Tracking Differential Gene Expression in MRL/MpJ Versus C57BL/6 Anergic B Cells: Molecular Markers of Autoimmunity

Background

Anergy is a key mechanism controlling expression of autoreactive B cells and a major site for failed regulation in autoimmune diseases. Yet the molecular basis for this differentiated cell state remains poorly understood. The current lack of well-characterized surface or molecular markers hinders the isolation of anergic cells for further study. Global gene profiling recently identified transcripts whose expression differentiates anergic from naïve B cells in model mouse systems. The objective of the current study was to evaluate the molecular and cellular processes that differentiate anergic cells that develop in the healthy C57BL/6 (B6) milieu from those that develop in the autoimmune-prone MRL/MpJ (MRL) background. This approach takes advantage of B6 and MRL mice bearing an anti-laminin Ig transgene with a well characterized anergic B cell phenotype.

Results

Global gene expression was evaluated in purified transgenic B cells using Operon version 3.0 oligonucleotide microarray assaying >31,000 oligoprobes. Genes with a 2-fold expression difference in B6 as compared to MRL anergic B cells were identified. Expression of selected genes was confirmed using quantitative RT-PCR. This approach identified 43 probes corresponding to 37 characterized genes, including Ptpn22, CD74, Birc1f/Naip, and Ctla4, as differentially expressed in anergic B cells in the two strains. Gene Ontology classification identified differentiation, cell cycle, proliferation, development, apoptosis, and cell death as prominently represented ontology groups. Ingenuity Pathway Analysis identified two major networks incorporating 27 qualifying genes. Network 1 centers on beta-estradiol and TP53, and Network 2 encompasses RB1, p38 MAPK, and NFkB cell growth, proliferation, and cell cycle signaling pathways.

Conclusion

Using microarray analysis we identified 37 characterized genes and two functional pathways engaged in maintenance of B cell anergy for which expression is distorted by underlying autoimmune genetic susceptibility. This approach identifes a new biological role for multiple genes and potential new therapeutic targets in autoimmunity.

Virus-like display of a neo-self antigen reverses B cell anergy in a B cell receptor transgenic mouse model

The ability to distinguish between self and foreign Ags is a central feature of immune recognition. For B cells, however, immune tolerance is not absolute, and factors that include Ag valency, the availability of T help, and polyclonal B cell stimuli can influence the induction of autoantibody responses. Here, we evaluated whether multivalent virus-like particle (VLP)-based immunogens could induce autoantibody responses in well-characterized transgenic (Tg) mice that express a soluble form of hen egg lysozyme (HEL) and in which B cell tolerance to HEL is maintained by anergy. Immunization with multivalent VLP-arrayed HEL, but not a trivalent form of HEL, induced high-titer Ab responses against HEL in both soluble HEL Tg mice and double Tg mice that also express a monoclonal HEL-specific BCR. Induction of autoantibodies against HEL was not dependent on coadministration of strong adjuvants, such as CFA. In contrast to previous data showing the T-independent induction of Abs to foreign epitopes on VLPs, the ability of HEL-conjugated VLPs to induce anti-HEL Abs in tolerant mice was dependent on the presence of CD4(+) Th cells, and could be enhanced by the presence of pre-existing cognate T cells. In in vitro studies, VLP-conjugated HEL was more potent than trivalent HEL in up-regulating surface activation markers on purified anergic B cells. Moreover, immunization with VLP-HEL reversed B cell anergy in vivo in an adoptive transfer model. Thus, Ag multivalency and T help cooperate to reverse B cell anergy, a major mechanism of B cell tolerance.

Cutting edge: Complement (C3d)-linked antigens break B cell anergy

C3dg adducts of Ag can coligate complement receptor type 2 (CR2; CD21) and the B cell Ag receptor. This interaction significantly amplifies BCR-mediated signals in Ag-naive wild-type mice, lowering the threshold for B cell activation and the generation of humoral immune responses as much as 1000-fold. In this study we demonstrate that CR2-mediated complementation of BCR signals can also overcome B cell anergy. Unlike Ag alone, BCR/CR2 costimulation (Ars-CCG/C3dg complexes) of anergic Ars/A1 B cells led to Ca(2+) mobilization in vitro and the production of autoantibodies in vivo. Interestingly, the in vivo immune response of anergic cells occurs without the formation of germinal centers. These results suggest that the Ag unresponsiveness of anergic B cells can be overcome by cross-reactive (self-mimicking) Ags that have been complement-opsonized. This mechanism may place individuals exposed to complement-fixing bacteria at risk for autoimmunity.

Molecular mechanisms of CD4+ T-cell anergy

Directing both innate and adaptive immune responses against foreign pathogens with correct timing, location and specificity is a fundamental objective for the immune system. Full activation of CD4+ T cells requires the binding of peptide-MHC complexes coupled with accessory signals provided by the antigen-presenting cell. However, aberrant activation of the T-cell receptor alone in mature T cells can produce a long-lived state of functional unresponsiveness, known as anergy. Recent studies probing both immune signalling pathways and the ubiquitin-proteasome system have helped to refine and elaborate current models for the molecular mechanisms underlying T-cell anergy. Controlling anergy induction and maintenance will be a key component in the future to mitigate unwanted T-cell activation that leads to autoimmune disease.