Regulation of Bim by TCR signals in CD4/CD8 double-positive thymocytes

Destined for the intestine: thymic selection of TCRαβ CD8αα intestinal intraepithelial lymphocytes

The immune system is composed of a variety of different T-cell lineages distributed through both secondary lymphoid tissue and non-lymphoid tissue. The intestinal epithelium is a critical barrier surface that contains numerous intraepithelial lymphocytes that aid in maintaining homeostasis at that barrier. This review focuses on T-cell receptor αβ (TCRαβ) CD8αα intraepithelial lymphocytes, and how recent advances in the field clarify how this unique T-cell subset is selected, matures, and functions in the intestines. We consider how the available evidence reveals a story of ontogeny starting from agonist selection of T cells in the thymus and finishing through the specific signaling environment of the intestinal epithelium. We conclude with how this story raises further key questions about the development of different ontogenic waves of TCRαβ CD8αα IEL and their importance for intestinal epithelial homeostasis.

Transforming Growth Factor-beta signaling in αβ thymocytes promotes negative selection

In the thymus, the T lymphocyte repertoire is purged of a substantial portion of highly self-reactive cells. This negative selection process relies on the strength of TCR-signaling in response to self-peptide-MHC complexes, both in the cortex and medulla regions. However, whether cytokine-signaling contributes to negative selection remains unclear. Here, we report that, in the absence of Transforming Growth Factor beta (TGF-β) signaling in thymocytes, negative selection is significantly impaired. Highly autoreactive thymocytes first escape cortical negative selection and acquire a Th1-like-phenotype. They express high levels of CXCR3, aberrantly accumulate at the cortico-medullary junction and subsequently fail to sustain AIRE expression in the medulla, escaping medullary negative selection. Highly autoreactive thymocytes undergo an atypical maturation program, substantially accumulate in the periphery and induce multiple organ-autoimmune-lesions. Thus, these findings reveal TGF-β in thymocytes as crucial for negative selection with implications for understanding T cell self-tolerance mechanisms.

Regulation of Bim in Health and Disease

The BH3-only Bim protein is a major determinant for initiating the intrinsic apoptotic pathway under both physiological and pathophysiological conditions. Tight regulation of its expression and activity at the transcriptional, translational and post-translational levels together with the induction of alternatively spliced isoforms with different pro-apoptotic potential, ensure timely activation of Bim. Under physiological conditions, Bim is essential for shaping immune responses where its absence promotes autoimmunity, while too early Bim induction eliminates cytotoxic T cells prematurely, resulting in chronic inflammation and tumor progression. Enhanced Bim induction in neurons causes neurodegenerative disorders including Alzheimer's, Parkinson's and Huntington's diseases. Moreover, type I diabetes is promoted by genetically predisposed elevation of Bim in β-cells. On the contrary, cancer cells have developed mechanisms that suppress Bim expression necessary for tumor progression and metastasis. This review focuses on the intricate network regulating Bim activity and its involvement in physiological and pathophysiological processes.

Bim mediates the elimination of functionally unfit Th1 responders from the memory pool

Selective clonal deletion in the CD4⁺ T cell compartment during the transition from effector to memory is accompanied by enhanced expression of the pro-apoptotic Bcl-2 family member Bim. Here, we show that Bim deficiency enables the survival of poorly functional Th1 responders that are normally eliminated during contraction. However, rescued bim^−/− CD4⁺ “memory” T cells continued to demonstrate deficient effector functions, poor sensitivity to antigen and an inability to respond to secondary challenge. Our results demonstrate that Bim activity plays a key role in shaping the CD4⁺ memory T cell repertoire, ensuring the emergence of highly functional CD4⁺ memory T cells and the elimination of Th1 effector cells with sub-optimal function. We propose that Bim is a key mediator of T cell death in the absence of appropriate TCR-driven activation and differentiation.

Chagasic thymic atrophy does not affect negative selection but results in the export of activated CD4+CD8+ T cells in severe forms of human disease

Extrathymic CD4⁺CD8⁺ double-positive (DP) T cells are increased in some pathophysiological conditions, including infectious diseases. In the murine model of Chagas disease, it has been shown that the protozoan parasite Trypanosoma cruzi is able to target the thymus and induce alterations of the thymic microenvironment and the lymphoid compartment. In the acute phase, this results in a severe atrophy of the organ and early release of DP cells into the periphery. To date, the effect of the changes promoted by the parasite infection on thymic central tolerance has remained elusive. Herein we show that the intrathymic key elements that are necessary to promote the negative selection of thymocytes undergoing maturation during the thymopoiesis remains functional during the acute chagasic thymic atrophy. Intrathymic expression of the autoimmune regulator factor (Aire) and tissue-restricted antigen (TRA) genes is normal. In addition, the expression of the proapoptotic Bim protein in thymocytes was not changed, revealing that the parasite infection-induced thymus atrophy has no effect on these marker genes necessary to promote clonal deletion of T cells. In a chicken egg ovalbumin (OVA)-specific T-cell receptor (TCR) transgenic system, the administration of OVA peptide into infected mice with thymic atrophy promoted OVA-specific thymocyte apoptosis, further indicating normal negative selection process during the infection. Yet, although the intrathymic checkpoints necessary for thymic negative selection are present in the acute phase of Chagas disease, we found that the DP cells released into the periphery acquire an activated phenotype similar to what is described for activated effector or memory single-positive T cells. Most interestingly, we also demonstrate that increased percentages of peripheral blood subset of DP cells exhibiting an activated HLA-DR⁺ phenotype are associated with severe cardiac forms of human chronic Chagas disease. These cells may contribute to the immunopathological events seen in the Chagas disease.

The thymus is a primary lymphoid organ that plays an important role on the development of the immune system and maturation of the T cell repertoire. During the normal life span, this organ undergoes involution during the aging and also in the presence of a wide variety of infectious diseases. It has been shown that the protozoan parasite Trypanosoma cruzi is able to target the thymus and induce alterations of the thymic microenvironment. In the acute phase, this results in a severe atrophy of the organ and early release of immature double-positive (DP) T cells into the periphery. The effect of the changes promoted by the parasite infection on thymic central tolerance has remained not clear. The present study shows that the intrathymic key elements that promote the negative selection of thymocytes during the thymopoiesis remains functional in the acute chagasic thymic atrophy. However, we found that the DP cells released into the periphery acquire an activated phenotype and its high frequency in the peripheral blood are associated with severe cardiac forms of human chronic Chagas disease.

Interleukin-7 regulates Bim proapoptotic activity in peripheral T-cell survival

Interleukin-7 (IL-7) is critical for T-cell development and peripheral T-cell homeostasis. The survival of pro-T cells and mature T cells requires IL-7. The survival function of IL-7 is accomplished partly through induction of the antiapoptotic protein Bcl-2 and inhibition of proapoptotic proteins Bax and Bad. We show here that the proapoptotic protein Bim, a BH3-only protein belonging to the Bcl-2 family, also plays a role in peripheral T-cell survival. Deletion of Bim partially protected an IL-7-dependent T-cell line and peripheral T cells, especially cells with an effector memory phenotype, from IL-7 deprivation. However, T-cell development in the thymus was not restored in IL-7(-/-) Rag2(-/-) mice reconstituted with Bim(-/-) bone marrow. IL-7 withdrawal altered neither the intracellular location of Bim, which was constitutively mitochondrial, nor its association with Bcl-2; however, a reduction in its association with the prosurvival protein Mcl-1 was observed. IL-7 withdrawal did not increase Bim mRNA or protein expression but did induce changes in the isoelectric point of Bim(EL) and its reactivity with an antiphosphoserine antibody. Our findings suggest that the maintenance of peripheral T cells by IL-7 occurs partly through inhibition of Bim activity at the posttranslational level.

Apoptotic signal transduction and T cell tolerance

The healthy immune system makes use of a variety of surveillance mechanisms at different stages of lymphoid development to prevent the occurrence and expansion of potentially harmful autoreactive T cell clones. Disruption of these mechanisms may lead to inappropriate activation of T cells and the development of autoimmune and lymphoproliferative diseases [such as multiple sclerosis, rheumatoid arthritis, lupus erythematosus, diabetes and autoimmune lymphoproliferative syndrome (ALPS)]. Clonal deletion of T cells with high affinities for self-peptide-MHC via programmed cell death (apoptosis) is an essential mechanism leading to self-tolerance. Referred to as negative selection, central tolerance in the thymus serves as the first checkpoint for the developing T cell repertoire and involves the apoptotic elimination of potentially autoreactive T cells clones bearing high affinity T cell receptors (TCR) that recognize autoantigens presented by thymic epithelial cells. Autoreactive T cells that escape negative selection are held in check in the periphery by either functional inactivation ("anergy") or extrathymic clonal deletion, both of which are dependent on the strength and frequency of the TCR signal and the costimulatory context, or by regulatory T cells. This review provides an overview of the different molecular executioners of cell death programs that are vital to intrathymic or extrathymic clonal deletion of T cells. Further, the potential involvement of various apoptotic signaling paradigms are discussed with respect to the genesis and pathophysiology of autoimmune disease.

A proapoptotic signaling pathway involving RasGRP, Erk, and Bim in B cells

OBJECTIVE

Bryostatin-1 and related diacylglycerol (DAG) analogues activate RasGRPs in lymphocytes, thereby activating Ras and mimicking some aspects of immune receptor signaling. To define the role of RasGRPs in lymphocyte apoptosis and to identify potential therapeutic uses for DAG analogues in lymphocyte disorders, we characterized the response of lymphoma-derived cell lines to DAG analogues.

MATERIALS AND METHODS

Human lymphoma-derived B cell lines and mouse primary B cells were treated with bryostatin-1 or its synthetic analogue "pico." Ras signaling partners and Bcl-2 family members were studied with biochemical assays. Cellular responses were monitored using growth and apoptosis assays.

RESULTS

Stimulation of B cells with DAG analogues results in activation of protein kinase C/RasGRP-Ras-Raf-Mek-Erk signaling and phosphorylation of the proapoptotic BH3-only protein Bim. In vitro, Bim is phosphorylated by Erk on sites previously associated with increased apoptotic activity. In Toledo B cells derived from a non-Hodgkin's lymphoma (B-NHL), DAG analogue stimulation leads to extensive apoptosis. Apoptosis can be suppressed by either downregulation of Bim or overexpression of Bcl-2. It is associated with the formation of Bak-Bax complexes and increased mitochondrial membrane permeability. Toledo B-NHL cell apoptosis shows a striking dependence on sustained signaling.

CONCLUSION

In B cells, Erk activation leads directly to phosphorylation of Bim on sites associated with activation of Bim. In Toledo B-NHL cells, the dependence of apoptosis on sustained signaling suggests that Bcl-2 family members could interpret signal duration, an important determinant of B cell receptor-mediated negative selection. Certain cases of B-NHL might respond to DAG analogue treatment by the mechanism outlined here.

Transforming growth factor beta (TGFbeta)-induced apoptosis: the rise & fall of Bim

Transforming growth factor beta (TGFbeta) regulates essential cellular functions such as cellular proliferation, differentiation and apoptosis. Multiple apoptotic mediators and signaling pathways have been implicated in TGFbeta-induced apoptosis. Bim, a BH3-only protein, is critical for apoptosis in a variety of cell types. In resting cells, BimEL expression levels, the major and most abundant isoform, are controlled by Erk1/2-mediated phosphorylation, which targets BimEL for ubiquitination and degradation. We previously reported that TGFbeta induces the expression of the pro-apoptotic protein Bim through a Smad3-dependent mechanism to induce cell death in B-lymphocytes. A number of studies have shown TGFbeta to cause transcriptional induction of Bim in many cell types. Recently, we demonstrated that, in addition to its transcriptional effects on Bim, TGFbeta induces a MAPK phosphatase (MKP), MKP2/DUSP4, to rapidly increase BimEL levels by inactivation of Erk1/2, resulting in dephosphorylation and escape of BimEL from ubiquitin-mediated degradation. Our findings are of importance not only in the context that we implicate TGFbeta to increase BimEL levels through both an immediate post-translational regulatory mechanism and a long-term effect through transcriptional induction, but also in the context of implicating MKPs as regulatory players in apoptosis. Here we summarize these recent findings and their significance to our understanding of how TGFbeta mediates apoptosis, and we explore the possible regulatory mechanisms controlling Bim expression levels.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

Central tolerance: what have we learned from mice?

Producing a healthy immune system capable of defending against pathogens, while avoiding autoimmunity, is dependent on thymic selection. Positive selection yields functional T cells that have the potential to recognize both self and foreign antigens. Therefore, negative selection exists to manage potentially self-reactive cells. Negative selection results from the induction of anergy, receptor editing, clonal diversion (agonist selection), and/or clonal deletion (apoptosis) in self-reactive clones. Clonal deletion has been inherently difficult to study because the cells of interest are undergoing apoptosis and being eliminated quickly. Furthermore, analysis of clonal deletion in humans has proved even more difficult due to availability of samples and lack of reagents. Mouse models have thus been instrumental in achieving our current understanding of central tolerance, and the evolution of elegant model systems has led to an explosion of new data to be assimilated. This review will focus on recent advances in the field of clonal deletion with respect to three aspects: the development of physiological model systems, signaling pathways that lead to apoptosis, and antigen presenting cell types involved in the induction of clonal deletion.

Multisite phosphorylation regulates Bim stability and apoptotic activity

The proapoptotic BH3-only protein Bim is established to be an important mediator of signaling pathways that induce cell death. Multisite phosphorylation of Bim by several members of the MAP kinase group is implicated as a regulatory mechanism that controls the apoptotic activity of Bim. To test the role of Bim phosphorylation in vivo, we constructed mice with a series of mutant alleles that express phosphorylation-defective Bim proteins. We show that mutation of the phosphorylation site Thr-112 causes decreased binding of Bim to the antiapoptotic protein Bcl2 and can increase cell survival. In contrast, mutation of the phosphorylation sites Ser-55, Ser-65, and Ser-73 can cause increased apoptosis because of reduced proteasomal degradation of Bim. Together, these data indicate that phosphorylation can regulate Bim by multiple mechanisms and that the phosphorylation of Bim on different sites can contribute to the sensitivity of cellular apoptotic responses.

During negative selection, Nur77 family proteins translocate to mitochondria where they associate with Bcl-2 and expose its proapoptotic BH3 domain

Apoptosis accompanying negative selection is a central but poorly understood event in T cell development. The Nur77 nuclear steroid receptor and Bim, a proapoptotic BH3-only member of the Bcl-2 family, are two molecules implicated in this process. However, how they relate to each other and how Nur77 induces apoptosis remain unclear. In thymocytes, Nur77 has been shown to induce cell death through a transcriptional-dependent pathway, but in cancer cell lines, Nur77 was reported to induce apoptosis through conversion of Bcl-2 into a killer protein at the mitochondria. Whether this Nur77 transcriptional-independent pathway actually occurs in vivo remains controversial. Using an optimized fractionation protocol for thymocytes, here we report that stimulation of CD4⁺CD8⁺ thymocytes results in translocation of Nur77 and its family member Nor-1 to the mitochondria, leading to their association with Bcl-2 and exposure of the Bcl-2 proapoptotic BH3 domain. In two T cell receptor transgenic models of negative selection, F5 and HY, a conformational change of the Bcl-2 molecule in the negatively selected T cell population was similarly observed. Thus, the Nur77 family and Bim pathways converge at mitochondria to mediate negative selection.

What do we know about the mechanisms of elimination of autoreactive T and B cells and what challenges remain

Tolerance to self-antigens within the adaptive immune system is safeguarded, at least in part, through deletion of autoreactive T and B lymphocytes. This deletion can occur during the development of these cells in primary lymphoid organs, the thymus or bone marrow, respectively, or at the mature stage in peripheral lymphoid tissues. Deletion of autoreactive lymphocytes is achieved to a large extent through apoptotic cell death. This review describes current understanding of the mechanisms that mediate apoptosis of autoreactive lymphocytes during their development in primary lymphoid organs and during their activation in the periphery. In particular, we discuss the roles of the proapoptotic Bcl-2 family member Bim and the small family of Nur77-related transcriptional regulators in lymphocyte negative selection. Finally, we speculate on the processes that may lead to the activation of Bim when antigen receptors are activated on autoreactive T or B cells.

Bim and Bcl-2 mutually affect the expression of the other in T cells

The life and death of T cells is controlled to a large extent by the relative amounts of Bcl-2-related proteins they contain. The antiapoptotic protein Bcl-2 and the proapoptotic protein Bim are particularly important in this process with the amount of Bcl-2 per cell dropping by about one-half when T cells prepare to die. In this study we show that Bcl-2 and Bim each control the expression of the other. Absence of Bim leads to a drop in the amount of intracellular Bcl-2 protein, while having no effect on the amounts of mRNA for Bcl-2. Conversely, high amounts of Bcl-2 per cell allow high amounts of Bim, although in this case the effect involves increases in Bim mRNA. These mutual effects occur even if Bcl-2 is induced acutely. Thus these two proteins control the expression of the other, at either the protein or mRNA level.

Transcriptional analysis of clonal deletion in vivo

Engagement of the TCR on CD4(+)CD8(+) thymocytes initiates either a program of survival and differentiation (positive selection) or death (clonal deletion), which is dictated in large part by the affinity of the TCR for self-peptide-MHC complexes. Although much is known about the factors involved in positive selection, little is understood about the molecular mechanism leading to clonal deletion. To gain further insight into this process, we used a highly physiological TCR transgenic mouse model to compare gene expression changes under conditions of nonselection, positive selection, and negative selection. We identified 388 genes that were differentially regulated in negative selection compared with either nonselection or positive selection. These regulated genes fall into many functional categories including cell surface and intracellular signal transduction, survival and apoptosis, transcription and translation, and adhesion and migration. Additionally, we have compared our transcriptional profile to profiles of negative selection in other model systems in an effort to identify those genes with a higher probability of being functionally relevant. These included three up-regulated genes, bim, nur77, and ian1, and one down-regulated gene, lip1. Collectively, these data provide a framework for understanding the molecular basis of clonal deletion.

The lymphocyte guard-IANs: regulation of lymphocyte survival by IAN/GIMAP family proteins

The life-or-death decision of immune cells makes an essential contribution to immune-system development and the regulation of immune responses. A new family of cell-survival regulators expressed in lymphocytes, termed immune-associated nucleotide-binding proteins (IANs) [also known as GTPase of immunity-associated proteins (GIMAPs)], has been described. The IAN/GIMAP family consists of GTP-binding proteins that share a unique primary structure and whose expression is finely regulated by T-cell receptor signals. Recent studies have shown that IAN/GIMAP family proteins crucially regulate the survival of T cells during development, selection and homeostasis, and are possibly linked to the onset of T-lymphopenia, leukemia and autoimmunity. IAN/GIMAP family proteins might also take part in mitochondrial regulation of lymphocyte apoptosis by interacting with Bcl-2 family proteins.

T-cell receptor triggering differentially regulates bim expression in human lymphocytes from healthy individuals and patients with infectious mononucleosis

Bim, a proapoptotic member of the Bcl-2 protein family, is a major regulator of central and peripheral T-cell deletion. Regulation of Bim activity by T-cell receptor (TCR) triggering is not well understood. We investigated expression of Bim in different subpopulations of ex vivo isolated human T cells from healthy donors and patients with infectious mononucleosis (IM). Upregulation of Bim expression in response to TCR-triggering was observed only in a small proportion of analyzed samples of peripheral blood lymphocytes (PBLs) from healthy donors and only occasionally upon longitudinal analysis of cells isolated from the same individuals. Populations of naive or memory T cells enriched on the basis of CD45RO or CD45RA expression showed only slight and comparable Bim upregulation. In contrast, ex vivo isolated PBLs from IM patients in the acute stage of the disease with significant expansions of CD8+ cells expressed increased levels of Bim, and lymphocytes from the majority of analyzed IM patients exhibited significant upregulation of all major Bim isoforms in response to TCR triggering. These results demonstrate that at least some antigen-induced expansions of human CD8+ T cells are associated with increased levels of Bim, and TCR triggering in effector T lymphocytes may increase Bim activity by upregulation of its expression.

Physiologic regulation of central and peripheral T cell tolerance: lessons for therapeutic applications

Immunologic tolerance is a state of unresponsiveness that is specific for a particular antigen. The immune system has an extraordinary potential for making T cell and B cell that recognize and neutralize any chemical entity and microbe entering the body. Certainly, some of these T cells and B cells recognize self-components; therefore, cellular mechanisms have evolved to control the activity of these self-reactive cells and achieve immunological self-tolerance. The most important in vivo biological significance of mechanisms regulating self-tolerance is to prevent the immune system from mounting an attack against the host's own tissues resulting in autoimmunity. This review summarizes recent developments in our understanding of T-helper cell tolerance and discusses how the new findings can be exploited to prevent and treat autoimmune diseases, allergy, cancer, and chronic infection, or establish donor-specific transplantation tolerance.

Thymocyte Negative Selection Is Mediated by Protein Kinase C- and Ca2+-Dependent Transcriptional Induction of Bim

The processes of positive and negative selection in the thymus both determine the population of T cells that will enter the peripheral immune system and eliminate self-reactive T cells by apoptosis. Substantial evidence indicates that TCR signal intensity mediates this cell fate choice: low-intensity signals lead to survival and differentiation, whereas high-intensity signals generated by self-Ag lead to cell death. The molecular mechanism by which these graded signals are converted to discrete outcomes is not understood. Positive selection requires the Ca(2+)-dependent phosphatase calcineurin, whereas negative selection requires the proapoptotic Bcl-2 family member Bcl-2-interacting mediator of cell death (Bim). In this study, we investigated the regulation of Bim expression and the role of Ca(2+) in mediating negative selection. Our results show that transcription is necessary for both negative selection and Bim induction. Surprisingly, we also found that Ca(2+) is necessary for Bim induction. Induction of bim transcription appears to involve protein kinase C, but not calcineurin, JNK, p38 MAPK, or MEK. These results localize the decision point in positive vs negative selection to a step downstream of Ca(2+) signaling and suggest that negative selection signals induce Ca(2+)-dependent bim transcription through PKC.

Commitment issues: linking positive selection signals and lineage diversification in the thymus

The thymus is responsible for the production of CD4+ helper and CD8+ cytotoxic T cells, which constitute the cellular arm of the immune system. These cell types derive from common precursors that interact with thymic stroma in a T-cell receptor (TCR)-specific fashion, generating intracellular signals that are translated into function-specific changes in gene expression. This overall process is termed positive selection, but it encompasses a number of temporally distinct and possibly mechanistically distinct cellular changes, including rescue from apoptosis, initiation of cell differentiation, and commitment to the CD4+ or CD8+ T-cell lineage. One of the puzzling features of positive selection is how specificity of the TCR controls lineage commitment, as both helper and cytolytic T cells utilize the same antigen-receptor components, with the exception of the CD4 or CD8 coreceptors themselves. In this review, we focus on the signals required for positive selection, particularly as they relate to lineage commitment. Identification of genes encoding transcriptional regulators that play a role in T-cell development has led to significant recent advances in the field. We also provide an overview of nuclear factors in this context and, where known, how their regulation is linked to the same TCR signals that have been implicated in initiating and regulating positive selection.