Pyrexia, anorexia, adipsia, and depressed motor activity in rats during systemic inflammation induced by the Toll-like receptors-2 and -6 agonists MALP-2 and FSL-1

Lipoteichoic acid, a cell wall component of Gram-positive bacteria, induces sleep and fever and suppresses feeding

Fragments of the bacterial cell wall are bioactive microbial molecules that have profound effects on the function of the brain. Some of the cell wall constituents are common to both Gram-positive and Gram-negative bacteria, e.g., peptidoglycans, while other cell wall components are specific to either Gram-positive or Gram-negative microbes. Lipopolysaccharide (LPS), also called endotoxin, is found exclusively in Gram-negative bacteria, while lipoteichoic acid (LTA) is specific to Gram-positive bacteria. The effects of peptidoglycans, their fragments, and LPS are well characterized, they induce sleep, fever and anorexia. In the present study, we investigated the sleep, body temperature and food intake modulating effects of LTA. We found that intraperitoneal injection of 100 and 250 μg LTA from B. subtilis and S. aureus increases non-rapid-eye movement sleep (NREMS) in mice. The effects were dose-dependent, and the changes were accompanied by decreased motor activity and feeding as well as febrile responses. Intraperitoneal injection of 10 μg LTA induced monophasic increases in body temperature, while 100 and 250 μg LTA from B. subtilis induced initial hypothermia followed by fever. Treatment with 250 μg LTA from S. aureus elicited monophasic hypothermia. Administration of 300 μg/kg LTA from S. aureus directly into the portal vein elicited similar sleep responses in rats but did not affect body temperature. The sleep-modulating effects of LTA were similar to that of LPS in mice, although LTA appears to be less potent. These findings suggest that the role of LTA in signaling by Gram-positive bacteria in the host body is analogous to the role of LPS/endotoxin in signaling by Gram-negative microbes. LTA may play a role in the development of sickness response in clinically manifest Gram-positive bacterial infections and may contribute to sleep signaling by the commensal intestinal microbiota.

Visceral Inflammation and Immune Activation Stress the Brain

Stress refers to a dynamic process in which the homeostasis of an organism is challenged, the outcome depending on the type, severity, and duration of stressors involved, the stress responses triggered, and the stress resilience of the organism. Importantly, the relationship between stress and the immune system is bidirectional, as not only stressors have an impact on immune function, but alterations in immune function themselves can elicit stress responses. Such bidirectional interactions have been prominently identified to occur in the gastrointestinal tract in which there is a close cross-talk between the gut microbiota and the local immune system, governed by the permeability of the intestinal mucosa. External stressors disturb the homeostasis between microbiota and gut, these disturbances being signaled to the brain via multiple communication pathways constituting the gut-brain axis, ultimately eliciting stress responses and perturbations of brain function. In view of these relationships, the present article sets out to highlight some of the interactions between peripheral immune activation, especially in the visceral system, and brain function, behavior, and stress coping. These issues are exemplified by the way through which the intestinal microbiota as well as microbe-associated molecular patterns including lipopolysaccharide communicate with the immune system and brain, and the mechanisms whereby overt inflammation in the GI tract impacts on emotional-affective behavior, pain sensitivity, and stress coping. The interactions between the peripheral immune system and the brain take place along the gut-brain axis, the major communication pathways of which comprise microbial metabolites, gut hormones, immune mediators, and sensory neurons. Through these signaling systems, several transmitter and neuropeptide systems within the brain are altered under conditions of peripheral immune stress, enabling adaptive processes related to stress coping and resilience to take place. These aspects of the impact of immune stress on molecular and behavioral processes in the brain have a bearing on several disturbances of mental health and highlight novel opportunities of therapeutic intervention.

Toll-Like Receptor 2 Is a Regulator of Circadian Active and Inactive State Consolidation in C57BL/6 Mice

Regulatory systems required to maintain behavioral arousal remain incompletely understood. We describe a previously unappreciated role that toll-like receptor 2 (Tlr2, a membrane bound pattern recognition receptor that recognizes specific bacterial, viral, and fungal peptides), contributes toward regulation of behavioral arousal. In 4–4.5 month old mice with constitutive loss of Tlr2 function (Tlr2^−/− mice), we note a marked consolidation in the circadian pattern of both active and inactive states. Specifically, Tlr2^−/− mice demonstrated significantly fewer but longer duration active states during the circadian dark cycle, and significantly fewer but longer duration inactive states during the circadian light cycle. Tlr2^−/− mice also consumed less food and water, and moved less during the circadian light cycle. Analysis of circadian rhythms further suggested that Tlr2^−/− mice demonstrated less day-to-day variability in feeding, drinking, and movement behaviors. Reevaluation of this same mouse cohort at age 8–8.5 months revealed a clear blunting of these differences. However, Tlr2^−/− mice were still noted to have fewer short-duration active states during the circadian dark cycle, and continued to demonstrate significantly less day-to-day variability in feeding, drinking, and movement behaviors. These results suggest that Tlr2 function may have a role in promoting transitions between active and inactive states. Prior studies have demonstrated that Tlr2 regulates sickness behaviors including hypophagia, hyperthermia, and decreased activity. Our work suggests that Tlr2 function also evokes behavioral fragmentation, another aspect of sickness behavior and a clinically significant problem of older adults.

Health-promoting and disease-preventive potential of Trianthema portulacastrum Linn. (Gadabani) -An Indian medicinal and dietary plant

It is estimated that 80% of the world population depends on traditional medicine for primary healthcare need. Trianthema portulacastrum Linn. (family: Aizoaceae) is a small perennial weed found in the Americas, Africa, India, and other regions of the world. This plant is used extensively in Indian traditional medicines and is also consumed as a vegetable throughout Asia for its perceived health benefits. Phytochemical analysis of T. portulacastrum reveals the presence of alkaloids, flavonoids, terpenoids, saponins, and phenolic compounds. Emerging studies demonstrate that crude extracts as well as bioactive phytoconstituents of T. portulacastrum exhibit potent antioxidant, anti-infective, analgesic, and anti-inflammatory activities. A growing number of in vitro and in vivo studies demonstrate various biological and pharmacological activities, including prevention and amelioration of hepatotoxicity, nephrotoxicity, hyperglycemia, hyperlipidemia, infectious diseases and cancer. This review aims to present and analyze available literature to understand the full potential of T. portulacastrum in health promotion and disease prevention. Current limitations and future directions of research on this medicinal and dietary plant are also critically discussed.

Lipoproteins/peptides are sepsis-inducing toxins from bacteria that can be neutralized by synthetic anti-endotoxin peptides

Sepsis, a life-threatening syndrome with increasing incidence worldwide, is triggered by an overwhelming inflammation induced by microbial toxins released into the bloodstream during infection. A well-known sepsis-inducing factor is the membrane constituent of Gram-negative bacteria, lipopolysaccharide (LPS), signalling via Toll-like receptor-4. Although sepsis is caused in more than 50% cases by Gram-positive and mycoplasma cells, the causative compounds are still poorly described. In contradicting investigations lipoproteins/-peptides (LP), lipoteichoic acids (LTA), and peptidoglycans (PGN), were made responsible for eliciting this pathology. Here, we used human mononuclear cells from healthy donors to determine the cytokine-inducing activity of various LPs from different bacterial origin, synthetic and natural, and compared their activity with that of natural LTA and PGN. We demonstrate that LP are the most potent non-LPS pro-inflammatory toxins of the bacterial cell walls, signalling via Toll-like receptor-2, not only in vitro, but also when inoculated into mice: A synthetic LP caused sepsis-related pathological symptoms in a dose-response manner. Additionally, these mice produced pro-inflammatory cytokines characteristic of a septic reaction. Importantly, the recently designed polypeptide Aspidasept^® which has been proven to efficiently neutralize LPS in vivo, inhibited cytokines induced by the various non-LPS compounds protecting animals from the pro-inflammatory activity of synthetic LP.

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.

Differences in PGE2 production between primary human monocytes and differentiated macrophages: role of IL-1β and TRIF/IRF3

Prostaglandin E2 (PGE₂) is induced in vivo by bacterial products including TLR agonists. To determine whether PGE₂ is induced directly or via IL-1β, human monocytes and macrophages were cultured with LPS or with Pam3CSK4 in presence of caspase-1 inhibitor, ZVAD, or IL-1R antagonist, Kineret. TLR agonists induced PGE₂ in macrophages exclusively via IL-1β-independent mechanisms. In contrast, ZVAD and Kineret reduced PGE₂ production in LPS-treated (but not in Pam3CSK4-treated) monocytes, by 30–60%. Recombinant human IL-1β augmented COX-2 and mPGES-1 mRNA and PGE₂ production in LPS-pretreated monocytes but not in un-primed or Pam3CSK4-primed monocytes. This difference was explained by the finding that LPS but not Pam3CSK4 induced phosphorylation of IRF3 in monocytes suggesting activation of the TRIF signaling pathway. Knocking down TRIF, TRAM, or IRF3 genes by siRNA inhibited IL-1β-induced COX-2 and mPGES-1 mRNA. Blocking of TLR4 endocytosis during LPS priming prevented the increase in PGE₂ production by exogenous IL-1β. Our data showed that TLR2 agonists induce PGE₂ in monocytes independently from IL-1β. In the case of TLR4, IL-1β augments PGE₂ production in LPS-primed monocytes (but not in macrophages) through a mechanism that requires TLR4 internalization and activation of the TRIF/IRF3 pathway. These findings suggest a key role for blood monocytes in the rapid onset of fever in animals and humans exposed to bacterial products and some novel adjuvants.

Toll-like receptors 2 and 4 modulate autonomic control of heart rate and energy metabolism

Toll-like receptors (TLR) are innate immune receptors typically activated by microbial-associated molecular patterns (MAMPs) during infection or damage-associated molecular patterns (DAMPs) as a result of tissue injury. Recent findings suggest that TLR2 and TLR4 signaling play important roles in developmental and adult neuroplasticity, and in learning and memory. In addition, activation of TLR2 and TLR4 worsens ischemic injury to the heart and brain in animal models of myocardial infarction and stroke. TLR activation is also implicated in thermoregulation and fever in response to infection. However, it is not known whether TLRs participate in the regulation of the sympathetic and/or parasympathetic components of the autonomic nervous system (ANS). Here we provide evidence that TLR2 and TLR4 influence autonomic regulation of heart rate (HR) body temperature and energy metabolism in mice. We show that mice lacking TLR2 or TLR4 exhibit reduced basal HR, which results from an increase of parasympathetic tone. In addition, thermoregulatory responses to stress are altered in TLR2-/- and TLR4-/- mice, and brown fat-dependent thermoregulation is altered in TLR4-/- mice. Moreover, TLR2-/- and TLR4-/- mice consume less food and exhibit a greater mass compared to wild type mice. Collectively, our findings suggest important roles for TLR2 and TLR4 in the ANS regulation of cardiovascular function, thermoregulation, and energy metabolism.

Immunostimulation by synthetic lipopeptide-based vaccine candidates: structure-activity relationships

Peptide-based vaccines offer several advantages over conventional whole organism or protein approaches by offering improved purity and specificity in inducing immune response. However, peptides alone are generally non-immunogenic. Concerns remain about the toxicity of adjuvants which are critical for immunogenicity of synthetic peptides. The use of lipopeptides in peptide vaccines is currently under intensive investigation because potent immune responses can be generated without the use of adjuvant (thus are self-adjuvanting). Several lipopeptides derived from microbial origin, and their synthetic versions or simpler fatty acid moieties impart this self-adjuvanting activity by signaling via Toll-like receptor 2 (TLR2). Engagement of this innate immune receptor on antigen-presenting cell leads to the initiation and development of potent immune responses. Therefore optimization of lipopeptides to enhance TLR2-mediated activation is a promising strategy for vaccine development. Considerable structure-activity relationships that determine TLR2 binding and consequent stimulation of innate immune responses have been investigated for a range of lipopeptides. In this mini review we address the development of lipopeptide vaccines, mechanism of TLR2 recognition, and immune activation. An overview is provided of the best studied lipopeptide vaccine systems.

Immune-neural connections: how the immune system's response to infectious agents influences behavior

Humans and animals use the classical five senses of sight, sound, touch, smell and taste to monitor their environment. The very survival of feral animals depends on these sensory perception systems, which is a central theme in scholarly research on comparative aspects of anatomy and physiology. But how do all of us sense and respond to an infection? We cannot see, hear, feel, smell or taste bacterial and viral pathogens, but humans and animals alike are fully aware of symptoms of sickness that are caused by these microbes. Pain, fatigue, altered sleep pattern, anorexia and fever are common symptoms in both sick animals and humans. Many of these physiological changes represent adaptive responses that are considered to promote animal survival, and this constellation of events results in sickness behavior. Infectious agents display a variety of pathogen-associated molecular patterns (PAMPs) that are recognized by pattern recognition receptors (PRRs). These PRR are expressed on both the surface [e.g. Toll-like receptor (TLR)-4] and in the cytoplasm [e.g. nucleotide-binding oligomerization domain (Nod)-like receptors] of cells of the innate immune system, primarily macrophages and dendritic cells. These cells initiate and propagate an inflammatory response by stimulating the synthesis and release of a variety of cytokines. Once an infection has occurred in the periphery, both cytokines and bacterial toxins deliver this information to the brain using both humoral and neuronal routes of communication. For example, binding of PRR can lead to activation of the afferent vagus nerve, which communicates neuronal signals via the lower brain stem (nucleus tractus solitarius) to higher brain centers such as the hypothalamus and amygdala. Blood-borne cytokines initiate a cytokine response from vascular endothelial cells that form the blood-brain barrier (BBB). Cytokines can also reach the brain directly by leakage through the BBB via circumventricular organs or by being synthesized within the brain, thus forming a mirror image of the cytokine milieu in the periphery. Although all cells within the brain are capable of initiating cytokine secretion, microglia have an early response to incoming neuronal and humoral stimuli. Inhibition of proinflammatory cytokines that are induced following bacterial infection blocks the appearance of sickness behaviors. Collectively, these data are consistent with the notion that the immune system communicates with the brain to regulate behavior in a way that is consistent with animal survival.

Exploratory clinical studies of a synthetic HIV-1 Tat epitope vaccine in asymptomatic treatment-naïve and antiretroviral-controlled HIV-1 infected subjects plus healthy uninfected subjects

TUTI-16 is a synthetic universal HIV-1 Tat epitope vaccine, designed to induce anti-Tat antibodies that block the function of circulating Tat, an HIV encoded protein secreted by HIV-1 infected cells. Circulating Tat activates CD4 T cells, permitting HIV replication and sustained viremia. Safety, immunogenicity and antiretroviral potential of TUTI-16 were explored in a randomized double-blind dose-escalating study in asymptomatic treatment-naïve HIV-1 infected subjects. TUTI-16 was safe, with mild local and systemic injection-related adverse reactions, but the antibody response was barely detectable. Surprisingly, a highly statistically significant reduction of HIV-1 viral load was found in the lowest 30 μg vaccine dose group (p < 0.01) but not at the higher doses. We posited that an anti-Tat antibody response below the limit of detection inhibited HIV viral load at this dose, an effect nullified at higher vaccine doses by activating cytokines induced by adjuvant components in TUTI-16. To clarify this immunogenicity/activation conundrum open label immunogenicity studies were performed in healthy HIV uninfected and aviremic ART-controlled HIV-infected subjects. These established that (1) healthy HIV negative subjects had robust antibody responses, maximal with 1 mg TUTI-16, (2) ART-controlled aviremic HIV infected subjects had similarly robust antibody responses, and (3) adjuvant-induced increases of HIV viral load did not occur in the presence of ART. These studies provided us a basis for the design of a protocol to explore the therapeutic potential of TUTI-16 vaccination to provide drug free control of HIV-1 viremia.

Cytokines and brain excitability

Cytokines are molecules secreted by peripheral immune cells, microglia, astrocytes and neurons in the central nervous system. Peripheral or central inflammation is characterized by an upregulation of cytokines and their receptors in the brain. Emerging evidence indicates that pro-inflammatory cytokines modulate brain excitability. Findings from both the clinical literature and from in vivo and in vitro laboratory studies suggest that cytokines can increase seizure susceptibility and may be involved in epileptogenesis. Cellular mechanisms that underlie these effects include upregulation of excitatory glutamatergic transmission and downregulation of inhibitory GABAergic transmission.

Fever, sickness behavior, and expression of inflammatory genes in the hypothalamus after systemic and localized subcutaneous stimulation of rats with the Toll-like receptor 7 agonist imiquimod

The Toll-like receptor 7 (TLR7) agonist imiquimod is used for topical treatment of skin cancers. We studied the consequences of injections of imiquimod into a subcutaneous (s.c.) air pouch or of intraperitoneal (i.p.) injections on the manifestation of fever, sickness behavior, and the peripheral and brain-intrinsic induction of a variety of inflammatory molecules. Rats were given imiqimod s.c. or i.p. (1 or 5 mg/kg). Body temperature, motor activity, and food and water intake were recorded by telemetric devices. Peripheral and brain-intrinsic induction of inflammatory mediators was analyzed by real-time polymerase chain reaction (RT-PCR), bioassays, enzyme-linked immunosorbent assays (ELISAs), and immunohistochemistry. Imiquimod is the first TLR-agonist to produce more potent effects with s.c. than i.p. administration. Peripheral induction of interferons (IFNs) and putative circulating pyrogens corresponded to the magnitude of the illness responses. In the brain, an expression of cytokines (TNFα, IL-1β, and IL-6) and inducible forms of enzymes for prostaglandin E2 synthesis (COX-2 and mPGES) occurred, which was accompanied by a moderate activation of the transcription factors NFκB and STAT3, and a strong activation of the transcription factor NF-IL6, in cells of specific areas with an open blood-brain barrier. These inflammatory responses noted within the brain were more marked after s.c. administration, than i.p. administration of imiquimod. At a dose of 5 mg/kg, imiquimod causes rather moderate brain-inflammatory responses, which are related to peripheral IFN-expression and possibly mediated by brain-intrinsic activation of NF-IL6 and induction of a proinflammatory cocktail. The lack of a septic-like state in imiquimod-treated rats reinforces the therapeutic use of this drug.

Dissociation between learning and memory impairment and other sickness behaviours during simulated Mycoplasma infection in rats

To investigate potential consequences for learning and memory, we have simulated the effects of Mycoplasma infection, in rats, by administering fibroblast-stimulating lipopepide-1 (FSL-1), a pyrogenic moiety of Mycoplasma salivarium. We measured the effects on body temperature, cage activity, food intake, and on spatial learning and memory in a Morris Water Maze. Male Sprague-Dawley rats had radio transponders implanted to measure abdominal temperature and cage activity. After recovery, rats were assigned randomly to receive intraperitoneal (I.P.) injections of FSL-1 (500 or 1000 μg kg(-1) in 1 ml kg(-1) phosphate-buffered saline; PBS) or vehicle (PBS, 1 ml kg(-1)). Body mass and food intake were measured daily. Training in the Maze commenced 18 h after injections and continued daily for four days. Spatial memory was assessed on the fifth day. In other rats, we measured concentrations of brain pro-inflammatory cytokines, interleukin (IL)-1β and IL-6, at 3 and 18 h after injections. FSL-1 administration induced a dose-dependent fever (∼1°C) for two days, lethargy (∼78%) for four days, anorexia (∼65%) for three days and body mass stunting (∼6%) for at least four days. Eighteen hours after FSL-1 administration, when concentrations of IL-1β, but not that of IL-6, were elevated in both the hypothalamus and the hippocampus, and when rats were febrile, lethargic and anorexic, learning in the Maze was unaffected. There also was no memory impairment. Our results support emerging evidence that impaired learning and memory is not inevitable during simulated infection.

Fever induction by systemic stimulation with macrophage-activating lipopeptide-2 depends upon TLR2 but not CD36

This study was designed to test the responses of TLR2-knockout mice (TLR2-KO) and wild- type mice (C57/BL-6), and of CD36 deficient spontaneously hypertensive rats (SHR) and their genetic controls [Wistar Kyoto (WKY) rats] to systemic stimulations with the TLR2/6 agonist MALP-2 and the TLR4 agonist LPS. Fever and formation of TNF-α and IL-6 induced by intraperitoneal injections of MALP-2 (1000 µg/kg) were completely blunted in TLR2-KO, while LPS (100 µg/kg)-induced responses were not abolished in these animals. In SHR lacking CD36, a reduction of fever was observed in response to MALP-2 (100 µg/kg), but LPS-fever was even more attenuated in SHR when compared with WKY controls. Concentrations of circulating IL-6 tended to be lower in SHR after stimulation with both pyrogens. However, the IL-6-mediated activation of the transcription factor STAT3 in the brain was identical in both strains, indicating that the brain-controlled inflammatory response to MALP-2 (and LPS) is not impaired in the absence of CD36. In addition, stimulation of peritoneal macrophages with LPS and MALP-2 (10 µg/ml) caused the appearance of similar concentrations of bioactive cytokines in the supernatants from cells of both rat strains. These results demonstrate that TLR2 is essential for the manifestation of MALP-2, but not LPS-induced inflammatory responses. A moderate participation of CD36 in MALP-2-induced sickness- and cytokine-responses can not be ruled out but is unlikely as LPS-induced inflammatory responses were also attenuated in SHR.

The intestinal microbiota affect central levels of brain-derived neurotropic factor and behavior in mice

BACKGROUND & AIMS

Alterations in the microbial composition of the gastrointestinal tract (dysbiosis) are believed to contribute to inflammatory and functional bowel disorders and psychiatric comorbidities. We examined whether the intestinal microbiota affects behavior and brain biochemistry in mice.

METHODS

Specific pathogen-free (SPF) BALB/c mice, with or without subdiaphragmatic vagotomy or chemical sympathectomy, or germ-free BALB/c mice received a mixture of nonabsorbable antimicrobials (neomycin, bacitracin, and pimaricin) in their drinking water for 7 days. Germ-free BALB/c and NIH Swiss mice were colonized with microbiota from SPF NIH Swiss or BALB/c mice. Behavior was evaluated using step-down and light preference tests. Gastrointestinal microbiota were assessed using denaturing gradient gel electrophoresis and sequencing. Gut samples were analyzed by histologic, myeloperoxidase, and cytokine analyses; levels of serotonin, noradrenaline, dopamine, and brain-derived neurotropic factor (BDNF) were assessed by enzyme-linked immunosorbent assay.

RESULTS

Administration of oral antimicrobials to SPF mice transiently altered the composition of the microbiota and increased exploratory behavior and hippocampal expression of BDNF. These changes were independent of inflammatory activity, changes in levels of gastrointestinal neurotransmitters, and vagal or sympathetic integrity. Intraperitoneal administration of antimicrobials to SPF mice or oral administration to germ-free mice did not affect behavior. Colonization of germ-free BALB/c mice with microbiota from NIH Swiss mice increased exploratory behavior and hippocampal levels of BDNF, whereas colonization of germ-free NIH Swiss mice with BALB/c microbiota reduced exploratory behavior.

CONCLUSIONS

The intestinal microbiota influences brain chemistry and behavior independently of the autonomic nervous system, gastrointestinal-specific neurotransmitters, or inflammation. Intestinal dysbiosis might contribute to psychiatric disorders in patients with bowel disorders.

Kinin B1 receptors mediate depression-like behavior response in stressed mice treated with systemic E. coli lipopolysaccharide

BACKGROUND

Kinin B1 receptors are inducible molecules up-regulated after inflammatory stimuli. This study evaluated the relevance of kinin B1 receptors in a mouse depression behavior model.

METHODS

Mice were exposed to a 5-min swimming session, and 30 min later they were injected with E. coli lipopolysaccharide (LPS). Depression-like behavior was assessed by determining immobility time in a tail suspension test. Different brain structures were collected for molecular and immunohistochemical studies. Anhedonia was assessed by means of a sucrose intake test.

RESULTS

Our protocol elicited an increase in depression-like behavior in CF1 mice, as assessed by the tail-suspension test, at 24 h. This behavior was significantly reduced by treatment with the selective B1 receptor antagonists R-715 and SSR240612. Administration of SSR240612 also prevented an increase in number of activated microglial cells in mouse hippocampus, but did not affect a reduction in expression of mRNA for brain-derived neurotrophic factor. The increased immobility time following LPS treatment was preceded by an enhancement of hippocampal and cortical B1 receptor mRNA expression (which were maximal at 1 h), and a marked production of TNFα in serum, brain and cerebrospinal fluid (between 1 and 6 h). The depression-like behavior was virtually abolished in TNFα p55 receptor-knockout mice, and increased B1 receptor mRNA expression was completely absent in this mouse strain. Furthermore, treatment with SSR240612 was also effective in preventing anhedonia in LPS-treated mice, as assessed using a sucrose preference test.

CONCLUSION

Our data show, for the first time, involvement of kinin B1 receptors in depressive behavioral responses, in a process likely associated with microglial activation and TNFα production. Thus, selective and orally active B1 receptor antagonists might well represent promising pharmacological tools for depression therapy.

Contribution of Toll-like receptor activation to lung damage after donor brain death

INTRODUCTION

Donor brain death is the first injurious event that can produce inflammatory dysfunction after pulmonary transplantation. This study was designed to determine whether stimulation of the toll-like receptor (TLR) system contributes to the changes produced by brain death.

MATERIALS AND METHODS

Rats were repeatedly treated with specific agonists for TLR4 or TLR2/6 to desensitize these receptors. Brain death was then induced by inflation of a balloon catheter within the extradural space. Mean arterial pressure changes and inflammatory markers were measured serially by protein and mRNA analysis.

RESULTS

Both desensitizing pretreatments prevented the neurogenic hypotension (P<0.001) and metabolic acidosis (P<0.001) observed in control animals after brain death. These treatments also reduced the levels of tumor necrosis factor-α and CXCL1 in serum and bronchoalveolar lavage fluid, although desensitization of TLR4 produced a greater inhibition than desensitization of TLR2. Desensitization of TLR4 also reduced (P<0.05) expression of the adhesive integrin CD11b on blood neutrophils after brain death. Examination of mRNA levels in lung tissue 5 hr after brain death showed that desensitization of TLR4 limited the expression of interferon (IFN)-γ, IFNβ, and CXCL10, whereas desensitization of TLR2/6 reduced only the expression of IFNγ.

CONCLUSION

These results indicate that activation of TLR signaling pathways can contribute to the lung damage produced by brain death; this may increase subsequent graft injury after transplantation.

Key role of CD36 in Toll-like receptor 2 signaling in cerebral ischemia

BACKGROUND AND PURPOSE

Toll-like receptors (TLRs) and the scavenger receptor CD36 are key molecular sensors for the innate immune response to invading pathogens. However, these receptors may also recognize endogenous "danger signals" generated during brain injury, such as cerebral ischemia, and trigger a maladaptive inflammatory reaction. Indeed, CD36 and TLR2 and 4 are involved in the inflammation and related tissue damage caused by brain ischemia. Because CD36 may act as a coreceptor for TLR2 heterodimers (TLR2/1 or TLR2/6), we tested whether such interaction plays a role in ischemic brain injury.

METHODS

The TLR activators FSL-1 (TLR2/6), Pam3 (TLR2/1), or lipopolysaccharide (TLR4) were injected intracerebroventricularly into wild-type or CD36-null mice, and inflammatory gene expression was assessed in the brain. The effect of TLR activators on the infarct produced by transient middle cerebral artery occlusion was also studied.

RESULTS

The inflammatory response induced by TLR2/1 activation, but not TLR2/6 or TLR4 activation, was suppressed in CD36-null mice. Similarly, TLR2/1 activation failed to increase infarct volume in CD36-null mice, whereas TLR2/6 or TLR4 activation exacerbated postischemic inflammation and increased infarct volume. In contrast, the systemic inflammatory response evoked by TLR2/6 activation, but not by TLR2/1 activation, was suppressed in CD36-null mice.

CONCLUSIONS

In the brain, TLR2/1 signaling requires CD36. The cooperative signaling of TLR2/1 and CD36 is a critical factor in the inflammatory response and tissue damage evoked by cerebral ischemia. Thus, suppression of CD36-TLR2/1 signaling could be a valuable approach to minimize postischemic inflammation and the attendant brain injury.

Effects of repeated injections of fibroblast-stimulating lipopeptide-1 on fever, formation of cytokines, and on the responsiveness to endotoxin in guinea-pigs

AIMS

We investigated, whether the Toll-like receptors (TLRs)-2/6-agonist fibroblast-stimulating lipopeptide-1 (FSL-1), like the TLR-4 agonist lipopolysaccharide (LPS), induces a state of tolerance. We further tested the influence of repeated pre-treatment with FSL-1 on the animals' responsiveness to LPS.

METHODS

Abdominal temperature was recorded in unrestrained guinea-pigs with intra-abdominally implanted radiotransmitters. Circulating concentrations of tumour necrosis factor (TNF) and interleukin-6 (IL-6) were measured with specific bioassays. We tested the effects of intra-arterial (i.a.) or intraperitoneal (i.p.) injections of 100 microg kg(-1) FSL-1, repeated five times at intervals of 3 days. The animals' responses to i.a. or i.p. injections of 10 microg kg(-1) LPS were determined another 3 days later and compared to those of naïve guinea-pigs.

RESULTS

The FSL-1-induced TNF peak was significantly attenuated starting with the third i.a. administration, while fever was unimpaired and the IL-6-peak just tended to decrease. Fever and IL-6 in response to i.a. injections of LPS were identical in both groups, while circulating TNF was higher in naïve compared to FSL-1 pre-treated animals. The effects of repeated i.p. injections of FSL-1 were more pronounced resulting in attenuation of fever as well as circulating TNF and IL-6, the strongest reduction observed after the third stimulation with FSL-1. Repeated i.p. pre-treatment with FSL-1 induced hyporesponsiveness to i.p. administration of LPS compared to naïve animals with regard to fever and especially with regard to LPS-induced formation of cytokines.

CONCLUSIONS

There is a development of tolerance to FSL-1 and cross-tolerance between FSL-1 and LPS depending on the route of administration of the respective TLR-2/6 and TLR-4 agonists.

Comparison of anorexia, lethargy, and fever induced by bacterial and viral mimetics in rats

Although it has been established that some acute phase responses present differently depending on whether a virus or bacteria activates the innate immune system, it has not yet been established whether fever and sickness behaviors, such as anorexia and lethargy, present differently. We therefore investigated the effects of administering lipopolysaccharide (LPS) and polyinosinic : polycytidylic acid (poly I:C) on body temperature, food intake, body mass, and activity (cage activity and wheel running). Male Sprague-Dawley rats were randomly assigned to receive an intraperitoneal injection of one of LPS (75 microg/kg or 250 microg/kg), poly I:C (3000 microg/kg or 4000 microg/kg), or saline. Administration of LPS or poly I:C induced fever, anorexia, and lethargy. Although voluntary wheel running and cage activity were both significantly reduced after administration of LPS or poly I:C, they were not affected equally. Indeed voluntary wheel running was decreased on average by approximately 30% more than cage activity regardless of the dose or type of mimetic administered. Our results indicate that poly I:C is less effective at inducing anorexia, lethargy, and fever in rats than is LPS, and that avoidance of exercise in animals and humans during infection is likely to be a more prominent feature of illness than is avoidance of routine daily activity.

Acute lung injury is reduced in fat-1 mice endogenously synthesizing n-3 fatty acids

RATIONALE

Acute lung injury (ALI) remains an important cause of mortality in intensive care units. Inflammation is controlled by cytokines and eicosanoids derived from the n-6 fatty acid (FA) arachidonic acid (AA). The n-3 FA eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and mediators derived from EPA and DHA possess reduced inflammatory potency.

OBJECTIVES

To determine whether the ability of fat-1 mice to endogenously convert n-6 to n-3 FA, and thus generate an increased ratio of n-3 to n-6 FA, impacts experimental ALI.

METHODS

We investigated ALI induced by intratracheal instillation of endotoxin in fat-1 and wild-type (WT) mice, assessing leukocyte numbers, protein concentration, and prostaglandin and cytokine levels in bronchoalveolar lavage fluid, as well as free FA in plasma, and lung ventilator compliance. Body temperature and motor activity of mice--markers of sickness behavior--were also recorded.

MEASUREMENTS AND MAIN RESULTS

In ALI, fat-1 mice exhibited significantly reduced leukocyte invasion, protein leakage, and macrophage inflammatory protein-2 and thromboxane B(2) levels in lavage fluid compared with WT mice. Free AA levels were increased in the plasma of WT mice in response to endotoxin, whereas EPA and DHA were increased in the fat-1 group. Ventilator compliance was significantly improved in fat-1 mice. Body temperature and motor activity were decreased in ALI. fat-1 Mice recovered body temperature and motor activity faster.

CONCLUSIONS

fat-1 Mice exhibited reduced features of ALI and sickness behavior. Increasing the availability of n-3 FA may thus be beneficial in critically ill patients with ALI.

Mast cell-dependent anorexia and hypothermia induced by mucosal activation of Toll-like receptor 7

Systemic viral infections produce a highly regulated set of responses in sickness behavior, such as fever, anorexia, and adipsia. Toll-like receptor (TLR)7, activated by viral RNA during infection, potently stimulates the innate and adaptive immune responses that aid in viral clearance. However, the physiological consequences of TLR7 activation have not been thoroughly studied. In these experiments, we used a potent synthetic TLR7 ligand, 9-benzyl-8-hydroxy-2-(2-methoxyethoxy)adenine (SM360320; 1V136), to investigate the consequences of TLR7 activation in genetically defined strains of mice. Administration of the drug by the nasal, intragastric, or intraperitoneal routes caused transient hypophagia, hypodypsia, and hypothermia. Analyses of mutant mouse strains indicated that these effects were dependent on the expression of TLR7, its adaptor protein MyD88, and TNF-alpha, and independent of IL-1beta, IL-6 and cyclo-oxygenase-1 (COX1). Partial roles were also implied for mast cells and COX2. Although plasma TNF-alpha levels were significantly higher after systemic drug delivery, the behavioral effects were maximal when the agent was administered to the mucosa. Tissue and mucosal mast cells are known to express high levels of TLR7 and to rapidly release TNF-alpha upon TLR7 ligation. Mice deficient in tissue mast cells, W/W(v), had significantly less anorexia after TLR7 activation, and this response was restored with mast cell reconstitution. Our results thus suggest that tissue mast cells may play a role in the anorexia induced by mucosal activation of TLR7.

Aberrant agouti-related protein system in the hypothalamus of the anx/anx mouse is associated with activation of microglia

Agouti-related protein (AgRP) is a key orexigenic neuropeptide expressed in the hypothalamic arcuate nucleus and a marker for neurons conveying hormonal signals of hunger to the brain. Mice homozygous for the anorexia (anx) mutation are characterized by decreased food intake, starvation, and death by 3-5 weeks of age. At this stage immunoreactivity for AgRP is increased in cell bodies but decreased in the nerve terminals. We studied when during early postnatal development the aberrant phenotype of the AgRP system becomes apparent in anx/anx mice and possible underlying mechanisms. AgRP and ionized calcium binding adapter molecule (Iba1), a marker for activated microglia, as well as Toll-like receptor 2 (TLR-2), were studied by immunohistochemistry at postnatal days P1, P5, P10, P12, P15 and P21 in anx/anx and wild-type mice. We found that the AgRP system in the anx/anx mouse develops similarly to the wild type until P12, when AgRP fibers in anx/anx mice cease to increase in density in the main projection areas. At P21, AgRP fiber density in anx/anx mice was significantly reduced vs. P15, in certain regions. At P21, many strongly AgRP-positive cell bodies were observed in the anx/anx arcuate nucleus vs. only few and weakly fluorescent ones in the wild type. The decrease in AgRP fiber density in anx/anx mice overlapped with an increase in Iba1 and TLR-2 immunoreactivities. Thus, the aberrant appearance of the AgRP system in the anx/anx mouse in the early postnatal development could involve a microglia-associated process and the innate immune system.

Cytokine induction by pyrogens: comparison of whole blood, mononuclear cells, and TLR-transfectants

Given the shortcomings in the measurement of pyrogenic contamination of pharmaceuticals and/or test substances by means of the rabbit pyrogen test and the Limulus amoebocyte lysate (LAL) test, several in vitro pyrogen tests have been developed based on the measurement of cytokine production by monocytes. In this study we measured cytokine production (IL-6, IL-8, IL-1beta, and TNF) in diluted whole blood (WB), mononuclear cells (MNC), and HEK cells stably transfected with CD14 and Toll-like Receptor-2 (TLR2) or TLR4, after stimulation with both standard pyrogens and contaminated substances. Our study demonstrated that in MNC, IL-6 production was more sensitive to pyrogen stimulation than IL-1beta and TNF production. The sensitivity of WB IL-8 production for pyrogens was comparable with that of MNC IL-6 production, but higher than WB IL-6 production. MNC IL-8 production as readout for pyrogenic stimulation was not useful due to high background IL-8 production. Surprisingly, contaminated culture media potently stimulated WB IL-8 production, but not MNC IL-6 production. Finally, the value of TLR-transfected HEK cells in the detection of pyrogenic contamination as well as the role of IL-10 in interindividual differences in cytokine production, is discussed. To summarize, the results presented herein together with literature data indicate that the measurement of WB IL-8 production may represent an advantageous alternative to the measurement of MNC IL-6 production, for the detection of pyrogenic contamination of pharmaceuticals.

TNF-alpha-mediated inflammation in cerebral aneurysms: a potential link to growth and rupture

Intracranial aneurysm (IA) rupture is one of the leading causes of stroke in the United States and remains a major health concern today. Most aneurysms are asymptomatic with a minor percentage of rupture annually. Regardless, IA rupture has a devastatingly high mortality rate and does not have specific drugs that stabilize or prevent aneurysm rupture, though other preventive therapeutic options such as clipping and coiling of incidental aneurysms are available to clinicians. The lack of specific drugs to limit aneurysm growth and rupture is, in part, attributed to the limited knowledge on the biology of IA growth and rupture. Though inflammatory macrophages and lymphocytes infiltrate the aneurysm wall, a link between their presence and aneurysm growth with subsequent rupture is not completely understood. Given our published results that demonstrate that the pro-inflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha), is highly expressed in human ruptured aneurysms, we hypothesize that pro-inflammatory cell types are the prime source of TNF-alpha that initiate damage to endothelium, smooth muscle cells (SMC) and internal elastic lamina (IEL). To gain insights into TNF-alpha expression in the aneurysm wall, we have examined the potential regulators of TNF-alpha and report that higher TNF-alpha expression correlates with increased expression of intracellular calcium release channels that regulate intracellular calcium (Ca2+), and Toll like receptors (TLR) that mediate innate immunity. Moreover, the reduction of tissue inhibitor of metalloproteinase-1 (TIMP-1) expression provides insights on why higher matrix metalloproteinase (MMP) activity is noted in ruptured IA. Because TNF-alpha is known to amplify several signaling pathways leading to inflammation, apoptosis and tissue degradation, we will review the potential role of TNF-alpha in IA formation, growth and rupture. Neutralizing TNF-alpha action in the aneurysm wall may have a beneficial effect in preventing aneurysm growth by reducing inflammation and arterial remodeling.

The reduction of voluntary physical activity after poly I:C injection is independent of the effect of poly I:C-induced interferon-beta in mice

One characteristic of sickness behavior in mice is demonstrated by a reduction in voluntary wheel-running activity during infection. Among synthetic double-stranded (ds) RNAs, polyriboinosinic: polyribocytidylic acid (poly I:C) activates to produce interferon (IFN) -beta, which plays an important role in anti-viral activity and host-defense. However, how voluntary wheel-running activity is regulated during poly I:C infection is unknown. To determine whether poly I:C-induced IFN-beta production is responsible for reduced spontaneous physical activity, we measured poly I:C-induced changes in voluntary wheel-running activity in mice. In this experiment, the mice were injected with poly I:C (0-5 mg/kg i.v.) and/or anti-IFN-beta neutralizing antibody (1.5x10(5) U/kg i.v.). We also observed the direct effect of injection of recombinant IFN-beta (rIFN-beta: 5.0x10(4) and 2.5x10(5) U/kg) on wheel-running behavior. Poly I:C treatment dose-dependently reduced wheel-running activity, and induced an increase in plasma IFN-beta in mice. However the activity was not attenuated by the neutralizing antibody specific to IFN-beta treatment. Additionally, the wheel-running activity in rIFN-beta treated mice was maintained, although they showed a higher IFN-gamma inducible protein (IP)-10 concentration in plasma compared with that of the vehicle group. Our results suggest that the transient reduction in physical activity after poly I:C injection is induced dose dependently, but that the mediator might not be poly I:C-induced IFN-beta.

Characterization of the febrile response induced by fibroblast-stimulating lipopeptide-1 in guinea pigs

Recently, it has been shown that the Toll-like receptors-2 and -6 agonist fibroblast-stimulating lipopeptide-1 (FSL-1) have the capacity to induce fever and sickness behavior in rats. Since the mechanisms of the fever-inducing effects of FSL-1 are still unknown, we tested the pyrogenic properties of FSL-1 in guinea pigs and assessed a role for TNF-alpha and prostaglandins in the manifestation of the febrile response to this substance. Intra-arterial and intraperitoneal injections of FSL-1 caused dose-dependent fevers that coincided with elevated plasma levels of TNF and IL-6, the intraperitoneal route of administration being more effective than the intra-arterial route. Intra-arterial or intraperitoneal injection of a soluble form of the TNF type 1 receptor, referred to as TNF binding protein (TNFbp), together with FSL-1, completely neutralized FSL-1-induced circulating TNF and reduced fever and circulating IL-6. Intra-arterial or intraperitoneal injection of the nonselective cyclooxygenase (COX)-inhibitor diclofenac depressed fever and FSL-1-induced elevations of circulating PGE2. Circulating TNF and IL-6, however, remained unimpaired by treatment with diclofenac. In conclusion, FSL-1-induced fever in guinea pigs depends, in shape and duration, on the route of administration and is, to a high degree, mediated by pyrogenic cytokines and COX products.

STAT3 and COX-2 activation in the guinea-pig brain during fever induced by the Toll-like receptor-3 agonist polyinosinic:polycytidylic acid

Intra-arterial injections of synthetic double-stranded RNA (polyinosinic:polycytidylic acid, PIPC) at a dose of 500 microg/kg evoked pronounced fever in guinea-pigs. PIPC-induced fever could be antagonized by treatment with the non-selective cyclooxygenase (COX) inhibitor diclofenac and was, in part, attenuated by the administration of the selective COX-2-inhibitor nimesulide (dose: 5 mg/kg for both COX inhibitors). We further investigated whether direct activation of brain cells during PIPC-induced fever could be demonstrated. Using radioactive in situ hybridization, we demonstrated that treatment with PIPC resulted in an upregulation of COX-2 and interleukin-1 beta mRNA in the guinea-pig brain. Thus, COX-2-specific hybridization signals seemed to be mainly associated with brain blood vessels. Intra-arterial injections of PIPC further induced the pronounced nuclear translocation of the transcription factor STAT3 in the endothelium of various fore- and hindbrain areas and in the meninges. In brain structures that lacked a tight blood-brain barrier, i.e. the sensory circumventricular organs (area postrema, vascular organ of laminae terminalis, subfornical organ), the astrocytes and a population of still undetermined cellular phenotype also showed marked STAT3 activation in response to PIPC. The Toll-like receptor-3 agonist PIPC therefore caused a similar activation of brain cells as that reported for other experimental models of systemic inflammation.

Synthesis and characterization of a dipalmitoylated lipopeptide derived from paralogous lipoproteins of Mycoplasma pneumoniae

Genomic analysis of Mycoplasma pneumoniae revealed the existence of a large number of putative lipoprotein genes compared with the numbers in other bacteria. However, the pathogenic roles of M. pneumoniae lipoproteins are still obscure. In this study, we synthesized a lipopeptide (designated M. pneumoniae paralogous lipoprotein 1 [MPPL-1]) in which an S-dipalmitoylglyceryl cysteine was coupled to a peptide with a consensus sequence of a putative paralogous lipoprotein group characteristic of M. pneumoniae. The cytokine-inducing activity of MPPL-1 in human monocytic cells was much weaker (approximately 700-fold weaker) than that of the known mycoplasmal S-dipalmitoylated lipopeptide FSL-1 or MALP-2. MPPL-1 required Toll-like receptor (TLR2) to activate NF-kappaB-dependent gene transcription in HEK293 cells, although a 1,000-fold-larger amount of MPPL-1 was needed to exert activity similar to that of FSL-1 in the cells. TLR2-mediated recognition of MPPL-1 was synergistically upregulated by TLR6 but not by TLR1 or TLR10, although the activity was still weak. In addition, MPPL-1 did not antagonize FSL-1 recognition in human monocytic cells and TLR2/TLR6-expressing HEK293 cells. Thus, these results suggest that there is preferential selective recognition of diacylated lipopeptides due to the magnitude of an affinity with TLR2 and TLR6 and the roles of increased paralogous lipoprotein genes of M. pneumoniae in evasion of TLR2 recognition.

Thermoregulation: some concepts have changed. Functional architecture of the thermoregulatory system

While summarizing the current understanding of how body temperature (Tb) is regulated, this review discusses the recent progress in the following areas: central and peripheral thermosensitivity and temperature-activated transient receptor potential (TRP) channels; afferent neuronal pathways from peripheral thermosensors; and efferent thermoeffector pathways. It is proposed that activation of temperature-sensitive TRP channels is a mechanism of peripheral thermosensitivity. Special attention is paid to the functional architecture of the thermoregulatory system. The notion that deep Tb is regulated by a unified system with a single controller is rejected. It is proposed that Tb is regulated by independent thermoeffector loops, each having its own afferent and efferent branches. The activity of each thermoeffector is triggered by a unique combination of shell and core Tbs. Temperature-dependent phase transitions in thermosensory neurons cause sequential activation of all neurons of the corresponding thermoeffector loop and eventually a thermoeffector response. No computation of an integrated Tb or its comparison with an obvious or hidden set point of a unified system is necessary. Coordination between thermoeffectors is achieved through their common controlled variable, Tb. The described model incorporates Kobayashi’s views, but Kobayashi’s proposal to eliminate the term sensor is rejected. A case against the term set point is also made. Because this term is historically associated with a unified control system, it is more misleading than informative. The term balance point is proposed to designate the regulated level of Tb and to attract attention to the multiple feedback, feedforward, and open-loop components that contribute to thermal balance.

Pyrogenicity of CpG-DNA in mice: role of interleukin-6, cyclooxygenases, and nuclear factor-κB

Bacterial DNA containing unmethylated cytosine-phosphate-guanosine motif (CpG-DNA) has been identified as a pathogen-associated molecular pattern, which is recognized by Toll-like receptors and activates immune cells to produce cytokines. The aim of the study was to characterize the ability of CpG-DNA to induce fever in mice. Intravenous administration of unmethylated CpG-DNA 1826 triggered an elevation of body temperature (Tb) lasting several hours. The magnitude of Tbelevation increased with an increase of dose of the oligonucleotide (administered in a range from 0.01 mg/kg to 1.0 mg/kg). A fever-like increase of Tbin mice was partially dependent on IL-6, as IL-6 deficient mice responded with reduced fever to the CpG-DNA 1826. Meloxicam and sulindac sulfide, inhibitors of cyclooxygenases, reduced fever in mice challenged with CpG-DNA 1826, indicating that the process may also depend on prostaglandins. In fact, plasma levels of prostaglandin E2, as well as IL-6, increased at 4 h postinjection of CpG-DNA 1826 into mice. These data demonstrate that the pathophysiological mechanism of the increase of Tbinduced by CpG-DNA 1826 is similar to fever induced by LPS. Both LPS and CpG-DNA 1826 failed to produce elevation of Tbin mice deficient for a nuclear factor-κB (NF-κB) gene, further supporting the hypothesis that the two pyrogens provoke fever, using the same components of the cellular signaling metabolism. However, parthenolide, an inhibitor of I-κB kinase reduced fever due to CpG-DNA 1826, and did not affect fever to LPS, suggesting that the two structurally dissimilar pyrogens may affect different intracellular pathways leading to the upregulation of NF-κB. In support of this hypothesis, we demonstrate that C3H/HeJ mice, known to exhibit a mutation in the Toll-like receptor-4 gene, do not respond with fever to LPS. They respond, however, with fever after injection of CpG-DNA 1826. We conclude that bacterial DNA shares with components of the bacterial wall the capacity to elicit fever and may, consequently, be part of a novel class of exogenous pyrogens.