Role of the 75-kDa tumor necrosis factor receptor: inhibition of early hematopoiesis

Role of the Interaction of Tumor Necrosis Factor-α and Tumor Necrosis Factor Receptors 1 and 2 in Bone-Related Cells

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine expressed by macrophages, monocytes, and T cells, and its expression is triggered by the immune system in response to pathogens and their products, such as endotoxins. TNF-α plays an important role in host defense by inducing inflammatory reactions such as phagocytes and cytocidal systems activation. TNF-α also plays an important role in bone metabolism and is associated with inflammatory bone diseases. TNF-α binds to two cell surface receptors, the 55kDa TNF receptor-1 (TNFR1) and the 75kDa TNF receptor-2 (TNFR2). Bone is in a constant state of turnover; it is continuously degraded and built via the process of bone remodeling, which results from the regulated balance between bone-resorbing osteoclasts, bone-forming osteoblasts, and the mechanosensory cell type osteocytes. Precise interactions between these cells maintain skeletal homeostasis. Studies have shown that TNF-α affects bone-related cells via TNFRs. Signaling through either receptor results in different outcomes in different cell types as well as in the same cell type. This review summarizes and discusses current research on the TNF-α and TNFR interaction and its role in bone-related cells.

Transmembrane TNF and Its Receptors TNFR1 and TNFR2 in Mycobacterial Infections

Tumor necrosis factor (TNF) is one of the main cytokines regulating a pro-inflammatory environment. It has been related to several cell functions, for instance, phagocytosis, apoptosis, proliferation, mitochondrial dynamic. Moreover, during mycobacterial infections, TNF plays an essential role to maintain granuloma formation. Several effector mechanisms have been implicated according to the interactions of the two active forms, soluble TNF (solTNF) and transmembrane TNF (tmTNF), with their receptors TNFR1 and TNFR2. We review the impact of these interactions in the context of mycobacterial infections. TNF is tightly regulated by binding to receptors, however, during mycobacterial infections, upstream activation signalling pathways may be influenced by key regulatory factors either at the membrane or cytosol level. Detailing the structure and activation pathways used by TNF and its receptors, such as its interaction with solTNF/TNFRs versus tmTNF/TNFRs, may bring a better understanding of the molecular mechanisms involved in activation pathways which can be helpful for the development of new therapies aimed at being more efficient against mycobacterial infections.

Current Perspectives on the Role of TNF in Hematopoiesis Using Mice With Humanization of TNF/LT System

TNF is a multifunctional cytokine with its key functions attributed to inflammation, secondary lymphoid tissue organogenesis and immune regulation. However, it is also a physiological regulator of hematopoiesis and is involved in development and homeostatic maintenance of various organs and tissues. Somewhat unexpectedly, the most important practical application of TNF biology in medicine is anti-TNF therapy in several autoimmune diseases. With increased number of patients undergoing treatment with TNF inhibitors and concerns regarding possible adverse effects of systemic cytokine blockade, the interest in using humanized mouse models to study the efficacy and safety of TNF-targeting biologics in vivo is justified. This Perspective discusses the main functions of TNF and its two receptors, TNFR1 and TNFR2, in steady state, as well as in emergency hematopoiesis. It also provides a comparative overview of existing mouse lines with humanization of TNF/TNFR system. These genetically engineered mice allow us to study TNF signaling cascades in the hematopoietic compartment in the context of various experimental disease models and for evaluating the effects of various human TNF inhibitors on hematopoiesis and other physiological processes.

Dual Role of TNF and LTα in Carcinogenesis as Implicated by Studies in Mice

Tumor necrosis factor (TNF) and lymphotoxin alpha (LTα) are two related cytokines from the TNF superfamily, yet they mediate their functions in soluble and membrane-bound forms via overlapping, as well as distinct, molecular pathways. Their genes are encoded within the major histocompatibility complex class III cluster in close proximity to each other. TNF is involved in host defense, maintenance of lymphoid tissues, regulation of cell death and survival, and antiviral and antibacterial responses. LTα, known for some time as TNFβ, has pleiotropic functions including control of lymphoid tissue development and homeostasis cross talk between lymphocytes and their environment, as well as lymphoid tissue neogenesis with formation of lymphoid follicles outside the lymph nodes. Along with their homeostatic functions, deregulation of these two cytokines may be associated with initiation and progression of chronic inflammation, autoimmunity, and tumorigenesis. In this review, we summarize the current state of knowledge concerning TNF/LTα functions in tumor promotion and suppression, with the focus on the recently uncovered significance of host-microbiota interplay in cancer development that may explain some earlier controversial results.

Necroinflammation emerges as a key regulator of hematopoiesis in health and disease

The hematopoietic system represents an organ system with an exceptional capacity for the production of mature blood cells from a small and mostly quiescent pool of hematopoietic stem cells (HSCs). This extraordinary capacity includes self-renewal but also the propensity to rapidly respond to extrinsic needs, such as acute infections, severe inflammation, and wound healing. In recent years, it became clear that inflammatory signals such as cytokines, chemokine and danger signals from pathogens (PAMPs) or dying cells (DAMPs) impact on HSCs, shaping their proliferation status, lineage bias, and repopulating ability and subsequently increasing the output of mature effector cells. However, inflammatory danger signals negatively impact on the capacity of HSCs to self-renew and to maintain their stem cell capabilities. This is evidenced in conditions of chronic inflammation where bone marrow failure may originate from HSC exhaustion. Even in hematopoietic cancers, inflammatory signals shape the phenotype of the malignant clone as exemplified by necrosome-dependent inflammation elicited during malignant transformation in acute myeloid leukemia. Accordingly, understanding the contribution of inflammatory signals, and specifically necroinflammation, to HSC integrity, HSC long-term functionality, and malignant transformation has attracted substantial research and clinical interest. In this review, we highlight recent developments and open questions at the interplay between inflammation, regulated necrosis, and HSC biology in the context of blood cell development, acute and chronic inflammation, and hematopoietic cancer.

Enhanced human hematopoietic stem and progenitor cell engraftment by blocking donor T cell-mediated TNFα signaling

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative therapy, but the large number of HSCs required limits its widespread use. Host conditioning and donor cell composition are known to affect HSCT outcomes. However, the specific role that the posttransplantation signaling environment plays in donor HSC fate is poorly understood. To mimic clinical HSCT, we injected human umbilical cord blood (UCB) cells at different doses and compositions into immunodeficient NOD/SCID/IL-2Rgc-null (NSG) mice. Surprisingly, higher UCB cell doses inversely correlated with stem and progenitor cell engraftment. This observation was attributable to increased donor cell-derived inflammatory signals. Donor T cell-derived tumor necrosis factor-α (TNFα) was specifically found to directly impair the survival and division of transplanted HSCs and progenitor cells. Neutralizing donor T cell-derived TNFα in vivo increased short-term stem and progenitor cell engraftment, accelerated hematopoietic recovery, and altered donor immune cell compositions. This direct effect of TNFα on transplanted cells could be decoupled from the indirect effect of alleviating graft-versus-host disease (GVHD) by interleukin-6 (IL-6) blockade. Our study demonstrates that donor immune cell-derived inflammatory signals directly influence HSC fate, and provides new clinically relevant strategies to improve engraftment efficiency during HSCT.

Pre-Transplantation Blockade of TNF-α-Mediated Oxygen Species Accumulation Protects Hematopoietic Stem Cells

Hematopoietic stem cell (HSC) transplantation (HSCT) for malignancy requires toxic pre-conditioning to maximize anti-tumor effects and donor-HSC engraftment. While this induces bone marrow (BM)-localized inflammation, how this BM environmental change affects transplanted HSCs in vivo remains largely unknown. We here report that, depending on interval between irradiation and HSCT, residence within lethally irradiated recipient BM compromises donor-HSC reconstitution ability. Both in vivo and in vitro we demonstrate that, among inflammatory cytokines, TNF-α plays a role in HSC damage: TNF-α stimulation leads to accumulation of reactive oxygen species (ROS) in highly purified hematopoietic stem/progenitor cells (HSCs/HSPCs). Transplantation of flow-cytometry-sorted murine HSCs reveals damaging effects of accumulated ROS on HSCs. Short-term incubation either with an specific inhibitor of tumor necrosis factor receptor 1 signaling or an antioxidant N-acetyl-L-cysteine (NAC) prevents TNF-α-mediated ROS accumulation in HSCs. Importantly, pre-transplantation exposure to NAC successfully demonstrats protective effects in inflammatory BM on graft-HSCs, exhibiting better reconstitution capability than that of nonprotected control grafts. We thus suggest that in vivo protection of graft-HSCs from BM inflammation is a feasible and attractive approach, which may lead to improved hematopoietic reconstitution kinetics in transplantation with myeloablative conditioning that inevitably causes inflammation in recipient BM. Stem Cells 2017;35:989-1002.

Immunological Basis of Bone Marrow Failure after Allogeneic Hematopoietic Stem Cell Transplantation

Bone marrow failure (BMF) syndromes are severe complications of allogeneic hematopoietic stem cell transplantation (allo-HSCT). In this paper, we distinguish two different entities, the graft failure (GF) and the poor graft function (PGF), and we review the current understanding of the interactions between the immune and hematopoietic compartments in these conditions. We first discuss how GF occurs as the result of classical alloreactive immune responses mediated by residual host cellular and humoral immunity persisting after conditioning and prevented by host and donor regulatory T cells. We next summarize the current knowledge about the contribution of inflammatory mediators to the development of PGF. In situations of chronic inflammation complicating allo-HSCT, such as graft-versus-host disease or infections, PGF seems to be essentially the result of a sustained impairment of hematopoietic stem cells (HSC) self-renewal and proliferation caused by inflammatory mediators, such as interferon-γ (IFN-γ) and tumor necrosis factor-α, and of induction of apoptosis through the Fas/Fas ligand pathway. Interestingly, the production of inflammatory molecules leads to a non-MHC restricted, bystander inhibition of hematopoiesis, therefore, representing a promising target for immunological interventions. Finally, we discuss immune-mediated impairment of bone marrow microenvironment as a potential mechanism hampering hematopoietic recovery. Better understanding of immunological mechanisms responsible for BMF syndromes after allo-HSCT may lead to the development of more efficient immunotherapeutic interventions.

Parallels between immune driven-hematopoiesis and T cell activation: 3 signals that relay inflammatory stress to the bone marrow

Quiescence, self-renewal, lineage commitment and differentiation of hematopoietic stem cells (HSCs) towards fully mature blood cells are a complex process that involves both intrinsic and extrinsic signals. During steady-state conditions, most hematopoietic signals are provided by various resident cells inside the bone marrow (BM), which establish the HSC micro-environment. However, upon infection, the hematopoietic process is also affected by pathogens and activated immune cells, which illustrates an effective feedback mechanism to hematopoietic stem and progenitor cells (HSPCs) via immune-mediated signals. Here, we review the impact of pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), costimulatory molecules and pro-inflammatory cytokines on the quiescence, proliferation and differentiation of HSCs and more committed progenitors. As modulation of HSPC function via these immune-mediated signals holds an interesting parallel with the "three-signal-model" described for the activation and differentiation of naïve T-cells, we propose a novel "three-signal" concept for immune-driven hematopoiesis. In this model, the recognition of PAMPs and DAMPs will activate HSCs and induce proliferation, while costimulatory molecules and pro-inflammatory cytokines confer a second and third signal, respectively, which further regulate expansion, lineage commitment and differentiation of HSPCs. We review the impact of inflammatory stress on hematopoiesis along these three signals and we discuss whether they act independently from each other or that concurrence of these signals is important for an adequate response of HSPCs upon infection.

TNF-α, a good or bad factor in hematological diseases?

Tumor necrosis factor-alpha (TNF-α) is a highly pleiotropic cytokine involved in a spectrum of physiological processes that control inflammation, anti-tumor responses and homeostasis through two receptors, TNF-R1 and TNF-R2. In general, TNF-R1 mediates cytotoxicity, resistance to infection and stimulation of NF-κB. By contrast, TNF-R2 has been implicated in proliferation of T-cell line, thymocytes and human mononuclear cells. Hematological malignancies are the types of cancer that affect normal hematopoiesis, have a speedy development, high lethal rate and until now still have no effective treatment. Several studies have shown that inflammatory cytokines play an important role in the onset and progress of these diseases. In this review, we summarize the recent studies and evaluate the positive or negative role of TNF-α in some hematological malignancies or diseases with a malignant tendency.

Activation and crosstalk between TNF family receptors in umbilical cord blood cells is not responsible for loss of engraftment capacity following culture

Umbilical cord blood (UCB) is a rich source of hematopoietic progenitors for transplantation. Murine and human progenitors are insensitive to apoptotic signaling mediated by the TNF family receptors, however extension of culture over 48 hours is accompanied by severe deterioration in engraftment and hematopoietic reconstituting capacity. In this study we assessed crosstalk between the Fas, TNF and TRAIL receptors, and questioned whether it contributes to increased mortality and decreased activity of UCB progenitors following extended ex vivo culture for 72 hours. The well-characterized TNF-induced expression of Fas is mediated by both TNF receptors, yet the TNF receptors determine survival rather than Fas: superior viability of TNF-R1 progenitors. Additional cross talk includes upregulation of TRAIL-R1 by Fas-ligand, mediated both by fast cycling and inductive crosstalk. These inductive interactions are not accompanied by concomitant sensitization of progenitors to receptor-mediated apoptosis during extended culture, but rather decreased fractional apoptosis in expanded progenitor subsets expressing the receptors. TRAIL upregulates both TRAIL-R1 and TRAIL-R2, accompanied by commensurate susceptibility to spontaneous apoptosis. The current data reveal inductive crosstalk between TNF family receptors, which are largely dissociated from the sensitivity of hematopoietic progenitors to apoptosis. Activation of Fas, TNF and TRAIL receptors and excessive apoptosis are not responsible for loss of engraftment and impaired reconstituting activity of UCB progenitors following extended culture.

TNF-α has tropic rather than apoptotic activity in human hematopoietic progenitors: involvement of TNF receptor-1 and caspase-8

Tumor necrosis factor-α (TNF-α) has been suggested to exert detrimental effects on hematopoietic progenitor function that might limit the success of transplants. In this study, we assessed the influences of TNF-α and its two cognate receptors on the function of fresh umbilical cord blood (UCB) and cryopreserved mobilized peripheral blood (mPB). CD34(+) progenitors from both sources are less susceptible to spontaneous apoptosis than lineage-committed cells and are not induced into apoptosis by TNF-α. Consequently, the activity of UCB-derived severe combined immune deficiency (SCID) reconstituting cells and long-term culture-initiating cells is unaffected by this cytokine. On the contrary, transient exposure of cells from both sources to TNF-α stimulates the activity of myeloid progenitors, which persists in vivo in UCB cell transplants. Progenitor stimulation is selectively mediated by TNF-R1 and involves activation of caspase-8, without redundant activity of TNF-R2. Despite significant differences between fresh UCB cells and cryopreserved mPB cells in susceptibility to apoptosis and time to activation, TNF-α is primarily involved in tropic signaling in hematopoietic progenitors from both sources. Cytokine-mediated tropism cautions against TNF-α neutralization under conditions of stress hematopoiesis and may be particularly beneficial in overcoming the limitations of UCB cell transplants.

Inflammatory signals regulate hematopoietic stem cells

Hematopoietic stem cells (HSCs) are the progenitors of all blood and immune cells, yet their role in immunity is not well understood. Most studies have focused on the ability of committed lymphoid and myeloid precursors to replenish immune cells during infection. Recent studies, however, have indicated that HSCs also proliferate in response to systemic infection and replenish effector immune cells. Inflammatory signaling molecules including interferons, tumor necrosis factor-α and Toll-like receptors are essential to the HSC response. Observing the biology of HSCs through the lens of infection and inflammation has led to the discovery of an array of immune-mediators that serve crucial roles in HSC regulation and function.

Loss of SIMPL compromises TNF-alpha-dependent survival of hematopoietic progenitors

OBJECTIVE

Emerging work has revealed an integral role of the tumor necrosis factor-alpha (TNF-alpha) nuclear factor (NF)-kappaB pathway in the regulation of hematopoiesis. TNF-alpha inhibition of hematopoietic stem/progenitor cell growth involves type I TNF-alpha receptor (TNF-RI) and type II TNF-alpha receptor (TNF-RII). However, the role of TNF-RI vs TNF-RII in mediating this response is less clear. Full induction of NF-kappaB-dependent gene expression through TNF-RI requires the transcriptional coactivator SIMPL (substrate that interacts with mouse pelle-like kinase). To address the role of SIMPL in TNF-alpha-dependent signaling in hematopoiesis, endothelial cells and hematopoietic progenitors expressing SIMPL short hairpin RNA were characterized.

MATERIAL AND METHODS

In vitro gene expression and progenitor assays employing SIMPL short hairpin RNA were used to examine the requirement for SIMPL in TNF-alpha-dependent effects upon cytokine gene expression and hematopoietic progenitor cell growth. Competitive repopulation studies were used to extend these studies in vivo.

RESULTS

SIMPL is required for full TNF-RI-dependent expression of NF-kappaB-controlled cytokines in endothelial cells. Hematopoietic progenitor cell expansion is not affected if progenitors lacked SIMPL or if progenitors are treated with human TNF-alpha, which signals through TNF-RI. In the absence of SIMPL, human TNF-alpha leads to a dramatic decrease in progenitor cell expansion that is not due to apoptosis. Loss of SIMPL does not affect the activity of transforming growth factor-beta1 and interferon-gamma, other known suppressors of hematopoiesis.

CONCLUSIONS

Suppression of myeloid progenitor cell expansion requires signaling through TNF-RI and TNF-RII. Signals transduced through the TNF-alpha-TNF-RI-SIMPL pathway support hematopoietic progenitor cell survival, growth and differentiation.

Lipopolysaccharide-induced upregulation of tumor necrosis factor-alpha (TNF-alpha) and TNF receptors in rat sciatic nerve

The proinflammatory and lipopolysaccharide (LPS)-inducible cytokine tumor necrosis factor alpha (TNF-alpha) has been shown to enhance primary sensory nociceptive signaling. However, the precise cellular sites of TNF-alpha and TNF receptors synthesis are still a matter of controversy. Therefore, we focused our study on TNF-alpha, TNFR1, and TNFR2 protein synthesis and expression patterns in sciatic nerve of controls and rats under systemic challenge with LPS. The enzyme-linked immunosorbent (ELISA) assay showed that the protein level of TNF-alpha reached peak at 6 h. Double immunofluorescence revealed that LPS-induced expression of TNF-alpha exclusively located in a subpopulation of Schwann cells, endothelial cells, and macrophages, which increased at late time point in the rat sciatic nerve. Positive staining of TNF receptors were also found in Schwann cells and a few endothelial cells. These observations have demonstrated the production of this proinflammatory cytokine by peripheral nerve glia especially Schwann cells. Synthesized TNF-alpha might directly act on peripheral nerve glia via TNF receptors, but the inherent mechanisms remain unknown. Further studies are needed to confirm the pathogenic role of tumor necrosis factor in the early stage of inflammation.

Intracellular application of TNF-alpha impairs cell to cell communication via gap junctions in glioma cells

Human gliomas are the most common class of brain neoplasm. In order to better characterize their response to inflammation, we evaluated the influence of tumor necrosis factor alpha (TNF-alpha) on the coupling behaviour and the membrane resting potential (MRP) of glioma cells (F98 glioma cell line) compared to primary astrocytes. In contrast to cultured primary astrocytes which exhibited a profound inhibition of gap junction mediated intercellular communication (GJIC), extracellular exposure of TNF-alpha to F98 glioma cells gained no effect on the functional coupling. Whereas, intracellular application of TNF-alpha into the glioma cells elicited similar effects as those found in primary astrocytes indicating a compromised accessibility of the TNF-alpha receptor in F98 cells. Western blotting, immunocytochemical staining and real time RT PCR analysis revealed a differential expression and distribution of TNF-alpha receptor 1 (TNFR1) in the glioma cells. Connexin 43 (Cx43) is the major astrocytic gap junction protein which when phosphorylated has been shown to reveal altered gating properties. Here we show that TNF-alpha increases the level of phosphorylated Cx43 in primary astrocytes but not in the F98 glioma cells. Our observations could account for the decreased regulatory effects of TNF-alpha on GJIC of F98 glioma cells.

IFN-gamma negatively modulates self-renewal of repopulating human hemopoietic stem cells

Whereas multiple growth-promoting cytokines have been demonstrated to be involved in regulation of the hemopoietic stem cell (HSC) pool, the potential role of negative regulators is less clear. However, IFN-gamma, if overexpressed, can mediate bone marrow suppression and has been directly implicated in a number of bone marrow failure syndromes, including graft-vs-host disease. Whether IFN-gamma might directly affect the function of repopulating HSCs has, however, not been investigated. In the present study, we used in vitro conditions promoting self-renewing divisions of human HSCs to investigate the effect of IFN-gamma on HSC maintenance and function. Although purified cord blood CD34(+)CD38(-) cells underwent cell divisions in the presence of IFN-gamma, cycling HSCs exposed to IFN-gamma in vitro were severely compromised in their ability to reconstitute long-term cultures in vitro and multilineage engraft NOD-SCID mice in vivo (>90% reduced activity in both HSC assays). In vitro studies suggested that IFN-gamma accelerated differentiation of targeted human stem and progenitor cells. These results demonstrate that IFN-gamma can negatively affect human HSC self-renewal.

Specific antibodies to mouse Sca-1- (Ly-6A/E) or Thy-1-positive haematopoietic progenitor cells induce formation of nitric oxide which inhibits subsequent colony formation

Mouse bone marrow cells were exposed to specific monoclonal antibodies, so that lineage positive (Lin+) cells could be removed with magnetic beads. The Lin- cells were cultured with Sca-1 or CD90 (Thy-1) monoclonal antibodies (MoAbs) in semi-solid medium for 7 d. We found that Sca-1 MoAb suppressed colony formation (20-30%), and the effect was largely abolished by N-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase. Similar results were obtained with antibodies to CD90. The findings suggest that the unknown physiological ligands to Sca-1 and Thy-1 markers on haematopoietic progenitor cells can inhibit colony formation, with NO as a pivotal mediator. Primitive progenitors may be a primary target of this Sca-1 ligand, as the Sca-1+ cell population contains the major part of the multipotent haematopoietic stem cells.

Tumor necrosis factor activates CRE-binding protein through a p38 MAPK/MSK1 signaling pathway in endothelial cells

Tumor necrosis factor (TNF) promotes immunity and modulates cell viability, in part, by promoting alterations of cellular gene expression. The mechanisms through which TNF communicates with the nucleus and alters gene expression are incompletely understood. Incubation of human umbilical vein endothelial cells (HUVEC) with TNF induces phosphorylation of the CRE-binding protein (CREB) transcription factor on serine 133 and increases CREB DNA binding and transactivation. Dominant negative CREB, an antagonist antibody directed against the type 1 TNF receptor, or pharmacological inhibition of p38 MAPK signaling blocked TNF-induced CREB activation as determined by phosphorylation and gene reporter assays. From among the kinases that can activate CREB, we found that downstream of p38 MAPK, MSK1 is activated by TNF to promote CREB activation. These observations show that CREB is activated by TNF/TNFR1 signaling through a p38MAPK/MSK1 signaling pathway.

Comparative analysis of genes regulated by PML/RAR alpha and PLZF/RAR alpha in response to retinoic acid using oligonucleotide arrays

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations involving retinoic acid receptor alpha (RAR alpha) and its fusion partners including promyelocytic leukemia (PML) and promyelocytic leukemia zinc finger (PLZF). Using oligonucleotide arrays, we examined changes in global gene expression mediated by the ectopic expression of either PML/RAR alpha (retinoid-sensitive) or PLZF/RAR alpha (retinoid-resistant) in U937 cells. Of more than 5000 genes analyzed, 16 genes were commonly up-regulated, and 57 genes were down-regulated by both fusion proteins suggesting their role in the APL phenotype. In our APL model, for example, TNFAIP2, TNFR2, ELF4, RAR gamma, and HoxA1 were down-regulated by both fusion proteins in the absence of retinoic acid (RA). RA strongly up-regulated these genes in PML/RAR alpha, but not in PLZF/RAR alpha expressing U937 cells. Expression studies in NB4, retinoid-resistant NB4-R2, normal human CD34+ cells, and APL patient samples strongly suggest their role in the regulation of granulocytic differentiation. Furthermore, combined treatment with tumor necrosis factor alpha (TNF alpha) and RA synergistically enhanced granulocytic differentiation in NB4 cells but not in NB4-R2 cells. Our data indicate that APL pathogenesis and retinoid-induced granulocytic differentiation of APL cells involve genes in the cell death pathway, and that cooperation between the RA and TNFalpha signaling pathways exists. Targeting both the retinoid-dependent differentiation and the cell death pathways may improve leukemic therapy, especially in retinoid-resistant acute myeloid leukemia.

TLR4-dependent lipopolysaccharide-induced shedding of tumor necrosis factor receptors in mouse bone marrow granulocytes

We reported previously that bone marrow granulocytes respond to small amounts of enterobacterial lipopolysaccharide (LPS) via a CD14-independent and TLR4-mediated mechanism by de novo expression of an inducible receptor (CD14) and by down-modulation of a constitutive receptor (L-selectin). In this report we address another effect of LPS: the down-regulation of receptors for tumor necrosis factor-alpha. In mouse bone marrow cells (BMC), this down-regulation is detectable soon (20 min) after exposure of the cells to low levels (0.5 ng/ml) of LPS. This temperature-dependent effect is rather selective for LPS and requires the presence of a conventional lipid A structure in the LPS molecule and a functional TLR4 molecule in the cells. The down-modulation, due to a shedding of the receptors, is blocked by p38 MAPK inhibitors, by a furin inhibitor, and by three metalloproteinase inhibitors (BB-3103, TIMP-2, and TIMP-3). In contrast, inhibitors of MEK, protein kinase C, cAMP-dependent protein kinase, and kinases of the Src family do not block the shedding. Analysis of BMC from mice lacking tumor necrosis factor receptor-1 (CD120a-/-) or tumor necrosis factor receptor-2 (CD120b-/-) indicates that the LPS-induced shedding is specific for CD120b. Thus, exposure of BMC to LPS triggers a rapid shedding of CD120b via a protein kinase C- and Src-independent pathway mediated by p38 MAPK, furin, and metalloproteinase. The additive effects of furin and metalloproteinase inhibitors suggest that these enzymes are involved in parallel shedding pathways.

Tumor necrosis factor-alpha expressed constitutively in erythroid cells or induced by erythropoietin has negative and stimulatory roles in normal erythropoiesis and erythroleukemia

Binding of erythropoietin (EPO) to its receptor (EPOR) on erythroid cells induces the activation of numerous signal transduction pathways, including the mitogen-activated protein kinase Jun-N-terminal kinase (JNK). In an effort to understand the regulation of EPO-induced proliferation and JNK activation, we have examined the role of potential autocrine factors in the proliferation of the murine erythroleukemia cell line HCD57. We report here that treatment of these cells with EPO induced the expression and secretion of tumor necrosis factor alpha (TNF-alpha). EPO-dependent proliferation was reduced by the addition of neutralizing antibodies to TNF-alpha, and exogenously added TNF-alpha induced proliferation of HCD57 cells. EPO also could induce TNF-alpha expression in BAF3 and DA3 myeloid cells ectopically expressing EPOR. Addition of TNF-alpha activated JNK in HCD57 cells, and the activity of JNK was partially inhibited by addition of a TNF-alpha neutralizing antibody. Primary human and murine erythroid progenitors expressed TNF-alpha in either an EPO-dependent or constitutive manner. However, TNF-alpha had an inhibitory effect on both immature primary human and murine cells, suggestive that the proliferative effects of TNF-alpha may be limited to erythroleukemic cells. This study suggests a novel role for autocrine TNF-alpha expression in the proliferation of erythroleukemia cells that is distinct from the effect of TNF-alpha in normal erythropoiesis.

An intrinsic thyrotropin-mediated pathway of TNF-alpha production by bone marrow cells

Recent studies have identified a role for thyroid-stimulating hormone (TSH; ie, thyrotropin) as an inductive signal for tumor necrosis factor-alpha (TNF-alpha) secretion by bone marrow (BM) cells, although the features of that activation pathway have not been defined. Using intracellular TSH staining and enzyme-linked immunoassay for detection of secreted TSH, we demonstrate that TSH synthesis in BM cells occurs within CD45(+) (leukocyte common antigen) hematopoietic cells and that the majority of that activity resides in a component of CD11b(+) BM cells that are not mature T cells, B cells, or Thy-1(+) cells in the BM. Conversely, TSH-responsive BM cells defined by expression of TSH receptor (TSHR) using flow cytometry were selectively associated with a nonerythroid CD11b(-) lymphocyte precursor population. In vitro culture of magnetic-activated cell sorted CD11b(-) and CD11b(+) cells with titrated amounts of purified TSH resulted in significantly higher levels of TNF-alpha secretion from CD11b(-) BM cells compared to non-TSH-treated cells, with no appreciable change in TNF-alpha production from CD11b(+) cells. These findings are the first to demonstrate TSH production by BM hematopoietic cells, and they demonstrate that TSH may be involved in the regulation of TNF-alpha by CD11b(-) BM cells. They also indicate that TSH-mediated regulation of TNF-alpha secretion within the BM most likely operates through an intrinsic network of TSH production and use between different types of BM cells, and they suggest that local TSH may be an important homeostatic regulator of hematopoiesis mediated by TNF-alpha.

Bone marrow B cell apoptosis during in vivo influenza virus infection requires TNF-alpha and lymphotoxin-alpha

Suppression of bone marrow myeloid and erythroid progenitor cells occurs after infection with a variety of different viruses. In this study, we characterize the alterations in bone marrow (BM) lymphocytes after influenza virus infection in mice. We found a severe loss of BM B cells, particularly CD43(low/-)B220(+) pre-B and immature B cells, in influenza virus-infected mice. Depletion of BM B lineage cells resulted primarily from cell cycle arrest and most likely apoptosis within the BM environment, rather than from increased trafficking of BM emigrants to peripheral lymphoid tissues. Use of gene-knockout mice indicates that depletion of BM B cells is dependent on TNF-alpha, lymphotoxin-alpha, and both TNF receptors, TNFR1-p55 and TNFR2-p75. Thus, TNF-alpha and lymphotoxin-alpha are required for loss of BM B lineage cells during respiratory infection with influenza virus.

Porcine hematopoiesis on primate stroma in long-term cultures: enhanced growth with neutralizing tumor necrosis factor-alpha and tumor growth factor-beta antibodies

BACKGROUND

Donor hematopoiesis is at a competitive disadvantage when bone marrow transplantation is across species barriers. This could present major limitations to xenogeneic stem cell transplantation as an approach to tolerance induction. An in vitro model of xenogeneic engraftment was established to identify inhibitors of porcine hematopoiesis in a primate environment.

METHODS

Porcine bone marrow cells (BMC), in the presence or absence of primate CD34+ positive cells, were cultured for 4-6 weeks on primate stroma with porcine cytokines. Cellularity and growth of colony-forming cells were indicators of hematopoietic growth. Effects of soluble factors were determined by using Transwell inserts to separate porcine cells from stroma. Neutralizing antibodies for human transforming growth factor-beta (TGF-beta) and tumor necrosis factor-alpha (TNF-alpha) were added to cultures.

RESULTS

Porcine hematopoiesis can be maintained in long-term cultures on primate stroma with pig cytokines. Adding BMC to the stroma below Transwell-containing porcine cells dramatically inhibited porcine hematopoiesis, showing an inhibitory role for soluble factors. Neutralizing antibodies against TNF-alpha or TGF-beta caused a modest enhancement of porcine hematopoiesis; however, the combination of both led to a substantial increase. Inhibitory effects of these cytokines were confirmed by adding TNF-alpha and TGF-beta to porcine cultures.

CONCLUSIONS

Porcine cells may be more sensitive to inhibitory effects of TNF-alpha and TGF-beta than primate cells and are at a disadvantage when in a primate environment. Potential implications of this observation are discussed in the context of establishing specific immune tolerance via mixed chimerism to facilitate xenotransplantation.

Roles of tumor necrosis factor-alpha receptor subtypes in the pathogenesis of the tristetraprolin-deficiency syndrome

Tristetraprolin (TTP) is a member of the CCCH tandem zinc-finger class of proteins. It can bind to and destabilize mRNAs encoding tumor necrosis factor-alpha (TNF-alpha) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Conversely, mice deficient in TTP develop a complex syndrome characterized by cachexia, myeloid hyperplasia, and joint and skin inflammation. Studies using anti-TNF-alpha neutralizing antibodies demonstrated that this syndrome, at least in part, is a consequence of the excess production of TNF-alpha in the absence of TTP. To evaluate the role played by each TNF-alpha receptor in the pathogenesis of this syndrome, mice were generated that were deficient in TTP and either or both of the known TNF-alpha receptors (TNFRs), type 1 (TNFR1) and type 2 (TNFR2). Mice deficient in TTP and TNFR1, or in TTP and both receptors, were protected from developing the TNF-alpha-induced cachexia and inflammation. In contrast, mice deficient in TNFR2 were more severely affected than mice deficient in TTP alone, suggesting that TNFR2 might play a protective role in the development of the syndrome. In cultured cells derived from these mice, apparent cooperation between the TNFRs was required to achieve normal TNF-alpha-induced expression of TTP, TNF-alpha, and GM-CSF mRNAs. Finally, the results showed that TNFR1 plays an important role in mediating TNF-alpha-induced changes in TNF-alpha and GM-CSF mRNA stability.

Targeting tumor necrosis factor alpha. New drugs used to modulate inflammatory diseases

Since its discovery, the understanding of the roles for TNF-alpha in human biology and disease has grown. Receptors for TNF are found on virtually all cell types, and many physiologic processes seem to be altered by TNF-alpha. The understanding of how TNF-alpha is involved in the pathophysiology of diseases, such as inflammatory diseases, has allowed the development of new drugs that can interfere with excess TNF-alpha and thus has allowed novel therapies for rheumatoid arthritis and Crohn's disease. As the role of TNF-alpha in other diseases becomes better understood, such TNF-alpha-modulating drugs may find further applications. In the skin, TNF-alpha is prominent cytokine that seems to be important in allergic and irritant contact dermatitis and inflammatory skin conditions. Modulating TNF-alpha activity in the skin may provide therapeutic benefits for a variety of skin conditions (Table 4). Tumor necrosis factor-alpha levels are elevated in skin lesions of psoriasis. A few reports have already suggested that etanercept and infliximab may offer a therapeutic effect in patients with psoriasis. Clinical studies evaluating the true efficacy of these drugs in psoriasis are under way. Specifically, the authors and others are involved in a double-blind, placebo-controlled study to assess the efficacy of etanercept for psoriasis. Thalidomide has been used off-label with some success to treat a number of dermatologic diseases, including several inflammatory skin conditions. Etanercept and infliximab might perhaps prove efficacious for inflammatory skin conditions as well. Finally, it is possible that drugs targeting TNF-alpha may have yet-unrecognized serious side effects. Because TNF-alpha seems to be a central cytokine in UVR-induced apoptosis, the chronic use of TNF-alpha-altering drugs might increase the risk for skin cancers. Tumor necrosis factor-alpha also plays some role in cutaneous wound healing; the effect these drugs might have on this process is also unknown at this time. Certainly, much is already [table: see text] known about TNF-alpha and how it plays many central roles. This understanding has allowed the development of useful new drugs for intractable disease. As the understanding of TNF-alpha and other cytokine biology increases, so will the number of potential therapeutic agents.

Self-renewal of multipotent long-term repopulating hematopoietic stem cells is negatively regulated by Fas and tumor necrosis factor receptor activation

Multipotent self-renewing hematopoietic stem cells (HSCs) are responsible for reconstitution of all blood cell lineages. Whereas growth stimulatory cytokines have been demonstrated to promote HSC self-renewal, the potential role of negative regulators remains elusive. Receptors for tumor necrosis factor (TNF) and Fas ligand have been implicated as regulators of steady-state hematopoiesis, and if overexpressed mediate bone marrow failure. However, it has been proposed that hematopoietic progenitors rather than stem cells might be targeted by Fas activation. Here, murine Lin(-)Sca1(+)c-kit(+) stem cells revealed little or no constitutive expression of Fas and failed to respond to an agonistic anti-Fas antibody. However, if induced to undergo self-renewal in the presence of TNF-alpha, the entire short and long-term repopulating HSC pool acquired Fas expression at high levels and concomitant activation of Fas suppressed in vitro growth of Lin(-)Sca1(+)c-kit(+) cells cultured at the single cell level. Moreover, Lin(-)Sca1(+)c-kit(+) stem cells undergoing self-renewal divisions in vitro were severely and irreversibly compromised in their short- and long-term multilineage reconstituting ability if activated by TNF-alpha or through Fas, providing the first evidence for negative regulators of HSC self-renewal.

Tumor necrosis factor (TNF)-mediated activation of the p55 TNF receptor negatively regulates maintenance of cycling reconstituting human hematopoietic stem cells

Hematopoietic stem cell (HSC) fate decisions between self-renewal and commitment toward differentiation are tightly regulated in vivo. Recent developments in HSC culture and improvements of human HSC assays have facilitated studies of these processes in vitro. Through such studies stimulatory cytokines critically involved in HSC maintenance in vivo have been demonstrated to also promote HSC self-renewing divisions in vitro. Evidence for negative regulators of HSC self-renewal is, however, lacking. Tumor necrosis factor (TNF), if overexpressed, has been implicated to mediate bone marrow suppression. However, whether and how TNF might affect the function of HSC with a combined myeloid and lymphoid reconstitution potential has not been investigated. In the present studies in vitro conditions recently demonstrated to promote HSC self-renewing divisions in vitro were used to study the effect of TNF on human HSCs capable of reconstituting myelopoiesis and lymphopoiesis in nonobese diabetic-severe combined immunodeficient (NOD-SCID) mice. Although all cord blood and adult bone marrow CD34(+)CD38(-) cells were capable of undergoing cell divisions in the presence of TNF, cycling HSCs exposed to TNF in vitro and in vivo were severely compromised in their ability to reconstitute NOD-SCID mice and long-term cultures. The negative effect of TNF was not dependent on the Fas pathway, and a similar effect could be observed using a mutant TNF exclusively targeting the p55 TNF receptor. TNF did not appear to enhance apoptosis or affect cell-cycle distribution of cultured progenitors, but rather promoted myeloid differentiation. Thus, TNF might regulate HSC fate by promoting their differentiation rather than self-renewal.

High blood soluble receptor p80 for tumour necrosis factor-alpha is associated with erythropoietin resistance in haemodialysis patients

BACKGROUND

Inflammation is one of the major causes of resistance to erythropoietin (rHuEpo) treatment. Tumour necrosis factor-alpha (TNF-alpha), one of the most potent proinflammatory cytokines, is known to inhibit human erythropoiesis directly in vitro. Although blood levels of soluble receptors for TNF-alpha (sTNFRs) are elevated in haemodialysis (HD) patients, the role of sTNFR for rHuEpo responsiveness in HD patients remains to be clarified.

METHODS

We measured serum sTNFR (p55 and p80) levels in 83 stable outpatients undergoing regular HD (age 62+/-1, HD duration 15+/-1 years). After dividing the patients into three groups according to rHuEpo dose: (low (L) <60, n=31; moderate (M) > or =60 to <120, n=31; high (H) > or =120 U/kg/week rHuEpo, n=21), we examined the relationship between serum sTNFR levels and the degree of renal anaemia and rHuEpo dosage.

RESULTS

Haemoglobin was significantly higher in patients receiving low rHuEpo dosage (L, 10.5+/-0.2; M, 9.7+/-0.1; H, 9.5+/-0.2 g/dl, P<0.01 vs M and H groups). There were no differences in blood TNF-alpha, sTNFR p55, C-reactive protein, albumin, ferritin, or intact parathyroid hormone levels among the three groups. Body mass index and creatinine generation rate, a marker of whole-body muscle volume, were significantly reduced in group H (P<0.01). Serum sTNFR p80 levels were significantly higher in group H (4.88+/-0.45 ng/ml) than in L (3.73+/-0.14 ng/ml) and M (3.67+/-0.21 ng/ml) groups (P<0.05). The blood interleukin (IL)-6 level was also increased in patients requiring high rHuEpo doses (L, 5.5+/-0.5; M, 6.4+/-0.5; H, 10.2+/-2.0 pg/ml, P<0.05 vs L and H groups). A stepwise regression analysis revealed that gender and sTNFR p80 were significant predictors of rHuEpo dosage. A significant direct relationship was found between rHuEpo dose and sTNFR p80 (r=0.499) and IL-6 (r=0.439) values in women (P<0.01) but not in men.

CONCLUSIONS

These findings suggest that high blood sTNFR p80 may contribute to the development of rHuEpo resistance in female patients undergoing long-term HD.

Collagen-induced arthritis in TNF receptor-1-deficient mice: TNF receptor-2 can modulate arthritis in the absence of TNF receptor-1

TNF is a potent proinflammatory cytokine important for the development of arthritis in human and animals. We have investigated the roles of TNF receptor-1 (TNFR1) and TNF receptor-2 (TNFR2) in collagen-induced arthritis (CIA) by inducing CIA in mice genetically deficient in TNFR1. TNFR1-/- mice developed arthritis with similar incidence and severity as TNFR1+/- littermates, indicating that TNFR1 is redundant for the development of CIA. Anti-type II collagen (CII) antibody levels and T cell responses to CII did not differ between TNFR1-/- mice and controls. Neutralization of TNF with soluble TNF binding protein suppressed the development of arthritis in TNFR1+/- mice but not in TNFR1-/- mice, indicating that TNFR2 cannot substitute for TNFR1 for the proinflammatory function. To further investigate the functions of TNFR2, TNFR1-/- mice were injected with murine TNF-alpha at different stages during the course of CIA. Repeated TNF-alpha injection during the early induction phase enhanced the development of arthritis, but inhibited arthritis when administered during the late progression phase. These results show that the engagement of TNFR2 by TNF is involved in the development of CIA in the absence of TNFR1 and that opposing signals can be transduced by TNFR2.

The role of tumour necrosis factor-alpha in the pathogenesis of complicated falciparum malaria

Plasmodium falciparum malaria is the most important parasitic infection of humans and is one of the most serious health problems facing the inhabitants of developing countries. It is responsible for about 2 million deaths every year. To date there is no specific treatment for the disease apart from anti-malarials. The declining sensitivity to these drugs is a serious therapeutic problem, while no safe and effective vaccine is likely to be available for general use in the near future. There is now abundant laboratory and clinical evidence to suggest that tumour necrosis factor-alpha (TNF-alpha) plays a major role in the pathogenesis of complicated falciparum malaria. Modulation of TNF-alpha response in combination with the current anti-malarial drugs, may represent a novel approach to the treatment of the serious complications associated with the disease.

Dissecting megakaryocytopoiesis in vivo with toxigenes

The genetic programs that regulate the commitment of a totipotent stem cell to the megakaryocytic lineage remain poorly defined and require appropriate in vivo models. Using a cell-specific obliteration technique, a transgenic mouse model was produced where perturbations of megakaryocytopoiesis and platelet production may be induced on demand. This was achieved by targeting the expression of the herpes virus thymidine kinase (HSV-tk) to megakaryocytes using the regulatory regions of the gene coding for the alphaIIb gene, an early marker of megakaryocytopoiesis, which encodes the alpha subunit of the platelet integrin alphaIIb beta3. The HSV-tk gene is not toxic by itself, but sensitizes the target cell to the effect of ganciclovir (GCV), leading to the inhibition of DNA synthesis in dividing cells. The programmed eradication of the megakaryocytic lineage was induced by treating transgenic mice bearing the hybrid construct (alphaIIb-tk) with GCV. After 10 days of treatment, the platelet number was reduced by greater than 96.5% and megakaryocytes were not detectable in the bone marrow (BM). After discontinuing GCV, BM was repopulated with megakaryocytes, and the platelet count was restored within seven days. The recovery was accelerated by the administration of interleukin 11. Prolonged GCV treatment induced erythropenia in the transgenic mice. Assays of myeloid progenitor cells in vitro demonstrated that the transgene was expressed in early erythro-megakaryocytic bipotent progenitor cells. The reversibility and facility of this system provide a powerful model to determine both the critical events in megakaryocytic and erythroid lineage development, and for evaluating the precise role that platelets play in the pathogenesis of a number of vascular occlusive disorders.

In vitro effects of recombinant TNF-alpha binding protein (rTBP-1) on hematopoiesis of HIV-infected patients

Tumor necrosis factor-alpha (TNF-alpha) is believed to contribute to the hematopoietic failure often observed in patients with AIDS. Soluble TNF receptors (sTNFR) compete for TNF-alpha with cell surface receptors and thus may block its activity. The effect of the p55 sTNFR (recombinant TNF-binding protein-1 [rTBP-1]) on the clonogenic growth of hematopoietic progenitor cells from 27 HIV-infected patients was evaluated in comparison with 11 normal study subjects. Peripheral blood-derived, myelopoietic (i.e., granulomonocytic colony-forming cells [GM-CFC]) and erythropoietic (i.e, burst-forming unit, erythroid [BFU-E]) colonies were grown in 10-day semisolid cultures with increasing concentrations of rTBP-1. Significantly, dose-dependent increases occurred in GM-CFC from 17 of 21 AIDS patients and 12 of 21 in BFU-E at rTBP-1 concentrations of 1microg/ml to 25 microg/ml. In contrast, rTBP-1 failed to induce any appreciably increased colony formation in normal cell cultures. In 6 patients treated with highly active antiretroviral treatment (HAART), TBP-1 alone did not demonstrate the in vitro hematopoiesis-enhancing effect. This study may provide an initial step in development of therapeutic use of TBP as a TNF-alpha antagonist in HIV-infected patients who do not benefit sufficiently from antiretroviral treatment, and in other conditions in which increased levels of TNF-alpha may contribute to hematopoietic deficiencies.

Tumor necrosis factor (TNF)-alpha and TNF receptors in viral pathogenesis

Tumor necrosis factor-alpha (TNF-alpha) and TNF receptors (TNFR) are members of the growing TNF ligand and receptor families that are involved in immune regulation. The present report will focus on the role of the prototypic ligand TNF and its two receptors, TNFR1 and TNFR2, in viral pathogenesis. Although TNF was reported years ago to modulate viral infections, recent findings on the molecular pathways involved in TNFR signaling have allowed a better understanding of the molecular interactions between cellular and viral factors within the infected cell. The interactions of viral proteins with intracellular components downstream of the TNFR have highlighted at the molecular level how viruses can manipulate the cellular machinery to escape the immune response and to favor the spread of the infection. We will review here the role of TNF and TNFR in immune response and the role of TNF and TNFR signaling in viral pathogenesis.

Tumor necrosis factor-alpha-induced lung cell expression of antiapoptotic genes TRAF1 and cIAP2

Tumor necrosis factor (TNF) receptor (TNFR)-associated factors 1 and 2 (TRAF1 and TRAF2) and inhibitor of apoptosis proteins cIAP1 (MIHB) and cIAP2 (MIHC) were recently identified as proteins that associate with the TNF-alpha receptors TNFRI (p55) and TNFRII (p75) and inhibit TNF-alpha-induced programmed cell death or apoptosis. In the original reports, TRAF1 expression, unlike the ubiquitous TRAF2, was restricted to specific tissues in the lung, spleen, and testis. TNF-alpha is increased in the lung in many forms of pulmonary disease. In the current study, Western analysis, immunohistochemistry, and ribonuclease protection assays were used to determine whether TNF-alpha regulates the expression of these TNFR-associated proteins in lung cells. We demonstrate for the first time TNF-alpha dose-dependent induction of TRAF1 protein and messenger RNA (mRNA) in human H441 and A549 pulmonary adenocarcinoma cell lines, as well as in lung cells of C57BL/6J mice after intratracheal administration of TNF-alpha. In contrast to the epithelial cells, TRAF1 was not induced by TNF-alpha in U937 cells, a human monocytic cell line, suggesting cell type-specific regulation. Similarly, cIAP2 mRNA was induced by TNF-alpha in both H441 and A549 pulmonary epithelial cells but not in U937 cells. TNF-alpha is a primary mediator of acute pulmonary inflammation and contributes to the pathophysiology of chronic lung diseases such as bronchopulmonary dysplasia (BPD), a fibrotic disease of prematurely born infants. Immunohistochemical staining of human neonatal lung tissue demonstrated increased TRAF1 in lungs of infants dying of pneumonia or BPD in comparison with those dying of congenital malformation. These studies support the hypothesis that the TRAF1 and cIAP2 genes are highly regulated in pulmonary cells and may play a role in human lung disease.

TNF-α-Induced Growth Suppression of CD34+ Myeloid Leukemic Cell Lines Signals Through TNF Receptor Type I and Is Associated with NF-κB Activation

Conflicting results have been reported regarding the effect of TNF-α on the growth of human primitive hemopoietic cells. In this study, we have examined the effect of TNF-α on the proliferation of several CD34+/CD38+ (KG-1, TF-1) and CD34+/CD38− (KG-1a, TF-1a) myeloid leukemic progenitor cell lines. Our data show that TNF-α markedly inhibits the growth of these cells in both liquid and soft agar cultures. Addition of GM-CSF or IL-3 does not prevent TNF-α-induced growth inhibition. Flow cytometry analyses of propidium iodide-stained cells demonstrated cell death of all four cell lines, as judged by the presence of cells with hypodiploid DNA content after exposure of cells to TNF-α for 4 days. Annexin V assays detected apoptosis in TF-1, but not in TF-1a, KG-1, and KG-1a cells in terms of translocation of phosphatidylserine shortly after TNF-α treatment. Neutralizing anti-TNF receptor type I (TNFR-I; p55) Ab almost completely reversed TNF-α-induced growth inhibition in both liquid and soft agar cultures, whereas anti-TNFR-II (p75) Ab had only a marginal effect. TNF-α rapidly induced marked activation of nuclear transcription factor NF-κB in all 4 cell lines. The majority of this effect was abolished by the type I receptor Ab, whereas the type II receptor neutralizing Ab had no effect. Our data also show that TNF-α is incapable of inducing activation of the mitogen-activated protein kinase pathway in these leukemic cell lines.

Effects of hTNFalpha expression in T cells on haematopoiesis in transgenic mice

A transgenic line of mice carrying one copy of the hTNFalpha gene under the control of its own promoter and the CD2 locus control region has been analysed for the effects of TNFalpha on haematopoiesis. A low level constitutive expression of hTNFalpha in lymphoid tissue was observed. Human TNFalpha binds to and activates the murine p55 receptor, but not the p75 receptor. This implies that the observed effects of hTNFalpha in mice were mediated only through the p55 receptor. Various lymphoid tissues were depleted of lymphocytes, especially thymus, spleen and peripheral blood. Effects on thymus development were detected already at 3 wk of age, more general effects on haematopoiesis were evident by 5 wk: a drop in total blood leukocytes, mainly due to a 67% decline in lymphocytes. At 16 wk the mice had developed anaemia, whereas platelets, neutrophils and monocytes had increased. The fall in lymphocytes was due to lowered levels of T cells as well as B cells. The cause of the shortened lifespan of the transgenic mice was probably not the haematological effects of hTNFalpha directly. Absence of trophic factors supplied by the normal T cell population remains possible.

Tumor necrosis factor p55 receptor (TNF-RI) mediates the in vitro inhibition of hepatic erythropoietin production

Tumor necrosis factor alpha (TNFalpha) is thought to contribute to the blunted erythropoietin (Epo) production in inflammatory diseases. The present study was carried out to find out as to whether the 55 kD (TNF-RI) or the 75 kD (TNF-RII) receptor is responsible for the TNFalpha-induced inhibition of hepatic Epo synthesis. When the effects of two receptor-specific mutants were compared, only the TNF-RI-specific isoform proved to suppress the formation of immunoreactive Epo in the human hepatoma cell lines HepG2 and Hep3B, similar to the effect of wild-type TNFalpha. Anti-TNFalpha antibody restored Epo production in TNFalpha- or TNF-RI mutant-treated cultures. By gel shift assay NF-kappaB binding to DNA was demonstrated following the addition of TNFalpha or TNF-RI-specific mutant to HepG2 cells, while the TNF-RII-specific mutant was ineffective. Finally, immunoreactive TNF-RI, but not TNF-RII, fragments were measurable in cell culture supernatants. Taken together, these results suggest that the inhibition of hepatic Epo production by TNFalpha is mediated by TNF-RI signaling.

Differential Effects of Anti-Fas Ligand and Anti-Tumor Necrosis Factor α Antibodies on Acute Graft-Versus-Host Disease Pathologies

Both tumor necrosis factor α (TNFα) and Fas ligand (FasL) have been implicated in the pathogenesis of graft-versus-host disease (GVHD). In this study, we examined the ameliorating effects of neutralizing anti-FasL and/or anti-TNFα monoclonal antibody (MoAb) in a lethal acute GVHD model in mice. Whereas the treatment with either anti-FasL or anti-TNFα MoAb alone significantly delayed the mortality and improved the body weight, a complete protection was achieved by the administration of both MoAbs. Pathological examination indicated differential effects of anti-FasL or anti-TNFα MoAb on GVHD-associated pathologies. Hepatic lesion was improved by anti-FasL but not anti-TNFα MoAb. In contrast, intestinal lesion was improved by anti-TNFα but not anti-FasL MoAb. Cutaneous and splenic lesions were improved by either MoAb. The combination of both MoAbs improved all these lesions. These results indicate that FasL and TNFα differentially contribute to the GVHD pathologies and a complete protection from mortality can be achieved by neutralization of both FasL and TNFα.

Differential Effects of Anti-Fas Ligand and Anti-Tumor Necrosis Factor α Antibodies on Acute Graft-Versus-Host Disease Pathologies

Both tumor necrosis factor α (TNFα) and Fas ligand (FasL) have been implicated in the pathogenesis of graft-versus-host disease (GVHD). In this study, we examined the ameliorating effects of neutralizing anti-FasL and/or anti-TNFα monoclonal antibody (MoAb) in a lethal acute GVHD model in mice. Whereas the treatment with either anti-FasL or anti-TNFα MoAb alone significantly delayed the mortality and improved the body weight, a complete protection was achieved by the administration of both MoAbs. Pathological examination indicated differential effects of anti-FasL or anti-TNFα MoAb on GVHD-associated pathologies. Hepatic lesion was improved by anti-FasL but not anti-TNFα MoAb. In contrast, intestinal lesion was improved by anti-TNFα but not anti-FasL MoAb. Cutaneous and splenic lesions were improved by either MoAb. The combination of both MoAbs improved all these lesions. These results indicate that FasL and TNFα differentially contribute to the GVHD pathologies and a complete protection from mortality can be achieved by neutralization of both FasL and TNFα.

Overexpression of the p80 TNF Receptor Leads to TNF-Dependent Apoptosis, Nuclear Factor-κB Activation, and c-Jun Kinase Activation

Because they have distinct intracellular domains, it has been proposed that the p60 and p80 forms of the TNF receptor mediate different signals. Several signaling proteins have been isolated that associate with either the p60 or the p80 receptor. By using TNF muteins specific to the p60 and p80 receptors, we have previously shown that cytotoxicity and nuclear factor-κB (NF-κB) activation are mediated through the p60 form of the endogenous receptor. What signals are mediated through the p80 receptor is less clear. This study was an effort to answer that question. HeLa cells, which express only p60 receptors, were transfected with p80 receptor cDNA and then examined for apoptosis, NF-κB activation, and c-Jun kinase activation induced by TNF and by p60 or p80 receptor-specific muteins. The p80 mutein, like TNF and the p60 mutein, induced apoptosis and activation of NF-κB and c-Jun kinase in cells overexpressing recombinant p80 receptor but had no effect on cells expressing a high level of endogenous p80 receptor. The apoptosis mediated through the p60 receptor was also potentiated after overexpression of the p80 receptor, suggesting a synergistic relationship between the two receptors. Interestingly, Abs to the p80 receptor blocked apoptosis induced by all ligands but by itself activated NF-κB in the p80-transfected cells. Overall, our results show that the p80 receptor, which lacks the death domain, mediated apoptosis, NF-κB activation, and c-Jun kinase activation, but only when it was overexpressed, whereas endogenous p60 receptor mediated similar signals without overexpression.

Suppression of rat bone marrow cells by Friend murine leukemia virus envelope proteins

In a retroviral rat model, we have investigated the nontransforming effects of murine leukemia virus FB29 on the bone marrow. Upon intraperitoneal inoculation with murine leukemia virus FB29 of either neonatal or adult rats, bone marrow cells became massively infected within the first 12 days postinoculation. In neonatally inoculated rats, a persistent productive bone marrow infection was established, whereas in rats inoculated as adults, no infected bone marrow cells could be detected beyond 12 days postinoculation. Retroviral infection was most likely cleared by an antiviral immune response (Hein et al., 1995, Virology 211, 408-417). Exposure to virus irreversibly decreased numbers of bone marrow cells staining with monoclonal antibody OX7 by 10-30%. Reduction of OX7+ bone marrow cells by 20% was also observed in vitro, after bone marrow cells from uninfected adult rats had been co-incubated with virus. FB29-envelope proteins were sufficient alone to reduce numbers of OX7+ bone marrow cells, both in vivo and in vitro. According to results on incorporation of propidium iodide, decreased numbers of OX7+ cells were due to cell death. By flow cytometric analyses OX7+ bone marrow cells as well as monocytes/macrophages were identified to be major target cells for infection with FB29 within the bone marrow. Thus, the mechanism(s) responsible for death of OX7+ bone marrow cells might be due to direct toxicity of viral envelope proteins and/or to interactions of viral envelope proteins with cells of the monocytic lineage.

Induction of Jak/STAT Signaling by Activation of the Type 1 TNF Receptor

Cellular responses to TNF are initiated by either of two cell surface receptors, the type 1 TNF receptor (TNFR1) and the type 2 TNF receptor (TNFR2). Although neither receptor contains an intrinsic protein tyrosine kinase, such activity has been implicated in TNF action. In this study, we show that murine TNF induces the tyrosine phosphorylation and activation of the intracellular Janus tyrosine kinases Jak1, Jak2, and Tyk2 in murine 3T3-L1 adipocytes. Activation of Jak kinases by TNF was associated with tyrosine phosphorylation of STAT1, STAT3, STAT5, and STAT6, but not STAT2 or STAT4, showing that TNF acts on a specific subset of these latent cytoplasmic transcription factors in 3T3-L1 adipocytes. Agonist antiserum to TNFR1 induced Jak kinase and STAT protein phosphorylation. Phosphorylation of Jak proteins was also induced by human TNF, which selectively binds to TNFR1 on murine cells. 35S-labeled Jak kinases were precipitated from a cell-free system and from lysates of 3T3-L1 adipocytes by a glutathione S-transferase fusion protein containing the cytoplasmic domain of TNFR1. These results suggest that the cytoplasmic domain of TNFR1 can directly interact with and form signaling complexes with Jak kinases. Jak2 was precipitated from HeLa cells by antiserum to TNFR1, directly demonstrating their association in vivo. Thus, TNF activates a Jak/STAT signal-transduction cascade by acting through TNFR1.

Endothelial cell contact potentiates release of soluble tumor necrosis factor (TNF) receptors from the monocyte-like cell line THP-1

The two tumor necrosis factor (TNF) receptors can undergo proteolytic cleavage to form soluble receptors, TNF-R55-BP and TNF-R75-BP, that can neutralize TNF. The aim was to study the release of soluble TNF receptor forms during monocyte-endothelial cell interaction. Monocytic THP-1 cells were used, and their release of TNF-R75-BP was determined. Contact between THP-1 cells and confluent endothelial cells induced fourfold higher release of TNF-R75-BP from the THP-1 cells than with these cells in suspension. The release was further increased up to eightfold after prestimulation of the endothelial cells with interleukin-1beta (IL-1beta). Prestimulation for 10 min gave maximal release of TNF-R75-BP from the attached THP-1 cells. IL-1beta by itself did not induce shedding of soluble TNF receptors in THP-1 cells. Blocking antibodies against the endothelial cell adhesion molecules VCAM, ICAM, and E-selectin did not affect the release of TNF-R75-BP from THP-1 cells attached to the endothelium. Conditioned medium from IL-1beta-stimulated endothelial cells increased the production of TNF-R75-BP from THP-1 cells in suspension. However, surface contact between endothelial cells and THP-1 cells was necessary for maximal production of TNF-R75-BP. TNF-alpha released from endothelial cells on IL-1beta stimulation did not promote shedding of TNF-R75 from THP-1 cells. Thus, endothelial cell contact potentiates the production of TNF-R75-BP in a monocyte-like cell line. The shedding of soluble TNF receptors observed in this case seems to be a result of both cell attachment and soluble factors.

Isolation and functional characterization of the mouse p75 TNF receptor promoter

Tumor necrosis factor (TNF) is a pleiotropic cytokine that plays an important role in immunological and inflammatory responses. It exerts its biological effects via two distinct membrane receptors of apparent molecular weight of 55 (p55TNFR) and 75 kDa (p75TNFR), respectively. Most cell lines and primary tissues express both receptor types. While the p55TNFR gene is constitutively expressed at rather low levels, the transcription of p75TNFR is strongly modulated by a number of stimulatory agents. To characterize the mouse p75TNFR gene expression on a molecular level, we screened a mouse genomic library using the 5' end of the p75TNFR cDNA as a probe. A 6.3kb genomic clone containing about 6 kb of 5' flanking region and 300 bp of 3' sequence including the translational start site and the first exon was isolated and subcloned. Primer extension analysis revealed three transcriptional start sites located at -35, -39, and -564 bp upstream of the ATG-containing first exon. To determine whether the 5' flanking region exerts functional promoter activity, we generated deletion mutants fused to the luciferase reporter gene. Transfection of mouse fibroblasts (NIH3T3) with these constructs showed functional promoter activity of the isolated 5' region. By further sequence analysis of the 5' flanking region a number of putative DNA-binding sites for transcription factors, e.g., Sp1, CREB, Yi, YY1, and IFN gamma-responsive element, were identified.

Growth factors and hematopoietic stem cells

The hematopoietic system is mediated in part by cell-to-cell interactions and soluble mediators or growth factors (cytokines). A large number of cytokines directly and potently control hematopoietic stem and precursor cell proliferation and differentiation. This review focuses on the recent studies devoted to the role of cytokines in the ex vivo expansion and differentiation of hematopoietic stem and precursor cells.

A Metalloproteinase Inhibitor Prevents Lethal Acute Graft-Versus-Host Disease in Mice

Tumor necrosis factor (TNF ) and Fas ligand (FasL) have been implicated in the pathogenesis of graft-versus-host disease (GVHD), which is a major complication after allogeneic bone marrow transplantation. We examined here the ameliorating effect of a metalloproteinase inhibitor (KB-R7785) that inhibits TNF-α and FasL release in a lethal acute GVHD model in mice. Administration of KB-R7785 into (BALB/c × C57BL/6) F1 that received C57BL/6 spleen cells markedly reduced the mortality and weight loss in association with minimal signs of GVHD pathology in the liver, intestine, and hematopoietic tissues. The ameliorating effect of KB-R7785 was superior to that of anti–TNF-α antibody. Our results suggest that KB-R7785 could be a potent therapeutic agent for GVHD.