Stimulation of muscle protein degradation and prostaglandin E2 release by leukocytic pyrogen (interleukin-1). A mechanism for the increased degradation of muscle proteins during fever

To clarify the mechanisms underlying the loss of body protein during fever and sepsis, we incubated rat muscles with highly purified human leukocytic pyrogen. This polypeptide, which appears identical to interleukin-1, is released by leukocytes and signals the onset of fever in the hypothalamus. In muscles incubated at 37 degrees C, leukocytic pyrogen stimulated net protein degradation by 62 to 118 per cent (P less than 0.001). Proteolysis increased, but rates of muscle-protein synthesis did not change. The pyrogen also dramatically stimulated muscle synthesis of prostaglandin E2, which promotes protein breakdown in this tissue. Addition of indomethacin with leukocytic pyrogen prevented prostaglandin E2 synthesis and abolished the increase in proteolysis. The acceleration of protein breakdown induced by pyrogen was also blocked by Ep-475, an inhibitor of lysosomal thiol proteases. When muscles were incubated at 39 degrees C to mimic fever, protein breakdown increased, but addition of leukocytic pyrogen caused a further marked increase in proteolysis and prostaglandin E2 production. Thus, human leukocytic pyrogen can act on skeletal muscle to stimulate intralysosomal proteolysis by increasing the production of prostaglandin E2. These findings suggest that cyclooxygenase inhibitors may be useful in the treatment of negative nitrogen balance in fever. In addition, the release of prostaglandin E2 induced by leukocytic pyrogen may account for the myalgia that accompanies fever.

Interleukin 6 is involved in interleukin 1-induced activities

Human monocytes produce a number of soluble mediators involved in regulation of inflammation and lymphocyte growth and differentiation such as interleukin 1 (IL 1) and tumor necrosis factor. Recently, the cDNA of another monocyte-derived factor, interleukin 6 (IL 6), was cloned. Herein we show that purified E. coli-derived recombinant IL 6 (rIL 6) is as active as IL 1 in the thymocyte assay. In addition, IL 1 and IL 6 synergize strongly in stimulating thymocyte proliferation. Another property shared by IL 1 and IL 6 is their pyrogenicity. Human rIL 6 induces a monophasic fever after i.v. injection into rabbits. Together with the observation that IL 1 induces IL 6 in a variety of cells including thymocytes, our data suggest that IL 6 is involved in many of the pleiotropic effects of IL 1.

Cytokines as endogenous pyrogens

Cytokines are pleiotropic molecules mediating several pathologic processes. Long before the discovery of cytokines as immune system growth factors or as bone marrow stimulants, investigators learned a great deal about cytokines when they studied them as the endogenous mediators of fever. The terms "granulocytic" or "endogenous pyrogen" were used to describe substances with the biologic property of fever induction. Today, we recognize that pyrogenicity is a fundamental biologic property of several cytokines and hence the clinically recognizeable property of fever links host perturbations during disease with fundamental perturbations in cell biology. In this review, the discoveries made on endogenous pyrogens are revisited, with insights into the importance of the earlier work to the present-day understanding of cytokines in health and in disease.

Molecular basis of fever in humans

This review presents several areas of research on the pathogenesis of fever in humans and updates new information concerning the role of fever in host defense mechanisms. Fever is mediated by a polypeptide of phagocytic cell origin called leukocytic pyrogen. Several agents and disease processes are associated with the synthesis and release of leukocytic pyrogen. Although the original studies on leukocytic pyrogen suggested that the neutrophil was the primary source, recent experiments indicate the mononuclear phagocyte to be the major producer of leukocytic pyrogen. The mechanism by which human monocytes are stimulated to produce leukocytic pyrogen is discussed, including the effects of corticosteroids, estrogens and antipyretics on the synthesis of leukocytic pyrogen in vitro. The ability of leukocytic pyrogen to alter the hypothalamic thermoregulatory center by increasing arachidonic acid metabolite levels is the most likely mechanism by which leukocytic pyrogen initiates fever. Antipyretics prevent the synthesis of certain cyclooxygenase metabolites, which accounts for their ability to reduce fever. Studies on the chemical and physical properties of human leukocytic pyrogen are reviewed and form the basis for current experiments on the similarities between leukocytic pyrogen and lymphocyte activating factor. These studies suggest that leukocytic pyrogen, in addition to producing fever, also stimulates non-hypothalamic cells involved in aspects of the acute-phase response. In this regard, leukocytic pyrogen may be an important mechanism for host defenses. Hyperthermia may also be beneficial to the host but is distinct from fever; the role of leukocytic pyrogen as well as hyperthermia as a defense mechanism is discussed.

Signaling the brain in systemic inflammation: role of sensory circumventricular organs

The sensory circumventricular organs (CVOs) are specialized brain regions that lack a tight blood-brain barrier. A role for these brain structures in signaling the brain during systemic inflammation is based on the following sets of observations. In spite of some conflicting data from literature, lesions of CVOs have been shown to block several components of brain controlled illness responses (i.e. fever or neuroendocrine modifications). Receptors for inflammatory cytokines and for bacterial fragments are constitutively expressed in cells within the sensory CVOs. The expression of most of these receptors is upregulated under conditions of systemic inflammation. Cellular responses in theses brain areas can be recorded and documented after stimulation of these respective receptors. Such responses include changes in electrical activity of neurons, induction of transcription factors leading to modifications in gene expression during inflammation and to a localized release of secondary signal molecules. These molecules may influence or even gain access to neural structures inside the blood-brain barrier, which can normally not directly be reached by circulating cytokines or bacterial fragments.

Vagotomy attenuates the behavioural but not the pyrogenic effects of interleukin-1 in rats

Vagal afferent signals, have been implicated in cytokine mediated interactions between the periphery and the central nervous system. Studies in experimental animals have shown that cytokine induced activation of brain mediated responses to infection such as fever, sickness behaviour and pituitary-adrenal activation, are inhibited by subdiaphragmatic vagotomy. We have previously proposed that the peripheral signal to the brain in fever is of a humoral nature while others have suggested that either neural afferents or a mixture of both humoral and neural signals may be involved. The objective of the present study was to examine further the role of vagal transmission, in mediating the febrile response to a systemic injection of IL-1beta in rats and to compare this with changes in social exploration behaviour. Intraperitoneal injection of IL-1beta (1.0-30.0 microg/kg) inhibited social exploration in rats and this was attenuated in vagotomized animals. Injection of increasing concentrations of IL-1beta (0.1-1.0 microg/rat) induced significant (P<0.001) increases in core body temperature. However, in contrast to effects on social exploration, the increase in temperature was not inhibited by vagotomy at any of the doses used. These observations demonstrate a dissociation between the two brain mediated events, one of which is dependent on the integrity of the vagus nerve (social exploration) while the other (fever) is apparently generated by different mechanisms which may include circulating pyrogens.

The pharmacology of fever

The ability to minimise, if not prevent, large variations in deep body temperature that would otherwise result from some environmental conditions is a homeostatic function of unquestioned benefit that is demonstrated only by the more highly evolved animals. Nevertheless, body temperature is raised above normal values in many pathological conditions. This increase in temperature or fever is an active and co-ordinated response, which indicates the involvement of the CNS. Central injection and lesion studies have shown that the brain, in particular the PO/AH, is the site of action of fever-inducing agents, termed pyrogens. Electrophysiological data show that pyrogens modify the activity of central thermosensitive neurones as if to increase heat gain and decrease heat loss. The common response of fever to pyrogens of diverse origins is attributable to fever being mediated by an endogenous pyrogen released by phagocytic cells in the host. The mechanism by which central neuronal function is disturbed by pyrogens present in the periphery is not known. Tracer studies have yet to demonstrate the passage of a pyrogen across the blood-brain barrier. The possible involvement of several putative neurotransmitters and modulators in fever has been reviewed here, but most compounds have not been studied sufficiently to allow firm conclusions to be drawn. Much of the data is limited to the effects of the putative mediators on normal thermoregulation but, even when the effect is hyperthermia, such observations do not necessarily indicate a role for the endogenous material in fever. Dose-response curves for agonists and the effects of antagonists are often undetermined. This shortfall in data is due to some extent to the nature of fever; a central response in vivo over several hours. Although fever may enhance other host reactions to combat infection and inflammation, neither this benefit nor the undesirability of antipyretic therapy has been demonstrated unequivocally in either homeothermic laboratory animals or humans. Consequently, antipyretic drugs continue to be used clinically to alleviate the fever, malaise and/or pain commonly associated with disease. The drugs in common usage are the nonsteroidal antipyretic analgesics, many of which also have an anti-inflammatory effect. The primary mode of action of these drugs as antipyretics appears at present to be the inhibition of cyclo-oxygenase and a consequent reduction of prostanoid material in pyrogen-sensitive areas of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasmodium falciparum produces prostaglandins that are pyrogenic, somnogenic, and immunosuppressive substances in humans

Plasmodium falciparum causes the most severe form of human malaria, which kills approximately 1.5-2.7 million people every year, but the molecular mechanisms underlying the clinical symptoms and the host-parasite interaction remain unclear. We show here that P. falciparum produces prostaglandins (PGs) D2, E2, and F2alpha. After incubation with 1 mM arachidonic acid (AA), cell homogenates of P. falciparum produced PGs as determined by enzyme immunoassay and gas chromatography-selected ion monitoring. PG production in the parasite homogenate was not affected by the nonsteroidal antiinflammatory drugs aspirin and indomethacin, and was partially heat resistant, whereas PG biosynthesis by mammalian cyclooxygenase was completely inhibited by these chemicals and by heat treatment. Addition of AA to the parasite cell culture markedly increased an ability of the parasite cell homogenate to produce PGs and of parasitized red blood cells to accumulate PGs in the culture medium. PGD2 and PGE2 accumulated in the culture medium at the stages of trophozoites and schizonts more actively than at the ring stage. These findings are the first evidence of the direct involvement of a malaria parasite in the generation of substances that are pyrogenic and injurious to the host defenses. We will discuss a possible contribution of the parasite-produced PGs to pathogenesis and host-parasite interaction of P. falciparum.

Fever and aging

The pathogenesis and clinical relevance of fever is reviewed. The interrelationship between fever and other biologic responses to infection is summarized. A blunted or absent fever response to infections observed in some elderly patients may be due to defects in thermoregulation. These abnormalities in thermoregulation may include impairment of both behavioral and physiologic responses.

Endogenous pyrogens in the CNS: role in the febrile response

The febrile reaction is an integrated endocrine, autonomic and behavioral response, coordinated by the hypothalamus, that includes certain components of the stress response, such as elevated corticosteroid secretion. It is produced by the actions of circulating cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor (TNF), on the organum vasculosum of the lamina terminalis (OVLT), resulting in the secretion of prostaglandin E2, which initiates a variety of responses, including elevation of body temperature and corticosteroid secretion. Although circulating cytokines apparently do not enter the brain, injections of IL-1 or TNF well within the blood-brain barrier produce identical effects. We have examined the localization of possible central sources of cytokines and prostaglandins, using immunohistochemistry, immunoblotting and enzyme assay. Our data indicate that in the brain cyclooxygenase, the key enzyme in the synthesis of prostaglandins, is found in neurons in the OVLT, but is also made by neurons in many sensory and visceral regulatory systems. We present evidence also that IL-1 beta in the human brain and TNF alpha in the mouse may be present in the central nervous system as neuromodulators that are important for producing the autonomic, endocrine and behavioral components of the febrile reaction. We propose a sequence of events in the febrile reaction involving: (1) action of circulating cytokines on cyclooxygenase containing neurons within the OVLT to produce local prostaglandin secretion; (2) local diffusion of prostaglandin E2 into the preoptic and anterior hypothalamic areas; (3) action of prostaglandin E2 on cytokine containing neurons in the preoptic and anterior hypothalamic areas; and (4) release of cytokines from neuronal terminals at distal sites involved in producing the autonomic, endocrine and behavioral components of the febrile reaction.

Thermoregulation and the pathogenesis of fever

Infections, trauma, inflammatory processes, and some malignant diseases induce a constellation of host responses that are collectively referred to as the "acute-phase response." Elevation of core temperature is certainly part of the acute-phase response, and cytokines that raise core temperature in pathologic states are some of the same cytokines that account for other manifestations of the acute-phase response. This article examines fever as a part of the acute-phase reaction and the role of cytokines in thermoregulation.

Fever, temperature, and the immune response

Fever's ability to manipulate the character and extent of physiological temperature gradients correlates with the unusual influence different physiological temperatures have upon model immune responses in vitro. This relationship may help to explain the remarkable evolutionary conservation of the febrile response to infection. A very restricted range of the upper physiological temperatures supports the activation of resting lymphocytes for proliferation and effector formation in the two major limbs of the immune system, cell-mediated immunity and humoral immunity. In contrast, once effectors are formed they can function in a fashion which is nearly independent of physiological temperature. This suggests that physiological temperature change acts to regulate the emergence of new immune responses but does not restrict the activity of existing effector mechanisms once they have been formed. The differential sensitivity of these processes to different physiological temperatures suggests that fever's biological purpose with respect to the immune system is the elimination of lower peripheral tissue temperatures rather than the elevation of core temperatures. However, further studies may reveal that some functions are amplified by the core temperature transitions while other functions are selectively regulated by peripheral tissue temperature transitions. The critical cell for the temperature dependence of immune responses seems to be the Th since its ability to produce cytokines is highly temperature dependent. In contrast, macrophages produce cytokines equally well at all temperatures except those of the febrile core, a feature which may serve to downregulate the production of endogenous pyrogens.

Streptococcal pyrogenic exotoxin B induces apoptosis and reduces phagocytic activity in U937 cells

Treatment of U937 human monocyte-like cells with Streptococcus pyogenes led to an induction of apoptosis in these cells. A comparison between the wild-type strain and its isogenic protease-negative mutant indicated that the production of streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease, caused a greater extent of apoptosis in U937 cells. Further study using purified SPE B showed that this protease alone could induce U937 cells to undergo apoptosis, which was characterized by morphologic changes, DNA fragmentation laddering on the gel, and an increase in the percentages of hypodiploid cells. The protease activity of SPE B was required for apoptosis to proceed, since treatment with cysteine protease inhibitor E64 or heat inactivation abrogated this death-inducing effect. The SPE B-induced apoptosis pathway was interleukin-1beta converting enzyme (ICE) family protease dependent. Further experiments showed that the phagocytic activity of U937 cells was reduced by SPE B. Treatment with E64 and heat inactivation both abrogated this phagocytosis-inhibitory effect. Taken together, the present data show that SPE B not only possesses the ability to induce apoptosis in monocytic cells but also helps bacteria to resist phagocytosis by host cells.

Characterization of a leukocyte-derived endogenous mediator responsible for increased plasma fibrinogen

Fibrinogen has been the plasma protein most frequently studied after tissue injury. This report presents evidence that leukocytic endogenous mediator (LEM) from macrophages promotes fibrinogen synthesis. LEM has a molecular weight of 13,000-16,000, an isoelectric point (pI) at pH 7.3, is heat labile, and is inactivated by trypsin or sulfhydryl reactive agents. LEM not only promotes increased synthesis of acute phase proteins, but also causes increased neutrophilia and alterations in metal metabolism. There is considerable evidence that LEM may be the same protein as endogenous pyrogen and Interleukin 1 (IL-1). There was no increase in plasma fibrinogen when endotoxin was injected in C3H/HeJ mice; however, this strain of mice responded the same as normal mice to injections of LEM. This provides further evidence that LEM is the endogenous mediator for acute phase protein synthesis during tissue injury. The half-life of LEM is still circulation following its iv injection into rats was less than 10 minutes. There is still considerable doubt about the mechanism LEM uses in promoting increased hepatocyte synthesis of fibrinogen. Some evidence indicates a direct action of LEM upon the hepatocyte, whereas other data suggest an indirect role through other mediators or the central nervous system. In addition to LEM with pI of 7.3, there are proteins with a pI near 5 that will increase plasma fibrinogen. These proteins also have a molecular weight between 13,000 to 16,000 but do not have essential sulfhydryl groups. These proteins also have pyrogenic and IL-1 activities. LEM shows a limited amount of species specificity. For example, the pI 7 LEM prepared from human monocytes or rabbit peritoneal leukocytes will increase plasma fibrinogen in rats, mice, and rabbits; but the pI 5 LEM from rabbits is inactive in rats.

Microbial cell-wall contaminants in peptides: a potential source of physiological artifacts

Microbial cell-wall products (MCWP) such as endotoxins are easily introduced into peptides produced under standard laboratory conditions. Because these products stimulate the induction of cytokines and other mediators, which, in turn, trigger a broad range of physiological responses. MCWP in peptide preparations are potential sources of artifacts. This brief tutorial outlines the physical/chemical nature of MCWP, some of their sources, their physiological effects, and a simple method to control for them in some peptide preparations.

Further similarities of endogenous pyrogen and leukocytic endogenous mediator

The release of endogenous pyrogen (EP) from rabbit peritoneal granulocytes was measured with a three-point log dose-response curve. Release of EP was inhibited when the cells were incubated in media containing potassium or calcium. Measurements of leukocytic endogenous mediator (LEM) activity, i.e., lowering of plasma iron and zinc and increases in blood neutrophils, were made on the same supernatant media. When EP release was inhibited there was a similar inhibition of LEM activity. These results indicate a similarity between the release of pyrogenic and LEM activities. Together with previous purification studies, the results suggest that EP and LEM are similar and may be identical factors.

Role of the nitric oxide/cyclic GMP/Ca2+ signaling pathway in the pyrogenic effect of interleukin-1beta

Interleukin-1beta (IL-1beta) has a wide spectrum of inflammatory, metabolic, haemopoietic, and immunological properties. Because it produces fever when injected into animals and humans, it is considered an endogenous pyrogen. There is evidence to suggest that Ca2+ plays a critical role in the central mechanisms of thermoregulation, and in the intracellular signaling pathways controlling fever induced by IL-1beta and other pyrogens. Data from different labs indicate that Ca2+ and Na+ determine the temperature set point in the posterior hypothalamus (PH) of various mammals and that changes in Ca2+ and PGE2 concentrations in the cerebrospinal fluid (CSF) of these animals are associated with IL-1beta-induced fever. Antipyretic drugs such as acetylsalicylic acid, dexamethasone, and lipocortin 5-(204-212) peptide counteract IL-1beta-induced fever and abolish changes in Ca2+ and PGE2 concentrations in CSF. In vitro studies have established that activation of the nitric oxide (NO)/cyclic GMP (cGMP) pathway is part of the signaling cascade transducing Ca2+ mobilization in response to IL-1beta and that the ryanodine (RY)- and inositol-(1,4,5)-trisphosphate (IP3)-sensitive pools are the main source of the mobilized Ca2+. It is concluded that the NO/cGMP/Ca2+ pathway is part of the signaling cascade subserving some of the multiple functions of IL-1beta.

The toxins of group A streptococcus, the flesh eating bacteria

Streptococcus pyogenes causes a wide variety of infections in individuals of all ages in most countries of the world. Because of the frequency with which these infections occur, physicians are quite familiar with the diversity of clinical presentations associated with the Group A streptococcus. Yet in the late 1980's, a severe form of streptococcal infection, the Streptococcal Toxic Shock Syndrome, emerged and has persisted for the last 10 years. This syndrome is associated with invasive soft tissue infections and the early onset of shock and organ failure. The purpose of this paper is to briefly describe the epidemiologic and clinical features of the Streptococcal Toxic Shock Syndromes and to emphasize the role that toxins produced by S. pyogenes play in the pathogenesis of this disease.

Antagonism of central glucagon-like peptide-1 receptors enhances lipopolysaccharide-induced fever

Lipopolysaccharide (LPS; a model of systemic bacterial infection) causes fever and activates glucagon-like peptide-1 (GLP-1) neurons in the caudal brainstem. The present study examined whether central GLP-1 receptor signaling plays a functional role in LPS-induced fever. Adult male Sprague-Dawley rats were injected i.p. with LPS (0 or 100 microg/kg), then were infused intracerebroventricularly with GLP-1 receptor antagonist (0 or 10 microg) delivered 2.5 h after injection of LPS or vehicle. Core body temperature was measured at 30-min intervals for 6.5 h after LPS treatment. Consistent with previous reports, body temperature was significantly elevated within 90 min and remained elevated for the remainder of the monitoring period. The pyrogenic effect of LPS was amplified in rats that received central infusion of GLP-1 receptor antagonist, although the antagonist by itself did not alter body temperature. These findings suggest that endogenous GLP-1 acts at central receptors to limit the fever response in rats after i.p. administration of LPS.