Characterisation of soluble murine CD137 and its association with systemic lupus

Immunological characterization of IgG subclass deficiency reveals decreased Tregs and increased circulating costimulatory and regulatory immune checkpoints

Background

Immunoglobulin G subclass deficiencies (IgGsd) comprise a wide clinical spectrum from no symptoms to repeated respiratory infections and risk for the development of lung damage. Our aims were to investigate whether the immunological phenotype of IgGsd patients on and off immunoglobulin replacement therapy (IgRT) was reflected in the clinical features of IgGsd.

Method

Thirty patients with IgGsd were included in this prospective study of 18 months of IgRT, followed by 7-18 months of IgRT discontinuation. Blood samples were collected when patients were on and off IgRT and compared with samples from 34 cross-sectional healthy controls. An in-depth lymphocyte phenotyping was performed by flow cytometry and plasma levels of immune checkpoints were assessed.

Results

IgG3 subclass deficiency was most common. Patients with IgGsd had decreased levels of activated T cells and B cells and plasma levels of negative immune checkpoint molecules correlated negatively with T cell and B cell activation. The decreased T cell activation level was unaffected by IgRT, while the B cell activation was partly restored. Of note, decreased levels of activated regulatory T cells (Tregs) were found in IgGsd patients and was partly restored during IgRT. The profile of comorbidities did not associate with Treg levels.

Discussion

IgGsd is associated with decreased B cell and T cell activation including Tregs, and increased plasma levels of negative immune checkpoint molecules. The consequence of reduced activated Tregs in IgGsd remains unclear. Decreased immune cell activation was partly restored during IgRT, demonstrating that IgRT may contribute to improved immune function in patients with IgGsd.

Soluble form of immune checkpoints in autoimmune diseases

Immune checkpoints are essential regulators of immune responses, either by activating or suppressing them. Consequently, they are regarded as pivotal elements in the management of infections, cancer, and autoimmune disorders. In recent years, researchers have identified numerous soluble immune checkpoints that are produced through various mechanisms and demonstrated biological activity. These soluble immune checkpoints can be produced and distributed in the bloodstream and various tissues, with their roles in immune response dysregulation and autoimmunity extensively documented. This review aims to provide a thorough overview of the generation of various soluble immune checkpoints, such as sPD-1, sCTLA-4, sTim-3, s4-1BB, sBTLA, sLAG-3, sCD200, and the B7 family, and their importance as indicators for the diagnosis and prediction of autoimmune conditions. Furthermore, the review will investigate the potential pathological mechanisms of soluble immune checkpoints in autoimmune diseases, emphasizing their association with autoimmune diseases development, prognosis, and treatment.

Characterization and application of a lactate and branched chain amino acid metabolism related gene signature in a prognosis risk model for multiple myeloma

BACKGROUND

About 10% of hematologic malignancies are multiple myeloma (MM), an untreatable cancer. Although lactate and branched-chain amino acids (BCAA) are involved in supporting various tumor growth, it is unknown whether they have any bearing on MM prognosis.

METHODS

MM-related datasets (GSE4581, GSE136337, and TCGA-MM) were acquired from the Gene Expression Omnibus (GEO) database and the Cancer Genome Atlas (TCGA) database. Lactate and BCAA metabolism-related subtypes were acquired separately via the R package "ConsensusClusterPlus" in the GSE4281 dataset. The R package "limma" and Venn diagram were both employed to identify lactate-BCAA metabolism-related genes. Subsequently, a lactate-BCAA metabolism-related prognostic risk model for MM patients was constructed by univariate Cox, Least Absolute Shrinkage and Selection Operator (LASSO), and multivariate Cox regression analyses. The gene set enrichment analysis (GSEA) and R package "clusterProfiler"were applied to explore the biological variations between two groups. Moreover, single-sample gene set enrichment analysis (ssGSEA), Microenvironment Cell Populations-counter (MCPcounte), and xCell techniques were applied to assess tumor microenvironment (TME) scores in MM. Finally, the drug's IC50 for treating MM was calculated using the "oncoPredict" package, and further drug identification was performed by molecular docking.

RESULTS

Cluster 1 demonstrated a worse prognosis than cluster 2 in both lactate metabolism-related subtypes and BCAA metabolism-related subtypes. 244 genes were determined to be involved in lactate-BCAA metabolism in MM. The prognostic risk model was constructed by CKS2 and LYZ selected from this group of genes for MM, then the prognostic risk model was also stable in external datasets. For the high-risk group, a total of 13 entries were enriched. 16 entries were enriched to the low-risk group. Immune scores, stromal scores, immune infiltrating cells (except Type 17 T helper cells in ssGSEA algorithm), and 168 drugs'IC50 were statistically different between two groups. Alkylating potentially serves as a new agent for MM treatment.

CONCLUSIONS

CKS2 and LYZ were identified as lactate-BCAA metabolism-related genes in MM, then a novel prognostic risk model was built by using them. In summary, this research may uncover novel characteristic genes signature for the treatment and prognostic of MM.

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.

The long and winding road: From mouse linkage studies to a novel human therapeutic pathway in type 1 diabetes

Autoimmunity involves a loss of immune tolerance to self-proteins due to a combination of genetic susceptibility and environmental provocation, which generates autoreactive T and B cells. Genetic susceptibility affects lymphocyte autoreactivity at the level of central tolerance (e.g., defective, or incomplete MHC-mediated negative selection of self-reactive T cells) and peripheral tolerance (e.g., failure of mechanisms to control circulating self-reactive T cells). T regulatory cell (Treg) mediated suppression is essential for controlling peripheral autoreactive T cells. Understanding the genetic control of Treg development and function and Treg interaction with T effector and other immune cells is thus a key goal of autoimmunity research. Herein, we will review immunogenetic control of tolerance in one of the classic models of autoimmunity, the non-obese diabetic (NOD) mouse model of autoimmune Type 1 diabetes (T1D). We review the long (and still evolving) elucidation of how one susceptibility gene, Cd137, (identified originally via linkage studies) affects both the immune response and its regulation in a highly complex fashion. The CD137 (present in both membrane and soluble forms) and the CD137 ligand (CD137L) both signal into a variety of immune cells (bi-directional signaling). The overall outcome of these multitudinous effects (either tolerance or autoimmunity) depends upon the balance between the regulatory signals (predominantly mediated by soluble CD137 via the CD137L pathway) and the effector signals (mediated by both membrane-bound CD137 and CD137L). This immune balance/homeostasis can be decisively affected by genetic (susceptibility vs. resistant alleles) and environmental factors (stimulation of soluble CD137 production). The discovery of the homeostatic immune effect of soluble CD137 on the CD137-CD137L system makes it a promising candidate for immunotherapy to restore tolerance in autoimmune diseases.

Soluble CD137 as a dynamic biomarker to monitor agonist CD137 immunotherapies

Background

On the basis of efficacy in mouse tumor models, multiple CD137 (4-1BB) agonist agents are being preclinically and clinically developed. The costimulatory molecule CD137 is inducibly expressed as a transmembrane or as a soluble protein (sCD137). Moreover, the CD137 cytoplasmic signaling domain is a key part in approved chimeric antigen receptors (CARs). Reliable pharmacodynamic biomarkers for CD137 ligation and costimulation of T cells will facilitate clinical development of CD137 agonists in the clinic.

Methods

We used human and mouse CD8 T cells undergoing activation to measure CD137 transcription and protein expression levels determining both the membrane-bound and soluble forms. In tumor-bearing mice plasma sCD137 concentrations were monitored on treatment with agonist anti-CD137 monoclonal antibodies (mAbs). Human CD137 knock-in mice were treated with clinical-grade agonist anti-human CD137 mAb (Urelumab). Sequential plasma samples were collected from the first patients intratumorally treated with Urelumab in the INTRUST clinical trial. Anti-mesothelin CD137-encompassing CAR-transduced T cells were stimulated with mesothelin coated microbeads. sCD137 was measured by sandwich ELISA and Luminex. Flow cytometry was used to monitor CD137 surface expression.

Results

CD137 costimulation upregulates transcription and protein expression of CD137 itself including sCD137 in human and mouse CD8 T cells. Immunotherapy with anti-CD137 agonist mAb resulted in increased plasma sCD137 in mice bearing syngeneic tumors. sCD137 induction is also observed in human CD137 knock-in mice treated with Urelumab and in mice transiently humanized with T cells undergoing CD137 costimulation inside subcutaneously implanted Matrigel plugs. The CD137 signaling domain-containing CAR T cells readily released sCD137 and acquired CD137 surface expression on antigen recognition. Patients treated intratumorally with low dose Urelumab showed increased plasma concentrations of sCD137.

Conclusion

sCD137 in plasma and CD137 surface expression can be used as quantitative parameters dynamically reflecting therapeutic costimulatory activity elicited by agonist CD137-targeted agents.

Soluble CD137 is a novel serum marker of liver cirrhosis in patients with hepatitis C and alcohol-associated disease etiology

Defective T-cell functions play a role in the persistence of HCV infection. Activated T cells express CD137, which costimulates antivirus T-cell responses, and this activity is antagonized by soluble CD137 (sCD137). Here, we show that in sera of 81 patients with chronic HCV, sCD137 levels did not correlate with measures of viral infection, and did not decline after virus eradication using direct-acting antivirals. Thus, serum sCD137 was similar in patients infected with HCV and in uninfected controls. Of note, in HCV patients with liver cirrhosis and patients with mostly alcohol-associated liver cirrhosis, sCD137 was increased. A negative association of sCD137 and albumin existed in both cohorts. sCD137 concentrations were similar in hepatic and portal vein blood excluding the liver as the origin of higher levels. Recombinant sCD137 reduced Th1 and Th2 but not Th17 cell polarization in vitro, and accordingly lowered IFN-γ, TNF, and IL-13 in cell media. Serum sCD137 is associated with inflammatory states, and positively correlated with serum TNF in cirrhotic HCV patients following virus eradication. Our study argues against a role of sCD137 in HCV infection and suggests a function of sCD137 in liver cirrhosis, which yet has to be defined.

Early loss of T lymphocyte 4-1BB receptor expression is associated with higher short-term mortality in alcoholic hepatitis

Objectives

In alcoholic hepatitis (AH), dysfunctional T lymphocytes may contribute to the high mortality from infections. T lymphocyte activation is governed by the expression of co-stimulatory receptors such as 4-1BB balanced by inhibitory receptors such as Programmed Death receptor 1 (PD-1). 4-1BB expression is unaccounted for in AH, while PD-1 is elevated. We characterized expression of 4-1BB and PD-1 and the associated T lymphocyte functional status in AH and investigated whether these were associated with short-term mortality.

Methods

Thirty-five patients with AH (at diagnosis and days 7 and 90) were compared with healthy controls (HC). Spontaneous and in vitro stimulated receptor expression were quantified by flow cytometry, and plasma proteins by ELISA.

Results

At diagnosis, the patients showed increased stimulated 4-1BB responses of CD4⁺ T lymphocytes. Also, the frequencies of PD-1⁺ T lymphocytes both with and without co-expressed 4-1BB were increased. Further, interferon-gamma was predominantly produced in T lymphocytes co-expressing 4-1BB. A decrease in the frequency of spontaneous 4-1BB⁺ T lymphocytes and an increase in soluble 4-1BB during the first week after diagnosis were associated with higher mortality at day 90 in AH. PD-1 expression showed no systematic dynamics related to mortality.

Conclusions

We found an increased stimulated 4-1BB response of T lymphocytes in AH and early loss of these lymphocytes was associated with a higher short-term mortality. This suggests a role of T lymphocyte 4-1BB expression in the progression of AH.

Role of ADAM10 and ADAM17 in Regulating CD137 Function

Human CD137 (4-1BB), a member of the TNF receptor family, and its ligand CD137L (4-1BBL), are expressed on immune cells and tumor cells. CD137/CD137L interaction mediates bidirectional cellular responses of potential relevance in inflammatory diseases, autoimmunity and oncology. A soluble form of CD137 exists, elevated levels of which have been reported in patients with rheumatoid arthritis and various malignancies. Soluble CD137 (sCD137) is considered to represent a splice variant of CD137. In this report, however, evidence is presented that A Disintegrin and Metalloproteinase (ADAM)10 and potentially also ADAM17 are centrally involved in its generation. Release of sCD137 by transfected cell lines and primary T cells was uniformly inhibitable by ADAM10 inhibition. The shedding function of ADAM10 can be blocked through inhibition of its interaction with surface exposed phosphatidylserine (PS), and this effectively inhibited sCD137 generation. The phospholipid scramblase Anoctamin-6 (ANO6) traffics PS to the outer membrane and thus modifies ADAM10 function. Overexpression of ANO6 increased stimulated shedding, and hyperactive ANO6 led to maximal constitutive shedding of CD137. sCD137 was functionally active and augmented T cell proliferation. Our findings shed new light on the regulation of CD137/CD137L immune responses with potential impact on immunotherapeutic approaches targeting CD137.

Elevated soluble 4-1BB is associated with serum markers of hepatitis B virus in patients with chronic hepatitis B

BACKGROUND

Previous studies have suggested that the costimulatory molecule 4-1BB plays pivotal roles in regulating immunity during chronic viral infection. However, up to now, there are few studies about 4-1BB in chronic hepatitis B (CHB).

AIM

To clarify this issue, we report our comprehensive study results on the expression levels of 4-1BB in patients with CHB.

METHODS

From September 2018 to June 2019, a total of 64 patients with CHB were recruited from the Department of Hepatology, The First Hospital of Jilin University. Peripheral blood samples were collected from 52 treatment-naïve and 12 entecavir-treated patients with CHB as well as 37 healthy donors (including 24 healthy adults and 13 healthy children). The levels of soluble 4-1BB (s4-1BB) in plasma were measured by ELISA. 4-1BB mRNA expression in peripheral blood mononuclear cells was detected by real-time quantitative PCR.

RESULTS

The s4-1BB levels in the plasma of patients with CHB were significantly higher than those in healthy adults (94.390 ± 7.393 ng/mL vs 8.875 ± 0.914 ng/mL, P < 0.001). In addition, the s4-1BB level in plasma was significantly increased in patients with a higher viral load and a disease flare up. However, there were no significant differences between treatment-naïve and entecavir-treated patients. Interestingly, among treatment-naïve patients with CHB, the levels of s4-1BB in plasma had a significant positive correlation with hepatitis B surface antigen, hepatitis B virus DNA, hepatitis B e antigen, and triglyceride levels (r = 0.748, P < 0.001; r = 0.406, P = 0.004; r = 0.356, P = 0.019 and r = -0.469, P = 0.007, respectively). The 4-1BB mRNA expression was higher in the peripheral blood mononuclear cells of patients with CHB than in the peripheral blood mononuclear cells of healthy adults, but the difference was not statistically significant.

CONCLUSION

These results suggest that the levels of s4-1BB may be associated with pathogenesis of hepatitis B virus and therefore may be a promising biomarker for disease progression.

CD137 / CD137 ligand signalling regulates the immune balance: A potential target for novel immunotherapy of autoimmune diseases

CD137 (TNFRSF9, 4-1BB) is a potent co-stimulatory molecule of the tumour necrosis factor receptor superfamily (TNFRSF) that is expressed by activated T cells. CD137/CD137 ligand (CD137L) signalling primarily induces a potent cell-mediated immune response, while signalling of cell surface-expressed CD137L into antigen presenting cells enhances their activation, differentiation and migratory capacity. Studies have shown that bidirectional CD137/CD137L signalling plays an important role in the pathogenesis of autoimmune diseases. This review discusses the mechanisms how CD137/CD137L signalling contributes to immune deviation of helper T cell pathways in various murine models, and the potential of developing immunotherapies targeting CD137/CD137L signalling for the treatment of autoimmune diseases.

The relevance of soluble CD137 in the regulation of immune responses and for immunotherapeutic intervention

CD137 is a potent costimulatory receptor. Several agonistic anti-CD137 antibodies are currently in clinical trials for tumor immunotherapy. Soluble forms of CD137 (sCD137) are generated by differential splicing and antagonize the activities of membrane-bound CD137 (mCD137) and of therapeutic CD137 agonists. sCD137 is found in sera of patients suffering from autoimmune diseases where it is a natural regulator of immune responses, and which has therapeutic potential for immune-mediated diseases. This review summarizes the current knowledge on sCD137, highlights its potential role in immunotherapy against cancer and in autoimmune diseases, and presents important issues to be addressed by future research.

Destroy, what destroys you

New evidence indicates the importance of CD137 for controlling Epstein-Barr virus (EBV) infections. (1) Mutations in CD137 predispose to EBV-associated diseases. (2) EBV induces ectopic CD137 expression, thereby activating a negative feed-back regulation and reducing T cell costimulation. These findings suggest CD137 agonists as new treatments for EBV-associated diseases.

Impact of immunosuppressive therapy on brain derived cytokines after liver transplantation

BACKGROUND

While acute neurotoxic side effects of calcineurin inhibitors (CNI) are well-known, data upon long-term effects on brain structure and function are sparse. We hypothesize that long-term CNI therapy affects the neuroimmune system, thereby, increasing the risk of neurodegeneration. Here, we measured the impact of CNI therapy on plasma levels of brain- and T cell-derived cytokines in a cohort of patients after liver transplantation (LT).

METHODS

Levels of T cell-mediated cytokines (e.g. Interferon-γ (IFN-γ)) and brain-derived cytokines (e.g. brain derived neurotrophic factor (BDNF), platelet derived growth factor (PDGF)) were measured by multiplex assays in plasma of 82 patients about 10 years after LT (17 with CNI free, 35 with CNI low dose, 30 with standard dose CNI immunosuppression) and 33 healthy controls. Data were related to psychometric test results and parameters of cerebral magnetic resonance imaging.

RESULTS

IFN-γ levels were significantly higher in the CNI free LT patient group (p=0.027) compared to healthy controls. BDNF levels were significantly lower in LT patients treated with CNI (CNI low: p<0.001; CNI standard: p=0.016) compared to controls. PDGF levels were significantly lower in the CNI low dose group (p=0.004) and for PDGF-AB/BB also in the CNI standard dose group (p=0.029) compared to controls. BDNF and PDGF negatively correlated with cognitive function and brain volume (p<0.05) in the CNI low dose group.

CONCLUSION

Our results imply that long-term treatment with CNI suppresses BDNF and PDGF expression, both crucial for neuronal signaling, cell survival and synaptic plasticity and thereby may lead to cognitive dysfunction and neurodegeneration.

The Progress of Investigating the CD137-CD137L Axis as a Potential Target for Systemic Lupus Erythematosus

Costimulatory molecules facilitate cross-talks among leukocytes via mutual stimulatory and inhibitory signalling, contributing to diverse immunological outcomes in normal physiological responses and pathological conditions. Systemic lupus erythematosus (SLE) is a complex multi-systemic autoimmune condition in which cellular communication through the involvement of costimulatory molecules is crucial in driving proinflammatory responses from the stage of autoantigen presentation to the subsequent process of pathogenic autoantibody production. While the physiology of the costimulatory systems including OX40-OX40L, CD28/CTLA-4-CD80/86, ICOS-B7RP1 and CD70-CD27 has been relatively well studied in SLE, recent data on the immunopathology of the CD137-CD137 ligand (CD137L) system in murine lupus models and patients with SLE highlight the critical role of this costimulatory system in initiating and perpetuating the diverse clinical and serological phenotypes of SLE. CD137, a membrane-bound receptor which belongs to the tumour necrosis factor receptor superfamily, is mainly expressed on activated T cells. Activation of the CD137 receptor via its interaction with CD137L which is expressed on antigen present cells (APC) including B cells, triggers bi-directional signalling; that is, signalling through CD137 as well as signalling through CD137L (reverse signalling), which further activates T cells and polarizes them to the Th1/Tc1 pathway. Further, via reverse CD137L signalling it enhances differentiation and maturation of the APC, particularly of dendritic cells, which subsequently drive proinflammatory cytokine production. In this review, recent data including our experience in the manipulation of CD137L signalling pertaining to the pathophysiology of SLE will be critically reviewed. More in-depth understanding of the biology of the CD137-CD137L co-stimulation system opens an opportunity to identify new prognostic biomarkers and the design of novel therapeutic approaches for advancing the management of SLE.

CD137L dendritic cells induce potent response against cancer-associated viruses and polarize human CD8+ T cells to Tc1 phenotype

Therapeutic tumor vaccination based on dendritic cells (DC) is safe; however, its efficacy is low. Among the reasons for only a subset of patients benefitting from DC-based immunotherapy is an insufficient potency of in vitro generated classical DCs (cDCs), made by treating monocytes with GM-CSF + IL-4 + maturation factors. Recent studies demonstrated that CD137L (4-1BBL, TNFSF9) signaling differentiates human monocytes to a highly potent novel type of DC (CD137L-DCs) which have an inflammatory phenotype and are closely related to in vivo DCs. Here, we show that CD137L-DCs induce potent CD8+ T-cell responses against Epstein-Barr virus (EBV) and Hepatitis B virus (HBV), and that T cells primed by CD137L-DCs more effectively lyse EBV+ and HBV+ target cells. The chemokine profile of CD137L-DCs identifies them as inflammatory DCs, and they polarize CD8+ T cells to a Tc1 phenotype. Expression of exhaustion markers is reduced on T cells activated by CD137L-DCs. Furthermore, these T cells are metabolically more active and have a higher capacity to utilize glucose. CD137L-induced monocyte to DC differentiation leads to the formation of AIM2 inflammasome, with IL-1beta contributing to CD137L-DCs possessing a stronger T cell activation ability. CD137L-DCs are effective in crosspresentation. PGE2 as a maturation factor is required for enhancing migration of CD137L-DCs but does not significantly reduce their potency. This study shows that CD137L-DCs have a superior ability to activate T cells and to induce potent Tc1 responses against the cancer-causing viruses EBV and HBV which suggest CD137L-DCs as promising candidates for DC-based tumor immunotherapy.

CD137 signaling in Hodgkin and Reed-Sternberg cell lines induces IL-13 secretion, immune deviation and enhanced growth

CD137 and its ligand, CD137L, are expressed on activated T cells and antigen-presenting cells (APC), respectively, and are powerful inducers of cellular, type 1 immune responses. CD137 is ectopically expressed by Hodgkin and Reed-Sternberg (HRS) cells, the malignant cells in Hodgkin lymphoma (HL). Here we report that CD137 transmits signals into HRS cells, which induce the secretion of IL-13. IL-13 in conditioned supernatants of HRS cell lines inhibits the secretion of IFNγ by peripheral blood mononuclear cells (PBMC). Since IFNγ is essential for the development of a type 1 immune response, CD137-induced IL-13 secretion facilitates escape from immune surveillance. Further, CD137-induced IL-13 enhances the growth of HRS cell lines. CD137, IL-13 double-positive cells could be detected in the majority (58%) of HL patient samples, providing clinical evidence for a role of IL-13 induction by CD137 during HL pathogenesis. This study validates CD137 as a candidate target for immunotherapy of HL.

CD137 and CD137L signals are main drivers of type 1, cell-mediated immune responses

CD137 is expressed on activated T cells and NK cells, among others, and is a potent co-stimulator of antitumor immune responses. CD137 ligand (CD137L) is expressed by antigen presenting cells (APC), and CD137L reverse signaling into APC enhances their activity. CD137-CD137L interactions as main driver of type 1, cell-mediated immune responses explains the puzzling observation that CD137 agonists which enhance antitumor immune responses also ameliorate autoimmune diseases. Upon co-stimulation by CD137, Th1 CD4⁺ T cells together with Tc1 CD8⁺ T cells and NK cells inhibit other T cell subsets, thereby promoting antitumor responses and mitigating non-type 1 auto-immune diseases.

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.

Hypoxia-induced soluble CD137 in malignant cells blocks CD137L-costimulation as an immune escape mechanism

Hypoxia is a common feature in solid tumors that has been implicated in immune evasion. Previous studies from our group have shown that hypoxia upregulates the co-stimulatory receptor CD137 on activated T lymphocytes and on vascular endothelial cells. In this study, we show that exposure of mouse and human tumor cell lines to hypoxic conditions (1% O₂) promotes CD137 transcription. However, the resulting mRNA is predominantly an alternatively spliced form that encodes for a soluble variant, lacking the transmembrane domain. Accordingly, soluble CD137 (sCD137) is detectable by ELISA in the supernatant of hypoxia-exposed cell lines and in the serum of tumor-bearing mice. sCD137, as secreted by tumor cells, is able to bind to CD137-Ligand (CD137L). Our studies on primed T lymphocytes in co-culture with stable transfectants for CD137L demonstrate that tumor-secreted sCD137 prevents co-stimulation of T lymphocytes. Such an effect results from preventing the interaction of CD137L with the transmembrane forms of CD137 expressed on T lymphocytes undergoing activation. Indeed, silencing CD137 with shRNA renders more immunogenic tumor-cell variants upon inoculation to immunocompetent mice but which readily grafted on immunodeficient or CD8⁺ T-cell-depleted mice. These mechanisms are interpreted as a molecular strategy deployed by tumors to repress lymphocyte co-stimulation via CD137/CD137L.

Trogocytic CD137 transfer causes an internalization of CD137 ligand on murine APCs leading to reduced T cell costimulation

CD137 ligand (CD137L) is expressed on APCs and crosslinks CD137, a powerful costimulatory molecule on T cells during cognate interactions, and thereby greatly enhances immune responses. We report that CD137 can be transferred from activated T cells and from tumor cells that express CD137 to other cells via trogocytosis. This trogocytic transfer is independent of CD137L expression by the recipient cell. However, if CD137L is present on the recipient cell, the transferred CD137 binds to CD137L and the CD137-CD137L complex becomes internalized. The removal of CD137L from the surface of APCs lowers their ability to costimulate T cells, as evidenced by a reduced IFN-γ secretion. Removal of CD137L on APCs by trogocytic transfer of CD137 occurs within 1 h and requires cell-cell contact and the continuous presence of CD137-expressing cells. Bidirectional signaling exists for the CD137 receptor/ligand system, because CD137L also signals into APCs. We propose that the trogocytic transfer of CD137 from activated T cells to APCs and the subsequent removal of CD137L from APCs is a physiologic regulatory mechanism that limits immune activity. Furthermore, we hypothesize that the trogocytic transfer of CD137 occurs in cancers and quenches the activity of APCs, contributing to the cancer cells escaping immune surveillance. Taken together, our findings demonstrate that the trogocytic transfer of CD137 leads to an internalization of CD137L on APCs and a reduction in immune activity.

Ectopic CD137 expression facilitates the escape of Hodgkin and Reed-Sternberg cells from immunosurveillance

CD137 is ectopically expressed on Hodgkin and Reed-Sternberg (HRS) cells, causing the removal of the immunostimulatory CD137 ligand from HRS cells as well as from surrounding antigen presenting cells. This inhibits T-cell co-stimulation and supports the immune evasion of Hodgkin’s lymphoma.

The B10 Idd9.3 locus mediates accumulation of functionally superior CD137(+) regulatory T cells in the nonobese diabetic type 1 diabetes model

CD137 is a T cell costimulatory molecule encoded by the prime candidate gene (designated Tnfrsf9) in NOD.B10 Idd9.3 congenic mice protected from type 1 diabetes (T1D). NOD T cells show decreased CD137-mediated T cell signaling compared with NOD.B10 Idd9.3 T cells, but it has been unclear how this decreased CD137 T cell signaling could mediate susceptibility to T1D. We and others have shown that a subset of regulatory T cells (Tregs) constitutively expresses CD137 (whereas effector T cells do not, and only express CD137 briefly after activation). In this study, we show that the B10 Idd9.3 region intrinsically contributes to accumulation of CD137(+) Tregs with age. NOD.B10 Idd9.3 mice showed significantly increased percentages and numbers of CD137(+) peripheral Tregs compared with NOD mice. Moreover, Tregs expressing the B10 Idd9.3 region preferentially accumulated in mixed bone marrow chimeric mice reconstituted with allotypically marked NOD and NOD.B10 Idd9.3 bone marrow. We demonstrate a possible significance of increased numbers of CD137(+) Tregs by showing functional superiority of FACS-purified CD137(+) Tregs in vitro compared with CD137(-) Tregs in T cell-suppression assays. Increased functional suppression was also associated with increased production of the alternatively spliced CD137 isoform, soluble CD137, which has been shown to suppress T cell proliferation. We show for the first time, to our knowledge, that CD137(+) Tregs are the primary cellular source of soluble CD137. NOD.B10 Idd9.3 mice showed significantly increased serum soluble CD137 compared with NOD mice with age, consistent with their increased numbers of CD137(+) Tregs with age. These studies demonstrate the importance of CD137(+) Tregs in T1D and offer a new hypothesis for how the NOD Idd9.3 region could act to increase T1D susceptibility.

The non-obese diabetic (NOD) mouse as a model of human type 1 diabetes

The non-obese diabetic (NOD) mouse spontaneously develops type 1 diabetes (T1D) and has thus served as a model for understanding the genetic and immunological basis, and treatment, of T1D. Since its initial description in 1980, however, the field has matured and recognized that prevention of diabetes in NOD mice (i.e., preventing the disease from occurring by an intervention prior to frank diabetes) is relatively easy to achieve and does not correlate well with curing the disease (after the onset of frank hyperglycemia). Hundreds of papers have described the prevention of diabetes in NOD mice but only a handful have described its actual reversal. The paradoxical conclusion is that preventing the disease in NOD mice does not necessarily tell us what caused the disease nor how to reverse it. The NOD mouse model is therefore best used now, with respect to human disease, as a way to understand the genetic and immunologic causes of and as a model for trying to reverse disease once hyperglycemia occurs. We describe how genetic approaches to identifying causative gene variants can be adapted to identify novel therapeutic agents for reversing new-onset T1D.

Tumor necrosis factor receptor subfamily 9 (Tnfrsf9) gene is expressed in distinct cell populations in mouse uterus and conceptus during implantation period of pregnancy

Tumor necrosis factor receptor subfamily 9 (TNFRSF9) plays a potentially important general role in immune function. Tnfrsf9 gene expression has previously been characterized in late pregnant mouse uterus and placenta. However, little is known about its expression in the uterus during the implantation phase of early pregnancy. We have assessed the levels and localization of Tnfrsf9 expression in the mouse uterus and conceptus during implantation. Relative Tnfrsf9 mRNA levels were significantly higher in implantation than in non-implantation site tissue on days 6.5-8.5 of pregnancy. This increase did not depend on the presence of the conceptus, as mRNA levels were not significantly different between pregnant implantation sites and artificially induced deciduomas. Localization by in situ hybridization revealed a subpopulation of endothelial and uterine natural killer cells expressing Tnfrsf9 in the endometrium during implantation. In the developing conceptus, primary trophoblast giant and ectoplacental cells expressed Tnfrsf9 on days 6.5-8.5, followed by expression in the trophoblast giant cell layers surrounding the conceptus on day 9.5 of pregnancy. Two main splice forms of Tnfrsf9 mRNA exist and encode proteins with distinct biological functions; both mRNA splice forms were present in uterine and conceptus tissues as determined by reverse transcription with the polymerase chain reaction. Thus, both membrane and soluble forms of Tnfrsf9 are expressed in specific cell types of the uterus and conceptus during the progression of implantation in mice and possibly have an important function in this process.

Agonist anti-CD137 mAb act on tumor endothelial cells to enhance recruitment of activated T lymphocytes

Agonist monoclonal antibodies (mAb) to the immune costimulatory molecule CD137, also known as 4-1BB, are presently in clinical trials for cancer treatment on the basis of their costimulatory effects on primed T cells and perhaps other cells of the immune system. Here we provide evidence that CD137 is selectively expressed on the surface of tumor endothelial cells. Hypoxia upregulated CD137 on murine endothelial cells. Treatment of tumor-bearing immunocompromised Rag(-/-) mice with agonist CD137 mAb did not elicit any measurable antiangiogenic effects. In contrast, agonist mAb stimulated tumor endothelial cells, increasing cell surface expression of the adhesion molecules intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, and E-selectin. When adoptively transferred into mice, activated T lymphocytes derived from CD137-deficient animals entered more avidly into tumor tissue after treatment with agonist mAb. This effect could be neutralized with anti-ICAM-1 and anti-VCAM-1 blocking antibodies. Thus, stimulation of CD137 not only enhanced T-cell activation but also augmented their trafficking into malignant tissue, through direct actions on the blood vessels that irrigate the tumor. Our findings identify an additional mechanism of action that can explain the immunotherapeutic effects of agonist CD137 antibodies.

Species difference of CD137 ligand signaling in human and murine monocytes

BACKGROUND

Stimulation of CD137 ligand on human monocytes has been shown to induce DC differentiation, and these CD137L-DCs are more potent than classical DCs, in stimulating T cell responses in vitro. To allow an in vivo evaluation of the potency of CD137L-DCs in murine models we aimed at generating murine CD137L-DCs.

METHODOLOGY/PRINCIPAL FINDINGS

When stimulated through CD137 ligand murine monocytes responded just as human monocytes with an increased adherence, morphological changes, proliferation and an increase in viable cell numbers. But CD137 ligand signaling did not induce expression of inflammatory cytokines and costimulatory molecules in murine monocytes and these cells had no T cell stimulatory activity. Murine monocytes did not differentiate to inflammatory DCs upon CD137 ligand signaling. Furthermore, while CD137 ligand signaling induces maturation of human immature classical DCs it failed to do so with murine immature classical DCs.

CONCLUSIONS/SIGNIFICANCE

These data demonstrate that both human and murine monocytes become activated by CD137 ligand signaling but only human and not murine monocytes differentiate to inflammatory DCs.

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.