Cytokine neurobiology: behavior and adaptive responses

Reactions of the brain to systemic LPS or IL-1 beta treatment were shown to have different thresholds, be mediated by different neurotransmitter systems, and have different mechanisms of realisation. Changes in behaviour and neurotransmitter systems activity of the hypothalamus induced by a systemic IL-1 beta treatment were shown to be mediated by its receptors in the brain. Expression of mRNA of the tumour necrosis factor was revealed in the rabbit brain following administration of high pyrogenic doses of the LPS. The data obtained corroborate the concept of the cytokines role in maintenance of the defence responses in activation of the immune system, as well as their probable role in normal mechanisms of physiological functions control.     

Neuropeptide Y regulates catecholamine release evoked by interleukin-1beta in mouse chromaffin cells

Activation of the hypothalamic-pituitary-adrenal gland (HPA) axis can modulate the immune system. Cytokines and neuropeptide Y (NPY) are potent regulators of the HPA axis and are both produced by the adrenal medulla. The cytokine interleukin-1beta (IL-1beta) belongs to the interleukin-1 family along with interleukin-1alpha and the interleukin receptor antagonist (IL-1ra). The aim of the present study was to determine the interaction between NPY and IL-1beta in catecholamine (norepinephrine, NE and epinephrine, EP) release from mouse chromaffin cells in culture. We found that IL-1beta increased the constitutive release of NPY, NE and EP from mouse chromaffin cells. This IL-1beta stimulatory effect was blocked by IL-1ra. The immunoneutralization of NPY and the use of the NPY Y(1) receptor antagonist (BIBP 3226) inhibited the stimulatory effect of IL-1beta on catecholamine release from these cells. The present work shows that IL-1beta induces catecholamine release, and in turn this peptide will induce an additional increase in catecholamine release acting through the Y(1) receptor. This work suggests that NPY is involved in the regulatory loop between the immune and the adrenal system in some pathophysiological conditions where plasmatic IL-1beta increases, like in sepsis, rheumatoid arthritis, stress or hypertension.     

Site-specific modulation of LPS-induced fever and interleukin-1 beta expression in rats by interleukin-10

Bacterial lipopolysaccharide (LPS) induces fever that is mediated by pyrogenic cytokines such as interleukin (IL)-1 beta. We hypothesized that the anti-inflammatory cytokine IL-10 modulates the febrile response to LPS by suppressing the production of pyrogenic cytokines. In rats, intravenous but not intracerebroventricular infusion of IL-10 was found to attenuate fever induced by peripheral administration of LPS (10 microg/kg iv). IL-10 also suppressed LPS-induced IL-1 beta production in peripheral tissues and in the brain stem. In contrast, central administration of IL-10 attenuated the febrile response to central LPS (60 ng/rat icv) and decreased IL-1 beta production in the hypothalamus and brain stem but not in peripheral tissues and plasma. Furthermore, intravenous LPS upregulated expression of IL-10 receptor (IL-10R1) mRNA in the liver, whereas intracerebroventricular LPS enhanced IL-10R1 mRNA in the hypothalamus. We conclude that IL-10 modulates the febrile response by acting in the periphery or in the brain dependent on the primary site of inflammation and that its mechanism of action most likely involves inhibition of local IL-1 beta production.     

Role of IL-6 and IL-1beta in reactivation by acetylcholine of latently infecting pseudorabies virus.

We previously reported that the latently infecting Pseudorabies virus (PrV) could be reactivated by injection of swine or mice with acetylcholine. However, the mechanism of the reactivation was not clear yet. In this study, we analyzed the kinetics of cytokines related to stress to clarify the relationship between virus reactivation by acetylcholine and the immune system. IL-6 and IL-1beta were detected in mice after stimulation with acetylcholine. This shows that acetylcholine induced physiological stress conditions. However, there seemed to be no relationship between the kinetics of the cytokine levels and PrV excretion. Moreover, neither IL-6 nor IL-1beta alone could reactivate latently infecting PrV. Thus, acetylcholine causes the reactivation of latent PrV via a mechanism not involving these immunological factors.     

Physical activity alters the brain Hsp72 and IL-1beta responses to peripheral E. coli challenge

Physically active rats have facilitated heat shock protein 72 (Hsp72) responses after stressor exposure in both brain and peripheral tissues compared with sedentary rats. This study verifies that physically active animals do not have elevated Hsp72 levels compared with sedentary animals in the hypothalamus, pituitary, or dorsal vagal complex. We then examined whether 1) physically active rats respond more efficiently than sedentary rats to a bacterial challenge; 2) peripheral immune challenge elicits brain induction of Hsp72; 3) this induction is facilitated by prior freewheel running; and 4) Hsp72 upregulation produced by peripheral immune challenge results in a commensurate decrease in the proinflammatory cytokine IL-1beta. Adult male Fischer 344 rats were housed with either a mobile or locked running wheel. Six weeks later, rats were injected intraperitoneally with saline or Escherichia coli and killed 30 min, 2.5 h, 6 h, and 24 h later. Serum endotoxin and IL-1beta, and peritoneal fluid endotoxin and E. coli colony-forming units (CFUs) were measured. Hsp72 and IL-1beta were measured in hypothalamus, pituitary, and dorsal vagal complex. The results were that physically active rats had a faster reduction in endotoxin and E. coli CFUs and lower levels of circulating endotoxin and cytokines compared with sedentary rats. E. coli challenge elicited significantly greater time-dependent increases of both Hsp72 and IL-1beta in hypothalamus, pituitary, and dorsal vagal complex of physically active animals but not sedentary animals. Contrary to our hypothesis, increases in Hsp72 were positively correlated with IL-1beta. This study extends our findings that physical activity facilitates stress-induced Hsp72 to include immunological stressors such as bacterial challenge and suggests that brain Hsp72 and IL-1beta responses to peripheral immune challenge may contribute to exercise-mediated resistance to long-term sickness.     

Interleukin-18 induces expression and release of cytokines from murine glial cells: interactions with interleukin-1 beta

Interleukin (IL)-18, a member of the IL-1 cytokine family, is an important mediator of peripheral inflammation and host defence responses. IL-1 is a key proinflammatory cytokine in the brain, but the role of IL-18 in the CNS is not yet clear. The objective of this study was to investigate the actions of IL-18 on mouse glial cells. IL-18 induced intracellular expression of IL-1 alpha and proIL-1 beta, and release of IL-6 from mixed glia. Treatment of lipopolysaccharide-primed microglia with adenosine triphosphate (ATP), an endogenous secondary stimulus, induced IL-1 beta and IL-18 release. Although deletion of the IL-18 gene did not affect IL-1 beta expression or release in this experimental paradigm, IL-1 beta knockout microglia released significantly less IL-18 compared to wild-type microglia. In addition, ATP induced release of mature IL-1 beta from IL-18-primed microglia. These data suggest that IL-18 may contribute to inflammatory responses in the brain, and demonstrate that, in spite of several common features, IL-18 and IL-1 beta differ in their regulation and actions.     

IL-1beta and LPS induce anorexia by distinct mechanisms differentially dependent on microsomal prostaglandin E synthase-1

Recent work demonstrated that the febrile response to peripheral immune stimulation with proinflammatory cytokine IL-1beta or bacterial wall lipopolysaccharide (LPS) is mediated by induced synthesis of prostaglandin E(2) by the terminal enzyme microsomal prostaglandin E synthase-1 (mPGES-1). The present study examined whether a similar mechanism might also mediate the anorexia induced by these inflammatory agents. Transgenic mice with a deletion of the Ptges gene, which encodes mPGES-1, and wild-type controls were injected intraperitoneally with IL-1beta, LPS, or saline. Mice were free fed, and food intake was continuously monitored with an automated system for 12 h. Body weight was recorded every 24 h for 4 days. The IL-1beta induced anorexia in wild-type but not knock-out mice, and so it was almost completely dependent on mPGES-1. In contrast, LPS induced anorexia of the same magnitude in both phenotypes, and hence it was independent of mPGES-1. However, when the mice were prestarved for 22 h, LPS induced anorexia and concomitant body weight loss in the knock-out animals that was attenuated compared with the wild-type controls. These data suggest that IL-1beta and LPS induce anorexia by distinct immune-to-brain signaling pathways and that the anorexia induced by LPS is mediated by a mechanism different from the fever induced by LPS. However, nutritional state and/or motivational factors also seem to influence the pathways for immune signaling to the brain. Furthermore, both IL-1beta and LPS caused reduced meal size but not meal frequency, suggesting that both agents exerted an anhedonic effect during these experimental conditions.     

Central and Peripheral Cytokines Mediate Immune-Brain Connectivity

The immune system is a homeostatic system that contributes to maintain the constancy of the molecular and cellular components of the organism. Immune cells can detect the intrusion of foreign antigens or alteration of self-components and send information to the central nervous system (CNS) about this kind of perturbations, acting as a receptor sensorial organ. The brain can respond to such signals by emitting neuro/endocrine signals capable of affecting immune reactivity. Thus, the immune system, as other physiologic systems, is under brain control. Under disease conditions, when priorities for survival change, the immune system can, within defined limits, reset brain-integrated neuro-endocrine mechanisms in order to favour immune processes at the expenses of other physiologic systems. In addition, some cytokines initially conceived as immune products, such as IL-1 and IL-6, are also produced in the “healthy” brain by glial cells and even by some neurons. These and other cytokines have the capacity to affect synaptic plasticity acting as mediators of interactions between astrocytes and pre- and post-synaptic neurons that constitute what is actually defined as a tripartite synapse. Since the production of cytokines in the brain is affected by peripheral immune and central neural signals, it is conceivable that tripartite synapses can, in turn, serve as a relay system in immune-CNS communication.     

Activation of neutral sphingomyelinase by IL-1beta requires the type 1 interleukin 1 receptor

The cytokine interleukin 1beta (IL-1beta) plays an important role in host defence reactions and neuro-immune interactions but it is still not clear which of the two interleukin 1 receptor subtypes is coupled to activation of neutral sphingomyelinase (nSMase) by IL-1beta. To investigate involvement of neutral sphingomyelinase (nSMase) in central IL-1beta effects we used P(2)fractions of brain cerebral cortex from wild-type mice and mice deficient in the type 1 IL-1 receptor. IL-1beta (human, recombinant) was shown to activate, in a dose-dependent manner, nSMase in the P(2)brain fraction of the wild-type mice while in the knock-out mice the stimulatory effect of IL-1beta on nSMase was absent. In the presence of an IL-1 receptor antagonist (IL-1ra), IL-1beta did not activate nSMase either in the cortex of wild-type or knock-out mice. These data suggest that nSMase, a key enzyme of the sphingomyelin signal transduction pathway, might be involved in IL-1beta signalling in the brain and that activation of the enzyme requires the IL-1 receptor type 1.     

Microglia proliferation is regulated by hydrogen peroxide from NADPH oxidase

Microglia are resident brain macrophages that become activated and proliferate following brain damage or stimulation by immune mediators, such as IL-1beta or TNF-alpha. We investigated the mechanisms by which microglial proliferation is regulated in primary cultures of rat glia. We found that basal proliferation of microglia was stimulated by proinflammatory cytokines IL-1beta or TNF-alpha, and this proliferation was completely inhibited by catalase, implicating hydrogen peroxide as a mediator of proliferation. In addition, inhibitors of NADPH oxidase (diphenylene iodonium or apocynin) also prevented microglia proliferation, suggesting that this may be the source of hydrogen peroxide. IL-1beta and TNF-alpha rapidly stimulated the rate of hydrogen peroxide produced by isolated microglia, and this was inhibited by diphenylene iodonium, implying that the cytokines were acting directly on microglia to stimulate the NADPH oxidase. Low concentrations of PMA or arachidonic acid (known activators of NADPH oxidase) or xanthine/xanthine oxidase or glucose oxidase (generating hydrogen peroxide) also increased microglia proliferation and this was blocked by catalase, showing that NADPH oxidase activation or hydrogen peroxide was sufficient to stimulate microglia proliferation. In contrast to microglia, the proliferation of astrocytes was unaffected by the presence of catalase. In conclusion, these findings indicate that microglial proliferation in response to IL-1beta or TNF-alpha is mediated by hydrogen peroxide from NADPH oxidase.     

The contribution of the vagus nerve in interleukin-1beta-induced fever is dependent on dose

It has been suggested that proinflammatory cytokines communicate to the brain via a neural pathway involving activation of vagal afferents by interleukin-1beta (IL-1beta), in addition to blood-borne routes. In support, subdiaphragmatic vagotomy blocks IL-1beta-induced, brain-mediated responses such as fever. However, vagotomy has also been reported to be ineffective. Neural signaling would be expected to be especially important at low doses of cytokine, when local actions could occur, but only very small quantities of cytokine would become systemic. Here, we examined core body temperature after intraperitoneal injections of three doses of recombinat human IL-1beta (rh-IL-1beta). Subdiaphragmatic vagotomy completely blocked the fever produced by 0.1 microg/kg, only partially blocked the fever produced by 0.5 microg/kg, and had no effect at all on the fever that followed 1.0 microg/kg rh-IL-1beta. Blood levels of rh-IL-1beta did not become greater than normal basal levels of endogenous rat IL-beta until the 0.5-microg/kg dose nor was IL-1beta induced in the pituitary until this dose. These results suggest that low doses of intraperitoneal IL-1beta induce fever via a vagal route and that dose may account for some of the discrepancies in the literature.     

Cytokine-induced production of IFN-beta 2/IL-6 by freshly explanted human endometrial stromal cells. Modulation by estradiol-17 beta

The cytokine IFN-beta 2/IL-6 has emerged as an important means of communication between cells--both within the immune system as well as outside it. In exploring the link between the endocrine and the immune systems, we have studied the secretion of IFN-beta 2/IL-6 by freshly explanted human endometrial stromal cells and its modulation by estrogens. Endometrial stromal cells produced IFN-beta 2/IL-6 in response to other inflammation-associated cytokines such as IL-1 alpha or beta, TNF, and IFN-gamma. This secretion was strongly inhibited by estradiol-17 beta at concentrations as low as 10(-9) M. Multiple species of stromal cell IFN-beta 2/IL-6 in the size range 23 to 30 kDa were detected using immunoprecipitation or immunoblotting procedures. The endometrial stromal cell IFN-beta 2/IL-6 species were phosphorylated and differentially glycosylated in a manner comparable to IFN-beta 2/IL-6 secreted by induced human peripheral blood monocytes or foreskin fibroblasts. However, in contrast to peripheral blood monocytes and fibroblasts, bacterial LPS did not induce IFN-beta 2/IL-6 production in endometrial stromal cells. Additionally, the IFN-beta 2/IL-6 identified in medium from IL-1 alpha-induced stromal cells is biologically active on hepatocytes. These observations, taken together with the observation that IFN-beta 2/IL-6 strongly inhibits the proliferation of human epithelial cells, suggest the possibility that stromal cell secreted IFN-beta 2/IL-6 may affect the physiology of the overlying epithelium in an hormonally modulated manner. Estrogen-regulated production of endometrial IFN-beta 2/IL-6 may participate in gender-specific systemic immunomodulation.     

A warmer ambient temperature increases the passage of interleukin-1beta into the brains of old rats

We have demonstrated that after intraperitoneal lipopolysaccharide (LPS) injection, old rats mount fevers similar to those of young rats at an ambient temperature (Ta) of 31 degrees C, but not at 21 degrees C. The same is true for intraperitoneal or intravenous IL-1beta administration. The underlying mechanism responsible for blunted fever in old rats may be a deficiency in communication between the periphery and the brain. Possibly, peripheral cytokine actions are altered in old rats, such that the signal that reaches the brain is diminished. Here, we hypothesized that at standard laboratory temperatures, not enough IL-1beta is reaching the brain for fever to occur and that a warmer Ta would increase the influx of IL-1beta into the brain, enabling old rats to generate fever. Young (3-5 mo) and old (23-29 mo) Long-Evans rats were maintained for 3 days at either Ta 21 or 31 degrees C prior to intravenous injection with radiolabeled IL-1beta to measure passage across the blood-brain barrier. Young rats showed similar influx of IL-1beta into the brain at the two Tas, but old rats showed significant influx only at the warmer Ta. These data suggest that the lack of fever at a cool Ta may be due to a reduced influx of IL-1beta into the brain.     

IL-1 beta enhances CD40 ligand-mediated cytokine secretion by human dendritic cells (DC): a mechanism for T cell-independent DC activation.

CD40 ligand (CD40L) is a membrane-bound molecule expressed by activated T cells. CD40L potently induces dendritic cell (DC) maturation and IL-12p70 secretion and plays a critical role during T cell priming in the lymph nodes. IFN-gamma and IL-4 are required for CD40L-mediated cytokine secretion, suggesting that T cells are required for optimal CD40L activity. Because CD40L is rapidly up-regulated by non-T cells during inflammation, CD40 stimulation may also be important at the primary infection site. However, a role for T cells at the earliest stages of infection is unclear. The present study demonstrates that the innate immune cell-derived cytokine, IL-1beta, can increase CD40L-induced cytokine secretion by monocyte-derived DC, CD34(+)-derived DC, and peripheral blood DC independently of T cell-derived cytokines. Furthermore, IL-1beta is constitutively produced by monocyte-derived DC and monocytes, and is increased in response to intact Escherichia coli or CD40L, whereas neither CD34(+)-derived DC nor peripheral blood DC produce IL-1beta. Finally, DC activated with CD40L and IL-1beta induce higher levels of IFN-gamma secretion by T cells compared with DC activated with CD40L alone. Therefore, IL-1beta is the first non-T cell-derived cytokine identified that enhances CD40L-mediated activation of DC. The synergy between CD40L and IL-1beta highlights a potent, T cell-independent mechanism for DC activation during the earliest stages of inflammatory responses.     

The influence of interleukin-1beta on gamma-glutamyl transpepidase activity in rat hippocampus

Brain infections as well as peripheral challenges to the immune system lead to an increased production of interleukin-1beta (IL-1beta), a cytokine involved in leukocyte-mediated breakdown of the blood-brain barrier. The effects of IL-1beta have been reported to depend on whether the route of administration is systemic or intracerebral. Using 50-day-old male rats, we compared the effects of IL-1beta on brain gamma-glutamyl transpeptidase (GGT; an enzymatic marker of brain capillary endothelium) at 2, 24 and 96 h after either an intravenous (i.v.) injection of 5 microg IL-1beta or an intracerebroventricular (i.c.v. - lateral ventricle) infusion of 50 ng IL-1beta. When the i.v. route was used, the GGT activity underwent small but significant changes; decreasing in the hippocampus 2 h after the i.v. injection, increasing 24 h later and returning to control levels at 96 h. No significant changes in the hippocampal GGT activity were observed at 2 and 24 h following the i.c.v. infusion. The GGT activity in the hypothalamus remained unchanged regardless of the route of IL-1beta administrations. Similar changes in GGT activity were revealed histochemically. The labeling was found mainly in the capillary bed, the changes being most evident in the hippocampal stratum radiatum and stratum lacunosum-moleculare. A transient increase in GGT activity at 24 h, together with a less sharp delineation of GGT-stained vessels, may reflect IL-1beta induced increased turnover of glutathione and/or oxidative stress, that may in turn, be related to altered permeability of the blood-brain barrier in some neurological and mental disorders, including schizophrenia.     

Interleukin‐1β: a bridge between inflammation and excitotoxicity?

Interleukin-1 (IL-1) is a proinflammatory cytokine released by many cell types that acts in both an autocrine and/or paracrine fashion. While IL-1 is best described as an important mediator of the peripheral immune response during infection and inflammation, increasing evidence implicates IL-1 signaling in the pathogenesis of several neurological disorders. The biochemical pathway(s) by which this cytokine contributes to brain injury remain(s) largely unidentified. Herein, we review the evidence that demonstrates the contribution of IL-1β to the pathogenesis of both acute and chronic neurological disorders. Further, we highlight data that leads us to propose IL-1β as the missing mechanistic link between a potential beneficial inflammatory response and detrimental glutamate excitotoxicity.