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

CTLA-4: From mechanism to autoimmune therapy

CD28 and CTLA-4 are both important stimulatory receptors for the regulation of T cell activation. Because receptors share common ligands, B7.1 and B7.2, the expression and biological function of CTLA-4 is important for the negative regulation of T cell responses. Therefore, elimination of CTLA-4 can result in the breakdown of immune tolerance and the development of several diseases such as autoimmunity. Inhibitory signals of CTLA-4 suppress T cell responses and protect against autoimmune diseases in many ways. In this review, we summarize the structure, expression and signaling pathway of CTLA-4. We also highlight how CTLA-4 defends against potentially self-reactive T cells. Finally, we discuss how the CTLA-4 regulates a number of autoimmune diseases that indicate manipulation of this inhibitory molecule is a promise as a strategy for the immunotherapy of autoimmune diseases.

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.

Check Point Inhibitors and Their Role in Immunosuppression in Sepsis

Checkpoint regulators are a group of membrane-bound receptors or ligands expressed on immune cells to regulate the immune cell response to antigen presentation and other immune stimuli, such as cytokines, chemokines, and complement. In the context of profound immune activation, such as sepsis, the immune system can be rendered anergic by these receptors to prevent excessive inflammation and tissue damage. If this septic immunosuppression is prolonged, the host is unable to mount the appropriate immune response to a secondary insult or infection. This article describes the manner in which major regulators in the B7-CD28 family and their ligands mediate immunosuppression in sepsis.

Immunological Targets for Immunotherapy: Inhibitory T Cell Receptors

Tumor development is characterized by the accumulation of mutational and epigenetic changes that transform normal cells and survival pathways into self-sustaining cells capable of untrammeled growth. Although multiple modalities including surgery, radiation, and chemotherapy are available for the treatment of cancer, the benefits conferred are often limited. The immune system is capable of specific, durable, and adaptable responses. However, cancers hijack immune mechanisms such as negative regulatory checkpoints that have evolved to limit inflammatory and immune responses to thwart effective antitumor immunity. The development of monoclonal antibodies against inhibitory receptors expressed by immune cells has produced durable responses in a broad array of advanced malignancies and heralded a new dawn in the cancer armamentarium. However, these remarkable responses are limited to a minority of patients and indications, highlighting the need for more effective and novel approaches. Preclinical and clinical studies with immune checkpoint blockade are exploring the therapeutic potential antibody-based therapy targeting multiple inhibitory receptors. In this chapter, we discuss the current understanding of the structure, ligand specificities, function, and signaling activities of various inhibitory receptors. Additionally, we discuss the current development status of various immune checkpoint inhibitors targeting these negative immune receptors and highlight conceptual gaps in knowledge.

What did we learn from CTLA-4 insufficiency on the human immune system?

Cytotoxic-T-lymphocyte-antigen-4 (CTLA-4) is a negative immune regulator constitutively expressed on regulatory T (Treg) cells and upregulated on activated T cells. CTLA-4 inhibits T cell activation by various suppressive functions including competition with CD28, regulation of the inhibitory function of Treg cells, such as transendocytosis, and the control of adhesion and motility. Intrinsic CTLA-4 signaling has been controversially discussed, but so far no distinct signaling pathway has been identified. The CTLA-4-mediated Treg suppression plays an important role in the maintenance of peripheral tolerance and the prevention of autoimmune diseases. Human CTLA-4 insufficiency is caused by heterozygous germline mutations in CTLA4 and characterized by a complex immune dysregulation syndrome. Clinical studies on CTLA4 mutation carriers showed a reduced penetrance and variable expressivity, suggesting modifying factor(s). One hundred and forty-eight CTLA4 mutation carriers have been reported; patients showed hypogammaglobulinemia, recurrent infectious diseases, various autoimmune diseases, and lymphocytic infiltration into multiple organs. The CTLA-4 expression level in Treg cells was reduced, while the frequency of Treg cells was increased in CTLA-4-insufficient patients. The transendocytosis assay is a specific functional test for the assessment of newly identified CTLA4 gene variants. Immunoglobulin substitution, corticosteroids, immunosuppressive therapy, and targeted therapy such as with CTLA-4 fusion proteins and mechanistic target of rapamycin (mTOR) inhibitors were applied; patients with life-threatening, treatment-resistant symptoms underwent hematopoietic stem cell transplantation. The fact that in humans CTLA-4 insufficiency causes severe disease taught us that the amount of CTLA-4 molecules present in/on T cells matters for immune homeostasis. However, whether the pathology-causing activated T lymphocytes in CTLA-4-insufficient patients are antigen-specific is an unsolved question. CTLA-4, in addition, has a role in autoimmune diseases and cancer. Anti-CTLA-4 drugs are employed as checkpoint inhibitors to target various forms of cancer. Thus, clinical research on human CTLA-4 insufficiency might provide us a deeper understanding of the mechanism(s) of the CTLA-4 molecule and immune dysregulation disorders.

Development of immune checkpoint therapy for cancer

Fritz and Lenardo discuss the basic science and clinical discoveries of immune checkpoint blockade, which boosts antitumor immunity and increases survival of patients with cancer.

Since the early 20th century, immunologists have investigated mechanisms that protect vertebrates from damaging immune responses against self-antigens by mature lymphocytes, i.e., peripheral tolerance. These mechanisms have been increasingly delineated at the molecular level, ultimately culminating in new therapeutics that have revolutionized clinical oncology. Here, we describe basic science and clinical discoveries that converge mainly on two molecules, CTLA-4 and PD-1, that were recognized with the 2018 Nobel Prize in Physiology or Medicine awarded to James Allison and Tasuku Honjo. We discuss their investigations and those of many others in the field that contravene tolerance through checkpoint inhibition to boost immune killing of malignant cells. We also discuss the mechanisms underlying each therapy, the efficacy achieved, and the complications of therapy. Finally, we hint at research questions for the future that could widen the success of cancer immunotherapy.

From immune checkpoints to vaccines: The past, present and future of cancer immunotherapy

Cancer is a worldwide medical problem with significant repercussions on individual patients and societies as a whole. In order to alter the outcomes of this deadly disease the treatment of cancer over the centuries has undergone a unique evolution. However, utilizing the best treatment modalities and achieving cures or long-term durable responses have been inconsistent and limited, that is until recently. Contemporary research has highlighted a fundamental gap in our understanding of how we approach treating cancer, by revealing the intricate relationship between the immune system and tumors. In this atmosphere, the growth of immunotherapy has not only forever changed our understanding of cancer biology, but the manner by which we treat patients. It's paradigm shifting success has led to the approval of over 10 different immunotherapeutic agents, including checkpoint inhibitors, vaccine-based therapies, oncolytic viruses and T cell directed therapies for nearly 20 different indications across countless tumor types. Despite the breakthroughs that have occurred in the field of immunotherapy, it has not been the panacea for all cancers. With a deeper understanding of the immune system we have been able to peer into tumor immune escape and therapy resistance. Simultaneously this understanding has paved the way for the investigation and development of novel immune system altering agents and combinatorial therapies. In this chapter we review the immune system and its intricate relationship with cancer, the evolution of immunotherapy, its current landscape, and future directions in the context of resistance mechanisms and the challenges faced by immunotherapy against cancer.

The CD28-B7 Family of Co-signaling Molecules

Immune responses are controlled by the optimal balance between protective immunity and immune tolerance. T-cell receptor (TCR) signals are modulated by co-signaling molecules, which are divided into co-stimulatory and co-inhibitory molecules. By expression at the appropriate time and location, co-signaling molecules positively and negatively control T-cell differentiation and function. For example, ligation of the CD28 on T cells provides a critical secondary signal along with TCR ligation for naive T-cell activation. In contrast, co-inhibitory signaling by the CD28-B7 family is important to regulate immune homeostasis and host defense, as these signals limit the strength and duration of immune responses to prevent autoimmunity. At the same time, microorganisms or tumor cells can use these pathways to establish an immunosuppressive environment to inhibit the immune responses against themselves. Understanding these co-inhibitory pathways will support the development of new immunotherapy for the treatment of tumors and autoimmune and infectious diseases. Here, we introduce diverse molecules belonging to the members of the CD28-B7 family.

Signal Transduction Via Co-stimulatory and Co-inhibitory Receptors

T-cell receptor (TCR)-mediated antigen-specific stimulation is essential for initiating T-cell activation. However, signaling through the TCR alone is not sufficient for inducing an effective response. In addition to TCR-mediated signaling, signaling through antigen-independent co-stimulatory or co-inhibitory receptors is critically important not only for the full activation and functional differentiation of T cells but also for the termination and suppression of T-cell responses. Many studies have investigated the signaling pathways underlying the function of each molecular component. Co-stimulatory and co-inhibitory receptors have no kinase activity, but their cytoplasmic region contains unique functional motifs and potential phosphorylation sites. Engagement of co-stimulatory receptors leads to recruitment of specific binding partners, such as adaptor molecules, kinases, and phosphatases, via recognition of a specific motif. Consequently, each co-stimulatory receptor transduces a unique pattern of signaling pathways. This review focuses on our current understanding of the intracellular signaling pathways provided by co-stimulatory and co-inhibitory molecules, including B7:CD28 family members, immunoglobulin, and members of the tumor necrosis factor receptor superfamily.

PP2A holoenzymes, substrate specificity driving cellular functions and deregulation in cancer

PP2A is a highly conserved eukaryotic serine/threonine protein phosphatase of the PPP family of phosphatases with fundamental cellular functions. In cells, PP2A targets specific subcellular locations and substrates by forming heterotrimeric holoenzymes, where a core dimer consisting of scaffold (A) and catalytic (C) subunits complexes with one of many B regulatory subunits. PP2A plays a key role in positively and negatively regulating a myriad of cellular processes, as it targets a very sizable fraction of the cellular substrates phosphorylated on Ser/Thr residues. This review focuses on insights made toward the understanding on how the subunit composition and structure of PP2A holoenzymes mediates substrate specificity, the role of substrate modulation in the signaling of cellular division, growth, and differentiation, and its deregulation in cancer.

CD6, a Rheostat-Type Signalosome That Tunes T Cell Activation

Following T cell receptor triggering, T cell activation is initiated and amplified by the assembly at the TCR/CD3 macrocomplex of a multitude of stimulatory enzymes that activate several signaling cascades. The potency of signaling is, however, modulated by various inhibitory components already at the onset of activation, long before co-inhibitory immune checkpoints are expressed to help terminating the response. CD5 and CD6 are surface glycoproteins of T cells that have determinant roles in thymocyte development, T cell activation and immune responses. They belong to the superfamily of scavenger receptor cysteine-rich (SRCR) glycoproteins but whereas the inhibitory role of CD5 has been established for long, there is still controversy on whether CD6 may have similar or antagonistic functions on T cell signaling. Analysis of the structure and molecular associations of CD5 and CD6 indicates that these molecules assemble at the cytoplasmic tail a considerable number of signaling effectors that can putatively transduce diverse types of intracellular signals. Biochemical studies have concluded that both receptors can antagonize the flow of TCR-mediated signaling; however, the impact that CD5 and CD6 have on T cell development and T cell-mediated immune responses may be different. Here we analyze the signaling function of CD6, the common and also the different properties it exhibits comparing with CD5, and interpret the functional effects displayed by CD6 in recent animal models.

Targeting mitochondria in cancer: current concepts and immunotherapy approaches

An essential advantage during eukaryotic cell evolution was the acquisition of a network of mitochondria as a source of energy for cell metabolism and contrary to conventional wisdom, functional mitochondria are essential for the cancer cell. Multiple aspects of mitochondrial biology beyond bioenergetics support transformation including mitochondrial biogenesis, fission and fusion dynamics, cell death susceptibility, oxidative stress regulation, metabolism, and signaling. In cancer, the metabolism of cells is reprogrammed for energy generation from oxidative phosphorylation to aerobic glycolysis and impacts cancer mitochondrial function. Furthermore cancer cells can also modulate energy metabolism within the cancer microenvironment including immune cells and induce "metabolic anergy" of antitumor immune response. Classical approaches targeting the mitochondria of cancer cells usually aim at inducing changing energy metabolism or directly affecting functions of mitochondrial antiapoptotic proteins but most of such approaches miss the required specificity of action and carry important side effects. Several types of cancers harbor somatic mitochondrial DNA mutations and specific immune response to mutated mitochondrial proteins has been observed. An attractive alternative way to target the mitochondria in cancer cells is the induction of an adaptive immune response against mutated mitochondrial proteins. Here, we review the cancer cell-intrinsic and cell-extrinsic mechanisms through which mitochondria influence all steps of oncogenesis, with a focus on the therapeutic potential of targeting mitochondrial DNA mutations or Tumor Associated Mitochondria Antigens using the immune system.

CTLA-4 and PD-1 Control of T-Cell Motility and Migration: Implications for Tumor Immunotherapy

CTLA-4 is a co-receptor on T-cells that controls peripheral tolerance and the development of autoimmunity. Immune check-point blockade (ICB) uses monoclonal antibodies (MAbs) to block the binding of inhibitory receptors (IRs) to their natural ligands. A humanized antibody to CTLA-4 was first approved clinically followed by the use of antibody blockade against PD-1 and its ligand PD-L1. Effective anti-tumor immunity requires the activation of tumor-specific effector T-cells, the blockade of regulatory cells and the migration of T-cells into the tumor. Here, we review data implicating CTLA-4 and PD-1 in the motility of T-cells with a specific reference to the potential exploitation of these pathways for more effective tumor infiltration and eradication.

Protein Phosphatase 1α and Cofilin Regulate Nuclear Translocation of NF-κB and Promote Expression of the Anti-Inflammatory Cytokine Interleukin-10 by T Cells

While several protein serine/threonine kinases control cytokine production by T cells, the roles of serine/threonine phosphatases are largely unexplored. Here, we analyzed the involvement of protein phosphatase 1α (PP1α) in cytokine synthesis following costimulation of primary human T cells. Small interfering RNA (siRNA)-mediated knockdown of PP1α (PP1^KD) or expression of a dominant negative PP1α (D95N-PP1) drastically diminished interleukin-10 (IL-10) production. Focusing on a key transcriptional activator of human IL-10, we demonstrate that nuclear translocation of NF-κB was significantly inhibited in PP1^KD or D95N-PP1 cells. Interestingly, knockdown of cofilin, a known substrate of PP1 containing a nuclear localization signal, also prevented nuclear accumulation of NF-κB. Expression of a constitutively active nonphosphorylatable S3A-cofilin in D95N-PP1 cells restored nuclear translocation of NF-κB and IL-10 expression. Subpopulation analysis revealed that defective nuclear translocation of NF-κB was most prominent in CD4⁺ CD45RA^- CXCR3^- T cells that included IL-10-producing T_H2 cells. Together these findings reveal novel functions for PP1α and its substrate cofilin in T cells namely the regulation of the nuclear translocation of NF-κB and promotion of IL-10 production. These data suggest that stimulation of PP1α could limit the overwhelming immune responses seen in chronic inflammatory diseases.

Role of TRAFs in Signaling Pathways Controlling T Follicular Helper Cell Differentiation and T Cell-Dependent Antibody Responses

Follicular helper T (T_FH) cells represent a highly specialized CD4⁺ T cell subpopulation that supports the generation of germinal centers (GC) and provides B cells with critical signals promoting antibody class switching, generation of high affinity antibodies, and memory formation. T_FH cells are characterized by the expression of the chemokine receptor CXCR5, the transcription factor Bcl-6, costimulatory molecules ICOS, and PD-1, and the production of cytokine IL-21. The acquisition of a T_FH phenotype is a complex and multistep process that involves signals received through engagement of the TCR along with a multitude of costimulatory molecules and cytokines receptors. Members of the Tumor necrosis factor Receptor Associated Factors (TRAF) represent one of the major classes of signaling mediators involved in the differentiation and functions of T_FH cells. TRAF molecules are the canonical adaptor molecules that physically interact with members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF) and actively modulate their downstream signaling cascades through their adaptor function and/or E3 ubiquitin ligase activity. OX-40, GITR, and 4-1BB are the TRAF-dependent TNFRSF members that have been implicated in the differentiation and functions of T_FH cells. On the other hand, emerging data demonstrate that TRAF proteins also participate in signaling from the TCR and CD28, which deliver critical signals leading to the differentiation of T_FH cells. More intriguingly, we recently showed that the cytoplasmic tail of ICOS contains a conserved TANK-binding kinase 1 (TBK1)-binding motif that is shared with TBK1-binding TRAF proteins. The presence of this TRAF-mimicking signaling module downstream of ICOS is required to mediate the maturation step during T_FH differentiation. In addition, JAK-STAT pathways emanating from IL-2, IL-6, IL-21, and IL-27 cytokine receptors affect T_FH development, and crosstalk between TRAF-mediated pathways and the JAK-STAT pathways can contribute to generate integrated signals required to drive and sustain T_FH differentiation. In this review, we will introduce the molecular interactions and the major signaling pathways controlling the differentiation of T_FH cells. In each case, we will highlight the contributions of TRAF proteins to these signaling pathways. Finally, we will discuss the role of individual TRAF proteins in the regulation of T cell-dependent humoral responses.

Alpha4-overexpressing HL7702 cells can counteract microcystin-LR effects on cytoskeletal structure

Our previous studies indicated that α4 was involved in the toxicity of MC-LR on the cytoskeleton via the change of PP2A activity in HEK 293. To explore the role of α4 in MC-LR toxicity via PP2A regulation in different cell lines, the HL7702 cell overexpressing α4 protein was exposed to MC-LR, and the change of PP2A, cytoskeletal structure, and cytoskeleton-related proteins were investigated. The results showed that PP2A activity was decreased, PP2A/C subunit expression and phosphorylation (Tyr307) increased significantly, but methylation (Leu 309)clearly decreased. The structure of the actin filaments and microtubules (MTs) remained unchanged, and the expression and phosphorylation of the cytoskeleton-related proteins showed different changes. In addition, the main components of the MAPK pathway, JNK, P38, and ERK1/2, were activated together. Our results indicated that elevated α4 expression did confer some resistance to MC-LR-induced cytoskeletal changes, but the responses of different cell lines to MC-LR, under the α4-overexpression condition, are not exactly the same.

Interventional therapy combined with immune checkpoint inhibitors: Emerging opportunities for cancer treatment in the era of immunotherapy

Immune checkpoint inhibitors-based immunotherapy offers a new effective modality in the treatment of advanced malignancies. Considering the remarkable efficacy of immune checkpoint inhibitors in clinical trials, the FDA has approved a variety of immune checkpoint inhibitors for the treatment of advanced tumors. However, only limited patients with certain cancers can benefit from monotherapy of immune checkpoint inhibitors. Interventional therapy for cancer can not only destroy the primary tumors, but also regulate the immune system through different mechanisms, which provides a potential possibility for the combination of immune checkpoint inhibitors and interventional modalities in cancer treatment. This article reviews the possible synergistic mechanisms of interventional therapy combined with immune checkpoint inhibitors and summarizes the research progress of the combined therapy in cancer treatment.

Influence of T Cell Coinhibitory Molecules on CD8+ Recall Responses

T cell co-signaling molecules play an important role in fine-tuning the strength of T cell activation during many types of immune responses, including infection, cancer, transplant rejection, and autoimmunity. Over the last few decades, intense research into these cosignaling molecules has provided rich evidence to suggest that cosignaling molecules may be harnessed for the treatment of immune-related diseases. In particular, coinhibitory molecules such as programmed-death 1, 2B4, BTLA, TIGIT, LAG-3, TIM-3, and CTLA-4 inhibit T cell responses by counteracting TCR and costimulatory signals, leading to the inhibition of proliferation and effector function and the downregulation of activation and adhesion molecules at the cell surface. While many reviews have focused on the role of coinhibitory molecules in modifying primary CD8⁺ T cell responses, in this review, we will consider the complex role of coinhibitory molecules in altering CD8⁺ T cell recall potential. As memory CD8⁺ T cell responses are critical for protective memory responses in infection and cancer and contribute to potentially pathogenic memory responses in transplant rejection and autoimmunity, understanding the role of coinhibitory receptor control of memory T cells may illuminate important aspects of therapeutically targeting these pathways.

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.

Antitumor T-cell Reconditioning: Improving Metabolic Fitness for Optimal Cancer Immunotherapy

With the rapid rise of immunotherapy for cancer treatment, attention has focused on gaining a better understanding of T-cell biology in the tumor microenvironment. Elucidating the factors underlying changes in their function will allow for the development of new therapeutic strategies that could expand the patient population benefiting from immunotherapy, as well as circumvent therapy resistance. Cancers go beyond avoiding immune recognition and inducing T-cell dysfunction through coinhibitory molecules. Recent work has demonstrated that the tumor microenvironment elicits metabolic changes in T cells that dampen their ability to respond and that manipulating these metabolic changes can strengthen an antitumor immune response. Here we review the metabolic status of various types of T cells, the energetic state of the tumor microenvironment, and proposed modalities for improvement of immunotherapy through metabolic remodeling. Clin Cancer Res; 24(11); 2473-81. ©2018 AACR.

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.

Membrane Rafts in T Cell Activation: A Spotlight on CD28 Costimulation

Spatiotemporal compartmentalization of signaling pathways and second messengers is pivotal for cell biology and membrane rafts are, therefore, required for several lymphocyte functions. On the other hand, T cells have the specific necessity of tuning signaling amplification depending on the context in which the antigen is presented. In this review, we discuss of membrane rafts in the context of T cell signaling, focusing on CD28-mediated costimulation.

CD28 Blockade Ex Vivo Induces Alloantigen-Specific Immune Tolerance but Preserves T-Cell Pathogen Reactivity

Donor T-cells contribute to reconstitution of protective immunity after allogeneic hematopoietic stem cell transplantation (HSCT) but must acquire specific tolerance against recipient alloantigens to avoid life-threatening graft-versus-host disease (GvHD). Systemic immunosuppressive drugs may abrogate severe GvHD, but this also impedes memory responses to invading pathogens. Here, we tested whether ex vivo blockade of CD28 co-stimulation can enable selective T-cell tolerization to alloantigens by facilitating CD80/86-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) signaling. Treatment of human allogeneic dendritic cell/T-cell co-cultures with a human CD28 blocking antibody fragment (α-huCD28) significantly abrogated subsequent allospecific immune responses, seen by decreased T-cell proliferation and of type 1 cytokine (IFN-γ and IL-2) expression. Allo-tolerization persisted after discontinuation of CD28 blockade and secondary alloantigen stimulation, as confirmed by enhanced CTLA-4 and PD-1 immune checkpoint signaling. However, T-cells retained reactivity to pathogens, supported by clonotyping of neo-primed and cross-reactive T-cells specific for Candida albicans or third-party antigens using deep sequencing analysis. In an MHC-mismatched murine model, we tolerized C57BL/6 T-cells by ex vivo exposure to a murine single chain Fv specific for CD28 (α-muCD28). Infusion of these cells, after α-muCD28 washout, into bone marrow-transplanted BALB/c mice caused allo-tolerance and did not induce GvHD-associated hepatic pathology. We conclude that selective CD28 blockade ex vivo can allow the generation of stably allo-tolerized T-cells that in turn do not induce graft-versus-host reactions while maintaining pathogen reactivity. Hence, CD28 co-stimulation blockade of donor T-cells may be a useful therapeutic approach to support the immune system after HSCT.

Coinhibitory Pathways in the B7-CD28 Ligand-Receptor Family

Immune responses need to be controlled for optimal protective immunity and tolerance. Coinhibitory pathways in the B7-CD28 family provide critical inhibitory signals that regulate immune homeostasis and defense and protect tissue integrity. These coinhibitory signals limit the strength and duration of immune responses, thereby curbing immune-mediated tissue damage, regulating resolution of inflammation, and maintaining tolerance to prevent autoimmunity. Tumors and microbes that cause chronic infections can exploit these coinhibitory pathways to establish an immunosuppressive microenvironment, hindering their eradication. Advances in understanding T cell coinhibitory pathways have stimulated a new era of immunotherapy with effective drugs to treat cancer, autoimmune and infectious diseases, and transplant rejection. In this review we discuss the current knowledge of the mechanisms underlying the coinhibitory functions of pathways in the B7-CD28 family, the diverse functional consequences of these inhibitory signals on immune responses, and the overlapping and unique functions of these key immunoregulatory pathways.

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.

Structural and functional observations of the P151L MID1 mutation reveal alpha4 plays a significant role in X-linked Opitz Syndrome

Mutations of human MID1 are associated with X-linked Opitz G Syndrome (XLOS), which is characterized by midline birth defects. XLOS-observed mutations within the MID1 B-box1 domain are associated with cleft lip/palate, wide-spaced eyes and hyperspadias. Three of the four XLOS-observed mutations in the B-box1 domain results in unfolding but the structural and functional effects of the P151L mutation is not characterized. Here, we demonstrate that the P151L mutation does not disrupt the overall tertiary structure of the B-box1 domain and the adjacent domains. In fact, MID1 E3 ligase activity is slightly enhanced. However, the P151L mutation disrupted the ability of MID1 to catalyze the poly-ubiquitination of alpha4, a novel regulator of PP2A. This observation is consistent with results observed with the other three structure-destabilizing B-box1 mutations in targeting alpha4 but not PP2A. Alpha4 is shown to bind and sequester the catalytic subunit of PP2A and protect it from MID1-mediated ubiquitination and as a result, an increase in alpha4 can contribute to an increase in PP2A, playing a greater role in midline development during embryogenesis.

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.

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.

HEK293 cells exposed to microcystin-LR show reduced protein phosphatase 2A activity and more stable cytoskeletal structure when overexpressing α4 protein

Microcystin-LR (MC-LR) is one of the most toxic members of microcystins released by freshwater cyanobacterial. The major mechanism of MC-LR toxicity has been attributed to its inhibition of protein phosphatases 1 (PP1) and 2A (PP2A). In our prior research, α4 protein, a regulator of PP2A, was found not only crucial for PP2A regulation but also for the overall response of HEK 293 cells encountering MC-LR. To explore the role of α4 in MC-LR toxicity via PP2A regulation, here, HEK 293 cells overexpressing α4 protein were exposed to MC-LR and PP2A, cytoskeletal organization, and cytoskeleton-related proteins were investigated. The results showed that PP2A activity decreased and PP2A/C subunit expression and phosphorylation at Tyr307 increased significantly in the group exposed to high MC-LR. Vimentin IF became concentrated and formed perinuclear bundles. However, the assembly of actin filament and microtubules remained unchanged and the expression and phosphorylation of the cytoskeleton-related proteins HSP27 and VASP did not increase significantly. Some of these results differ from those of our previous study in which normal HEK293 cells were exposed to MC-LR. Our results indicate that elevated α4 expression confers some resistance to MC-LR-induced cytoskeletal change These new findings provide helpful insights into the mechanism of MC-LR toxicity and the role of α4 in regulating PP2A function. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 255-264, 2017.

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.

[Progress in Immunotherapy for Squamous Non-small Cell Lung Cancer]

近几年来，肺鳞癌在化疗及靶向治疗上的进展不够显著，但免疫治疗却在肺鳞癌的治疗上取得了突破性的进展。免疫治疗通过免疫系统来清除肿瘤细胞，主要分为免疫检查点抑制剂及治疗性疫苗。免疫检查点抑制剂，包括抗细胞毒性T淋巴细胞抗原4（cytotoxic T-lymphocyte associated antigen 4, CTLA-4）抗体与抗程序性死亡受体-1（programmed death receptor 1, PD-1）抗体等多种药物已进行了肺鳞癌的Ⅱ期、Ⅲ期临床试验，并取得了一定成果。免疫治疗将成为肺鳞癌治疗的一种重要手段。

Suspension survival mediated by PP2A-STAT3-Col XVII determines tumour initiation and metastasis in cancer stem cells

Targeting tumour-initiating cells (TICs) would lead to new therapies to cure cancer. We previously demonstrated that TICs have the capacity to survive under suspension conditions, while other cells undergo anoikis. Here we show that TICs exhibit increased phosphorylation levels of S727STAT3 because of PP2A inactivation. Collagen 17 gene expression is upregulated in a STAT3-dependent manner, which also stabilizes laminin 5 and engages cells to form hemidesmosome-like junctions in response. Blocking the PP2A-S727STAT3-collagen 17 pathway inhibits the suspension survival of TICs and their ability to form tumours in mice, while activation of the same pathway increases the suspension survival and tumour-initiation capacities of bulk cancer cells. The S727STAT3 phosphorylation levels correlate with collagen 17 expression in colon tumour samples, and correlate inversely with survival. Finally, this signalling axis enhances the ability of TIC to form tumours in mouse models of malignant lung cancer pleural effusion and spontaneous colon cancer metastasis.

CIP2A Promotes T-Cell Activation and Immune Response to Listeria monocytogenes Infection

The oncoprotein Cancerous Inhibitor of Protein Phosphatase 2A (CIP2A) is overexpressed in most malignancies and is an obvious candidate target protein for future cancer therapies. However, the physiological importance of CIP2A-mediated PP2A inhibition is largely unknown. As PP2A regulates immune responses, we investigated the role of CIP2A in normal immune system development and during immune response in vivo. We show that CIP2A-deficient mice (CIP2A^HOZ) present a normal immune system development and function in unchallenged conditions. However when challenged with Listeria monocytogenes, CIP2A^HOZ mice display an impaired adaptive immune response that is combined with decreased frequency of both CD4⁺ T-cells and CD8⁺ effector T-cells. Importantly, the cell autonomous effect of CIP2A deficiency for T-cell activation was confirmed. Induction of CIP2A expression during T-cell activation was dependent on Zap70 activity. Thus, we reveal CIP2A as a hitherto unrecognized mediator of T-cell activation during adaptive immune response. These results also reveal CIP2A^HOZ as a possible novel mouse model for studying the role of PP2A activity in immune regulation. On the other hand, the results also indicate that CIP2A targeting cancer therapies would not cause serious immunological side-effects.

Feeding an army: The metabolism of T cells in activation, anergy, and exhaustion

Through the direct control of infection or by providing cytokine signals to other cellular players, T cells play a central role in the orchestration of the immune response. However, in many disease states, T cells are rendered dysfunctional, unable to carry out their effector functions. As T cell activation is bioenergetically demanding, some T cell dysfunction can have metabolic underpinnings. In this review, we will discuss how T cells are programmed to fuel their effector response, and how programmed or pathologic changes can disrupt their ability to generate the energy needed to proliferate and carry out their critical functions.

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.

Immune Checkpoint Modulators: An Emerging Antiglioma Armamentarium

Immune checkpoints have come to the forefront of cancer therapies as a powerful and promising strategy to stimulate antitumor T cell activity. Results from recent preclinical and clinical studies demonstrate how checkpoint inhibition can be utilized to prevent tumor immune evasion and both local and systemic immune suppression. This review encompasses the key immune checkpoints that have been found to play a role in tumorigenesis and, more specifically, gliomagenesis. The review will provide an overview of the existing preclinical and clinical data, antitumor efficacy, and clinical applications for each checkpoint with respect to GBM, as well as a summary of combination therapies with chemotherapy and radiation.

Programmed cell death protein-1/programmed cell death ligand-1 pathway inhibition and predictive biomarkers: understanding transforming growth factor-beta role

A deeper understanding of the key role of the immune system in regulating tumor growth and progression has led to the development of a number of immunotherapies, including cancer vaccines and immune checkpoint inhibitors. Immune checkpoint inhibitors target molecular pathways involved in immunosuppression, such as cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) and programmed cell death protein-1 (PD-1)/programmed cell death ligand-1 (PD-L1) pathway, with the goal to enhance the host's own immune anticancer response. In phase I-III trials, anti-PD-1/PD-L1 antibodies have demonstrated to be effective treatment strategies by inducing significant durable tumor responses, with manageable toxicities, in patients with various malignancies, including those traditionally considered non-immunogenic, such as non-small cell lung cancer (NSCLC). Identification of predictive biomarkers to select patients for immune therapies is currently being investigated to improve their therapeutic efficacy. Transforming growth factor-β (TGF-β), a pleiotropic cytokine with immunosuppressive effects on multiple cell types of the innate and adaptive immune system, has emerged as one of the potential key factors modulating response to immune checkpoint inhibitors. However, due to the complexity of the anti-cancer immune response, the predictive value of many other factors related to cancer cells or tumor microenvironment needs to be further explored.

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.

Biochemical signaling of PD-1 on T cells and its functional implications

Maintenance of peripheral tolerance is essential for homeostasis of the immune system. While central tolerance mechanisms result in deletion of the majority of self-reactive T cells, T lymphocytes specific for self-antigens also escape this process and circulate in the periphery. To control the development of autoimmunity, multiple mechanisms of peripheral tolerance have evolved, including T cell anergy, deletion, and suppression by regulatory T (Treg) cells. The pathway consisting of the programmed cell death 1 (PD-1) receptor (CD279) and its ligands PD-L1 (B7-H1, CD274) and PD-L2 (B7-DC; CD273) plays a vital role in the induction and maintenance of peripheral tolerance. This pathway also regulates the balance between stimulatory and inhibitory signals needed for effective immunity and maintenance of T cell homeostasis. In contrast to this important beneficial role in maintaining T cell homeostasis, PD-1 mediates potent inhibitory signals that prevent the expansion and function of T effector cells and have detrimental effects on antiviral and antitumor immunity. Despite the compelling studies on the significant functional role of PD-1 in mediating inhibition of activated T cells, little is known about how PD-1 blocks T cell activation. Here, we will provide a brief overview of the signaling events that are regulated by PD-1 triggering, and we will discuss their implications on cell intrinsic and extrinsic mechanisms that determine the fate and function of T effector cells.

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.