From humoral fever to neuroimmunological control of fever

Selected contribution: role of IL-6 in LPS-induced nuclear STAT3 translocation in sensory circumventricular organs during fever in rats

Interleukin-6 (IL-6) is regarded as an endogenous mediator of lipopolysaccharide (LPS)-induced fever. IL-6 is thought to act on the brain at sites that lack a blood-brain barrier, the circumventricular organs (CVOs). Cells that are activated by IL-6 respond with nuclear translocation of the signal transducer and activator of transcription 3 molecule (STAT3) and can be detected by immunohistochemistry. We investigated whether the LPS-induced release of IL-6 into the systemic circulation was accompanied by a nuclear STAT3 translocation within the sensory CVOs. Treatment with LPS (100 microg/kg) led to a slight (1 h) and then a strong increase (2-8 h) in plasma IL-6 levels, which started to decline at the end of the febrile response. Administration of both pyrogens LPS and IL-6 (45 microg/kg) induced a febrile response with IL-6, causing a rather moderate fever compared with the LPS-induced fever. Nuclear STAT3 translocation in response to LPS was observed within the vascular organ of the lamina terminalis (OVLT) and the subfornical organ (SFO) 2 h after LPS treatment. To investigate whether this effect was mediated by IL-6, the cytokine itself was systemically applied and indeed an identical pattern of nuclear STAT3 translocation was observed. However, nuclear STAT3 translocation already occurred 1 h after IL-6 application and proved to be less effective compared with LPS treatment when analyzing OVLT and SFO cell numbers that showed nuclear STAT3 immunoreactivity after the respective pyrogen treatment. Our observations represent the first molecular evidence for an IL-6-induced STAT3-mediated genomic activation of OVLT and SFO cells and support the proposed role of these brain areas as sensory structures for humoral signals created by the activated immune system and resulting in the generation of fever.

Inhibition of nitric oxide synthase delays the development of tolerance to LPS in rats

The role of nitric oxide (NO) was investigated in endotoxin (lipopolysaccharide, LPS) tolerance in freely moving biotelemetered rats. We monitored changes in febrile response and feeding behavior (food intake, water intake) during the development of tolerance to repeated intraperitoneal injections of LPS (50 microg/kg) along with injections of N(omega)-nitro-L-arginine methyl ester (L-NAME; 50 mg/kg), an inhibitor of NO synthase. Rats were treated with LPS and L-NAME for three consecutive days. On the fourth day, all rats were injected with LPS alone. Control rats were injected with saline along with saline or with L-NAME for four consecutive days. Rats repeatedly injected with LPS became tolerant to pyrogenic and hypophagic/cachexic effects of LPS as early as on the second day of experiment. The treatment with L-NAME prevented the attenuation of febrile response following the second LPS injection. Moreover, the depressive effects of LPS on body weight as well as on water and food intake were prolonged in rats treated with a combination of L-NAME and LPS. Injection of LPS caused a 3.5-fold increase in plasma nitrite within 3 h and nitrite levels remained significantly elevated 6 and 24 h after LPS. Rats injected secondly with LPS did have still 2.5- to 3-fold increase in plasma nitrite levels 3 and 6 h, but not 24 h, after injection. Third injection of LPS did not elevate nitrite level in plasma. Taken together, presented data provide clear evidence that NO formation is involved in mechanisms responsible for development of early-stage tolerance to endotoxin.

The influence of bacterial lipopolysaccharide on the thermoregulation of the box turtle Terrapene carolina

Ectotherms can adjust their thermoregulatory set points in response to bacterial infection; the result may be similar to endothermic fever. We examined the influence of dose on the set point of body temperature (T(b)) in Terrapene carolina. After acclimating postprandial turtles to 20 degrees C, we injected them with two doses of bacterial endotoxin (LPS; lipopolysaccharide from Escherichia coli), 0.0025 or 0.025 mg LPS/g nonshell body mass, or with reptilian saline (control group). We placed the animals singly in linear thigmothermal gradients and recorded their T(b)'s for 48 h. The turtles showed dose-influenced thermal selection. Turtles injected with the high dose had T(b)'s significantly higher than control turtles, whereas low-dose turtles had T(b)'s significantly lower than control turtles. Also, there was a low daily effect on the T(b) of the turtles injected with the high dose. High-dose turtles had significantly higher T(b)'s than the control turtles during the first day but not during the second. Our results support the prediction of Romanovsky and Székely that an infectious agent may elicit opposite thermoregulatory responses depending on quality and quantity of the agent and the host health status.

Some historical perspectives on thermoregulation

In this paper, selected historical aspects of thermoregulation and fever are presented as background to the application of molecular biology to thermoregulation. Temperature-sensing mechanisms, coordination of thermal information, thermoregulatory circuitry, efferent responses to thermal stimuli, set point mechanisms, and some of the mechanisms and consequences of fever and hyperthermia are highlighted. Neurotransmitters used in thermoregulatory circuits are also discussed. An attempt is made to include information from comparative physiological sources. Possible future avenues of research in the light of recent new technologies are also presented.

From farm to table to brain: foodborne pathogen infection and the potential role of the neuro-immune-endocrine system in neurotoxic sequelae

The American diet is among the safest in the world; however, diseases transmitted by foodborne pathogens (FBPs) still pose a public health hazard. FBPs are the second most frequent cause of all infectious illnesses in the United States. Numerous anecdotal and clinical reports have demonstrated that central nervous system inflammation, infection, and adverse neurological effects occur as complications of foodborne gastroenteritis. Only a few well-controlled clinical or experimental studies, however, have investigated the neuropathogenesis. The full nature and extent of neurological involvement in foodborne illness is therefore unclear. To our knowledge, this review and commentary is the first effort to comprehensively discuss the issue of FBP induced neurotoxicity. We suggest that much of this information supports the role of a theoretical model, the neuro-immune-endocrine system, in organizing and helping to explain the complex pathogenesis of FBP neurotoxicity.

An explanation for the seasonality of acute upper respiratory tract viral infections

Despite a great increase in our understanding of the molecular biology of the viruses associated with acute upper respiratory tract viral infections (URTIs) there is a remarkable lack of knowledge and ideas about why URTI should exhibit a seasonal incidence. Most publications in this area either acknowledge a complete lack of any explanation for the seasonality of URTI or put forward an explanation relating to an increased "crowding" of susceptible persons in winter. This review will discuss some of the ideas concerning the seasonality of URTI and put forward a new hypothesis for discussion, namely that seasonal exposure to cold air causes an increase in the incidence of URTI due to cooling of the nasal airway. The hypothesis is supported by literature reports demonstrating that inhalation of cold air causes cooling of the nasal epithelium, and that this reduction in nasal temperature is sufficient to inhibit respiratory defences against infection such as mucociliary clearance and the phagocytic activity of leukocytes. A case is also made to suggest that warming of the nasal airway during fever and nasal congestion may help to resolve a current URTI.

Effects of heme oxygenase system on the cyclooxygenase in the primary cultured hypothalamic cells

Endogenous carbon monoxide (CO) shares with nitric oxide (NO) a role as a putative neural messenger in the brain. Both gases are believed to modulate CNS function via an increase in cytoplasmic cGMP concentrations secondary to the activation of soluble guanylate cyclase (sGC). Recently CO and NO were proposed as a possible mediator of febrile response in hypothalamus. NO has been reported to activate both the constitutive and inducible isoform of the cyclooxygenase (COX). Thus, we investigated whether CO arising from heme catabolism by heme oxygenase (HO) is involved in the febrile response via the activation of COX in the hypothalamus. PGE2 which is a final mediator of febrile response released from primary cultured hypothalamic cells was taken as a marker of COX activity. PGE2 concentration was measured with EIA kits. Exogenous CO (CO-saturated medium) and hemin (a substrate and potent inducer of HO) evoked an increase in PGE2 release from hypothalamic cells, and these effects were blocked by methylene blue (an inhibitor of sGC). And membrane permeable cGMP analogue, dibutyryl-cGMP elicited significant increases in PGE2 release. These results suggest that there may be a functional link between HO and COX enzymatic activities. The gaseous product of hemin through the HO pathway, CO, might play a role through the modulation of the COX activity in the hypothalamus.

Fever induction pathways: evidence from responses to systemic or local cytokine formation

The immune and central nervous systems are functionally connected and interacting. The concept that the immune signaling to the brain which induces fever during infection and inflammation is mediated by circulating cytokines has been traditionally accepted. Administration of bacterial lipopolysaccharide (LPS) induces the appearance of a so-termed "cytokine cascade" in the circulation more or less concomitantly to the developing febrile response. Also, LPS-like fever can be induced by systemic administration of key cytokines (IL-1 beta, TNF-alpha, and others). However, anti-cytokine strategies against IL-1 beta or TNF-alpha along with systemic injections of LPS frequently lead to attenuation of the later stages of the febrile response but not of the initial phase of fever, indicating that cytokines are rather involved in the maintenance than in the early induction of fever. Within the last years experimental evidence has accumulated indicating the existence of neural transport pathways of immune signals to the brain. Because subdiaphragmatic vagotomy prevents or attenuates fever in response to intraperitoneal or intravenous injections of LPS, a role for vagal afferent nerve fibers in fever induction has been proposed. Also other sensory nerves may participate in the manifestation of febrile responses under certain experimental conditions. Thus, injection of a small dose of LPS into an artificial subcutaneous chamber results in fever and formation of cytokines within the inflamed tissue around the site of injection. This febrile response can be blocked in part by injection of a local anesthetic into the subcutaneous chamber, indicating a participation of cutaneous afferent nerve signals in the manifestation of fever in this model. In conclusion, humoral signals and an inflammatory stimulation of afferent sensory nerves can participate in the generation and maintenance of a febrile response.

Role of endogenous interleukin-1 receptor antagonist in regulating fever induced by localised inflammation in the rat

1. Interleukin (IL)-1 is a mediator of host defence responses to inflammation and injury, including fever, but its sites of synthesis and action have not been fully elucidated. The actions of IL-1 are antagonised by IL-1 receptor antagonist (IL-1ra). The present study tested the hypothesis that IL-1 and IL-1ra are produced locally at sites of peripheral inflammation in rats, and that endogenous IL-1ra acts to limit the fever resulting from the inflammation. 2. Injection of lipopolysaccharide (LPS; 100 microg kg-1) into a subcutaneous air pouch (I.PO.) of rats induced a significant increase in body temperature. Virtually all (approximately 85 %) of the injected LPS was recovered from the pouch between 1 and 8 h (when the experiment was terminated) after injection of LPS, but LPS was undetectable (< 50 pg ml-1) in plasma at any time. Concentrations of immunoreactive IL-1alpha and IL-1beta were increased significantly in the pouch at 1, 2, 3, 5 and 8 h after injection of LPS, corresponding with the rise in body temperature and the fever peak. The appearance of IL-1ra was delayed until 2 h. Thereafter, the concentrations of IL-1beta and IL-1ra increased in parallel with the development of fever, while the concentrations of IL-1alpha remained constant. IL-1ra, but not IL-1alpha or IL-1bet, was detected in significant quantities in the plasma of LPS-injected animals. 3. Treatment of rats with an anti-IL-1ra serum (2 ml, I.PO.) at the time of injection of LPS (10 or 100 microg kg-1, I.PO.) abolished the appearance of IL-1ra in the circulation. Although neutralisation of endogenous IL-1ra did not affect the maximum body temperature reached after injection of submaximum (10 microg kg-1, I.PO.) or maximum (100 microg kg-1, I.PO.) doses of LPS, the duration of the fever was significantly prolonged, and was associated with a 3- to 4-fold increase in immunoreactive IL-1beta concentrations in the pouch fluid, but not in the plasma, at the 8 h time point. 4. These data show that effects of local (I.PO.) injection of LPS are not due to its action in the circulation or at distant sites (such as at the blood-brain barrier). These data also show that locally produced IL-1ra, in response to injection (I.PO.) of LPS, inhibits the production and/or action of locally produced IL-1beta. The ability of IL-1ra to limit the duration, rather than the magnitude of the fever, is consistent with its delayed production, relative to IL-IL-1ra, therefore, appears to play a key role in the resolution of fever induced by localised inflammatory responses.

Afferent nerves are involved in the febrile response to injection of LPS into artificial subcutaneous chambers in guinea pigs

In guinea pigs, fever was induced by injections of 100 or 10 microgram/kg lipopolysaccharide (LPS) into artificial subcutaneous chambers and analysed under the influence of the local anesthetic, ropivacaine (ROPI), which was administered into the chamber at a dose of 10 mg/kg 30 min prior to LPS. In response to injections of 100 microgram/kg LPS into the subcutaneous chambers, fever was not modified by pretreatment with ROPI. High amounts of bioactive tumor necrosis factor (TNF) alpha and interleukin-6 (IL-6) were measured in the lavage of the chambers after administration of LPS. Comparatively low concentrations of both cytokines (0.5-4% of the concentrations in the lavage fluid) were detected in blood plasma simultaneously. In response to injections of 10 microgram/kg LPS into the subcutaneous chambers, fever was significantly reduced by pretreatment with ROPI to about 60% of the febrile response of control animals. Levels of TNF and IL-6 were lower in response to the reduced dose of LPS. TNF in plasma was even below the limit of detection. The suppression of fever by the local anesthetic was not observed when ROPI was subcutaneously injected into the contralateral site of the chamber position so that a systemic effect of ROPI in the reduction of fever can be excluded. The results indicate a participation of afferent neural signals in the manifestation of fever. This effect becomes obvious only if the dose of the applied inflammatory stimulus (LPS) is not high enough to activate a systemic generalised inflammatory response.

Interleukin-1beta reduces temperature sensitivity but elevates thermal thresholds in different populations of warm-sensitive hypothalamic neurons in rat brain slices

Extracellularly recorded firing rates of neurons in slices of the preoptic area and anterior hypothalamus (PO/AH) of the rat were determined during thermal stimulation. Human recombinant interleukin-1beta (20 ng/ml) did not influence temperature-insensitive neurons, but reduced the firing rate and thermosensitivity in linear warm-sensitive neurons, and shifted the thermal thresholds of activation in threshold warm- and cold-sensitive neurons by 1.1-2.3 degrees C to hyperthermic temperatures. The data support the suggestion that endogenous pyrogens may act on different populations of thermosensitive PO/AH neurons to induce fever. The shift of the thermal thresholds of activation of threshold warm- and cold-sensitive neurons in combination with the otherwise maintained temperature sensitivity of these neurons appears to play a major part for the controlled shift of body temperature and the maintenance of the elevated body temperature during cytokine-induced fever.