Cutting edge: CTLA-4 (CD152) differentially regulates mitogen-activated protein kinases (extracellular signal-regulated kinase and c-Jun N-terminal kinase) in CD4+ T cells from receptor/ligand-deficient mice

Immune checkpoint inhibitors associated cardiovascular immune-related adverse events

Immune checkpoint inhibitors (ICIs) are specialized monoclonal antibodies (mAbs) that target immune checkpoints and their ligands, counteracting cancer cell-induced T-cell suppression. Approved ICIs like cytotoxic T-lymphocyte antigen-4 (CTLA-4), programmed death-1 (PD-1), its ligand PD-L1, and lymphocyte activation gene-3 (LAG-3) have improved cancer patient outcomes by enhancing anti-tumor responses. However, some patients are unresponsive, and others experience immune-related adverse events (irAEs), affecting organs like the lung, liver, intestine, skin and now the cardiovascular system. These cardiac irAEs include conditions like myocarditis, atherosclerosis, pericarditis, arrhythmias, and cardiomyopathy. Ongoing clinical trials investigate promising alternative co-inhibitory receptor targets, including T cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) and T cell immunoreceptor with immunoglobulin and ITIM domain (TIGIT). This review delves into the mechanisms of approved ICIs (CTLA-4, PD-1, PD-L1, and LAG-3) and upcoming options like Tim-3 and TIGIT. It explores the use of ICIs in cancer treatment, supported by both preclinical and clinical data. Additionally, it examines the mechanisms behind cardiac toxic irAEs, focusing on ICI-associated myocarditis and atherosclerosis. These insights are vital as ICIs continue to revolutionize cancer therapy, offering hope to patients, while also necessitating careful monitoring and management of potential side effects, including emerging cardiac complications.

The Role of the CD28 Family Receptors in T-Cell Immunomodulation

The CD28 family receptors include the CD28, ICOS (inducible co-stimulator), CTLA-4 (cytotoxic T-lymphocyte antigen-4), PD-1 (programmed cell death protein 1), and BTLA (B- and T-lymphocyte attenuator) molecules. They characterize a group of molecules similar to immunoglobulins that control the immune response through modulating T-cell activity. Among the family members, CD28 and ICOS act as enhancers of T-cell activity, while three others-BTLA, CTLA-4, and PD-1-function as suppressors. The receptors of the CD28 family interact with the B7 family of ligands. The cooperation between these molecules is essential for controlling the course of the adaptive response, but it also significantly impacts the development of immune-related diseases. This review introduces the reader to the molecular basis of the functioning of CD28 family receptors and their impact on T-cell activity.

Modulation of T cell function and survival by the tumor microenvironment

Cancer immunotherapy is shifting paradigms in cancer care. T cells are an indispensable component of an effective antitumor immunity and durable clinical responses. However, the complexity of the tumor microenvironment (TME), which consists of a wide range of cells that exert positive and negative effects on T cell function and survival, makes achieving robust and durable T cell responses difficult. Additionally, tumor biology, structural and architectural features, intratumoral nutrients and soluble factors, and metabolism impact the quality of the T cell response. We discuss the factors and interactions that modulate T cell function and survive in the TME that affect the overall quality of the antitumor immune response.

Anti-CD40 predominates over anti-CTLA-4 to provide enhanced antitumor response of DC-CIK cells in renal cell carcinoma

Cytokine-induced killer cells (CIK) in combination with dendritic cells (DCs) have shown favorable outcomes in renal cell carcinoma (RCC), yet some patients exhibit recurrence or no response to this therapy. In a broader perspective, enhancing the antitumor response of DC-CIK cells may help to address this issue. Considering this, herein, we investigated the effect of anti-CD40 and anti-CTLA-4 antibodies on the antitumor response of DC-CIK cells against RCC cell lines. Our analysis showed that, a) anti-CD40 antibody (G28.5) increased the CD3+CD56+ effector cells of CIK cells by promoting the maturation and activation of DCs, b) G28.5 also increased CTLA-4 expression in CIK cells via DCs, but the increase could be hindered by the CTLA-4 inhibitor (ipilimumab), c) adding ipilimumab was also able to significantly increase the proportion of CD3+CD56+ cells in DC-CIK cells, d) anti-CD40 antibodies predominated over anti-CTLA-4 antibodies for cytotoxicity, apoptotic effect and IFN-γ secretion of DC-CIK cells against RCC cells, e) after ipilimumab treatment, the population of Tregs in CIK cells remained unaffected, but ipilimumab combined with G28.5 significantly reduced the expression of CD28 in CIK cells. Taken together, we suggest that the agonistic anti-CD40 antibody rather than CTLA-4 inhibitor may improve the antitumor response of DC-CIK cells, particularly in RCC. In addition, we pointed towards the yet to be known contribution of CD28 in the crosstalk between anti-CTLA-4 and CIK cells.

Immunotherapy and Antivascular Targeted Therapy in Patients’ Treatment with Concurrent Malignant Tumors after Organ Transplantation: Opportunity or Challenge

Objective

To analyze the therapeutic effects and organ rejection of anti-PD-1 immunotherapy or antivascular targeting therapy on patients with combined malignancies after organ transplantation.

Methods

We collected retrospective studies on “post-transplantation, cancer, immunotherapy, and vascular targeting therapy” in Embase, Wanfang database, Cochrane Library, VIP databases, CNKI, and PubMed, and the case data were organized and analyzed.

Results

Data from only 40 papers met our requirements, which included 2 literature reviews, 4 original researches, and 34 case reports from 2016 to 2020. A total of 40 studies involving 66 patients were included, who were divided into 3 groups (patients using CTLA-4 inhibitors, group 1; patients who received sequential or concurrent anti-PD-1 and anti-CTLA-4 therapy, group 2; and patients using PD-1/PD-L1 inhibitors, group 3). There was no statistical difference in patients' DCR between the three groups (P > 0.05). Also, compared with group 2, there was no statistically significant difference in recipient organ rejection in group 1 and group 3 (P > 0.05). The DCR rate for antivascular targeted therapy is approximately 60%.

Conclusions

Immunotherapy should be carefully selected for patients with combined malignancies after organ transplantation. Antivascular targeted therapy is one of the options worth considering; the risk of side effects of drug therapy is something that needs to be closely monitored when combined with immunotherapy.

Inborn Errors of Immunity and Their Phenocopies: CTLA4 and PD-1

Elucidating links between genotype and phenotype in patients with rare inborn errors of immunity (IEIs) provides insights into mechanisms of immune regulation. In many autosomal dominant IEIs, however, variation in expressivity and penetrance result in complex genotype-phenotype relations, while some autosomal recessive IEIs are so rare that it is difficult to draw firm conclusions. Phenocopies arise when an environmental or non-genetic factor replicates a phenotype conferred by a specific genotype. Phenocopies can result from therapeutic antibodies or autoantibodies that target a protein to replicate aspects of the phenotype conferred by mutations in the gene encoding the same protein. Here, we consider IEIs arising from rare genetic variants in CTLA4 and PDCD1 and compare clinical and laboratory manifestations arising as drug-induced phenocopies (immune related adverse events, IRAEs) in cancer patients treated with immune checkpoint inhibitors (ICI) and identify outstanding questions regarding mechanism of disease.

Enhancement of interleukin-2 production by bovine peripheral blood mononuclear cells treated with the combination of anti-programmed death-ligand 1 and cytotoxic T lymphocyte antigen 4 chimeric monoclonal antibodies

Our previous studies demonstrate the therapeutic efficacy against bovine diseases of an anti-bovine programmed death-ligand 1 (PD-L1) chimeric antibody. In humans, PD-1 and PD-L1 antibodies are more effective when combined with an antibody targeting cytotoxic T lymphocyte antigen 4 (CTLA-4) and these combination therapies are therefore clinically used. Here we generated an anti-bovine CTLA-4 chimeric antibody (chAb) to enhance the therapeutic efficacy of the PD-L1 antibody. We further analyzed the effects of dual blockade of CTLA-4 and PD-1 pathways on T-cell responses. The established anti-bovine CTLA-4 chAb showed comparable blocking activity on the binding of bovine CTLA-4 to CD80 and CD86 as the anti-bovine CTLA-4 mouse monoclonal antibody. Anti-bovine CTLA-4 chAb also significantly increased IL-2 production from bovine peripheral blood mononuclear cells (PBMCs). Further, the combination of anti-CTLA-4 chAb with anti-PD-L1 chAb significantly upregulated IL-2 production by PBMCs. These results suggest that the combination of antibodies have higher potential to enhance immune responses against pathogens compared with single administration.

Nature vs. Nurture: The Two Opposing Behaviors of Cytotoxic T Lymphocytes in the Tumor Microenvironment

Similar to Janus, the two-faced god of Roman mythology, the tumor microenvironment operates two opposing and often conflicting activities, on the one hand fighting against tumor cells, while on the other hand, favoring their proliferation, survival and migration to other sites to establish metastases. In the tumor microenvironment, cytotoxic T cells-the specialized tumor-cell killers-also show this dual nature, operating their tumor-cell directed killing activities until they become exhausted and dysfunctional, a process promoted by cancer cells themselves. Here, we discuss the opposing activities of immune cells populating the tumor microenvironment in both cancer progression and anti-cancer responses, with a focus on cytotoxic T cells and on the molecular mechanisms responsible for the efficient suppression of their killing activities as a paradigm of the power of cancer cells to shape the microenvironment for their own survival and expansion.

Costimulation molecules differentially regulate the ERK-Zfp831 axis to shape T follicular helper cell differentiation

T follicular helper (Tfh) cells play essential roles in regulating humoral immunity, especially germinal center reactions. However, how CD4⁺ T cells integrate the antigenic and costimulatory signals in Tfh cell development is still poorly understood. Here, we found that phorbol 12-myristate 13-acetate (PMA) + ionomycin (P+I) stimulation, together with interleukin-6 (IL-6), potently induce Tfh cell-like transcriptomic programs in vitro. The ERK kinase pathway was attenuated under P+I stimulation; ERK2 inhibition enhanced Tfh cell development in vitro and in vivo. We observed that inducible T cell costimulator (ICOS), but not CD28, lacked the ability to activate ERK, which was important in sustaining Tfh cell development. The transcription factor Zfp831, whose expression was repressed by ERK, promoted Tfh cell differentiation by directly upregulating the expression of the transcription factors Bcl6 and Tcf7. We have hence identified an ERK-Zfp831 axis, regulated by costimulation signaling, in critical regulation of Tfh cell development.

Epigenetic mechanisms of liver tumor resistance to immunotherapy

Hepatocellular carcinoma (HCC) is the most common primary liver tumor, which stands fourth in rank of cancer-related deaths worldwide. The incidence of HCC is constantly increasing in correlation with the epidemic in diabetes and obesity, arguing for an urgent need for new treatments for this lethal cancer refractory to conventional treatments. HCC is the paradigm of inflammation-associated cancer, since more than 80% of HCC emerge consecutively to cirrhosis associated with a vast remodeling of liver microenvironment. In the recent decade, immunomodulatory drugs have been developed and have given impressive results in melanoma and later in several other cancers. In the present review, we will discuss the recent advancements concerning the use of immunotherapies in HCC, in particular those targeting immune checkpoints, used alone or in combination with other anti-cancers agents. We will address why these drugs demonstrate unsatisfactory results in a high proportion of liver cancers and the mechanisms of resistance developed by HCC to evade immune response with a focus on the epigenetic-related mechanisms.

Manipulation of the Immune System for Cancer Defeat: A Focus on the T Cell Inhibitory Checkpoint Molecules

The immune system actively counteracts the tumorigenesis process; a breakout of the immune system function, or its ability to recognize transformed cells, can favor cancer development. Cancer becomes able to escape from immune system control by using multiple mechanisms, which are only in part known at a cellular and molecular level. Among these mechanisms, in the last decade, the role played by the so-called "inhibitory immune checkpoints" is emerging as pivotal in preventing the tumor attack by the immune system. Physiologically, the inhibitory immune checkpoints work to maintain the self-tolerance and attenuate the tissue injury caused by pathogenic infections. Cancer cell exploits such immune-inhibitory molecules to contrast the immune intervention and induce tumor tolerance. Molecular agents that target these checkpoints represent the new frontier for cancer treatment. Despite the heterogeneity and multiplicity of molecular alterations among the tumors, the immune checkpoint targeted therapy has been shown to be helpful in selected and even histologically different types of cancer, and are currently being adopted against an increasing variety of tumors. The most frequently used is the moAb-based immunotherapy that targets the Programmed Cell Death 1 protein (PD-1), the PD-1 Ligand (PD-L1) or the cytotoxic T lymphocyte antigen-4 (CTLA4). However, new therapeutic approaches are currently in development, along with the discovery of new immune checkpoints exploited by the cancer cell. This article aims to review the inhibitory checkpoints, which are known up to now, along with the mechanisms of cancer immunoediting. An outline of the immune checkpoint targeting approaches, also including combined immunotherapies and the existing trials, is also provided. Notwithstanding the great efforts devoted by researchers in the field of biomarkers of response, to date, no validated FDA-approved immunological biomarkers exist for cancer patients. We highlight relevant studies on predictive biomarkers and attempt to discuss the challenges in this field, due to the complex and largely unknown dynamic mechanisms that drive the tumor immune tolerance.

Early astragaloside IV administration attenuates experimental autoimmune encephalomyelitis in mice by suppressing the maturation and function of dendritic cells

AIMS

Dendritic cells (DCs) actively participate in the pathogenesis of multiple sclerosis (MS), an autoimmune disease. Astragaloside IV (ASI), an active monomer isolated from the Chinese medicine Astragalus membranaceus, has a wide range of pharmacological effects. We aimed to elucidate the effects of ASI on the development of DCs in the early stage of MS/EAE.

MAIN METHODS

The mice were administered with ASI (20 mg/kg) daily 3 days in advance of EAE induction and continuously until day 7 post-immunization. The effect of ASI on CD11c⁺ DC cells from bone marrow (BMDCs) or the spleen of EAE mice at day 7 post-immunization were investigated respectively by flow cytometry, ELISA, western blot, real-time PCR and immunofluorescence.

KEY FINDINGS

ASI administration in the early stage of EAE was demonstrated to delay the onset and alleviate the severity of the disease. ASI inhibited the maturation and the antigen presentation of DCs in spleen of EAE mice and LPS-stimulated BMDCs, as evidenced by decreased expressions of CD11c, CD86, CD40 and MHC II. Accordingly, DCs treated by ASI secreted less IL-6 and IL-12, and prevented the differentiation of CD4⁺ T cells into Th1 and Th17 cells, which was probably through inhibiting the activation of NFκB and MAPKs signaling pathways.

SIGNIFICANCE

Our results implicated the alleviative effect of early ASI administration on EAE might be mediated by suppressing the maturation and function of DCs. The novel findings may add to our knowledge of ASI in the potentially clinical treatment of MS.

Expanding the Scope of Immunotherapy in Colorectal Cancer: Current Clinical Approaches and Future Directions

The success of immune checkpoint inhibitors (ICIs) in an increasing range of heavily mutated tumor types such as melanoma has culminated in their exploration in different subsets of patients with metastatic colorectal cancer (mCRC). As a result of their dramatic and durable response rates in patients with chemorefractory, mismatch repair-deficient-microsatellite instability-high (dMMR-MSI-H) mCRC, ICIs have become potential alternatives to classical systemic therapies. The anti-programmed death-1 (PD-1) agents, Pembrolizumab and Nivolumab, have been granted FDA approval for this subset of patients. Unfortunately, however, not all CRC cases with the dMMR-MSI-H phenotype respond well to ICIs, and ongoing studies are currently exploring biomarkers that can predict good response to them. Another challenge lies in developing novel treatment strategies for the subset of patients with the mismatch repair-proficient-microsatellite instability-low (pMMR-MSI-L) phenotype that comprises 95% of all mCRC cases in whom treatment with currently approved ICIs has been largely unsuccessful. Approaches aiming at overcoming the resistance of tumors in this subset of patients are being developed including combining different checkpoint inhibitors with either chemotherapy, anti-angiogenic agents, cancer vaccines, adoptive cell transfer (ACT), or bispecific T-cell (BTC) antibodies. This review describes the rationale behind using immunotherapeutics in CRC. It sheds light on the progress made in the use of immunotherapy in the treatment of patients with dMMR-MSI-H CRC. It also discusses emerging approaches and proposes potential strategies for targeting the immune microenvironment in patients with pMMR-MSI-L CRC tumors in an attempt to complement immune checkpoint inhibition.

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.

Immune Checkpoint Inhibitors: Basics and Challenges

Cancer is one of the most deadly diseases in the modern world. The last decade has witnessed dramatic advances in cancer treatment through immunotherapy. One extremely promising means to achieve anti-cancer immunity is to block the immune checkpoint pathways - mechanisms adopted by cancer cells to disguise themselves as regular components of the human body. Many review articles have described a variety of agents that are currently under extensive clinical evaluation. However, while checkpoint blockade is universally effective against a broad spectrum of cancer types and is mostly unrestricted by the mutation status of certain genes, only a minority of patients achieve a complete response. In this review, we summarize the basic principles of immune checkpoint inhibitors in both antibody and smallmolecule forms and also discuss potential mechanisms of resistance, which may shed light on further investigation to achieve higher clinical efficacy for these inhibitors.

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.

Current status of immunotherapy in metastatic colorectal cancer

BACKGROUND

Immunotherapy focuses on selectively enhancing the host's immune response against malignant disease. It has been investigated as an important treatment modality against malignant disease for many years, but until recently its use was mostly limited to a few cancers. The advent of new immunemodulating agents in the recent past has changed the landscape for management of many solid tumors. Currently, immunotherapy offers a valuable, and in many cases, a more effective alternate to the conventional cytotoxic therapy. Colorectal cancer is a leading cause of cancer-related death. Despite progress in systemic therapy, most patients with metastatic colorectal cancer die of their disease. There is an unmet need for more effective treatments for patients with metastatic colorectal cancer. The current data support that colorectal tumors are immunoresponsive and a subset of patients with advanced disease achieve long term benefit with immunotherapy.

OBJECTIVES

This review aims to provide the current status of immunotherapy in patients with metastatic colorectal cancer.

METHODS

We researched sources published in the English language between January 2000 and August 2018 and listed within the PubMed database using combinations of the key words and reviewed the proceedings of international cancer conferences and current guidelines made by major cancer societies.

RESULTS

In this review, we summarize the current status of research on immunotherapy in metastatic colorectal cancer and discuss various treatment modalities including checkpoint inhibitors, cancer vaccines, adoptive cell transfer, oncolytic virus therapy, and various other agents that are under investigation with a special emphasis on immune checkpoint inhibitors. Since the toxicity profile of immunotherapy is very different from conventional cytotoxic agents and could involve any organ system, we briefly review common adverse effects and their management.

Tune Up In Situ Autovaccination against Solid Tumors with Oncolytic Viruses

With the progress of immunotherapy in cancer, oncolytic viruses (OVs) have attracted more and more attention during the past decade. Due to their cancer-selective and immunogenic properties, OVs are considered ideal candidates to be combined with immunotherapy to increase both specificity and efficacy in cancer treatment. OVs preferentially replicate in and lyse cancer cells, resulting in in situ autovaccination leading to adaptive anti-virus and anti-tumor immunity. The main challenge in OV approaches is how to redirect the host immunity from anti-virus to anti-tumor and optimize the clinical outcome of cancer patients. Here, we summarize the conceptual updates on oncolytic virotherapy and immunotherapy in cancer, and the development of strategies to enhance the virus-mediated anti-tumor immune response, including: (1) arm OVs with cytokines to modulate innate and adaptive immunity; (2) combining OVs with immune checkpoint inhibitors to release T cell inhibition; (3) combining OVs with immune co-stimulators to enhance T cell activation. Future studies need to be enforced on developing strategies to augment the systemic effect on metastasized tumors.

A novel role for coinhibitory receptors/checkpoint proteins in the immunopathology of sepsis

Coinhibitory molecules, such as PD-1, CTLA-4, 2B4, and BTLA, are an important new family of mediators in the pathophysiology of severe bacterial and/or fungal infection, as well as the combined insults of shock and sepsis. Further, the expression of these molecules may serve as indicators of the immune status of the septic individual. Using PD-1:PD-L as an example, we discuss in this review how such checkpoint molecules may affect the host response to infection by mediating the balance between effective immune defense and immune-mediated tissue injury. Additionally, we explore how the up-regulation of PD-1 and/or PD-L1 expression on not only adaptive immune cells (e.g., T cells), but also on innate immune cells (e.g., macrophages, monocytes, and neutrophils), as well as nonimmune cells during sepsis and/or shock contributes to functional alterations often with detrimental sequelae.

Therapeutic Antibodies in Cancer Therapy

The therapeutic arsenal in solid tumors comprises different anticancer strategies with diverse chemotherapeutic agents and a growing number of biological substances. Large clinical study-based chemotherapeutic protocols combined with biologicals have become an important component in (neo-) adjuvant therapy alongside surgery in solid cancers as well as radiation therapy in some instances. In recent years, monoclonal antibodies have entered the mainstream of cancer therapy. Their first use was as antagonists of oncogenic receptor tyrosine kinases, but today monoclonal antibodies have emerged as long-sought vehicles for the targeted delivery of potent chemotherapeutic agents and as powerful tools to manipulate anticancer immune responses. There is a growing number of FDA approved monoclonal antibodies and small molecules targeting specific types of cancer suggestive of the clinical relevance of this approach.Targeted cancer therapies , also referred to as personalized medicine, are being studied for use alone, in combination with other targeted therapies, and in combination with chemotherapy. The use of monoclonal antibodies in colorectal and gastric cancer for example have shown best outcome when combined with chemotherapy, even though single agent anti-EGFR antibodies seem to be active in particular setting of metastatic colorectal cancer patients. However, it is not well defined whether the addition of anti-VEGF - and anti-EGFR strategies to chemotherapy could improve outcome in those patients susceptible to colorectal cancer-related metastases resection. Among the most promising approaches to activating therapeutic antitumor immunity is the blockade of immune checkpoints, exemplified by the recently FDA-approved agent, Ipilimumab, an antibody that blocks the coinhibitory receptor CTLA-4. Capitalizing on the success of Ipilimumab, agents that target a second coinhibitory receptor, PD-1, or its ligand, PD-L1, are in clinical development. This section attempts to discuss recent progress of targeted agents and in tackling a more general target applicable to gastrointestinal cancer .

Molecular Markers for Colorectal Cancer

Colorectal cancers develop through at least 3 major pathways, including chromosomal instability, mismatch repair, and methylator phenotype. These pathways can coexist in a single individual and occur in both sporadic and inherited colorectal cancers. In spite of the unique molecular and genetic signatures of colorectal cancers, nonspecific chemotherapy based on the antineoplastic effects of 5-fluorouracil is the cornerstone of therapy for stage III and some stage II disease. Techniques to recognize colorectal cancer at the molecular level have facilitated development of new signature drugs designed to inhibit the unique pathways of colorectal cancer growth and immunity.

Stereotactic radiosurgery and immunotherapy for metastatic spinal melanoma

The management of metastatic spinal melanoma involves maximizing local control, preventing recurrence, and minimizing treatment-associated toxicity and spinal cord damage. Additionally, therapeutic measures should promote mechanical stability, facilitate rehabilitation, and promote quality of life. These objectives prove difficult to achieve given melanoma's elusive nature, radioresistant and chemoresistant histology, vascular character, and tendency for rapid and early metastasis. Different therapeutic modalities exist for metastatic spinal melanoma treatment, including resection (definitive, debulking, or stabilization procedures), stereotactic radiosurgery, and immunotherapeutic techniques, but no single treatment modality has proven fully effective. The authors present a conceptual overview and critique of these techniques, assessing their effectiveness, separately and combined, in the treatment of metastatic spinal melanoma. They provide an up-to-date guide for multidisciplinary treatment strategies. Protocols that incorporate specific, goal-defined surgery, immunotherapy, and stereotactic radiosurgery would be beneficial in efforts to maximize local control and minimize toxicity.

Immunotherapy for lung cancer: for whom the bell tolls?

Lung cancer is the leading cause of cancer-related death and accounts for approximately 30% of all cancer deaths. Despite the recent developments in personalized therapy, the prognosis in lung cancer is still very poor. Immunotherapy is now emerging as a new hope for patients with lung cancer. It is well known that standard chemotherapeutic regimens have devastating effects for the patient's immune system. Therefore, the aim of immunotherapy is to specifically enhance the immune response against the tumour. Recently, many trials addressed the role of such therapies for metastatic non-small cell lung cancer (NSCLC) treatment: ipilimumab, tremelimumab, nivolumab and pembrolizumab are immunotherapeutic agents of high relevance in this field. Anti-tumour vaccines, as well as dendritic cell-based therapies, have emerged as potent inducers of immune response against the tumour. Herein, we will review some of the most promising cancer immunotherapies, highlighting their advantages and try to understand, in an immunological perspective, the missteps associated with the current treatments for cancer.

Alternative splice forms of CTLA-4 induced by antisense mediated splice-switching influences autoimmune diabetes susceptibility in NOD mice

Activated and regulatory T cells express the negative co-stimulatory molecule cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) that binds B7 on antigen-presenting cells to mediate cellular responses. Single nucleotide polymorphisms in the CTLA-4 gene have been found to affect alternative splicing and are linked to autoimmune disease susceptibility or resistance. Increased expression of a soluble splice form (sCTLA-4), lacking the transmembrane domain encoded by exon 3, has been shown to accelerate autoimmune pathology. In contrast, an exon 2-deficient form lacking the B7 ligand binding domain (liCTLA-4), expressed by diabetes resistant mouse strains has been shown to be protective when expressed as a transgene in diabetes susceptible non-obese diabetic (NOD) mice. We sought to employ an antisense-targeted splice-switching approach to independently produce these CTLA-4 splice forms in NOD mouse T cells and observe their relative impact on spontaneous autoimmune diabetes susceptibility. In vitro antisense targeting of the splice acceptor site for exon 2 produced liCTLA-4 while targeting exon 3 produced the sCTLA-4 form in NOD T cells. The liCTLA-4 expressing T cells exhibited reduced activation, proliferation and increased adhesion to intercellular adhesion molecule-1 (ICAM-1) similar to treatment with agonist α-CTLA-4. Mice treated to produce liCTLA-4 at the time of elevated blood glucose levels exhibited a significant reduction in the incidence of insulitis and diabetes, whereas a marked increase in the incidence of both was observed in animals treated to produce sCTLA-4. These findings provide further support that alternative splice forms of CTLA-4 affects diabetes susceptibility in NOD mice and demonstrates the therapeutic utility of antisense mediated splice-switching for modulating immune responses.

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.

Sepsis-induced potentiation of peritoneal macrophage migration is mitigated by programmed cell death receptor-1 gene deficiency

The effect of programmed cell death receptor-1 (PD-1) on phagocyte function has not been extensively described. Here we report that experimental mouse sepsis, cecal ligation and puncture (CLP), induced a marked increase in peritoneal macrophage random migration, motility and cell spread, but these changes were lost in the absence of PD-1. Alternatively, phagocytic activity was inversely affected. In vitro cell culture imaging studies, with the macrophage cell line J774, documented that blocking PD-1 with antibody led to aggregation of the cytoskeletal proteins α-actinin and F-actin. Further experiments looking at ex vivo peritoneal macrophages from mice illustrated that a similar pattern of α-actinin and F-actin was evident on cells from wild-type CLP mice but not PD-1-/- CLP mouse cells. We also observed that fMLP-induced migration by J774 cells was markedly attenuated using PD-1 blocking antibodies, a nonselective phosphatase inhibitor and a selective Ras-related protein 1 inhibitor. Finally, peritoneal macrophages derived from CLP as opposed to Sham mice demonstrated aspects of both cell surface co-localization with CD11b and internalization of PD-1 within vacuoles independent of CD11b staining. Together, we believe the data support a role for PD-1 in mediating aspects of innate macrophage immune dysfunction during sepsis, heretofore unappreciated.

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.

The blockade of immune checkpoints in cancer immunotherapy

Among the most promising approaches to activating therapeutic antitumour immunity is the blockade of immune checkpoints. Immune checkpoints refer to a plethora of inhibitory pathways hardwired into the immune system that are crucial for maintaining self-tolerance and modulating the duration and amplitude of physiological immune responses in peripheral tissues in order to minimize collateral tissue damage. It is now clear that tumours co-opt certain immune-checkpoint pathways as a major mechanism of immune resistance, particularly against T cells that are specific for tumour antigens. Because many of the immune checkpoints are initiated by ligand-receptor interactions, they can be readily blocked by antibodies or modulated by recombinant forms of ligands or receptors. Cytotoxic T-lymphocyte-associated antigen 4 (CTLA4) antibodies were the first of this class of immunotherapeutics to achieve US Food and Drug Administration (FDA) approval. Preliminary clinical findings with blockers of additional immune-checkpoint proteins, such as programmed cell death protein 1 (PD1), indicate broad and diverse opportunities to enhance antitumour immunity with the potential to produce durable clinical responses.

Immunotherapy earns its spot in the ranks of cancer therapy

Since it became clear that all cancer cells express tumor-specific and tumor-selective antigens generated by genetic alterations and epigenetic dysregulation, the immunology community has embraced the possibility of designing therapies to induce targeted antitumor immune responses. The potential therapeutic specificity and efficacy of such treatments are obvious to anyone who studies the exquisite specificity and cytocidal potency of immune responses. However, the value assigned to a therapeutic modality by the oncology community at large does not depend on scientific principle; all that matters is how patients respond. The bar for the ultimate acceptance of a therapy requires more than anecdotal clinical responses; rather, the major modalities of cancer therapeutics, including surgery, chemotherapy, radiation therapy, and, more recently, drugs targeting oncogenes, have earned their place only after producing dramatic frequent clinical responses or demonstrating statistically significant survival benefits in large randomized phase 3 clinical trials, leading to FDA approval. Although tumor-targeted antibodies have certainly cleared this bar, immunotherapies aimed at harnessing antitumor cellular responses have not-until now.

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.

CD152 (CTLA-4) determines CD4 T cell migration in vitro and in vivo

BACKGROUND

Migration of antigen-experienced T cells to secondary lymphoid organs and the site of antigenic-challenge is a mandatory prerequisite for the precise functioning of adaptive immune responses. The surface molecule CD152 (CTLA-4) is mostly considered as a negative regulator of T cell activation during immune responses. It is currently unknown whether CD152 can also influence chemokine-driven T cell migration.

METHODOLOGY/PRINCIPAL FINDINGS

We analyzed the consequences of CD152 signaling on Th cell migration using chemotaxis assays in vitro and radioactive cell tracking in vivo. We show here that the genetic and serological inactivation of CD152 in Th1 cells reduced migration towards CCL4, CXCL12 and CCL19, but not CXCL9, in a G-protein dependent manner. In addition, retroviral transduction of CD152 cDNA into CD152 negative cells restored Th1 cell migration. Crosslinking of CD152 together with CD3 and CD28 stimulation on activated Th1 cells increased expression of the chemokine receptors CCR5 and CCR7, which in turn enhanced cell migration. Using sensitive liposome technology, we show that mature dendritic cells but not activated B cells were potent at inducing surface CD152 expression and the CD152-mediated migration-enhancing signals. Importantly, migration of CD152 positive Th1 lymphocytes in in vivo experiments increased more than 200% as compared to CD152 negative counterparts showing that indeed CD152 orchestrates specific migration of selected Th1 cells to sites of inflammation and antigenic challenge in vivo.

CONCLUSIONS/SIGNIFICANCE

We show here, that CD152 signaling does not just silence cells, but selects individual ones for migration. This novel activity of CD152 adds to the already significant role of CD152 in controlling peripheral immune responses by allowing T cells to localize correctly during infection. It also suggests that interference with CD152 signaling provides a tool for altering the cellular composition at sites of inflammation and antigenic challenge.

CD28 and CTLA-4 coreceptor expression and signal transduction

SUMMARY

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

CTLA-4 on alloreactive CD4 T cells interacts with recipient CD80/86 to promote tolerance

Although the inhibitory receptor CTLA-4 (CD152) has been implicated in peripheral CD4 T-cell tolerance, its mechanism of action remains poorly defined. We analyzed mechanisms of CD4 cell tolerance in a model of tolerance induction involving establishment of mixed hematopoietic chimerism in recipients of fully MHC-mismatched allogeneic bone marrow cells with anti-CD154 mAb. Animals lacking CD80 and CD86 failed to achieve chimerism. We detected no T cell-intrinsic requirement for CD28 for chimerism induction. However, a CD4 T cell-intrinsic signal through CTLA-4 was shown to be essential within the first 48 hours of exposure to alloantigen for the establishment of tolerance and mixed chimerism. This signal must be provided by a recipient CD80/86(+) non-T-cell population. Donor CD80/86 expression was insufficient to achieve tolerance. Together, our findings demonstrate a surprising role for interactions of CTLA-4 expressed by alloreactive peripheral CD4 T cells with CD80/86 on recipient antigen-presenting cells (APCs) in the induction of early tolerance, suggesting a 3-cell tolerance model involving directly alloreactive CD4 cells, donor antigen-expressing bone marrow cells, and recipient antigen-presenting cells. This tolerance is independent of regulatory T cells and culminates in the deletion of directly alloreactive CD4 T cells.

CTLA-4 activation of phosphatidylinositol 3-kinase (PI 3-K) and protein kinase B (PKB/AKT) sustains T-cell anergy without cell death

The balance of T-cell proliferation, anergy and apoptosis is central to immune function. In this regard, co-receptor CTLA-4 is needed for the induction of anergy and tolerance. One central question concerns the mechanism by which CTLA-4 can induce T-cell non-responsiveness without a concurrent induction of antigen induced cell death (AICD). In this study, we show that CTLA-4 activation of the phosphatidylinositol 3-kinase (PI 3-K) and protein kinase B (PKB/AKT) sustains T-cell anergy without cell death. CTLA-4 ligation induced PI 3K activation as evidenced by the phosphorylation of PKB/AKT that in turn inactivated GSK-3. The level of activation was similar to that observed with CD28. CTLA-4 induced PI 3K and AKT activation also led to phosphorylation of the pro-apoptotic factor BAD as well as the up-regulation of BcL-XL. In keeping with this, CD3/CTLA-4 co-ligation prevented apoptosis under the same conditions where T-cell non-responsiveness was induced. This effect was PI 3K and PKB/AKT dependent since inhibition of these enzymes under conditions of anti-CD3/CTLA-4 co-ligation resulted in cell death. Our findings therefore define a mechanism by which CTLA-4 can induce anergy (and possibly peripheral tolerance) by preventing the induction of cell death.

CTLA-4 trafficking and surface expression

The T-cell co-receptor cytotoxic T-cell antigen 4 (CTLA-4) has a strong inhibitory role as shown by the lymphoproliferative phenotype of CTLA-4-deficient mice. Despite its potent effects on T-cell function, CTLA-4 is primarily an intracellular antigen whose surface expression is tightly regulated by restricted trafficking to the cell surface and rapid internalisation. Recently, several signalling molecules such as Trim, PLD, ARF-1 and TIRC7 have been described to be involved in the transport of CTLA-4 to the cell surface. Minor changes in surface expression levels have major effects on the outcome of T-cell activation. Optimal regulation of CTLA-4 surface expression is crucial for the balance of stimulatory and inhibitory signals to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity.

The reverse stop-signal model for CTLA4 function

Activation of the T-cell co-receptor cytotoxic T-lymphocyte antigen 4 (CTLA4) has a pivotal role in adjusting the threshold for T-cell activation and in preventing autoimmunity and massive tissue infiltration by T cells. Although many mechanistic models have been postulated, no single model has yet accounted for its overall function. In this Opinion article, I outline the strengths and weaknesses of the current models, and present a new 'reverse stop-signal model' to account for CTLA4 function.

Cytotoxic T-Lymphocyte–Associated Antigen-4: Fig. 1

Previously, the development of immune-based therapies has primarily focused on vaccines and cytokines, yielding benefit in a small percentage of patients. Recent advances in our understanding of the function of costimulatory molecules have revitalized enthusiasm in the development of immune therapies for cancer. This family of proteins possesses properties involved in both lymphocyte activation and immune-inhibitory functions. The costimulatory molecule with the greatest translation into the clinic thus far is CTL-associated antigen-4 (CTLA-4). CTLA-4 engagement leads to T-cell inhibition by two principle mechanisms. The first involves competitive binding with CD28 for B7 on the antigen-presenting cell. The second is direct intracellular inhibitory signals mediated by the CTLA-4 cytoplasmic tail. Numerous clinical trials testing the blockade of CTLA-4 signaling with fully human monoclonal antibodies have treated a variety of cancers, with the most experience in the treatment of metastatic melanoma. Significant antitumor activity as well as potential autoimmune-related toxicities have been observed. Further clinical investigation with CTLA-4 blockade, planned clinical trials testing manipulation of other costimulatory molecules, and continued improvement in understanding of costimulatory pathways present a new era of immune therapies for cancer patients.

CTL-associated antigen-4 ligation induces rapid T cell polarization that depends on phosphatidylinositol 3-kinase, Vav-1, Cdc42, and myosin light chain kinase

CTLA-4 can negatively regulate cytokine production and proliferation, increase motility, and override the TCR-induced stop-signal needed for stable T cell-APC conjugation. Despite this, little is known regarding whether CTLA-4 can alter T cell morphology and the nature of the signaling events that could account for this event. In this study, we demonstrate that anti-CTLA-4 and CD3/CTLA-4 induce rapid T cell polarization (i.e., within 15-30 min) with increases in lamellipodia, filopodia, and uropod formation. This was observed with anti-CTLA-4 and CD80-Ig ligation of CTLA-4, but not with anti-CD3 alone, or anti-CD3/CD28 coligation. Polarization required PI3K, the guanine nucleotide exchange factor Vav1, the GTP-binding protein Cdc42, as well as myosin L chain kinase. By contrast, a key downstream target of PI3K, protein kinase B, as well as Rho kinase and RhoA, were not needed. Our results demonstrate that CTLA-4 is a potent activator T cell polarization needed for motility, and this process involves specific set of signaling proteins that might contribute to coreceptor regulation of T cell function.

CTLA-4 ablation and interleukin-12 driven differentiation synergistically augment cardiac pathogenicity of cytotoxic T lymphocytes

CD8+ cytotoxic T lymphocytes contribute to viral and autoimmune myocarditis and cardiac allograft rejection. The role of cytotoxic T-lymphocyte-associated antigen (CTLA)-4 as a negative regulator of CD4+ T cells is well defined, yet CTLA-4 regulation of CD8+ T cells is less clear. We studied CTLA-4 regulation of cytotoxic T lymphocytes in a transgenic model of CD8+ T-cell-mediated myocarditis. We generated CTLA-4(-/-) Rag 2(-/-) OT-1 mice, the CD8+ T cells of which express an ovalbumin (OVA) peptide-specific, class I major histocompatibility complex-restricted T-cell receptor. CTLA-4(-/-Tc12) OT-1 effectors, differentiated with interleukin-12 present, are hyperproliferative in vitro, compared with CTLA-4(+/+)Tc12 OT-1 controls. Transfer of low doses of CTLA-4(-/-Tc12) OT-1 cells to cMy-mOVA mice, which express OVA on cardiac myocytes, causes severe myocarditis, with 99% mortality, compared with no mortality after transfer of low doses of CTLA-4(+/+)Tc12 OT-1 cells. High doses of CTLA-4(+/+)Tc12 cells cause lethal myocarditis in cMy-mOVA mice, but high doses of CTLA-4(+/+)Tc0 CTL, generated without interleukin-12, are hypoproliferative within the cardiac-draining lymph node and do not significantly infiltrate the heart. In contrast, CTLA-4(-/-Tc0) cytotoxic T lymphocytes do proliferate in the cardiac-draining lymph node and diffusely infiltrate the heart. Nonetheless, high doses of CTLA-4(-/-Tc0) cells cause only limited tissue damage, and the disease is not lethal. These data show that CTLA-4 regulates myocarditic CD8+ T cell responses and that CTLA-4 deficiency partly overcomes a differentiation block that exists when naïve CD8+ T cells are stimulated without interleukin-12. Therefore, targeting CTLA-4 solely or in conjunction with interleukin-12 could influence effector CD8+ T cell responses in therapeutically beneficial ways.

ICER/CREM-mediated transcriptional attenuation of IL-2 and its role in suppression by regulatory T cells

Here, we report that inducible cAMP early repressor/cAMP response element modulator (ICER/CREM) is induced early in CD25(+)CD4(+) regulatory T cell (T(R)) assays mainly in activated Foxp3(-) effector T cells and this induction correlates with sharp decrease in number of IL-2-expressing T cells. Importantly, RNAi targeting of ICER/CREM in responder CD25(-)CD4(+) T cells antagonizes T(R)-mediated suppression. Moreover, forced expression of Foxp3 in naive CD25(-) T cells induces constitutive expression of ICER/CREM in T cells with a regulatory phenotype. Foxp3 facilitates expression of ICER/CREM both in Foxp3 transductants as well as CD25(-) responder T cells suggesting that induction of T(R) function in suppression assays may utilize contact-dependent interaction. Indeed, CTLA-4 blockade or use of B7-deficient CD25(-) responder T cells prevents ICER/CREM accumulation and leads to the rescue of IL-2 expression. Therefore, we propose that CTLA-4 binding to B7 ligands expressed on activated ligand-bearing Foxp3(-) effector T cells results in ICER/CREM-mediated transcriptional attenuation of IL-2. Collectively, these data suggest that Foxp3 expression in T(R) cells imposes suppression in contact-dependent fashion by induction of constitutive ICER/CREM expression in activated CD25(+) Foxp3(-) T cell effectors thus preventing them from producing IL-2.

Regulatory T cell-mediated suppression: potential role of ICER

How regulatory T (TR) cells dampen T cell responses remains unclear. Multiple modes of action have been proposed, including cell contact-dependent and/or cytokine-dependent mechanisms. Suppression may involve direct contact between TR cells and responder T cells. Alternatively, TR cells may act on dendritic cells to reduce their ability to prime T cells by modulating costimulation, inducing the secretion of suppressive cytokines or the increase of tryptophan metabolism. Here, we review emerging, novel mechanisms involved in contact-dependent, TR-mediated suppression of IL-2 production in responder CD25- T lymphocytes and the potential involvement of inducible cAMP early repressor (ICER) in this suppression. Finally, cytokines such as TGF-beta and IL-10, produced by TR cells or other cells, may exert local suppression, which can be conveyed by basic mechanism(s) acting in a similar manner as contact-dependent, TR-mediated suppression.

Reversal of the TCR stop signal by CTLA-4

The coreceptor cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) is pivotal in regulating the threshold of signals during T cell activation, although the underlying mechanism is still not fully understood. Using in vitro migration assays and in vivo two-photon laser scanning microscopy, we showed that CTLA-4 increases T cell motility and overrides the T cell receptor (TCR)-induced stop signal required for stable conjugate formation between T cells and antigen-presenting cells. This event led to reduced contact periods between T cells and antigen-presenting cells that in turn decreased cytokine production and proliferation. These results suggest a fundamentally different model of reverse stop signaling, by which CTLA-4 modulates the threshold for T cell activation and protects against autoimmunity.

A molecular perspective of CTLA-4 function

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

Charting protein complexes, signaling pathways, and networks in the immune system

Systematic deciphering of protein-protein interactions has the potential to generate comprehensive and instructive signaling networks and to fuel new therapeutic and diagnostic strategies. Here, we describe how recent advances in high-throughput proteomic technologies, involving biochemical purification methods and mass spectrometry analysis, can be applied systematically to the characterization of protein complexes and the computation of molecular networks. The networks obtained form the basis for further functional analyses, such as knockdown by RNA interference, ultimately leading to the identification of nodes that represent candidate targets for pharmacological exploitation. No individual experimental approach can accurately elucidate all critical modulatory components and biological aspects of a signaling network. Such functionally annotated protein-protein interaction networks, however, represent an ideal platform for the integration of additional datasets. By providing links between molecules, they also provide links to all previous observations associated with these molecules, be they of genetic, pharmacological, or other origin. As exemplified here by the analysis of the tumor necrosis factor (TNF)-alpha/nuclear factor-kappaB (NF-kappaB) signaling pathway, the approach is applicable to any mammalian cellular signaling pathway in the immune system.

CTLA-4 up-regulation of lymphocyte function-associated antigen 1 adhesion and clustering as an alternate basis for coreceptor function

Although cytotoxic T lymphocyte antigen-4 (CTLA-4) negatively regulates T cell activation, the full range of functions mediated by this coreceptor has yet to be established. In this study, we report the surprising finding that CTLA-4 engagement by soluble antibody or CD80 potently up-regulates lymphocyte function-associated antigen 1 (LFA-1) adhesion to intercellular adhesion molecule-1 (ICAM-1) and receptor clustering concurrent with IL-2 inhibition. This effect was also observed with CTLA-4 ligation and not with other coreceptors. T cell antigen receptor (TcR)-induced lymphocyte function-associated antigen 1 function was also dependent on CTLA-4 expression as observed with reduced adhesion/clustering on CTLA-4(-/-) primary T cells. CTLA-4 up-regulated adhesion was mediated by regulator for cell adhesion and polarization type 1 (Rap-1) as shown by anti-CTLA-4-induced Rap-1 activation as well as Rap-1-N17 blockade and Rap-1-V12 mimicry of adhesion/clustering. Our findings identify a potent role for CTLA-4 in directing integrin adhesion and provide an alternate mechanism to account for aspects of CTLA-4 function in T cell immunity.

Multiple roles of Rap1 in hematopoietic cells: complementary versus antagonistic functions

Small G proteins serve as critical control points in signal transduction, integrating a wide range of stimuli to dictate discrete cellular outcomes. The outcomes of small G-protein signaling can both potentiate and antagonize one another. Studies in hematopoietic cells have uncovered multiple functions for the small G protein, Rap1 (Ras-proximate-1). Because Rap1 can regulate cell proliferation, differentiation, and adhesion through distinct mechanisms, it serves as a paradigm for the need for tight cellular control of small G-protein function. Rap1 has received recent attention for its role in enhancing integrin-dependent signals. This action of Rap1 augments a variety of processes that characterize hematopoietic-cell function, including aggregation, migration, extravasation, and homing to target tissues. Rap1 may also regulate cellular differentiation and proliferation via pathways that are distinct from those mediating adhesion, and involve regulation of the mitogen-activated protein (MAP) kinase or ERK (extracellular signal-regulated kinase) cascade. These actions of Rap1 occur in selected cell types to enhance or diminish ERK signaling, depending on the expression pattern of the MAP kinase kinase kinases of the Raf family: Raf-1 and B-Raf. This review will examine the functions of Rap1 in hematopoietic cells, and focus on 3 cellular scenarios where the multiple actions of Rap1 function have been proposed. Recent studies implicating Rap1 in the maturation of megakaryocytes, the pathogenesis of chronic myelogenous leukemia (CML), and activation of peripheral T cells will receive particular attention.

Regulation of the small GTPase Rap1 and extracellular signal-regulated kinases by the costimulatory molecule CTLA-4

The mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) is activated following engagement of the T-cell receptor and is required for interleukin 2 (IL-2) production and T-cell proliferation. This activation is enhanced by stimulation of the coreceptor CD28 and inhibited by the coreceptor CTLA-4. We show that the small G protein Rap1 is regulated in the opposite manner; it is inhibited by CD28 and activated by CTLA-4. Together, CD3 and CTLA-4 activate Rap1 in a sustained manner. To delineate T-cell function in the absence of Rap1 activity, we generated transgenic mice expressing Rap1GAP1, a Rap1-specific GTPase-activating protein. Transgenic mice showed lymphadenopathy, and transgenic T cells displayed increased ERK activation, proliferation, and IL-2 production. More significantly, the inhibitory effect of CTLA-4 on T-cell function in Rap1GAP1-transgenic T cells was reduced. We demonstrate that CTLA-4 activates Rap1, and we propose that intracellular signals from CTLA-4 antagonize CD28, at least in part, at the level of Rap1.

Down-regulation of myasthenogenic T cell responses by a dual altered peptide ligand via CD4+CD25+-regulated events leading to apoptosis

The myasthenogenic peptides p195-212 and p259-271 are sequences of the human acetylcholine receptor and were shown to induce myasthenia gravis-associated immune responses in mice. A dual altered peptide ligand (APL) composed of the two APLs of the myasthenogenic peptides inhibited, in vitro and in vivo, those responses. The aims of this study were to elucidate the events that follow the in vivo treatment with the dual APL and to characterize the cell population that is induced by the latter. We demonstrate here that s.c. administration of the dual APL up-regulates CD4+CD25+ regulatory T cells that are characterized by up-regulated expression of cytotoxic T lymphocyte-associated antigen 4, intracellular and membranal TGF-beta, and Foxp3. Administration of the dual APL to mice concomitant with the immunization with either of the myasthenogenic peptides resulted also in the up-regulation of c-Jun-NH2-terminal kinase activity and of Fas signaling pathway molecules as determined by measuring Fas, Fas ligand, and caspase 8. Thus, our results suggest that the suppression of myasthenia gravis-associated T cell responses exerted by the dual APL is mediated by the CD4+CD25+ immunoregulatory T cell function via TGF-beta or cytotoxic T lymphocyte-associated antigen 4, which further stimulate a cascade of events that up-regulates apoptosis.