In vivo calcium elevations in thymocytes with T cell receptors that are specific for self ligands

Dominant negative variants in ITPR3 impair T cell Ca2+ dynamics causing combined immunodeficiency

The importance of calcium (Ca2+) as a second messenger in T cell signaling is exemplified by genetic deficiencies of STIM1 and ORAI1, which abolish store-operated Ca2+ entry (SOCE) resulting in combined immunodeficiency (CID). We report five unrelated patients with de novo missense variants in ITPR3, encoding a subunit of the inositol 1,4,5-trisphosphate receptor (IP3R), which forms a Ca2+ channel in the endoplasmic reticulum (ER) membrane responsible for the release of ER Ca2+ required to trigger SOCE, and for Ca2+ transfer to other organelles. The patients presented with CID, abnormal T cell Ca2+ homeostasis, incompletely penetrant ectodermal dysplasia, and multisystem disease. Their predominant T cell immunodeficiency is characterized by significant T cell lymphopenia, defects in late stages of thymic T cell development, and impaired function of peripheral T cells, including inadequate NF-κB- and NFAT-mediated, proliferative, and metabolic responses to activation. Pathogenicity is not due to haploinsufficiency, rather ITPR3 protein variants interfere with IP3R channel function leading to depletion of ER Ca2+ stores and blunted SOCE in T cells.

Synthetic Biology Meets Ca2+ Release-Activated Ca2+ Channel-Dependent Immunomodulation

Many essential biological processes are triggered by the proximity of molecules. Meanwhile, diverse approaches in synthetic biology, such as new biological parts or engineered cells, have opened up avenues to precisely control the proximity of molecules and eventually downstream signaling processes. This also applies to a main Ca2+ entry pathway into the cell, the so-called Ca2+ release-activated Ca2+ (CRAC) channel. CRAC channels are among other channels are essential in the immune response and are activated by receptor-ligand binding at the cell membrane. The latter initiates a signaling cascade within the cell, which finally triggers the coupling of the two key molecular components of the CRAC channel, namely the stromal interaction molecule, STIM, in the ER membrane and the plasma membrane Ca2+ ion channel, Orai. Ca2+ entry, established via STIM/Orai coupling, is essential for various immune cell functions, including cytokine release, proliferation, and cytotoxicity. In this review, we summarize the tools of synthetic biology that have been used so far to achieve precise control over the CRAC channel pathway and thus over downstream signaling events related to the immune response.

CRAC Channels and Calcium Signaling in T Cell-Mediated Immunity

Calcium (Ca²⁺) signals play fundamental roles in immune cell function. The main sources of Ca²⁺ influx in mammalian lymphocytes following antigen receptor stimulation are Ca²⁺ release-activated Ca²⁺ (CRAC) channels. These are formed by ORAI proteins in the plasma membrane and are activated by stromal interaction molecules (STIM) located in the endoplasmic reticulum (ER). Human loss-of-function (LOF) mutations in ORAI1 and STIM1 that abolish Ca²⁺ influx cause a unique disease syndrome called CRAC channelopathy that is characterized by immunodeficiency autoimmunity and non-immunological symptoms. Studies in mice lacking Stim and Orai genes have illuminated many cellular and molecular mechanisms by which these molecules control lymphocyte function. CRAC channels are required for the differentiation and function of several T lymphocyte subsets that provide immunity to infection, mediate inflammation and prevent autoimmunity. This review examines new insights into how CRAC channels control T cell-mediated immunity.

The Role of Calcium-Calcineurin-NFAT Signaling Pathway in Health and Autoimmune Diseases

Calcium (Ca²⁺) is an essential signaling molecule that controls a wide range of biological functions. In the immune system, calcium signals play a central role in a variety of cellular functions such as proliferation, differentiation, apoptosis, and numerous gene transcriptions. During an immune response, the engagement of T-cell and B-cell antigen receptors induces a decrease in the intracellular Ca²⁺ store and then activates store-operated Ca²⁺ entry (SOCE) to raise the intracellular Ca²⁺ concentration, which is mediated by the Ca²⁺ release-activated Ca²⁺ (CRAC) channels. Recently, identification of the two critical regulators of the CRAC channel, stromal interaction molecule (STIM) and Orai1, has broadened our understanding of the regulatory mechanisms of Ca²⁺ signaling in lymphocytes. Repetitive or prolonged increase in intracellular Ca²⁺ is required for the calcineurin-mediated dephosphorylation of the nuclear factor of an activated T cell (NFAT). Recent data indicate that Ca²⁺-calcineurin-NFAT1 to 4 pathways are dysregulated in autoimmune diseases. Therefore, calcineurin inhibitors, cyclosporine and tacrolimus, have been used for the treatment of such autoimmune diseases as systemic lupus erythematosus and rheumatoid arthritis. Here, we review the role of the Ca²⁺-calcineurin–NFAT signaling pathway in health and diseases, focusing on the STIM and Orai1, and discuss the deregulated calcium-mediated calcineurin-NFAT pathway in autoimmune diseases.

STIM- and Orai-mediated calcium entry controls NF-κB activity and function in lymphocytes

The central role of Ca²⁺ signaling in the development of functional immunity and tolerance is well established. These signals are initiated by antigen binding to cognate receptors on lymphocytes that trigger store operated Ca²⁺ entry (SOCE). The underlying mechanism of SOCE in lymphocytes involves TCR and BCR mediated activation of Stromal Interaction Molecule 1 and 2 (STIM1/2) molecules embedded in the ER membrane leading to their activation of Orai channels in the plasma membrane. STIM/Orai dependent Ca²⁺ signals guide key antigen induced lymphocyte development and function principally through direct regulation of Ca²⁺ dependent transcription factors. The role of Ca²⁺ signaling in NFAT activation and signaling is well known and has been studied extensively, but a wide appreciation and mechanistic understanding of how Ca²⁺ signals also shape the activation and specificity of NF-κB dependent gene expression has lagged. Here we discuss and interpret what is known about Ca²⁺ dependent mechanisms of NF-kB activation, including what is known and the gaps in our understanding of how these signals control lymphocyte development and function.

Ion channels in innate and adaptive immunity

Ion channels and transporters mediate the transport of charged ions across hydrophobic lipid membranes. In immune cells, divalent cations such as calcium, magnesium, and zinc have important roles as second messengers to regulate intracellular signaling pathways. By contrast, monovalent cations such as sodium and potassium mainly regulate the membrane potential, which indirectly controls the influx of calcium and immune cell signaling. Studies investigating human patients with mutations in ion channels and transporters, analysis of gene-targeted mice, or pharmacological experiments with ion channel inhibitors have revealed important roles of ionic signals in lymphocyte development and in innate and adaptive immune responses. We here review the mechanisms underlying the function of ion channels and transporters in lymphocytes and innate immune cells and discuss their roles in lymphocyte development, adaptive and innate immune responses, and autoimmunity, as well as recent efforts to develop pharmacological inhibitors of ion channels for immunomodulatory therapy.

Agonist-selected T cell development requires strong T cell receptor signaling and store-operated calcium entry

T cell receptor (TCR) signaling driven by interaction of the TCR with specific complexes of self-peptide and the major histocompatibility complex determines T cell fate in thymic development. However, the signaling pathway through which TCR signal strength regulates distinct T cell lineages remains unknown. Here we have used mice lacking the endoplasmic reticulum Ca2+ sensors stromal interaction molecule 1 (STIM1) and STIM2 to show that STIM-induced store-operated Ca2+ entry is not essential for thymic development of conventional TCRαβ+ T cells but is specifically required for the development of agonist-selected T cells (regulatory T cells, invariant natural killer T cells, and TCRαβ+ CD8αα+ intestinal intraepithelial lymphocytes). The severe impairment of agonist-selected T cell development is mainly due to a defect in interleukin-2 (IL-2) or IL-15 signaling. Thus, STIM1 and STIM2-mediated store-operated Ca2+ influx, leading to efficient activation of NFAT (nuclear factor of activated T cells), is critical for the postselection maturation of agonist-selected T cells.

Calcium signalling and the regulation of apoptosis

Apoptotic cell death is characterized by cell shrinkage, chromatin condensation and fragmentation, formation of apoptotic bodies and phagocytosis (Kerr et al., 1972). At the molecular level, activation of a family of cysteine proteases, caspases, related to interleukin-1beta-converting enzyme is believed to be a crucial event in apoptosis. This is associated with the proteolysis of nuclear and cytoskeletal proteins, cell shrinkage, glutathione efflux, exposure of phosphatidylserine on the cell surface, membrane blebbing, etc. In CD95- or TNF-mediated apoptosis, the proteolytic cascade is believed to be triggered directly by caspase binding to the activated plasma membrane receptor complex. In other forms of apoptosis, the mechanisms of activation of the proteolytic cascade are less well established but may involve imported proteases, such as granzyme B, or factors released from the mitochondria and, possibly, other organelles. Recently, the possibility that cytochrome c released from the mitochondria may serve to activate dormant caspases in the cytosol, and thereby to propagate the apoptotic process, has attracted considerable attention. A perturbation of intracellular Ca(2+) homeostasis has been found to trigger apoptosis in many experimental systems, and the apoptotic process has been related to either a sustained increase in cytosolic free Ca(2+) level or a depletion of intracellular Ca(2+) stores. Although many of the biochemical events involved in the apoptotic process are Ca(2+) dependent, the exact mechanism by which Ca(2+) triggers apoptosis remains unknown. The bcl-2 gene family, which includes both inhibitors and inducers of apoptosis, appears to regulate intracellular Ca(2+) compartmentalization. The induction of apoptosis by Ca(2+)-mobilizing agents results in caspase activation, which is similar to what is seen with other inducers of apoptosis. In addition, Ca(2+)-dependent proteases, such as calpain and a Ca(2+)-dependent nuclear scaffold-associated serine protease, are also activated by Ca(2+) signalling in some cell types where they appear to be involved in alpha-fodrin and lamin beta cleavage, respectively. Thus, a spectrum of proteases are activated during apoptosis depending on both cell type and inducer. This proteolytic cascade can involve both caspases and Ca(2+)-dependent proteases, which seem to interact during the apoptotic process.

Differential STIM1 expression in T and B cell subsets suggests a role in determining antigen receptor signal amplitude

Ca(2+) acts ubiquitously as a second messenger in transmembrane signal transduction. In lymphocytes, calcium mobilization is triggered by antigen and chemokine receptors, among others, and controls cell functions ranging from proliferation to migration. The primary mechanism of extracellular Ca(2+) entry in lymphocytes is the CRAC influx. STIM1 is a crucial component of the CRAC influx mechanism in lymphocytes, acting as a sensor of low Ca(2+) concentration in the ER and an activator of the Ca(2+) selective channel ORAI1 in the plasma membrane. While STIM1 function has been studied extensively, little is known regarding whether it is differentially expressed and thereby affects the magnitude of calcium mobilization responses. We report here that STIM1 expression differs in murine T and B lymphocytes, and in respective subsets. For example, mature T cells express ∼4 times more STIM1 than mature B cells. Furthermore, we show that through the physiologic range of expression, STIM1 levels determine the magnitude of Ca(2+) influx responses that follow BCR-induced intracellular store depletion. Considered in view of previous reports that differences in amplitude of lymphocyte Ca(2+) mobilization determine alternate biological responses, these findings suggest that differential STIM1 expression may be important determinant of biological responses.

The essential role of evasion from cell death in cancer

The link between evasion of apoptosis and the development of cellular hyperplasia and ultimately cancer is implicitly clear if one considers how many cells are produced each day and, hence, how many cells must die to make room for the new ones (reviewed in Raff, 1996). Furthermore, cells are frequently experiencing noxious stimuli that can cause lesions in their DNA and faults in DNA replication can occur during cellular proliferation. Such DNA damage needs to be repaired efficiently or cells with irreparable damage must be killed to prevent subsequent division of aberrant cells that may fuel tumorigenesis (reviewed in Weinberg, 2007). The detection of genetic lesions in human cancers that activate prosurvival genes or disable proapoptotic genes have provided the first evidence that defects in programmed cell death can cause cancer (Tagawa et al., 2005; Tsujimoto et al., 1984; Vaux, Cory, and Adams, 1988) and this concept was proven by studies with genetically modified mice (Egle et al., 2004b; Strasser et al., 1990a). It is therefore now widely accepted that evasion of apoptosis is a requirement for both neoplastic transformation and sustained growth of cancer cells (reviewed in Cory and Adams, 2002; Hanahan and Weinberg, 2000; Weinberg, 2007). Importantly, apoptosis is also a major contributor to anticancer therapy-induced killing of tumor cells (reviewed in Cory and Adams, 2002; Cragg et al., 2009). Consequently, a detailed understanding of apoptotic cell death will help to better comprehend the complexities of tumorigenesis and should assist with the development of improved targeted therapies for cancer based on the direct activation of the apoptotic machinery (reviewed in Lessene, Czabotar, and Colman, 2008).

The VDAC2-BAK rheostat controls thymocyte survival

The proapoptotic proteins BAX and BAK constitute the mitochondrial apoptotic gateway that executes cellular demise after integrating death signals. The lethal BAK is kept in check by voltage-dependent anion channel 2 (VDAC2), a mammalian-restricted VDAC isoform. Here, we provide evidence showing a critical role for the VADC2-BAK complex in determining thymocyte survival in vivo. Genetic depletion of Vdac2 in the thymus resulted in excessive cell death and hypersensitivity to diverse death stimuli including engagement of the T cell receptor. These phenotypes were completely rescued by the concurrent deletion of Bak but not that of Bax. Thus, the VDAC2-BAK axis provides a mechanism that governs the homeostasis of thymocytes. Our study reveals a sophisticated built-in rheostat that likely fine-tunes immune competence to balance autoimmunity and immunodeficiency.

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.

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.

Cannabinoid-mediated elevation of intracellular calcium: a structure-activity relationship

This laboratory has reported previously that Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and cannabinol (CBN) robustly elevate intracellular calcium ([Ca(2+)](i)) in resting human and murine T cells, whereas CP55,940 [5-(1,1-dimethylheptyl)-2-(5-hydroxy-2-(3-hydroxypropyl)cyclohexyl)phenol], a high-affinity ligand for CB1 and CB2, does not. In light of our previous studies, the objective of the present investigation was to examine the ability of various cannabinoid compounds to elevate [Ca(2+)](i) in the CB2 receptor-expressing human peripheral blood acute lymphoid leukemia T cell line and the dependence of structural similarity to Delta(9)-THC therein. The present studies demonstrate that CBN and HU-210 [(6aR,10aR)-3-(1,1-dimethylbutyl)-6a,7,10,10a-tetrahydro-6,6-dimethyl-6H-dibenzo[b,d]pyran-9-methanol], both tricyclic and in that respect structurally similar to Delta(9)-THC, elevate [Ca(2+)](i). The [Ca(2+)](i) elevation elicited by both CBN and HU-210 was attenuated upon removal of extracellular calcium and upon pretreatment with SK&F96365 [1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole], an inhibitor of receptor-operated cation channels. In addition, pretreatment with either CB1 or CB2 receptor antagonists attenuated the CBN- and HU-210-mediated [Ca(2+)](i) elevation. Further investigation of the dependence of Delta(9)-THC, CBN, and HU-210 on cannabinoid receptors using splenocytes from wild-type and CB1(-/-)/CB2(-/-) mice showed that the [Ca(2+)](i) elevation elicited by all three tricyclic cannabinoids was independent of CB1 and CB2. Moreover, both the CB1 and CB2 receptor antagonists attenuated that rise in [Ca(2+)](i) elicited by the tricyclic cannabinoids in the wild-type and CB1(-/-)/CB2(-/-) mouse splenocytes. Taken together, the present results demonstrate that classic tricyclic cannabinoids with structural similarity to Delta(9)-THC elicit a robust influx of calcium in T cells putatively through receptor-operated cation channels in a manner sensitive to the cannabinoid receptor antagonists, but independent of the CB1 and CB2 receptors.

Bcl-2 differentially regulates Ca2+ signals according to the strength of T cell receptor activation

To investigate the effect of Bcl-2 on Ca²⁺ signaling in T cells, we continuously monitored Ca²⁺ concentration in Bcl-2–positive and –negative clones of the WEHI7.2 T cell line after T cell receptor (TCR) activation by anti-CD3 antibody. In Bcl-2–negative cells, high concentrations of anti-CD3 antibody induced a transient Ca²⁺ elevation, triggering apoptosis. In contrast, low concentrations of anti-CD3 antibody induced Ca²⁺ oscillations, activating the nuclear factor of activated T cells (NFAT), a prosurvival transcription factor. Bcl-2 blocked the transient Ca²⁺ elevation induced by high anti-CD3, thereby inhibiting apoptosis, but did not inhibit Ca²⁺ oscillations and NFAT activation induced by low anti-CD3. Reduction in the level of all three inositol 1,4,5-trisphosphate (InsP₃) receptor subtypes by small interfering RNA inhibited the Ca²⁺ elevation induced by high but not low anti-CD3, suggesting that Ca²⁺ responses to high and low anti-CD3 may have different requirements for the InsP₃ receptor. Therefore, Bcl-2 selectively inhibits proapoptotic Ca²⁺ elevation induced by strong TCR activation without hindering prosurvival Ca²⁺ signals induced by weak TCR activation.

Regulation of peripheral T cell activation by calreticulin

Regulated expression of positive and negative regulatory factors controls the extent and duration of T cell adaptive immune response preserving the organism's integrity. Calreticulin (CRT) is a major Ca2+ buffering chaperone in the lumen of the endoplasmic reticulum. Here we investigated the impact of CRT deficiency on T cell function in immunodeficient mice reconstituted with fetal liver crt-/- hemopoietic progenitors. These chimeric mice displayed severe immunopathological traits, which correlated with a lower threshold of T cell receptor (TCR) activation and exaggerated peripheral T cell response to antigen with enhanced secretion of inflammatory cytokines. In crt-/- T cells TCR stimulation induced pulsatile cytosolic elevations of Ca2+ concentration and protracted accumulation of nuclear factor of activated T cells in the nucleus as well as sustained activation of the mitogen-activated protein kinase pathways. These observations support the hypothesis that CRT-dependent shaping of Ca2+ signaling critically contributes to the modulation of the T cell adaptive immune response.

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.

LAT-mediated signaling in CD4+CD25+ regulatory T cell development

Engagement of the T cell receptor for antigen (TCR) induces formation of signaling complexes mediated through the transmembrane adaptor protein, the linker for activation of T cells (LAT). LAT plays an important role in T cell development, activation, and homeostasis. A knock-in mutation at Tyr136, which is the phospholipase C (PLC)-γ1–binding site in LAT, leads to a severe autoimmune disease in mice. In this study, we show that CD4⁺CD25⁺ T reg cells that expressed Foxp3 transcription factor were nearly absent in both thymus and peripheral lymphoid organs of LAT^Y136F mice. This defect was not a result of the autoimmune environment as LAT^Y136F T reg cells also failed to develop in healthy LAT^−/− mice that received mixed wild-type and LAT^Y136F bone marrow cells. Moreover, adoptive transfer of normal CD4⁺CD25⁺ T reg cells protected neonatal LAT^Y136F mice from developing this disease. These T reg cells effectively controlled expansion of CD4⁺ T cells in LAT^Y136F mice likely via granzymes and/or TGF-β–mediated suppression. Furthermore, ectopic expression of Foxp3 conferred a suppressive function in LAT^Y136F T cells. Our data indicate that the LAT–PLC-γ1 interaction plays a critical role in Foxp3 expression and the development of CD4⁺CD25⁺ T reg cells

Induction of intracellular calcium elevation by Δ9-tetrahydrocannabinol in T cells involves TRPC1 channels

We have reported previously that Delta9-tetrahydrocannabinol (Delta9-THC) treatment of resting human and murine splenic T cells robustly elevated intracellular calcium ([Ca2+]i). The objective of the present investigation was to examine the putative role of [Ca2+]i store depletion and store-operated calcium (SOC) and receptor-operated cation (ROC) channels in the mechanism by which Delta9-THC increases [Ca2+]i in the cannabinoid-2 receptor-expressing human peripheral blood-acute lymphoid leukemia (HPB-ALL) human T cell line. By using the smooth endoplasmic reticulum Ca2+-ATPase pump inhibitor, thapsigargin, and the ryanodine receptor antagonist, 8-bromo-cyclic adenosine diphosphate ribose, we demonstrate that the Delta9-THC-mediated elevation in [Ca2+]i occurs independently of [Ca2+]i store depletion. Furthermore, the ROC channel inhibitor, SK&F 96365 was more efficacious at attenuating the Delta9-THC-mediated elevation in [Ca2+]i than SOC channel inhibitors, 2-aminoethoxydiphenyl borate and La3+. Recently, several members of the transient receptor potential canonical (TRPC) channel subfamily have been suggested to operate as SOC or ROC channels. In the present studies, treatment of HPB-ALL cells with 1-oleoyl-2-acetyl-sn-glycerol (OAG), a cell-permeant analog of diacylglycerol (DAG), which gates several members of the TRPC channel subfamily, rapidly elevated [Ca2+]i, as well as prevented a subsequent, additive elevation in [Ca2+]i by Delta9-THC, independent of protein kinase C. Reverse transcriptase-polymerase chain reaction analysis for TRPC1-7 showed that HPB-ALL cells express detectable mRNA levels of only TRPC1. Finally, small interference RNA knockdown of TRPC1 attenuated the Delta9-THC-mediated elevation of [Ca2+]i. Collectively, these results suggest that Delta9-THC-induced elevation in [Ca2+]i is attributable entirely to extracellular calcium influx, which is independent of [Ca2+]i store depletion, and is mediated, at least partially, through the DAG-sensitive TRPC1 channels.

Calcium oscillations regulate thymocyte motility during positive selection in the three-dimensional thymic environment

The three-dimensional thymic microenvironment and calcium signaling pathways are essential for driving positive selection of developing T cells. However, the nature of calcium signals and the diversity of their effects in the thymus are unknown. We describe here a thymic slice preparation for visualizing thymocyte motility and signaling in real time with two-photon microscopy. Naive thymocytes were highly motile at low intracellular calcium concentrations, but during positive selection cells became immobile and showed sustained calcium concentration oscillations. Increased intracellular calcium was necessary and sufficient to arrest thymocyte motility. The calcium dependence of motility acts to prolong thymocyte interactions with antigen-bearing stromal cells, promoting sustained signaling that may enhance the expression of genes underlying positive selection.

Cannabinoid receptor-mediated regulation of intracellular calcium by Δ9-tetrahydrocannabinol in resting T cells

Cannabinoids exhibit broad immune modulating activity by targeting many cell types within the immune system, including T cells, which exhibit sensitivity, as evidenced by altered activation, proliferation, and cytokine expression. As a result of the critical role calcium plays in T cell function coupled with previous findings demonstrating disruption of the calcium-regulated transcription factor, nuclear factor of activated T cells, by cannabinoid treatment, the objective of the present investigation was to perform an initial characterization of the role of the cannabinoid receptors in the regulation of the intracellular calcium concentration ([Ca(2+)](i)) by delta(9)-tetrahydrocannabinol (delta(9)-THC) in T lymphocytes. Here, we demonstrate that delta(9)-THC robustly elevates [Ca(2+)](i) in purified murine splenic T cells and in the human peripheral blood acute lymphoid leukemia (HPB-ALL) human T cell line but only minimally elevates [Ca(2+)](i) in Jurkat E6-1 (dysfunctional cannabinoid receptor 2-expressing) human T cells. Removal of extracellular calcium severely attenuated the delta(9)-THC-mediated rise in [Ca(2+)](i) in murine splenic T cells and HPB-ALL cells. Pretreatment with cannabinoid receptor antagonists, SR144528 and/or SR141716A, led to an attenuation of delta(9)-THC-mediated elevation in [Ca(2+)](i) in splenic T cells and HPB-ALL cells but not in Jurkat E6-1 cells. Furthermore, pretreatment of HPB-ALL cells with SR144528 antagonized the small rise in [Ca(2+)](i) elicited by delta(9)-THC in the absence of extracellular calcium. These findings suggest that delta(9)-THC induces an influx of extracellular calcium in resting T cells in a cannabinoid receptor-dependent manner.

The control of apoptosis in lymphocyte selection

The stochastic nature of rearrangement and diversification of the gene segments encoding immunoglobulins (Igs) and T cell receptors (TCRs) inevitably gives rise to immature B and T lymphocytes that lack antigen receptors or express useless or dangerous (self-antigen-specific) ones. Signaling through antigen receptors promotes survival, proliferative expansion and further differentiation of useful cells and deletion of the useless and dangerous ones. During immune responses, pathogen-specific B and T lymphocytes, as well as cells of the innate immune system, undergo extensive proliferation and develop effector functions, such as antibody secretion, cytotoxicity or cytokine production. To prevent tissue damage by these effector molecules, activated lymphocytes are removed when an infection has been overcome. Together with other mechanisms, including developmental arrest and induction of unresponsiveness (anergy), programmed cell death (apoptosis) of autoreactive lymphocytes safeguards immunological tolerance to self and assists in the development of an effective immune system. We have been investigating the molecular mechanisms that control programmed cell death. This review describes some of our experiments using transgenic and knockout mice, which overexpress or lack apoptosis regulators, that led to discoveries on how life and death decisions are made during development and functioning of the immune system.

T cell hyporesponsiveness induced by oral administration of ovalbumin is associated with impaired NFAT nuclear translocation and p27kip1 degradation

Oral tolerance is an important physiological component of the immune system whereby the organism avoids dangerous reactions such as hypersensitivity to ingested food proteins and other luminal Ags which may cause tissue damage and inflammation. In addition, it has been shown in animal models and in humans that oral tolerance can be applied to controlling undesired immune responses, including autoimmune diseases, allergies, and organ transplant rejections. However, the molecular mechanisms of oral tolerance have been poorly defined. In this study, we investigated the molecular basis underlying the hyporesponsiveness of orally tolerant CD4 T cells using a TCR transgenic mouse system in which oral tolerance was induced by long-term feeding with high dose Ag. We demonstrate that the hyporesponsive state of the CD4 T cells was maintained by a selective impairment in the TCR-induced calcium/NFAT signaling pathway and in the IL-2R-induced degradation of p27(kip1) and cell cycle progression. Thus, physiological mucosal tolerance is revealed to be associated with a unique type of T cell hyporesponsiveness which differs from previously described anergic T cells.

BH3-only Bcl-2 family member Bim is required for apoptosis of autoreactive thymocytes

During lymphocyte development, the assembly of genes coding for antigen receptors occurs by the combinatorial linking of gene segments. The stochastic nature of this process gives rise to lymphocytes that can recognize self-antigens, thereby having the potential to induce autoimmune disease. Such autoreactive lymphocytes can be silenced by developmental arrest or unresponsiveness (anergy), or can be deleted from the repertoire by cell death. In the thymus, developing T lymphocytes (thymocytes) bearing a T-cell receptor (TCR)-CD3 complex that engages self-antigens are induced to undergo programmed cell death (apoptosis), but the mechanisms ensuring this 'negative selection' are unclear. We now report that thymocytes lacking the pro-apoptotic Bcl-2 family member Bim (also known as Bcl2l11) are refractory to apoptosis induced by TCR-CD3 stimulation. Moreover, in transgenic mice expressing autoreactive TCRs that provoke widespread deletion, Bim deficiency severely impaired thymocyte killing. TCR ligation upregulated Bim expression and promoted interaction of Bim with Bcl-XL, inhibiting its survival function. These findings identify Bim as an essential initiator of apoptosis in thymocyte-negative selection.

Requirement for Ca2+/calmodulin-dependent kinase type IV/Gr in setting the thymocyte selection threshold

The outcome of thymocyte selection is influenced by the nature of Ca2+ signals transduced by the TCR. Robust Ca2+ responses characterize high-affinity, negatively selecting peptide/TCR interactions, while modest responses typify lower-affinity, positively selecting interactions. To elucidate mechanisms by which thymocytes decode distinct Ca2+ signals, we examined selection events in mice lacking Ca2+/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr), which is enriched in thymocytes. CaMKIV/Gr-deficient thymocytes exhibited impaired positive selection and defective Ca2+-dependent gene transcription. Significantly, CaMKIV/Gr deficiency raised the selection threshold of peptide/TCR interactions such that a peptide that normally induced weak negative selection instead promoted positive selection. These results demonstrate an important role for CaMKIV/Gr in sensitizing thymocytes to selection by low-affinity peptides.

Increased expression of apoptosis-linked gene 2 (ALG2) in the rat brain after temporary focal cerebral ischemia

Calcium is an important mediator of programmed cell death induced by transient cerebral ischemia, and calcium-binding proteins have been implicated in calcium-regulated signal transduction. Apoptosis-linked gene 2 is a calcium-binding protein required for cell death induced by different apoptotic stimuli. By Western blot analysis, we found that apoptosis-linked gene 2 protein was expressed in normal brains, and that expression increased in ischemic brains after 20 or 90 min of transient focal cerebral ischemia. Immunocytochemistry showed increased apoptosis-linked gene 2 protein expression in frontal cortex, a region where neurons underwent ischemic stress but still survived, after 20 or 90 min of focal cerebral ischemia. Apoptosis-linked gene 2 protein was also up-regulated in the ischemic border-zone of parietal cortex 24h after 20 min of focal ischemia, and was remarkably over-expressed in the caudate-putamen and parietal cortex, (where cells are destined to die) 24h after 90 min of ischemia. The expression pattern of apoptosis-linked gene 2 protein was similar to that of deoxyribonucleic acid damage detected by Klenow labeling assay. Our results suggest that apoptosis-linked gene 2 may be involved in the regulation of cell death after transient focal cerebral ischemia.

A role for the Tec family tyrosine kinase Txk in T cell activation and thymocyte selection

Recent data indicate that several members of the Tec family of protein tyrosine kinases function in antigen receptor signal transduction. Txk, a Tec family protein tyrosine kinase, is expressed in both immature and mature T cells and in mast cells. By overexpressing Txk in T cells throughout development, we found that Txk specifically augments the phospholipase C (PLC)-gamma1-mediated calcium signal transduction pathway upon T cell antigen receptor (TCR) engagement. Although Txk is structurally different from inducible T cell kinase (Itk), another Tec family member expressed in T cells, expression of the Txk transgene could partially rescue defects in positive selection and signaling in itk(-)(/)(-) mice. Conversely, in the itk(+/+) (wild-type) background, overexpression of Txk inhibited positive selection of TCR transgenic thymocytes, presumably due to induction of cell death. These results identify a role for Txk in TCR signal transduction, T cell development, and selection and suggest that the Tec family kinases Itk and Txk perform analogous functions.

Receptor avidity and costimulation specify the intracellular Ca2+ signaling pattern in CD4(+)CD8(+) thymocytes

Thymocyte maturation is governed by antigen-T cell receptor (TCR) affinity and the extent of TCR aggregation. Signals provided by coactivating molecules such as CD4 and CD28 also influence the fate of immature thymocytes. The mechanism by which differences in antigen-TCR avidity encode unique maturational responses of lymphocytes and the influence of coactivating molecules on these signaling processes is not fully understood. To better understand the role of a key second messenger, calcium, in governing thymocyte maturation, we measured the intracellular free calcium concentration ([Ca2+]i) response to changes in TCR avidity and costimulation. We found that TCR stimulation initiates either amplitude- or frequency-encoded [Ca2+]i changes depending on (a) the maturation state of stimulated thymocytes, (b) the avidity of TCR interactions, and (c) the participation of specific coactivating molecules. Calcium signaling within immature but not mature thymocytes could be modulated by the avidity of CD3/CD4 engagement. Low avidity interactions induced biphasic calcium responses, whereas high avidity engagement initiated oscillatory calcium changes. Notably, CD28 participation converted the calcium response to low avidity receptor engagement from a biphasic to oscillatory pattern. These data suggest that calcium plays a central role in encoding the nature of the TCR signal received by thymocytes and, consequently, a role in thymic selection.

Signals involved in thymocyte positive and negative selection

Extensive research has focused upon understanding how thymocytes distinguish between interactions that lead to positive or negative selection. Various intracellular pathways that are activated after TCR engagement are outlined in this review, and their contribution to thymocyte selection is discussed. Although thymocyte fate is generally governed by a quantitative/avidity model, this largely reflects the interactions that occur at the cell surface. Therefore, we outline possible models of how different intercellular interactions are translated into intracellular signals that diverge and lead to thymocyte survival or death.

ATP-evoked Ca2+ transients and currents in murine thymocytes: possible role for P2X receptors in death by neglect

The P2X family of ATP receptors (P2XR) have been implicated in thymocyte death in vitro and in vivo. We characterized ATP-evoked Ca2+ transients and membrane currents in thymocytes to better understand the role of P2XR during thymocyte development. ATP4-, but not UTP or GTP, activated a sustained non-selective cation current in voltage-clamped CD4- CD8- and CD4+ CD8+ thymocytes that was reversed by apyrase, which hydrolyzes ATP, and by the P2XR antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). The more selective P2XR agonist alphabeta-methylene ATP activated a smaller rapidly decaying current in both thymocyte populations. Reverse transcription-PCR results indicate that P2X1, P2X2, P2X6, and/or P2X7 are expressed in thymocytes. Finally, we used PPADS to examine the role of P2XR during thymocyte development in situ. PPADS-treated thymi yielded significantly more thymocytes (38%), due to a selective increase in CD4+ CD8+ cells. Together these data suggest that one or more PPADS-sensitive P2XR (P2X1, P2X2, P2X7) are involved in thymocyte apoptosis, and we propose more specifically a role associated with death by neglect.

Cloning of AIP1, a novel protein that associates with the apoptosis-linked gene ALG-2 in a Ca2+-dependent reaction

ALG-2 is a 22-kDa calcium-binding protein necessary for cell death induced by different stimuli in 3DO T-cell hybridoma. 3DO cell clones depleted of ALG-2 protein exhibit normal caspases activation, suggesting that ALG-2 function is required downstream or is independent of caspase proteases activity for apoptosis to occur. Using the yeast two-hybrid screening system, we have isolated and characterized the mouse cDNA encoding for ALG-2 interacting protein 1 (AIP1), a novel protein that interacts with ALG-2. ALG-2 and AIP1 colocalize in the cytosol and the presence of calcium is an indispensable requisite for their association. Sequence alignment shows that AIP1 is highly similar to BRO1, a yeast protein related to components of the Pkc1p-MAP kinase cascade. Overexpression of a truncated form of AIP1 protects two different cell types from death induced by trophic factors withdrawal; thus, our data indicate that AIP1 cooperates with ALG-2 in executing the calcium-dependent requirements along the cell death pathway.

Vav regulates peptide-specific apoptosis in thymocytes

The protooncogene Vav functions as a GDP/GTP exchange factor (GEF) for Rho-like small GTPases involved in cytoskeletal reorganization and cytokine production in T cells. Gene-targeted mice lacking Vav have a severe defect in positive and negative selection of T cell antigen receptor transgenic thymocytes in vivo, and vav-/- thymocytes are completely resistant to peptide-specific and anti-CD3/anti-CD28-mediated apoptosis. Vav acts upstream of mitochondrial pore opening and caspase activation. Biochemically, Vav regulates peptide-specific Ca2+ mobilization and actin polymerization. Peptide-specific cell death was blocked both by cytochalasin D inhibition of actin polymerization and by inhibition of protein kinase C (PKC). Activation of PKC with phorbol ester restored peptide-specific apoptosis in vav-/- thymocytes. Vav was found to bind constitutively to PKC-theta in thymocytes. Our results indicate that peptide-triggered thymocyte apoptosis is mediated via Vav activation, changes in the actin cytoskeleton, and subsequent activation of a PKC isoform.

Degree of TCR Internalization and Ca2+ Flux Correlates with Thymocyte Selection

Recent evidence suggests that TCR down-regulation directly reflects the number of TCRs that have engaged MHC/peptide ligand complexes. Here, we examined the influence of defined peptides on thymic selection based on their ability to induce differential TCR internalization. Our results demonstrate that there is a direct correlation: peptides that induce strong TCR down-regulation are most efficient at mediating negative selection, whereas peptides that induce suboptimal TCR internalization are more efficient at triggering positive selection. As a consequence of suboptimal TCR internalization, a proportion of TCR complexes that remain on the cell surface may be able to relay continual signals required for survival and differentiation. In addition, we show that the magnitude of Ca2+ influx set by these peptides reflects the hierarchy of TCR down-regulation and correlates with positive vs negative selection of transgenic thymocytes. Together, our data suggest that T cell selection is mediated by differing intensities of the same TCR-mediated signal, rather than by distinct signals.

Thymocyte selection in Vav and IRF-1 gene-deficient mice

T cells undergo a defined program of phenotypic and genetic changes during differentiation within the thymus. These changes define commitment of T-cell receptor (TCR) gamma delta and TCR alpha beta cells and lineage differentiation into CD4+ T helper and CD8+ cytotoxic T cells. T-cell differentiation and selection in the thymus constitute a tightly co-ordinated multistep journey through a network that can be envisaged as a three-dimensional informational highway made up of stromal cells and extracellular matrix molecules. This intrathymic journey is controlled by information exchange, with thymocytes depending on two-way cellular interactions with thymic stromal cells in order to receive essential signals for maturation and selection. Genetic inactivation of surface receptors, signal transduction molecules, and transcription factors using homologous recombination has provided novel insight into the signaling cascades that relay surface receptor engagement to gene transcription and subsequent progression of the developmental program. In this review we discuss molecular mechanisms of T lymphocyte development in mice that harbour genetic mutations in the guanine nucleotide exchange factor Vav and the interferon regulatory transcription factor 1 (IRF-1). We also propose a novel model of T-cell selection based on TCR alpha chain-directed signals for allelic exclusion and TCR alpha-based selection for single receptor usage.

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.

The role of calcium in the regulation of apoptosis

Apoptosis (programmed cell death) has gained widespread attention due to its roles in a variety of physiological and pathological processes, yet precisely how apoptosis is regulated by external and internal cues remains unclear. Work from our laboratories and others has implicated alterations in intracellular Ca2+ in apoptosis, and more recent work has defined particular biochemical processes that are targeted by Ca2+ in apoptotic cells. This review will summarize the role of Ca2+ in apoptosis within the context of what is known about the core components of the effector machinery for apoptosis.

Functional cleavage of the common cytokine receptor gamma chain (gammac) by calpain

The small subunit of calpain, a calcium-dependent cysteine protease, was found to interact with the cytoplasmic domain of the common cytokine receptor gamma chain (gammac) in a yeast two-hybrid interaction trap assay. This interaction was functional as demonstrated by the ability of calpain to cleave in vitro-translated wild-type gammac, but not gammac containing a mutation in the PEST (proline, glutamate, serine, and threonine) sequence in its cytoplasmic domain, as well as by the ability of endogenous calpain to mediate cleavage of gammac in a calcium-dependent fashion. In T cell receptor-stimulated murine thymocytes, calpain inhibitors decreased cleavage of gammac. Moreover, in single positive CD4(+) thymocytes, not only did a calpain inhibitor augment CD3-induced proliferation, but antibodies to gammac blocked this effect. Finally, treatment of cells with ionomycin could inhibit interleukin 2-induced STAT protein activation, but this inhibition could be reversed by calpain inhibitors. Together, these data suggest that calpain-mediated cleavage of gammac represents a mechanism by which gammac-dependent signaling can be controlled.

A requirement for the Rho-family GTP exchange factor Vav in positive and negative selection of thymocytes

The T cell repertoire is shaped by positive and negative selection of thymocytes that express low levels of T cell receptor (TCR) and both CD4 and CD8. TCR-mediated signals that determine these selection processes are only partly understood. Vav, a GDP-GTP exchange factor for Rho-family proteins, is tyrosine phosphorylated following TCR stimulation, suggesting that it may transduce TCR signals. We now demonstrate that mice lacking Vav are viable and display a profound defect in the positive selection of both class I- and class II-restricted T cells. In contrast, Vav is not essential for negative selection, though in its absence negative selection is much less effective. Vav may influence the efficiency of TCR-induced selection events by regulating the intracellular calcium flux of thymocytes.

Magnitude of protein tyrosine phosphorylation-linked signals determines growth versus death of thymic T lymphocytes

Using concanavalin A (Con A) as a multireceptor-reactive agonist, we studied the relationship between the growth or death of thymic T lymphocytes and the agonist concentration-dependent magnitude of the intracellularly delivered signal. Both immature and mature thymic T lymphocytes were subjected to a high concentration of Con A-mediated signal for apoptotic cell death. In this model, a number of cellular proteins including mitogen activated protein kinases were phosphorylated at tyrosine depending on the concentration of Con A. This effect was followed by corresponding increase in serine 73 phosphorylation of c-jun and transcription of c-fos. DNA fragmentation and cell membrane disruption developed concomitantly after stimulation with high concentrations of Con A. The addition of inhibitors of protein kinases which completely inhibited the growth of cells stimulated with low concentrations of Con A only partially prevented death, and even promoted DNA fragmentation of cells stimulated with high concentrations of Con A. The dissociated sensitivities of Con A-mediated cell growth and cell death to the inhibitors were, however, shown to be due to the different efficiency of inhibition of high and low levels of intracellularly delivered signals. The results indicate that the magnitude of signaling could be the principal element that determines the growth versus death of thymic T lymphocytes.

Death of germinal center B cells without DNA fragmentation

During the selection of B cells within germinal centers (GC) on the basis of their affinity for T-dependent antigen, B cells not positively selected are eliminated within GC. This process of B cell death has been considered to be apoptosis. In a recent study, we have reported that, although a substantial number of thymocytes were considered to be dead because of their extremely small cell size and heavy chromatin condensation even though they were not yet phagocytosed (pyknosis), they were devoid of DNA fragmentation, the most characteristic feature for apoptosis. In this study, we examined in vivo the mechanism of B cell death within GC by using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) method to detect DNA double-strand breaks. TUNEL+ B cells were scattered throughout the upper dark and the light zones of GC. Double staining of the sections by the TUNEL method and acid phosphatase (AcP) activities showed that all the TUNEL+ B cells were phagocytosed by macrophages. Light microscopic and ultrastructural studies revealed the presence of small unphagocytosed B cells within the light zone. These cells are undoubtedly dead because they were much smaller than surrounding lymphoid cells and have a heavy chromatin condensation. Furthermore, ultrastructural detection of DNA fragmentation confirmed that these small unphagocytosed B cells were TUNEL-, implying that DNA fragmentation is not primarily involved in the cell death process of these small dead B cells. These results indicate that most B cells, not positively selected and thus destined to be eliminated, die within GC without DNA fragmentation, and are subsequently phagocytosed by macrophages and become TUNEL+. Typical apoptosis, characterized by DNA fragmentation in situ, is not the predominant type of cell death that occurs during the selection of B cells in GC.

Apoptosis--molecular mechanisms and biomedical implications

Apoptosis is a distinct form of cell death of importance in tissue development and homeostasis and in several diseases. This review summarizes current knowledge about the regulation and molecular mechanisms of apoptosis and discusses the potential role of disregulated apoptosis in several major diseases. Finally, we speculate that modulation of apoptosis may be a target in future drug therapy.

Calcium/calmodulin-dependent protein kinase II downregulates both calcineurin and protein kinase C-mediated pathways for cytokine gene transcription in human T cells

Engagement of the T cell receptor for antigen activates phospholipase C resulting in an increase in intracellular free calcium concentration ([Ca2+]i) and activation of protein kinase C (PKC). Increased [Ca2+]i activates Ca2+/calmodulin-dependent kinases including the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaM-K II), as well as calcineurin, a type 2B protein phosphatase. Recent studies have identified calcineurin as a key enzyme for interleukin (IL)-2 and IL-4 promoter activation. However, the role of CaM-K II remains unknown. We have used mutants of these kinases and phosphatases (gamma B*CaM-K and delta CaM-AI, respectively) to explore their relative role in cytokine gene transcription and their interactions with PKC-dependent signaling systems. gamma B*CaM-K and delta CaM-AI, known to exhibit constitutive Ca(2+)-independent activity, were cotransfected (alone or in combination) in Jurkat T cells with a plasmid containing the intact IL-2 promoter driving the expression of the chloramphenicol acetyltransferase reporter gene. Cotransfection of gamma B*CaM-K with the IL-2 promoter construct downregulated its transcription in response to stimulation with ionomycin and phorbol myristate acetate (PMA). The inhibitory effect of CaM-K II on IL-2 promoter was associated with decreased transcription of its AP-1 and NF-AT transactivating pathways. Under the same conditions, delta CaM-AI superinduced IL-2 promoter activity (approximately twofold increase). When both mutants were used in combination, gamma B*CaM-K inhibited the induction of the IL-2 promoter by delta CaM-AI. Similar results were obtained when a construct containing the IL-4 promoter also was used. gamma B*CaM-K also downregulated the activation of AP-1 in response to transfection with a constitutively active mutant of PKC or stimulation with PMA. These results suggest that CaM-K II may exert negative influences on cytokine gene transcription in human T cells, and provide preliminary evidence for negative cross-talk with the calcineurin- and PKC-dependent signaling systems.

T cell receptor-mediated signaling events in CD4+CD8+ thymocytes undergoing thymic selection: requirement of calcineurin activation for thymic positive selection but not negative selection

The goal of this study was to identify the differences of intracellular signals between the processes of thymic positive and negative selection. The activation of calcineurin, a calcium- and calmodulin-dependent phosphatase, is known to be an essential event in T cell activation via the T cell receptor (TCR). The effect of FK506, an inhibitor of calcineurin activation, on positive and negative selection in CD4+CD8+ double positive (DP) thymocytes was examined in normal mice and in a TCR transgenic mouse model. In vivo FK506 treatment blocked the generation of mature TCRhighCD4+CD8- and TCRhighCD4-CD8+ thymocytes, and the induction of CD69 expression on DP thymocytes. In addition, the shutdown of recombination activating gene 1 (RAG-1) transcription and the downregulation of CD4 and CD8 expression were inhibited by FK506 treatment suggesting that the activation of calcineurin is required for the first step (or the very early intracellular signaling events) of TCR-mediated positive selection of DP thymocytes. In contrast, FK506-sensitive calcineurin activation did not appear to be required for negative selection based on the observations that negative selection of TCR alpha beta T cells in the H-2b male thymus (a negative selecting environment) was not inhibited by in vivo treatment with FK506 and that there was no rescue of the endogenous superantigen-mediated clonal deletion of V beta 6 and V beta 11 thymocytes in FK506-treated CBA/J mice. DNA fragmentation induced by TCR activation of DP thymocytes in vitro was not affected by FK506. In addition, different effects of FK506 from Cyclosporin A on the T cell development in the thymus were demonstrated. The results of this study suggest that different signaling pathways work in positive and negative selection and that there is a differential dependence on calcineurin activation in the selection processes.

Activation-induced apoptosis in lymphocytes

Activation-induced apoptosis has been proposed as a mechanism for purging the immune repertoire of anti-self specificities, not only in development but also during the generation of somatic mutation in germinal centers. The pathways involved in driving immature and mature T and B cells to programmed cell death are reviewed with respect to two hypotheses, the pre-emptive death model, in which certain signals are obligatory for programmed cell death, and the two signal: death/survival model. Depending on the system, some data support the former pathway, in which certain signals are obligatory for programmed cell death, whereas other data are consistent with the two signal hypothesis. Moreover, recent data suggests that the c-myc protein plays a pivotal role in controlling this process. Finally conflicting roles of protein kinases, bcl-2 and p53 are reviewed and contrasted for involvement in activation-induced cell death in T and B lymphocytes.