Complement component c5a is integral to the febrile response of mice to lipopolysaccharide

Mononuclear phagocyte morphological response to chemoattractants is dependent on geranylgeranyl pyrophosphate

Statins are used to treat hypercholesterolemia and function by inhibiting the production of the rate-limiting metabolite mevalonate. As such, statin treatment not only inhibits de novo synthesis of cholesterol but also isoprenoids that are involved in prenylation, the posttranslational lipid modification of proteins. The immunomodulatory effects of statins are broad and often conflicting. Previous work demonstrated that statins increased survival and inhibited myeloid cell trafficking in a murine model of sepsis, but the exact mechanisms underlying this phenomenon were unclear. Herein, we investigated the role of prenylation in chemoattractant responses. We found that simvastatin treatment abolished chemoattractant responses induced by stimulation by C5a and FMLP. The inhibitory effect of simvastatin treatment was unaffected by the addition of either farnesyl pyrophosphate (FPP) or squalene but was reversed by restoring geranylgeranyl pyrophosphate (GGPP). Treatment with prenyltransferase inhibitors showed that the chemoattractant response to both chemoattractants was dependent on geranylgeranylation. Proteomic analysis of C15AlkOPP-prenylated proteins identified several geranylgeranylated proteins involved in chemoattractant responses, including RHOA, RAC1, CDC42, and GNG2. Chemoattractant responses in THP-1 human macrophages were also geranylgeranylation dependent. These studies provide data that help clarify paradoxical findings on the immunomodulatory effects of statins. Furthermore, they establish the role of geranylgeranylation in mediating the morphological response to chemoattractant C5a.NEW & NOTEWORTHY The immunomodulatory effect of prenylation is ill-defined. We investigated the role of prenylation on the chemoattractant response to C5a. Simvastatin treatment inhibits the cytoskeletal remodeling associated with a chemotactic response. We showed that the chemoattractant response to C5a was dependent on geranylgeranylation, and proteomic analysis identified several geranylgeranylated proteins that are involved in C5a receptor signaling and cytoskeletal remodeling. Furthermore, they establish the role of geranylgeranylation in mediating the response to chemoattractant C5a.

Bupleurum chinense exerts a mild antipyretic effect on LPS-induced pyrexia rats involving inhibition of peripheral TNF-α production

ETHNOPHARMACOLOGICAL RELEVANCE

Bupleuri Radix, the dried roots of Bupleurum chinense DC. (BC) or Bupleurum scorzonerifolium Willd., is one of the most frequently used traditional Chinese medicines. As the species in Xiao-Chai-Hu decoction, BC has been used as an antipyretic medicine with a long history. However, its antipyretic characteristics and underlying mechanism(s) remain unclear.

AIM OF THE STUDY

To elucidate the antipyretic characteristics and mechanism(s) of BC used in its traditional way.

METHODS

The water extract of BC (BCE) was prepared according to the traditional decocting mode. Murine fever and endotoxemia models were induced by intravenous injection of lipopolysaccharide (LPS). In vitro complement activation assay and the levels of TNF-α, IL-6, IL-1β, and C5a were determined by ELISA.

RESULTS

BCE exerted a confirmed but mild antipyretic effect on LPS-induced fever of rat. In vitro, it significantly lowered LPS-elevated TNF-α in the supernatant of rat complete blood cells and THP-1 cells, but failed to decrease IL-6 and IL-1β. In murine endotoxemia models, BCE markedly decreased serum TNF-α, but had no impact on IL-6 and IL-1β. BCE also restricted complement activation in vitro and in vivo. Nevertheless, the mixture of saikosaponin A and D could not suppress supernatant TNF-α of monocytes and serum TNF-α of endotoxemia mice.

CONCLUSIONS

The present study dissects the peripheral mechanism for the antipyretic effect of BC used in the traditional way. Our findings indicate that BCE directly suppresses monocyte-produced TNF-α, thus decreasing circulating TNF-α, which may be responsible for its mild but confirmed antipyretic action.

Circulating and broncho-alveolar interleukin-6 in relation to body temperature in an experimental model of bovine Chlamydia psittaci infection

In rodent models of experimentally induced fever, the important role of interleukin-6 (IL-6) as a circulating endogenous pyrogen is well established. Studies employing larger animal species and real infections are scarce. Therefore, we assessed bioactive IL-6 in peripheral blood and in broncho-alveolar lavage fluid (BALF) of calves after intra-bronchial inoculation with vital Chlamydia psittaci (Cp), with inactivated Cp, or with BGM cells. Only calves inoculated with vital Cp developed fever (peak at 2-3 days after challenge) and significantly increased IL-6 activity. Controls inoculated with either inactivated Cp or BGM cells also expressed increased bioactive IL-6, but no fever developed. Activity of IL-6 in BALF was significantly higher compared to blood serum. This experimental model of Cp infection revealed no apparent relation between IL-6 in blood and body temperature, but did reveal a relation between IL-6 and other markers of inflammation in BALF. We conclude that a local inflammatory response in the lungs of infected calves caused fever, which developed by mechanisms including other mediators besides IL-6.

Polysaccharides from Arnebia euchroma Ameliorated Endotoxic Fever and Acute Lung Injury in Rats Through Inhibiting Complement System

Arnebiaeuchroma (Royle) Johnst (Ruanzicao) is a traditional Chinese herbal medicine (TCM). It is extensively used in China and other countries for treatment of inflammatory diseases. It is known that hyper-activated complement system involves in the fever and acute lung injury (ALI) in rats. In our preliminary studies, anti-complementary activity of crude Arnebiaeuchroma polysaccharides (CAEP) had been demonstrated in vitro. This study aimed to investigate the role and mechanism of crude Arnebiaeuchroma polysaccharides (CAEP) using two animal models, which relate with inappropriate activation of complement system. In lipopolysaccharide (LPS)-induced fever model, the body temperature and leukocytes of peripheral blood in rats were significantly increased, while the complement levels of serum were remarkably decreased. CAEP administration alleviated the LPS-induced fever, reduced the number of leukocytes, and improved the levels of complement. Histological assay showed that there were severe damages and complement depositions in lung of the ALI rats. Further detection displayed that the oxidant stress was enhanced, and total hemolytic activity and C3/C4 levels in serum were decreased significantly in the ALI model group. Remarkably, CAEP not only attenuated the morphological injury, edema, and permeability in the lung but also significantly weakened the oxidant stress in bronchoalveolar lavage fluid (BALF) in the ALI rats. The levels of complement and complement depositions were improved by the CAEP treatment. In conclusion, the CAEP treatment ameliorated febrile response induced by LPS and acute lung injury induced by LPS plus ischemia-reperfusion. CAEP exerted beneficial effects on inflammatory disease potentially via inhibiting the inappropriate activation of complement system.

PKR deficiency alters E. coli-induced sickness behaviors but does not exacerbate neuroimmune responses or bacterial load

Background

Systemic inflammation induces neuroimmune activation, ultimately leading to sickness (e.g., fever, anorexia, motor impairments, exploratory deficits, and social withdrawal). In this study, we evaluated the role of protein kinase R (PKR), a serine-threonine kinase that can control systemic inflammation, on neuroimmune responses and sickness.

Methods

Wild-type (WT) PKR+/+ mice and PKR−/− mice were subcutaneously injected with live Escherichia coli (E. coli) or vehicle. Food consumption, rotarod test performance, burrowing, open field activity, object investigation, and social interaction were monitored. Plasma TNF-α and corticosterone were measured by ELISA. The percentage of neutrophils in blood was deduced from blood smears. Inflammatory gene expression (IL-1β, TNF-α, IL-6, cyclooxygenase (COX)-2, iNOS) in the liver and the brain (hypothalamus and hippocampus) were quantified by real-time PCR. Blood and lavage fluid (injection site) were collected for microbiological plate count and for real-time PCR of bacterial 16S ribosomal DNA (rDNA). Corticotrophin-releasing hormone (CRH) expression in the hypothalamus was also determined by real-time PCR.

Results

Deficiency of PKR diminished peripheral inflammatory responses following E. coli challenge. However, while the core components of sickness (anorexia and motor impairments) were similar between both strains of mice, the behavioral components of sickness (reduced burrowing, exploratory activity deficits, and social withdrawal) were only observable in PKR−/− mice but not in WT mice. Such alteration of behavioral components was unlikely to be caused by exaggerated neuroimmune activation, by an impaired host defense to the infection, or due to a dysregulated corticosterone response, because both strains of mice displayed similar neuroimmune responses, bacterial titers, and plasma corticosterone profiles throughout the course of infection. Nevertheless, the induction of hypothalamic corticotrophin-releasing hormone (CRH) by E. coli was delayed in PKR−/− mice relative to WT mice, suggesting that PKR deficiency may postpone the CRH response during systemic inflammation.

Conclusions

Taken together, our findings show that (1) loss of PKR could alter E. coli-induced sickness behaviors and (2) this was unlikely to be due to exacerbated neuroimmune activation, (3) elevated bacterial load, or (4) dysregulation in the corticosterone response. Further studies can address the role of PKR in the CRH response together with its consequence on sickness.

Sickness: From the focus on cytokines, prostaglandins, and complement factors to the perspectives of neurons

Systemic inflammation leads to a variety of physiological (e.g. fever) and behavioral (e.g. anorexia, immobility, social withdrawal, depressed mood, disturbed sleep) responses that are collectively known as sickness. While these phenomena have been studied for the past few decades, the neurobiological mechanisms by which sickness occurs remain unclear. In this review, we first revisit how the body senses and responds to infections and injuries by eliciting systemic inflammation. Next, we focus on how peripheral inflammatory molecules such as cytokines, prostaglandins, and activated complement factors communicate with the brain to trigger neuroinflammation and sickness. Since depression also involves inflammation, we further elaborate on the interrelationship between sickness and depression. Finally, we discuss how immune activation can modulate neurons in the brain, and suggest future perspectives to help unravel how changes in neuronal functions relate to sickness responses.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

Role of C3, C5 and anaphylatoxin receptors in acute lung injury and in sepsis

The complement system plays a major role in innate immune defenses against infectious agents, but exaggerated activation of complement can lead to severe tissue injury. Systemic (intravascular) activation of complement can, via C5a, lead to neutrophil (PMN) activation, sequestration and adhesion to the pulmonary capillary endothelium, resulting in damage and necrosis of vascular endothelial cells and acute lung injury (ALI). Intrapulmonary (intraalveolar) activation of complement can cause ALI that is complement and PMN-dependent, resulting in a cytokine/chemokine storm that leads to intense ALI. Surprisingly, C3(-/-) mice develop the full intensity of ALI in a C5a-dependent manner due to the action of thrombin that generates C5a directly from C5. There is conflicting evidence on the role of the second C5a receptor, C5L2 in development of ALI. There is accumulating evidence that C5a may suppress inflammatory responses or divert them from Th1 to Th2 responses, impacting the innate immune system. Finally, in experimental polymicrobial sepsis, there is evidence that many of the adverse outcomes can be linked to the roles of C5a and engagement of its two receptors, C5aR and C5L2. These observations underscore the diversity of effects of C5a in a variety of inflammatory settings.

Early life activation of toll-like receptor 4 reprograms neural anti-inflammatory pathways

A single postnatal exposure to the bacterial endotoxin, lipopolysaccharide (LPS), reduces the neuroimmune response to a subsequent LPS exposure in the adult rat. The attenuated fever and proinflammatory response is caused by a paradoxical, amplified, early corticosterone response to LPS. Here we identify the mechanisms underlying the heightened corticosterone response to LPS in adults after early life exposure to LPS. In postnatal LPS-treated rats, hypothalamic corticotrophin-releasing hormone mRNA, pituitary proopiomelanocortin mRNA, and circulating adrenocorticotrophic hormone were all increased after adult exposure to LPS without significant modification to hippocampal or hypothalamic glucocorticoid receptor mRNA or protein or vagally mediated afferent signaling to the brain. Postnatal LPS administration did cause a persistent upregulation of the LPS Toll-like receptor-4 (TLR4) mRNA in liver and spleen, but not in brain, pituitary, or adrenal gland. In addition, cyclooxygenase-2 (COX-2), which is a prostaglandin biosynthetic enzyme and is normally undetectable in most peripheral tissue, was constitutively expressed in the liver. Adult immune activation of the upregulated TLR4 and COX-2 caused a rapid, amplified rise in circulating, but not brain, prostaglandin E(2) that induced an early, enhanced activation of the hypothalamic-pituitary-adrenal (HPA) axis. Thus, postnatal LPS reprograms the neuroimmune axis by priming peripheral tissues to create a novel, prostaglandin-mediated activation of the HPA axis brought about by increased constitutive expression of TLR4 and COX-2.

Effects of lipopolysaccharide on the induction of mixed chimerism in cyclophosphamide-induced tolerance

Cyclophosphamide (CP)-induced tolerance is a mixed chimerism-based tolerance and is one of the strategies used to induce transplant tolerance. Toll-like receptor (TLR) agonists are reportedly able to abrogate the induction of tolerance by activating alloreactive T cells, or by inhibiting Treg cells. However, little is known about the effect of the immune response mediated by TLR on mixed chimerism-based tolerance protocols. In this study, we evaluated the influence of lipopolysaccharide (LPS), which is best known as an TLR4 agonist, on CP-induced tolerance. BALB/c (H-2(d)) mice received a conditioning regimen consisting of 10(8) donor DBA/2 (H-2(d)) spleen cells (SC) on day 0 and 200 mg/kg CP on day 2. A single dose of 20 microg LPS was injected on day -2, 0, 7, or 35. Our results showed that LPS infusion at any time point resulted in chronic rejection of donor skin grafts and the abrogation of mixed chimerism in 33-60% of recipients. We found a correlation between skin graft acceptance and higher levels of mixed chimerism. Flow cytometric analysis revealed that donor-reactive T cells were permanently eliminated, regardless of LPS infusion. In conclusion, LPS-infusion had little influence on the immune response of donor-reactive T cells, but had a significant effect on the induction and maintenance of mixed chimerism in CP-induced tolerance.

Sepsis-associated delirium: the pro and con of C5a blockade

The intimate mechanisms of sepsis-induced delirium are unknown. Among the potential contributing factors, the breakdown of the blood–brain barrier is considered a key determinant of brain dysfunction. The complement activation is paramount to an appropriate activation of the central nervous system during stress. C3a and C5a have been extensively studied and may be involved in sepsis-induced delirium. Here we discuss the pro and con for inhibiting C5a to attenuate brain damage during sepsis. In particular, we discuss the hypothesis that C5a increased blood–brain barrier permeability amy ease the brain to mount an appropriate response to sepsis. Thus, blockade of C5a may be detrimental, resulting in an attenuated response of the stress system.

Living history: Clark M. Blatteis

In 2005, the American Physiological Society (APS) initiated the Living History Project to recognize senior members who have made extraordinary contributions during their career to the advancement of the discipline and profession of physiology. During 2007, the APS Section of Environmental and Exercise Physiology selected Clark M. Blatteis to be profiled in Advances in Physiology Education.

Acute lung injury induced by lipopolysaccharide is independent of complement activation

Although acute lung injury (ALI) is an important problem in humans, its pathogenesis is poorly understood. Airway instillation of bacterial LPS, a known complement activator, represents a frequently used model of ALI. In the present study, pathways in the immunopathogenesis of ALI were evaluated. ALI was induced in wild-type, C3(-/-), and C5(-/-) mice by airway deposition of LPS. To assess the relevant inflammatory mediators, bronchoalveolar lavage fluids were evaluated by ELISA analyses and various neutralizing Abs and receptor antagonists were administered in vivo. LPS-induced ALI was neutrophil-dependent, but it was not associated with generation of C5a in the lung and was independent of C3, C5, or C5a. Instead, LPS injury was associated with robust generation of macrophage migration inhibitory factor (MIF), leukotriene B(4) (LTB4), and high mobility group box 1 protein (HMGB1) and required engagement of receptors for both MIF and LTB4. Neutralization of MIF or blockade of the MIF receptor and/or LTB4 receptor resulted in protection from LPS-induced ALI. These findings indicate that the MIF and LTB4 mediator pathways are involved in the immunopathogenesis of LPS-induced experimental ALI. Most strikingly, complement activation does not contribute to the development of ALI in the LPS model.

Sub-pyrogenic systemic inflammation impacts on brain and behavior, independent of cytokines

Systemic inflammation impacts on the brain and gives rise to behavioral changes, often referred to as 'sickness behavior'. These symptoms are thought to be mainly mediated by pro-inflammatory cytokines. We have investigated the communication pathways between the immune system and brain following sub-pyrogenic inflammation. Low grade systemic inflammation was induced in mice using lipopolysaccharide (LPS); 1-100 microg/kg to mimic aspects of bacterial infection. Changes in fever, open-field activity, burrowing and consumption of glucose solution were assessed and immune activation was studied in the periphery and brain by measuring cytokine production, and immunohistochemistry to study changes in immune cell phenotype. Sub-pyrogenic inflammation resulted in changes in a species-typical, untrained behavior (burrowing) that depends on the integrity of the hippocampus. Increased expression of cytokines was observed in the periphery and selected regions of the brain which coincided with changes in behavior. However, peripheral neutralization of LPS-induced pro-inflammatory cytokines IL-1beta, IL-6 and TNF-alpha did not abrogate the LPS-induced behavioral changes nor affect CNS cytokine synthesis. In contrast, pretreatment of mice with indomethacin completely prevented LPS-induced behavior changes, without affecting cytokine levels. Taken together, these experiments suggest a key role for prostaglandins, rather than cytokines, in communicating to the brain.

Endotoxic fever: New concepts of its regulation suggest new approaches to its management

Endotoxic fever is regulated by endogenous factors that provide pro- and anti-pyretic signals at different points along the febrigenic pathway, from the periphery to the brain. Current evidence indicates that the febrile response to invading Gram-negative bacteria and their products is initiated upon their arrival in the liver via the circulation and their uptake by Kupffer cells (Kc). These pathogens activate the complement cascade on contact, hence generating complement component 5a. It, in turn, very rapidly stimulates Kc to release prostaglandin (PG)E2. Pyrogenic cytokines (TNF-alpha, etc.) are produced later and are no longer considered to be the immediate triggers of fever. The Kc-generated PGE2 either (1) may be transported by the bloodstream to the ventromedial preoptic-anterior hypothalamus (POA, the locus of the temperature-regulating center), presumptively diffusing into it and acting on thermoregulatory neurons; PGE2 is thus taken to be the final, central fever mediator. Or (2) it may activate hepatic vagal afferents projecting to the medulla oblongata, thence to the POA via the ventral noradrenergic bundle. Norepinephrine consequently secreted stimulates alpha1-adrenoceptors on thermoregulatory neurons, rapidly evoking an initial rise in core temperature (Tc) not associated with any change in POA PGE2; this neural, PGE2-independent signaling pathway is quicker than the blood-borne route. Elevated POA PGE2 and a secondary Tc rise occur later, consequent to alpha2 stimulation. Endogenous counter-regulatory factors are also elaborated peripherally and centrally at different points during the course of the febrile response; they are, therefore, anti-pyretic. These multiple interacting pathways are the subject of this review.

Kupffer cell-generated PGE2triggers the febrile response of guinea pigs to intravenously injected LPS

Because the onset of fever induced by intravenously (iv) injected bacterial endotoxic lipopolysaccharides (LPS) precedes the appearance in the bloodstream of pyrogenic cytokines, the presumptive peripheral triggers of the febrile response, we have postulated previously that, in their stead, PGE2could be the peripheral fever trigger because it appears in blood coincidentally with the initial body core temperature (Tc) rise. To test this hypothesis, we injected Salmonella enteritidis LPS (2 μg/kg body wt iv) into conscious guinea pigs and measured their plasma levels of LPS, PGE2, TNF-α, IL-1β, and IL-6 before and 15, 30, 60, 90, and 120 min after LPS administration; Tcwas monitored continuously. The animals were untreated or Kupffer cell (KC) depleted; the essential involvement of KCs in LPS fever was shown previously. LPS very promptly (<10 min) induced a rise of Tcthat was temporally correlated with the elevation of plasma PGE2. KC depletion prevented the Tcand plasma PGE2rises and slowed the clearance of LPS from the blood. TNF-α was not detectable in plasma until 30 min and in IL-1β and IL-6 until 60 min after LPS injection. KC depletion did not alter the times of appearance or magnitudes of rises of these cytokines, except TNF-α, the maximal level of which was increased approximately twofold in the KC-depleted animals. In a follow-up experiment, PGE2antiserum administered iv 10 min before LPS significantly attenuated the febrile response to LPS. Together, these results support the view that, in guinea pigs, PGE2rather than pyrogenic cytokines is generated by KCs in immediate response to iv LPS and triggers the febrile response.

C3a expressed in the central nervous system protects against LPS-induced shock

Complement is implicated in the pathogenesis of inflammatory disorders of the central nervous system (CNS), like multiple sclerosis, Alzheimer's disease, and trauma. The anaphylatoxins C3a and C5a are thought to be the major contributors to complement-mediated inflammation in the CNS, likely mediating their effects via their ability to attract and activate leukocytes and common capacity to augment inflammation. For example, in experimental autoimmune encephalomyelitis, the animal model of multiple sclerosis, CNS-specific expression of C3a in C3a/GFAP transgenic mice renders them prone to massive cellular infiltration of the CNS and increases their mortality. In contrast, other studies have suggested that C3a can function in an anti-inflammatory fashion in the CNS, by inducing neurotrophin production and preventing NMDA-mediated neurotoxicity. To further investigate the seemingly paradoxical role of C3a in acute inflammation of the brain, we studied the pathogenesis of endotoxin shock in C3a/GFAP transgenic, C3a receptor-deficient (C3aR-/-) and C3a/GFAPxC3aR-/- mutant mice. Here we report that C3a/GFAP mice were significantly more resistant to endotoxin-induced lethality than wild-type and C3aR-/- mice. Surprisingly, C3a/GFAPxC3aR-/- hybrids were also significantly protected, indicating that C3a exerts its protective anti-inflammatory effect either directly or via an as yet unidentified non-canonical C3aR.

LPS-activated complement, not LPS per se, triggers the early release of PGE2by Kupffer cells

The intravenous injection of LPS rapidly evokes fever. We have hypothesized that its onset is mediated by prostaglandin (PG)E2quickly released by Kupffer cells (Kc). LPS, however, does not stimulate PGE2production by Kc as rapidly as it induces fever; but complement (C) activated by LPS could be the exciting agent. To test this hypothesis, we injected LPS (2 or 8 μg/kg) or cobra venom factor (CVF, an immediate activator of the C cascade that depletes its substrate, ultimately causing hypocomplementemia; 25 U/animal) into the portal vein of anesthetized guinea pigs and measured the appearance of PGE2, TNF-α, IL-1β, and IL-6 in the inferior vena cava (IVC) over the following 60 min. LPS (at both doses) and CVF induced similar rises in PGE2within the first 5 min after treatment; the rises in PGE2due to CVF returned to control in 15 min, whereas PGE2rises due to LPS increased further, then stabilized. LPS given 3 h after CVF to the same animals also elevated PGE2, but after a 30- to 45-min delay. CVF per se did not alter basal PGE2and cytokine levels and their responses to LPS. These in vivo effects were substantiated by the in vitro responses of primary Kc from guinea pigs to C (0.116 U/ml) and LPS (200 ng/ml). These results indicate that LPS-activated C rather than LPS itself triggers the early release of PGE2by Kc.

Cytokines, PGE2 and endotoxic fever: a re-assessment

The innate immune system serves as the first line of host defense against the deleterious effects of invading infectious pathogens. Fever is the hallmark among the defense mechanisms evoked by the entry into the body of such pathogens. The conventional view of the steps that lead to fever production is that they begin with the biosynthesis of pyrogenic cytokines by mononuclear phagocytes stimulated by the pathogens, their release into the circulation and transport to the thermoregulatory center in the preoptic area (POA) of the anterior hypothalamus, and their induction there of cyclooxygenase (COX)-2-dependent prostaglandin (PG)E(2), the putative final mediator of the febrile response. But data accumulated over the past 5 years have gradually challenged this classical concept, due mostly to the temporal incompatibility of the newer findings with this concatenation of events. Thus, the former studies generally overlooked that the production of cytokines and the transduction of their pyrogenic signals into fever-mediating PGE(2) proceed at relatively slow rates, significantly slower certainly than the onset latency of fever produced by the i.v. injection of bacterial endotoxic lipopolysaccharides (LPS). Here, we review the conflicts between the earlier and the more recent findings and summarize new data that reconcile many of the contradictions. A unified model based on these data explicating the generation and maintenance of the febrile response is presented. It postulates that the steps in the production of LPS fever occur in the following sequence: the immediate activation by LPS of the complement (C) cascade, the stimulation by the anaphylatoxic C component C5a of Kupffer cells, their consequent, virtually instantaneous release of PGE(2), its excitation of hepatic vagal afferents, their transmission of the induced signals to the POA via the ventral noradrenergic bundle, and the activation by the thus, locally released norepinephrine (NE) of neural alpha(1)- and glial alpha(2)-adrenoceptors. The activation of the first causes an immediate, PGE(2)-independent rise in core temperature (T(c)) [the early phase of fever; an antioxidant-sensitive PGE(2) rise, however, accompanies this first phase], and of the second a delayed, PGE(2)-dependent T(c) rise [the late phase of fever]. Meanwhile-generated pyrogenic cytokines and their consequent upregulation of blood-brain barrier cells COX-2 also contribute to the latter rise. The consecutive steps that initiate the febrile response to LPS would now appear, therefore, to occur in an order different than conceived originally.