4-1BB ligand stimulation enhances myeloid dendritic cell maturation from human umbilical cord blood CD34+ progenitor cells

The Murine CD137/CD137 Ligand Signalosome: A Signal Platform Generating Signal Complexity

CD137, a member of the TNFR family, is a costimulatory receptor, and CD137L, a member of the TNF family, is its ligand. Studies using CD137- and CD137L-deficient mice and antibodies against CD137 and CD137L have revealed the diverse and paradoxical effects of these two proteins in various cancers, autoimmunity, infections, and inflammation. Both their cellular diversity and their spatiotemporal expression patterns indicate that they mediate complex immune responses. This intricacy is further enhanced by the bidirectional signal transduction events that occur when these two proteins interact in various types of immune cells. Here, we review the biology of murine CD137/CD137L, particularly, the complexity of their proximal signaling pathways, and speculate on their roles in immune responses.

A Distinct Transcriptional Program in Human CAR T Cells Bearing the 4-1BB Signaling Domain Revealed by scRNA-Seq

T cells engineered to express chimeric antigen receptors (CARs) targeting CD19 have produced impressive outcomes for the treatment of B cell malignancies, but different products vary in kinetics, persistence, and toxicity profiles based on the co-stimulatory domains included in the CAR. In this study, we performed transcriptional profiling of bulk CAR T cell populations and single cells to characterize the transcriptional states of human T cells transduced with CD3ζ, 4-1BB-CD3ζ (BBζ), or CD28-CD3ζ (28ζ) co-stimulatory domains at rest and after activation by triggering their CAR or their endogenous T cell receptor (TCR). We identified a transcriptional signature common across CARs with the CD3ζ signaling domain, as well as a distinct program associated with the 4-1BB co-stimulatory domain at rest and after activation. CAR T cells bearing BBζ had increased expression of human leukocyte antigen (HLA) class II genes, ENPP2, and interleukin (IL)-21 axis genes, and decreased PD1 compared to 28ζ CAR T cells. Similar to previous studies, we also found BBζ CAR CD8 T cells to be enriched in a central memory cell phenotype and fatty acid metabolism genes. Our data uncovered transcriptional signatures related to costimulatory domains and demonstrated that signaling domains included in CARs uniquely shape the transcriptional programs of T cells.

CD137L-DCs, Potent Immune-Stimulators-History, Characteristics, and Perspectives

Dendritic cell (DC)-based immunotherapies are being explored for over 20 years and found to be very safe. Most often, granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4)-induced monocyte-derived DCs (moDCs) are being used, which have demonstrated some life-prolonging benefit to patients of multiple tumors. However, the limited clinical response and efficacy call for the development of more potent DCs. CD137L-DC may meet this demand. CD137L-DCs are a novel type of monocyte-derived inflammatory DCs that are induced by CD137 ligand (CD137L) agonists. CD137L is expressed on the surface of antigen-presenting cells, including monocytes, and signaling of CD137L into monocytes induces their differentiation to CD137L-DCs. CD137L-DCs preferentially induce type 1 T helper (Th1) cell polarization and strong type 1 CD8⁺ T cell (Tc1) responses against tumor-associated viral antigens. The in vitro T cell-stimulatory capacity of CD137L-DCs is superior to that of conventional moDCs. The transcriptomic profile of CD137L-DC is highly similar to that of in vivo DCs at sites of inflammation. The strict activation dependence of CD137 expression and its restricted expression on activated T cells, NK cells, and vascular endothelial cells at inflammatory sites make CD137 an ideally suited signal for the induction of monocyte-derived inflammatory DCs in vivo. These findings and their potency encouraged a phase I clinical trial of CD137L-DCs against Epstein-Barr virus-associated nasopharyngeal carcinoma. In this review, we introduce and summarize the history, the characteristics, and the transcriptional profile of CD137L-DC, and discuss the potential development and applications of CD137L-DC.

Transcriptional and functional characterization of CD137L-dendritic cells identifies a novel dendritic cell phenotype

The importance of monocyte-derived dendritic cells (DCs) is evidenced by the fact that they are essential for the elimination of pathogens. Although in vitro DCs can be generated by treatment of monocytes with GM-CSF and IL-4, it is unknown what stimuli induce differentiation of DCs in vivo. CD137L-DCs are human monocyte-derived DC that are generated by CD137 ligand (CD137L) signaling. We demonstrate that the gene signature of in vitro generated CD137L-DCs is most similar to those of GM-CSF and IL-4-generated immature DCs and of macrophages. This is reminiscent of in vivo inflammatory DC which also have been reported to share gene signatures with monocyte-derived DCs and macrophages. Performing direct comparison of deposited human gene expression data with a CD137L-DC dataset revealed a significant enrichment of CD137L-DC signature genes in inflammatory in vivo DCs. In addition, surface marker expression and cytokine secretion by CD137L-DCs resemble closely those of inflammatory DCs. Further, CD137L-DCs express high levels of adhesion molecules, display strong attachment, and employ the adhesion molecule ALCAM to stimulate T cell proliferation. This study characterizes the gene expression profile of CD137L-DCs, and identifies significant similarities of CD137L-DCs with in vivo inflammatory monocyte-derived DCs and macrophages.

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.

Genes specifically modulated in sensitized skins allow the detection of sensitizers in a reconstructed human skin model. Development of the SENS-IS assay

Analysis of genes modulated during the sensitization process either on mice (LLNA) or human (blisters) combined with data mining has allowed the definition of a comprehensive panel of sensitization biomarkers. This set of genes includes already identified markers such as the ARE family and others not yet associated with the sensitization process (the so-called SENS-IS gene subset). The expression of this set of genes has been measured on reconstituted human epidermis models (Episkin) exposed to various sensitizers and non-sensitizers. Fine analysis of their expression pattern indicates that it is the number of modulated genes rather than the intensity of up-regulation that correlates best with the sensitization potential of a chemical. Moreover, sensitizers that are weak inductors of ARE genes tend to be relevant modulators of the SENS-IS subset. By combining the expression data obtained with both gene subsets, it is now possible to identify a wide variety of sensitizers on a test system (in vitro reconstructed human epidermis) that is very similar to the in vivo situation and compatible with a large variety of test substance characteristics.

4-1BB ligand signaling to T cells limits T cell activation

4-1BB ligand (4-1BBL) and its receptor, 4-1BB, are both induced on T cells after activation, but little is known about the role of 4-1BBL. In this study we show that 4-1BBL can transmit signals that limit T cell effector activity under tolerogenic conditions. Cross-linking 4-1BBL inhibited IL-2 production in vitro, primarily with suboptimal TCR stimulation. Furthermore, naive 4-1BBL-deficient OT-II transgenic T cells displayed a greater conversion to effector T cells in vivo when responding to soluble OVA peptide in wild-type hosts, whereas development of Foxp3(+) regulatory T cells was not altered. A greater number of effector T cells also differentiated from naive wild-type OT-II T cells when transferred into 4-1BB-deficient hosts, suggesting that APC-derived 4-1BB is likely to trigger 4-1BBL. Indeed, effector T cells that could not express 4-1BBL accumulated in larger numbers in vitro when stimulated with 4-1BB-expressing mesenteric lymph node dendritic cells. 4-1BBL was expressed on T cells when Ag presentation was limiting, and 4-1BBL was aberrantly expressed at very high levels on T cells that could not express 4-1BB. Trans-ligation, Ab capture, and endocytosis experiments additionally showed that T cell-intrinsic 4-1BB regulated internalization of membrane 4-1BBL, implying that the strong induction of 4-1BB on T cells may counteract the suppressive function of 4-1BBL by limiting its availability. These data suggest that 4-1BBL expressed on T cells can restrain effector T cell development, creating a more favorable regulatory T cell to effector cell balance under tolerogenic conditions, and this may be particularly active in mucosal barrier tissues where 4-1BB-expressing regulatory dendritic cells present Ag.

CD137 ligand signalling induces differentiation of primary acute myeloid leukaemia cells

CD137 ligand (CD137L), a member of the tumour necrosis factor family, is expressed as a cell surface molecule. Engagement of CD137L on haematopoietic progenitor cells induces monocytic differentiation, and in peripheral monocytes CD137L signalling promotes differentiation to mature dendritic cells. We hypothesized that CD137L signalling would also induce differentiation in transformed myeloid cells. Here we show that recombinant CD137 protein, which crosslinks CD137L and initiates reverse CD137L signalling in myeloid cells, induces morphological changes (adherence, spreading), loss of progenitor markers (CD117), expression of maturation markers (CD11b, CD13) and secretion of cytokines that are indicative of myeloid differentiation. Under the influence of CD137L signalling, acute myeloid leukaemia (AML) cells acquired expression of co-stimulatory molecules (CD80, CD86, CD40), the dendritic cell marker CD83 and dendritic cell activities, enabling them to stimulate T cells. CD137L signalling induced differentiation in 71% (15 of 21) of AML samples, irrespective of French-American-British classification and CD137L expression level. However, the type of response varied with the AML subtype and patient sample. In summary, this study demonstrated that CD137L signalling induced differentiation in malignant cells of AML patients, and suggests that it may be worthwhile to investigate treatment with recombinant CD137 protein as a potential novel therapeutic approach for AML.

CD137L-stimulated dendritic cells are more potent than conventional dendritic cells at eliciting cytotoxic T-cell responses

Dendritic cells (DCs) are highly potent initiators of adaptive immune responses and, as such, represent promising tools for immunotherapeutic applications. Despite their potential, the current efficacy of DC-based immunotherapies is poor. CD137 ligand (CD137L) signaling has been used to derive a novel type of DCs from human peripheral blood monocytes, termed CD137L-DCs. Here, we report that CD137L-DCs induce more potent cytotoxic T-cell responses than classical DCs (cDCs). Furthermore, in exploring several DC maturation factors for their ability to enhance the potency of CD137L-DCs, we found the combination of interferon γ (IFNγ) and the mixed Toll-like receptor (TLR)7/8 agonist R848, to display the highest efficacy in potentiating the T-cell co-stimulatory activity of CD137L-DCs. Of particular importance, CD137L-DCs were found to be more efficient than cDCs in activating autologous T cells targeting the cytomegalovirus (CMV)-derived protein pp65. Specifically, CD137L-DC-stimulated T cells were found to secrete higher levels of IFNγ and killed 2-3 times more HLA-matched, pp65-pulsed target cells than T cells activated by cDCs. Finally, in addition to stimulating CD8⁺ T cells, CD137L-DCs efficiently activated CD4⁺ T cells. Taken together, these findings demonstrate the superior potency of CD137L-stimulated DCs in activating CMV-specific, autologous T cells, and encourage the further development of CD137L-DCs for antitumor immunotherapy.

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.

Regulation of granulocyte and macrophage populations of murine bone marrow cells by G-CSF and CD137 protein

Background

Granulocytes and monocytes/macrophages differentiate from common myeloid progenitor cells. Granulocyte colony-stimulating factor (G-CSF) and CD137 (4-1BB, TNFRSF9) are growth and differentiation factors that induce granulocyte and macrophage survival and differentiation, respectively. This study describes the influence of G-CSF and recombinant CD137-Fc protein on myelopoiesis.

Methodology/Principal Findings

Both, G-CSF and CD137 protein support proliferation and survival of murine bone marrow cells. G-CSF enhances granulocyte numbers while CD137 protein enhances macrophage numbers. Both growth factors together give rise to more cells than each factor alone. Titration of G-CSF and CD137 protein dose-dependently changes the granulocyte/macrophage ratio in bone marrow cells. Both factors individually induce proliferation of hematopoietic progenitor cells (lin^-, c-kit⁺) and differentiation to granulocytes and macrophages, respectively. The combination of G-CSF and CD137 protein further increases proliferation, and results in a higher number of macrophages than CD137 protein alone, and a lower number of granulocytes than G-CSF alone demonstrating that CD137 protein-induced monocytic differentiation is dominant over G-CSF-induced granulocytic differentiation. CD137 protein induces monocytic differentiation even in early hematopoietic progenitor cells, the common myeloid progenitors and the granulocyte macrophage progenitors.

Conclusions/Significance

This study confirms earlier data on the regulation of myelopoiesis by CD137 receptor - ligand interaction, and extends them by demonstrating the restriction of this growth promoting influence to the monocytic lineage.

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.

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.

Expression of 4-1BB and 4-1BBL in thymocytes during thymus regeneration

4-1BB, a member of the tumor necrosis factor receptor (TNFR) superfamily, is a major costimulatory receptor that is rapidly expressed on the surface of CD4(+) and CD8(+) T cells after antigen- or mitogen-induced activation. The interaction of 4-1BB with 4-1BBL regulates immunity and promotes the survival and expansion of activated T cells. In this study, the expression of 4-1BB and 4-1BBL was examined during regeneration of the murine thymus following acute cyclophosphamide- induced involution. Four-color flow cytometry showed that 4-1BB and 4-1BBL were present in the normal thymus and were preferentially expressed in the regenerating thymus, mainly in CD4(+)CD8(+) double-positive (DP) thymocytes. Furthermore, the CD4(lo)CD8(lo), CD4(+)CD8(lo) and CD4(lo)CD8(+) thymocyte subsets, representing stages of thymocyte differentiation intermediate between DP and single-positive (SP) thymocytes, also expressed 4-1BB and 4-1BBL during thymus regeneration but to a lesser degree. Interestingly, the 4-1BB and 4-1BBL positive cells among the CD4(+)CD8(+) DP thymocytes present during thymus regeneration were TCR(hi) and CD69(+) unlike the corresponding controls. Moreover, the 4-1BB and 4-1BBL positive cells among the intermediate subsets present during thymus regeneration also exhibited TCR(hi/int+) and CD69(+/int) phenotypes, indicating that 4-1BB and 4-1BBL are predominantly expressed by the positively selected population of the CD4(+)CD8(+) DP and the intermediate thymocytes during thymus regeneration. RT-PCR and Western blot analyses confirmed the presence and elevated levels of 4-1BB and 4-1BBL mRNA and protein in thymocytes during thymus regeneration. We also found that the interaction of 4-1BB with 4-1BBL promoted thymocyte adhesion to thymic epithelial cells. Our results suggest that 4-1BB and 4-1BBL participate in T lymphopoiesis associated with positive selection during recovery from acute thymic involution.

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.

CD137 ligand reverse signaling has multiple functions in human dendritic cells during an adaptive immune response

T cell activation via dendritic cells (DC) is an important step in the adaptive immune response, which requires DC maturation, migration to lymph nodes and presentation of antigen to T cells. CD137 receptor expressed on activated T cells is a potent costimulatory molecule. Here, we investigated the functions of CD137 ligand (CD137L) in human monocyte-derived DC during an immune response. Cross-linking of CD137L on DC leads to cell maturation in an autocrine fashion, mostly via release of TNF-alpha. Reverse signaling of CD137L also mediates migration of DC via up-regulation of the CCR7 chemokine receptor, demonstrated by an in vivo MIP-3beta-dependent SCID mouse migration model. Finally, CD137L-activated DC induce differentiation of human T cells into potent Th1 effectors. Cocultivation of autologous T cells and CD137L-activated DC in an antigen-specific reaction leads to T cell proliferation and the release of IL-12p70 and IFN-gamma. These findings deliver new insights into the multiple effects of reverse signaling of CD137L in human DC during the initiation of an adaptive immune response, including the key features of DC maturation, migration and, ultimately, antigen-specific T cell differentiation.

Absence of 4-1BB increases cell influx into the peritoneal cavity in response to LPS stimulation by decreasing macrophage IL-10 levels

Peritoneal injection of lipopolysaccharide (LPS) increased the influx of polymorphonuclear leukocytes and macrophages into the peritoneal cavity (PEC), with significantly higher cell numbers in the 4-1BB-deficient (KO) mice than in wild-type (WT) mice. The peritoneal macrophages of KO mice contained less IL-10 transcripts and protein than those of WT after LPS treatment, and immobilization of 4-1BB-Fc increased the level of IL-10. Injection of IL-10 resulted in lower cell numbers into the PEC of KO mice, suggesting that lower level of IL-10 is responsible for stimulated cell influx in KO mice due to lack of 4-1BB and 4-1BBL interaction.

Soluble PD-1 Facilitates 4-1BBL–Triggered Antitumor Immunity against Murine H22 Hepatocarcinoma In vivo

PURPOSE

The use of costimulatory molecules targeting distinct T-cell signaling pathways has provided a means for triggering and enhancing antitumor immunity; however, it is still not fully understood what types of costimulatory molecules are suitable for the combination in tumor therapy. Our purpose in this study is to establish an effective antitumor immune approach by using costimulatory molecule 4-1BBL in combination with soluble PD-1.

EXPERIMENTAL DESIGN

The murine H22 hepatocarcinoma served as an ectopic tumor model. Local gene transfer was done by injection with naked plasmid p4-1BBL and/or psPD-1. The synergistic mechanism of dual-gene therapy was elucidated by detecting the change of gene expression of immunoregulatory factors in tumor microenvironment. The effects of immunotherapy were evaluated by testing the function of tumor-specific T cells, measuring tumor weight or volume, survival of mice, and H&E staining of tissues.

RESULTS

4-1BBL expressed by normal nonimmune cells effectively enhanced antitumor immune response but up-regulated PD-L1 and did not reduce IL-10 and transforming growth factor-beta (TGF-beta). sPD-1 synergized with 4-1BBL to establish efficient antitumor immune environment, including down-regulation of IL-10 and TGF-beta, further up-regulation of interleukin (IL)-2 and IFN-gamma, and higher CD8(+) T-cell infiltration. The combined treatment by 4-1BBL/sPD-1 eradicated tumors from mice with small amounts of preexistent tumor cells or tumors from approximately 60% of individuals with larger amounts of preexistent tumor cells.

CONCLUSIONS

Our findings in this report imply a great potential of 4-1BBL in combination with sPD-1 in tumor therapeutics with the in vivo existent tumor cells as antigens.

CD137 is expressed on blood vessel walls at sites of inflammation and enhances monocyte migratory activity

The cytokine receptor CD137 is a member of the TNF receptor family and a potent T cell costimulatory molecule. Its ligand is expressed on antigen presenting cells as a transmembrane protein and it too can deliver signals into the cells it is expressed on (reverse signaling). In monocytes, immobilized CD137 protein induces activation, prolongation of survival and proliferation. Here we show that recombinant immobilized CD137 protein enhances migration of monocytes in vitro. Further, CD137 expression on spheroids leads to a significantly enhanced infiltration by monocytes. The migration-inducing activity of CD137 could be confirmed in vivo. Matrigel, which was coated with recombinant CD137 protein and was inserted into the flanks of mice attracted large numbers of monocytes and was heavily infiltrated by these cells. In vivo, expression of CD137 by blood vessel walls at sites of inflammation was detectable by immunohistochemistry. CD137 expression is inducible by proinflammatory cytokines in endothelial cells, suggesting that a physiological function of CD137 may be the facilitation of monocyte extravasation in inflammatory tissues.

Effects of mesenchymal stem cells on differentiation, maturation, and function of human monocyte-derived dendritic cells

Mesenchymal stem cells (MSCs) reportedly inhibit the mixed lymphocyte reaction. Whether this effect is mediated by dendritic cells (DCs) is still unknown. In this study, we used an in vitro model to observe the effects of MSCs and their supernatants on the development of monocyte-derived DCs. Phenotypes and the endocytosic ability of harvested DCs were determined by flow cytometry; interleukin 12 (IL-12) secreted by DCs was evaluated by enzyme-linked immunosorbent assay (ELISA); and the antigen-presenting function of DCs was evaluated by MLR. Our results show that MSCs inhibit the up-regulation of CD1a, CD40, CD80, CD86, and HLA-DR during DC differentiation and prevent an increase of CD40, CD86, and CD83 expression during DC maturation. MSCs supernatants had no effect on DCs differentiation, but they inhibited the up-regulation of CD83 during maturation. Both MSCs and their supernatants interfered with endocytosis of DCs, decreased their capacity to secret IL-12 and activate alloreactive T cells. Thus, effects of MSCs on DCs contribute to immunoregulation and development.

Impaired maturation and altered regulatory function of plasmacytoid dendritic cells in multiple sclerosis

Plasmacytoid dendritic cells (pDCs) represent a DC subtype that exerts divergent functions in innate and adoptive immunity including the immediate reaction to microbial factors and the induction of immunoregulatory responses. It is thought that different DC subtypes may be critically involved in the pathogenesis of multiple sclerosis (MS). In our study we assessed the phenotype, maturation and functional properties of peripheral blood pDCs from 35 clinically stable, untreated multiple sclerosis patients, 30 healthy controls and 9 patients with pneumonia, which was used as a non-specific inflammatory condition (NIC). Ex vivo expression of CD86 and 4-1BBL was significantly lower on pDCs from multiple sclerosis patients than from controls and patients with NIC (22 versus 47 versus 41% and 12 versus 35 versus 32%, respectively). When stimulated with IL-3 and CD40L, pDCs of multiple sclerosis patients showed inefficient maturation as demonstrated by significantly lower or delayed upregulation of CD86, 4-1BBL, CD40 and CD83. Additionally, in multiple sclerosis, stimulation of pDCs by unmethylated cytosine-phosphate-guanosine oligodeoxynucleotides (CpG ODN) resulted in a significantly lower interferon (IFN) alpha secretion than in controls. In multiple sclerosis, but not in controls, pDCs failed to upregulate proliferative responses and IFN-gamma secretion of autologous peripheral blood mononuclear cells (PBMC) in a co-culture system. Moreover, depletion of pDCs in multiple sclerosis patients, but not in controls, had no effect on generation of CD4+Foxp3+ regulatory T cells. We also provide data showing that glatiramer acetate (GA) treatment partially restores phenotype and function of pDCs in multiple sclerosis patients. These findings suggest functional abnormalities of pDCs in these patients, which might be of importance in the understanding of the development of immune dysregulation in this disease.

A signal through 4-1BB ligand inhibits receptor for activation of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis by increasing interferon (IFN)-beta production

We tested whether any intracellular signals are transmitted through 4-1BB/CD137 ligand (4-1BBL), using a 4-1BB-Fc fusion protein and 4-1BB-deficient mice. Immobilized 4-1BB-Fc fusion protein strongly inhibited osteoclastogenesis induced by macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappaB ligand (RANKL) derived from bone marrow macrophages (BMM). Incubation of BMM with M-CSF increased 4-1BBL mRNA and surface expression of 4-1BBL protein. Cross-linking 4-1BBL with immobilized 4-1BB-Fc also dramatically reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells (MNC) derived from the BMM from 4-1BB-deficient mice, suggesting that the inhibitory effect of immobilized 4-1BB on osteoclastogenesis is due to a signal through 4-1BBL. Reverse signaling by 4-1BB-Fc increased the level of interferon (IFN)-beta in BMM and neutralization of IFN-beta reversed the inhibitory effect of immobilized 4-1BB-Fc. Inhibition of osteoclastogenesis by immobilized 4-1BB-Fc is, therefore, at least in part, due to elevation of the level of the negative regulator, IFN-beta in BMM.

Hematopoietic stem cells from NOD mice exhibit autonomous behavior and a competitive advantage in allogeneic recipients

Type 1 diabetes is a systemic autoimmune disease that can be cured by transplantation of hematopoietic stem cells (HSCs) from disease-resistant donors. Nonobese diabetic (NOD) mice have a number of features that distinguish them as bone marrow transplant recipients that must be understood prior to the clinical application of chimerism to induce tolerance. In the present studies, we characterized NOD HSCs, comparing their engraftment characteristics to HSCs from disease-resistant strains. Strikingly, NOD HSCs are significantly enhanced in engraftment potential compared with HSCs from disease-resistant donors. Unlike HSCs from disease-resistant strains, they do not require graft-facilitating cells to engraft in allogeneic recipients. Additionally, they exhibit a competitive advantage when coadministered with increasing numbers of syngeneic HSCs, produce significantly more spleen colony-forming units (CFU-Ss) in vivo in allogeneic recipients, and more granulocyte macrophage–colony-forming units (CFU-GMs) in vitro compared with HSCs from disease-resistant controls. NOD HSCs also exhibit significantly enhanced chemotaxis to a stromal cell–derived factor 1 (SDF-1) gradient and adhere significantly better on primary stroma. This enhanced engraftment potential maps to the insulin-dependent diabetes locus 9 (Idd9) locus, and as such the tumor necrosis factor (TNF) receptor family as well as ski/sno genes may be involved in the mechanism underlying the autonomy of NOD HSCs. These findings may have important implications to understand the evolution of autoimmune disease and impact on potential strategies for cure.

Modulation of cord blood CD8+ T-cell effector differentiation by TGF-β1 and 4-1BB costimulation

Transforming growth factor-β1 (TGF-β1), an immunosuppressive cytokine, inhibits cytotoxic T cell (CTL) immune responses. In contrast, 4-1BB (CD137), a costimulatory molecule in the tumor necrosis factor (TNF) receptor family, amplifies CTL-mediated antitumor immune responses. We investigated whether TGF-β1 responses could be reversed by 4-1BB costimulation during in vitro differentiation of naive CD8+ T cells into effector CTL cells. TGF-β1 potently suppressed CTL differentiation of human cord blood naive CD8+ T cells as determined by reduced induction of characteristic phenotypes of effector cells and cytotoxic activity. TGF-β1-mediated suppression of CTL differentiation was abrogated by 4-1BB costimulation but not by CD28 or another member in the TNF receptor family, CD30. 4-1BB costimulation suppressed Smad2 phosphorylation induced by TGF-β1, suggesting that 4-1BB effects were at the level of TGF-β1 signaling. 4-1BB effects on the TGF-β1-mediated suppression were enhanced by interleukin 12 (IL-12) but counteracted by IL-4; 4-1BB expression was up- or down-regulated, respectively, by IL-12 and IL-4. IL-4 was more dominant than IL-12 when both cytokines were present during 4-1BB costimulation in the presence of TGF-β1. This indicates critical roles for IL-4 and IL-12 in regulating 4-1BB effects on TGF-β1-mediated suppression. (Blood. 2005;105:274-281)

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.

4-1BB-ligand is regulated on human dendritic cells and induces the production of IL-12

Co-stimulation via 4-1BB and its ligand 4-1BB ligand (4-1BB-L) plays an important role in cytotoxic and pro-inflammatory immune responses. 4-1BB-L is generally described on activated antigen-presenting cells but there is limited information on its expression and function in human dendritic cells (DC). We herein compared purified CD1a+CD14- DC issued from monocytes or from hematopoietic progenitor cells (HPC). These DC expressed 4-1BB-L mRNA transcripts with highest cell surface levels on HPC-derived DC cultured with IL-1. Pro-inflammatory activation, particularly CD40 ligand+IL-1, up-regulated 4-1BB-L on DC. We confirmed reverse signaling via 4-1BB-L as immobilized 4-1BB in conjunction with CD40-L, enhanced IL-12beta mRNA and the secretion of IL-12 p70 in various APC, including monocytes. Altogether, DC may differ in T cell co-stimulation properties due to variable and regulated levels of 4-1BB-L. Data illustrate reciprocal stimulations between T cells and APC via up-regulated receptor/ligands and production of key cytokines that consolidate cellular immune responses.