LIGHT, a novel ligand for lymphotoxin beta receptor and TR2/HVEM induces apoptosis and suppresses in vivo tumor formation via gene transfer

Overexpression of bcl-2 enhances LIGHT- and interferon-gamma -mediated apoptosis in Hep3BT2 cells

LIGHT is a member of the tumor necrosis factor superfamily and is the ligand for LT-betaR, HVEM, and decoy receptor 3. LIGHT has a cytotoxic effect, which is further enhanced by the presence of interferon-gamma (IFN-gamma). Although LIGHT/IFN-gamma can activate caspase activity, neither benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone nor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone can completely inhibit LIGHT/IFN-gamma-mediated apoptosis. Moreover, overexpression of Bcl-2 further enhances LIGHT/IFN-gamma-mediated apoptosis. It appears that LIGHT and IFN-gamma act synergistically to activate caspase-3, with the resultant cleavage of Bcl-2, removal of the BH4 domain, leading to conversion of Bcl-2 from an antiapoptotic to a proapoptotic form in p53-deficient hepatocellular carcinoma Hep3BT2 cells. Thus, LIGHT seems to be able to override the protective effect of Bcl-2 and induce cell death. Although benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone can prevent the cleavage of Bcl-2 by LIGHT/IFN-gamma, they only partially inhibit apoptosis in Hep3BT2 cells that are overexpressing Bcl-2. In contrast, both LIGHT/IFN-gamma-mediated apoptosis and Bcl-2 cleavage are inhibited by free radical scavengers, indicating that free radicals may play an essential role in LIGHT/IFN-gamma-mediated apoptosis at a step upstream of caspase-3 activation. These results suggest that LIGHT signaling may diverge into multiple, separate processes.

Reciprocal expression of the TNF family receptor herpes virus entry mediator and its ligand LIGHT on activated T cells: LIGHT down-regulates its own receptor

The TNF receptor (TNFR) family plays a central role in the development of the immune response. Here we describe the reciprocal regulation of the recently identified TNFR superfamily member herpes virus entry mediator (HVEM) (TR2) and its ligand LIGHT (TL4) on T cells following activation and the mechanism of this process. T cell activation resulted in down-regulation of HVEM and up-regulation of LIGHT, which were both more pronounced in CD8(+) than CD4(+) T lymphocytes. The analysis of HVEM and LIGHT mRNA showed an increase in the steady state level of both mRNAs following stimulation. LIGHT, which was present in cytoplasm of resting T cells, was induced both in cytoplasm and at the cell surface. For HVEM, activation resulted in cellular redistribution, with its disappearance from cell surface. HVEM down-regulation did not rely on de novo protein synthesis, in contrast to the partial dependence of LIGHT induction. Matrix metalloproteinase inhibitors did not modify HVEM expression, but did enhance LIGHT accumulation at the cell surface. However, HVEM down-regulation was partially blocked by a neutralizing mAb to LIGHT or an HVEM-Fc fusion protein during activation. As a model, we propose that following stimulation, membrane or secreted LIGHT binds to HVEM and induces receptor down-regulation. Degradation or release of LIGHT by matrix metalloproteinases then contributes to the return to baseline levels for both LIGHT and HVEM. These results reveal a self-regulating ligand/receptor system that contributes to T cell activation through the interaction of T cells with each other and probably with other cells of the immune system.

Costimulatory wars: the tumor menace

Advances in our understanding of T cell costimulatory molecules have provided a vast array of novel approaches to tumor immunotherapy. In the past year, combinatorial immunotherapy based on earlier studies of CTLA-4 blockade, the identification of novel B7-family members, the modulation of CD40 to reverse tolerance to tumor-associated antigens and the use of OX40 to enhance antitumor responses of CD4+ T cells have all contributed to the development of more-powerful immunomodulatory cancer therapies.

The three HveA receptor ligands, gD, LT-alpha and LIGHT bind to distinct sites on HveA

The herpes virus entry mediator A (HveA), a member of the tumor necrosis factor receptor (TNFR) superfamily, interacts with three different protein ligands; lymphotoxin-alpha (LT-alpha) and LIGHT (LIGHT stands for lymphotoxin homolog, which exhibits inducible expression and competes with HSV glycoprotein D for HveA and is expressed on T-lymphocytes) from the host and the herpes simplex virus (HSV) surface glycoprotein gD. It has been reported that the gD binding site on HveA is located within the receptor's two N-terminal CRP domains, and that gD and LIGHT compete for their binding to HveA. However, whether these ligands interact with the same or different sites on the receptor is unclear. We analyzed and compared the sites of interaction between HveA and its TNF ligands, by using two recombinant forms of the receptor, comprising the full-receptor ectodomain (HveA (200t)) and its two first CRP domains (HveA (120t)), as well as several monoclonal antibodies recognizing HveA. Two HveA peptide ligands (BP-1 and BP-2) that differentially inhibit binding of soluble gD and LT-alpha to the receptor were also used to demonstrate that gD, LIGHT and LT-alpha bind to distinct sites on the receptor. Our results suggest that binding of a ligand to HveA may alter the conformation of this receptor, thereby affecting its interaction with its other ligands.

T cell life and death signalling via TNF-receptor family members

An effective immune response requires the rapid and accurate mobilisation of millions of effector cells in an antigen driven fashion. These effector cells must be kept alive long enough to fulfil their function but the majority must then be eliminated, a process known as activation-induced cell death. Recent advances in the field of lymphocyte biology have shed light onto how this balance is maintained and onto the consequences for disease if the homeostatic mechanisms become disturbed.

The lymphotoxin-beta receptor is necessary and sufficient for LIGHT-mediated apoptosis of tumor cells

LIGHT is a tumor necrosis factor (TNF) ligand superfamily member, which binds two known cellular receptors, lymphotoxin-beta receptor (LTbetaR) and the herpesvirus entry mediator (HveA). LIGHT is a homotrimer that activates proapoptotic and integrin-inducing pathways. Receptor binding residues via LIGHT were identified by introducing point mutations in the A' --> A" and D --> E loops of LIGHT, which altered binding to LTbetaR and HveA. One mutant of LIGHT exhibits selective binding to HveA and is inactive triggering cell death in HT29.14s cells or induction of ICAM-1 in fibroblasts. Studies with HveA- or LTbetaR-specific antibodies further indicated that HveA does not contribute, either cooperatively or by direct signaling, to the death pathway activated by LIGHT. LTbetaR, not HveA, recruits TNF receptor-associated factor-3 (TRAF3), and LIGHT-induced death is blocked by a dominant negative TRAF3 mutant. Together, these results indicate that TRAF3 recruitment propagates death signals initiated by LIGHT-LTbetaR interaction and implicates a distinct biological role for LIGHT-HveA system.

LIGHT, a TNF-like molecule, costimulates T cell proliferation and is required for dendritic cell-mediated allogeneic T cell response

LIGHT is a recently identified member of the TNF superfamily and its receptors, herpesvirus entry mediator and lymphotoxin beta receptor, are found in T cells and stromal cells. In this study, we demonstrate that LIGHT is selectively expressed on immature dendritic cells (DCs) generated from human PBMCs. In contrast, LIGHT is not detectable in DCs either freshly isolated from PBMCs or rendered mature in vitro by LPS treatment. Blockade of LIGHT by its soluble receptors, lymphotoxin beta receptor-Ig or HVEM-Ig, inhibits the induction of DC-mediated primary allogeneic T cell response. Furthermore, engagement of LIGHT costimulates human T cell proliferation, amplifies the NF-kappaB signaling pathway, and preferentially induces the production of IFN-gamma, but not IL-4, in the presence of an antigenic signal. Our results suggest that LIGHT is a costimulatory molecule involved in DC-mediated cellular immune responses.

Modulation of T-cell-mediated immunity in tumor and graft-versus-host disease models through the LIGHT co-stimulatory pathway

LIGHT was recently described as a member of the tumor necrosis factor (TNF) 'superfamily'. We have isolated a mouse homolog of human LIGHT and investigated its immunoregulatory functions in vitro and in vivo. LIGHT has potent, CD28-independent co-stimulatory activity leading to T-cell growth and secretion of gamma interferon and granulocyte-macrophage colony-stimulating factor. Gene transfer of LIGHT induced an antigen-specific cytolytic T-cell response and therapeutic immunity against established mouse P815 tumor. In contrast, blockade of LIGHT by administration of soluble receptor or antibody led to decreased cell-mediated immunity and ameliorated graft-versus-host disease. Our studies identify a previously unknown T-cell co-stimulatory pathway as a potential therapeutic target.

Development of Peyer's patches, follicle-associated epithelium and M cell: lessons from immunodeficient and knockout mice

Studies with immunodeficient and knockout mice have revealed that the development of mucosa-associated lymphoid tissues (MALT) and peripheral lymphoid nodes share common mechanisms, but also require distinct signals. Gene disruption of lymphotoxins or their cognate receptors affects both Peyer's patch and lymph node organogenesis. Disruption of the osteoprotegerin TNF-family member gene does not impair Peyer's patch development, but prevents formation of peripheral lymph nodes. Peyer's patch do not form in mice with a deleted gene encoding a B lymphocyte-specific chemokine receptor, while most peripheral lymph nodes, except inguinal, are normal in numbers and architecture. In B or T lymphocyte-deficient mice, Peyer's patches, with their overlying follicle-associated epithelium (FAE), are present although reduced in number and size. No Peyer's patches develop in RAG deficient mice. Formation of FAE with typical M cells has not been analyzed in these mice. M cell formation requires the close association of immune cells with differentiated enterocytes and their conversion appears to be transcriptionally regulated. The development of MALT, FAE and probably M cells is a multistep process that requires signalling pathways common to all secondary lymphoid tissues, but also MALT-specific factors.

A newly identified member of tumor necrosis factor receptor superfamily (TR6) suppresses LIGHT-mediated apoptosis

TR6 (decoy receptor 3 (DcR3)) is a new member of the tumor necrosis factor receptor (TNFR) family. TR6 mRNA is expressed in lung tissues and colon adenocarcinoma, SW480. In addition, the expression of TR6 mRNA was shown in the endothelial cell line and induced by phorbol 12-myristate 13-acetate/ionomycin in Jurkat T leukemia cells. The open reading frame of TR6 encodes 300 amino acids with a 29-residue signal sequence but no transmembrane region. Using histidine-tagged recombinant TR6, we screened soluble forms of TNF-ligand proteins with immunoprecipitation. Here, we demonstrate that TR6 specifically binds two cellular ligands, LIGHT (herpes virus entry mediator (HVEM)-L) and Fas ligand (FasL/CD95L). These bindings were confirmed with HEK 293 EBNA cells transfected with LIGHT cDNA by flow cytometry. TR6 inhibited LIGHT-induced cytotoxicity in HT29 cells. It has been shown that LIGHT triggers apoptosis of various tumor cells including HT29 cells that express both lymphotoxin beta receptor (LTbetaR) and HVEM/TR2 receptors. Our data suggest that TR6 inhibits the interactions of LIGHT with HVEM/TR2 and LTbetaR, thereby suppressing LIGHT- mediated HT29 cell death. Thus, TR6 may play a regulatory role for suppressing in FasL- and LIGHT-mediated cell death.

Bovine herpesvirus 1-induced apoptotic cell death: role of glycoprotein D

Bovine herpesvirus 1 (BHV-1) induces apoptotic cell death in peripheral blood mononuclear cells and in bovine B lymphoma (BL-3) cells. Attachment but not penetration of BHV-1 is necessary to induce apoptosis in target cells, suggesting that one or more BHV-1 envelope glycoproteins could be involved in the activation of the apoptotic process. In the present study, we demonstrate that, although BHV-1 virions devoid of glycoprotein D (BHV-1 gD-/-) still bind to BL-3 cells, they are no longer able to induce apoptosis. In contrast, virions that contain glycoprotein D (gD) in the viral envelope but do not genetically encode gD (BHV-1 gD-/+) induce a level of apoptosis comparable to that produced by wild-type (wt) BHV-1. In addition, monoclonal antibodies directed against gD, but not against gB or gC, strongly reduced the high levels of apoptosis induced by BHV-1. These observations demonstrate that the induction of apoptosis is directly due to BHV-1 viral particles harboring gD in the viral envelope. Interestingly, binding of affinity-purified gD to BL-3 cells did not induce apoptosis but inhibited the ability of wt BHV-1 to induce apoptosis. Altogether, these results provide evidence for the direct or indirect involvement of gD in the mechanism by which BHV-1 induces apoptosis.

The cytoplasmic domain of the lymphotoxin-beta receptor mediates cell death in HeLa cells

Activation of lymphotoxin-beta receptor (LT-betaR) by conjugation with heterotrimeric lymphotoxin, LT-alpha1/beta2, or by cross-linking with anti-LT-betaR antibodies can trigger apoptosis. We have observed that overexpression of either LT-betaR or the cytoplasmic domain of LT-betaR (LT-betaR(CD)) also induces apoptosis, which may be attributed to the tendency of LT-betaR(CD) to self-associate. The self-association domain of LT-betaR(CD) was mapped to amino acids 324-377, a region of the protein that is also essential for LT-betaR-induced apoptosis. Furthermore, we have shown that LT-betaR(CD)-induced apoptosis could be inhibited by a TRAF3 dominant negative mutant and by the caspase inhibitors Z-VAD-FMK, DEVD-FMK, and CrmA. The ligand-independent apoptosis induced by LT-betaR(CD) will help us to further dissect LT-betaR signaling pathway.

VEGI, a novel cytokine of the tumor necrosis factor family, is an angiogenesis inhibitor that suppresses the growth of colon carcinomas in vivo

A novel member of the tumor necrosis factor (TNF) family has been identified from the human umbilical vein endothelial cell cDNA library, named vascular endothelial growth inhibitor (VEGI). The VEGI gene was mapped to human chromosome 9q32. The cDNA for VEGI encodes a protein of 174 amino acid residues with the characteristics of a type II transmembrane protein. Its amino acid sequence is 20-30% identical to other members of the TNF family. Unlike other members of the TNF family, VEGI is expressed predominantly in endothelial cells. Local production of a secreted form of VEGI via gene transfer caused complete suppression of the growth of MC-38 murine colon cancers in syngeneic C57BL/6 mice. Histological examination showed marked reduction of vascularization in MC-38 tumors that expressed soluble but not membrane-bound VEGI or were transfected with control vector. The conditioned media from soluble VEGI-expressing cells showed marked inhibitory effect on in vitro proliferation of adult bovine aortic endothelial cells. Our data suggest that VEGI is a novel angiogenesis inhibitor of the TNF family and functions in part by directly inhibiting endothelial cell proliferation. The results further suggest that VEGI maybe highly valuable toward angiogenesis-based cancer therapy.