Lisa M. Sedger

TNF and TNF-receptors: From mediators of cell death and inflammation to therapeutic giants – past, present and future

Tumor Necrosis Factor (TNF), initially known for its tumor cytotoxicity, is a potent mediator of inflammation, as well as many normal physiological functions in homeostasis and health, and anti-microbial immunity. It also appears to have a central role in neurobiology, although this area of TNF biology is only recently emerging. Here, we review the basic biology of TNF and its normal effector functions, and discuss the advantages and disadvantages of therapeutic neutralization of TNF - now a commonplace practice in the treatment of a wide range of human inflammatory diseases. With over ten years of experience, and an emerging range of anti-TNF biologics now available, we also review their modes of action, which appear to be far more complex than had originally been anticipated. Finally, we highlight the current challenges for therapeutic intervention of TNF: (i) to discover and produce orally delivered small molecule TNF-inhibitors, (ii) to specifically target selected TNF producing cells or individual (diseased) tissue targets, and (iii) to pre-identify anti-TNF treatment responders. Although the future looks bright, the therapeutic modulation of TNF now moves into the era of personalized medicine with societys challenging expectations of durable treatment success and of achieving long-term disease remission.

Characterization of the in vivo function of TNF-α-related apoptosis-inducing ligand, TRAIL/Apo2L, using TRAIL/Apo2L gene-deficient mice

To define the normal physiological role for the TRAIL/Apo2L in vivo, we generated TRAIL/Apo2L gene‐targeted mice. These mice develop normally and show no defects in lymphoid or myeloid cell homeostasis or function. Although TRAIL/Apo2L kills transformed cells in vitro, TRAIL/Apo2L–/– mice do not spontaneously develop overt tumors at an early age. However, in the A20 B cell lymphoma‐transferred tumor model, TRAIL/Apo2L–/– mice are clearly more susceptible to death from overwhelming tumor burden, due to increased lymphoma load in the liver. A20 tumors are susceptible to TRAIL/Apo2L killing in vitro, indicating that TRAIL/Apo2L may act directly to control A20 cells in vivo. Despite the fact that TRAIL binds osteoprotegerin and osteoprotegerin‐transgenic mice are osteopetrotic, TRAIL/Apo2L–/– mice show no evidence of altered gross bone density, and no alterations in frequency or in vitro differentiationof bone marrow precursor osteoclasts. Moreover, leucine zipper TRAIL has no toxicity when repeatedly administered to osteoprotegerin–/– mice. Thus, TRAIL/Apo2L is important in controlling tumors in vivo, but is not an essential regulator of osteoprotegerin‐mediated biology, under normal physiological conditions.

Apoptosis in experimental NASH is associated with p53 activation and TRAIL receptor expression

Background and Aims:  We examined extrinsic and intrinsic (endogenous) mitochondrial apoptosis pathways in experimental non‐alcoholic steatohepatitis (NASH).

Bone Marrow B Cell Apoptosis During In Vivo Influenza Virus Infection Requires TNF-α and Lymphotoxin-α

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 CD43low/−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-α, lymphotoxin-α, and both TNF receptors, TNFR1-p55 and TNFR2-p75. Thus, TNF-α and lymphotoxin-α are required for loss of BM B lineage cells during respiratory infection with influenza virus.

Poxvirus Tumor Necrosis Factor Receptor (TNFR)-Like T2 Proteins Contain a Conserved Preligand Assembly Domain That Inhibits Cellular TNFR1-Induced Cell Death

ABSTRACT The poxvirus tumor necrosis factor receptor (TNFR) homologue T2 has immunomodulatory properties; secreted myxoma virus T2 (M-T2) protein binds and inhibits rabbit TNF-α, while intracellular M-T2 blocks virus-induced lymphocyte apoptosis. Here, we define the antiapoptotic function as inhibition of TNFR-mediated death via a highly conserved viral preligand assembly domain (vPLAD). Jurkat cell lines constitutively expressing M-T2 were generated and shown to be resistant to UV irradiation-, etoposide-, and cycloheximide-induced death. These cells were also resistant to human TNF-α, but M-T2 expression did not alter surface expression levels of TNFRs. Previous studies indicated that T2s antiapoptotic function was conferred by the N-terminal region of the protein, and further examination of this region revealed a highly conserved N-terminal vPLAD, which is present in all poxvirus T2-like molecules. In cellular TNFRs and TNF-α-related apoptosis-inducing ligand (TRAIL) receptors (TRAILRs), PLAD controls receptor signaling competency prior to ligand binding. Here, we show that M-T2 potently inhibits TNFR1-induced death in a manner requiring the M-T2 vPLAD. Furthermore, we demonstrate that M-T2 physically associates with and colocalizes with human TNFRs but does not prevent human TNF-α binding to cellular receptors. Thus, M-T2 vPLAD is a species-nonspecific dominant-negative inhibitor of cellular TNFR1 function. Given that the PLAD is conserved in all known poxvirus T2-like molecules, we predict that it plays an important function in each of these proteins. Moreover, that the vPLAD confers an important antiapoptotic function confirms this domain as a potential target in the development of the next generation of TNF-α/TNFR therapeutics.

microRNA control of interferons and interferon induced anti-viral activity

Interferons (IFNs) are cytokines that are spontaneously produced in response to virus infection. They act by binding to IFN-receptors (IFN-R), which trigger JAK/STAT cell signalling and the subsequent induction of hundreds of IFN-inducible genes, including both protein-coding and microRNA genes. IFN-induced genes then act synergistically to prevent virus replication and create an anti-viral state. miRNA are therefore integral to the innate response to virus infection and are important components of IFN-mediated biology. On the other hand viruses also encode miRNAs that in some cases interfere directly with the IFN response to infection. This review summarizes the important roles of miRNAs in virus infection acting both as IFN-stimulated anti-viral molecules and as critical regulators of IFNs and IFN-stimulated genes. It also highlights how recent knowledge in RNA editing influence miRNA control of virus infection.

M-T2: A poxvirus TNF receptor homologue with dual activities

Poxviruses are experts at manipulating and evading the hosts immune response. They have acquired a number of open reading frames which specifically confer direct anti‐immune properties, either by mimicking cytokine receptors and growth factors or by disarming cytokine regulatory cascades. The Myxoma T2 protein (M‐T2), a TNF receptor homologue is secreted from virus infected cells and can bind TNF‐α with high affinity, and thereby inhibit TNF‐α‐mediated cytotoxicity. M‐T2 also acts to inhibit virus‐induced lymphocyte apoptosis by an as yet undefined mechanism. As such, T2 constitutes a significant virulence factor for poxviruses, influencing the outcome of infection, both in vitro and in vivo.

B-lymphopoiesis is stopped by mobilizing doses of G-CSF and is rescued by overexpression of the anti-apoptotic protein Bcl2

Osteoblasts are necessary to B lymphopoiesis and mobilizing doses of G-CSF or cyclophosphamide inhibit osteoblasts, whereas AMD3100/Plerixafor does not. However, the effect of these mobilizing agents on B lymphopoiesis has not been reported. Mice (wild-type, knocked-out for TNF-α and TRAIL, or over-expressing Bcl-2) were mobilized with G-CSF, cyclophosphamide, or AMD3100. Bone marrow, blood, spleen and lymph node content in B cells was measured. G-CSF stopped medullar B lymphopoiesis with concomitant loss of B-cell colony-forming units, pre-pro-B, pro-B, pre-B and mature B cells and increased B-cell apoptosis by an indirect mechanism. Overexpression of the anti-apoptotic protein Bcl2 in transgenic mice rescued B-cell colony forming units and pre-pro-B cells in the marrow, and prevented loss of all B cells in marrow, blood and spleen. Blockade of endogenous soluble TNF-α with Etanercept, or combined deletion of the TNF-α and TRAIL genes did not prevent B lymphopoiesis arrest in response to G-CSF. Unlike G-CSF, treatments with cyclophosphamide or AMD3100 did not suppress B lymphopoiesis but caused instead robust B-cell mobilization. G-CSF, cyclophosphamide and AMD3100 have distinct effects on B lymphopoiesis and B-cell mobilization with: 1) G-CSF inhibiting medullar B lymphopoiesis without mobilizing B cells in a mechanism distinct from the TNF-α-mediated loss of B lymphopoiesis observed during inflammation or viral infections; 2) CYP mobilizing B cells but blocking their maturation; and 3) AMD3100 mobilizing B cells without affecting B lymphopoiesis. These results suggest that blood mobilized with these three agents may have distinct immune properties.

Myxoma T2 protein as a model for poxvirus TNF receptor homologs

Many poxviruses encode a plethora of immunomodulatory proteins, including homologs of cellular cytokine receptors. These receptor mimics, also referred to as viroceptors, are believed to function by binding and sequestering host cytokines thus preventing their signaling cascade prior to receptor engagement. The M-T2 protein of myxoma virus is a TNF receptor homolog that has two distinct activities: the secreted dimeric M-T2 protein binds and inhibits TNF alpha while the intracellular version permits myxoma virus replication in infected T-lymphocytes by blocking the cellular apoptosis response to the virus infection. Studies with M-T2 mutants reveal that distinct protein domains mediate these two anti-immune properties of this protein.

Extreme lymphoproliferative disease and fatal autoimmune thrombocytopenia in FasL and TRAIL double-deficient mice

To delineate the relative roles of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and Fas ligand in lymphocyte biology and lymphoproliferative disease, we generated mice defective in both molecules. B6.GT mice develop severe polyclonal lymphoproliferative disease because of accumulating CD3(+)CD4(-)CD8(-)B220(+) T cells, CD4(+) and CD8(+) T cells, and follicular B cells, and mice die prematurely from extreme lymphocytosis, thrombocytopenia, and hemorrhage. Accumulating lymphocytes resembled antigen-experienced lymphocytes, consistent with the maximal resistance of B6.GT CD4(+) and CD8(+) T cell to activation-induced cell death. More specifically, we show that TRAIL contributes to Fas ligand-mediated activation-induced cell death and controls lymphocyte apoptosis in the presence of interferon-gamma once antigen stimulation is removed. Furthermore, dysregulated lymphocyte homeostasis results in the production of anti-DNA and rheumatoid factor autoantibodies, as well as antiplatelet IgM and IgG causing thrombocytopenia. Thus, B6.GT mice reveal new roles for TRAIL in lymphocyte homeostasis and autoimmune lymphoproliferative syndromes and are a model of spontaneous idiopathic thrombocytopenia purpura secondary to lymphoproliferative disease.