Regulation of T cell receptor signaling by tyrosine phosphatase SYP association with CTLA-4

Chimeric switch receptor: switching for improved adoptive T-cell therapy against cancers

Adoptive T-lymphocyte transfer-based immunotherapy for cancers has seen huge leaps with both CARs and engineered TCRs. Despite this, issues relating to safety and efficacy persist. To address this, chimeric switch receptors have been created to reverse the outcomes of their original signaling pathways in order to confer immune cells with the ability to overcome the immunosuppressive tumor microenvironment and to allow them to have greater in vivo persistence. Activating switch receptors exploit the inhibitory molecules expressed by cancer cells to further stimulate the tumor antigen-specific T lymphocytes. On the other hand, inhibitory switch receptors inhibit the effects of tumor-reactive T lymphocytes on unintended targets. This paper reviews the switch receptors reported thus far, and lists out potential improvements and future works.

Immune checkpoint receptors: homeostatic regulators of immunity

Alcoholic liver disease (ALD) is an escalating global problem accounting for more than 3 million deaths annually. Bacterial infections are diagnosed in 25-47% of hospitalized patients with cirrhosis and represent the most important trigger for acute decompensation, multi-organ failure, septic shock and death. Current guidelines recommend intensive antibiotic therapy, but this has led to the emergence of multi-drug resistant bacteria, which are associated with increased morbidity and mortality rates. As such, there is a pressing need to explore new paradigms for anti-infective therapy and host-directed immunomodulatory therapies are a promising approach. Paradoxically, cirrhotic patients are characterised by heightened immune activity and exacerbated inflammatory processes but are unable to contend with bacterial infection, demonstrating that whilst immune effector cells are primed, their antibacterial effector functions are switched-off, reflecting a skewed homeostatic balance between anti-pathogen immunity and host-induced immunopathology. Preservation of this equilibrium physiologically is maintained by multiple immune-regulatory checkpoints and these feedback receptors serve as pivotal regulators of the host immunity. Checkpoint receptor blockade is proving to be effective at rescuing deranged/exhausted immunity in pre-clinical studies for chronic viral infection and sepsis. This approach has also obtained FDA approval for restoring anti-tumor immunity, with improved response rates and good safety profiles. To date, no clinical studies have investigated checkpoint blockade in ALD, highlighting an area for development of host-targeted immunotherapeutic strategies in ALD, for which there are no current specific treatment options. This review aims at framing current knowledge on immune checkpoints and the possibility of their therapeutic utility in ALD-associated immune dysfunctions.

A reappraisal of CTLA-4 checkpoint blockade in cancer immunotherapy

It is assumed that anti-CTLA-4 antibodies cause tumor rejection by blocking negative signaling from B7-CTLA-4 interactions. Surprisingly, at concentrations considerably higher than plasma levels achieved by clinically effective dosing, the anti-CTLA-4 antibody Ipilimumab blocks neither B7 trans-endocytosis by CTLA-4 nor CTLA-4 binding to immobilized or cell-associated B7. Consequently, Ipilimumab does not increase B7 on dendritic cells (DCs) from either CTLA4 gene humanized (Ctla4^h/h) or human CD34⁺ stem cell-reconstituted NSG™ mice. In Ctla4^h/m mice expressing both human and mouse CTLA4 genes, anti-CTLA-4 antibodies that bind to human but not mouse CTLA-4 efficiently induce Treg depletion and Fc receptor-dependent tumor rejection. The blocking antibody L3D10 is comparable to the non-blocking Ipilimumab in causing tumor rejection. Remarkably, L3D10 progenies that lose blocking activity during humanization remain fully competent in inducing Treg depletion and tumor rejection. Anti-B7 antibodies that effectively block CD4 T cell activation and de novo CD8 T cell priming in lymphoid organs do not negatively affect the immunotherapeutic effect of Ipilimumab. Thus, clinically effective anti-CTLA-4 mAb causes tumor rejection by mechanisms that are independent of checkpoint blockade but dependent on the host Fc receptor. Our data call for a reappraisal of the CTLA-4 checkpoint blockade hypothesis and provide new insights for the next generation of safe and effective anti-CTLA-4 mAbs.

The molecular basis of immune regulation in autoimmunity

Autoimmune diseases can be triggered and modulated by various molecular and cellular characteristics. The mechanisms of autoimmunity and the pathogenesis of autoimmune diseases have been investigated for several decades. It is well accepted that autoimmunity is caused by dysregulated/dysfunctional immune susceptible genes and environmental factors. There are multiple physiological mechanisms that regulate and control self-reactivity, but which can also lead to tolerance breakdown when in defect. The majority of autoreactive T or B cells are eliminated during the development of central tolerance by negative selection. Regulatory cells such as Tregs (regulatory T) and MSCs (mesenchymal stem cells), and molecules such as CTLA-4 (cytotoxic T-lymphocyte associated antigen 4) and IL (interleukin) 10 (IL-10), help to eliminate autoreactive cells that escaped to the periphery in order to prevent development of autoimmunity. Knowledge of the molecular basis of immune regulation is needed to further our understanding of the underlying mechanisms of loss of tolerance in autoimmune diseases and pave the way for the development of more effective, specific, and safer therapeutic interventions.

Antagonist Anti-CD28 Therapeutics for the Treatment of Autoimmune Disorders

The effector functions of T lymphocytes are responsible for most autoimmune disorders and act by directly damaging tissues or by indirectly promoting inflammation and antibody responses. Co-stimulatory and co-inhibitory T cell receptor molecules are the primary pharmacological targets that enable interference with immune-mediated diseases. Among these, selective CD28 antagonists have drawn special interest, since they tip the co-stimulation/co-inhibition balance towards efficiently inhibiting effector T cells while promoting suppression by pre-existing regulatory T-cells. After having demonstrated outstanding therapeutic efficacy in multiple models of autoimmunity, inflammation and transplantation, and safety in phase-I studies in humans, selective CD28 antagonists are currently in early clinical development for the treatment of systemic lupus erythematous and rheumatoid arthritis. Here, we review the available proof of concept studies for CD28 antagonists in autoimmunity, with a special focus on the mechanisms of action.

High-affinity human PD-L1 variants attenuate the suppression of T cell activation

The activated T cells can be suppressed by programed death-1 (PD-1) axis through low affinity interaction between PD-1 and PD-ligand 1 (PD-L1) in solution or on antigen presenting cells. In clinic, the concentration of soluble PD-L1 in peripheral blood negatively correlates with cancer prognosis. However, there is little information about the relation between the affinity of PD-1/PD-L1 interaction and the suppressive capacity of PD-1 axis. In this study, we analyzed inhibitory roles of high affinity soluble human PD-L1 (hPD-L1) variants, which were generated with directed molecular evolution. Resultant two clones L3C7-hPD-L1 and L3B3-hPD-L1 showed over 20 folds greater affinity than that of native hPD-L1. We found that L3B3-hPD-L1 and L3C7-hPD-L1 could compete with an anti-PD-1 antibody (EH12.1) for binding to hPD-1. More importantly, although native soluble hPD-L1 can induce suppressive effects on activated T cells, we found L3B3-hPD-L1 and L3C7-hPD-L1 attenuated the strength of PD-1 axis for suppressing the proliferation and interferon γ (IFN-γ) secretion of PBMC. In conclusion, our data provide direct evidence in which immune checkpoint receptor-ligand interactive strength can alter the the suppressive function, in particular, the suppressive capacity of PD-1 axis could be decreased with enhanced affinity of soluble PD-L1 and PD-1 interaction. Our study might provide a new direction for manipulating immune checkpoints.

Checkpoint Inhibitors: Applications for Autoimmunity

To limit excessive T cell-mediated inflammatory responses, the immune system has a milieu of inhibitory receptors, called immune checkpoints. Cancer cells have evolved to seize those inhibitory pathways and to prevent T cell-mediated killing of tumor cells. Therefore, immune checkpoint inhibitors (ICI) consisting of blocking antibodies against these receptors present an exciting avenue in the fight against cancer. The last decade has seen the implementation of ICI against a variety of cancer indications that have improved the overall anti-tumor responses and patient survival. However, inflammatory toxicities and autoimmunity are a significant adverse event of ICI therapies. In this review, we will discuss the biology of immune checkpoints, highlight research strategies that may help reduce the incidence of immune-related adverse events associated with ICI therapies, and also suggest investigational approaches to manipulate immune checkpoints to treat primary autoimmune disorders.

Immunomodulatory Drugs: Immune Checkpoint Agents in Acute Leukemia

Intrinsic immune responses to acute leukemia are inhibited by a variety of mechanisms, such as aberrant antigen expression by leukemia cells, secretion of immunosuppressive cytokines and expression of inhibitory enzymes in the tumor microenvironment, expansion of immunoregulatory cells, and activation of immune checkpoint pathways, all leading to T cell dysfunction and/or exhaustion. Leukemic cells, similar to other tumor cells, hijack these inhibitory pathways to evade immune recognition and destruction by cytotoxic T lymphocytes. Thus, blockade of immune checkpoints has emerged as a highly promising approach to augment innate anti-tumor immunity in order to treat malignancies. Most evidence for the clinical efficacy of this immunotherapeutic strategy has been seen in patients with metastatic melanoma, where anti-CTLA-4 and anti-PD-1 antibodies have recently revolutionized treatment of this lethal disease with otherwise limited treatment options. To meet the high demand for new treatment strategies in acute leukemia, clinical testing of these promising therapies is commencing. Herein, we review the biology of multiple inhibitory checkpoints (including CTLA-4, PD-1, TIM-3, LAG-3, BTLA, and CD200R) and their contribution to immune evasion by acute leukemias. In addition, we discuss the current state of preclinical and clinical studies of immune checkpoint inhibition in acute leukemia, which seek to harness the body's own immune system to fight leukemic cells.

Limitations and opportunities for immune checkpoint inhibitors in pediatric malignancies

Immune checkpoint inhibitors (ICI) have shown great promise in a wide spectrum of adult solid and hematological malignancies, achieving objective tumor responses and prolonging survival. However, there is limited clinical success amongst pediatric patients. In this review, we summarize the current understanding of ICI and present an up-to-date overview of recent and ongoing clinical trials of ICI in pediatric malignancies. In addition, we will discuss immunologic and clinical difficulties in this young population, as well as future prospects for combination of ICI with other immune-based and conventional treatments.

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.

T cell costimulatory receptor CD28 is a primary target for PD-1-mediated inhibition

Programmed cell death-1 (PD-1) is a coinhibitory receptor that suppresses T cell activation and is an important cancer immunotherapy target. Upon activation by its ligand PD-L1, PD-1 is thought to suppress signaling through the T cell receptor (TCR). By titrating PD-1 signaling in a biochemical reconstitution system, we demonstrate that the co-receptor CD28 is strongly preferred over the TCR as a target for dephosphorylation by PD-1-recruited Shp2 phosphatase. We also show that CD28, but not the TCR, is preferentially dephosphorylated in response to PD-1 activation by PD-L1 in an intact cell system. These results reveal that PD-1 suppresses T cell function primarily by inactivating CD28 signaling, suggesting that costimulatory pathways play key roles in regulating effector T cell function and responses to anti-PD-L1/PD-1 therapy.

Is the Genetic Background of Co-Stimulatory CD28/CTLA-4 Pathway the Risk Factor for Prostate Cancer?

The impairment of immunological surveillance caused by aberrant T cell activation can lead to an inadequate anti-tumor response. Therefore, deregulation in co-stimulatory pathway might be associated with cancer susceptibility. Here we undertook a prospective study to investigate whether genetic variations in gene encoding molecule CD28 and CTLA-4 playing pivotal role in regulating adoptive immune response can influence susceptibility to prostate cancer. Single nucleotide polymorphisms (SNPs) in CTLA-4 and CD28 genes were genotyped in 301 prostate cancer (PCa) patients and 301 controls. The distributions of the genotypes and haplotypes in the CTLA-4/CD28 SNPs were similar in both studied groups. However, the overrepresentation of carriers of CTLA-4c.49A>G[A] allele and carriers of CTLA-4g.319C>T[T] allele in PCa as compared to controls was observed (p = 0.082 and p = 0.13, respectively). The risk of disease was higher (OR 1.78) for carriers of both susceptibility alleles as compared to carriers of protective genotypes (p = 0.03). The CTLA-4c.49A>G and CTLA-4g.319C>T SNPs might be considered as low risk susceptibility locus for PCa.

TCR Signaling and CD28/CTLA-4 Signaling Cooperatively Modulate T Regulatory Cell Homeostasis

Foxp3⁺ T regulatory cells (Tregs), conventional CD4⁺Foxp3^- T cells, and CD8⁺ T cells represent heterogeneous populations composed of naive phenotype (NP, CD44^low) and memory phenotype (MP, CD44^high) subpopulations. NP and MP subsets differ in their activation state, contribution to immune function, and capacity to proliferate in vivo. To further understand the factors that contribute to the differential homeostasis of NP/MP subsets, we examined the differential effects of CD28 and CTLA-4 interaction with CD80/CD86, as well as MHC class II-TCR interaction within mouse Treg pools and CD4⁺ and CD8⁺ T cell pools. Blockade of CD80/CD86 with CTLA-4-Ig markedly reduced the cycling and absolute numbers of MP Tregs and MP CD4⁺ T cells, with minimal effect on the NP T cell subpopulations. Blockade of MHC class II-TCR interaction led to selective expansion of MP Tregs and MP CD4⁺ and CD8⁺ T cells that was reversed upon cotreatment with CTLA-4-Ig. Treatment with anti-CTLA-4 mAb altered MP Treg and MP CD4⁺ and CD8⁺ T cell homeostasis in a manner similar to that observed with anti-MHC class II. We postulate a complex pathway in which CD28 is the primary driver of Treg proliferation and CTLA-4 functions as the main brake but is likely dependent on TCR signals and CD80/CD86. These findings have important implications for the use of biologic agents targeting such pathways to modulate autoimmune and neoplastic disease.

Mining the Complex Family of Protein Tyrosine Phosphatases for Checkpoint Regulators in Immunity

The family of protein tyrosine phosphatases (PTPs) includes 107 genes in humans that are diverse in their structures and expression profiles. The majority are present in immune cells and play various roles in either inhibiting or promoting the duration and amplitude of signaling cascades. Several PTPs, including TC-PTP (PTPN2) and SHP-1 (PTPN6), have been recognized as being crucial for maintaining proper immune response and self-tolerance, and have gained recognition as true immune system checkpoint modulators. This chapter details the most recent literature on PTPs and immunity by examining their known functions in regulating signaling from either established checkpoint inhibitors or by their intrinsic properties, as modulators of the immune response. Notably, we review PTP regulatory properties in macrophages, antigen-presenting dendritic cells, and T cells. Overall, we present the PTP gene family as a remarkable source of novel checkpoint inhibitors wherein lies a great number of new targets for immunotherapies.

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.

Mechanisms of immune evasion and current status of checkpoint inhibitors in non-small cell lung cancer

In the past several years, immunotherapy has emerged as a viable treatment option for patients with advanced non‐small cell lung cancer (NSCLC) without actionable driver mutations that have progressed on standard chemotherapy. We are also beginning to understand the methods of immune evasion employed by NSCLC which likely contribute to the 20% response rate to immunotherapy. It is also yet unclear what tumor or patient factors predict response to immunotherapy. The objectives of this review are (1) review the immunogenicity of NSCLC (2) describe the mechanisms of immune evasion (3) summarize efforts to target the anti‐program death‐1 (PD‐1) and anti‐program death‐ligand 1(PD‐L1) pathway (4) outline determinants of response to PD‐1/PD‐L1 therapy and (5) discuss potential future areas for research.

Glutamate Receptor Interacting Protein 1 Regulates CD4(+) CTLA-4 Expression and Transplant Rejection

PDZ domains are common 80- to 90-amino-acid regions named after the first three proteins discovered to share these domains: postsynaptic density 95, discs large, and zonula occludens. PDZ domain-containing proteins typically interact with the C-terminus of membrane receptors. Glutamate receptor interacting protein 1 (GRIP1), a seven-PDZ domain protein scaffold, regulates glutamate receptor surface expression and trafficking in neurons. We have found that human and mouse T cells also express GRIP1. T cell-specific GRIP1(-/-) mice >11 weeks old had prolonged cardiac allograft survival. Compared with wild-type T cells, in vitro stimulated GRIP1(-/-) T cells had decreased expression of activation markers and increased apoptotic surface marker expression. Surface expression of the strong T cell inhibitory molecule cytotoxic T lymphocyte antigen-4 (CTLA-4) was increased on GRIP1(-/-) T cells from mice >11 weeks old. CTLA-4 increases with T cell stimulation and its surface expression on GRIP1(-/-) T cells remained high after stimulation was removed, indicating a possible internalization defect in GRIP1-deficient T cells. CTLA-4-blocking antibody treatment following heart transplantation led to complete rejection in T cell GRIP1(-/-) mice, indicating that increased CTLA-4 surface expression contributed to the extended graft survival. Our data indicate that GRIP1 regulates T cell activation by regulating CTLA-4 surface expression.

Immune checkpoint therapy for non-small-cell lung cancer: an update

The role of immunotherapy in treatment of non-small-cell lung cancer (NSCLC) has been gaining interest over the past few years. This has been driven primarily by promising results from trials evaluating antagonist antibodies that target co-inhibitory immune checkpoints expressed on tumor cells and immune cells within the tumor microenvironment. Immune checkpoints exist to dampen or terminate immune activity to guard against autoimmunity and allow for self-tolerance. However, tumors can take advantage of these immune checkpoint pathways to evade destruction. Antibodies that block inhibitory checkpoints, such as anti-CTLA-4, anti-PD1 and anti-PD-L1 antibodies have demonstrated delayed tumor growth and increased survival. Novel therapies are now investigating combining checkpoint inhibitors with chemotherapy, targeted therapy, radiation and vaccines to produce synergistic antitumor activity.

In situ vaccination by radiotherapy to improve responses to anti-CTLA-4 treatment

Targeting immune checkpoint receptors has emerged as an effective strategy to induce immune-mediated cancer regression in the subset of patients who have significant pre-existing anti-tumor immunity. For the remainder, effective anti tumor responses may require vaccination. Radiotherapy, traditionally used to achieve local tumor control, has acquired a new role, that of a partner for immunotherapy. Ionizing radiation has pro-inflammatory effects that facilitate tumor rejection. Radiation alters the tumor to enhance the concentration of effector T cells via induction of chemokines, cytokines and adhesion molecules. In parallel, radiation can induce an immunogenic death of cancer cells, promoting cross-presentation of tumor-derived antigens by dendritic cells to T cells. Newly generated anti-tumor immune responses have been demonstrated post-radiation in both murine models and occasional patients, supporting the hypothesis that the irradiated tumor can become an in situ vaccine. It is in this role, that radiation can be applied to induce anti-tumor T cells in lymphocyte-poor tumors, and possibly benefit patients who would otherwise fail to respond to immune checkpoint inhibitors. This review summarizes preclinical and clinical data demonstrating that radiation acts in concert with antibodies targeting the immune checkpoint cytotoxic T-lymphocyte antigen-4 (CTLA-4), to induce therapeutically effective anti-tumor T cell responses in tumors otherwise non responsive to anti-CTLA-4 therapy.

Deletion of CTLA-4 on regulatory T cells during adulthood leads to resistance to autoimmunity

Paterson et al. demonstrate that, in contrast to CTLA-4 germline knockout mice, conditional deletion on T reg cells during adulthood confers protection from EAE and does not increase resistance to tumors.

Cytotoxic T lymphocyte antigen-4 (CTLA-4) is an essential negative regulator of T cell responses. Germline Ctla4 deficiency is lethal, making investigation of the function of CTLA-4 on mature T cells challenging. To elucidate the function of CTLA-4 on mature T cells, we have conditionally ablated Ctla4 in adult mice. We show that, in contrast to germline knockout mice, deletion of Ctla4 during adulthood does not precipitate systemic autoimmunity, but surprisingly confers protection from experimental autoimmune encephalomyelitis (EAE) and does not lead to increased resistance to MC38 tumors. Deletion of Ctla4 during adulthood was accompanied by activation and expansion of both conventional CD4⁺Foxp3⁻ (T conv) and regulatory Foxp3⁺ (T reg cells) T cell subsets; however, deletion of CTLA-4 on T reg cells was necessary and sufficient for protection from EAE. CTLA-4 deleted T reg cells remained functionally suppressive. Deletion of Ctla4 on T reg cells alone or on all adult T cells led to major changes in the Ctla4 sufficient T conv cell compartment, including up-regulation of immunoinhibitory molecules IL-10, LAG-3 and PD-1, thereby providing a compensatory immunosuppressive mechanism. Collectively, our findings point to a profound role for CTLA-4 on T reg cells in limiting their peripheral expansion and activation, thereby regulating the phenotype and function of T conv cells.

Cytotoxic T lymphocyte antigen-4 and immune checkpoint blockade

The relationship between cancer and the immune system is complex and provides unique therapeutic opportunities. Cytotoxic T lymphocyte antigen-4 (CTLA-4) is a regulatory molecule that suppresses T cell effector function following initial activation by costimulatory signals. Fully human monoclonal antibodies targeting CTLA-4 have been shown to increase T cell function and antitumor responses in patients with advanced metastatic melanoma. Responses observed with such immune checkpoint therapy can follow a different pattern from that seen with cytotoxic chemotherapy or targeted therapy and may continue after therapy is discontinued. In addition, the toxicities that are associated with anti-CTLA-4 therapy may differ from those of conventional therapies and consist of inflammatory events in parts of the body that do not contain cancerous cells. Early recognition of these inflammatory events and intervention is important, and the identification of predictive biomarkers continues to be an unfulfilled need in the field of immunotherapy. Combinatorial approaches with targeted therapies, radiation therapy, chemotherapy, or other immune checkpoint agonists/antagonists have the potential to increase the efficacy of CTLA-4 blockade.

Combination of radiotherapy and immune checkpoint inhibitors

The ability of ionizing radiation to cause cell death and inflammatory reactions has been known since the beginning of its therapeutic use in oncology. However, only recently this property of radiation has attracted the attention of immunologists seeking to induce or improve antitumor immunity. As immune checkpoint inhibitors are becoming mainstream cancer treatments, radiation oncologists have begun to observe unexpected out-of-the-field (abscopal) responses in patients receiving radiation therapy during immunotherapy. These unexpected responses were predicted by experimental work in preclinical tumor models and have clear biological bases. Accumulating experimental evidence that radiation induces an immunogenic cell death and promotes recruitment and function of T cells within the tumor microenvironment supports the hypothesis that radiation can convert the tumor into an in situ individualized vaccine. This property of radiation is key to its synergy with immune checkpoint inhibitors, antibodies targeting inhibitory receptors on T cells such as cytotoxic T lymphocyte antigen-4 and programmed death-1. By removing the obstacles hindering the activation and function of antitumor T cells, these agents benefit patients with pre-existing antitumor immunity but are ineffective in patients lacking these spontaneous responses. Radiation induces antitumor T cells complementing the activity of immune checkpoint inhibitors.

Immune-related strategies driving immunotherapy in breast cancer treatment: a real clinical opportunity

Because its original use as a treatment for hematologic disease, more recently immunotherapy has emerged as a novel effective therapeutic strategy for solid malignancies, such as melanoma and prostate carcinoma. For breast carcinoma, an immunologic therapeutic approach has not been well evaluated, even though there is evidence to suggest it would be a successful novel strategy, especially taking into account the high mortality rate of the most aggressive variants of this heterogeneous disease. Here, we briefly describe the most recently awarded immune-based therapies with a consolidated or potential implication for the treatment of solid malignancies. We focus on immune checkpoints and on the clinical potential of their abrogation, with a further overview of novel vaccine-based approaches and the most relevant immunotherapeutic techniques. We aim to provide an exhaustive review of the most promising immune-therapeutic agents that may have implications for breast cancer treatment.

Present and future of immune checkpoint blockade: Monotherapy to adjuvant approaches

Immune regulation of aggressive tumor growth is often outpaced by tumor up-regulation of ligands that inhibit effector immune responses through the activation of immune checkpoints. A few of such checkpoints include programmed death-1 (PD-1), cytotoxic T lymphocyte associated antigen-4 (CTLA-4), lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, Glucocorticoid-induced TNFR family-related receptor (GITR), and killer cell immunoglobulin like receptor. With the exception of GITR, after binding to their respective ligands these checkpoints induce down-modulation of immune responses to prevent autoimmunity. However, such immune mechanisms are co-opted by tumors to allow rapid tumor cell proliferation. Pre-clinical studies in antibody blockade of PD-1 and CTLA-4 have led to promising augmentation of effector immune responses in murine tumor models, and human antibodies against PD-1 and CTLA-4 alone or in combination have demonstrated tumor regression in clinical trials. The development of immune checkpoint blockade as a potential future immunotherapy has led to increasing interest in combining treatment modalities. Combination checkpoint blockade with chemotherapy and radiation therapy has shown synergistic effects in pre-clinical and clinical studies, and combination checkpoint blockade with bacterial vaccine vectors have produced increased effector immune responses in pre-clinical models. The future of immune checkpoint blockade may be as a powerful adjuvant alongside the current standard of care.

Immune checkpoint blockade

Since the development and approval of Ipilimumab, the first immune checkpoint inhibitor licensed for the treatment of metastatic melanoma, clinicians have gained a better understanding of the mode of action, management of toxicities, and assessment of response to this class of drugs. Several antibodies are now in development, aimed at blocking novel immune checkpoint molecules, such as PD-1 and it's corresponding ligand PD-L1. This article summarizes the mechanism of action, preclinical development, and subsequent clinical studies of immune checkpoint antibodies in melanoma.

Confusing signals: recent progress in CTLA-4 biology

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Cell extrinsic and cell intrinsic mechanisms of action of CTLA-4 are unclear.

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Data suggest that the extracellular domain of CTLA4 elicits regulatory function.

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The function of CTLA-4 tail may lie in regulating localization rather than signaling.

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Membrane levels of CTLA-4 directly impact access of CD28 to shared ligands.

The mechanism of action of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) remains surprisingly unclear. Regulatory T (Treg) cells can use CTLA-4 to elicit suppression; however, CTLA-4 also operates in conventional T cells, reputedly by triggering inhibitory signals. Recently, interactions mediated via the CTLA-4 cytoplasmic domain have been shown to preferentially affect Treg cells, yet other evidence suggests that the extracellular domain of CTLA-4 is sufficient to elicit suppression. Here, we discuss these paradoxical findings in the context of CTLA-4-mediated ligand regulation. We propose that the function of CTLA-4 cytoplasmic domain is not to transmit inhibitory signals but to precisely control the turnover, cellular location, and membrane delivery of CTLA-4 to facilitate its central function: regulating the access of CD28 to their shared ligands.

The New Era of Cancer Immunotherapy: Manipulating T-Cell Activity to Overcome Malignancy

Using the immune system to control cancer has been investigated for over a century. Yet it is only over the last several years that therapeutic agents acting directly on the immune system have demonstrated improved overall survival for cancer patients in phase III clinical trials. Furthermore, it appears that some patients treated with such agents have been cured of metastatic cancer. This has led to increased interest and acceleration in the rate of progress in cancer immunotherapy. Most of the current immunotherapeutic success in cancer treatment is based on the use of immune-modulating antibodies targeting critical checkpoints (CTLA-4 and PD-1/PD-L1). Several other immune-modulating molecules targeting inhibitory or stimulatory pathways are being developed. The combined use of these medicines is the subject of intense investigation and holds important promise. Combination regimens include those that incorporate targeted therapies that act on growth signaling pathways, as well as standard chemotherapy and radiation therapy. In fact, these standard therapies have intrinsic immune-modulating properties that can support antitumor immunity. In the years ahead, adoptive T-cell therapy will also be an important part of treatment for some cancer patients. Other areas which are regaining interest are the use of oncolytic viruses that immunize patients against their own tumors and the use of vaccines against tumor antigens. Immunotherapy has demonstrated unprecedented durability in controlling multiple types of cancer and we expect its use to continue expanding rapidly.

Immunotherapy of hepatocellular carcinoma

Newer immunotherapy agents may break the barrier that tumors create to evade the attack from the immune system. Dendritic cell vaccination has shown encouraging clinical activity and a favorable safety profile in advanced tumor stages. However, optimal cell maturation status, choice of tumor antigens and route of administration have not been established. Single or multiple peptides derived from tumor-associated antigens may also be used for cancer vaccination. Intratumoral delivery of oncolytic viruses expressing immunostimulating cytokines like GM-CSF have produced stimulating clinical results that need further verification. But it is probably T-cell checkpoint modulation with monoclonal antibodies that has attracted the highest expectations. Promising activity has been reported for tremelimumab, a CTLA-4 inhibitor, and a clinical trial testing the PD-1 antibody nivolumab is underway. Future progress will probably come from a better understanding of the mechanisms of cancer-related immunosuppression, improvement in agents and strategies and combination of the available therapeutic tools.

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.

Humanized mice as a model for aberrant responses in human T cell immunotherapy

Immune-deficient mice, reconstituted with human stem cells, have been used to analyze human immune responses in vivo. Although they have been used to study immune responses to xenografts, allografts, and pathogens, there have not been models of autoimmune disease in which the mechanisms of the pathologic process can be analyzed. We have found that reconstituted "humanized" mice treated with anti-CTLA-4 Ab (ipilimumab) develop autoimmune disease characterized by hepatitis, adrenalitis, sialitis, anti-nuclear Abs, and weight loss. Induction of autoimmunity involved activation of T cells and cytokine production, and increased infiltration of APCs. When anti-CTLA-4 mAb-treated mice were cotreated with anti-CD3 mAb (teplizumab), hepatitis and anti-nuclear Abs were no longer seen and weight loss did not occur. The anti-CD3 blocked proliferation and activation of T cells, release of IFN-γ and TNF, macrophage infiltration, and release of IP-10 that was induced with anti-CTLA-4 mAb. We also found increased levels of T regulatory cells (CD25(+)CD127(-)) in the spleen and mesenteric lymph nodes in the mice treated with both Abs and greater constitutive phosphorylation of STAT5 in T regulatory cells in spleen cells compared with mice treated with anti-CTLA-4 mAb alone. We describe a model of human autoimmune disease in vivo. Humanized mice may be useful for understanding the mechanisms of biologics that are used in patients. Hepatitis, lymphadenopathy, and other inflammatory sequelae are adverse effects of ipilimumab treatment in humans, and this study may provide insights into this pathogenesis and the effects of immunologics on autoimmunity.

Rab8 binding to immune cell-specific adaptor LAX facilitates formation of trans-Golgi network-proximal CTLA-4 vesicles for surface expression

Despite playing a central role in tolerance, little is known regarding the mechanism by which intracellular CTLA-4 is shuttled from the trans-Golgi network to the surfaces of T cells. In this context, Ras-related GTPase Rab8 plays an important role in the intracellular transport, while we have previously shown that CTLA-4 binds to the immune cell adaptor TRIM in T cells. In this study, we demonstrate that CTLA-4 forms a multimeric complex comprised of TRIM and related LAX that in turn binds to GTP bound Rab8 for post-Golgi transport to the cell surface. LAX bound via its N terminus to active GTP-Rab8, as well as the cytoplasmic tail of CTLA-4. TRIM required LAX for binding to Rab8 in a complex. Wild-type LAX or its N terminus (residues 1 to 77) increased CTLA-4 surface expression, whereas small interfering RNAs of Rab8 or LAX or disruption of LAX/Rab8 binding reduced numbers of CTLA-4-containing vesicles and its coreceptor surface expression. LAX also promoted the polarization of CTLA-4 and the reorientation of the microtubule-organizing center to the site of T-cell receptor engagement. Our results identify a novel CTLA-4/TRIM/LAX/Rab8 effector complex in the transport of CTLA-4 to the surfaces of T cells.

Orchestrating immune check-point blockade for cancer immunotherapy in combinations

Inhibitory receptors on immune system cells respond to membrane-bound and soluble ligands to abort or mitigate the intensity of immune responses by raising thresholds of activation, halting proliferation, favoring apoptosis or inhibiting/deviating effector function differentiation. Such evolutionarily selected inhibitory mechanisms are termed check-points and therefore check-point inhibitors empower any ongoing anti-cancer immune response that might have been too weak or exhausted. Monoclonal antibodies (mAb) interfering with CTLA-4-CD80/86, PD-1 - PD-L1, TIM-3-GAL9 and LAG3-MHC-II belong to this category of check-point inhibitors. The anti-CTLA-4 mAb ipilimumab has been approved for metastatic melanoma. Anti-PD-1 and anti-PD-L1 mAbs have shown extremely encouraging clinical activity. The potential of combination strategies with these agents has recently been highlighted by clinical observations on CTLA-4+PD-1 combined blockade in melanoma patients.

Targeting CD28 to prevent transplant rejection

INTRODUCTION

The pivotal role of costimulatory pathways in regulating T-cell activation versus tolerance has stimulated tremendous interest in their manipulation for therapeutic purposes. Of these, the CD28-B7 pathway is arguably the most important and best studied. Therapeutic targets of CD28 are currently used in the treatment of melanoma, autoimmune diseases and in transplantation.

AREAS COVERED

In this review, we summarize our current knowledge of CD28 and cytotoxic T-lymphocyte antigen-4 (CTLA-4) signaling, and review the current state and challenges of harnessing them to promote transplant tolerance.

EXPERT OPINION

Despite the success of belatacept, a first-in-class CTLA-4 fusion protein now clinically used in transplantation, it is apparent that we have only scratched the surface in understanding the complexities of how costimulatory pathways modulate the immune system. Our initial assumption that positive costimulators activate effector T cells and prevent tolerance, while negative costimulators inhibit effector T cells and promote tolerance, is clearly an oversimplified view. Indeed, belatacept is not only capable of blocking deleterious CD28-B7 interactions that promote effector T-cell responses but can also have undesired effects on tolerogenic regulatory T-cell populations.

Molecular mechanisms of T-cell anergy

Anergy is a long-term stable state of T-lymphocyte unresponsiveness to antigenic stimulation associated with the blockade of IL-2 production and proliferation. Anergy is a pathway of peripheral tolerance formation. In this review, mechanisms underlying T-cell tolerization are considered in a classical in vitro model of clonal anergy, and these mechanisms are compared with different pathways of anergy induction in vivo. Special attention is given to regulatory T-lymphocytes because, on one hand, anergy is a specific feature of these cells, and on the other hand anergy is also a mechanism of their action on target cells - effector T-lymphocytes. The role of this phenomenon in the differentiation of regulatory T-cells and also in the development of activation-induced apoptosis in effector T-lymphocytes is discussed.

Programmed death 1 and B and T lymphocyte attenuator immunoreceptors and their association with malignant T-lymphoproliferative disorders: brief review

Malignant T-cell lymphoproliferative diseases are relatively rare. T cells are activated through the T-cell receptor with the aid of costimulating molecules that can be either excitatory or inhibitory. Such pathways have been also implicated in mechanisms of malignant T-cell lymphoproliferative diseases' persistence and relapse by circumventing immune responses. To date, three major immunoinhibitory molecules have been recognized, namely programmed cell death-1 (PD-1), B and T lymphocyte attenuator (BTLA) and cytotoxic T lymphocyte antigen 4 (CTLA-4). Although CTLA-4 is considered the 'gatekeeper' of immune tolerance, PD-1 negatively regulates immune responses broadly, whereas BTLA activation has been shown to inhibit CD8+ cancer-specific T cells. Both PD-1 and BTLA downregulate proximal T-cell receptor signalling cascade and are involved in immune evasion of leukaemias and lymphomas, even after allogeneic stem cell transplantation. These immunoregulatory molecules can have seemingly a synergistic effect on weakening the immune response of patients with haematological malignancies, and their manipulation represents a very active field of preclinical as well as clinical interest.

Molecular pathways: coexpression of immune checkpoint molecules: signaling pathways and implications for cancer immunotherapy

The expression of immune checkpoint molecules on T cells represents an important mechanism that the immune system uses to regulate responses to self-proteins. Checkpoint molecules include cytotoxic T lymphocyte antigen-4, programmed death-1, lymphocyte activation gene-3, T-cell immunoglobulin and mucin protein-3, and several others. Previous studies have identified individual roles for each of these molecules, but more recent data show that coexpression of checkpoint molecules occurs frequently on cancer-specific T cells as well as on pathogen-specific T cells in chronic infections. As the signaling pathways associated with each checkpoint molecule have not been fully elucidated, blocking multiple checkpoints with specific monoclonal antibodies results in improved outcomes in several chronic viral infections as well as in a wide array of preclinical models of cancer. Recent clinical data suggest similar effects in patients with metastatic melanoma. These findings support the concept that individual immune checkpoint molecules may function through nonoverlapping molecular mechanisms. Here, we review current data regarding immune checkpoint molecule signaling and coexpression, both in cancer and infectious disease, as well as the results of preclinical and clinical manipulations of checkpoint proteins.

A clinical trial of CTLA-4 blockade with tremelimumab in patients with hepatocellular carcinoma and chronic hepatitis C

BACKGROUND & AIMS

Tremelimumab is a monoclonal antibody that blocks cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), an inhibitory co-receptor that interferes with T cell activation and proliferation. The purpose of this pilot clinical trial was to test the antitumor and antiviral effect of tremelimumab in patients with hepatocellular carcinoma (HCC) and chronic hepatitis C virus (HCV) infection; and to study the safety of its administration to cirrhotic patients.

METHODS

Tremelimumab at a dose of 15 mg/kg IV every 90 days was administered until tumor progression or severe toxicity. Twenty patients were assessable for toxicity and viral response and 17 were assessable for tumor response. Most patients were in the advanced stage and 43% had an altered liver function (Child-Pugh class B).

RESULTS

A good safety profile was recorded and no patient needed steroids because of severe immune-mediated adverse events. Some patients had a transient albeit intense elevation of transaminases after the first dose, but not following subsequent cycles. Partial response rate was 17.6% and disease control rate was 76.4%. Time to progression was 6.48 months (95% CI 3.95-9.14). A significant drop in viral load was observed while new emerging variants of the hypervariable region 1 of HCV replaced the predominant variants present before therapy, particularly in those patients with a more prominent drop in viral load. This antiviral effect was associated with an enhanced specific anti-HCV immune response.

CONCLUSIONS

Tremelimumab safety profile and antitumor and antiviral activity, in patients with advanced HCC developed on HCV-induced liver cirrhosis, support further investigation.

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.

Modulation of tumor immunity by soluble and membrane-bound molecules at the immunological synapse

To circumvent pathology caused by infectious microbes and tumor growth, the host immune system must constantly clear harmful microorganisms and potentially malignant transformed cells. This task is accomplished in part by T-cells, which can directly kill infected or tumorigenic cells. A crucial event determining the recognition and elimination of detrimental cells is antigen recognition by the T cell receptor (TCR) expressed on the surface of T cells. Upon binding of the TCR to cognate peptide-MHC complexes presented on the surface of antigen presenting cells (APCs), a specialized supramolecular structure known as the immunological synapse (IS) assembles at the T cell-APC interface. Such a structure involves massive redistribution of membrane proteins, including TCR/pMHC complexes, modulatory receptor pairs, and adhesion molecules. Furthermore, assembly of the immunological synapse leads to intracellular events that modulate and define the magnitude and characteristics of the T cell response. Here, we discuss recent literature on the regulation and assembly of IS and the mechanisms evolved by tumors to modulate its function to escape T cell cytotoxicity, as well as novel strategies targeting the IS for therapy.

The B7-independent isoform of CTLA-4 functions to regulate autoimmune diabetes

The critical role of CTLA-4 in inhibiting Ag-driven T cell responses upon engagement with its ligands, B7-1 and B7-2 and its importance for peripheral T cell tolerance and T cell homeostasis has been studied intensively. The CTLA-4 splice variant ligand-independent (li)-CTLA-4 is expressed in naive and activated T cells and can actively alter T cell signaling despite its lack of a B7 binding domain. To study the effect of li-CTLA-4 in regulating T cell responses in the context of autoimmunity, we engineered a B6.CTLA-4 (floxed-Exon2)-BAC-transgene, resulting in selective expression of li-CTLA-4 upon Cre-mediated deletion of Exon 2. Introducing the B6.BAC into the NOD background, which is genetically deficient for li-CTLA-4, restores mRNA levels of li-CTLA-4 to those observed in C57BL/6 mice. Furthermore, re-expressing this ligand nonbinding isoform in NOD mice reduced IFN-γ production in T effector cells accompanied by a significant decrease in insulitis and type 1 diabetes frequency. However, selective expression of li-CTLA-4 could not fully rescue the CTLA-4 knockout disease phenotype when bred onto NOD.BDC2.5.CTLA-4 knockout background because of the requirement of the full-length, B7-binding CTLA-4 molecule on T effector cells. Thus, the li-CTLA-4 form, when expressed at physiologic levels in the CTLA-4-sufficient NOD background can suppress autoimmunity; however, the functionality of the li-CTLA-4 isoform depends on the presence of the full-length molecule to alter effector T cell signaling.

Comparative antitumor effect of preventive versus therapeutic vaccines employing B16 melanoma cells genetically modified to express GM-CSF and B7.2 in a murine model

Cancer vaccines have always been a subject of gene therapy research. One of the most successful approaches has been working with genetically modified tumor cells. In this study, we describe our approach to achieving an immune response against a murine melanoma model, employing B16 tumor cells expressing GM-CSF and B7.2. Wild B16 cells were injected in C57BL6 mice to cause the tumor. Irradiated B16 cells transfected with GM-CSF, B7.2, or both, were processed as a preventive and therapeutic vaccination. Tumor volumes were measured and survival curves were obtained. Blood samples were taken from mice, and IgGs of each treatment group were also measured. The regulatory T cells (Treg) of selected groups were quantified using counts of images taken by confocal microscopy. Results: one hundred percent survival was achieved by preventive vaccination with the group of cells transfected with p2F_GM-CSF. Therapeutic vaccination achieved initial inhibition of tumor growth but did not secure overall survival of the animals. Classical Treg cells did not vary among the different groups in this therapeutic vaccination model.

Murine regulatory T cells differ from conventional T cells in resisting the CTLA-4 reversal of TCR stop-signal

CTLA-4 inhibits T-cell activation and protects against the development of autoimmunity. We and others previously showed that the coreceptor can induce T-cell motility and shorten dwell times with dendritic cells (DCs). However, it has been unclear whether this property of CTLA-4 affects both conventional T cells (Tconvs) and regulatory T cells (Tregs). Here, we report that CTLA-4 had significantly more potent effects on the motility and contact times of Tconvs than Tregs. This was shown firstly by anti-CTLA-4 reversal of the anti-CD3 stop-signal on FoxP3-negative cells at concentrations that had no effect on FoxP3-positive Tregs. Secondly, the presence of CTLA-4 reduced the contact times of DO11.10 x CD4(+)CD25(-) Tconvs, but not DO11.10 x CD4(+)CD25(+) Tregs, with OVA peptide presenting DCs in lymph nodes. Thirdly, blocking of CTLA-4 with anti-CTLA-4 Fab increased the contact times of Tconvs, but not Tregs with DCs. By contrast, the presence of CD28 in a comparison of Cd28(-/-) and Cd28(+/+) DO11.10 T cells had no detectable effect on the contact times of either Tconvs or Tregs with DCs. Our findings identify for the first time a mechanistic explanation to account for CTLA-4-negative regulation of Tconv cells but not Tregs in immune responses.

Basis of CTLA-4 function in regulatory and conventional CD4(+) T cells

CTLA-4 proteins contribute to the suppressor function of regulatory T cells (Tregs), but the mechanism by which they do so remains incompletely understood. In the present study, we assessed CTLA-4 protein function in both Tregs and conventional (Tconv) CD4(+) T cells. We report that CTLA-4 proteins are responsible for all 3 characteristic Treg functions of suppression, TCR hyposignaling, and anergy. However, Treg suppression and anergy only required the external domain of CTLA-4, whereas TCR hyposignaling required its internal domain. Surprisingly, TCR hyposignaling was neither required for Treg suppression nor anergy because costimulatory blockade by the external domain of CTLA-4 was sufficient for both functions. We also report that CTLA-4 proteins were localized in Tregs in submembrane vesicles that rapidly recycled to/from the cell surface, whereas CTLA-4 proteins in naive Tconv cells were retained in Golgi vesicles away from the cell membrane and had no effect on Tconv cell function. However, TCR signaling of Tconv cells released CTLA-4 proteins from Golgi retention and caused activated Tconv cells to acquire suppressor function. Therefore, the results of this study demonstrate the importance of intracellular localization for CTLA-4 protein function and reveal that CTLA-4 protein externalization imparts suppressor function to both regulatory and conventional CD4(+) T cells.

The emerging role of CTLA4 as a cell-extrinsic regulator of T cell responses

The T cell protein cytotoxic T lymphocyte antigen 4 (CTLA4) was identified as a crucial negative regulator of the immune system over 15 years ago, but its mechanisms of action are still under debate. It has long been suggested that CTLA4 transmits an inhibitory signal to the cells that express it. However, not all the available data fit with a cell-intrinsic function for CTLA4, and other studies have suggested that CTLA4 functions in a T cell-extrinsic manner. Here, we discuss the data for and against the T cell-intrinsic and -extrinsic functions of CTLA4.

Cell-autonomous and -non-autonomous roles of CTLA-4 in immune regulation

It is controversial how cytotoxic T lymphocyte antigen (CTLA)-4, a co-inhibitory molecule, contributes to immunological tolerance and negative control of immune responses. Its role as an inducer of cell-intrinsic negative signals to activated effector T cells is well documented. However, there is accumulating evidence that CTLA-4 is essential for the function of naturally occurring Foxp3(+) regulatory T (Treg) cells, which constitutively express the molecule. CTLA-4 deficiency in Foxp3(+) Treg cells indeed impairs their in vivo and in vitro suppressive function. Further, Treg cells can modulate the function of CD80- and CD86-expressing antigen-presenting cells via CTLA-4. Here we discuss how CTLA-4 expression by one T cell can influence the activation of another in a cell non-autonomous fashion and thus control immune responses.