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Whether interleukin (IL)-6 contributes to tumor necrosis factor (TNF)-induced lethal shock or whether, on the contrary, it is part of a protective feedback system, remains unresolved. Here, we report experiments with IL-6 gene-disrupted mice (IL-6(0/0)). We have tested the susceptibility of these to TNF-induced metabolic changes and lethality in different models, and compared the results with those obtained with IL-6+/+ wild-type mice. We studied the response to TNF in three different models: (i) murine TNF administration; (ii) TNF in galactosamine (GalN)-sensitized mice; (iii) TNF in Bacillus Calmette-Guérin-sensitized mice. We observed no significant difference between the two types of mice in any of the three models. Furthermore, IL-6(0/0) mice could be equally well desensitized (by IL-1) to TNF/GalN-induced lethality and tolerized to TNF-induced shock as IL-6+/+ mice. We also observed that, in response to turpentine, TNF or IL-1, IL-6(0/0) mice produced significantly less acute phase proteins (APP) than IL-6+/+ mice. In IL-6(0/0) mice, less corticosterone was induced by TNF than in the control mice, while the response to adrenocorticotropic hormone was the same. The results indicate that IL-6 is not contributing in a major way to the pathogenesis leading to TNF-induced shock, and that neither IL-6 nor the APP studied are essential for a protective feedback system.

Several studies in human patients and in laboratory animals have revealed a correlation between serum interleukin (IL)-6 levels and outcome in clinical sepsis and in related animal models, respectively. In the present study, two monoclonal antibodies were used to investigate the contribution of IL-6 in the lethal action of tumor necrosis factor (TNF) and of lipopolysaccharide (LPS) in mice. We studied the potential protective properties of an anti-murine (m) IL-6 antibody and of an anti-mIL-6 receptor antibody. In controlled experiments, we observed that both monoclonal antibodies conferred a dose-dependent protection to a lethal dose of mTNF. Detailed studies with the monoclonal antibodies indicate, however, that protection was no longer observed when the mTNF dose was slightly higher than the lethal dose. Likewise, the anti-IL-6 monoclonal antibody protected against injections of LPS at a lethal-dose concentration, but here too failed to protect against higher doses of LPS. The anti-IL-6 monoclonal antibody was unable to protect against mTNF in mice sensitized by galactosamine, the corticoid receptor antagonist RU38486 or human (h) IL-1 beta. Protection did not correlate with the serum concentrations of IL-6. Finally, we demonstrate that hIL-6 injection did not change the sensitivity of mice towards mTNF. We conclude that, although IL-6 levels may be of value as a marker for the outcome in septic shock, this cytokine contributes only marginally in the pathogenesis leading to death. The small, but real, contribution of IL-6 in some situations might be due to its ability to up-regulate the level of TNF receptors.

Although tumor necrosis factor (TNF) is undoubtedly a major mediator of the antitumor and shock-inducing activities of lipopolysaccharide (LPS), the outcome of a challenge with TNF is highly dependent on the presence or absence of other substances or conditions. We have previously shown that to obtain lethality in mice after TNF administration both TNF receptor (TNF-R) types have to be triggered. This is illustrated by the fact that recombinant human (rh) TNF, which is a selective murine (m) TNF-R55 agonist, is not lethal, whereas mTNF, which binds both mTNF-R55 and mTNF-R75, is lethal in mice. Triggering of TNF-R75 is, however, no longer needed when sensitizers such as galactosamine or low doses of LPS or interleukin (IL)-1 are also present. Here, we report that this selective species specificity of TNF is also reflected in patterns of induced IL-6: both rmTNF and rhTNF could induce considerable IL-6 peak levels in the plasma (up to 10 ng/ml) 2 to 3 h after TNF administration. However, only rmTNF was capable of inducing the same pattern of sustained IL-6 levels previously observed after lethal LPS doses, while rhTNF only caused induction of transient IL-6 levels, as found after nonlethal LPS doses. We also observed that the sensitizer IL-1 could complement rhTNF to induce such a sustained IL-6 induction. Since we were interested in sensitizers with a defined mechanism of action, we further investigated the effects of the glucocorticoid and progesterone inhibitor RU38486 on the lethal and IL-6-inducing properties of TNF. We observed that RU38486 closely mimicked IL-1: both had similar effects on IL-6 induction and sensitized mice to the lethal effects of TNF with comparable efficiency and kinetics. Using a monoclonal anti-IL-1R antibody, we finally observed that the effects of RU38486 were most probably not mediated by IL-1. These observations suggest that a glucocorticoid-antagonistic activity might be a key factor in the pathways leading to septic shock and that such activity could be a key target for the pharmacological manipulation of sepsis.

Interleukin (IL) 6 is a pleistropic cytokine with activities, among others, on immune cells, hematopoietic precursor cells and hepatocytes. We have investigated the kinetics and amplitude of its in vivo induction in mice after injection of four different IL 1 species as well as murine (m) and human (h) tumor necrosis factor (TNF) and bacterial lipopolysaccharide (LPS) using a sensitive bioassay on 7TD1 cells to measure the IL 6 concentrations. Recombinant mIL 1 beta, administered as a single i.v. injection in mice, induced the appearance of IL 6 in the plasma with peak levels observed after 2 h. A dose-response correlation was found between serum IL 6 levels and injected IL 1 alpha concentrations at 3 and 8 h after the injection. We then compared the ability of h/mIL 1 alpha, h/mIL 1 beta, h/mTNF and LPS to induce IL 6 in mice. We found: (a) LPS is the most potent inducer of IL 6; (b) 3 h after injection, the four IL 1 preparations had induced IL 6 levels comparable with the IL 6 levels observed after TNF injection; (c) high doses of mIL 1, alpha or beta, but not hIL 1, resulted in a high IL 6 level persisting for over 8 h. We conclude that IL 1 is a potent inducer of IL 6 in vivo and that no major differences are observed between the four IL 1 preparations, as evaluated at 3 h after the injection. However, mIL 1 alpha and mIL 1 beta, in contrast to hIL 1 alpha and hIL 1 beta, induced a sustained IL 6 level over a longer time period. This pattern of prolonged IL 6 induction is even much more pronounced after mTNF injection, but not after hTNF injection.