Ligands working as receptors: reverse signaling by members of the TNF superfamily enhance the plasticity of the immune system

Reverse signalling of membrane-integrated tumour necrosis factor differentially regulates alloresponses of CD4+ and CD8+ T cells against human microvascular endothelial cells

Reverse signalling of membrane-integrated ligands is a common phenomenon in the tumour necrosis factor (TNF) family and contributes to the pleiotropy of this pro-inflammatory cytokine and to the plasticity of the immune system in general. Transmembrane TNF (mTNF) itself can induce resistance to bacterial endotoxin in monocytes and can stimulate the immune activity of mitogen-activated, as well as of virus-infected, T cells. The aim of the present study was to investigate the influence of reverse signalling of mTNF on the allogeneic activity of CD4+ and CD8+ T cells against human microvascular endothelial cells (HMEC), as targets of various inflammatory responses. The proliferative potential of CD4+ T cells towards HMEC was attenuated by mTNF signalling, whereas stimulation of mTNF on CD8+ T cells increased their cytotoxic potential against HMEC. These effects were specific for reverse signalling of mTNF, as a blockade of the classical TNF-TNF receptor interaction by a neutralizing TNF receptor antibody had no effect. Cytokine profiling of the effector cells revealed that the anti-endothelial CD4+ T cells were of a T helper 2 (Th2) phenotype, whereas CD8+ T cells mainly produced cytotox. T cell 1 (Tc1) cytokines. From the results obtained in this study, we conclude that reverse signalling of mTNF differentially modulates CD4+ and CD8+ T-cell activity against allogeneic endothelial cells, which should be taken into account in settings of therapeutic cytokine antagonisms.

OX40 ligand and CD30 ligand are expressed on adult but not neonatal CD4+CD3- inducer cells: evidence that IL-7 signals regulate CD30 ligand but not OX40 ligand expression

In this report, we have examined the expression of the T cell survival signals, OX40 ligand (OX40L) and CD30 ligand (CD30L) on CD4(+)CD3(-)CD11c(-)B220(-)IL-7Ralpha(+) inducer cells from birth to adulthood in mice. We found that adult but not neonatal inducer cells expressed high levels of OX40L and CD30L, whereas their expression of TNF-related activation-induced cytokine (TRANCE) and receptor activator of NF-kappaB (RANK) was comparable. The failure of neonatal inducer cells to express the ligands that rescue T cells helps to explain why exposure to Ag in neonatal life induces tolerance rather than immunity. The expression of OX40L and CD30L on inducer cells increased gradually in the first few weeks of life achieving essentially normal levels around the time mice were weaned. We found that IL-7 signaling through the common cytokine receptor gamma-chain was critical for the optimal expression of both TNF-related activation-induced cytokine and CD30L but not OX40L. Furthermore, glucocorticoids, which potently suppress T effector function, did not influence the expression of OX40L and CD30L in the presence of IL-7.

Contribution of nuclear factor of activated T cells c1 to the transcriptional control of immunoreceptor osteoclast-associated receptor but not triggering receptor expressed by myeloid cells-2 during osteoclastogenesis

Bone homeostasis depends on the coordination of osteoclastic bone resorption and osteoblastic bone formation. Receptor activator of NF-kappaB ligand (RANKL) induces osteoclast differentiation through activating a transcriptional program mediated by the key transcription factor nuclear factor of activated T cells (NFAT) c1. Immunoreceptors, including osteoclast-associated receptor (OSCAR) and triggering receptor expressed by myeloid cells (TREM)-2, constitute the co-stimulatory signals required for RANKL-mediated activation of calcium signaling, which leads to the activation of NFATc1. However, it remains unknown whether the expression of immunoreceptors are under the control of NFATc1. Here we demonstrate that the expression of OSCAR, but not that of TREM-2, is up-regulated during osteoclastogenesis and markedly suppressed by the calcineurin inhibitor FK506, suggesting that OSCAR is transcriptionally regulated by NFATc1. NFATc1 expression results in the activation of the OSCAR promoter, which was found to be further enhanced by co-expression of PU.1 and microphthalmia-associated transcription factor (MITF). We further provide evidence that NFATc1 specifically regulates OSCAR by chromatin immunoprecipitation assay and quantification of OSCAR and TREM-2 mRNA in NFATc1-/- cells. Thus, OSCAR but not TREM-2 is involved in the positive feedback loop of the immunoreceptor-NFATc1 pathway during osteoclastogenesis. Although several immunoreceptors have been identified as co-stimulatory molecules for RANKL, the expression and function are differentially regulated. These mechanisms, possibly together with the delicate regulation of their ligands on osteoblasts, may provide the exquisite machinery for the modulation of osteoclastogenesis in the maintenance of bone homeostasis.

The intriguing biology of the tumour necrosis factor/tumour necrosis factor receptor superfamily: players, rules and the games

The members of the tumour necrosis factor (TNF)/tumour necrosis factor receptor (TNFR) superfamily are critically involved in the maintenance of homeostasis of the immune system. The biological functions of this system encompass beneficial and protective effects in inflammation and host defence as well as a crucial role in organogenesis. At the same time, members of this superfamily are responsible for host damaging effects in sepsis, cachexia, and autoimmune diseases. This review summarizes recent progress in the immunobiology of the TNF/TNFR superfamily focusing on results obtained from animal studies using gene targeted mice. The different modes of signalling pathways affecting cell proliferation, survival, differentiation, apoptosis, and immune organ development as well as host defence are reviewed. Molecular and cellular mechanisms that demonstrate a therapeutic potential by targeting individual receptors or ligands for the treatment of chronic inflammatory or autoimmune diseases are discussed.

Effects of elastase and cathepsin G on the levels of membrane and soluble TNFα

Neutrophil elastase (NE) and cathepsin G (CG), the proteolytic enzymes localized in azurophil granules of neutrophils (PMN), are involved in PMN responses to various stimuli. When released at sites of inflammation, they participate in the degradation of numerous proteins involved in the regulation of the immune response. In this study, we employed ADAM17(-/-) fibroblasts stably transfected with cDNA of human pro-tumor necrosis factor alpha (proTNFalpha) (ADAM17(-/-)TNF(+)) to investigate the effects of NE and CG on shedding and degradation of TNFalpha. Both NE and CG were found to diminish the level of membrane TNFalpha (mTNFalpha) as measured by flow cytometry. This process was accompanied by the accumulation of biologically active soluble TNFalpha (sTNFalpha) in the culture medium, as determined by an increase in both the cytotoxic activity of TNFalpha and its ability to serve as a co-stimulator in the induction of inducible nitric oxide synthase (iNOS). However, in contrast to CG, NE at high concentrations was able to degrade sTNFalpha released from the cell surface. Using soluble recombinant human TNFalpha, we identified Val(93)-Ala(94) and Val(117)-Glu(118) as the NE cleavage sites within the sTNFalpha molecule. Taken together, the ability of NE and CG to modulate levels of membrane and soluble forms of TNFalpha may contribute to the proinflammatory activity of neutrophils.

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.