Targeted inactivation of genes in the tumor necrosis factor (TNF)/lymphotoxin (LT) ligand and receptor system has recently revealed essential roles for these molecules in lymphoid tissue development and organization. Lymphotoxin-alphabeta (LTalphabeta)/lymphotoxin-beta receptor (LTbeta-R) signaling is critical for the organogenesis of lymph nodes and Peyer's patches and for the structural compartmentalization of the splenic white pulp into distinct B and T cell areas and marginal zones. Moreover, an essential role has been demonstrated for TNF/p55 tumor necrosis factor receptor (p55TNF-R) signaling in the formation of splenic B lymphocyte follicles, follicular dendritic cell networks, and germinal centers. In contrast to a previously described essential role for the p55TNF-R in Peyer's patch organogenesis, we show in this report that Peyer's patches are present in both TNF and p55TNF-R knockout mice, demonstrating that these molecules are not essential for the organogenesis of this lymphoid organ. Furthermore, we show that in the absence of TNF/p55TNF-R signaling, lymphocytes segregate normally into T and B cell areas and a normal content and localization of dendritic cells is observed in both lymph nodes and Peyer's patches. However, although B cells are found to home normally within Peyer's patches and in the outer cortex area of lymph nodes, organized follicular structures and follicular dendritic cell networks fail to form. These results show that in contrast to LTalphabeta signaling, TNF signaling through the p55TNF-R is not essential for lymphoid organogenesis but rather for interactions that determine the cellular and structural organization of B cell follicles in all secondary lymphoid tissues.

In a model of concanavalin A (Con A)-induced, CD4(+) T cell-dependent experimental hepatitis in mice, in which TNF is a central mediator of apoptotic and necrotic liver damage, we now provide evidence for an essential in vivo function of TNFR2 in this pathophysiological process. We demonstrate that a cooperation of TNFR1 and TNFR2 is required for hepatotoxicity as mice deficient of either receptor were resistant against Con A. A significant role of TNFR2 for Con A-induced hepatitis is also shown by the enhanced sensitivity of transgenic mice overexpressing the human TNFR2. The ligand for cytotoxic signaling via both TNF receptors is the precursor of soluble TNF, i. e. transmembrane TNF Indeed, transmembrane TNF is sufficient to mediate hepatic damage, as transgenic mice deficient in wild-type soluble TNF but expressing a mutated nonsecretable form of TNF developed inflammatory liver disease. The significance of tumor necrosis factor receptor 1 (TNFR1) for TNF function in vivo is well documented, whereas the role of TNFR2 so far remains obscure.

BACKGROUND & AIMS:Treatment with tumor necrosis factor (TNF) induces murine hepatocyte apoptosis in vitro and in vivo when sensitizing concentrations of toxins are present. The aim of this study was to investigate whether endogenously formed TNF contributes to liver failure caused by hepatotoxins.METHODS:The extent of liver damage, induced by alpha-amanitin or actinomycin D (ActD), was examined under various experimental conditions, preventing the action of TNF on hepatocytes.RESULTS:TNF induced apoptosis of murine hepatocytes or human hepatoma cells in the presence of alpha-amanitin or ActD. TNF and alpha-amanitin induced such hepatotoxicity also in vivo in a synergistic way. After in vivo administration of high doses of ActD or alpha-amanitin alone, hepatic TNF-messenger RNA was increased and hepatocytes underwent apoptosis. A neutralizing antiserum against TNF-alpha prevented the liver injury. Hepatotoxicity of ActD or alpha-amanitin also was prevented by pretreatment of mice with low doses of the tolerizing cytokine interleukin 1. Mice deficient for the 55-kilodalton TNF receptor were protected from ActD- or alpha-amanitin-induced toxicity. Endotoxin-unresponsive C3H/HeJ mice also had liver failure after ActD treatment, and this damage was prevented by treatment with anti-TNF antiserum.CONCLUSIONS:Hepatotoxins such as alpha-amanitin may induce liver failure by an indirect mechanism involving sensitization of parenchymal cells toward endogenously produced TNF.

BACKGROUND:Activation of either the 55-kD tumor necrosis factor receptor (TNF-R1) or CD95 (Fas/Apo-1) causes apoptosis of cells and liver failure in mice, and has been associated with human liver disorders. The aim of this study was first to clarify the association between CD95 activation, hepatocyte apoptosis, and fulminant liver failure. Next, we investigated whether TNF-R1 and CD95 operate independently of each other in the induction of hepatocyte apoptosis.MATERIALS AND METHODS:Using both mice and primary liver cell cultures deficient in either TNF-R1 or functional CD95, the induction of apoptosis and hepatocyte death following activation of TNF-R1 or CD95 were studied in vitro and in various in vivo models of acute liver failure.RESULTS:In vivo or in vitro stimulation of CD95 caused apoptosis of wild-type (wt) murine hepatocytes which had not been sensitized by blocking transcription. Time course studies showed that DNA fragmentation and chromatin condensation preceded, respectively, membrane lysis in vitro and necrosis in vivo. Similar results were obtained after CD95 activation in hepatocytes or livers lacking TNF-R1. Conversely, hepatocytotoxicity due to endogenous or exogenous TNF was not affected in animals or liver cell cultures lacking the expression of functional CD95.CONCLUSIONS:TNF-R1 and CD95 are independent and differentially regulated triggers of murine apoptotic liver failure.