CD137 and CD137 ligand constitutively coexpressed on human T and B leukemia cells signal proliferation and survival

A unique human cord blood CD8+CD45RA+CD27+CD161+ T-cell subset identified by flow cytometric data analysis using Seurat

Advances in single-cell level analytical techniques, especially cytometric approaches, have led to profound innovation in biomedical research, particularly in the field of clinical immunology. This has resulted in an expansion of high-dimensional data, posing great challenges for comprehensive and unbiased analysis. Conventional manual analysis is thus becoming untenable to handle these challenges. Furthermore, most newly developed computational methods lack flexibility and interoperability, hampering their accessibility and usability. Here, we adapted Seurat, an R package originally developed for single-cell RNA sequencing (scRNA-seq) analysis, for high-dimensional flow cytometric data analysis. Based on a 20-marker antibody panel and analyses of T-cell profiles in both adult blood and cord blood (CB), we showcased the robust capacity of Seurat in flow cytometric data analysis, which was further validated by Spectre, another high-dimensional cytometric data analysis package, and conventional manual analysis. Importantly, we identified a unique CD8+ T-cell population defined as CD8+CD45RA+CD27+CD161+ T cell that was predominantly present in CB. We characterised its IFN-γ-producing and potential cytotoxic properties using flow cytometry experiments and scRNA-seq analysis from a published dataset. Collectively, we identified a unique human CB CD8+CD45RA+CD27+CD161+ T-cell subset and demonstrated that Seurat, a widely used package for scRNA-seq analysis, possesses great potential to be repurposed for cytometric data analysis. This facilitates an unbiased and thorough interpretation of complicated high-dimensional data using a single analytical pipeline and opens a novel avenue for data-driven investigation in clinical immunology.

Human/mouse CD137 agonist, JNU-0921, effectively shrinks tumors through enhancing the cytotoxicity of CD8+ T cells in cis and in trans

Agonistic antibodies against CD137 have been demonstrated to completely regress established tumors through activating T cell immunity. Unfortunately, current CD137 antibodies failed to benefit patients with cancer. Moreover, their antitumor mechanisms in vivo remain to be determined. Here, we report the development of a small molecular CD137 agonist, JNU-0921. JNU-0921 effectively activates both human and mouse CD137 through direct binding their extracellular domains to induce oligomerization and signaling and effectively shrinks tumors in vivo. Mechanistically, JNU-0921 enhances effector and memory function of cytotoxic CD8+ T cells (CTLs) and alleviates their exhaustion. JNU-0921 also skews polarization of helper T cells toward T helper 1 type and enhances their activity to boost CTL function. Meanwhile, JNU-0921 attenuates the inhibitory function of regulatory T cells on CTLs. Our current work shows that JNU-0921 shrinks tumors by enhancing the cytotoxicity of CTLs in cis and in trans and sheds light on strategy for developing CD137 small molecular agonists.

Cysteine-binding adjuvant enhances survival and promotes immune function in a murine model of acute myeloid leukemia

ABSTRACT

Therapeutic vaccination has long been a promising avenue for cancer immunotherapy but is often limited by tumor heterogeneity. The genetic and molecular diversity between patients often results in variation in the antigens present on cancer cell surfaces. As a result, recent research has focused on personalized cancer vaccines. Although promising, this strategy suffers from time-consuming production, high cost, inaccessibility, and targeting of a limited number of tumor antigens. Instead, we explore an antigen-agnostic polymeric in situ cancer vaccination platform for treating blood malignancies, in our model here with acute myeloid leukemia (AML). Rather than immunizing against specific antigens or targeting adjuvant to specific cell-surface markers, this platform leverages a characteristic metabolic and enzymatic dysregulation in cancer cells that produces an excess of free cysteine thiols on their surfaces. These thiols increase in abundance after treatment with cytotoxic agents such as cytarabine, the current standard of care in AML. The resulting free thiols can undergo efficient disulfide exchange with pyridyl disulfide (PDS) moieties on our construct and allow for in situ covalent attachment to cancer cell surfaces and debris. PDS-functionalized monomers are incorporated into a statistical copolymer with pendant mannose groups and TLR7 agonists to target covalently linked antigen and adjuvant to antigen-presenting cells in the liver and spleen after IV administration. There, the compound initiates an anticancer immune response, including T-cell activation and antibody generation, ultimately prolonging survival in cancer-bearing mice.

4-1BB: A promising target for cancer immunotherapy

Immunotherapy, powered by its relative efficacy and safety, has become a prominent therapeutic strategy utilized in the treatment of a wide range of diseases, including cancer. Within this class of therapeutics, there is a variety of drug types such as immune checkpoint blockade therapies, vaccines, and T cell transfer therapies that serve the purpose of harnessing the body’s immune system to combat disease. Of these different types, immune checkpoint blockades that target coinhibitory receptors, which dampen the body’s immune response, have been widely studied and established in clinic. In contrast, however, there remains room for the development and improvement of therapeutics that target costimulatory receptors and enhance the immune response against tumors, one of which being the 4-1BB (CD137/ILA/TNFRSF9) receptor. 4-1BB has been garnering attention as a promising therapeutic target in the setting of cancer, amongst other diseases, due to its broad expression profile and ability to stimulate various signaling pathways involved in the generation of a potent immune response. Since its discovery and demonstration of potential as a clinical target, major progress has been made in the knowledge of 4-1BB and the development of clinical therapeutics that target it. Thus, we seek to summarize and provide a comprehensive update and outlook on those advancements in the context of cancer and immunotherapy.

Tumor-mediated 4-1BB induces tumor proliferation and metastasis in the colorectal cancer cells

AIMS

4-1BB is a member of the tumor necrosis factor receptor superfamily that mainly expressed on activated T-cells and plays important roles in cell proliferation and survival of T-cells and natural killer cells. The roles of 4-1BB in immune cells have been intensively studied, whereas little is known about the expression and roles of 4-1BB in cancer cells.

MAIN METHODS

In the present study, we investigated 4-1BB expression in colorectal cancer tissues from human patients and established colorectal cancer cells, using mRNA expression, FACS, and immunostaining. Cancer cell proliferation and metastasis regulated by transfected 4-1BB was evaluated by cell growth rate, colony forming assay, cell migration, and Western blot with antibodies which are involved in epithelial-mesenchymal transition and anti-apoptosis. Expression of 4-1BB was knockdown by 4-1BB shRNA to prove that 4-1BB was involved in the cell proliferation. In vivo, 4-1BB transfected cancer cells were injected into mice, to induce tumor local region or lung.

KEY FINDINGS

We found that colorectal cancer tissues from human patients and established colorectal cancer cells expressed 4-1BB at the high level. The higher expression of 4-1BB proliferated faster. In addition, we identified two forms of 4-1BB detected in colorectal cancer cells: full length form that was located on the plasma membrane and a short soluble form in the cytosol. The soluble form was also detected in the plasma from the mice with tumor xenografts expressed 4-1BB.

SIGNIFICANCE

Tumor-mediated 4-1BB expression in the colorectal cancer cells showed effects on cancer cell proliferation, invasion, and metastasis.

Roles of OX40 and OX40 Ligand in Mycosis Fungoides and Sézary Syndrome

Mycosis fungoides (MF) and Sézary syndrome (SS), the most common types of cutaneous T-cell lymphoma (CTCL), are characterized by proliferation of mature CD4+ T-helper cells. Patients with advanced-stage MF and SS have poor prognosis, with 5-year survival rates of 52%. Although a variety of systemic therapies are currently available, there are no curative options for such patients except for stem cell transplantation, and thus the treatment of advanced MF and SS still remains challenging. Therefore, elucidation of the pathophysiology of MF/SS and development of medical treatments are desired. In this study, we focused on a molecule called OX40. We examined OX40 and OX40L expression and function using clinical samples of MF and SS and CTCL cell lines. OX40 and OX40L were co-expressed on tumor cells of MF and SS. OX40 and OX40L expression was increased and correlated with disease severity markers in MF/SS patients. Anti-OX40 antibody and anti-OX40L antibody suppressed the proliferation of CTCL cell lines both in vitro and in vivo. These results suggest that OX40–OX40L interactions could contribute to the proliferation of MF/SS tumor cells and that the disruption of OX40–OX40L interactions could become a new therapeutic strategy for the treatment of MF/SS.

A prognostic gene signature for predicting survival outcome in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) is an heterogenous cancer that can have profound differences in survival outcomes. Molecular profiling has allowed for the identification of DLBCL subclasses, and together with clinical prognostic factors, such as the international prognostic index, have improved clinical care and survival. Despite these advances, a gene signature that is associated with overall survival (OS) and is reproducible across different DLBCL studies could better classify risk and predict OS. Here, we have identified genes that are associated with OS in DLBCL using data from the Lymphoma/Leukemia Molecular Profiling Project and developed a prognostic gene signature consisting of 33 genes that - when transformed into a risk score - can stratify individuals into high or low risk groups that have significantly different OS. The prognostic gene signature was associated with OS in multiple clinical studies, and when used in conjunction with DLBCL molecular subtype and IPI score, significantly predicted OS. Thus, we identified a potential prognostic gene signature that can discriminate high-risk from low-risk DLBCL patients.

Lipid Metabolism and Immune Checkpoints

Immune checkpoints are essential for the regulation of immune cell functions. Although the abrogation of immunosurveillance of tumor cells is known, the regulators of immune checkpoints are not clear. Lipid metabolism is one of the important metabolic activities in organisms. In lipid metabolism, a large number of metabolites produced can regulate the gene expression and activation of immune checkpoints through various pathways. In addition, increasing evidence has shown that lipid metabolism leads to transient generation or accumulation of toxic lipids that result in endoplasmic reticulum (ER) stress and then regulate the transcriptional and posttranscriptional modifications of immune checkpoints, including transcription, protein folding, phosphorylation, palmitoylation, etc. More importantly, the lipid metabolism can also affect exosome transportation of checkpoints and the degradation of checkpoints by affecting ubiquitination and lysosomal trafficking. In this chapter, we mainly empathize on the roles of lipid metabolism in the regulation of immune checkpoints, such as gene expression, activation, and degradation.

Identification of CD137-Expressing B Cells in Multiple Sclerosis Which Secrete IL-6 Upon Engagement by CD137 Ligand

The potent costimulatory effect of CD137 has been implicated in several murine autoimmune disease models. CD137 costimulates and polarizes antigen-specific T cells toward a potent Th1/Tc1 response, and is essential for the development of experimental autoimmune encephalomyelitis (EAE), a murine model of Multiple Sclerosis (MS). This study aimed to investigate a role of CD137 in MS. Immunohistochemical and immunofluorescence staining of MS brain tissues was used to identify expression of CD137. CD137⁺ cells were identified in MS brain samples, with active lesions having the highest frequency of CD137⁺ cells. CD137 expression was found on several leukocyte subsets, including T cells, B cells and endothelial cells. In particular, CD137⁺ B cells were found in meningeal infiltrates. In vitro experiments showed that CD137 engagement on activated B cells increased early TNF and persistent IL-6 secretion with increased cell proliferation. These CD137⁺ B cells could interact with CD137L-expressing cells, secrete pro-inflammatory cytokines and accumulate in the meningeal infiltrate. This study demonstrates CD137 expression by activated B cells, enhancement of the inflammatory activity of B cells upon CD137 engagement, and provides evidence for a pathogenic role of CD137⁺ B cells in MS.

The potentials of immune checkpoints for the treatment of blood malignancies

Immune checkpoints are the regulators of the immune system, which include stimulatory and inhibitory receptors. They play substantial roles in the maintenance of immune system homeostasis and the prevention of autoimmunity and cancer. In the current review, immune checkpoints roles are surveyed in the initiation, progression, and treatment of blood malignancies. The significant roles of immune checkpoints are discussed as clinical markers in the diagnosis and prognosis of a plethora of blood malignancies and also as potential targets for the treatment of these malignancies. It could be concluded that the regulation of immune checkpoints in various blood cancers can be employed as a novel strategy to obtain effective results in leukemia treatment and introduce immune checkpoint inhibitors as sufficient weapons against blood cancers in the future.

CD137, an attractive candidate for the immunotherapy of lung cancer

Immunotherapy has become a hotspot in cancer therapy in recent years. Several immune checkpoints inhibitors have been used to treat lung cancer. CD137 is a kind of costimulatory molecule that mediates T cell activation, which regulates the activity of immune cells in a variety of physiological and pathological processes. Targeting CD137 or its ligand (CD137L) has been studied, aiming to enhance anticancer immune responses. Accumulating studies show that anti-CD137 mAbs alone or combined with other drugs have bright antitumor prospects. In the following, we reviewed the biology of CD137, the antitumor effects of anti-CD137 Ab monotherapy and the combined therapy in lung cancer.

Functions of Immune Checkpoint Molecules Beyond Immune Evasion

Immune checkpoint molecules, including inhibitory and stimulatory immune checkpoint molecules, are defined as ligand-receptor pairs that exert inhibitory or stimulatory effects on immune responses. Most of the immune checkpoint molecules that have been described so far are expressed on cells of the adaptive immune system, particularly on T cells, and of the innate immune system. They are crucial for maintaining the self-tolerance and modulating the length and magnitude of immune responses of effectors in different tissues to minimize the tissue damage. More and more evidences have shown that inhibitory or stimulatory immune checkpoint molecules are expressed on a sizeable fraction of tumor types. Although the main function of tumor cell-associated immune checkpoint molecules is considered to mediate the immune evasion, it has been reported that the immune checkpoint molecules expressed on tumor cells also play important roles in the maintenance of many malignant behaviors, including self-renewal, epithelial-mesenchymal transition, metastasis, drug resistance, anti-apoptosis, angiogenesis, or enhanced energy metabolisms. In this section, we mainly focus on delineating the roles of the tumor cell-associated immune checkpoint molecules beyond immune evasion, such as PD-L1, PD-1, B7-H3, B7-H4, LILRB1, LILRB2, TIM3, CD47, CD137, and CD70.

Regulation of CD137 expression through K-Ras signaling in pancreatic cancer cells

BACKGROUND

The interaction between CD137 and its ligand (CD137L) plays a major role in the regulation of immune functions and affects cancer immunotherapy. CD137 is a cell surface protein mainly located on activated T cells, and its regulation and functions in immune cells are well established. However, the expression of CD137 and its regulation in cancer cells remain poorly understood. The main purposes of this study were to examine the expression of CD137 in pancreatic cancer cells and to investigate its underlying mechanisms.

METHODS

Cells containing inducible K-Ras^G12V expression vector or with different K-Ras mutational statuses were used as in vitro models to examine the regulation of CD137 expression by K-Ras. Various molecular assays were employed to explore the regulatory mechanisms. Tumor specimens from 15 pancreatic cancer patients and serum samples from 10 patients and 10 healthy donors were used to test if the expression of CD137 could be validated in clinical samples.

RESULTS

We found that the CD137 protein was expressed on the cell surface in pancreatic cancer tissues and cancer cell lines. Enzyme-linked immunosorbent assay revealed no difference in the levels of secreted CD137 in the sera of patients and healthy donors. By using the K-Ras inducible cell system, we further showed that oncogenic K-Ras up-regulated CD137 through the activation of MAPK (mitogen-activated protein kinases) and NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) pathways, as evidenced by significantly reduced CD137 mRNA expression led by genetic silencing of MAPK1 and p65, the key proteins involved in the respective pathways. Furthermore, we also found that the NF-κB pathway was mainly stimulated by the K-Ras-induced secretion of interleukin-1α (IL-1α) which promoted the transcription of the CD137 gene in pancreatic cancer cell lines. Analysis of the TCGA (the cancer genome atlas) database also revealed a significant correlation between IL-1α and CD137 expression (r = 0.274) in tumor samples from pancreatic cancer patients (P < 0.001).

CONCLUSIONS

The present study has demonstrated that the CD137 protein was expressed on pancreatic cancer cell surface, and has identified a novel mechanism by which K-Ras regulates CD137 in pancreatic cancer cells through MAPK and NF-κB pathways stimulated by IL-1α.

Aberrant CD137 ligand expression induced by GATA6 overexpression promotes tumor progression in cutaneous T-cell lymphoma

CD137 and its ligand, CD137L, are expressed on activated T cells and antigen-presenting cells, respectively. Recent studies have shown that CD137L and CD137 are aberrantly expressed by tumor cells, especially in some hematopoietic malignancies, and interactions between these molecules on tumor cells promote tumor growth. In this study, we investigated the roles of CD137L and CD137 in cutaneous T-cell lymphoma (CTCL), represented by mycosis fungoides and Sézary syndrome. Flow cytometric analysis showed that primary Sézary cells and CTCL cell lines (Hut78, MyLa, HH, SeAx, and MJ) aberrantly expressed CD137L. CD137L expression by tumor cells in CTCL was also confirmed by immunohistochemistry. Anti-CD137L-neutralizing antibody inhibited proliferation, survival, CXCR4-mediated migration, and in vivo growth in CTCL cell lines through inhibition of phosphorylation of AKT, extracellular signal-regulated kinase 1/2, p38 MAPK, and JNK. Moreover, suppression of CD137L signaling decreased antiapoptotic proteins Bcl-2 and phosphorylated Bad. We also explored the transcription factor regulating CD137L expression. Because GATA6 has been proposed as an oncogene in many types of tumors with aberrant CD137L expression, we examined GATA6 expression and the involvement of GATA6 in CD137L expression in CTCL. DNA hypomethylation and histone acetylation induced GATA6 overexpression in CTCL cells. Furthermore, chromatin immunoprecipitation, luciferase reporter assay, and knockdown by short hairpin RNA showed that GATA6 directly upregulated CD137L expression. Inhibition of GATA6 resulted in decreased survival and in vivo growth in CTCL cells. Collectively, our findings prompt a novel therapeutic approach to CTCL based on the discovery that the GATA6/CD137L axis plays an important role in the tumorigenesis of CTCL.

CD137: A checkpoint regulator involved in atherosclerosis

Inflammation is associated with atherosclerotic plaque development and precipitation of myocardial infarction and stroke, and anti-inflammatory therapy may reduce disease severity. Costimulatory molecules are key regulators of immune cell activity and inflammation, and are associated with disease development in atherosclerosis. Accumulating evidence indicates that a costimulatory molecule of the Tumor Necrosis Factor Receptor superfamily, the checkpoint regulator CD137, promotes atherosclerosis and vascular inflammation in experimental models. In light of the burgeoning consideration of CD137-targeted therapy in the clinic, it will be important to better understand costimulator immunobiology in development of cardiovascular disease. Here, we review available data on the costimulator CD137 and its potential role in atherosclerosis.

T-cell responses against CD19+ pediatric acute lymphoblastic leukemia mediated by bispecific T-cell engager (BiTE) are regulated contrarily by PD-L1 and CD80/CD86 on leukemic blasts

T-cell immunotherapies are promising options in relapsed/refractory B-precursor acute lymphoblastic leukemia (ALL). We investigated the effect of co-signaling molecules on T-cell attack against leukemia mediated by CD19/CD3-bispecific T-cell engager. Primary CD19+ ALL blasts (n≥10) and physiologic CD19+CD10+ bone marrow precursors were screened for 20 co-signaling molecules. PD-L1, PD-1, LAG-3, CD40, CD86, CD27, CD70 and HVEM revealed different stimulatory and inhibitory profiles of pediatric ALL compared to physiologic cells, with PD-L1 and CD86 as most prominent inhibitory and stimulatory markers. PD-L1 was increased in relapsed ALL patients (n=11) and in ALLs refractory to Blinatumomab (n=5). Exhaustion markers (PD-1, TIM-3) were significantly higher on patients' T cells compared to physiologic controls. T-cell proliferation and effector function was target-cell dependent and correlated to expression of co-signaling molecules. Blockade of inhibitory PD-1-PD-L and CTLA-4-CD80/86 pathways enhanced T-cell function whereas blockade of co-stimulatory CD28-CD80/86 interaction significantly reduced T-cell function. Combination of Blinatumomab and anti-PD-1 antibody was feasible and induced an anti-leukemic in vivo response in a 12-year-old patient with refractory ALL. In conclusion, ALL cells actively regulate T-cell function by expression of co-signaling molecules and modify efficacy of therapeutic T-cell attack against ALL. Inhibitory interactions of leukemia-induced checkpoint molecules can guide future T-cell therapies.

Tumor cell-associated immune checkpoint molecules - Drivers of malignancy and stemness

Inhibitory or stimulatory immune checkpoint molecules are expressed on a sizeable fraction of tumor cells in different tumor types. It was thought that the main function of tumor cell-associated immune checkpoint molecules would be the modulation (down- or upregulation) of antitumor immune responses. In recent years, however, it has become clear that the expression of immune checkpoint molecules on tumor cells has important consequences on the biology of the tumor cells themselves. In particular, a causal relationship between the expression of these molecules and the acquisition of malignant traits has been demonstrated. Thus, immune checkpoint molecules have been shown to promote the epithelial-mesenchymal transition of tumor cells, the acquisition of tumor-initiating potential and resistance to apoptosis and antitumor drugs, as well as the propensity to disseminate and metastasize. Herein, we review this evidence, with a main focus on PD-L1, the most intensively investigated tumor cell-associated immune checkpoint molecule and for which most information is available. Then, we discuss more concisely other tumor cell-associated immune checkpoint molecules that have also been shown to induce the acquisition of malignant traits, such as PD-1, B7-H3, B7-H4, Tim-3, CD70, CD28, CD137, CD40 and CD47. Open questions in this field as well as some therapeutic approaches that can be derived from this knowledge, are also addressed.

TNFSF9 exerts an inhibitory effect on hepatocellular carcinoma

OBJECTIVE

Tumor necrosis factor superfamily member 9 (TNFSF9), also known as 4-1BBL and CD137L, has been implicated in cancer immunotherapy due to its function as a T-cell co-stimulator. We aimed to investigate the role of TNFSF9 in the cancer pathogenesis in hepatocellular carcinoma (HCC).

METHODS

TNFSF9 expression was examined by immunohistochemistry in 106 pairs of HCC and adjacent non-tumorous tissues, and by quantitative polymerase chain reaction and Western blot in HCC cell lines. The impact of TNFSF9 on the proliferation, migration and invasion of HCC cells was determined using the 3-(4,5-diethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt (MTS) and transwell assays in vitro. We also assessed the influence of TNFSF9 on the growth and metastasis of HCC tumors in an orthotopic mouse model of human HCC.

RESULTS

TNFSF9 expression was downregulated in approximately 70% of HCC tissues. A decreased expression of TNFSF9 was also consistently observed in all the four HCC cell lines. Either the overexpression of TNFSF9 or treatment with recombinant TNFSF9 protein could significantly inhibit the proliferation, migration and invasion of Huh7 and SMMC-7721 HCC cells in vitro. The inhibitory effect of TNFSF9 on HCC was further confirmed in vivo. Mice orthotopically transplanted with TNFSF9-overexpressing Huh7 cells developed significantly smaller tumors with less intrahepatic metastasis and distant metastasis compared with the control group.

CONCLUSIONS

TNFSF9 may be a tumor suppressor in HCC. Based on its immune stimulatory aspect and the tumor inhibition property, TNFSF9 may be a promising therapeutic target for HCC.

Therapeutic potential of anti-CD137 (4-1BB) monoclonal antibodies

INTRODUCTION

4-1BB (CD137) is an important T-cell stimulating molecule. The 4-1BB mAb or its variants have shown remarkable therapeutic activity against autoimmunity, viral infections, and cancer. Antibodies to 4-1BB have recently entered clinical trials for the treatment of cancer with favorable toxicity profile. In this article, we present a review documenting the efficacy and pitfalls of 4-1BB therapy.

AREAS COVERED

An extensive literature search has been made on 4-1BB, spanning two decades, and a comprehensive report is presented here highlighting the origins, biological effects, therapeutic potential, and mechanistic basis of targeting 4-1BB as well as the side effects associated with such therapy.

EXPERT OPINION

Research so far indicates that 4-1BB is highly protective against various pathological conditions including cancer. However, a few important side effects of 4-1BB therapy such as liver toxicity, thrombocytopenia, anemia, and suppressive effects on certain immune competent cells should be taken into consideration before it is used for human therapy.

4-1BB (CD137), an inducible costimulatory receptor, as a specific target for cancer therapy

Although considerable progress has been made in understanding how tumors evade immune surveillance, measures to counter the same have not kept pace with the advances made in designing effective strategies. 4-1BB (CD137; TNFRS9), an activation-induced costimulatory molecule, is an important regulator of immune responses. Targeting 4-1BB or its natural ligand 4-1BB ligand (4-1BBL) has important implications in many clinical conditions, including cancer. In-depth analysis revealed that 4-1BB-mediated anti-cancer effects are based on its ability to induce activation of cytotoxic T lymphocytes (CTL), and among others, high amounts of IFN-γ. In this review, we will discuss the various aspects of 4-1BB-mediated anti-tumor responses, the basis of such responses, and future directions. [BMB Reports 2014; 47(3): 122-129]

Immunotherapy of cancer with 4-1BB

4-1BB (CD137), a member of the TNF receptor superfamily, is an activation-induced T-cell costimulatory molecule. Signaling via 4-1BB upregulates survival genes, enhances cell division, induces cytokine production, and prevents activation-induced cell death in T cells. The importance of the 4-1BB pathway has been underscored in a number of diseases, including cancer. Growing evidence indicates that anti-4-1BB monoclonal antibodies possess strong antitumor properties, which in turn are the result of their powerful CD8+ T-cell activating, IFN-γ producing, and cytolytic marker-inducing capabilities. In addition, combination therapy of anti-4-1BB with other anticancer agents, such as radiation, has robust tumor-regressing abilities against nonimmunogenic or poorly immunogenic tumors. Furthermore, the adoptive transfer of ex vivo anti-4-1BB-activated CD8+ T cells from previously tumor-treated animals efficiently inhibits progression of tumors in recipient mice that have been inoculated with fresh tumors. In addition, targeting of tumors with variants of 4-1BBL directed against 4-1BB also have potent antitumor effects. Currently, a humanized anti-4-1BB is in clinical trials in patients with solid tumors, including melanoma, renal carcinoma, and ovarian cancer, and so far seems to have a favorable toxicity profile. In this review, we discuss the basis of the therapeutic potential of targeting the 4-1BB-4-1BBL pathway in cancer treatment.

4-1BB ligand modulates direct and Rituximab-induced NK-cell reactivity in chronic lymphocytic leukemia

NK cells play an important role in tumor immunosurveillance and largely contribute to the therapeutic success of anti-tumor antibodies like Rituximab. Here, we studied the role of the TNF family member 4-1BB ligand (4-1BBL) during the interaction of NK cells with chronic lymphocytic leukemia (CLL) cells. 4-1BBL was highly expressed on patient B-CLL cells in all 56 investigated cases. Signaling via 4-1BBL following interaction with 4-1BB, which was detected on NK cells of CLL patients but not healthy individuals, led to the release of immunoregulatory cytokines including TNF by CLL cells. CLL patient sera contained elevated levels of TNF and induced 4-1BB upregulation on NK cells, which in turn impaired direct and Rituximab-induced NK-cell reactivity against 4-1BBL-expressing targets. NK-cell reactivity was not only enhanced by blocking the interaction of NK cell-expressed 4-1BB with 4-1BBL expressed by CLL cells, but also by preventing 4-1BB upregulation on NK cells via neutralization of TNF in patient serum with Infliximab. Our data indicate that 4-1BBL mediates NK-cell immunosubversion in CLL, and thus might contribute to the reportedly compromised efficacy of Rituximab to induce NK-cell reactivity in the disease, and that TNF neutralization may serve to enhance the efficacy of Rituximab treatment in CLL.

CD137 ligand, a member of the tumor necrosis factor family, regulates immune responses via reverse signal transduction

CD137 (4-1BB, TNFR superfamily 9) and its ligand are members of the TNFR and TNF families, respectively, and are involved in the regulation of a wide range of immune activities. CD137 ligand cross-links its receptor, CD137, which is expressed on activated T cells, and costimulates T cell activities. CD137 ligand can also be expressed as a transmembrane protein on the cell surface and transmit signals into the cells on which it is expressed (reverse signaling). CD137 ligand expression is found on most types of leukocytes and on some nonimmune cells. In monocytic cells (monocytes, macrophages, and DCs), CD137 ligand signaling induces activation, migration, survival, and differentiation. The activities of T cells, B cells, hematopoietic progenitor cells, and some malignant cells are also influenced by CD137 ligand, but the physiological significance is understood only partly. As CD137 and CD137 ligand are regarded as valuable targets for immunotherapy, it is pivotal to determine which biological effects are mediated by which of the 2 molecules.

CD137 ligand mediates opposite effects in human and mouse NK cells and impairs NK-cell reactivity against human acute myeloid leukemia cells

Natural killer (NK) cells play an important role in the immunosurveillance of leukemia. Their reactivity is governed by a balance of activating and inhibitory receptors including various members of the tumor necrosis factor receptor (TNFR) family. Here we report that human NK cells acquire expression of the TNFR family member CD137 upon activation, and NK cells of acute myeloid leukemia (AML) patients display an activated phenotype with substantial CD137 expression. CD137 ligand (CD137L) was detectable on leukemic cells in 35% of 65 investigated AML patients, but not on healthy CD34(+) cells, and expression was associated with monocytic differentiation. Bidirectional signaling following CD137-CD137L interaction induced the release of the immunomodulatory cytokines interleukin-10 and TNF by AML cells and directly diminished granule mobilization, cytotoxicity, and interferon-gamma production of human NK cells, which was restored by blocking CD137. Cocultures of NK cells with CD137L transfectants confirmed that human CD137 inhibits NK-cell reactivity, while activating signals were transduced by its counterpart on NK cells in mice. Our data underline the necessity to study the function of seemingly analog immunoregulatory molecules in mice compared with men and demonstrate that CD137-CD137L interaction enables immune evasion of AML cells by impairing NK-cell tumor surveillance in humans.

The role of soluble and cell-surface expressed 4-1BB ligand in patients with malignant hemopoietic disorders

The TNFR family member 4-1BB and its ligand 4-1BBL are involved in the costimulation of T-cells and tumor-derived soluble (s)4-1BBL may influence the interaction of malignant cells with the immune system. Here, we report that cell-surface-expressed (c)4-1BBL can be expressed on mononuclear blood cells from patients with acute myeloid leukemia (AML) (n = 35), myelodysplasia (n = 5) or non-Hodgkin lymphoma (n = 11) and can be coexpressed on varying proportions of lymphoid or myeloid malignant cells and on dendritic cells differentiated from AML-blasts. Direct correlations between c- and s4-1BBL were not found in the investigated cases. Up to now expression of 4-1BBL has not been described on primary myeloid malignant cells, but only on malignant cells of lymphoid or solid tumor origin or on tumor cell lines. With our work we further contribute to the understanding of the potential role of c/s4-1BBL in immune reactions and its influence on the interaction of tumor and immunoreactive cells.

Serum levels of sCD137 (4-1BB) ligand are prognostic factors for progression in acute myeloid leukemia but not in non-Hodgkin's lymphoma

CD178 (Fas/APO-1 ligand) and CD137 ligand (CD137L) have previously been described in sera of patients with various malignancies and play an important role in the pathogenesis of various diseases. Recently, we demonstrated that low levels of soluble (s) CD137L and high levels of sCD178 correlate significantly with a long progression free survival in patients with myelodysplastic syndrome (MDS). In this study, we correlated sCD137L and sCD178 levels in sera of 42 samples of patients with acute myeloid leukemia (AML) and 46 samples of patients with non-Hodgkin's lymphoma (NHL) with stages, subtypes, and the clinical course of the diseases and determined cut-off values with maximum probability for significant differentiation between cases with higher/lower probability for progress free survival. In contrast to patients with MDS, surprisingly no correlation between sCD178 levels and different subtypes and stages or with prognosis in AML or NHL were observed. Regarding sCD137L, NHL-patients displayed lower levels compared with AML. Statistically significant higher median levels of sCD137L are present in patients with undifferentiated AML (M1/M2, 1,470 pg/mL), poor cytogenetic risk (288 pg/mL) and higher levels of BM-blasts (186 pg/mL) compared with patients with monocytoid AML (M4/M5, 89 pg/mL), intermediate cytogenetic risk (59 pg/mL) and lower levels of BM-blasts (14 pg/mL) respectively. Furthermore, in AML patients sCD137L levels correlate significantly with the probabilities to achieve complete remission (CR), stay in CR or with progress of the disease. Taken together, our data demonstrate that sCD137L can be used as a prognostic factor not only in MDS but also in AML.

Mantle cell lymphomas acquire increased expression of CCL4, CCL5 and 4-1BB-L implicated in cell survival

We have analyzed mantle cell lymphomas (MCLs), using high-density DNA microarrays, and confirmed the expression of differentially regulated antigens, using flow cytometry and immunohistochemistry. The results show that MCLs acquire expression of molecules that normally are involved in interaction with other immune cells and, thus, might affect the ability of the tumor to survive. The MCL signature is represented by the overexpression of the chemokine CCL4 (MIP-1beta), implicated in the recruitment of regulatory T cells, as well as CCL5 and 4-1BB-L. The latter molecules are normally involved in chemotaxis of T cells and B cell activation, respectively. Signaling through 4-1BB-L allows B cells to proliferate and the expression of its ligand, by the intra-tumoral mesh of follicular dendritic cells (FDC), could thus serve as a paracrine loop facilitating growth and survival of MCL cells.

Fibromodulin as a novel tumor-associated antigen (TAA) in chronic lymphocytic leukemia (CLL), which allows expansion of specific CD8+ autologous T lymphocytes

Fibromodulin (FMOD) was shown to be highly overexpressed in chronic lymphocytic leukemia (CLL) cells compared with normal B lymphocytes by gene expression profiling. Therefore FMOD might serve as potential tumor-associated antigen (TAA) in CLL, enabling expansion of FMOD-specific T cells. In CLL samples derived from 16 different patients, high expression of FMOD by real-time reverse transcriptase–polymerase chain reaction (RT-PCR) was detectable in contrast to normal B lymphocytes. We used unpulsed native CLL cells and CD40 ligand (CD40L)–stimulated CLL cells as antigen-presenting cells (APCs) to expand autologous T cells from 13 patients. The number of T cells during 4 weeks of in vitro culture increased 2- to 3.5-fold and the number of T cells recognizing FMOD peptides bound to HLA-A2 dimers increased 10-fold. The expanded T cells also were able to secrete interferon-γ (IFN-γ) upon recognition of the antigen demonstrated by IFN-γ ELISPOT assays. T cells not only recognized HLA-A2–binding FMOD peptides presented by transporter-associated with antigen-processing (TAP)–deficient T2 cells, but also FMOD overexpressing autologous CLL cells in an HLA-A2–restricted manner. In summary, FMOD was shown for the first time to be naturally processed and presented as TAA in primary CLL cells, enabling the expansion of autologous tumor-specific T cells.

Co-stimulation: novel methods for preventing viral-induced lung inflammation

Respiratory infections cause significant morbidity and mortality worldwide. Although an immune response is required to eliminate respiratory pathogens, if unchecked, it can damage surrounding tissues and block primary lung function. Based on our knowledge of immune T-cell activation, there are several pathways to which immune intervention could be applied. However, relatively few interventions target only those immune cells that are responding to antigens. OX40 and 4-1BB are members of the tumour necrosis factor receptor family and are expressed on the surface of T cells in several inflammatory conditions. Recently, the inhibition of OX40 has proved beneficial during influenza virus infection. This review highlights the recent advances in the manipulation of such molecules and how they have been applied to inflammatory conditions that are caused by viruses in the lung.

Biological activities of reverse signal transduction through CD137 ligand

CD137 is a member of the tumor necrosis factor receptor family and a potent regulator of T cell activities. Agonists of CD137 have been used widely and successfully to treat cancer in animal models, and recently, it has become evident that CD137 agonists can also be used to treat autoimmune disease. An aspect of the CD137 receptor/ligand system, which has been comparatively little-explored, is its ability of reverse signaling. Just as is CD137, the CD137 ligand is also expressed as a cell membrane protein, and it too can transduce signals into the cells on which it is expressed. This bidirectional signaling capacity allows the CD137 receptor/ligand system to mediate extensive cross-talk between immune cells and between immune and nonimmune cells. This review summarizes the known activities of the CD137 ligand on the different leukocyte subsets and on cancer cells and discusses their influence on the course of immune responses.