CARD11 gain-of-function mutation drives cell-autonomous accumulation of PD-1+ ICOShigh activated T cells, T-follicular, T-regulatory and T-follicular regulatory cells

Introduction

Germline CARD11 gain-of-function (GOF) mutations cause B cell Expansion with NF-κB and T cell Anergy (BENTA) disease, whilst somatic GOF CARD11 mutations recur in diffuse large B cell lymphoma (DLBCL) and in up to 30% of the peripheral T cell lymphomas (PTCL) adult T cell leukemia/lymphoma (ATL), cutaneous T cell lymphoma (CTCL) and Sezary Syndrome. Despite their frequent acquisition by PTCL, the T cell-intrinsic effects of CARD11 GOF mutations are poorly understood.

Methods

Here, we studied B and T lymphocytes in mice with a germline Nethyl-N-nitrosourea (ENU)-induced Card11^M365K mutation identical to a mutation identified in DLBCL and modifying a conserved region of the CARD11 coiled-coil domain recurrently mutated in DLBCL and PTCL.

Results and discussion

Our results demonstrate that CARD11.M365K is a GOF protein that increases B and T lymphocyte activation and proliferation following antigen receptor stimulation. Germline Card11^M365K mutation was insufficient alone to cause B or T-lymphoma, but increased accumulation of germinal center (GC) B cells in unimmunized and immunized mice. Card11^M365K mutation caused cell-intrinsic over-accumulation of activated T cells, T regulatory (T_REG), T follicular (T_FH) and T follicular regulatory (T_FR) cells expressing increased levels of ICOS, CTLA-4 and PD-1 checkpoint molecules. Our results reveal CARD11 as an important, cell-autonomous positive regulator of T_FH, T_REG and T_FR cells. They highlight T cell-intrinsic effects of a GOF mutation in the CARD11 gene, which is recurrently mutated in T cell malignancies that are often aggressive and associated with variable clinical outcomes.

A divergent transcriptional landscape underpins the development and functional branching of MAIT cells

MR1-restricted mucosal-associated invariant T (MAIT) cells play a unique role in the immune system. These cells develop intrathymically through a three-stage process, but the events that regulate this are largely unknown. Here, using bulk and single-cell RNA sequencing-based transcriptomic analysis in mice and humans, we studied the changing transcriptional landscape that accompanies transition through each stage. Many transcripts were sharply modulated during MAIT cell development, including SLAM (signaling lymphocytic activation molecule) family members, chemokine receptors, and transcription factors. We also demonstrate that stage 3 "mature" MAIT cells comprise distinct subpopulations including newly arrived transitional stage 3 cells, interferon-γ-producing MAIT1 cells and interleukin-17-producing MAIT17 cells. Moreover, the validity and importance of several transcripts detected in this study are directly demonstrated using specific mutant mice. For example, MAIT cell intrathymic maturation was found to be halted in SLAM-associated protein (SAP)-deficient and CXCR6-deficient mouse models, providing clear evidence for their role in modulating MAIT cell development. These data underpin a model that maps the changing transcriptional landscape and identifies key factors that regulate the process of MAIT cell differentiation, with many parallels between mice and humans.

Massively parallel sequencing of the mouse exome to accurately identify rare, induced mutations: an immediate source for thousands of new mouse models

Accurate identification of sparse heterozygous single-nucleotide variants (SNVs) is a critical challenge for identifying the causative mutations in mouse genetic screens, human genetic diseases and cancer. When seeking to identify causal DNA variants that occur at such low rates, they are overwhelmed by false-positive calls that arise from a range of technical and biological sources. We describe a strategy using whole-exome capture, massively parallel DNA sequencing and computational analysis, which identifies with a low false-positive rate the majority of heterozygous and homozygous SNVs arising de novo with a frequency of one nucleotide substitution per megabase in progeny of N-ethyl-N-nitrosourea (ENU)-mutated C57BL/6j mice. We found that by applying a strategy of filtering raw SNV calls against known and platform-specific variants we could call true SNVs with a false-positive rate of 19.4 per cent and an estimated false-negative rate of 21.3 per cent. These error rates are small enough to enable calling a causative mutation from both homozygous and heterozygous candidate mutation lists with little or no further experimental validation. The efficacy of this approach is demonstrated by identifying the causative mutation in the Ptprc gene in a lymphocyte-deficient strain and in 11 other strains with immune disorders or obesity, without the need for meiotic mapping. Exome sequencing of first-generation mutant mice revealed hundreds of unphenotyped protein-changing mutations, 52 per cent of which are predicted to be deleterious, which now become available for breeding and experimental analysis. We show that exome sequencing data alone are sufficient to identify induced mutations. This approach transforms genetic screens in mice, establishes a general strategy for analysing rare DNA variants and opens up a large new source for experimental models of human disease.

A repository of ENU mutant mouse lines and their potential for male fertility research

Many of the proteins and their encoding genes involved in spermatogenesis are unknown, making the specific diagnosis and treatment of infertility in males difficult and highlighting the importance of identifying new genes that are involved in spermatogenesis. Through genome-wide chemical mutagenesis using N-ethyl-N-nitrosourea (ENU) and a three-generation breeding scheme to isolate recessive mutations, we have identified mouse lines with a range of abnormalities relevant to human male fertility. Abnormal phenotypes included hypospermatogenesis, Sertoli cell-only (SCO) seminiferous tubules, germ-cell arrest and abnormal spermiogenesis and were accompanied, in some, with abnormal serum levels of reproductive hormones. In total, from 65 mouse lines, 14 showed a reproductive phenotype consistent with a recessive mutation. This study shows that it is feasible to use ENU mutagenesis as an effective and rapid means of generating mouse models relevant to furthering our understanding of human male infertility. Spermatozoa and genomic DNA from all mouse lines, including those with abnormal reproductive tract parameters, have been cryopreserved for the regeneration of lines as required. This repository will form a valuable resource for the identification and analysis of key regulators of multiple aspects of male fertility.

Genome-wide ENU mutagenesis to reveal immune regulators

A complete list of molecular components for immune system function is now available with the completion of the human and mouse genome sequences. However, identification and functional annotation of genes involved in immunological processes require a discovery methodology that can efficiently and broadly analyze the complex interplay of these components in vivo. Our recent experience indicates that genome-wide chemical mutagenesis in the mouse is an extremely powerful methodology for the identification of genes required for complex immunological processes.

Pathways for self-tolerance and the treatment of autoimmune diseases

Antigen delivers both immunogenic and tolerogenic signals to lymphocytes. The outcome of antigen exposure represents a complex integration of the timing of antigen binding with signals from many other immunogenic and tolerogenic costimulatory pathways. A road map of these signalling pathways is only beginning to be charted, revealing the mechansim of action and limitations of current immunotherapeutic agents and the points of attack for new agents. Ciclosporin and tacrolimus interfere with tolerogenic signals from antigen in addition to blocking immunogenic signals, thus preventing active establishment of tolerance. Corticosteroids inhibit a key immunogenic pathway, NFkappaB, and more specific inhibitors of this pathway may allow tolerance to be actively established while immune responses are blocked. New experimental therapies aim to mimic tolerogenic antigen signals by chronically stimulating antigen receptors with antigen or antibodies to the receptor, or aim to block costimulatory pathways involving CD40 ligand, B7, or interleukin 2. Obtaining the desired response with these strategies is unpredictable because many of these signals have both tolerogenic and immunogenic roles. The cause of autoimune diseases has been determined for several rare monogenic disorders, revealing inherited deficiencies in tolerogenic costimulatory pathways such as FAS. Common autoimmune disorders may have a biochemically related pathogenesis.

Single epitope multiple staining to detect ultralow frequency B cells

Here we describe a method for detecting ultralow frequency target cells from within a high background of irrelevant cells by a novel method, single epitope multiple staining (SEMS). Samples of murine splenocytes were seeded with a low number of splenocytes from mice transgenic for a hen eggwhite lysozyme (HEL)-specific immunoglobulin (Ig). These samples were stained with two reagents specific for the same epitope expressed by the transgenic B cells, which had been conjugated to two different detectable labels (FITC and biotin). This dual staining of a single epitope allowed us to reduce the background due both to non-specific binding of reagents and to probabilistic distribution of the cells. We also were able to detect the cells based on knowing only one thing about them, namely, their antigen specificity. The SEMS method allowed us to reproducibly detect transgenic cells at frequencies below one cell in one million cells. SEMS could be used to increase the sensitivity of numerous fluorescence-based applications in addition to the detection and isolation of antigen-specific lymphocytes, including the detection and highly specific isolation of genetically modified cells, transformed cells, stem cells, fetal cells, or infectious organisms.

A genomic view of immunology

The outstanding problems facing immunology are whole system issues: curing allergic and autoimmune disease and developing vaccines to stimulate stronger immune responses against pathogenic organisms and cancer. We hope that the human genome sequence will reveal the molecular checks and balances that ensure both an effective immunogenic response against pathogenic microorganisms and a suitably tolerogenic response to self antigens and innocuous environmental antigens. Three synergistic approaches--sequence homology searches, messenger RNA expression profiling on microarrays, and mutagenesis in mice--provide the best opportunities to reveal, in the genome sequence, key proteins and pathways for targeting by new immunomodulatory treatments.

B-lymphocyte quiescence, tolerance and activation as viewed by global gene expression profiling on microarrays

Self-tolerance is achieved by deleting or regulating self-reactive lymphocytes at a series of cellular checkpoints placed at many points along the developmental pathways to plasma cells and effector T cells. At each checkpoint, what are the molecular pathways that determine whether a lymphocyte remains quiescent, begins dividing, differentiates or dies? In splenic B cells, the decision between quiescence, tolerance by anergy, and activation provides a tractable setting to explore these issues by global gene expression profiling on DNA microarrays. Here we discuss the application of microarrays to illuminate a set of cell fate decisions that appear to be determined by summation of numerous small changes in expression of stimulatory and inhibitory genes. Many genes with known or predicted inhibitory functions are highly expressed in naive, quiescent B cells, notably the signal inhibitor SLAP and DNA-binding proteins of the Kruppel family (LKLF, BKLF, GKLF), Tsc-22, GILZ, Id-3, and GADD45. Activation of naive B cells, triggered by acute binding of antigen to the B-cell receptor, involves a rapid decrease in expression of these inhibitory genes. Promitotic genes are induced in parallel, including c myc, LSIRF/IRF4, cyclin D2, Egr-1 and Egr-2, as are the anti-apoptotic gene A1 and genes for the T-cell-attracting chemokines MIP-1alpha and beta. B-cell tolerance through the process of anergy, induced by chronic binding of self antigen, maintains expression of the inhibitory genes found in quiescent B cells and induces an additional set of inhibitory genes. The latter include inhibitors of signaling - CD72, neurogranin, pcp4 - and additional inhibitors of gene expression such as SATB1, MEF2C, TGIF and Nab-2. The effects of tolerance, the immunosuppressive drug FK506 and other modulators of calcium or MAPK signaling allow individual gene responses to be linked to different signal transduction pathways. The global molecular profiles obtained illustrate how quiescence and anergy are actively maintained in circulating B cells, how these states are switched to clonal expansion and how they could be better emulated by pro-tolerogenic drugs.

Genomic-scale gene expression analysis of lymphocyte growth, tolerance and malignancy

Immunologists are already comfortable with the need for monitoring many different gene products simultaneously. It is a common challenge to remember what CD-one-hundred-and-something is, and an ever-increasing number of colours are required for identification on the flow cytometer. Gene expression arrays now offer the possibility of extending this approach beyond the cell surface and expanding it dramatically to survey the entire catalogue of gene transcripts in a lymphoid cell.

Role of Syk in B-cell development and antigen-receptor signaling

Antigen receptors (BCRs) on developing B lymphocytes play two opposing roles-promoting survival of cells that may later bind a foreign antigen and inhibiting survival of cells that bind too strongly to self-antigens. It is not known how these opposing outcomes are signaled by BCRs on immature B cells. Here we analyze the effect of a null mutation in the Syk tyrosine kinase on maturing B cells displaying a transgene-encoded BCR that binds hen egg lysozyme (HEL). In the absence of HEL antigen, HEL-specific BCRs are expressed normally on the surface of Syk-deficient immature B-lineage cells, but this fails to promote maturation beyond the earliest stages of B-lineage commitment. Binding of HEL antigen, nevertheless, triggers phosphorylation of CD79alpha/beta BCR subunits and modulation of receptors from the surface in Syk-deficient cells, but it cannot induce an intracellular calcium response. Continuous binding of low- or high-avidity forms of HEL, expressed as self-antigens, fails to restore the signal needed for maturation. Compared with the effects in the same system of null mutations in other BCR signaling elements, such as CD45 and Lyn kinase, these results indicate that Syk is essential for transmitting a signal that initiates the program of B-lymphocyte maturation.

How self-tolerance and the immunosuppressive drug FK506 prevent B-cell mitogenesis

Therapy for transplant rejection, autoimmune disease and allergy must target mature lymphocytes that have escaped censoring during their development. FK506 and cyclosporin are immunosuppressants which block three antigen-receptor signalling pathways (NFAT, NFkappaB and JNK), through inhibition of calcineurin, and inhibit mature lymphocyte proliferation to antigen. Neither drug induces long-lived tolerance in vivo, however, necessitating chronic use with adverse side effects. Physiological mechanisms of peripheral tolerance to self-antigens provide an opportunity to emulate these processes pharmacologically. Here we use gene-expression arrays to provide a molecular explanation for the loss of mitogenic response in peripheral B-cell anergy, one aspect of immunological tolerance. Self-antigen induces a set of genes that includes negative regulators of signalling and transcription but not genes that promote proliferation. FK506 interferes with calcium-dependent components of the tolerance response and blocks an unexpectedly small fraction of the activation response. Many genes that were not previously connected to self-tolerance are revealed, and our findings provide a molecular fingerprint for the development of improved immunosuppressants that prevent lymphocyte activation without blocking peripheral tolerance.

Growing up on the streets: why B-cell development differs from T-cell development

B-cell development differs significantly from T-cell development in that negative selection of autoreactive B cells can occur in the same microenvironment in which productive immune responses begin. Here, Sarah Townsend and colleagues discuss how this 'growing up on the streets' might provide a mechanism that fills holes in the B-cell repertoire, much as major histocompatibility complex polymorphism fills holes in the T-cell repertoire.

A critical role for complement in maintenance of self-tolerance

The role of complement in the maintenance of self-tolerance has been examined in two models: an immunoglobulin transgenic model of peripheral tolerance and a lupus-like murine model of CD95 (Fas) deficiency. We find that self-reactive B lymphocytes deficient in complement receptors CD21/CD35 or transferred into mice deficient in the complement protein C4 are not anergized by soluble self-antigen. In the second model, deficiency in CD21/CD35 or C4 combined with CD95 deficiency results in high titers of anti-nuclear antibodies leading to severe lupus-like disease. These findings suggest a novel role for the complement system in B cell tolerance and provide insight into the genetic association of complement deficiency with susceptibility to systemic lupus erythematosus.

Regulation of mouse CD72 gene expression during B lymphocyte development

CD72 is a 45-kDa transmembrane glycoprotein that is predominantly expressed on cells of the B lineage except plasma cells. Previously, we identified the 255-bp minimal mouse CD72 promoter capable of tissue-specific and developmental stage-specific expression. DNase I footprinting analysis of the 255-bp CD72 promoter revealed three protected elements, footprint (FP) I, FP II, and FP III. FP II, which extends from nucleotide -189 to -169 of the mouse CD72 promoter, exhibited both tissue-specific and developmental stage-specific activity that was reflective of the activity of the CD72 gene in vivo. In this report, we show that FP II is specifically recognized by the transcription factor B cell-specific activator protein (BSAP). Mutations eliminating the binding of BSAP in reporter constructs also eliminated the increase of reporter activity in B cells. In addition, cotransfections with reporter constructs plus different amounts of expression plasmids for BSAP showed that CD72 promoter activity was up-regulated by BSAP in plasmacytoma cells and T cells in a dose-dependent manner. Moreover, the expression level of CD72 decreased 10-fold on normal plasma cells. Compared with the presence of BSAP binding in mature B cells, the binding of BSAP was undetectable in those plasma cells. This study strongly suggests that BSAP-FP II interaction plays a critical role in determining the cell-type specificity of the CD72 promoter. The absence of positive factors such as BSAP accounts for at least part of the loss of mouse CD72 expression in plasma cells and thus might be common for the down-regulation of many molecules at the plasma cell stage.

Quantitative and qualitative control of antigen receptor signalling in tolerant B lymphocytes

Lymphocyte antigen receptors, such as the B cell antigen receptor (BCR), have the ability to promote or inhibit immune responses. This functional plasticity is exemplified by BCR-induced mitosis in naïve but not tolerant B cells and is correlated with biochemical differences in the signals triggered by foreign and self antigens. Acute stimulation of naïve B cells with foreign antigen induces a biphasic Ca2+ flux, and activates nuclear signalling through NF-AT, NF-kappa B, JNK and ERK. In tolerant B lymphocytes, by contrast, self antigen triggers only a low Ca2+ plateau, NF-AT and ERK. After removal from self antigen, the BCRs on tolerant B cells reacquire the ability to stimulate a biphasic Ca2+ flux and to promote proliferation. The differences in nuclear signalling between naïve and tolerant cells is brought about in part by differences in the magnitude of the Ca2+ signal. A low, sustained Ca2+ signal, such as that seen in tolerant B cells, activates NF-AT, whereas, a high but transient Ca2+ spike, which resembles that triggered in naïve B cells, activates NF-kappa B and JNK. These findings demonstrate that the quantitative differences in Ca2+ signalling between naïve and tolerant B cells are reversible and contribute to the differential triggering of nuclear signals. The activation of selected transcription factors may in turn account for the different functional responses triggered in naïve and tolerant lymphocytes.

B cell antigen receptor signalling in the balance of tolerance and immunity

The quantity and quality of signals from the B cell antigen receptor (BCR) drives the positive and negative selection of B lymphocytes and establishes the balance of tolerance and immunity. Experiments using immunoglobulin transgenic mice and mutations in key BCR signalling components have given insight into how the antigen receptor is tuned and how thresholds for qualitatively different outcomes are established and maintained. This research also describes how genetic variants can shift the balance between autoimmunity and tolerance.

The in vivo balance between B cell clonal expansion and elimination is regulated by CD95 both on B cells and in their micro-environment

The expression of CD95 (Fas/APO-1) on B cells has been shown to play a direct role in their fate. B cells that chronically bind antigen due to prolonged antigen exposure, such as self-reactive B cells, are induced to express CD95 by CD40 ligand (CD40L) and are subsequently eliminated by CD95 ligand (CD95L) when they present antigen to CD4+ T cells. B cells that bind antigen acutely due to sudden antigen encounter, such as foreign antigen reactive B cells, up-regulate CD95, but are normally protected from CD95L-mediated apoptosis. Here, however, it is shown in vivo that foreign antigen-specific B cells fail to be protected from CD95-dependent elimination in a host that is CD95 deficient, regardless of antigenic challenge. These data indicate that B cell antigen receptor (BCR)-induced protection against CD95L-mediated apoptosis is not absolute but depends upon other micro-environmental factors in vivo. The normal balance between T cell-dependent humoral immunity and tolerance is thus regulated intrinsically by CD95 expression on responding B cells, and extrinsically by CD95-mediated control of CD95L or other molecules in the lymphoid micro-environment.

Repression of B7.2 on self-reactive B cells is essential to prevent proliferation and allow Fas-mediated deletion by CD4(+) T cells

Peripheral tolerance mechanisms normally prevent delivery of T cell help to anergic self-reactive B cells that accumulate in the T zones of spleen and lymph nodes. Chronic exposure to self-antigens desensitizes B cell antigen receptor (BCR) signaling on anergic B cells so that they are not stimulated into clonal expansion by CD4(+) T cells but instead are eliminated by Fas (CD95)-induced apoptosis. Because a range of BCR-induced signals and responses are repressed in anergic B cells, it is not known which of these are critical to regulate for Fas-mediated peripheral tolerance. Display of the costimulatory molecule, B7.2 (CD86), represents a potentially important early response to acute BCR engagement that is poorly induced by antigen on anergic B cells. We show here that restoring B7.2 expression on tolerant B cells using a constitutively expressed B7.2 transgene is sufficient to prevent Fas-mediated deletion and to trigger extensive T cell-dependent clonal expansion and autoantibody secretion in the presence of specific T cells. Dysregulated expression of B7.2 on tolerant B cells caused a more extreme reversal of peripheral tolerance than that caused by defects in Fas or Fas ligand, and resulted in T cell-dependent clonal expansion and antibody secretion comparable in magnitude to that made by foreign antigen-specific B cells. These findings demonstrate that repression of B7.2 is critical to eliminate autoreactive B cells by Fas in B cell-T cell interactions. The possible role of B7.2 dysregulation in systemic autoimmune diseases is discussed.

Immune Responses: costimulatory receptors have their say

Lymphocytes are more likely to make an immune response if costimulatory and antigen receptors coincidently signal; the way the signals are integrated illustrates how a lymphocyte learns to distinguish self from foreign antigens, and provides a model for coincident signaling through more than one receptor.

Self-reactive B lymphocytes overexpressing Bcl-xL escape negative selection and are tolerized by clonal anergy and receptor editing

Self-reactive B cells Tg for both a bcl-xL death inhibitory gene and an Ig receptor recognizing hen egg lysozyme (HEL-Ig) efficiently escaped developmental arrest and deletion in mice expressing membrane-bound self-antigen (mHEL). In response to the same antigen, Tg HEL-Ig B cells not expressing bcl-xL were deleted, while cells expressing bcl-2 were arrested at the immature B stage. Bcl-xL Tg B cells escaping negative selection were anergic in both in vitro and in vivo assays and showed some evidence for receptor editing. These studies suggest that Bcl-x may have a distinct role in controlling survival at the immature stage of B cell development and demonstrate that tolerance is preserved when self-reactive B cells escape central deletion.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Abortive proliferation of rare T cells induced by direct or indirect antigen presentation by rare B cells in vivo

Antigen-specific B cells are implicated as antigen-presenting cells in memory and tolerance responses because they capture antigens efficiently and localize to T cell zones after antigen capture. It has not been possible, however, to visualize the effect of specific B cells on specific CD4+ helper T cells under physiological conditions. We demonstrate here that rare T cells are activated in vivo by minute quantities of antigen captured by antigen-specific B cells. Antigen-activated B cells are helped under these conditions, whereas antigen-tolerant B cells are killed. The T cells proliferate and then disappear regardless of whether the B cells are activated or tolerant. We show genetically that T cell activation, proliferation, and disappearance can be mediated either by transfer of antigen from antigen-specific B cells to endogenous antigen-presenting cells or by direct B-T cell interactions. These results identify a novel antigen presentation route, and demonstrate that B cell presentation of antigen has profound effects on T cell fate that could not be predicted from in vitro studies.

Dependence of germinal center B cells on expression of CD21/CD35 for survival

Affinity-driven selection of B lymphocytes within germinal centers is critical for the development of high-affinity memory cells and host protection. To investigate the role of the CD21/CD35 coreceptor in B cell competition for follicular retention and survival within the germinal center, either Cr2+ or Cr2null lysozyme-specific transgenic B cells were adoptively transferred into normal mice immunized with duck (DEL) or turkey (TEL) lysozyme, which bind with different affinities. In mice injected with high-affinity turkey lysozyme, Cr2null B cells responded by follicular retention; however, they could not survive within germinal centers. This suggests that CD21 provides a signal independent of antigen that is required for survival of B cells in the germinal center.

Polygenic autoimmune traits: Lyn, CD22, and SHP-1 are limiting elements of a biochemical pathway regulating BCR signaling and selection

A B lymphocyte hyperactivity syndrome resembling systemic lupus erythematosus characterizes mice lacking the src-family kinase Lyn. Lyn is not required to initiate B cell antigen receptor (BCR) signaling but is an essential inhibitory component. lyn-/- B cells have a delayed but increased calcium flux and exaggerated negative selection responses in the presence of antigen and spontaneous hyperactivity in the absence of antigen. As in invertebrates, genetic effects of loci with only one functional allele can be used to analyze signaling networks in mice, demonstrating that negative regulation of the BCR is a complex quantitative trait in which Lyn, the coreceptor CD22, and the tyrosine phosphatase SHP-1 are each limiting elements. The biochemical basis of this complex trait involves a pathway requiring Lyn to phosphorylate CD22 and recruit SHP-1 to the CD22/BCR complex.

Spontaneous follicular exclusion of SHP1-deficient B cells is conditional on the presence of competitor wild-type B cells

Engagement of antigen receptors on mature B lymphocytes is known to block cell entry into lymphoid follicles and promote accumulation in T cell zones, yet the molecular basis for this change in cell distribution is not understood. Previous studies have shown that follicular exclusion requires a threshold level of antigen receptor engagement combined with occupancy of follicles by B cells without equivalent receptor engagement. The possibility has been raised that follicular composition affects B cell positioning by altering the amount of available antigen and the degree of receptor occupancy. Here we show that follicular composition affects migration of mature B cells under conditions that are independent of antigen receptor occupancy. B cells deficient in the negative regulatory protein tyrosine phosphatase, SHP1, which have elevated intracellular signaling by the B cell receptor, are shown to accumulate in the T zone in the absence of their specific antigen. Follicular exclusion of SHP1-deficient B cells was found to be conditional on the presence of excess B cells that lack elevated intracellular signaling, and was not due to a failure of SHP-1-deficient cells to mature and express the follicle-homing chemokine receptor Burkitt's lymphoma receptor 1. These findings strongly suggest that signals that are negatively regulated by SHP1 promote B cell localization in T cell zones by reducing competitiveness for follicular entry, and provide further evidence that follicular composition influences the positioning of antigen-engaged B cells.

Self-reactive B cells are not eliminated or inactivated by autoantigen expressed on thyroid epithelial cells

Graves' Disease results from the production of autoantibodies against receptors for thyroid stimulating hormone (TSH) on thyroid epithelial cells, and represents the prototype for numerous autoimmune diseases caused by autoantibodies that bind to organ-specific cell membrane antigens. To study how humoral tolerance is normally maintained to organ-specific membrane antigens, transgenic mice were generated selectively expressing membrane-bound hen egg lysozyme (mHEL) on the thyroid epithelium. In contrast to the deletion of autoreactive B cells triggered by systemic mHEL (Hartley, S.B., J. Crosbie, R. Brink, A.B. Kantor, A. Basten, and C.C. Goodnow. 1991. Nature. 353:765-769), selective expression of mHEL autoantigen on thyroid cells did not trigger elimination or inactivation of circulating HEL-reactive B cells. These results provide evidence that tolerance is not actively acquired to organ-specific antigens in the preimmune B cell repertoire, underscoring the importance of maintaining tolerance to such antigens by other mechanisms. The role of an intact endothelial barrier in sequestering organ-specific antigens from circulating preimmune B cells is discussed.

CD19-regulated signaling thresholds control peripheral tolerance and autoantibody production in B lymphocytes

The CD19 cell surface molecule regulates signal transduction events critical for B lymphocyte development and humoral immunity. Increasing the density of CD19 expression renders B lymphocytes hyper-responsive to transmembrane signals, and transgenic mice that overexpress CD19 have increased levels of autoantibodies. The role of CD19 in tolerance regulation and autoantibody generation was therefore examined by crossing mice that overexpress a human CD19 transgene with transgenic mice expressing a model autoantigen (soluble hen egg lysozyme, sHEL) and high-affinity HEL-specific IgMa and IgDa (IgHEL) antigen receptors. In this model of peripheral tolerance, B cells in sHEL/IgHEL double-transgenic mice are functionally anergic and do not produce autoantibodies. However, it was found that overexpression of CD19 in sHEL/IgHEL double-transgenic mice resulted in a breakdown of peripheral tolerance and the production of anti-HEL antibodies at levels similar to those observed in IgHEL mice lacking the sHEL autoantigen. Therefore, altered signaling thresholds due to CD19 overexpression resulted in the breakdown of peripheral tolerance. Thus, CD19 overexpression shifts the balance between tolerance and immunity to autoimmunity by augmenting antigen receptor signaling.

Redundant expression but selective utilization of nuclear factor of activated T cells family members

Nuclear factor of activated T cells (NF-AT) complexes regulate the induction of many early T cell activation molecules. Four related proteins can function as the cytoplasmic subunit of NF-AT, and their overlapping expression patterns and the mild phenotype of the NF-ATp null mice suggest that they may be functionally redundant. We characterized the distribution and activation of cytoplasmic NF-AT proteins in mature lymphocytes and found that NF-ATc, NF-ATp, and NF-AT4/x/c3 are co-expressed and co-regulated in mature T and B cells. Each protein forms independent DNA binding complexes, and at physiologic concentrations, NF-ATc and NF-ATp complexes out-compete NF-AT4/x/c3 for occupancy of NF-AT sites from the IL-2, IL-3/granulocyte-macrophage CSF, IL-4, and CD40 ligand genes. This predicts heavily redundant immune regulatory functions of NF-ATp and NF-ATc, but distinct activities for NF-AT4/x/c3. Additionally, Ab interaction with NF-ATp induces high affinity NF-kappaB site interaction, suggesting that nuclear partners may dramatically vary the specificity of the NF-AT family.

Redundant expression but selective utilization of nuclear factor of activated T cells family members

Nuclear factor of activated T cells (NF-AT) complexes regulate the induction of many early T cell activation molecules. Four related proteins can function as the cytoplasmic subunit of NF-AT, and their overlapping expression patterns and the mild phenotype of the NF-ATp null mice suggest that they may be functionally redundant. We characterized the distribution and activation of cytoplasmic NF-AT proteins in mature lymphocytes and found that NF-ATc, NF-ATp, and NF-AT4/x/c3 are co-expressed and co-regulated in mature T and B cells. Each protein forms independent DNA binding complexes, and at physiologic concentrations, NF-ATc and NF-ATp complexes out-compete NF-AT4/x/c3 for occupancy of NF-AT sites from the IL-2, IL-3/granulocyte-macrophage CSF, IL-4, and CD40 ligand genes. This predicts heavily redundant immune regulatory functions of NF-ATp and NF-ATc, but distinct activities for NF-AT4/x/c3. Additionally, Ab interaction with NF-ATp induces high affinity NF-kappaB site interaction, suggesting that nuclear partners may dramatically vary the specificity of the NF-AT family.

A range of CD4 T cell tolerance: partial inactivation to organ-specific antigen allows nondestructive thyroiditis or insulitis

T cell receptor (TCR) transgenic mice specific for hen egg lysozyme (HEL) were crossed with mice expressing HEL on the thyroid epithelium, on pancreatic islet beta cells, or systemically. Depending on the pattern of HEL expression, deletion of double-positive thymocytes ranged from minimal to complete, and peripheral CD4 cells exhibited graded reduction in TCR expression, in vitro responsiveness, and in vivo helper ability. CD4 cells were least tolerant in TCR/thyroid-HEL and TCR/islet-HEL mice, which developed an extensive lymphocytic thyroiditis or insulitis that nevertheless did not eliminate HEL-expressing endocrine cells. Autoreactive CD4 clones thus escape the thymus under a range of circumstances, retain sufficient function to initiate subclinical autoimmune inflammation when self-antigens are concentrated in the thyroid or pancreas, and may regulate progression of subclinical inflammation to destructive autoimmune disease.

Antigen-specific B cells preferentially induce CD4+ T cells to produce IL-4

The role of Ag presentation by B cells in regulating the development of T cells with restricted cytokine profiles remains controversial. In this report, we compared Ag presentation by naive polyclonal B cells, naive Ag-specific B cells (from Ig receptor transgenic mice), or splenic adherent cells (SAC) and examined the capacity of these cells to influence cytokine production by CD4+ T cells. Freshly isolated naive B cells stimulated vigorous T cell proliferation and very strong T cell cytokine responses, but only when cultured with Ag recognized by the B cell Ig receptor (cognate Ag) and not when cultured with a noncognate Ag. Under these conditions, B cells activated by Ig receptor-mediated endocytosis of Ag induced both naive and Ag-primed CD4+ T cells to produce high levels of IL-4 (300-4000 pg/ml). In contrast, SAC induced the production of very low levels of IL-4 (<100 pg/ml) but much higher maximal levels of IFN-gamma than did Ag-specific B cells. The induction of IL-4 synthesis by Ag-specific B cells was significantly reduced by blocking CD40-CD40 ligand (CD40L) interactions or by the addition of small quantities of rIL-12. These results suggest that B cells activated by their cognate Ag preferentially induce IL-4 synthesis as a result of the interaction of CD40L on T cells with CD40, whereas SAC preferentially induce IFN-gamma synthesis by T cells as a result of their greater production of IL-12 and their limited capacity to trigger CD40L on T cells.

Antigen-specific B cells preferentially induce CD4+ T cells to produce IL-4

The role of Ag presentation by B cells in regulating the development of T cells with restricted cytokine profiles remains controversial. In this report, we compared Ag presentation by naive polyclonal B cells, naive Ag-specific B cells (from Ig receptor transgenic mice), or splenic adherent cells (SAC) and examined the capacity of these cells to influence cytokine production by CD4+ T cells. Freshly isolated naive B cells stimulated vigorous T cell proliferation and very strong T cell cytokine responses, but only when cultured with Ag recognized by the B cell Ig receptor (cognate Ag) and not when cultured with a noncognate Ag. Under these conditions, B cells activated by Ig receptor-mediated endocytosis of Ag induced both naive and Ag-primed CD4+ T cells to produce high levels of IL-4 (300-4000 pg/ml). In contrast, SAC induced the production of very low levels of IL-4 (&lt;100 pg/ml) but much higher maximal levels of IFN-gamma than did Ag-specific B cells. The induction of IL-4 synthesis by Ag-specific B cells was significantly reduced by blocking CD40-CD40 ligand (CD40L) interactions or by the addition of small quantities of rIL-12. These results suggest that B cells activated by their cognate Ag preferentially induce IL-4 synthesis as a result of the interaction of CD40L on T cells with CD40, whereas SAC preferentially induce IFN-gamma synthesis by T cells as a result of their greater production of IL-12 and their limited capacity to trigger CD40L on T cells.

Differential activation of transcription factors induced by Ca2+ response amplitude and duration

An increase in the intracellular calcium ion concentration ([Ca2+]i) controls a diverse range of cell functions, including adhesion, motility, gene expression and proliferation. Calcium signalling patterns can occur as single transients, repetitive oscillations or sustained plateaux, but it is not known whether these patterns are responsible for encoding the specificity of cellular responses. We report here that the amplitude and duration of calcium signals in B lymphocytes controls differential activation of the pro-inflammatory transcriptional regulators NF-kappaB, c-Jun N-terminal kinase (JNK) and NFAT. NF-kappaB and JNK are selectively activated by a large transient [Ca2+]i rise, whereas NFAT is activated by a low, sustained Ca2+ plateau. Differential activation results from differences in the Ca2+ sensitivities and kinetic behaviour of the three pathways. Our results show how downstream effectors can decode information contained in the amplitude and duration of Ca2+ signals, revealing a mechanism by which a multifunctional second messenger such as Ca2+ can achieve specificity in signalling to the nucleus.

Different nuclear signals are activated by the B cell receptor during positive versus negative signaling

It is not known how immunogenic versus tolerogenic cellular responses are signaled by receptors such as the B cell antigen receptor (BCR). Here we compare BCR signaling in naive cells that respond positively to foreign antigen and self-tolerant cells that respond negatively to self-antigen. In naive cells, foreign antigen triggered a large biphasic calcium response and activated nuclear signals through NF-AT, NF-kappa B, JNK, and ERK/pp90rsk. In tolerant B cells, self-antigen stimulated low calcium oscillations and activated NF-AT and ERK/pp90rsk but not NF-kappa B or JNK. Self-reactive B cells lacking the phosphatase CD45 did not exhibit calcium oscillations or ERK/pp90rsk activation, nor did they repond negatively to self-antigen. These data reveal striking biochemical differences in BCR signaling to the nucleus during positive selection by foreign antigens and negative selection by self-antigens.

Lymphocyte homing: the scent of a follicle

The recent discovery of a receptor needed for lymphocyte migration into lymphoid follicles indicates that multiple chemoattractive gradients allow lymphocytes to navigate to specialized niches in lymphoid organs.

T cell-mediated elimination of B7.2 transgenic B cells

Transgenic mice were generated to explore the effects on lymphoid development and immune function of constitutive expression of murine B7.2 on B and T cells. The number of B lymphocytes in primary and secondary lymphoid tissues is normal in B7.2 transgenic lines expressing low levels of B7.2 on B cells, but markedly reduced in transgenic lines expressing moderate to high levels of the transgene on B cells. This reduction is not due to an intrinsic abnormality of the transgenic B cells, but is rather the consequence of an elimination by an immune mechanism requiring the engagement of CD28 on T cells. Interestingly, during cognate antigen-specific interaction with T cells in vivo, B7.2 transgenic B cells are not eliminated, but proliferate and differentiate normally. Our findings suggest that, in the absence of high affinity ligand for the TCR, the CD28-B7.2 system participates in the regulation of B cell homeostasis.

Glimpses into the balance between immunity and self-tolerance

The need to maintain self-tolerance is at odds with the need to draw upon antibody and T cell receptor diversity to fight infection. Advances in genetic manipulation of the mouse have at last brought into view the clonal selection mechanisms that underpin self-tolerance, confirming in general terms the notion of clonal deletion and clonal anergy put forward by Burnet and Nossal. The image that has emerged, however, is much more sophisticated than could have been imagined, revealing that self-reactive clones are deleted or held back in a remarkable series of culling checkpoints placed at many steps along the pathway to antibody production. These checkpoints act in concert to balance the nature and size of the holes in the repertoire generated by self-tolerance against the need to draw upon as many clones as possible for immunity to infection. Spontaneous and induced mutations in the mouse, such as Fas, PTP1C and CD45 mutations, have just begun to yield a few glimpses into the molecular circuitry underpinning these cellular checkpoints. Much more extensive genetic analysis, made possible by the genome project, will be needed to illuminate the details of those circuits and the factors that lead them to fail in autoimmune disease.

Expansion or elimination of B cells in vivo: dual roles for CD40- and Fas (CD95)-ligands modulated by the B cell antigen receptor

Signals from CD4+ T cells induce two opposite fates in B cells: clonal proliferation of B cells that bind specifically to foreign antigens and clonal deletion of equivalent B cells that bind self-antigens. This B cell fate decision is determined by the concerted action of two surface proteins on activated T cells, CD40-and Fas-ligands (CD40L and FasL), whose effects are switched by signals from the B cell antigen receptor (BCR). Foreign antigens that stimulate the BCR acutely cause CD40L and FasL to promote clonal proliferation. CD40L and FasL trigger deletion, however, when the BCRs become desensitized by chronic stimulation with self-antigens or when BCRs have not bound an antigen. The need for both Fas and CD40L to correctly regulate self-reactive B cell fate may explain the severe autoantibody disorders in Fas- or CD40L-deficient children.

Regulation of B-lymphocyte negative and positive selection by tyrosine phosphatase CD45

Elimination of self-reactive B cells must be balanced against the need for B-cell diversity for antibody responses to pathogens. To analyse factors that determine the extent of B-cell negative selection, we crossed CD45-deficient mice with mice carrying immunoglobulin transgenes specific for hen egg lysozyme (HEL). CD45 positively regulates antigen-receptor signalling and CD45-deficient HEL-specific B cells gave diminished signalling in response to HEL. Significantly, few mature CD45-/- B cells accumulated, despite normal immature B-cell production. Circulating HEL autoantigen mediates negative selection of mature CD45+/+ HEL-binding B cells but, in striking contrast, the autoantigen positively selected CD45-/- HEL-binding B cells, promoting their accumulation as long-lived IgD(hi) cells. These findings are consistent with a signal-threshold model for B-cell selection and demonstrate that changes in antigen receptor signalling can cause high-affinity self-reactive B cells to be actively retained instead of eliminated, thus revealing a potential mechanism for inherited susceptibility to autoimmune disease.

Danger - pathogen on the premises! Immunological tolerance

Recent results show that immune responses can be induced in neonatal mice. Do they really refute the traditional view that the ability to discriminate between 'self' and 'non-self' is a fundamental property of the immune system?

Intravenous injection of soluble antigen induces thymic and peripheral T-cells apoptosis

The mechanism by which tolerance is induced via systemic administration of high doses of aqueous antigen has been analyzed by using mice transgenic for a T-cell receptor specific for the influenza virus hemagglutinin (HA) peptide comprising amino acids 126-138. After intravenous injection of 750 (but not 75) micrograms of HA peptide, a state of hyporesponsiveness was rapidly induced. In the thymus, in situ apoptosis in the cortex and at the corticomedullary junction was responsible for a synchronous and massive deletion of CD4+ CD8+ thymocytes. In secondary lymphoid organs, HA-reactive T cells were initially activated but were hyporesponsive at the single cell level. After 3 days, however, those cells were rapidly deleted, at least partially, through an apoptotic process. Therefore, both thymic and peripheral apoptosis, in addition to T-cell receptor desensitization, contribute to high-dose tolerance.

Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Immunological self-tolerance is ensured by eliminating or inhibiting self-reactive lymphocyte clones, creating physical or functional holes in the B- and T-lymphocyte antigen receptor repertoires. The nature and size of these gaps in our immune defenses must be balanced against the necessity of mounting rapid immune responses to an everchanging array of foreign pathogens. To achieve this balance, only a fraction of particularly hazardous self-reactive clones appears to be physically eliminated from the repertoire in a manner that fully prevents their recruitment into an antimicrobial immune response. Many self-reactive cells are retained with a variety of conditional and potentially flexible restraints: (i) their ability to be triggered by antigen is diminished by mechanisms that tune down signaling by their antigen receptors, (ii) their ability to carry out inflammatory effector functions can be inhibited, and (iii) their capacity to migrate and persist is constrained. This balance between tolerance and immunity can be shifted, altering susceptibility to autoimmune disease and to infection by genetic or environmental differences either in the way antigens are presented, in the tuning molecules that adjust triggering set points for lymphocyte responses to antigen, or in the effector molecules that eliminate, retain, or expand particular clones.