Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Mechanisms involved in the p62-73 idiopeptide-modulated delay of lupus nephritis in SNF(1) mice

The F(1) progeny of the (SWR × NZB) cross develop a lupus-like disease with high serum titers of autoantibodies, and increased frequency and severity of immune complex-mediated glomerulonephritis in females. In previous work, we found that an idiotypic peptide corresponding to aa62-73 (p62-73) of the heavy chain variable region of autoantibody 540 (Id(LN)F(1)) induced the proliferation of p62-73 idiotype-reactive T cell clones. Further, monthly immunization of pre-nephritic SNF(1) female mice with p62-73 resulted in decreased nephritis and prolonged life spans. Here we show that this treatment modulated proliferative responses to Id(LN)F(1) antigen, including a reduction in the population of idiopeptide-presenting antigen-presenting cells (APCs), as early as two weeks after immunization (10 weeks of age). Th1-type cytokine production was increased at 12 weeks of age. The incidence and severity of nephritis was reduced by 14 weeks compared to controls. Clinical indicators of nephritis, specifically histological evidence of glomerulonephritis and urine protein levels, were reduced by 20 weeks. Together these data suggest that events involved in the mechanism(s) whereby p62-73 immunization delayed nephritis occurred early after immunization, and involved modulation of APCs, B and T cell populations.

The Fas/CD95 Receptor Regulates the Death of Autoreactive B Cells and the Selection of Antigen-Specific B Cells

Cell death receptors have crucial roles in the regulation of immune responses. Here we review recent in vivo data confirming that the Fas death receptor (TNFSR6) on B cells is important for the regulation of autoimmunity since the impairment of only Fas function on B cells results in uncontrolled autoantibody production and autoimmunity. Fas plays a role in the elimination of the non-specific and autoreactive B cells in germinal center, while during the selection of antigen-specific B cells different escape signals ensure the resistance to Fas-mediated apoptosis. Antigen-specific survival such as BCR or MHCII signal or coreceptors (CD19) cooperating with BCR inhibits the formation of death inducing signaling complex. Antigen-specific survival can be reinforced by antigen-independent signals of IL-4 or CD40 overproducing the anti-apoptotic members of the Bcl-2 family proteins.

Autoimmunity in wiskott-Aldrich syndrome: an unsolved enigma

Wiskott-Aldrich Syndrome (WAS) is a severe X-linked Primary Immunodeficiency that affects 1-10 out of 1 million male individuals. WAS is caused by mutations in the WAS Protein (WASP) expressing gene that leads to the absent or reduced expression of the protein. WASP is a cytoplasmic protein that regulates the formation of actin filaments in hematopoietic cells. WASP deficiency causes many immune cell defects both in humans and in the WAS murine model, the Was(-/-) mouse. Both cellular and humoral immune defects in WAS patients contribute to the onset of severe clinical manifestations, in particular microthrombocytopenia, eczema, recurrent infections, and a high susceptibility to develop autoimmunity and malignancies. Autoimmune diseases affect from 22 to 72% of WAS patients and the most common manifestation is autoimmune hemolytic anemia, followed by vasculitis, arthritis, neutropenia, inflammatory bowel disease, and IgA nephropathy. Many groups have widely explored immune cell functionality in WAS partially explaining how cellular defects may lead to pathology. However, the mechanisms underlying the occurrence of autoimmune manifestations have not been clearly described yet. In the present review, we report the most recent progresses in the study of immune cell function in WAS that have started to unveil the mechanisms contributing to autoimmune complications in WAS patients.

Immunological aspects in chronic lymphocytic leukemia (CLL) development

Chronic lymphocytic leukemia (CLL) is unique among B cell malignancies in that the malignant clones can be featured either somatically mutated or unmutated IGVH genes. CLL cells that express unmutated immunoglobulin variable domains likely underwent final development prior to their entry into the germinal center, whereas those that express mutated variable domains likely transited through the germinal center and then underwent final development. Regardless, the cellular origin of CLL remains unknown. The aim of this review is to summarize immunological aspects involved in this process and to provide insights about the complex biology and pathogenesis of this disease. We propose a mechanistic hypothesis to explain the origin of B-CLL clones into our current picture of normal B cell development. In particular, we suggest that unmutated CLL arises from normal B cells with self-reactivity for apoptotic bodies that have undergone receptor editing, CD5 expression, and anergic processes in the bone marrow. Similarly, mutated CLL would arise from cells that, while acquiring self-reactivity for autoantigens-including apoptotic bodies-in germinal centers, are also still subject to tolerization mechanisms, including receptor editing and anergy. We believe that CLL is a proliferation of B lymphocytes selected during clonal expansion through multiple encounters with (auto)antigens, despite the fact that they differ in their state of activation and maturation. Autoantigens and microbial pathogens activate BCR signaling and promote tolerogenic mechanisms such as receptor editing/revision, anergy, CD5+ expression, and somatic hypermutation in CLL B cells. The result of these tolerogenic mechanisms is the survival of CLL B cell clones with similar surface markers and homogeneous gene expression signatures. We suggest that both immunophenotypic surface markers and homogenous gene expression might represent the evidence of several attempts to re-educate self-reactive B cells.

Defective B cell tolerance in adenosine deaminase deficiency is corrected by gene therapy

Adenosine deaminase (ADA) gene defects are among the most common causes of SCID. Restoration of purine metabolism and immune functions can be achieved by enzyme replacement therapy, or more effectively by bone marrow transplant or HSC gene therapy (HSC-GT). However, autoimmune complications and autoantibody production, including anti-nuclear antibodies (ANAs), frequently occur in ADA-SCID patients after treatment. To assess whether ADA deficiency affects the establishment of B cell tolerance, we tested the reactivity of recombinant antibodies isolated from single B cells of ADA-SCID patients before and after HSC-GT. We found that before HSC-GT, new emigrant/transitional and mature naive B cells from ADA-SCID patients contained more autoreactive and ANA-expressing clones, indicative of defective central and peripheral B cell tolerance checkpoints. We further observed impaired B cell receptor (BCR) and TLR functions in B cells after ADA inhibition, which may underlie the defects in B cell tolerance. Strikingly, after HSC-GT, ADA-SCID patients displayed quasi-normal early B cell tolerance checkpoints, as evidenced by restored removal of developing autoreactive and ANA-expressing B cells. Hence, ADA plays an essential role in controlling autoreactive B cell counterselection by regulating BCR and TLR functions.

The establishment of early B cell tolerance in humans: lessons from primary immunodeficiency diseases

Patients with primary immunodeficiency (PID) provide rare opportunities to study the impact of specific gene mutations on the regulation of human B cell tolerance. Alterations in B cell receptor and Toll-like receptor signaling pathways result in a defective central checkpoint and a failure to counterselect developing autoreactive B cells in the bone marrow. In contrast, CD40L- and MHC class II-deficient patients only displayed peripheral B cell tolerance defects, suggesting that decreased numbers of regulatory T cells and increased concentration of B cell activating factor (BAFF) may interfere with the peripheral removal of autoreactive B cells. The pathways regulating B cell tolerance identified in PID patients are likely to be affected in patients with rheumatoid arthritis, systemic lupus erythematosus, and type 1 diabetes who display defective central and peripheral B cell tolerance checkpoints. Indeed, risk alleles encoding variants altering BCR signaling, such as PTPN22 alleles associated with the development of these diseases, interfere with the removal of developing autoreactive B cells. Hence, insights into B cell selection from PID patients are highly relevant to the understanding of the etiology of autoimmune conditions.

Role of Epac proteins in mechanisms of cAMP-dependent immunoregulation

This review presents observations on the role of Epac proteins (exchange protein directly activated by cAMP) in immunoregulation mechanisms. Signaling pathways that involve Epac proteins and their domain organization and functions are considered. The role of Epac1 protein expressed in the immune system cells is especially emphasized. Molecular mechanisms of the cAMP-dependent signal via Epac1 are analyzed in monocytes/macrophages, T-cells, and B-lymphocytes. The role of Epac1 is shown in the regulation of adhesion, leukocyte chemotaxis, as well as in phagocytosis and bacterial killing. The molecular cascade initiated by Epac1 is examined under conditions of antigen activation of T-cells and immature B-lymphocytes.

Autoimmune dysregulation and purine metabolism in adenosine deaminase deficiency

Genetic defects in the adenosine deaminase (ADA) gene are among the most common causes for severe combined immunodeficiency (SCID). ADA-SCID patients suffer from lymphopenia, severely impaired cellular and humoral immunity, failure to thrive, and recurrent infections. Currently available therapeutic options for this otherwise fatal disorder include bone marrow transplantation (BMT), enzyme replacement therapy with bovine ADA (PEG-ADA), or hematopoietic stem cell gene therapy (HSC-GT). Although varying degrees of immune reconstitution can be achieved by these treatments, breakdown of tolerance is a major concern in ADA-SCID. Immune dysregulation such as autoimmune hypothyroidism, diabetes mellitus, hemolytic anemia, and immune thrombocytopenia are frequently observed in milder forms of the disease. However, several reports document similar complications also in patients on long-term PEG-ADA and after BMT or GT treatment. A skewed repertoire and decreased immune functions have been implicated in autoimmunity observed in certain B-cell and/or T-cell immunodeficiencies, but it remains unclear to what extent specific mechanisms of tolerance are affected in ADA deficiency. Herein we provide an overview about ADA-SCID and the autoimmune manifestations reported in these patients before and after treatment. We also assess the value of the ADA-deficient mouse model as a useful tool to study both immune and metabolic disease mechanisms. With focus on regulatory T- and B-cells we discuss the lymphocyte subpopulations particularly prone to contribute to the loss of self-tolerance and onset of autoimmunity in ADA deficiency. Moreover we address which aspects of immune dysregulation are specifically related to alterations in purine metabolism caused by the lack of ADA and the subsequent accumulation of metabolites with immunomodulatory properties.

Altered B cell homeostasis is associated with type I diabetes and carriers of the PTPN22 allelic variant

The PTPN22 genetic variant 1858T, encoding Lyp620W, is associated with multiple autoimmune disorders for which the production of autoantibodies is a common feature, suggesting a loss of B cell tolerance. Lyp620W results in blunted BCR signaling in memory B cells. Because BCR signal strength is tightly coupled to central and peripheral tolerance, we examined whether Lyp620W impacts peripheral B cell homeostasis in healthy individuals heterozygous for the PTPN221858T variant. We found that these subjects display alterations in the composition of the B cell pool that include specific expansion of the transitional and anergic IgD(+)IgM(-)CD27(-) B cell subsets. The PTPN22 1858T variant was further associated with significantly diminished BCR signaling and a resistance to apoptosis in both transitional and naive B cells. Strikingly, parallel changes in both BCR signaling and composition of B cell compartment were observed in type 1 diabetic subjects, irrespective of PTPN22 genotype, revealing a novel immune phenotype and likely shared mechanisms leading to a loss of B cell tolerance. Our combined findings suggest that Lyp620W-mediated effects, due in part to the altered BCR signaling threshold, contribute to breakdown of peripheral tolerance and the entry of autoreactive B cells into the naive B cell compartment.

Exploiting bias in a non-immune human antibody library to predict antigenicity

Non-immune human antibody fragment libraries have generated antigen-binding proteins useful as prospective research, imaging, diagnostic and therapeutic agents. However, because the generation of such libraries relies on cloning antibody sequences from the circulating immune repertoire rather than truly naïve, germline sequences, their composition may reflect the deletion of autoreactive sequences, making them less suited for isolating binding clones to human antigens, but perhaps useful in applications where an in vitro handle on representative circulating antibody diversity is desired. Here we demonstrate that a large non-immune human scFv library is relatively depleted of sequences capable of recognizing human antigens as compared with orthologs antigens. Additionally, because this non-naïve, non-immune library may capture a representative section of antibody diversity, we explore its possible utility in conducting early pre-screens to predict the antigenicity of prospective therapeutics and find a correlation between the clinical immunogenicity of a small panel of protein therapeutics with their propensity for interacting with the library.

Activation-induced cytidine deaminase (AID) is required for B-cell tolerance in humans

Impaired immune functions leading to primary immunodeficiencies often correlate with paradoxical autoimmune complications; patients with hyper-IgM syndromes who are deficient in activation-induced cytidine deaminase (AID), which is required for class-switch recombination and somatic hypermutation, are prone to develop autoimmune diseases. To investigate the impact of AID-deficiency on early B-cell tolerance checkpoints in humans, we tested by ELISA the reactivity of recombinant antibodies isolated from single B cells from AID-deficient patients. New emigrant/transitional and mature naive B cells from AID-deficient patients express an abnormal Ig repertoire and high frequencies of autoreactive antibodies, demonstrating that AID is required for the establishment of both central and peripheral B-cell tolerance. In addition, B-cell tolerance was further breached in AID-deficient patients as illustrated by the detection of anti-nuclear IgM antibodies in the serum of all patients. Thus, we identified a major and previously unsuspected role for AID in the removal of developing autoreactive B cells in humans.

Anergic responses characterize a large fraction of human autoreactive naive B cells expressing low levels of surface IgM

B cell anergy represents an important mechanism of peripheral immunological tolerance for mature autoreactive B cells that escape central tolerance enforced by receptor editing and clonal deletion. Although well documented in mice, the extent of its participation in human B cell tolerance remains to be fully established. In this study, we characterize the functional behavior of strictly defined human naive B cells separated on the basis of their surface IgM (sIgM) expression levels. We demonstrate that cells with lower sIgM levels (IgM(lo)) are impaired in their ability to flux calcium in response to either anti-IgM or anti-IgD cross-linking and contain a significantly increased frequency of autoreactive cells compared with naive B cells with higher levels of sIgM. Phenotypically, in healthy subjects, IgM(lo) cells are characterized by the absence of activation markers, reduction of costimulatory molecules (CD19 and CD21), and increased levels of inhibitory CD22. Functionally, IgM(lo) cells display significantly weaker proliferation, impaired differentiation, and poor Ab production. In aggregate, the data indicate that hyporesponsiveness to BCR cross-linking associated with sIgM downregulation is present in a much larger fraction of all human naive B cells than previously reported and is likely to reflect a state of anergy induced by chronic autoantigen stimulation. Finally, our results indicate that in systemic lupus erythematosus patients, naive IgM(lo) cells display increased levels of CD95 and decreased levels of CD22, a phenotype consistent with enhanced activation of autoreactive naive B cells in this autoimmune disease.

Trafficking of B cell antigen in lymph nodes

The clonal selection theory first proposed by Macfarlane Burnet is a cornerstone of immunology (1). At the time, it revolutionized the thinking of immunologists because it provided a simple explanation for lymphocyte specificity, immunological memory, and elimination of self-reactive clones (2). The experimental demonstration by Nossal & Lederberg (3) that B lymphocytes bear receptors for a single antigen raised the central question of where B lymphocytes encounter antigen. This question has remained mostly unanswered until recently. Advances in techniques such as multiphoton intravital microscopy (4, 5) have provided new insights into the trafficking of B cells and their antigen. In this review, we summarize these advances in the context of our current view of B cell circulation and activation.

IVIg modulates BCR signaling through CD22 and promotes apoptosis in mature human B lymphocytes

Among various mechanisms for interactions with B cells, intravenous immunoglobulin (IVIg) may operate through the insertion of its Fc part into the Fc-γ receptor, or the binding of its sialic acid (SA)–bearing glycans to the negatively regulating CD22 lectin. It appeared that IVIg reduces B lymphocyte viability in a dose- and time-dependent manner. Furthermore, we show by confocal microscopy that SA-positive IgG, but not SA-negative IgG bind to CD22. This interaction reduces the strength of B-cell receptor–mediated signaling trough down-regulating tyrosine phosphorylation of Lyn and the B-cell linker proteins, and up-regulating phospholipase Cγ2 activation. This cascade resulted in a sustained activation of Erk 1/2 and arrest of the cell cycle at the G1 phase. These changes may be accounted for the efficacy of IVIg in autoimmune diseases.

A life of adventure in immunobiology

This article outlines my early start in medicine, a late start in immunology research, and my efforts to integrate the two activities. I first describe some of the background information, excitement, and implications of the recognition of T and B cells as separate but functionally intertwined arms of the adaptive immune system. The article continues with a brief account of my colleagues' and my efforts to use the model of hematopoietic stem cell differentiation along T and B cell lines to gain a better understanding of immunodeficiency diseases and lymphoid malignancies. It concludes with the discovery of a more ancient adaptive immune system in which T-like and B-like cells in jawless vertebrates use variable lymphocyte receptors constructed with leucine-rich-repeat sequences to recognize antigens.

Lentiviral vector transduction of fetal mesenchymal stem cells

Human fetal mesenchymal stem cells (hfMSC) demonstrate extensive expansion and differentiation capacities and are hence being studied for use in stem cell therapeutics, including gene delivery. With advanced prenatal diagnosis, fetal gene therapy represents an additional avenue for the treatment of inherited deficiencies. We have recently demonstrated harvesting of first-trimester fMSC from fetal blood for ex vivo genetic engineering to introduce genes of interest, and finally intra-uterine transplantation (IUT) of these cells to the fetus. Here we discuss methods in the harvesting of hfMSC, lentiviral transduction to introduce genes of interest, and in vitro methods to characterise transgene expression.

Autoimmune and infectious skin diseases that target desmogleins

Desmosomes are intercellular adhesive junctions of epithelial cells that contain two major transmembrane components, the desmogleins (Dsg) and desmocollins (Dsc), which are cadherin-type cell-cell adhesion molecules and are anchored to intermediate filaments of keratin through interactions with plakoglobin and desmoplakin. Desmosomes play an important role in maintaining the proper structure and barrier function of the epidermis and mucous epithelia. Four Dsg isoforms have been identified to date, Dsg1-Dsg4, and are involved in several skin and heart diseases. Dsg1 and Dsg3 are the two major Dsg isoforms in the skin and mucous membranes, and are targeted by IgG autoantibodies in pemphigus, an autoimmune disease of the skin and mucous membranes. Dsg1 is also targeted by exfoliative toxin (ET) released by Staphylococcus aureus in the infectious skin diseases bullous impetigo and staphylococcal scalded skin syndrome (SSSS). ET is a unique serine protease that shows lock and key specificity to Dsg1. Dsg2 is expressed in all tissues possessing desmosomes, including simple epithelia and myocardia, and mutations in this gene are responsible for arrhythmogenic right ventricular cardiomyopathy/dysplasia. Dsg4 plays an important adhesive role mainly in hair follicles, and Dsg4 mutations cause abnormal hair development. Recently, an active disease model for pemphigus was generated by a unique approach using autoantigen-deficient mice that do not acquire tolerance against the defective autoantigen. Adoptive transfer of Dsg3(-/-) lymphocytes into mice expressing Dsg3 induces stable anti-Dsg3 IgG production with development of the pemphigus phenotype. This mouse model is a valuable tool with which to investigate immunological mechanisms of harmful IgG autoantibody production in pemphigus. Further investigation of desmoglein molecules will continue to provide insight into the unsolved pathophysiological mechanisms of diseases and aid in the development of novel therapeutic strategies with minimal side effects.

The role of Epac proteins, novel cAMP mediators, in the regulation of immune, lung and neuronal function

Chronic degenerative inflammatory diseases, such as chronic obstructive pulmonary disease and Alzheimer's dementia, afflict millions of people around the world, causing death and debilitation. Despite the global impact of these diseases, there have been few innovative breakthroughs into their cause, treatment or cure. As with many debilitating disorders, chronic degenerative inflammatory diseases may be associated with defective or dysfunctional responses to second messengers, such as cyclic adenosinemonophosphate (cAMP). The identification of the cAMP-activated guanine nucleotide exchange factors for Ras-like GTPases, Epac1 (also known as cAMP-GEF-I) and Epac2 (also known as cAMP-GEF-II), profoundly altered the prevailing assumptions concerning cAMP signalling, which until then had been solely associated with protein kinase A (PKA). Studies of the molecular mechanisms of Epac-related signalling have demonstrated that these novel cAMP sensors regulate many physiological processes either alone and/or in concert with PKA. These include calcium handling, cardiac and smooth muscle contraction, learning and memory, cell proliferation and differentiation, apoptosis, and inflammation. The diverse signalling properties of cAMP might be explained by spatio-temporal compartmentalization, as well as A-kinase anchoring proteins, which seem to coordinate Epac signalling networks. Future research should focus on the Epac-regulated dynamics of cAMP, and, hopefully, the development of compounds that specifically interfere with the Epac signalling system in order to determine the precise significance of Epac proteins in chronic degenerative inflammatory disorders.

Cells with regulatory function of the innate and adaptive immune system in primary Sjögren's syndrome

The aim of the present study was to describe subsets of cells with regulatory properties in primary Sjögren's syndrome (pSS), and to correlate these cell populations with clinical symptoms. Among the 32 investigated patients, 23 had extraglandular manifestations (EGMs), while nine had only glandular symptoms. Twenty healthy individuals served as controls. The percentages of natural killer (NK), natural killer T cells (NK T), interleukin (IL)-10 producing T regulatory type 1 (Tr1) cells and CD4(+)CD25(+) regulatory T cells (T(reg)) cells were determined by flow cytometry and serum cytokine levels of IL-4, IL-6, IL-10, tumour necrosis factor (TNF)-alpha and interferon (IFN)-gamma were evaluated by enzyme-linked immunosorbent assay (ELISA). Functional tests were carried out to assess the suppressor properties of T(reg) cells in patients and controls. Peripheral NK, NK T and Tr1 cell percentages were elevated in pSS, while CD4(+)CD25(+) T(reg) cells showed reduced frequencies in patients compared to controls. In pSS, elevated percentages of NK T, Tr1 and CD4(+)CD25(+) T(reg) cells were observed in patients with EGMs, when compared to patients with sicca symptoms only. CD4(+)CD25(+) T(reg) cell percentages showed a negative correlation with sialometry values. The in vitro functional assay demonstrated lower suppression activity of CD4(+)CD25(+) T(reg) cells in patients compared to controls. Serum IL-6 and TNF-alpha levels were elevated, while IL-10 was decreased in patients compared to controls. Negative correlation was found between IL-10 levels and the percentages of Tr1 cells. Changes in the investigated subsets of regulatory cells in pSS may contribute to the development and progression of the disease.

Removal of B cell epitopes as a practical approach for reducing the immunogenicity of foreign protein-based therapeutics

Immunogenicity of non-human proteins with useful therapeutic properties has prevented their development for use in the therapy of disease. However, this class of proteins could be very useful, if their immunogenicity could be markedly reduced so that many treatment cycles could be administered. One approach to reduce the immunogenicity of foreign proteins is to identify B cell epitopes on the protein and eliminate them by mutagenesis. In this article, theoretical aspects and experimental evidence for the feasibility of B cell epitope removal is reviewed. A special focus is given to our results with deimmunization of recombinant immunotoxins in which Fvs are fused to a 38kDa portion of the bacterial protein, Pseudomonas exotoxin A (PE38). Immunotoxins targeting CD22 and CD25 have produced complete remissions in many patients with drug resistant Hairy Cell Leukemia and are being evaluated in other malignancies. Experimental data summarized in this review indicates that removal of B cell epitopes is a practical approach for making less immunogenic protein therapeutics from non-human functional proteins. This approach requires grouping of the epitopes to identify targets for deimmunization followed by quantitative analysis of the decrease in affinity produced by the mutations in B cell epitopes.

Restricted autoantigen recognition associated with deletional and adaptive regulatory mechanisms

Autoimmune diabetes (T1D) is characterized by CD4(+) T cell reactivity to a variety of islet-associated Ags. At-risk individuals, genetically predisposed to T1D, often have similar T cell reactivity, but nevertheless fail to progress to clinically overt disease. To study the immune tolerance and regulatory environment permissive for such autoreactive T cells, we expressed TCR transgenes derived from two autoreactive human T cells, 4.13 and 164, in HLA-DR4 transgenic mice on a C57BL/6-derived "diabetes-resistant" background. Both TCR are responsive to an immunodominant epitope of glutamic acid decarboxylase 65(555-567), which is identical in sequence between humans and mice, is restricted by HLA-DR4, and is a naturally processed self Ag associated with T1D. Although both TCR use the identical Valpha and Vbeta genes, differing only in CDR3, we found stark differences in the mechanisms utilized in vivo in the maintenance of immune tolerance. A combination of thymic deletion (negative selection), TCR down-regulation, and peripheral activation-induced cell death dominated the phenotype of 164 T cells, which nevertheless still maintain their Ag responsiveness in the periphery. In contrast, 4.13 T cells are much less influenced by central and deletional tolerance mechanisms, and instead display a peripheral immune deviation including differentiation into IL-10-secreting Tr1 cells. These findings indicate a distinct set of regulatory alternatives for autoreactive T cells, even within a single highly restricted HLA-peptide-TCR recognition profile.

Sialylated multivalent antigens engage CD22 in trans and inhibit B cell activation

CD22 is an inhibitory coreceptor on the surface of B cells that attenuates B cell antigen receptor (BCR) signaling and, therefore, B cell activation. Elucidating the molecular mechanisms underlying the inhibitory activity of CD22 is complicated by the ubiquity of CD22 ligands. Although antigens can display CD22 ligands, the receptor is known to bind to sialylated glycoproteins on the cell surface. The propinquity of CD22 and cell-surface glycoprotein ligands has led to the conclusion that the inhibitory properties of the receptor are due to cis interactions. Here, we examine the functional consequences of trans interactions by employing sialylated multivalent antigens that can engage both CD22 and the BCR. Exposure of B cells to sialylated antigens results in the inhibition of key steps in BCR signaling. These results reveal that antigens bearing CD22 ligands are powerful suppressors of B cell activation. The ability of sialylated antigens to inhibit BCR signaling through trans CD22 interactions reveals a previously unrecognized role for the Siglec-family of receptors as modulators of immune signaling.

T-cell receptor signaling is mediated by transient Lck activity, which is inhibited by inorganic mercury

Genetically susceptible rodents exposed to low nontoxic levels of inorganic mercury (Hg(2+)) develop idiosyncratic autoimmune disease associated with defective T-cell function. However, the molecular mechanisms underlying this phenomenon remain mostly unexplained. Brief exposure of T cells to micromolar concentrations of Hg(2+) leads to physiologically relevant nontoxic cellular mercury burdens, and as we have previously reported, attenuates T-cell receptor (TCR) signal strength by approximately 50%. We have found this to be the result of an inadequate activation of the tyrosine kinase ZAP-70, which is hypophosphorylated following TCR stimulation in Hg(2+) burdened cells when compared to untreated controls. In T cells, ZAP-70 phosphorylation is dependent on lymphocyte-specific protein tyrosine kinase (Lck) activity, which in turn is either positively or negatively regulated by the phosphorylation of specific Lck tyrosine residues. In particular, the general belief is that Lck is negatively regulated by phosphorylation of tyrosine 192 (Y192). We now demonstrate by Western blotting that, in Jurkat T cells, TCR signal transduction (and ZAP-70 phosphorylation) was positively associated with a rapid transient phosphorylation of Y192, which was inhibited in cells that were briefly (5 min) exposed to 5 microM Hg(2+). Thus, Hg(2+) inhibits a critical activating role played by Lck Y192 during the most proximal events of the TCR-induced cell signaling.

B cell receptor-induced growth arrest and apoptosis in WEHI-231 immature B lymphoma cells involve cyclic AMP and Epac proteins

Signaling by the B cell antigen receptor (BCR) is essential for B lymphocyte homeostasis and immune function. In immature B cells, ligation of the BCR promotes growth arrest and apoptosis, and BCR-driven balancing between pro-apoptotic extracellular signal-regulated kinase 1 and 2 (ERK1/2) and anti-apoptotic phosphoinositide 3-kinase-dependent Akt seems to define the final cellular apoptotic response. Dysfunction of these late BCR signaling events can lead to the development of immunological diseases. Here we report on novel cyclic AMP-dependent mechanisms of BCR-induced growth arrest and apoptosis in the immature B lymphoma cell line WEHI-231. BCR signaling to ERK1/2 and Akt requires cyclic AMP-regulated Epac, the latter acting as a guanine nucleotide exchange factor for Rap1 and H-Ras independent of protein kinase A. Importantly, activation of endogenously expressed Epac by a specific cyclic AMP analog enhanced the induction of growth arrest (reduced DNA synthesis) and apoptosis (nuclear condensation, annexin V binding, caspase-3 cleavage and poly-ADP-ribose polymerase processing) by the BCR. Our data indicate that cyclic AMP-dependent Epac signals to ERK1/2 and Akt upon activation of Rap1 and H-Ras, and is involved in BCR-induced growth arrest and apoptosis in WEHI-231 cells.

Influence of B cell antigen receptor expression level on pathways of B cell tolerance induction

We have described an Ig-transgenic, autoreactive B cell clonotype that undergoes a novel tolerance pathway. Early in development this clonotype expresses average BCR levels, but these levels are progressively down-regulated as development proceeds efficiently to the mature, follicular compartment. This clonotype does not display conventional features of anergy and can be induced to undergo apoptosis and receptor editing in in vitro bone marrow cultures, but these pathways are not taken in vivo. These data suggested that autoantigen-driven down-regulation of BCR levels and, hence, avidity for autoantigen allows this clonotype to bypass conventional tolerance mechanisms. To test this idea, we enforced elevated levels of expression of BCR in this clonotype by making the transgenic Igh locus homozygous. This resulted in retarded clonotype development and L chain receptor editing in vivo. These data support a pivotal role for adaptive, autoantigen-induced adjustment of BCR expression levels in the regulation of primary B cell development and tolerance.

T cell-independent spontaneous loss of tolerance by anti-double-stranded DNA B cells in C57BL/6 mice

Systemic lupus erythematosus is characterized by loss of tolerance to DNA and other nuclear Ags. To understand the role of T cells in the breaking of tolerance, an anti-DNA site-specific transgenic model of spontaneous lupus, B6x56R, was studied. T cells were eliminated by crossing B6x56R with CD4(-/)(-) or TCRbeta(-/-)delta(-/-) mice, and the effects on anti-dsDNA serum levels, numbers of anti-dsDNA Ab-secreting cells, and isotypes of anti-dsDNA were analyzed. In addition, the development and activation of B cells in these mice were examined. Surprisingly, the presence of T cells made little difference in the development and character of the serum anti-dsDNA Ab in B6x56R mice. At 1 mo of age, anti-dsDNA Abs were somewhat lower in mice deficient in alphabeta and gammadelta T cells. Levels of Abs later were not affected by T cells, nor was autoantibody class switching. B cell activation was somewhat diminished in T cell-deficient mice. Thus, in the B6 background, the presence of an anti-dsDNA transgene led the production of autoantibodies with a specificity and isotype characteristic of murine systemic lupus erythematosus with little influence from T cells. TLR9 also did not appear to play a role. Although we do not yet understand the mechanism of this failure of immunoregulation, these results suggest that similar processes may influence autoimmunity associated with clinical disease.

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.

CD5: a safeguard against autoimmunity and a shield for cancer cells

CD5 is a pan T cell marker also expressed at various developmental and activation stages on human B cells. CD5 is a well established negative regulator of TCR and BCR signalling. In the last years, great advances have been made in understanding the molecular mechanisms behind this regulatory function. Using animal models, it was demonstrated that increased expression of CD5 on either T cells or B cells protects against autoimmunity; as a consequence of an increase of the threshold needed for TCR- or a BCR-mediated activation following antigen recognition. CD5-positive cells may also prevent the emergence of autoimmunity by provision of cytokines such as IL-10. Although the physiological role of CD5 might be to control the generation of aberrant immune responses, the expansion of CD5+ lymphocytes may be deleterious. The other side of the coin is that in cancers, CD5 expression plays a role in the fate of tumour-specific T cells, rendering lymphocytes tolerant and unable to recognize and eliminate malignant cells. IL-10-producing B-cells may also inhibit anti-tumoural immune responses. An important issue is therefore to better understand the molecules produced by tolerogenic or immunogenic microenvironments that respectively turn on or downregulate CD5 expression, with potential therapeutic implications.

IRAK-4- and MyD88-dependent pathways are essential for the removal of developing autoreactive B cells in humans

Most autoreactive B cells are normally counterselected during early B cell development. To determine whether Toll-like receptors (TLRs) regulate the removal of autoreactive B lymphocytes, we tested the reactivity of recombinant antibodies from single B cells isolated from patients deficient for interleukin-1 receptor-associated kinase 4 (IRAK-4), myeloid differentiation factor 88 (MyD88), and UNC-93B. Indeed, all TLRs except TLR3 require IRAK-4 and MyD88 to signal, and UNC-93B-deficient cells are unresponsive to TLR3, TLR7, TLR8, and TLR9. All patients suffered from defective central and peripheral B cell tolerance checkpoints, resulting in the accumulation of large numbers of autoreactive mature naive B cells in their blood. Hence, TLR7, TLR8, and TLR9 may prevent the recruitment of developing autoreactive B cells in healthy donors. Paradoxically, IRAK-4-, MyD88-, and UNC-93B-deficient patients did not display autoreactive antibodies in their serum or develop autoimmune diseases, suggesting that IRAK-4, MyD88, and UNC-93B pathway blockade may thwart autoimmunity in humans.

B cell receptor revision diminishes the autoreactive B cell response after antigen activation in mice

Autoreactive B cells are regulated in the BM during development through mechanisms, including editing of the B cell receptor (BCR), clonal deletion, and anergy. Peripheral B cell tolerance is also important for protection from autoimmune damage, although the mechanisms are less well defined. Here we demonstrated, using a mouse model of SLE-like serology, that during an autoimmune response, RAG was reinduced in antigen-activated early memory or preplasma B cells. Expression of RAG was specific to antigen-reactive B cells, required the function of the IL-7 receptor (IL-7R), and contributed to maintenance of humoral tolerance. We also showed that soluble antigen could diminish a non-autoreactive antibody response through induction of BCR revision. These data suggest that tolerance induction operates in B cells at a postactivation checkpoint and that BCR revision helps regulate autoreactivity generated during an ongoing immune response.

Defining a transcriptional fingerprint of murine splenic B-cell development

B-cell development occurs in a stepwise fashion that can be followed by the expression of B cell-specific surface markers. In this study, we wished to identify proteins that could contribute to the changes in expression of such markers. By using RNA from freshly isolated B220+ cells, we hoped to reduce the effect of artifacts that occur during the isolation and amplification steps necessary to use flow cytometry analysis-sorted subsets in microarray experiments. Analyses comparing expression patterns from B220+ 2-week bone marrow (pro-B, pre-B, immature B cells), 2-week spleen (predominantly transitional cells) and 8-week spleen (mainly mature B cells) yielded hundreds of genes. We also examined the B cell-activating factor (BAFF)-dependent effects on immature splenic B cells by comparing expression patterns in the spleen between 2-week A/J vs 2-week A/WySnJ mice, which lack functional BAFF receptor signaling. Genes that showed the expression differences between spleen and bone marrow samples were then analyzed through quantitative PCR on B-cell subsets isolated using two different sorting protocols. A comparison of the results from our study with the results from other analyses showed not only some overlap of preferentially expressed genes but also an expansion of other genes potentially involved in B-cell development.

The role of B cells in systemic sclerosis

Systemic sclerosis (SSc) is a connective disease characterized by features of autoimmunity, vasculopathy, inflammation, and fibrosis. The disease typically starts with Raynaud's phenomenon, followed by skin thickening in the extremities due to inflammation and fibrosis. Fibrosis results from excessive collagen production by fibroblasts, which constitutes the final common pathway of complex cellular interactions including B cells. Several studies have indicated that B cells may play a role in SSc. Lesional skin infiltrates from SSc patients consist of a variety of cells, including eosinophils, neutrophils, lymphocytes, plasma cells, and macrophages. Autoantibodies of several specificities are present in the serum of SSc patients of which antitopoisomerase 1 is the most common, and evidence has been gathered for a potential pathogenic role of some autoantibodies, eg, anti-PDGF antibodies. The blood of SSc patients contains an increased proportion of naïve B cells but a decreased proportion of memory B cells. Furthermore, serum levels of interleukin-6, an important pro-inflammatory cytokine, have been shown to correlate with skin fibrosis. Animal models of SSc have provided more in-depth information on the role of B lymphocytes, eg, through disruption of B cell function. In this review we will discuss the evidence that B cells are involved in the pathogenesis of SSc.

Villous B cells of the small intestine are specialized for invariant NK T cell dependence

B cells are important in mucosal microbial homeostasis through their well-known role in secretory IgA production and their emerging role in mucosal immunoregulation. Several specialized intraintestinal B cell compartments have been characterized, but the nature of conventional B cells in the lamina propria is poorly understood. In this study, we identify a B cell population predominantly composed of surface IgM(+) IgD(+) cells residing in villi of the small intestine and superficial lamina propria of the large intestine, but distinct from the intraepithelial compartment or organized intestinal lymphoid structures. Small intestinal (villous) B cells are diminished in genotypes that alter the strength of BCR signaling (Bruton tyrosine kinase(xid), Galphai2(-/-)), and in mice lacking cognate BCR specificity. They are not dependent on enteric microbial sensing, because they are abundant in mice that are germfree or genetically deficient in TLR signaling. However, villous B cells are reduced in the absence of invariant NK T cells (Jalpha18(-/-) or CD1d(-/-) mice). These findings define a distinct population of conventional B cells in small intestinal villi, and suggest an immunologic link between CD1-restricted invariant NK T cells and this B cell population.

Characterization of a late transitional B cell population highly sensitive to BAFF-mediated homeostatic proliferation

We have characterized a distinct, late transitional B cell subset, CD21^int transitional 2 (T2) B cells. In contrast to early transitional B cells, CD21^int T2 B cells exhibit augmented responses to a range of potential microenvironmental stimuli. Adoptive transfer studies demonstrate that this subset is an immediate precursor of both follicular mature and marginal zone (MZ) B cells. In vivo, a large percentage of CD21^int T2 B cells has entered the cell cycle, and the cycling subpopulation exhibits further augmentation in mitogenic responses and B cell-activating factor of the TNF family (BAFF) receptor expression. Consistent with these features, CD21^int T2 cells exhibit preferential responses to BAFF-facilitated homeostatic signals in vivo. In addition, we demonstrate that M167 B cell receptor (BCR) idiotypic-specific B cells are first selected within the cycling CD21^int T2 population, ultimately leading to preferential enrichment of these cells within the MZ B cell compartment. These data, in association with the coordinate role for BAFF and microenvironmental cues in determining the mature BCR repertoire, imply that this subset functions as a unique selection point in peripheral B cell development.

Transfusion-induced autoantibodies and differential immunogenicity of blood group antigens: a novel hypothesis

Blood bank serology has identified hundreds of red blood cell (RBC) antigens contained within numerous blood group systems. Although most blood group antigens are defined by amino acid polymorphisms in the extracellular domain of membrane proteins, it is also possible that additional nonexofacial polymorphisms (NEPs) may exist within cytoplasmic or transmembrane domains. To assess this possibility, we analyzed several blood group molecules by searching the SNPper database for nonsynonymous single-nucleotide polymorphisms. We report the identification of a number of NEPs in the Kell, Kidd, and Duffy molecules. Because the identified NEPs are not exposed on the surface of intact RBCs and are, thus, not accessible to recipient antibodies, they would neither be detected by blood bank serology in vitro, nor would they be recognized targets in hemolytic transfusion reactions in vivo. The presentation of peptides containing NEPs by recipient MHC Class II molecules, however, would nevertheless produce helper T-cell epitopes. In addition to identifying NEPs in human blood group molecules, we explore a novel hypothesis that the presence of NEPs contributes to the immunogenicity of blood group antigens. We further hypothesize that NEPs provide a mechanism by which transfusion can lead to anti-RBC autoantibodies, which are known to occur in humans after transfusion. The scientific basis, existing evidence, approaches to testing, and predicted biology of this hypothesis are presented.

PECAM-1-regulated signalling thresholds control tolerance in anergic transgenic B-cells

Platelet Endothelial Cell Adhesion Molecule-1 (PECAM-1/CD31) is an immunoglobulin (Ig)-immunoreceptor tyrosine based inhibitory motif (Ig-ITIM) superfamily member that recruits and activates protein-tyrosine phosphatases, predominantly SHP-2 and to a lesser extent, SHP-1. Previously, we have shown that deletion of PECAM-1 results in a hyper-proliferative B-cell phenotype. We wanted to test whether the Ig-ITIM superfamily member, PECAM-1 maintains peripheral tolerance by regulating signalling thresholds of B-cells that control autoantibody production or relaxed negative selection of autoreactive B-cells in bone marrow. In order to address this issue, we utilised the classical model of lysozyme/immunoglobulin transgenic mouse model that defines thresholds for eliminating or inactivating self-reactive B-cells. In this study, we show that breeding of double transgenes: soluble hen egg lysozyme (HEL) and its corresponding high-affinity receptor (HEL-Ig) onto PECAM-1 null background resulted in a spontaneous loss of B-cell tolerance in vivo. The resultant PECAM-1(-/-) Dbl Tg mice displayed elevated levels of anti-HEL immunoglobulin M (IgM) antibodies in the serum compared to PECAM-1+/+ anergic counterparts. Dbl Tg B-cells lacking PECAM-1 showed enhanced B-cell proliferation and calcium flux responses to LPS, IL-4 alone, IgM cross-linking and IL-4 indicating augmentation of antigen-receptor signalling. Thus, PECAM-1 is important in maintaining peripheral tolerance in Dbl Tg B-cells.

Measurement of natural (CD4+CD25high) and inducible (CD4+IL-10+) regulatory T cells in patients with systemic lupus erythematosus

Abnormalities of regulatory T cells may play an important role in the loss of self-tolerance, which is a major characteristic of lupus. The objective of this study was to determine the ratio and the number of natural CD4+CD25highFoxp3+ and inducible CD4+IL-10+ regulatory T cells in lupus patients and to search correlation with disease activity. Seventy-two Hungarian lupus patients were enrolled in the study. Fourty-one age- and sex matched healthy donors served as controls. Flow cytometry was used for the quantification of CD4+CD25high Foxp3+ (nTreg) and CD4+IL-10+ (iTreg) cells. The ratio (3.06 +/- 1.45%) and the number (0.019 +/- 0.012 x 10(9)/L) of nTreg cells decreased in lupus significantly (P < 0.001 in both) as compared to normal controls (4.26 +/- 1.01% and 0.039 +/- 0.017 x 10(9)/L). The ratio of iTreg cells were significantly higher in patients than in controls (20.92 +/- 14.02% versus 15.49 +/- 11.65%, P < 0.03), but the number of these cell type did not differ in significant manner (0.314 +/- 0.236 x 10(9)/L versus 0.259 +/- 0.183 x 10(9)/L). The 19 active patients were characterised by significantly higher disease activity index (SLEDAI 8.63 +/- 2.95 versus 1.74 +/- 1.68, P < 0.001) and anti-DNA concentration (117.85 +/- 145.89 versus 37.36 +/- 68.85 IU/mL, P = 0.001) as compered to the 52 inactive patients. Furthermore, active patients required higher dose of methylprednisolon than inactive ones (14.8 +/- 10.6 versus 4.8 +/- 3.4 mg/day, P < 0.001). However, we did not find statistical significant difference in the number and ratio of the examined cell populations regarding to disease activity. Altered ratio and number of both natural and inducible regulatory T cells may play a role in the pathogenesis of lupus. There are small but appreciable difference in the number of regulatory T cells between inactive patients and healthy controls. It suggests that immunoregulatory deficiencies are present in the inactive stage of the disease also.