Organization of proximal signal initiation at the TCR:CD3 complex

Characteristics of TCRζ, ZAP-70, and FcɛRIγ gene expression in patients with T- and NK/T-cell lymphoma

Abnormal expression of key signaling molecules and defective T-cell function play a crucial role in the pathogenesis of T-cell immunodeficiency in hematological malignancies. To understand the molecular basis of T-cell signaling abnormalities and TCRζ chain deficiencies in T- and NK/T-cell lymphoma, the expression level of the TCRζ, ZAP-70, and FcɛRIγ genes in peripheral blood mononuclear cells from 25 patients with T-cell lymphoma, 16 patients with NK/T-cell lymphoma (NK/T-CL), and 26 healthy individuals was determined. In addition, their relationship with disease stage and TCRζ 3' untranslated region (3'UTR) splice variants was analyzed in this study. The expression level of all three genes was significantly altered with disease progression, and a decreasing trend was found in patients compared with healthy controls. TCRζ and ZAP-70 were significantly positively related in all samples, and a negative relationship between TCRζ and FcɛRIγ was significantly lost in NK/T-CL patients. Moreover, distinct expression patterns were defined for patient groups with different TCRζ 3'UTR isoforms. In conclusion, a lower expression pattern for all three genes may indicate a weaker immune status based on reduced TCRζ and ZAP-70 expression without the complementary effects of FcɛRIγ, while aberrant TCRζ 3'UTR splicing may contribute to T-cell receptor (TCR) signaling regulation in T cells from patients with T- and NK/T-cell lymphoma.

Modulatory effect of celastrol on Th1/Th2 cytokines profile, TLR2 and CD3+ T-lymphocyte expression in a relapsing-remitting model of multiple sclerosis in rats

Multiple sclerosis (MS) is an autoimmune inflammatory demyelinating disease of brain and spinal cord that has an increasing incidence worldwide and classically presents in a relapsing-remitting form. This study was designed to induce a relapsing-remitting model of experimental autoimmune encephalomyelitis (EAE) to investigate the possible modulatory effect of celastrol on Th1/Th2 cytokines profile, immunohistochemical expression of TLR2, and CD3+T-lymphocytic count. Eighteen female Sprague Dawley rats were divided into 3 groups; where group I served as normal control, group II as EAE+vehicle, and group III as EAE treated by celastrol (1mg/kg/day, i.p.) started at 10th day till 42nd day post-immunization. The clinical score of rats in group II (EAE+vehicle) was relapsed after the re-challenge at the 35th day post-immunization and exhibited significant positive association with serum TNF-α, NF-κB expression and nitrites levels in brain and spinal cord, and CD3+ T-lymphocytic count in brain tissues while serum IL-10 showed significant negative association. Treatment of EAE by celastrol caused amelioration of the clinical score and inhibited the relapse. It caused significant shift in cytokines profile from Th1 by decrease in TNF-α towards Th2 pattern by increase in IL-10. Moreover, celastrol treatment resulted in significant reduction in NF-κB expression, nitrites levels, as well as immunohistochemical expression of TLR2 and CD3+ T-lymphocytic count. The beneficial effect of celastrol was further confirmed histopathologically by reduction in H&E score. Collectively, these results provide a promising pre-clinical evidence and conclusion about use of celastrol in treatment of multiple sclerosis that must be accessed in further clinical studies.

Large scale characterization of the LC13 TCR and HLA-B8 structural landscape in reaction to 172 altered peptide ligands: a molecular dynamics simulation study

The interplay between T cell receptors (TCRs) and peptides bound by major histocompatibility complexes (MHCs) is one of the most important interactions in the adaptive immune system. Several previous studies have computationally investigated their structural dynamics. On the basis of these simulations several structural and dynamical properties have been proposed as effectors of the immunogenicity. Here we present the results of a large scale Molecular Dynamics simulation study consisting of 100 ns simulations of 172 different complexes. These complexes consisted of all possible point mutations of the Epstein Barr Virus peptide FLRGRAYGL bound by HLA-B*08:01 and presented to the LC13 TCR. We compare the results of these 172 structural simulations with experimental immunogenicity data. We found that simulations with more immunogenic peptides and those with less immunogenic peptides are in fact highly similar and on average only minor differences in the hydrogen binding footprints, interface distances, and the relative orientation between the TCR chains are present. Thus our large scale data analysis shows that many previously suggested dynamical and structural properties of the TCR/peptide/MHC interface are unlikely to be conserved causal factors for peptide immunogenicity.

Membrane association of the CD3ε signaling domain is required for optimal T cell development and function

The TCR:CD3 complex transduces signals that are critical for optimal T cell development and adaptive immunity. In resting T cells, the CD3ε cytoplasmic tail associates with the plasma membrane via a proximal basic-rich stretch (BRS). In this study, we show that mice lacking a functional CD3ε-BRS exhibited substantial reductions in thymic cellularity and limited CD4- CD8- double-negative (DN) 3 to DN4 thymocyte transition, because of enhanced DN4 TCR signaling resulting in increased cell death and TCR downregulation in all subsequent populations. Furthermore, positive, but not negative, T cell selection was affected in mice lacking a functional CD3ε-BRS, which led to limited peripheral T cell function and substantially reduced responsiveness to influenza infection. Collectively, these results indicate that membrane association of the CD3ε signaling domain is required for optimal thymocyte development and peripheral T cell function.

Rho-GTPases as key regulators of T lymphocyte biology

Rho-GTPases belong to the Ras superfamily and are crucial signal transducing proteins downstream of many receptors. In general, the Rho-GTPases function as molecular switches, cycling between inactive (GDP-bound) and active (GTP-bound) states. The activated GTP bound Rho-GTPases interact with a broad spectrum of effectors to regulate a plethora of biological pathways including cytoskeletal dynamics, motility, cytokinesis, cell growth, apoptosis, transcriptional activity and nuclear signaling. Recently, gene targeting in mice allowed the selective inactivation of different Rho-GTPases and has advanced our understanding of the physiological role of these proteins, particularly in the immune system. Particularly, these proteins are key signaling molecules in T lymphocytes, which are generated in the thymus and are major players in the immune system. The scope of this review is to discuss recent data obtained in Rho-GTPases deficient mice by focusing on the role-played by Rho-GTPases in T-lymphocyte development, migration, activation and differentiation.

Diacylglycerol promotes centrosome polarization in T cells via reciprocal localization of dynein and myosin II

Centrosome reorientation to the immunological synapse maintains the specificity of T-cell effector function by facilitating the directional release of cytokines and cytolytic factors toward the antigen-presenting cell. This polarization response is driven by the localized accumulation of diacylglycerol, which recruits multiple protein kinase (PK)C isozymes to the synaptic membrane. Here, we used T-cell receptor (TCR) photoactivation and imaging methodology to demonstrate that PKCs control centrosome dynamics through the reciprocal localization of two motor complexes, dynein and nonmuscle myosin (NM)II. Dynein accumulated in the region of TCR stimulation, whereas NMII clustered in the back of the cell, behind the polarizing centrosome. PKC activity, which shaped both dynein and NMII accumulation within this framework, controlled NMII localization directly by phosphorylating inhibitory sites within the myosin regulatory light chain, thereby suppressing NMII clustering in the region of TCR stimulation. Concurrently, phosphorylation of distinct sites within myosin regulatory light chain by Rho kinase drove NMII clustering in areas behind the centrosome. These results reveal a role for NMII in T-cell polarity and demonstrate how it is regulated by upstream signals.

Early relaxation dynamics in the LC 13 T cell receptor in reaction to 172 altered peptide ligands: a molecular dynamics simulation study

The interaction between the T cell receptor and the major histocompatibility complex is one of the most important events in adaptive immunology. Although several different models for the activation process of the T cell via the T cell receptor have been proposed, it could not be shown that a structural mechanism, which discriminates between peptides of different immunogenicity levels, exists within the T cell receptor. In this study, we performed systematic molecular dynamics simulations of 172 closely related altered peptide ligands in the same T cell receptor/major histocompatibility complex system. Statistical evaluations yielded significant differences in the initial relaxation process between sets of peptides at four different immunogenicity levels.

Molecular mechanisms involved in the aging of the T-cell immune response

T-lymphocytes play a central role in the effector and regulatory mechanisms of the adaptive immune response. Upon exiting the thymus they begin to undergo a series of phenotypic and functional changes that continue throughout the lifetime and being most pronounced in the elderly. The reason postulated for this is that the dynamic processes of repeated interaction with cognate antigens lead to multiple division cycles involving a high degree of cell differentiation, senescence, restriction of the T-cell receptor (TCR) repertoire, and cell cycle arrest. This cell cycle arrest is associated with the loss of telomere sequences from the ends of chromosomes. Telomere length is reduced at each cell cycle, and critically short telomeres recruit components of the DNA repair machinery and trigger replicative senescence or apoptosis. Repetitively stimulated T-cells become refractory to telomerase induction, suffer telomere erosion and enter replicative senescence. The latter is characterized by the accumulation of highly differentiated T-cells with new acquired functional capabilities, which can be caused by aberrant expression of genes normally suppressed by epigenetic mechanisms in CD4+ or CD8+ T-cells. Age-dependent demethylation and overexpression of genes normally suppressed by DNA methylation have been demonstrated in senescent subsets of T-lymphocytes. Thus, T-cells, principally CD4+CD28^null T-cells, aberrantly express genes, including those of the KIR gene family and cytotoxic proteins such as perforin, and overexpress CD70, IFN-γ, LFA-1 and others. In summary, owing to a lifetime of exposure to and proliferation against a variety of pathogens, highly differentiated T-cells suffer molecular modifications that alter their cellular homeostasis mechanisms.

Loss of TCR-beta F1 and/or EZRIN expression is associated with unfavorable prognosis in nodal peripheral T-cell lymphomas

Nodal peripheral T-cell lymphoma (nodal PTCL) has an unfavorable prognosis, and specific pathogenic alterations have not been fully identified. The biological and clinical relevance of the expression of CD30/T-cell receptor (TCR) genes is a topic under active investigation. One-hundred and ninety-three consecutive nodal PTCLs (89 angioimmunoblastic T-cell lymphomas (AITL) and 104 PTCL-unspecified (PTCL-not otherwise specified (NOS)) cases) were analyzed for the immunohistochemical expression of 19 molecules, involving TCR/CD30 pathways and the associations with standard prognostic indices. Mutually exclusive expression was found between CD3 and TCR-beta F1 with CD30 expression. Taking all PTCL cases together, logistic regression identified a biological score (BS) including TCR molecules (TCR-beta F1 and EZRIN) that separates two subgroups of patients with a median survival of 34.57 and 5.20 months (P<0.001). Multivariate analysis identified BS and the prognostic index for PTCL (PIT) score as independent prognostic factors. This BS maintained its significance in multivariate analysis only for the PTCL-NOS subgroup of tumors. In AITL cases, only a high level of ki67 expression was related to prognosis. A BS including molecules involved in the TCR signaling pathway proved to be an independent prognostic factor of poor outcome in a multivariate analysis, specifically in PTCL-NOS patients. Nevertheless, validation in an independent series of homogeneously treated PTCL patients is required to confirm these data.

The membrane adaptor LAT is proteolytically cleaved following Fas engagement in a tyrosine phosphorylation-dependent fashion

Engagement of the TCR (T-cell receptor) induces tyrosine phosphorylation of the LAT (linker for the activation of T-cells) adaptor, and thereby it recruits several cytosolic mediators for downstream signalling pathways. The Fas protein is essential for T-lymphocyte apoptosis, and following Fas engagement, many proteins are proteolytically cleaved, including several molecules that are important for the transduction of TCR intracellular signals. In the present study, we demonstrate that the adaptor LAT is also subject to a proteolytic cleavage in mature T-lymphocytes and thymocytes in response to Fas engagement, and also on TCR stimulation, and we identify three aspartic acid residues at which LAT is cleaved. Interestingly, these aspartic acid residues are located in proximity to several functionally important tyrosine residues of LAT, raising the possibility that their phosphorylation could modulate LAT cleavage. Consistent with that hypothesis, we show that induction of phosphorylation by pervanadate or H2O2 in Jurkat cells and thymocytes inhibits Fas-mediated cleavage of LAT. Moreover, we show that LAT proteolysis is also enhanced during anergy induction of primary human T-cells, suggesting that LAT cleavage may act as a regulator of TCR-mediated activation of T-cells and not only as a transducer of cell death promoting stimuli.

Systems model of T cell receptor proximal signaling reveals emergent ultrasensitivity

Receptor phosphorylation is thought to be tightly regulated because phosphorylated receptors initiate signaling cascades leading to cellular activation. The T cell antigen receptor (TCR) on the surface of T cells is phosphorylated by the kinase Lck and dephosphorylated by the phosphatase CD45 on multiple immunoreceptor tyrosine-based activation motifs (ITAMs). Intriguingly, Lck sequentially phosphorylates ITAMs and ZAP-70, a cytosolic kinase, binds to phosphorylated ITAMs with differential affinities. The purpose of multiple ITAMs, their sequential phosphorylation, and the differential ZAP-70 affinities are unknown. Here, we use a systems model to show that this signaling architecture produces emergent ultrasensitivity resulting in switch-like responses at the scale of individual TCRs. Importantly, this switch-like response is an emergent property, so that removal of multiple ITAMs, sequential phosphorylation, or differential affinities abolishes the switch. We propose that highly regulated TCR phosphorylation is achieved by an emergent switch-like response and use the systems model to design novel chimeric antigen receptors for therapy.

Recognition of antigen by the T cell antigen receptor (TCR) is a central event in the initiation of adaptive immune responses and for this reason the TCR has been extensively studied. Multiple studies performed over the past 20 years have revealed intriguing findings that include the observation that the TCR has multiple phosphorylation sites that are sequentially phosphorylated by the kinase Lck and that ZAP-70, a cytosolic kinase, binds to these sites with different affinities. The purpose of multiple sites, their sequential phosphorylation by Lck, and the differential binding affinities of ZAP-70 are unknown. Using a novel mechanistic model that incorporates a high level of molecular detail, we find, unexpectedly, that all factors are critical for producing ultrasensitivity (switch-like response) and therefore this signaling architecture exhibits systems-level emergent ultrasensitivity. We use the model to study existing therapeutic chimeric antigen receptors and in the design of novel ones. The work also has direct implications to the study of many other immune receptors.

Piecing together the family portrait of TCR-CD3 complexes

The pre-T-cell receptor (TCR)-, αβTCR-, and γδTCR-CD3 complexes are members of a family of modular biosensors that are responsible for driving T-cell development, activation, and effector functions. They inform essential checkpoint decisions by relaying key information from their ligand-binding modules (TCRs) to their signaling modules (CD3γε + CD3δε and CD3ζζ) and on to the intracellular signaling apparatus. Their actions shape the T-cell repertoire, as well as T-cell-mediated immunity; yet, the mechanisms that underlie their activity remain an enigma. As with any molecular machine, understanding how they function depends upon understanding how their parts fit and work together. In the 30 years since the initial biochemical and genetic characterizations of the αβTCR, the structure and function of the individual components of these family members have been extensively characterized. Cumulatively, this information has allowed us to piece together a portrait of the αβTCR-CD3 complex and outline the form of the remaining family members. Here we review the known structural and functional characteristics of the components of these TCR-CD3 complex family members. We then discuss how these data have informed our understanding of the architecture of the αβTCR-CD3 complex as well as their implications for the other family members. The intent is to provide a framework for considering: (i) how these thematically similar complexes diverge to execute their specific functions and (ii) how our knowledge of the form and function of these distinct family members can cross-inform our understanding of the other family members.

Distinct TCR signaling pathways drive proliferation and cytokine production in T cells

The physiological basis and mechanistic requirements for a large number of functional immunoreceptor tyrosine-based activation motifs (ITAMs; high ITAM multiplicity) in the complex of the T cell antigen receptor (TCR) and the invariant signaling protein CD3 remain obscure. Here we found that whereas a low multiplicity of TCR-CD3 ITAMs was sufficient to engage canonical TCR-induced signaling events that led to cytokine secretion, a high multiplicity of TCR-CD3 ITAMs was required for TCR-driven proliferation. This was dependent on the formation of compact immunological synapses, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate the recruitment and activation of the oncogenic transcription factor Notch1 and, ultimately, proliferation induced by the cell-cycle regulator c-Myc. Analogous mechanistic events were also needed to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and are coordinated by the multiplicity of phosphorylated ITAMs in TCR-CD3.

Singularities of calcium signaling in effector T-lymphocytes

CD4(+) helper T (Th) lymphocytes orchestrate the immune response and include several types of effectors such as Th1, Th17 and Th2 cells. They fight against intracellular, extracellular pathogens and parasites respectively. They may also cause distinct immunopathological disorders. Th1 and Th17 are implicated in the development of autoimmune diseases while Th2 cells can initiate allergic diseases. These subsets differ by their TCR-associated signaling. In addition, the regulation of intracellular calcium concentration is not the same in Th1, Th2 and 17 cells. Our group showed that Th2 cells selectively overexpressed voltage-activated calcium (Cav1)-related channels. An increasing number of groups report the presence of Cav1-related products in T-lymphocyte subsets. This is a matter of debate since these calcium channels are classically defined as activated by high cell membrane depolarization in excitable cells. However, the use of mice with ablation of some Cav1 subunits shows undoubtedly an immune phenotype raising the question of how Cav1 channels are regulated in lymphocytes. We showed that knocking down Cav1.2 and/or Cav1.3 subunits impairs the functions of Th2 lymphocytes and is beneficial in experimental models of asthma, while it has no effect on Th1 cell functions. Beyond the role of Cav1 channels in T-lymphocytes, the identification of key components selectively implicated in one or the other T cell subset paves the way for the design of new selective therapeutic targets in the treatment of immune disorders while preserving the other T-cell subsets. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Development of promyelocytic leukemia zinc finger-expressing innate CD4 T cells requires stronger T-cell receptor signals than conventional CD4 T cells

MHC class II-expressing thymocytes and thymic epithelial cells can mediate CD4 T-cell selection resulting in functionally distinct thymocyte-selected CD4 (T-CD4) and epithelial-selected CD4 (E-CD4) T cells, respectively. However, little is known about how T-cell receptor (TCR) signaling influences the development of these two CD4 T-cell subsets. To study TCR signaling for T-CD4 T-cell development, we used a GFP reporter system of Nur77 in which GFP intensity directly correlates with TCR signaling strength. T-CD4 T cells expressed higher levels of GFP than E-CD4 T cells, suggesting that T-CD4 T cells received stronger TCR signaling than E-CD4 T cells during selection. Elimination of Ras GTPase-activating protein enhanced E-CD4 but decreased T-CD4 T-cell selection efficiency, suggesting a shift to negative selection. Conversely, the absence of IL-2-inducible T-cell kinase that causes poor E-CD4 T-cell selection due to insufficient TCR signaling improved T-CD4 T-cell generation, consistent with rescue from negative selection. Strong TCR signaling during T-CD4 T-cell development correlates with the expression of the transcription factor promyelocytic leukemia zinc finger protein. However, although modulation of the signaling strength affected the efficiency of T-CD4 T-cell development during positive and negative selection, the signaling strength is not as important for the effector function of T-CD4 T cells. These findings indicate that innate T-CD4 T cells, together with invariant natural killer T cells and γδ T cells, receive strong TCR signals during their development and that signaling requirements for the development and the effector functions are distinct.

Signal transduction via the T cell antigen receptor in naïve and effector/memory T cells

T cells play an indispensable role in immune defense against infectious agents, but can also be pathogenic. These T cells develop in the thymus, are exported into the periphery as naïve cells and participate in immune responses. Upon recognition of antigen, they are activated and differentiate into effector and memory T cells. While effector T cells carry out the function of the immune response, memory T cells can last up to the life time of the individual, and are activated by subsequent antigenic exposure. Throughout this life cycle, the T cell uses the same receptor for antigen, the T cell Receptor, a complex multi-subunit receptor. Recognition of antigen presented by peptide/MHC complexes on antigen presenting cells unleashes signaling pathways that control T cell activation at each stage. In this review, we discuss the signals regulated by the T cell receptor in naïve and effector/memory T cells.

End-binding protein 1 controls signal propagation from the T cell receptor

The role of microtubules (MTs) in the control and dynamics of the immune synapse (IS) remains unresolved. Here, we show that T cell activation requires the growth of MTs mediated by the plus-end specific protein end-binding 1 (EB1). A direct interaction of the T cell receptor (TCR) complex with EB1 provides the molecular basis for EB1 activity promoting TCR encounter with signalling vesicles at the IS. EB1 knockdown alters TCR dynamics at the IS and prevents propagation of the TCR activation signal to LAT, thus inhibiting activation of PLCγ1 and its localization to the IS. These results identify a role for EB1 interaction with the TCR in controlling TCR sorting and its connection with the LAT/PLCγ1 signalosome.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

A functional insertion/deletion polymorphism in the proximal promoter of CD3G is associated with susceptibility for hepatocellular carcinoma in Chinese population

Hepatocellular carcinoma (HCC) represents the most common primary malignancy of the liver with a worldwide increasing incidence. Although the risk factors for HCC are well characterized, the molecular mechanisms responsible for malignant transformation of hepatocytes are not well understood. In this study, a case-control study including 291 HCC patients and 294 healthy controls was conducted to investigate the association between HCC susceptibility and with a 4-bp insertion/deletion polymorphism (rs66465034) in the proximal promoter of CD3G. Logistic regression analysis showed that the heterozygote and the homozygote 4-bp ins/ins confer a significantly increased risk of HCC after controlling for other covariates (adjusted odds ratio [OR]=1.51, 95% confidence interval [C.I.] 1.01-2.27, p=0.040; OR=1.71, 95% C.I. 1.07-2.89, p=0.025, respectively). Carriage of the 4-bp insertion allele was associated with a greatly increased risk of developing the disease (OR=1.30, 95% C.I. 1.02-1.64, p=0.027). Moreover, hepatitis B virus (HBV) stratification analysis showed that the differences between cases and controls were more obvious in HBV-positive than in the HBV-negative population, suggesting a possible role of this polymorphism in the immune regulation during HBV infection. Further, luciferase-based transient transfection assays revealed that rs66465034 can affect promoter activity of CD3G, indicating its possible functional significance. Our data suggested that common genetic polymorphisms in CD3G may influence HCC risk in Chinese population. Considering the relative small sample size, replication in other populations with larger sample size and further functional analysis are required for fully understanding the roles of CD3G polymorphisms in predisposition for HCC.

In Vivo Consequences of Disrupting SH3-Mediated Interactions of the Inducible T-Cell Kinase

ITK-SH3-mediated interactions, both with exogenous ligands and via intermolecular self-association with ITK-SH2, have been shown to be important for regulation of ITK activity. The biological significance of these competing SH3 interactions is not completely understood. A mutant of ITK where substitution of the SH3 domain with that of the related kinase BTK (ITK-BTK_(SH3)) was used to disrupt intermolecular self-association of ITK while maintaining canonical binding to exogenous ligands such as SLP-76. ITK-BTK_(SH3) displays reduced association with SLP-76 leading to inefficient transphosphorylation, reduced phosphorylation of PLCγ1, and diminished Th₂ cytokine production. In contrast, ITK-BTK_(SH3) displays no defect in its localization to the T-cell-APC contact site. Another mutation, Y511F, in the activation loop of ITK, impairs ITK activation. T cells expressing ITK-Y511F display defective phosphorylation of ITK and its downstream target PLCγ1, as well as significant inhibition of Th₂ cytokines. In contrast, the inducible localization of ITK-Y511F to the T cell-APC contact site and its association with SLP-76 are not affected. The presented data lend further support to the hypothesis that precise interactions between ITK and its signaling partners are required to support ITK signaling downstream of the TCR.

Role of ITAM signaling module in signal integration

Diverse cell types use a small number of evolutionarily conserved signaling modules to integrate external cues and elicit distinct functions. A question thus arises as to how does a receptor, which contains a single signaling module, produce distinct outcomes to diverse signals, particularly if such module is shared amongst a family of receptors? Emerging data suggest that many immunoreceptors, all of which use a conserved ITAM-module for their signaling, can couple with members of additional classes of membrane receptors to deliver unique signal(s) to the cell. We discuss the possible biological purposes and mechanisms behind these interactions at the plasma membrane. We offer a conceptual framework to understand information processing within the immune system and discuss the new biology of old receptors involving their structural and functional collaborations that evolved to deliver unique signal(s) to the cell using a limited set of conserved signaling modules.

Receptor signaling in immune cell development and function

Immune cell development and function must be tightly regulated through cell surface receptors to ensure proper responses to pathogen and tolerance to self. In T cells, the signal from the T-cell receptor is essential for T-cell maturation, homeostasis, and activation. In mast cells, the high-affinity receptor for IgE transduces signal that promotes mast cell survival and induces mast cell activation. In dendritic cells and macrophages, the toll-like receptors recognize microbial pathogens and play critical roles for both innate and adaptive immunity against pathogens. Our research explores how signaling from these receptors is transduced and regulated to better understand these immune cells. Our recent studies have revealed diacylglycerol kinases and TSC1/2-mTOR as critical signaling molecules/regulators in T cells, mast cells, dendritic cells, and macrophages.

A live imaging cell motility screen identifies prostaglandin E2 as a T cell stop signal antagonist

The T cell migration stop signal is a central step in T cell activation and inflammation; however, its regulatory mechanisms remain largely unknown. Using a live-cell, imaging-based, high-throughput screen, we identified the PG, PGE(2), as a T cell stop signal antagonist. Src kinase inhibitors, microtubule inhibitors, and PGE(2) prevented the T cell stop signal, and impaired T cell-APC conjugation and T cell proliferation induced by primary human allogeneic dendritic cells. However, Src inhibition, but not PGE(2) or microtubule inhibition, impaired TCR-induced ZAP-70 signaling, demonstrating that T cell stop signal antagonists can function either upstream or downstream of proximal TCR signaling. Moreover, we found that PGE(2) abrogated TCR-induced activation of the small GTPase Rap1, suggesting that PGE(2) may modulate T cell adhesion and stopping through Rap1. These results identify a novel role for PGs in preventing T cell stop signals and limiting T cell activation induced by dendritic cells.

Dendritic cells expressing BTLA induces CD8+ T cell tolerance and attenuates the severity of diabetes

Numerous evidence demonstrate Type 1 diabetes (T1D) is due to a loss of immune tolerance to islet antigens, and CD8(+) T cells play an important role in the development of T1D in NOD mice. The novel coinhibitory receptor BTLA may have a regulatory role in maintaining peripheral tolerance, however, its role in autoimmune diabetes is unknown. To explore whether the generation of tolerance aiming at BTLA will help therapeutic intervention in T1D, the NOD mice were treated with genetically modified dendritic cells (DCs) expressing BTLA. The results demonstrated that transfer of modified DCs significantly induced CD8(+) T cell tolerance and attenuated the severity of diabetes. The findings suggest that genetically modified DC therapies enhancing the BTLA negative cosignal may prove valuable in treating T1D and other autoimmune diseases.

Impact of age on T cell signaling: a general defect or specific alterations?

Decreased immune responsiveness associated with aging is generally termed "immunosenescence". Several theories have been proposed to explain age-related declines in immune responses. Here, we will focus on and describe potential defects in T cell signal transduction from the membrane to the nucleus, leading to changes in the type, intensity and duration of the response as a major factor contributing to immunosenescence. We will first detail T cell signaling through the T cell receptor (TCR), CD28 and IL-2 receptor (IL-2R) and then discuss the observed age-related alterations to these signaling pathways. The role of membrane rafts in T cell signaling and T cell aging will be described. These factors will be considered in the context of the notion that age-related changes to T cell signaling may be attributed to changes in the functionality of the T cells due to shifts in T cell subpopulations with age. For this reason, we conclude by highlighting the application of multiparametric signaling analysis in leukocyte subsets using flow cytometry as a means to obtain a clearer picture with respect to age-related changes to immune signaling.

Calcium signalling in T-lymphocytes

Calcium signalling is essential for most of the biological T-cell activities, including in Th2 lymphocytes, a T-cell subset that produce interleukin 4, 5 and 13 and which is involved in allergic diseases. T-cell receptor engagement induces the production of inositol trisphosphate that binds to its receptor, releasing intracellular Ca(2+) stores. STIM in the endo (sarco) plasmic reticulum (ER/SR) is a Ca(2+) sensor that perceives the depletion of intracellular Ca(2+) stores, localizes near the cell membrane and allows the activation of ORAI, the main calcium channels at the cell membrane. However, other calcium channels at the membrane of intracellular compartments and at the cell membrane can also contribute to the TCR-driven intracellular Ca(2+) rise. Among them, voltage-dependent calcium (Ca(v)1) channels have been reported in several types of T-lymphocytes, although how they are gated in these non-excitable cells remains unsolved. We have shown that Cav1 channel expression was selectively up regulated in Th2 lymphocytes. In this review, we will discuss about the diversity of the Ca(2+) channels responsible for Ca(2+) homeostasis in the different cell subsets and the interactions between these molecules, which can account for the variety of the calcium responses depending upon the functions of effector T-cells.

Intraflagellar transport: a new player at the immune synapse

The assembly and maintenance of primary cilia, which orchestrate signaling pathways centrally implicated in cell proliferation, differentiation and migration, are ensured by multimeric protein particles in a process known as intraflagellar transport (IFT). It has recently been demonstrated that a number of IFT components are expressed in hematopoietic cells, which have no cilia. Here, we summarize data for an unexpected role of IFT proteins in immune synapse assembly and intracellular membrane trafficking in T lymphocytes, and discuss the hypothesis that the immune synapse could represent the functional homolog of the primary cilium in these cells.

Ultrasensitivity in multisite phosphorylation of membrane-anchored proteins

Cellular signaling is initially confined to the plasma membrane, where the cytoplasmic tails of surface receptors and other membrane-anchored proteins are phosphorylated in response to ligand binding. These proteins often contain multiple phosphorylation sites that are regulated by membrane-confined enzymes. Phosphorylation of these proteins is thought to be tightly regulated, because they initiate and regulate signaling cascades leading to cellular activation, yet how their phosphorylation is regulated is poorly understood. Ultrasensitive or switchlike responses in their phosphorylation state are not expected because the modifying enzymes are in excess. Here, we describe a novel mechanism of ultrasensitivity exhibited by multisite membrane-anchored proteins, but not cytosolic proteins, even when enzymes are in excess. The mechanism underlying this concentration-independent ultrasensitivity is the local saturation of a single enzyme by multiple sites on the substrate. Local saturation is a passive process arising from slow membrane diffusion, steric hindrances, and multiple sites, and therefore may be widely applicable. Critical to this ultrasensitivity is the brief enzymatic inactivation that follows substrate modification. Computations are presented using ordinary differential equations and stochastic spatial simulations. We propose a new role, to our knowledge, for multisite membrane-anchored proteins, discuss experiments that can be used to probe the model, and relate our findings to previous theoretical work.

Molecular evolution of immunoglobulin superfamily genes in primates

Genes of the immunoglobulin superfamily (IgSF) have a wide variety of cellular activities. In this study, we investigated molecular evolution of IgSF genes in primates by comparing orthologous sequences of 249 IgSF genes among human, chimpanzee, orangutan, rhesus macaque, and common marmoset. To evaluate the non-synonymous/synonymous substitution ratio (ω), we applied Bn-Bs program and PAML program. IgSF genes were classified into 11 functional categories based on the Gene Ontology (GO) database. Among them, IgSF genes in three functional categories, immune system process (GO:0002376), defense response (GO:0006952), and multi-organism process (GO:0051704), which are tightly linked to the regulation of immune system had much higher values of ω than genes in the other GO categories. In addition, we estimated the average values of ω for each primate lineage. Although each primate lineage had comparable average values of ω, the human lineage showed the lowest ω value for the immune-related genes. Furthermore, 11 IgSF genes, SIGLEC5, SLAMF6, CD33, CD3E, CEACAM8, CD3G, FCER1A, CD48, CD4, TIM4, and FCGR2A, were implied to have been under positive selective pressure during the course of primate evolution. Further sequence analyses of CD3E and CD3G from 23 primate species suggested that the Ig domains of CD3E and CD3G underwent the positive Darwinian selection.

The signaling tool box for tyrosine-based costimulation of lymphocytes

Triggering lymphocyte effector functions is controlled by a diverse array of immune cell coreceptors that dampen or potentiate the primary activation signal from antigen receptors. Attenuation of lymphocyte activation has been shown to be accomplished by immunoreceptor tyrosine-based inhibition motifs that upon phosphorylation recruit protein or lipid phosphatases. By contrast, a general concept of signal amplification and/or diversification is still out. However, the recent discovery of antigen receptor-intrinsic costimulation by membrane-bound immunoglobulins in class-switched memory B cells identified a consensus phosphorylation motif that can boost antigen-induced signal chains and is also employed by costimulatory receptors on T and Natural Killer cells to provide secondary signals for cellular activation. Here we define a common basis of tyrosine-based lymphocyte costimulation comprising immunoglobulin tail tyrosine (ITT)-like phosphorylation motifs and their proximal effectors, growth factor receptor-bound protein (Grb) 2 and phosphatidylinositol-3 kinase (PI3K) enzymes of class IA.

Heterologous immunity: immunopathology, autoimmunity and protection during viral infections

Heterologous immunity is a common phenomenon present in all infections. Most of the time it is beneficial, mediating protective immunity, but in some individuals that have the wrong crossreactive response it leads to a cascade of events that result in severe immunopathology. Infections have been associated with autoimmune diseases such as diabetes, multiple sclerosis and lupus erythematosis, but also with unusual autoimmune like pathologies where the immune system appears dysregulated, such as, sarcoidosis, colitis, panniculitis, bronchiolitis obliterans, infectious mononucleosis and even chronic fatigue syndrome. Here we review the evidence that to better understand these autoreactive pathologies it requires an evaluation of how T cells are regulated and evolve during sequential infections with different pathogens under the influence of heterologous immunity.

The TCR-mediated signaling pathways that control the direction of helper T cell differentiation

In the periphery, upon antigen recognition by alphabetaTCR, naïve CD4 T cells undergo functional differentiation and acquire the ability to produce a specific set of cytokines. At least four Th cell subsets, i.e., Th1, Th2, Th17 and iTreg cells have so far been identified and the differentiation of each subset is driven by distinct cytokine sets. Antigen recognition by TCR and the activation of the TCR-mediated signaling pathways that follows, however, are most critical for initiating Th cell differentiation. This review focuses on the TCR signal strength and the TCR-mediated signaling pathways that control the differentiation into these four Th cell subsets.

Dynamics of subsynaptic vesicles and surface microclusters at the immunological synapse

Imaging studies have identified clusters of kinases and adaptor proteins that serve as centers of signaling at the contact points between T cells and antigen-presenting cells (APCs). Here, we report that the kinase ZAP-70 and the adaptor proteins LAT and SLP-76 accumulated in separate clusters at the interface between T cells and coverslips coated with a stimulatory antibody against CD3, a component of the T cell antigen receptor complex. A fraction of LAT was detected in motile vesicles that repeatedly moved to surface microclusters of SLP-76 and the adaptor protein GADS (growth factor receptor-bound protein-related adaptor downstream of Shc), where they exhibited decreased motility. LAT molecules in which the residues tyrosine 171 and tyrosine 191 (which are required for the binding of LAT to GADS) were mutated to phenylalanine did not dwell at clusters of SLP-76. At immunological synapses, LAT-containing vesicles also colocalized with microclusters of SLP-76, as detected in experiments in which laser tweezers were used to position T cell-APC conjugates vertically for high-resolution imaging. Phosphorylation of LAT was most prominent when vesicular LAT colocalized with SLP-76. Indeed, the abundance of phosphorylated LAT within a microcluster of SLP-76 was greatest in those clusters that had more recent interactions with LAT-containing vesicles. Finally, negative signals by the inhibitory receptor ILT2 disrupted the assembly of SLP-76-containing microclusters. Together, these data show that the movement of LAT-containing vesicles is linked to the organization of protein microclusters and suggest an important role for vesicular LAT in the SLP-76 signalosome.