The role of tumor necrosis factor (TNF) in the febrile and metabolic responses of rats to intraperitoneal injection of a high dose of lipopolysaccharide Injection of a high dose of lipopolysaccharide (LPS) induces a septic-shock-like state, which can be accompanied by phases of hypothermia and phases of fever. In the present study we monitored body core temperature and locomotor activity, both by remote radiotelemetry, as well as changes in food intake, body mass and water intake for 3 days after an intraperitoneal (i.p.) injection of a high dose of LPS (5 mg/kg) along with sterile 0.9% saline or a neutralizing form of the soluble tumor necrosis factor (TNF) type 1 receptor (referred to as TNF-binding protein, TNF bp). Intraperitoneal injection of LPS rapidly induced high concentrations of TNF in the plasma and peritoneal lavage fluid. TNF was undetectable in the plasma and peritoneal lavage fluid of animals co-injected with LPS and TNF bp, implying neutralization of peripheral bioactive TNF. Administration of LPS induced hypothermia by about 1.5 degrees C, which lasted for 5 h after injection. During the light-time periods of days 2 and 3 after injection, the rats developed a robust fever. Treatment with TNF bp resulted in a faster recovery from the LPS-induced hypothermia so that the rats developed a pronounced fever on the day of injection. Locomotor activity during night-time periods was suppressed in LPS-treated animals. The LPS-induced depression of night-time activity was not antagonized by co-injection of TNF bp. On day 1 after the injection of LPS, food intake reduced to virtually zero, water intake fell to about 30% of the control value and body mass dropped by 25 g (about 10% of total body mass). With the exception of body mass, these variables recovered slowly during days 2 and 3 after LPS injection, but did not reach the control values. The LPS-induced decreases in food intake, body mass and water intake were significantly attenuated by the treatment with TNF bp. These results confirm that TNF contributes significantly to the rats' responses to intraperitoneal injection of a high dose of LPS. The fact that treatment with TNF bp accelerated and improved the rats' ability to develop a febrile response supports the view that the fever is beneficial, since all other metabolic responses measured in this study were normalized more effectively in those rats that developed a faster and more pronounced increase in body temperature.

A soluble form of the tumour necrosis factor (TNF) type 1 receptor (referred to as TNF binding protein, TNF‐bp) at a dose of 1 mg per animal, or an equivalent volume of solvent, was injected together with 10 μg kg−1 lipopolysaccharide (LPS) or 50 μg kg−1 muramyl‐dipeptide (MDP) directly into the arterial circulation of guinea‐pigs and the effects on circulating TNF or interleukin‐6 (IL‐6) and on abdominal temperature were studied. At 15 or 60 min after injection, LPS‐induced and MDP‐induced circulating TNF was below the detection limit of the assay and thus completely neutralized in animals treated with TNF‐bp. In the control group, TNF was still below the limit of detection in most animals 15 min after LPS was injected; in some animals small traces of TNF could already be detected at that time. However, 60 min after administration of LPS, large amounts of TNF (19508 ± 4682 pg ml−1) were measured in the control group. MDP‐induced TNF in plasma was below the limit of detection 15 min after MDP was injected, and rose to 10862 ± 3029 pg ml−1 60 min after injection. Low levels of circulating IL‐6 (20‐40 international units (IU) ml−1) were measured in all groups of animals 15 min after injection of LPS or MDP. This value corresponds to the baseline activity of IL‐6 in plasma of guinea‐pigs. One hour after administration of LPS, IL‐6 rose to 5442 ± 1662 IU ml−1 in the control group and to a significantly lower value of 1485 ± 179 IU ml−1 in guinea‐pigs treated with TNF‐bp. One hour after injection of MDP, circulating IL‐6 was 2614 ± 506 IU ml−1 in the control group, while the corresponding value in animals treated with TNF‐bp again was significantly lower (873 ± 312 IU ml−1). The second phase of the characteristic biphasic LPS fever in guinea‐pigs was significantly attenuated in animals treated with TNF‐bp. The shorter first phase of the febrile response to LPS was identical in both groups of animals. The late phase of MDP‐induced fever (7‐22 h after injection) was depressed by treatment with TNF‐bp, while the first phase of MDP‐induced fever (0‐7 h after injection) was significantly enhanced by the neutralization of TNF by TNF‐bp.

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In guinea pigs, lipopolysaccharide (LPS) from gram-negative bacteria and muramyl dipeptide (MDP) from gram-positive bacteria are potent inducers of systemic production of proinflammatory cytokines and fever. However, there is a striking difference between these two bacterial pyrogens in so far as repeated administration of LPS, but not of MDP, in short-term intervals induces tolerance by a progressive downregulation of the systemic cytokine network. In the present study, we investigated MDP-induced fever and the systemic release of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in LPS-tolerant guinea pigs in comparison with naive animals. Endotoxin tolerance was induced by repeated intramuscular injections of 20 μg/kg LPS at intervals of 3 days. In response to the last of five injections with LPS, systemic production of TNF-α and IL-6 as well as the development of a febrile response was abrogated almost completely. Those guinea pigs that had developed an LPS tolerance could, however, produce the same amounts of TNF-α and IL-6 as naive animals in response to a challenge with MDP. Also, MDP-induced fever was identical in LPS-tolerant and naive guinea pigs. These results provide evidence for a lack of cross tolerance between LPS and MDP in induction of circulating cytokines and fever in guinea pigs.