Src family tyrosine kinases associate with and phosphorylate CTLA-4 (CD152)

Soluble immune checkpoints: implications for cancer prognosis and response to immune checkpoint therapy and conventional therapies

Longitudinal sampling of tumor tissue from patients with solid cancers, aside from melanoma and a few other cases, is often unfeasible, and thus may not capture the plasticity of interactions between the tumor and immune system under selective pressure of a given therapy. Peripheral blood analyses provide salient information about the human peripheral immunome while offering technical and practical advantages over traditional tumor biopsies, and should be utilized where possible alongside interrogation of the tumor. Some common blood-based biomarkers used to study the immune response include immune cell subsets, circulating tumor DNA, and protein analytes such as cytokines. With the recent explosion of immune checkpoint inhibitors (ICI) as a modality of treatment in multiple cancer types, soluble immune checkpoints have become a relevant area of investigation for peripheral immune-based biomarkers. However, the exact functions of soluble immune checkpoints and their roles in cancer for the most part remain unclear. This review discusses current literature on the production, function, and expression of nine soluble immune checkpoints - sPD-L1, sPD-1, sCTLA4, sCD80, sTIM3, sLAG3, sB7-H3, sBTLA, and sHVEM - in patients with solid tumors, and explores their role as biomarkers of response to ICI as well as to conventional therapies (chemotherapy, radiotherapy, targeted therapy, and surgery) in cancer patients.

Current understanding of CTLA-4: from mechanism to autoimmune diseases

Autoimmune diseases (ADs) are characterized by the production of autoreactive lymphocytes, immune responses to self-antigens, and inflammation in related tissues and organs. Cytotoxic T-lymphocyte antigen 4 (CTLA-4) is majorly expressed in activated T cells and works as a critical regulator in the inflammatory response. In this review, we first describe the structure, expression, and how the signaling pathways of CTLA-4 participate in reducing effector T-cell activity and enhancing the immunomodulatory ability of regulatory T (Treg) cells to reduce immune response, maintain immune homeostasis, and maintain autoimmune silence. We then focused on the correlation between CTLA-4 and different ADs and how this molecule regulates the immune activity of the diseases and inhibits the onset, progression, and pathology of various ADs. Finally, we summarized the current progress of CTLA-4 as a therapeutic target for various ADs.

Tumor microenvironment signaling and therapeutics in cancer progression

Tumor development and metastasis are facilitated by the complex interactions between cancer cells and their microenvironment, which comprises stromal cells and extracellular matrix (ECM) components, among other factors. Stromal cells can adopt new phenotypes to promote tumor cell invasion. A deep understanding of the signaling pathways involved in cell-to-cell and cell-to-ECM interactions is needed to design effective intervention strategies that might interrupt these interactions. In this review, we describe the tumor microenvironment (TME) components and associated therapeutics. We discuss the clinical advances in the prevalent and newly discovered signaling pathways in the TME, the immune checkpoints and immunosuppressive chemokines, and currently used inhibitors targeting these pathways. These include both intrinsic and non-autonomous tumor cell signaling pathways in the TME: protein kinase C (PKC) signaling, Notch, and transforming growth factor (TGF-β) signaling, Endoplasmic Reticulum (ER) stress response, lactate signaling, Metabolic reprogramming, cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) and Siglec signaling pathways. We also discuss the recent advances in Programmed Cell Death Protein 1 (PD-1), Cytotoxic T-Lymphocyte Associated Protein 4 (CTLA4), T-cell immunoglobulin mucin-3 (TIM-3) and Lymphocyte Activating Gene 3 (LAG3) immune checkpoint inhibitors along with the C-C chemokine receptor 4 (CCR4)- C-C class chemokines 22 (CCL22)/ and 17 (CCL17), C-C chemokine receptor type 2 (CCR2)- chemokine (C-C motif) ligand 2 (CCL2), C-C chemokine receptor type 5 (CCR5)- chemokine (C-C motif) ligand 3 (CCL3) chemokine signaling axis in the TME. In addition, this review provides a holistic understanding of the TME as we discuss the three-dimensional and microfluidic models of the TME, which are believed to recapitulate the original characteristics of the patient tumor and hence may be used as a platform to study new mechanisms and screen for various anti-cancer therapies. We further discuss the systemic influences of gut microbiota in TME reprogramming and treatment response. Overall, this review provides a comprehensive analysis of the diverse and most critical signaling pathways in the TME, highlighting the associated newest and critical preclinical and clinical studies along with their underlying biology. We highlight the importance of the most recent technologies of microfluidics and lab-on-chip models for TME research and also present an overview of extrinsic factors, such as the inhabitant human microbiome, which have the potential to modulate TME biology and drug responses.

Immune checkpoint proteins: Signaling mechanisms and molecular interactions in cancer immunotherapy

Immune checkpoint proteins (ICP) are currently one of the most novel and promising areas of immune-oncology research. This novel way of targeting tumor cells has shown favorable success over the past few years with some FDA approvals such as Ipilimumab, Nivolumab, Pembrolizumab etc. Currently, more than 3000 clinical trials of immunotherapeutic agents are ongoing with majority being ICPs. However, as the number of trials increase so do the challenges. Some challenges such as adverse side effects, non-specific binding on healthy tissues and absence of response in some subset populations are critical obstacles. For a safe and effective further therapeutic development of molecules targeting ICPs, understanding their mechanism at molecular level is crucial. Since ICPs are mostly membrane bound receptors, a number of downstream signaling pathways divaricate following ligand-receptor binding. Most ICPs are expressed on more than one type of immune cell populations. Further, the expression varies within a cell type. This naturally varied expression pattern adds to the difficulty of targeting specific effector immune cell types against cancer. Hence, understanding the expression pattern and cellular mechanism helps lay out the possible effect of any immunotherapy. In this review, we discuss the signaling mechanism, expression pattern among various immune cells and molecular interactions derived using interaction database analysis (BioGRID).

Current Understanding of Cytotoxic T Lymphocyte Antigen-4 (CTLA-4) Signaling in T-Cell Biology and Disease Therapy

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is an immune checkpoint molecule that is mainly expressed on activated T cells and regulatory T (Treg) cells that inhibits T-cell activation and regulates immune homeostasis. Due to the crucial functions of CTLA-4 in T-cell biology, CTLA-4-targeted immunotherapies have been developed for autoimmune disease as well as cancers. CTLA-4 is known to compete with CD28 to interact with B7, but some studies have revealed that its downstream signaling is independent of its ligand interaction. As a signaling domain of CTLA-4, the tyrosine motif plays a role in inhibiting T-cell activation. Recently, the lysine motif has been shown to be required for the function of Treg cells, emphasizing the importance of CTLA-4 signaling. In this review, we summarize the current understanding of CTLA-4 biology and molecular signaling events and discuss strategies to target CTLA-4 signaling for immune modulation and disease therapy.

Differential regulation of CTLA4 expression through BTK-dependent and independent mechanisms in CLL

Ibrutinib suppresses CLL cell CTLA4 expression in vitro and in vivo.

CTLA4 expression on CLL is regulated by non-BTKs that differ from T-cell CTLA4 regulation.

Cytotoxic T lymphocyte antigen 4 (CTLA4) is a major immune checkpoint and target for cancer immunotherapy. Although originally discovered and primarily studied on T cells, its role on other cell types has also been recognized in recent years. Here we describe an unexpected interaction between ibrutinib (a targeted inhibitor of Bruton tyrosine kinase [BTK]) and CTLA4 expression on malignant chronic lymphocytic leukemia (CLL) cells. Although BTK itself does play a role in CTLA4 expression in CLL, we demonstrate that ibrutinib’s main suppressive effect on CTLA4 protein expression and trafficking occurs through non-BTK targets influenced by this drug. This suppression is not seen in T cells, indicating a different mechanism of CTLA4 regulation in CLL vs T cells. Appreciating this distinct mechanism and the beneficial non-BTK effects of ibrutinib may contribute to understanding the immune benefits of ibrutinib treatment and lead to therapeutic approaches to improve immune function in patients with CLL by suppressing CTLA4 expression.

Targeting protein kinases benefits cancer immunotherapy

Small-molecule kinase inhibitors have been well established and successfully developed in the last decades for cancer target therapies. However, intrinsic or acquired drug resistance is becoming the major barrier for their clinical application. With the development of immunotherapies, in particular the discovery of immune checkpoint inhibitors (ICIs), the combination of ICIs with other therapies have recently been extensively explored, among which combination of ICIs with kinase inhibitors achieves promising clinical outcome in a plethora of cancer types. Here we comprehensively summarize the potent roles of protein kinases in modulating immune checkpoints both in tumor and immune cells, and reshaping tumor immune microenvironments by evoking innate immune response and neoantigen generation or presentation. Moreover, the clinical trial and approval of combined administration of kinase inhibitors with ICIs are collected, highlighting the precise strategies to benefit cancer immune therapies.

Impact of checkpoint blockade on cancer vaccine-activated CD8+ T cell responses

Immune and molecular profiling of CD8 T cells of patients receiving DC vaccines expressing three full-length melanoma antigens (MAs) was performed. Antigen expression levels in DCs had no significant impact on T cell or clinical responses. Patients who received checkpoint blockade before DC vaccination had higher baseline MA-specific CD8 T cell responses but no evidence for improved functional responses to the vaccine. Patients who showed the best clinical responses had low PD-1 expression on MA-specific T cells before and after DC vaccination; however, blockade of PD-1 during antigen presentation by DC had minimal functional impact on PD-1high MA-specific T cells. Gene and protein expression analyses in lymphocytes and tumor samples identified critical immunoregulatory pathways, including CTLA-4 and PD-1. High immune checkpoint gene expression networks correlated with inferior clinical outcomes. Soluble serum PD-L2 showed suggestive positive association with improved outcome. These findings show that checkpoint molecular pathways are critical for vaccine outcomes and suggest specific sequencing of vaccine combinations.

Short linear motif candidates in the cell entry system used by SARS-CoV-2 and their potential therapeutic implications

The first reported receptor for SARS-CoV-2 on host cells was the angiotensin-converting enzyme 2 (ACE2). However, the viral spike protein also has an RGD motif, suggesting that cell surface integrins may be co-receptors. We examined the sequences of ACE2 and integrins with the Eukaryotic Linear Motif (ELM) resource and identified candidate short linear motifs (SLiMs) in their short, unstructured, cytosolic tails with potential roles in endocytosis, membrane dynamics, autophagy, cytoskeleton, and cell signaling. These SLiM candidates are highly conserved in vertebrates and may interact with the μ2 subunit of the endocytosis-associated AP2 adaptor complex, as well as with various protein domains (namely, I-BAR, LC3, PDZ, PTB, and SH2) found in human signaling and regulatory proteins. Several motifs overlap in the tail sequences, suggesting that they may act as molecular switches, such as in response to tyrosine phosphorylation status. Candidate LC3-interacting region (LIR) motifs are present in the tails of integrin β3 and ACE2, suggesting that these proteins could directly recruit autophagy components. Our findings identify several molecular links and testable hypotheses that could uncover mechanisms of SARS-CoV-2 attachment, entry, and replication against which it may be possible to develop host-directed therapies that dampen viral infection and disease progression. Several of these SLiMs have now been validated to mediate the predicted peptide interactions.

Isolation of Two Novel Human Anti-CTLA-4 mAbs with Intriguing Biological Properties on Tumor and NK Cells

The cytotoxic T lymphocyte-antigen 4 (CTLA-4) has been considered an IC exclusively expressed on T cells, where it counteracts the co-stimulatory CD28 receptor, by competing for its binding to CD-80 and CD-86. We recently found that it is expressed also on tumor and NK cells, suggesting other possible unknown roles of CTLA-4. To shed light on these novel aspects of CTLA-4, we used Ipilimumab, the first FDA approved human antibody targeting CTLA-4, in parallel studies with two novel human mAbs we isolated by using an efficient phage display selection strategy on live activated lymphocytes and purified mouse and human CTLA-4. The selection for cross-reactive mAbs was guaranteed by a high throughput sequencing to identify the sequences commonly enriched by two parallel pannings on human and mouse CTLA-4. Two isolated antibodies were found to bind with high affinity to both human and mouse CTLA-4 and lymphocytes, showing nanomolar or sub-nanomolar Kd values. They were able to kill Treg cells by ADCC, and to activate both human and mouse PBMCs, by strongly increasing cytokines secretion. Interestingly, they activated NK cells, exhibited cytotoxicity against cancer cells by inducing ADCC and inhibited tumor cell growth by affecting CTLA-4 downstream pathways in a similar fashion to CD-80 and CD-86 ligands and differently from Ipilimumab. Moreover, the novel mAbs showed a reduced ability to interfere in the binding of CD-80 ligands to CTLA-4 on T cells with respect to Ipilimumab, suggesting that they could allow for anti-tumor effects without the irAEs associated with the potent antagonistic activity of Ipilimumab.

Phosphorylation: A Fast Switch For Checkpoint Signaling

Checkpoint signaling involves a variety of upstream and downstream factors that participate in the regulation of checkpoint expression, activation, and degradation. During the process, phosphorylation plays a critical role. Phosphorylation is one of the most well-documented post-translational modifications of proteins. Of note, the importance of phosphorylation has been emphasized in aspects of cell activities, including proliferation, metabolism, and differentiation. Here we summarize how phosphorylation of specific molecules affects the immune activities with preference in tumor immunity. Of course, immune checkpoints are given extra attention in this book. There are many common pathways that are involved in signaling of different checkpoints. Some of them are integrated and presented as common activities in the early part of this chapter, especially those associated with PD-1/PD-L1 and CTLA-4, because investigations concerning them are particularly abundant and variant. Their distinct regulation is supplementarily discussed in their respective section. As for checkpoints that are so far not well explored, their related phosphorylation modulations are listed separately in the later part. We hope to provide a clear and systematic view of the phosphorylation-modulated immune signaling.

Immunometabolic Regulations Mediated by Coinhibitory Receptors and Their Impact on T Cell Immune Responses

Host immunity provides wide spectrum protection that serves to eradicate pathogens and cancer cells, while maintaining self-tolerance and immunological homeostasis. Ligation of the T cell receptor (TCR) by antigen activates signaling pathways that coordinately induce aerobic glycolysis, mitochondrial activity, anabolic metabolism, and T effector cell differentiation. Activation of PI3K, Akt, and mTOR triggers the switch to anabolic metabolism by inducing transcription factors such as Myc and HIF1, and the glucose transporter Glut1, which is pivotal for the increase of glucose uptake after T cell activation. Activation of MAPK signaling is required for glucose and glutamine utilization, whereas activation of AMPK is critical for energy balance and metabolic fitness of T effector and memory cells. Coinhibitory receptors target TCR-proximal signaling and generation of second messengers. Imbalanced activation of such signaling pathways leads to diminished rates of aerobic glycolysis and impaired mitochondrial function resulting in defective anabolic metabolism and altered T cell differentiation. The coinhibitory receptors mediate distinct and synergistic effects on the activation of signaling pathways thereby modifying metabolic programs of activated T cells and resulting in altered immune functions. Understanding and therapeutic targeting of metabolic programs impacted by coinhibitory receptors might have significant clinical implications for the treatment of chronic infections, cancer, and autoimmune diseases.

CTLA-4, an Essential Immune-Checkpoint for T-Cell Activation

The response of peripheral T lymphocytes (T cell) is controlled by multiple checkpoints to avoid unwanted activation against self-tissues. Two opposing costimulatory receptors, CD28 and CTLA-4, on T cells bind to the same ligands (CD80 and CD86) on antigen-presenting cells (APCs), and provide positive and negative feedback for T-cell activation, respectively. Early studies suggested that CTLA-4 is induced on activated T cells and binds to CD80/CD86 with much stronger affinity than CD28, providing a competitive inhibition. Subsequent studies by many researchers revealed the more complex mode of T-cell inhibition by CTLA-4. After T-cell activation, CTLA-4 is stored in the intracellular vesicles, and recruited to the immunological synapse formed between T cells and APCs, and inhibits further activation of T cells by blocking signals initiated by T-cell receptors and CD28. CTLA-4-positive cells can also provide cell-extrinsic regulation on other autoreactive T cells, and are considered to provide an essential regulatory mechanism for FoxP3+ regulatory T cells. Genetic deficiency of CTLA-4 leads to CD28-mediated severe autoimmunity in mice and humans, suggesting its function as a fundamental brake that restrains the expansion and activation of self-reactive T cells. In cancer, therapeutic approaches targeting CTLA-4 by humanized blocking antibodies has been demonstrated to be an effective immunotherapy by reversing T-cell tolerance against tumors. This chapter introduces CTLA-4 biology, including its discovery and mechanism of action, and discusses questions related to CTLA-4.

Immune checkpoints aberrations and gastric cancer; assessment of prognostic value and evaluation of therapeutic potentials

Till now, the prognosis of advanced gastric cancer looked dreadful; thus the search for newer better approaches for this lethal disease has been a strategic target for cancer researchers. In recent years, important immunobiological aspects of the tumor have been revealed with the subsequent proposal of immune check point inhibitors to target these pathways. Clinically, unselected use of immune checkpoint inhibitors in gastric cancer has been deemed with failure; in contrast to the clear success of more recent studies reporting on the use of pembrolizumab in molecularly selected patients. This may illustrate that any future use of immune checkpoint inhibitors in gastric cancer has to be molecularly supported. This review provides a delicate dissection of the clinical and immunobiological considerations underlying the use of these agents in addition to a thorough review of the published clinical data of immune checkpoint inhibitors in gastric cancer.

Diverse mechanisms regulate the surface expression of immunotherapeutic target ctla-4

T-cell co-receptor cytotoxic T-cell antigen-4 (CTLA-4) is a critical inhibitory regulator of T-cell immunity and antibody blockade of the co-receptor has been shown to be effective in tumor immunotherapy. Paradoxically, the majority of CTLA-4 is located in intracellular compartments from where it is transported to the cell surface and rapidly internalized. The intracellular trafficking pathways that control transport of the co-receptor to the cell surface ensures the appropriate balance of negative and positive signaling for a productive immune response with minimal autoimmune disorders. It will also influence the degree of inhibition and the potency of antibody checkpoint blockade in cancer immunotherapy. Current evidence indicates that the mechanisms of CTLA-4 transport to the cell surface and its residency are multifactorial involving a combination of immune cell-specific adapters such as TRIM and LAX, the small GTPase Rab8 as well as generic components such as ARF-1, phospholipase D, and the heterotetrameric AP1/2 complex. This review covers the recent developments in our understanding of the processes that control the expression of this important co-inhibitory receptor for the modulation of T-cell immunity. Interference with the processes that regulate CTLA-4 surface expression could provide an alternate therapeutic approach in the treatment of cancer and autoimmunity.

At the bench: preclinical rationale for CTLA-4 and PD-1 blockade as cancer immunotherapy

Tumors can avoid immune surveillance by stimulating immune inhibitory receptors that function to turn off established immune responses. By blocking the ability of tumors to stimulate inhibitory receptors on T cells, sustained, anti-tumor immune responses can be generated in animals. Thus, therapeutic blockade of immune inhibitory checkpoints provides a potential method to boost anti-tumor immunity. The CTLA-4 and PD-1Rs represent two T cell-inhibitory receptors with independent mechanisms of action. Preclinical investigations revealed that CTLA-4 enforces an activation threshold and attenuates proliferation of tumor-specific T lymphocytes. In contrast, PD-1 functions primarily as a stop signal that limits T cell effector function within a tumor. The unique mechanisms and sites of action of CTLA-4 and PD-1 suggest that although blockade of either has the potential to promote anti-tumor immune responses, combined blockade of both might offer even more potent anti-tumor activity. See related review At the Bedside: CTLA-4 and PD-1 blocking antibodies in cancer immunotherapy.

PAG/Cbp suppression reveals a contribution of CTLA-4 to setting the activation threshold in T cells

Background

PAG/Cbp represents a ubiquitous mechanism for regulating Src family kinases by recruiting Csk to the plasma membrane, thereby controlling cellular activation. Since Src kinases are known oncogenes, we used RNA interference in primary human T cells to test whether the loss of PAG resulted in lymphocyte transformation.

Results

PAG-depletion enhanced Src kinase activity and augmented proximal T-cell receptor signaling; exactly the phenotype expected for loss of this negative regulator. Surprisingly, rather than becoming hyper-proliferative, PAG-suppressed T cells became unresponsive. This was mediated by a Fyn-dependent hyper-phosphorylation of the inhibitory receptor CTLA-4, which recruited the protein tyrosine phosphatase Shp-1 to lipid rafts. Co-suppression of CTLA-4 abrogates this inhibition and restores proliferation to T cells.

Conclusion

We have identified a fail-safe mechanism as well as a novel contribution of CTLA-4 to setting the activation threshold in T cells.

Constitutive clathrin-mediated endocytosis of CTLA-4 persists during T cell activation

CTLA-4 is one of the most important negative regulators of the T cell immune response. However, the subcellular distribution of CTLA-4 is unusual for a receptor that interacts with cell surface transmembrane ligands in that CTLA-4 is rapidly internalized from the plasma membrane. It has been proposed that T cell activation can lead to stabilization of CTLA-4 expression at the cell surface. Here we have analyzed in detail the internalization, recycling, and degradation of CTLA-4. We demonstrate that CTLA-4 is rapidly internalized from the plasma membrane in a clathrin- and dynamin-dependent manner driven by the well characterized YVKM trafficking motif. Furthermore, we show that once internalized, CTLA-4 co-localizes with markers of recycling endosomes and is recycled to the plasma membrane. Although we observed limited co-localization of CTLA-4 with lysosomal markers, CTLA-4 was nonetheless degraded in a manner inhibited by lysosomal blockade. T cell activation stimulated mobilization of CTLA-4, as judged by an increase in cell surface expression; however, this pool of CTLA-4 continued to endocytose and was not stably retained at the cell surface. These data support a model of trafficking whereby CTLA-4 is constitutively internalized in a ligand-independent manner undergoing both recycling and degradation. Stimulation of T cells increases CTLA-4 turnover at the plasma membrane; however, CTLA-4 endocytosis continues and is not stabilized during activation of human T cells. These findings emphasize the importance of clathrin-mediated endocytosis in regulating CTLA-4 trafficking throughout T cell activation.

SHP-2 expression negatively regulates NK cell function

Src homology region 2-containing protein tyrosine phosphatase-2 (SHP-2) is required for full activation of Ras/ERK in many cytokine and growth factor receptor signaling pathways. In contrast, SHP-2 inhibits activation of human NK cells upon recruitment to killer cell Ig-like receptors (KIR). To determine how SHP-2 impacts NK cell activation in KIR-dependent or KIR-independent signaling pathways, we employed knockdown and overexpression strategies in NK-like cell lines and analyzed the consequences on functional responses. In response to stimulation with susceptible target cells, SHP-2-silenced NK cells had elevated cytolytic activity and IFN-gamma production, whereas cells overexpressing wild-type or gain-of-function mutants of SHP-2 exhibited dampened activities. Increased levels of SHP-2 expression over this range significantly suppressed microtubule organizing center polarization and granzyme B release in response to target cells. Interestingly, NK-target cell conjugation was only reduced by overexpressing SHP-2, but not potentiated in SHP-2-silenced cells, indicating that conjugation is not influenced by physiological levels of SHP-2 expression. KIR-dependent inhibition of cytotoxicity was unaffected by significant reductions in SHP-2 levels, presumably because KIR were still capable of recruiting the phosphatase under these limiting conditions. In contrast, the general suppressive effect of SHP-2 on cytotoxicity and cytokine release was much more sensitive to changes in cellular SHP-2 levels. In summary, our studies have identified a new, KIR-independent role for SHP-2 in dampening NK cell activation in response to tumor target cells in a concentration-dependent manner. This suppression of activation impacts microtubule organizing center-based cytoskeletal rearrangement and granule release.

Autoimmunity risk alleles in costimulation pathways

SUMMARY

The basis for susceptibility to common autoimmune diseases is a complex interplay between multiple genetic and environmental risk factors. We have now entered a new generation of genetic study designs which has not only furthered our understanding of the individual mechanisms involved in the common human autoimmune diseases but also has pointed towards common pathways. In this review we focus on costimulatory mechanisms with the most convincing association results in large collections of patients and control subjects. These include the genes encoding cytotoxic T-lymphocyte antigen-4, CD58, CD40, inducible T-cell costimulator ligand, CD244, CD226, tumor necrosis factor (TNF) (ligand) superfamily member 4, TNF superfamily member 15, and programmed cell death 1. The unbiased genome-wide association scans suggest that indeed immune related genes underlie the pathogenesis of human autoimmune disease with common involvement of costimulatory pathways. The identification of allelic variants associated with disease risk followed by understanding their functional outcomes and affected pathways provides a rationale approach for drug design.

CD28 and CTLA-4 coreceptor expression and signal transduction

SUMMARY

T-cell activation is mediated by antigen-specific signals from the TCRzeta/CD3 and CD4-CD8-p56lck complexes in combination with additional co-signals provided by coreceptors such as CD28, inducible costimulator (ICOS), cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death (PD-1), and others. CD28 and ICOS provide positive signals that promote and sustain T-cell responses, while CTLA-4 and PD-1 limit responses. The balance between stimulatory and inhibitory co-signals determines the ultimate nature of T-cell responses where response to foreign pathogen is achieved without excess inflammation and autoimmunity. In this review, we outline the current knowledge of the CD28 and CTLA-4 signaling mechanisms [involving phosphatidylinositol 3 kinase (PI3K), growth factor receptor-bound protein 2 (Grb2), Filamin A, protein kinase C theta (PKCtheta), and phosphatases] that control T-cell immunity. We also present recent findings on T-cell receptor-interacting molecule (TRIM) regulation of CTLA-4 surface expression, and a signaling pathway involving CTLA-4 activation of PI3K and protein kinase B (PKB)/AKT by which cell survival is ensured under conditions of anergy induction.

T Cell Receptor-Interacting Molecule Acts as a Chaperone to Modulate Surface Expression of the CTLA-4 Coreceptor

The costimulatory molecule CTLA-4 is a potent downregulator of T cell responses. Although localized mostly in intracellular compartments, little is understood regarding the mechanism that regulates its transport to the cell surface. In this study, we demonstrated that the adaptor TRIM (T cell receptor-interacting molecule) bound to CTLA-4 in the trans Golgi network (TGN) and promoted transport of CTLA-4 to the surface of T cells. Increased TRIM expression augmented surface CTLA-4 expression, and pulse-chase analysis showed a more rapid transport of CTLA-4 to the cell surface. A reduction of TRIM expression by small hairpin RNAs reduced the expression of surface CTLA-4. This resulted in a more localized pattern of CTLA-4 in the TGN. Altered CTLA-4 expression by TRIM was accompanied by corresponding changes in coreceptor-mediated effects on cytokine production and proliferation. Our findings identify a role for TRIM as a chaperone in regulating CTLA-4 expression and function by enhancing CTLA-4 transport to the surface of T cells.

Balancing co-stimulation and inhibition with BTLA and HVEM

The interaction between B- and T-lymphocyte attenuator (BTLA), an inhibitory receptor whose extracellular domain belongs to the immunoglobulin superfamily, and herpesvirus-entry mediator (HVEM), a co-stimulatory tumour-necrosis factor receptor, is unique in that it is the only receptor-ligand interaction that directly bridges these two families of receptors. This interaction has raised many questions about how receptors from two different families could interact and what downstream signalling events might occur as a result of receptor ligation. As we discuss, recent studies show that engagement of HVEM with its endogenous ligand (LIGHT) from the tumour-necrosis factor family induces a powerful immune response, whereas HVEM interactions with BTLA negatively regulate T-cell responses.

High-throughput kinase assay based on surface plasmon resonance suitable for native protein substrates

We report a novel in vitro high-throughput (HTP) kinase assay using surface plasmon resonance (SPR). In vitro tyrosine phosphorylation was performed in a microtiter plate, after which the substrate was captured with an antibody on a sensor chip and phosphotyrosine (pTyr) was detected with an anti-pTyr antibody. The capture and pTyr detection steps were performed using a Biacore A100, which is a sensitive and high-performance flow-cell-based SPR biosensor. This system allowed multiple sample processing (1000 samples/day) and high-quality data sampling. We compared the abilities of the HTP-SPR method and a standard radioisotope assay by measuring the phosphorylation of several substrate proteins by the Fyn tyrosine kinase. Similar results were obtained with both methods, suggesting that the HTP-SPR method is reliable. Therefore, the HTP-SPR method described in this study can be a powerful tool for a variety of screening analyses, such as kinase activity screening, kinase substrate profiling, and kinase HTP screening of kinase inhibitors.

A molecular perspective of CTLA-4 function

Within the paradigm of the two-signal model of lymphocyte activation, the interest in costimulation has witnessed a remarkable emergence in the past few years with the discovery of a large array of molecules that can serve this role, including some with an inhibitory function. Interest has been further enhanced by the realization of these molecules' potential as targets to modulate clinical immune responses. Although the therapeutic translation of mechanistic knowledge in costimulatory molecules has been relatively straightforward, the capacity to target their inhibitory counterparts has remained limited. This limited capacity is particularly apparent in the case of the cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), a major negative regulator of T cell responses. Because there have been several previous comprehensive reviews on the function of this molecule, we focus here on the physiological implications of its structural features. Such an exercise may ultimately help us to design immunotherapeutic agents that target CTLA-4.

BTLA and HVEM Cross Talk Regulates Inhibition and Costimulation

Recently a new inhibitory immunoglobulin domain-containing lymphocyte receptor was identified on the basis of its T helper 1 (T(H)1)-selective expression in murine T cell lines, which was named B and T lymphocyte attenuator (BTLA). Several groups have confirmed the initial characterization of BTLA as an inhibitory receptor, which was initially inferred from the mild increases in several parameters of BTLA-deficient mice. The initial expectation that BTLA would interact with a B7 family ligand, such as the B7x protein, was surprisingly overturned with the functional cloning of the actual BTLA ligand as herpesvirus entry mediator (HVEM). This was unexpected largely due to the fact that this interaction represents the convergence of two very different, although each quite extensive, families of receptors and ligands. The interaction of BTLA, which belongs to the CD28 family of the immunoglobulin superfamily, and HVEM, a costimulatory tumor-necrosis factor (TNF) receptor (TNFR), is quite unique in that it is the only receptor-ligand interaction that directly bridges these two families of receptors. This interaction has raised many questions about how receptors from two different families could interact and which are the signaling events downstream of receptor ligation. As we discuss here and recently demonstrated, HVEM interaction with BTLA serves to negatively regulate T cell responses, in contrast to the strong activation observed when HVEM engages its endogenous ligand from the TNF family. Finally, as studies of BTLA are just now beginning to extend beyond the initial characterizations, it is becoming clear that there are many complex issues remaining to be resolved, particularly potential polymorphisms that may engender disease susceptibility in the human.

CTLA4 gene polymorphism and autoimmunity

CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA4) are two receptors that have critical but opposing functions in T-cell stimulation. CD28 promotes a number of T-cell activities, whereas in contrast CTLA4 is an essential inhibitor of T-cell responses. Because of its inhibitory role, CTLA4 is a strong candidate susceptibility gene in autoimmunity and several studies suggest disease-associated polymorphisms. In this review, we discuss recent progress in relating CTLA4 polymorphisms to disease susceptibility and consider the putative mechanisms by which CTLA4 may act to inhibit autoimmunity.

Multiple functions for CD28 and cytotoxic T lymphocyte antigen-4 during different phases of T cell responses: implications for arthritis and autoimmune diseases

Chronic T cell responses, as they occur in rheumatoid arthritis, are complex and are likely to involve many mechanisms. There is a growing body of evidence that, in concert with the T cell antigen receptor signal, CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4; CD152) are the primary regulators of T cell responses. Whereas CD28 primarily activates T cell processes, CTLA-4 inhibits them. The mechanism for this dichotomy is not fully understood, especially as CD28 and CTLA-4 recruit similar signalling molecules. In addition, recent studies demonstrate that CD28 and CTLA-4 have multiple functions during T cell responses. In particular, CTLA-4 exerts independent distinct effects during different phases of T cell responses that could be exploited for the treatment of rheumatoid arthritis.

CTLA-4 and tolerance: the biochemical point of view

Potentially autoreactive T cells that escape negative selection in the thymus must be strictly controlled in the periphery to avoid autoimmune disease. The most robust regulatory process controlling autoreactivity is mediated by the CTLA-4-B7 pathway. The critical homeostasis mediated by CTLA-4 was proven using monoclonal antibodies and genetically disrupted CTLA-4 knockout mice that develop polyclonal lymphocyte activation and proliferation leading to massively enlarged lymph nodes and spleen and fatal multiorgan lymphocytic infiltrates. CTLA-4 ligation following T-cell activation downregulates cytokine production and cell-cycle progression, however, the proximal biochemical basis for robust T-cell regulation remains unclear. In this review, we summarize studies supporting a dynamic role for CTLA-4 at the immunological synapse leading to direct attenuation of early cell signals. A model is proposed based on these observations, which proposes that CTLA-4 may, in fact, function under some circumstances in a ligand-independent manner.

Unifying concepts in CD28, ICOS and CTLA4 co-receptor signalling

Many studies have shown the central importance of the co-receptors CD28, inducible costimulatory molecule (ICOS) and cytotoxic T lymphocyte antigen 4 (CTLA4) in the regulation of many aspects of T-cell function. CD28 and ICOS have both overlapping and distinct functions in the positive regulation of T-cell responses, whereas CTLA4 negatively regulates the response. The signalling pathways that underlie the function of each of the co-receptors indicate their shared and unique properties and provide compelling hints of functions that are as yet uncovered. Here, we outline the shared and distinct signalling events that are associated with each of the co-receptors and provide unifying concepts that are related to signalling functions of these co-receptors.

Negative feedback of T cell activation through inhibitory adapters and costimulatory receptors

Antigen recognition by the T cell receptor (TCR) complex induces the formation of a TCR signalosome by recruiting various signaling molecules, generating the recognition signals for T cell activation. The activation status and functional outcome are positively and negatively regulated by dynamic organization of the signalosome and by costimulation signals. We have studied the negative regulation of T cell activation, particularly through inhibitory adapters and costimulation receptors that are little expressed in resting cells but are induced upon T cell activation. We described Grb-associated binder 2 (Gab2) and cytotoxic T lymphocyte antigen-4 (CTLA-4) as a representative inhibitory adapter and a negative costimulation receptor, respectively, both of which exhibit negative feedback. Gab2 functions as a signal branch for activation vs. inhibition, as phosphorylation of either Src homology 2 (SH2) domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) or Gab2 by zeta-associated protein of 70 kDa (ZAP-70) determines the fate of the response. As a professional inhibitory receptor, CTLA-4 inhibits T cell response by competition of ligand binding with positive costimulator receptor CD28, and also induces inhibitory signaling. The trafficking and the cell surface expression of CTLA-4 are dynamically regulated and induced. CTLA-4 is accumulated in lysosomes and secreted to the T cell-APC contact site upon TCR stimulation. As T cell activation proceeds, these inhibitory adapters and costimulation receptors are induced and suppress/regulate the responses as negative feedback.

Re-establishing peripheral tolerance in the absence of CTLA-4: complementation by wild-type T cells points to an indirect role for CTLA-4

CTLA-4 plays an important role in the down-regulation of activated T cells and in the establishment of peripheral tolerance. It has been hypothesized that CTLA-4 on the cell surface signals directly into T cells during primary immune responses, resulting in intrinsic T cell down-regulation. It is not known, however, whether CTLA-4 directly inhibits the less intense activating signals received by autoreactive T cells in the periphery. We investigated whether CTLA-4 acts intrinsically upon self-reactive cells in vivo, or whether it inhibits autoreactive cells indirectly, in a non-cell autonomous manner. The adoptive transfer of CTLA-4-deficient splenocytes or Thy 1(+) cells into recombinase-activating gene 2-deficient mice resulted in fatal inflammation and tissue destruction similar to that seen in CTLA-4-deficient mice. When an equivalent number of splenocytes or Thy 1(+) cells from wild-type animals was transferred with the CTLA-4-deficient cells, recipient mice survived indefinitely. Since CTLA-4 was absent in the T cells responsible for the inflammatory phenotype, the down-regulation of these autoreactive cells must have been facilitated indirectly by wild-type Thy 1(+) cells. In addition, a rapid reduction in the ratio of CTLA-4-deficient to wild-type cells was observed. We propose two possible indirect mechanisms by which CTLA-4 may function in the establishment and maintenance of peripheral tolerance.

L1 endocytosis is controlled by a phosphorylation-dephosphorylation cycle stimulated by outside-in signaling by L1

Dynamic regulation of the cell surface expression of adhesion molecules is an important mechanism for controlling neuronal growth cone motility and guidance. Clathrin-mediated vesicular internalization of L1 via the tyrosine-based endocytosis motif YRSL regulates adhesion and signaling by this Ig superfamily molecule. Here, we present evidence that tyrosine-1176 (Y1176) of the YRSL motif is phosphorylated in vivo. The nonreceptor tyrosine kinase (p60src) is implicated in L1-mediated neurite outgrowth, and we find that p60src phosphorylates Y1176 in vitro. Phosphorylation of Y1176 prevents L1 binding to AP-2, an adaptor required for clathrin-mediated internalization of L1. mAb 74-5H7 recognizes the sequence immediately NH2-terminal to the tyrosine-based motif and binds L1 only when Y1176 is dephosphorylated. 74-5H7 identifies a subset of L1 present at points of cell-cell contact and in vesicle-like structures that colocalize with an endocytosis marker. L1-L1 binding or L1 cross-linking induces a rapid increase in 74-5H7 immunoreactivity. Our data suggest a model in which homophilic binding or L1 cross-linking triggers transient dephosphorylation of the YRSL motif that makes L1 available for endocytosis. Thus, the regulation of L1 endocytosis through dephosphorylation of Y1176 is a critical regulatory point of L1-mediated adhesion and signaling.

Inhibition of CTLA-4 function by the regulatory subunit of serine/threonine phosphatase 2A

The catalytic subunit of the serine/threonine phosphatase 2A (PP2A) can interact with the cytoplasmic tail of CTLA-4. However, the molecular basis and the biological significance of this interaction are unknown. In this study, we report that the regulatory subunit of PP2A (PP2AA) also interacts with the cytoplasmic tail of CTLA-4. Interestingly, TCR ligation induces tyrosine phosphorylation of PP2AA and its dissociation from CTLA-4 when coligated. The association between PP2AA and CTLA-4 involves a conserved three-lysine motif in the juxtamembrane portion of the cytoplasmic tail of CTLA-4. Mutations of these lysine residues prevent the binding of PP2AA and enhance the inhibition of IL-2 gene transcription by CTLA-4, indicating that PP2A represses CTLA-4 function. Our data imply that the lysine-rich motif in CTLA-4 may be used to identify small molecules that block its binding to PP2A and act as agonists for CTLA-4 function.

Intracellular distribution of lysosomal sialidase is controlled by the internalization signal in its cytoplasmic tail

Sialidase (neuraminidase), encoded by the neu-1 gene in the major histocompatibility complex locus catalyzes the intralysosomal degradation of sialylated glycoconjugates. Inherited deficiency of sialidase results in sialidosis or galactosialidosis, both severe metabolic disorders associated with lysosomal storage of oligosaccharides and glycopeptides. Sialidase also plays an important role in cellular signaling and is specifically required for the production of cytokine interleukin-4 by activated T lymphocytes. In these cells, neu-1-encoded sialidase activity is increased on the cell surface, suggesting that a specific mechanism regulates sorting of this enzyme to the plasma membrane. We investigated that mechanism by first showing that sialidase contains the internalization signal found in lysosomal membrane proteins targeted to endosomes via clathrin-coated pits. The signal consists of a C-terminal tetrapeptide (412)YGTL(415), with Tyr(412) and Leu(415) essential for endocytosis of the enzyme. We further demonstrated that redistribution of sialidase from lysosomes to the cell surface of activated lymphocytes is accompanied by increased reactivity of the enzyme with anti-phosphotyrosine antibodies. We speculate that phosphorylation of Tyr(412) results in inhibition of sialidase internalization in activated lymphocytes.

T-cell regulation by CD28 and CTLA-4

Activation of T lymphocytes is thought to require at least two signals, one delivered by the T-cell receptor complex after antigen recognition, and one provided on engagement of co-stimulatory receptors, such as CD28. Recent studies are providing clues as to the specific signalling roles of co-stimulatory receptors. Furthermore, superimposition of inhibitory signals, such as those delivered by cytotoxic T-lymphocyte antigen 4 (CTLA-4), leads to a complex network of positive and negative co-stimulatory signals, the integration of which modulates immune responses.

Src kinases Fyn and Lck facilitate the accumulation of phosphorylated CTLA-4 and its association with PI-3 kinase in intracellular compartments of T-cells

Src kinases bind to surface receptors and mediate signaling events at the surface of cells. Little is known regarding whether these kinases can mediate events within intracellular compartments. The T-cell antigen CTLA-4 resides primarily in the trans-Golgi network (TGN), and as such could serve as a model to study the intracellular function of src kinases in their ability to phosphorylate the receptor. In this study, we show that tyrosine kinases p56lck and p59fyn phosphorylate the cytoplasmic domain of CTLA-4 in T-cells. Most interestingly, these kinases are also found in the Golgi apparatus, the intracellular compartment where most of CTLA-4 is localized. Transfection of Lck or Fyn resulted in increased phosphorylation of intracellular CTLA-4 and recruitment of PI-3 kinase. By contrast, phosphorylation did not influence the presence of the receptor in the TGN. These data demonstrate that src kinases operate to modulate receptor binding to intracellular signaling proteins introducing the possibility that intracellular forms of receptors may generate growth signals.

A regulatory role for cytoplasmic YVKM motif in CTLA-4 inhibition of TCR signaling

CTLA-4 negatively regulates TCR signaling, although the molecular basis for this effect has yet to be elucidated. The cytoplasmic YVKM motif, while binding to phosphatidylinositol 3-kinase, SHP-2 and the AP-1/AP-2 clathrin adaptor complexes, has been reported to play no role in CTLA-4 function. In contrast, in this study, we demonstrate that, although not essential, the YVKM motif contributes to optimal CTLA-4 blockage of TCRzeta or combined TCRzeta/CD28 signaling. Significantly, dependency on the YVKM motif varied with the mode of anti-receptor presentation, where soluble antibody ligation was more dependent on the presence of the motif than immobilized antibody. Previous studies have mainly relied on the use of immobilized antibody. Neither SHP-2 binding, alterations in TCRzeta chain phosphorylation, nor ZAP-70 recruitment was involved in CTLA-4 wild-type or mutant inhibition. Overall, our findings clearly implicate the YVKM motif in optimal CTLA-4 function.

The CD28 and CTLA-4 receptors associate with the serine/threonine phosphatase PP2A

CD28 and CTLA-4 are related members of a family of T lymphocyte cell surface receptors that function to regulate T cell activation. We have found that the cytoplasmic domains of both CTLA-4 and CD28 can associate with members of the PP2A family of serine/threonine phosphatases. The association of PP2A with CD28 was negatively regulated by tyrosine phosphorylation of the CD28 cytoplasmic domain. Inhibition of PP2A activity in Jurkat leukemia T cells by treatment with okadaic acid or by expression of a dominant-negative mutant enhanced T cell activation induced by CD28 engagement. Interactions between cell surface receptors such as CTLA-4 and CD28 and serine/threonine phosphatases may represent a novel mechanism for modulating the intracellular signal transduction pathways associated with cell activation.

Tyrosine phosphatase SHP-2 binding to CTLA-4: absence of direct YVKM/YFIP motif recognition

CTLA-4 is well documented in its negative regulation of T-cell proliferation. However, little is known regarding the signaling mechanisms induced by CTLA-4. CTLA-4 associates with the phosphatidylinositol 3-kinase, the phosphatase SHP-2 and the clathrin adaptor complexes AP-1 and AP-2. SHP-2 SH2 domain binding to CTLA-4 is unusual given the absence of a I/VxYxxI/V/L motif. Here, we demonstrate that the phosphorylation of CTLA-4 tyrosines (YVKM and YFIP) fails to allow for single or tandem SHP-2 SH2 domain binding. This was observed using wild-type and inactive SHP-2 as well as a construct with the isolated two SH2 domains. The phosphorylated YVKM and YFIP motifs therefore do not appear to represent novel binding motifs for SHP-2 SH2 domains. At the same time, we could confirm that SHP-2 can associate with CTLA-4 in murine T-cells indicating that the interaction between the phosphatase and CTLA-4 is an indirect event, possibly mediated by PI 3-kinase/SHP-2 binding.

Cutting edge: tyrosine-independent transmission of inhibitory signals by CTLA-4

CTLA-4 is an important inhibitor of T cell activation. We used Jurkat cells expressing mutants of murine CTLA-4 to study the structural requirements for inhibitory signaling. We find that signals for the inhibition of IL-2 secretion are delivered efficiently by a CTLA-4 mutant in which both cytoplasmic tyrosines have been replaced by phenylalanines. A CTLA-4 mutant that lacks the carboxyl-terminal half of the intracellular domain also retains the ability to inhibit, but deletion of an additional 11 aa completely abrogates that capability. We conclude that delivery of an inhibitory signal requires the membrane-proximal region of the CTLA-4 cytoplasmic domain and does not depend upon the tyrosine phosphorylation of CTLA-4.

The inhibitory function of CTLA-4 does not require its tyrosine phosphorylation

CTLA-4 is a negative regulator of T cell responses. Sequence analysis of this molecule reveals the presence of two cytoplasmic tyrosine residues at positions 165 and 182 that are potential Src homology (SH)-2 domain binding sites. The role of phosphorylation of these residues in CTLA-4-mediated signaling is unknown. Here, we show that sole TCR ligation induces zeta-associated protein (ZAP)-70-dependent tyrosine phosphorylation of CTLA-4 that is important for cell surface retention of this molecule. However, CTLA-4 tyrosine phosphorylation is not required for down-regulation of T cell activation following CD3-CTLA-4 coengagement. Specifically, inhibition of extracellular signal-regulated kinase (ERK) activation and of IL-2 production by CTLA-4-mediated signaling occurs in T cells expressing mutant CTLA-4 molecules lacking the cytoplasmic tyrosine residues, and in lck-deficient or ZAP-70-deficient T cells. Therefore, CTLA-4 function involves interplay between two different levels of regulation: phosphotyrosine-dependent cell surface retention and phosphotyrosine-independent association with signaling molecules.

Cytotoxic T lymphocyte antigen 4 (CTLA-4) engagement delivers an inhibitory signal through the membrane-proximal region in the absence of the tyrosine motif in the cytoplasmic tail

Cytotoxic T lymphocyte antigen 4 (CTLA-4) is a T cell costimulation receptor that delivers inhibitory signals upon activation. Although the tyrosine-based motif ((165)YVKM) within its cytoplasmic tail has been shown to associate in vitro with Src homology 2 domain-containing tyrosine phosphatase (SHP-2) and phosphatidylinositol 3 kinase upon phosphorylation, the mechanism of negative signaling remains unclear. Here, we report a new mechanism of negative signaling based on the analysis of murine T cell clones transfected with various mutants of CTLA-4. Upon T cell activation by cross-linking with anti-CD3 and anti-CD28 antibodies, CTLA-4 engagement inhibited both proliferation and interleukin 2 production in tyrosine mutants as well as in wild-type CTLA-4 transfectants. Furthermore, the mutant CTLA-4 lacking most of the cytoplasmic region strongly suppressed interleukin 2 production as well. These data suggest that negative signals by CTLA-4 could be mediated through the membrane-proximal region of CTLA-4 but not through the YVKM motif and that the association of CTLA-4 with SHP-2 is not required for CTLA-4-mediated suppression of T cell activation.

Costimulatory regulation of T cell function

During the past several years, the critical role of costimulatory molecules in regulating T cell responses has been demonstrated. Costimulatory molecule CD28 enhances whereas CTLA-4 downmodulates T cell responses. An understanding of the integration of the signals mediated by costimulatory molecules and the T cell receptor at the cellular and molecular levels is just beginning to be achieved.

The role of CTLA-4 in the regulation of T cell immune responses

Over the past few years a great deal of research has examined how T cell-dependent immune responses are initiated and subsequently regulated. Ligation of the TCR with an antigenic peptide bound to an MHC protein on a professional APC provides the crucial antigen-specific stimulus required for T cell activation. Interaction of CD28 with CD80 or CD86 molecules on APC initiates a costimulatory or second signal within the T cell which augments and sustains T cell activation initiated through the TCR. However, recently it has become clear that T cell immune responses are a result of a balance between stimulatory and inhibitory signals. Cytotoxic T lymphocyte-associated molecule-4 (CTLA-4) is a cell surface molecule that is expressed nearly exclusively on CD4+ and CD8+ T cells. Investigation into the role of CTLA-4 in the regulation of T cell immune responses has revealed that CTLA-4 is a very important molecule involved in the maintenance of T cell homeostasis. In the present review, evidence for the proposed inhibitory role of CTLA-4 is examined and a model suggesting a role for CTLA-4 in both early and late stages of T cell activation is presented.

Role of the CTLA-4 receptor in T cell activation and immunity. Physiologic function of the CTLA-4 receptor

Costimulatory molecules of the B7 family regulate the activation of T lymphocytes. T cell activation is promoted by binding of B7 molecules to CD28 and inhibited by binding to CTLA-4 (CD152). The balance between positive signals through CD28 and negative signals through CTLA-4 is critical for the fate of the T cell and is subject to tight regulation. Recent in vitro and in vivo studies have significantly advanced our understanding of the function of the CTLA-4 receptor. The results of these experiments suggest that CTLA-4 is critical for the induction of self-tolerance, and that it may have distinct signaling functions in resting and activated T cells. In resting T cells, CTLA-4 crosslinking leads to cell-cycle arrest, whereas in activated T cells, CTLA-4 crosslinking induces apoptosis. In this article, we will review the physiologic functions of the CTLA-4 receptor.