4-1BB (CD137) is required for rapid clearance of Listeria monocytogenes infection

Soluble CD137: A Potential Prognostic Biomarker in Critically Ill Patients

T cell depletion and functional impairment are characteristics of sepsis. CD137 is a costimulatory receptor on activated T cells, while soluble CD137 (sCD137) inhibits CD137 signaling. This study found elevated sCD137 levels in the plasma of patients with systemic inflammatory response syndrome (SIRS), sepsis, or septic shock compared to healthy controls. The sCD137 levels negatively correlated with the C-reactive protein and positively with procalcitonin and interleukin-6. There was no difference in sCD137 levels based on ventilation, dialysis, or vasopressor treatment. Patients with SARS-CoV-2, Gram-positive, or Gram-negative bacterial infections had similar sCD137 levels as noninfected individuals. Notably, higher plasma sCD137 levels were observed in non-survivors compared to survivors in both the SIRS/sepsis group and the SARS-CoV-2 subgroup. In conclusion, plasma sCD137 levels are associated with severe illness and survival in critically ill patients.

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.

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.

CD137 Signaling Is Critical in Fungal Clearance during Systemic Candida albicans Infection

Invasive fungal infections by Candida albicans frequently cause mortality in immunocompromised patients. Neutrophils are particularly important for fungal clearance during systemic C. albican infection, yet little has been known regarding which surface receptor controls neutrophils’ antifungal activities. CD137, which is encoded by Tnfrsf9, belongs to the tumor necrosis receptor superfamily and has been shown to regulate neutrophils in Gram-positive bacterial infection. Here, we used genetic and immunological tools to probe the involvement of neutrophil CD137 signaling in innate defense mechanisms against systemic C. albicans infection. We first found that Tnfrsf9^−/− mice were susceptible to C. albicans infection, whereas injection of anti-CD137 agonistic antibody protected the host from infection, suggesting that CD137 signaling is indispensable for innate immunity against C. albicans infection. Priming of isolated neutrophils with anti-CD137 antibody promoted their phagocytic and fungicidal activities through phospholipase C. In addition, injection of anti-CD137 antibody significantly augmented restriction of fungal growth in Tnfrsf9^−/− mice that received wild-type (WT) neutrophils. In conclusion, our results demonstrate that CD137 signaling contributes to defense mechanisms against systemic C. albicans infection by promoting rapid fungal clearance.

TLR2 Promotes Monocyte/Macrophage Recruitment Into the Liver and Microabscess Formation to Limit the Spread of Listeria Monocytogenes

TLR2 signaling plays a critical protective role against acute Listeria monocytogenes (Lm) infection by up-regulating inflammatory cytokines and promoting macrophage antimicrobial capabilities. However, the underlying mechanism by which TLR2 regulates hepatic macrophage-mediated anti-Lm immune responses remains poorly understood. In this study, we found that both the absolute number and proportion of monocyte/macrophage (Mo/MΦ) in the liver and spleen of Tlr2 ^-/- mice were significantly lower compared to wild type mice. Changes in TLR2 signaling in both hepatocytes and Mo/MΦs were associated with the infiltration of Mo/MΦs in response to Lm-infection. Analyses by proteome profiler array and ELISA revealed that hepatocytes recruited Mo/MΦs via TLR2-dependent secretion of CCL2 and CXCL1, which was confirmed by receptor blocking and exogenous chemokine administration. Importantly, we found that TLR2 contributed to macrophage mobility in the liver through a TLR2/NO/F-actin pathway, facilitating the formation of macrophage-associated hepatic microabscesses. Moreover, TLR2 activation induced the expression of several PRRs on hepatic macrophages associated with the recognition of Lm and augmented macrophage bacterial clearance activity. Our findings provide insight into the intrinsic mechanisms of TLR2-induced Mo/MΦ migration and mobility, as well as the interaction between macrophages and hepatocytes in resistance to Lm infection.

4-1BB expression on MAIT cells is associated with enhanced IFN-γ production and depends on IL-2

The role of MAIT cells in immunity against Mycobacterium tuberculosis infection in humans is still largely unexplored. In this study, we investigated the functional role of 4-1BB on MAIT cells. We found that 4-1BB was highly up-regulated on MAIT cells from tuberculous pleural effusions following Mtb antigen stimulation and its level of expression correlated with IFN-γ and IL-17 production. 4-1BB expression on MAIT cells in response to Mtb antigens was partially dependent on IL-2 and was associated with common γ chain receptor. By transcriptome sequencing, we identified numerous differentially expressed genes between 4-1BB^- and 4-1BB⁺ MAIT cells. GO enrichment and KEGG pathway analysis of differentially expressed genes identified enriched pathways that included T-cell receptor and NF-κB signaling pathways. It is concluded that 4-1BB has the potential to be used as a biomarker to identify MAIT cells with enhanced IFN-γ and IL-17 responses that might be associated with tuberculosis infection control.

The Essential Role of Neutrophils during Infection with the Intracellular Bacterial Pathogen Listeria monocytogenes

Neutrophils have historically been characterized as first responder cells vital to host survival because of their ability to contain and eliminate bacterial and fungal pathogens. However, recent studies have shown that neutrophils participate in both protective and detrimental responses to a diverse array of inflammatory and infectious diseases. Although the contribution of neutrophils to extracellular infections has been investigated for decades, their specific role during intracellular bacterial infections has only recently been appreciated. During infection with the Gram-positive intracellular pathogen Listeria monocytogenes, neutrophils are recruited from the bone marrow to sites of infection where they use novel bacterial-sensing pathways leading to phagocytosis and production of bactericidal factors. This review summarizes the requirement of neutrophils during L. monocytogenes infection by examining both neutrophil trafficking and function during primary and secondary infection.

4-1BB Agonists: Multi-Potent Potentiators of Tumor Immunity

Immunotherapy is a rapidly expanding field of oncology aimed at targeting, not the tumor itself, but the immune system combating the cancerous lesion. Of the many approaches currently under study to boost anti-tumor immune responses; modulation of immune co-receptors on lymphocytes in the tumor microenvironment has thus far proven to be the most effective. Antibody blockade of the T cell co-inhibitory receptor cytotoxic T lymphocyte antigen-4 (CTLA-4) has become the first FDA approved immune checkpoint blockade; however, tumor infiltrating lymphocytes express a diverse array of additional stimulatory and inhibitory co-receptors, which can be targeted to boost tumor immunity. Among these, the co-stimulatory receptor 4-1BB (CD137/TNFSF9) possesses an unequaled capacity for both activation and pro-inflammatory polarization of anti-tumor lymphocytes. While functional studies of 4-1BB have focused on its prominent role in augmenting cytotoxic CD8 T cells, 4-1BB can also modulate the activity of CD4 T cells, B cells, natural killer cells, monocytes, macrophages, and dendritic cells. 4-1BB’s expression on both T cells and antigen presenting cells, coupled with its capacity to promote survival, expansion, and enhanced effector function of activated T cells, has made it an alluring target for tumor immunotherapy. In contrast to immune checkpoint blocking antibodies, 4-1BB agonists can both potentiate anti-tumor and anti-viral immunity, while at the same time ameliorating autoimmune disease. Despite this, 4-1BB agonists can trigger high grade liver inflammation which has slowed their clinical development. In this review, we discuss how the underlying immunobiology of 4-1BB activation suggests the potential for therapeutically synergistic combination strategies in which immune adverse events can be minimized.

SA-4-1BBL as a novel adjuvant for the development of therapeutic cancer vaccines

Tumor associated antigen (TAA)-based therapeutic vaccines have great potential as a safe, practical, and cost-efficient alternative to standard treatments for cancer. Clinical efficacy of TAA-based vaccines, however, has yet to be realized and will require adjuvants with pleiotropic functions on immune cells. Such adjuvants need not only to generate/boost T cell responses, but also reverse intrinsic/extrinsic tumor immune evasion mechanisms for therapeutic efficacy. This review focuses on a novel agonistic ligand, SA-4-1BBL, for 4-1BB costimulatory receptor as an adjuvant of choice because of its ability to: i) serve as a vehicle to deliver TAAs to dendritic cells (DCs) for antigen uptake and cross-presentation to CD8(+) T cells; ii) augment adaptive Th1 and innate immune responses; and iii) overcome various immune evasion mechanisms, cumulatively translating into therapeutic efficacy in preclinical tumor models.

T cells and costimulation in cancer

Optimal T cell response is dependent not only on T cell receptor activation, but also on additional signaling from coreceptors. The main coreceptors include B7 and tumor necrosis factor family members. They exert costimulatory or coinhibitory effects, and their balance determines the fate of T cell response. In normal conditions, costimulators facilitate the development of protective immune response, whereas coinhibitors dampen inflammation to avoid organ/tissue damage from excessive immune reaction. In the tumor microenvironment, the balance is garbled: inhibitory pathways predominate, and T cell response is impaired. The importance of cosignaling in the tumor immune response has been experimentally and clinically demonstrated. New therapeutic strategies targeting T cell cosignaling, especially coinhibitory molecules, are under active experimental and clinical investigation. This review summarizes the functions of main T cell cosignaling axes and discusses their clinical application.

Regulation of myelopoiesis by CD137L signaling

CD137 ligand (CD137L) has emerged as a powerful regulator of myelopoiesis that links emergency situations, such as infections, to the generation of additional myeloid cells, and to their activation and maturation. CD137L is expressed on the cell surface of hematopoietic stem and progenitor cells (HSPC) and antigen presenting cells (APC) as a transmembrane protein. The signaling of CD137L into HSPC induces their proliferation and differentiation to monocytes and macrophages, and in monocytes CD137L signaling induces differentiation to potent dendritic cells (DC). CD137L signaling is initiated by CD137 which is expressed by T cells, once they become activated. Some of these activated, CD137-expressing T cells migrate from the site of infection to the bone marrow where they interact with HSPC to induce myelopoiesis, or they induce monocyte to DC differentiation locally at the site of infection. Therapeutically, induction of CD137L signaling can be utilized to reinitiate myeloid differentiation in acute myeloid leukemia cells, and to generate potent DC for immunotherapy.

Transcriptome profile of the murine macrophage cell response to Candida parapsilosis

Candida parapsilosis is a human fungal pathogen with increasing global significance. Understanding how macrophages respond to C. parapsilosis at the molecular level will facilitate the development of novel therapeutic paradigms. The complex response of murine macrophages to infection with C. parapsilosis was investigated at the level of gene expression using an Agilent mouse microarray. We identified 155 and 511 differentially regulated genes at 3 and 8h post-infection, respectively. Most of the upregulated genes encoded molecules involved in immune response and inflammation, transcription, signaling, apoptosis, cell cycle, electron transport and cell adhesion. Typical of the classically activated macrophages, there was significant upregulation of genes coordinating the production of inflammatory cytokines such as TNF, IL-1 and IL-15. Further, we used both primary murine macrophages and macrophages differentiated from human peripheral mononuclear cells to confirm the upregulation of the TNF-receptor family member TNFRSF9 that is associated with Th1 T-helper cell responses. Additionally, the microarray data indicate significant differences between the response to C. parapsilosis infection and that of C. albicans.

Role of the CD137 ligand (CD137L) signaling pathway during Mycobacterium tuberculosis infection

The role of the CD137-CD137 ligand (CD137L) signaling pathway in T cell co-stimulation has been well established. Dysregulated CD137 or CD137L stimulation can lead to pathological conditions such as inflammatory diseases or cancer. However, the contribution of CD137-CD137L interaction to the control of infectious diseases has not been extensively studied, with the few available reports focusing mainly on viral infections. Here we investigated the role of the CD137-CD137L interactions during Mycobacterium tuberculosis infection. Using CD137L-deficient mice, we found that absence of the CD137L-mediated signaling pathway during M. tuberculosis infection resulted in delayed activation of CD4(+) T cells in the draining lymph nodes. This finding was supported by an in vitro mixed lymphocyte reaction assay that revealed impaired priming of T cells by CD137L-deficient dendritic cells upon mycobacterial infection. In addition, greater numbers of CD4(+) T cells and antigen presenting cells were measured in the lungs of CD137L-deficient mice. Strikingly, the lung cytokine production profile was profoundly altered in M. tuberculosis-infected CD137L-deficient mice with lower levels of TNF-α, IL-12 and IL-6 and elevated concentrations of IL-17 compared to their wild type counterparts. However and surprisingly, these tangible immunological disorders translated only into a mild and transient increase in the bacterial loads and a higher number of granulomatous lesions with impaired architecture in the lungs of the CD137L-deficient infected mice. Together, while our data support the engagement of the CD137L signaling pathway during M. tuberculosis infection, they underscore the functional redundancy and robustness of the host defense arsenal deployed against mycobacterial infection.

CD137 expressed on neutrophils plays dual roles in antibacterial responses against Gram-positive and Gram-negative bacterial infections

Severe sepsis and septic shock caused mainly by bacterial infections are life-threatening conditions that urge the development of novel therapies. However, host responses to and pathophysiology of sepsis have not been clearly understood, which remains a major obstacle for the development of effective therapeutics. Recently, we have shown that stimulation of a costimulatory molecule, CD137, enhanced survival of mice infected with the Gram-positive (G(+)) intracellular bacterium Listeria monocytogenes but decreased survival in a polymicrobial sepsis model. Herein, we report that CD137 deficiency or blocking of CD137 signaling decreased antibacterial responses of mice infected with G(+) bacteria (Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus faecalis) but increased these responses in mice infected with Gram-negative (G(-)) bacteria (Escherichia coli, Pseudomonas aeruginosa, and Salmonella enterica serovar Typhimurium). Consistent with these findings, stimulation of CD137 by administration of agonistic antibody enhanced responses against G(+) bacteria, whereas it decreased these responses against G(-) bacteria. Neutrophils were responsible for CD137-mediated opposite roles in control of G(+) and G(-) bacterial infections. Stimulation of CD137 enhanced activities of neutrophils against S. aureus but decreased these activities against E. coli, while CD137 blocking produced opposite results with the stimulation of CD137 in vivo and in vitro. Furthermore, we found that combined signaling of CD137 and Toll-like receptor 2 (TLR2) induced synergistic production of tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6) by neutrophils, but combined signaling of CD137 and TLR4 did not. Our data strongly suggest that CD137 may play a dual role in sepsis in association with TLRs.

Systemic 4-1BB activation induces a novel T cell phenotype driven by high expression of Eomesodermin

4-1BB agonist antibody treatment induces a population of KLRG1(+) T cells that infiltrate melanoma tumors. We investigated the origin and function of these cells, as well as their place within established T cell paradigms. We find that these T cells, particularly the CD4 lineage, represent a novel phenotype characterized by enhanced, multipotent cytotoxicity. Distinct from described polarities, this T cell phenotype is driven by the T-box transcription factor Eomesodermin. Formation of this phenotype requires 4-1BB signaling on both T and antigen-presenting cells and the resulting production of the cytokines IL-27, IL-15, and IL-10. Furthermore, we find CD4(+) T cells bearing the signature features of this phenotype in the livers of mice infected with both bacterial and viral intracellular pathogens, suggesting a role for these cells in infectious immunity. These T cells constitute a novel phenotype that resolves multiple questions associated with 4-1BB activation, including how 4-1BB enhances tumor-specific cytotoxicity and how 4-1BB can promote tumor immunity while repressing autoimmunity.

Effects of Acer okamotoanum sap on the function of polymorphonuclear neutrophilic leukocytes in vitro and in vivo

Sap is a plant fluid that primarily consists of water and small amounts of mineral elements, sugars, hormones and other nutrients. Acer mono (A. mono) is an endemic Korean mono maple which was recently suggested to have health benefits due to its abundant calcium and magnesium ion content. In the present study, we examined the effects of sap from Acer okamotoanum (A. okamotoanum) on the phagocytic response of mouse neutrophils in vivo and rat and canine neutrophils in vitro. We tested the regulation of phagocytic activity, oxidative burst activity (OBA) and the levels of filamentous polymeric actin (F-actin) in the absence and presence of dexamethasone (DEX) in vitro and in vivo. Our results showed that DEX primarily reduced OBA in the mouse neutrophils, and that this was reversed in the presence of the sap. By contrast, the phagocytic activity of the mouse cells was not regulated by either DEX or the sap. Rat and canine polymorphonuclear neutrophilic leukocytes (PMNs) responded in vitro to the sap in a similar manner by increasing OBA. However, regulation of phagocytic activity by the sap was different between the species. In canine PMNs, phagocytic activity was enhanced by the sap at a high dose, while it did not significantly modulate this activity in rat PMNs. These findings suggest that the sap of A. okamotoanum stimulates neutrophil activity in the mouse, rat and canine by increasing OBA in vivo and in vitro, and thus may have a potential antimicrobial effect in the PMNs of patients with infections.

Regulation of Inflammation by Bidirectional Signaling through CD137 and Its Ligand

Although the majority of research on CD137 has been directed to T cells, it is becoming clear that this molecule has distinct functions in other lineages of cells, including non-hematopoietic cells. In particular, emerging evidence suggests that the CD137-its ligand (CD137L) network involving immune cells and non-immune cells, directly or indirectly regulates inflammation in both positive and negative manners. Bidirectional signaling through both CD137 and CD137L is critical in the evolution of inflammation: 1) CD137L signaling plays an indispensible role in the activation and recruitment of neutrophils by inducing the production of proinflammatory cytokines and chemokines in hematopoietic and non-hematopoietic cells such as macrophages, endothelial cells and epithelial cells; 2) CD137 signaling in NK cells and T cells is required for their activation and can influence other cells participating in inflammation via either their production of proinflammatory cytokines or engagement of CD137L by their cell surface CD137: 3) CD137 signaling can suppress inflammation by controlling regulatory activities of dendritic cells and regulatory T cells. As recognition grows of the role of dysregulated CD137 or CD137L stimulation in inflammatory diseases, significant efforts will be needed to develop antagonists to CD137 or CD137L.

T-cell intrinsic effects of GITR and 4-1BB during viral infection and cancer immunotherapy

GITR [glucocorticoid inducible tumor necrosis factor receptor (TNFR)-related protein] and 4-1BB are costimulatory TNFR family members that are expressed on regulatory and effector T cells as well as on other cells of the immune system. Here we discuss the role of GITR and 4-1BB on T cells during viral infections and in cancer immunotherapy. Systemic treatment with agonistic anti-4-1BB antibody leads to a number of immune system abnormalities, and clinical trials of anti-4-1BB have been terminated. However, other modes of 4-1BB ligation may be less toxic. To date, similar toxicities have not been reported for anti-GITR treatment of mice, although anti-GITR antibodies can exacerbate mouse autoimmune models. Intrinsic effects of GITR and 4-1BB on effector T cells appear to predominate over their effects on other cell types in some models. Despite their similarities in enhancing T-cell survival, 4-1BB and GITR are clearly not redundant, and both pathways are required for maximal CD8(+) T-cell responses and mouse survival following severe respiratory influenza infection. GITR uses TNFR-associated factor (TRAF) 2 and TRAF5, whereas 4-1BB recruits TRAF1 and TRAF2 to mediate survival signaling in T cells. The differential use of signaling adapters combined with their differential expression may explain the non-redundant roles of GITR and 4-1BB in the immune system.

Expression of CD137 in the cerebral artery after experimental subarachnoid hemorrhage in rats: a pilot study

Inflammation and immunity play a crucial role in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage (SAH). CD137 is recognized as an independent costimulatory molecule of T cells and activator of monocytes. A growing body of evidence indicates that CD137 is vital for inflammation and immunity. Therefore, this study aimed to investigate the expression of CD137 in the basilar artery in a rat SAH model and to clarify the potential role of CD137 in cerebral vasospasm. A total of 107 rats were randomly divided into four groups: control group; day 3, day 5, and day 7 groups. Day 3, day 5, and day 7 groups were all SAH groups. The animals in SAH groups were subjected to injection of autologous blood into cisterna magna twice on day 0 and day 2 and were sacrificed on days 3, 5, and 7, respectively. Cross-sectional area of basilar artery was measured and the CD137 expression was assessed by quantitative real-time PCR, Western blot and immunohistochemistry. The cross-sectional area of basilar artery was found to be 57,944±5581μm(2) in control group, 26,100±2639μm(2) in day 3, 19,723±2412μm(2) in day 5, and 28,800±2980μm(2) in day 7 group, respectively. The basilar artery exhibited vasospasm after SAH and became more severe on day 5. The elevated mRNA and protein of CD137 were detected after SAH and peaked on day 5. CD137 is increasingly expressed in a parallel time course to the development of cerebral vasospasm in a rat experimental model of SAH. These findings indicate the possible role of CD137 in the pathogenesis of cerebral vasospasm after SAH.

Regulation of mouse 4-1BB expression: multiple promoter usages and a splice variant

The expression of 4-1BB has been known to be dependent on T cell activation. Recent studies have, however, revealed that 4-1BB expression is not restricted to T cells. We sought to determine the molecular basis for the differential gene expression. Here we report the expression pattern of two mouse 4-1BB transcripts, type I and type II. Whereas the type I transcript was specifically expressed on immune organ as previously reported, the type II transcript was ubiquitously expressed in tissues and various cell lines. However, both type I and type II transcript were highly induced on activated T cells. Primer extension assay of the two 4-1BB transcripts suggested that mouse 4-1BB had more than two transcripts. Using luciferase assay we have identified three promoter regions (PI, PII and PIII), which located on upstream region of second exon 1, first exon 1, and exon 2, respectively. In particular, the type I transcript was preferentially induced when naïve T cells are stimulated by anti-CD3 monoclonal antibody (mAb) since NF-κB specifically binds to the putative NF-κB element of PI. We have also shown that a splice variant, in which the transmembrane domain was deleted, could inhibit 4-1BB signaling. The splicing variant was highly induced by TCR stimulation. Our results reveal 4-1BB also has a negative regulation system through soluble 4-1BB produced from a splice variant induced under activation conditions.

CD137 is required for M cell functional maturation but not lineage commitment

Mucosal immune surveillance depends on M cells that reside in the epithelium overlying Peyer's patch and nasopharyngeal associated lymphoid tissue to transport particles to underlying lymphocytes. M cell development is associated with B lymphocytes in a basolateral pocket, but the interactions between these cells are poorly understood. In a cell culture model of M cell differentiation, we found lymphotoxin/tumor necrosis factor alpha induction of CD137 (TNFRSF9) protein on intestinal epithelial cell lines, raising the possibility that CD137 on M cells in vivo might interact with CD137L expressed by B cells. Accordingly, while CD137-deficient mice produced UEA-1+ M cell progenitors in nasopharyngeal associated lymphoid tissue and Peyer's patch epithelium, they showed an abnormal morphology, including the absence of basolateral B cell pockets. More important, CD137-deficient nasopharyngeal associated lymphoid tissue M cells were defective in microparticle transcytosis. Bone marrow irradiation chimeras confirmed that while induction of UEA-1+ putative M cell precursors was not CD137-dependent, full M cell transcytosis function required expression of CD137 by radioresistant stromal cells as well as by bone marrow-derived cells. These results are consistent with a two-step model of M cell differentiation, with initial CD137-independent commitment to the M cell lineage followed by a CD137-CD137L interaction of M cells with CD137-activated B lymphocytes or dendritic cells for functional maturation.

4-1BB as a therapeutic target for human disease

4-1BB (CD137) is being thought of as an attractive target for immunotherapy of many human immune diseases based on encouraging results with 4-1BB agonistic antibody treatment in mouse models of cancer, autoimmune disease, asthma and additionally as a means to improve vaccination. In this review, we will summarize the results of basic research on 4-1BB and 4-1BB immunotherapy of disease and provide some potential mechanistic insights into the many stimulatory and regulatory functions of 4-1BB.

Immune regulation by 4-1BB and 4-1BBL: complexities and challenges

SUMMARY

The tumor necrosis factor receptor family member 4-1BB plays a key role in the survival of activated and memory CD8(+) T cells. Depending on the disease model, 4-1BB can participate at different stages and influence different aspects of the immune response, likely due to the differential expression of receptor and ligand relative to other costimulatory molecules. Studies comparing mild versus severe influenza infection of mice suggest that the immune system uses inducible receptors such as 4-1BB to prolong the immune response when pathogens take longer to clear. The expression of 4-1BB on diverse cell types, evidence for bidirectional as well as receptor-independent signaling by 4-1BBL, the unexpected hyperproliferation of 4-1BB-deficient T cells, and complex effects of agonistic anti-4-1BB therapy have revealed additional roles for the 4-1BB/4-1BBL receptor/ligand pair in the immune system. In this review, we discuss these diverse roles of 4-1BB and its ligand in the immune response, exploring possible mechanisms for the observed complexities and implications for therapeutic applications of 4-1BB/4-1BBL.

TNFR and LTbetaR agonists induce follicle-associated epithelium and M cell specific genes in rat and human intestinal epithelial cells

M cells assist mucosal immune surveillance by transcytosis of particles to underlying lymphoid tissue, but the mechanisms of M cell differentiation are poorly understood. To develop a better defined cell culture model of M cell differentiation, we treated human (Caco-2BBe) and rat (IEC-6) intestinal epithelial cell lines with lymphotoxin beta receptor (LTbetaR) and TNF receptor (TNFR) agonists. Treated cells were studied for regulation of genes associated with M cell and follicle-associated epithelium (FAE). We found that LTbetaR and TNFR agonists induce transcription of FAE-specific genes (Ccl20 and Lamb3) in Caco2-BBe cells and IEC-6 cells as well as rodent M cell specific genes such as Sgne-1/Scg5, Cldn4, and Gp2. The cytokines have distinct but complementary effects; TNFR agonists mainly induced FAE-specific genes, while the LTbetaR agonist induced M cell specific genes. The combination of cytokines showed additive induction of the FAE-associated Ccl20, Lamb3 and a surprising induction of CD137/Tnfrsf9. On the other hand TNF agonists appeared to suppress expression of some LTbetaR-induced genes. Functionally, cytokine treatment led to the reorganization of microvilli and Claudin-4 redistribution. These studies suggest complex interactions between these cytokines in the context of either inflammation or tissue differentiation.

CD137-CD137 Ligand Interactions in Inflammation

The main stream of CD137 studies has been directed to the function of CD137 in CD8⁺ T-cell immunity, including its anti-tumor activity, and paradoxically the immunosuppressive activity of CD137, which proves to be of a great therapeutic potential for animal models of a variety of autoimmune and inflammatory diseases. Recent studies, however, add complexes to the biology of CD137. Accumulating is evidence supporting that there exists a bidirectional signal transduction pathway for the CD137 receptor and its ligand (CD137L). CD137/CD137L interactions are involved in the network of hematopoietic and nonhematopoietic cells in addition to the well characterized antigen-presenting cell-T cell interactions. Signaling through CD137L plays a critical role in the differentiation of myeloid cells and their cellular activities, suggesting that CD137L signals trigger and sustain inflammation. The overall consequence might be that the amplified inflammation by CD137L enhances the T-cell activity together with CD137 signals by upregulating costimulatory molecules, MHC molecules, cell adhesion molecules, cytokines, and chemokines. Solving this outstanding issue is urgent and will have an important clinical implication.

Blockade of CD137 signaling counteracts polymicrobial sepsis induced by cecal ligation and puncture

Sepsis, a leading cause of death worldwide, involves proinflammatory responses and inefficient bacterial clearance. Previously, we have shown that CD137 (4-1BB), a member of the tumor necrosis factor receptor superfamily, plays critical roles in eradicating infective Listeria monocytogenes, a gram-positive bacterium, and that stimulation of CD137 protects mice from sepsis-induced death. In this study, we unexpectedly found that CD137 activation aggravated polymicrobial sepsis due to mixed gram-positive and gram-negative bacterial infection induced by cecal ligation and puncture (CLP). CD137-deficient (CD137(-/-)) mice showed significantly lower mortality than CD137-sufficient (CD137(+/+)) mice in the CLP model. Administration of an agonistic anti-CD137 monoclonal antibody (MAb) to CD137(+/+) mice decreased their survival in this infection model, while administration of a blocking anti-CD137 ligand MAb (TKS-1) to such mice increased their survival. CD137(-/-) mice and TKS-1-treated CD137(+/+) mice had lower levels of chemokines/proinflammatory cytokines (monocyte chemoattractant protein 1, interleukin-6 [IL-6], tumor necrosis factor alpha, IL-12) and an anti-inflammatory cytokine (IL-10), exhibited improved bacterial clearance in the peritoneum, liver, and blood, and had greater numbers of infiltrated peritoneal neutrophils and macrophages in the CLP model than control mice. Our data suggest that CD137 activation aggravates polymicrobial sepsis induced by CLP.

The role of TNF superfamily members in T-cell function and diseases

Interactions that occur between several tumour necrosis factor (TNF)-TNF receptors that are expressed by T cells and various other immune and non-immune cell types are central to T-cell function. In this Review, I discuss the biology of four different ligand-receptor interactions - OX40 ligand and OX40, 4-1BB ligand and 4-1BB, CD70 and CD27, and TL1A and death receptor 3 - and their potential to be exploited for therapeutic benefit. Manipulating these interactions can be effective for treating diseases in which T cells have an important role, including inflammatory conditions, autoimmunity and cancer. Here, I explore how blocking or inducing the signalling pathways that are triggered by these different interactions can be an effective way to modulate immune responses.

Stimulation of the molecule 4-1BB enhances host defense against Listeria monocytogenes infection in mice by inducing rapid infiltration and activation of neutrophils and monocytes

The tumor necrosis factor receptor family molecule 4-1BB (CD137) has diverse roles in adaptive and innate immune responses. However, little is known of its role in bacterial infections. Previously, we showed that 4-1BB-deficient mice have enhanced susceptibility to Listeria monocytogenes infection, and mice pretreated with agonistic anti-4-1BB antibody (3E1) were much more resistant to L. monocytogenes infection than mice treated with control antibody. In this study, we report that stimulating 4-1BB by administering 3E1 in the early phase of L. monocytogenes infection is critical for promoting the survival of mice by inducing rapid infiltration of neutrophils and monocytes into L. monocytogenes-infected livers. The levels of tumor necrosis factor alpha, interleukin 6, and monocyte chemoattractant protein 1 in the livers of 3E1-treated mice increased as early as 30 min postinfection and peaked by 1 to 2 h, while those in mice treated with control antibody started to increase only at 16 h postinfection. Monocytes and neutrophils from the 3E1-treated mice had higher levels of activation markers, phagocytic activity, and reactive oxygen species than those from control mice. In vitro stimulation of 4-1BB induced the production of the inflammatory cytokines/chemokines of neutrophils, but not those of monocytes. These results suggest that 4-1BB stimulation of neutrophils in the early phase of L. monocytogenes infection causes rapid production of inflammatory cytokines/chemokines and that the subsequent infiltration of neutrophils and monocytes is crucial for eliminating the infecting L. monocytogenes.

Identification of regulatory functions for 4-1BB and 4-1BBL in myelopoiesis and the development of dendritic cells

The costimulatory molecule 4-1BB and its ligand 4-1BBL can control adaptive immunity, but here we show that their interaction also suppressed myelopoiesis. We found that 4-1BBL was expressed on hematopoietic stem cells, differentiating common myeloid progenitors and granulocyte-macrophage progenitors, and 4-1BB was inducible on activated myeloid progenitors. Steady-state numbers of granulocyte-macrophage progenitors, myeloid-lineage cells and mature dendritic cells were higher in 4-1BB- and 4-1BBL-deficient mice, indicative of a negative function, and we confirmed that result with bone marrow chimeras and in vitro, where the absence of interactions between 4-1BB and 4-1BBL led to enhanced differentiation into dendritic cell lineages. The regulatory activity was mediated by 4-1BBL, with binding by 4-1BB inhibiting differentiation of myeloid progenitors. Thus, 4-1BB and 4-1BBL have a previously unknown function in limiting myelopoiesis and the development of dendritic cells.

Cellular immune response to an engineered cell-based tumor vaccine at the vaccination site

The engineered expression of the immune co-stimulatory molecules CD80 and CD137L on the surface of a neuroblastoma cell line converts this tumor into a cell-based cancer vaccine. The mechanism by which this vaccine activates the immune system was investigated by capturing and analyzing immune cells responding to the vaccine cell line embedded in a collagen matrix and injected subcutaneously. The vaccine induced a significant increase in the number of activated CD62L(-) CCR7(-) CD49b(+) CD8 effector memory T cells captured in the matrix. Importantly, vaccine responsive cells could be detected in the vaccine matrix within a matter of days as demonstrated by IFN-gamma production. The substitution of unmodified tumor cells for the vaccine during serial vaccination resulted in a significant decrease in activated T cells present in the matrix, indicating that immune responses at the vaccine site are a dynamic process that must be propagated by continued co-stimulation.

CD8+ T cell dysfunction and increase in murine gammaherpesvirus latent viral burden in the absence of 4-1BB ligand

Studies of costimulatory receptors belonging to the TNFR family have revealed their diverse roles in affecting different stages of the T cell response. The 4-1BB ligand (4-1BBL)/4-1BB pathway has emerged as a receptor-ligand pair that impacts not the initial priming, but later phases of the T cell response, such as sustaining clonal expansion and survival, maintaining memory CD8(+) T cells, and supporting secondary expansion upon Ag challenge. Although the role of this costimulatory pathway in CD8(+) T cell responses to acute viral infections has been well-studied, its role in controlling chronic viral infections in vivo is not known to date. Using the murine gammaherpesvirus-68 (MHV-68) model, we show that 4-1BBL-deficient mice lack control of MHV-68 during latency and show significantly increased latent viral loads. In contrast to acute influenza infection, the numbers of MHV-68-specific memory CD8(+) T cells were maintained during latency. However, the virus-specific CD8(+) T cells showed defects in function, including decreased cytolytic function and impaired secondary expansion. Thus, 4-1BBL deficiency significantly affects the function, but not the number, of virus-specific CD8(+) T cells during gammaherpesvirus latency, and its absence results in an increased viral burden. Our study suggests that the 4-1BB costimulatory pathway plays an important role in controlling chronic viral infections.

Functional Dichotomy between OX40 and 4-1BB in Modulating Effector CD8 T Cell Responses

Members of the TNFR family are thought to deliver costimulatory signals to T cells and modulate their function and survival. In this study, we compare the role of two closely related TNFR family molecules, OX40 and 4-1BB, in generating effector CD8 T cells to Ag delivered by adenovirus. OX40 and 4-1BB were both induced on responding naive CD8 T cells, but 4-1BB exhibited faster and more sustained kinetics than OX40. OX40-deficient CD8 T cells initially expanded normally; however, their accumulation and survival at late times in the primary response was significantly impaired. In contrast, 4-1BB-deficient CD8 T cells displayed hyperresponsiveness, expanding more than wild-type cells. The 4-1BB-deficient CD8 T cells also showed enhanced maturation attributes, whereas OX40-deficient CD8 T cells had multiple defects in the expression of effector cell surface markers, the synthesis of cytokines, and in cytotoxic activity. These results suggest that, in contrast to current ideas, OX40 and 4-1BB can have a clear functional dichotomy in modulating effector CD8 T cell responses. OX40 can positively regulate effector function and late accumulation/survival, whereas 4-1BB can initially operate in a negative manner to limit primary CD8 responses.