- Email: [email protected]
Cytokines and sickness behavior: implications from knockout animal models
Research Update
References 1 Tisch, R. and McDevitt, H. (1996) Insulindependent diabetes mellitus. Cell 85, 291–297 2 Tian, J. et al. (1997) Determinant spreading of T helper cell 2 (Th2) responses to pancreatic islet autoantigens. J. Exp. Med. 186, 2039–2043 3 Bradley, L.M. et al. (1999) Islet-specific Th1, but not Th2, cells secrete multiple chemokines and promote rapid induction of autoimmune diabetes. J. Immunol. 162, 2511–2520 4 Katz, J.D. et al. (1995) T helper cell subsets in insulin-dependent diabetes. Science 268, 1185–1188 5 Poulin, M. and Haskins, K. (2000) Induction of diabetes in nonobese diabetic mice by Th2 T-cell clones from a TCR-transgenic mouse. J. Immunol. 164, 3072–3078 6 Concannon, P. et al. (1998) A second-generation screen of the human genome for susceptibility to insulin-dependent diabetes mellitus. Nat. Genet. 19, 292–296 7 Mein, C.A. et al. (1998) A search for type 1 diabetes susceptibility genes in families from the United Kingdom. Nat. Genet. 19, 297–300 8 She, J.X. (1996) Susceptibility to type 1 diabetes: HLA-DQ and DR revisited. Immunol. Today 17, 323–329
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9 Undlien, D.E. et al. (2001) HLA complex genes in type 1 diabetes and other autoimmune diseases. Which genes are involved? Trends Genet. 17, 93–100 10 Thorsby, E. (1997) HLA-associated diseases. Anniversary review article. Hum. Immunol. 53, 1–11 11 Sheehy, M.J. et al. (1989) A diabetes-susceptible HLA haplotype is best defined by a combination of HLA-DR and -DQ alleles. J. Clin. Invest. 83, 830–835 12 Caillat-Zucman, S. et al. (1992) Age-dependent HLA genetic heterogeneity of type 1 insulindependent diabetes mellitus. J. Clin. Invest. 90, 2242–2250 13 Cucca, F. et al. (1995) The distribution of DR4 haplotypes in Sardinia suggests a primary association of type 1 diabetes with DRB1 and DQB1 loci. Hum. Immunol. 43, 301–308 14 Undlien, D.E. et al. (1997) HLA-encoded genetic predisposition in insulin-dependent diabetes mellitus (IDDM). DR4 subtypes may be associated with different degrees of protection. Diabetes 46, 143–149 15 Sheehy, M.J. (1992) HLA and insulin-dependent diabetes. A protective perspective. Diabetes 41, 123–129
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16 Wen, L. et al. (2001) The regulatory role of DR4 in a spontaneous diabetes DQ8 transgenic model. J. Clin. Invest. 107, 871–880 17 Wen, L. et al. (2000) In vivo evidence for the contribution of human histocompatibility leukocyte antigen (HLA)-DQ molecules to the development of diabetes. J. Exp. Med. 191, 97–104 18 Abraham, R.S. et al. (2000) Coexpression of HLA DR3 and DQ8 results in the development of spontaneous insulitis and loss of tolerance to GAD65 in transgenic mice. Diabetes 49, 548–554 19 Schmidt, D. et al. (1997) A mechanism for the major histocompatibility complex-linked resistance to autoimmunity. J. Exp. Med. 186, 1059–1075 20 Sonderstrup, G. and McDevitt, H.O. (2001) DR, DQ and you: MHC alleles and autoimmunity. J. Clin. Invest. 107, 795–796
Dag E. Undlien* Erik Thorsby Institute of Immunology, Rikshospitalet University Hospital, N-0027 Oslo, Norway. *e-mail: [email protected]
Cytokines and sickness behavior: implications from knockout animal models Akio Inui Cytokines are key regulators of host defenses mediated by the central nervous system and are important in the development of signs of sickness, such as anorexia and fever. Despite the welldocumented redundancy in the cytokine cascade, recent studies with knockout animals have demonstrated successfully considerable functional specificity of cytokines in normal, as well as sickness, behavior.
Cytokines are a heterogeneous group of polypeptide mediators that have been associated classically with the activation of the immune system and inflammatory responses1–3. An increasing number of related mediators are now included in this category, and most of them have been shown to act on a variety of tissues, including the central nervous system (CNS). The CNS mediates a coordinated set of autonomic, endocrine and behavioral responses, which constitute the cerebral component of the acute-phase reaction1,4. Virtually all known cytokines and their receptors have been found in many different types of CNS cell, http://immunology.trends.com
including those in the hypothalamus5, as well as in cells of organs of the gastrointestinal tract. …IL-1β β is a crucial neurocytokine for the ‘… regulation of food intake, particularly when inflammatory stimuli are localized in the CNS.’
Novel techniques for genetic manipulation now permit researchers to generate a precise and long-lasting alteration in the expression level of a specific cytokine, so that its functions can be deduced in vivo3,6–8. The analysis of such mutants has increased our understanding of the roles of cytokines in defense mechanisms substantially, not only in the immune system but also in the CNS, where sickness behavior is evoked during infection and inflammation to regulate the pathological processes and maintain body homeostasis (Fig. 1). Anorexia and body-weight loss
Interleukin-1 (IL-1) plays a crucial role in the development of the pathophysiological responses to infection and inflammation.
Biologically active IL-1 consists of two distinct forms, IL-1α and IL-1β, the latter of which is the major form of IL-1 released from cells. Following treatment with turpentine, which causes localized inflammation and tissue injury, knockout mice lacking IL-1β or its type I receptor (IL-1RI) lost the elevation of levels of plasma IL-6 seen in wild-type mice and had defective production of acute-phase proteins in the liver9. The control mice lost a significant amount of body weight following turpentine treatment, and decreased body weight correlated well with a decreased intake of food and water. However, IL-1β-deficient mice were completely resistant to turpentineinduced anorexia and body-weight loss, although they did develop anorexia and weight loss in response to influenza virus or systemic inflammation induced by injection of the bacterial endotoxin lipopolysaccharide (LPS)9. Similar results were obtained from IL-6-deficient mice10, indicating that IL-1β and IL-6 are interrelated in the cytokine network, with IL-1β being a potent inducer of IL-6, and that both cytokines are involved in
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(a) Sickness behavior Fever
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IL-1β
Anorexia
IL-6
Aggression
IL-10 Sleep
(b)
TNF-α IL-2
Spatial memory
LIF
HPA axis
Immune system Glucocorticoids
Adrenal gland
Cytokine production
SNS (vagus) Pituitary ACTH
Hypothalamus
Infectious and/or inflammatory stimuli [e.g. influenza (viral), LPS (bacterial) and terpentine (abscess formation)]
CNS
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Fig. 1. (a) Sickness behavior induced by cytokines. The stimulatory or inhibitory effects of cytokines on sickness behavior are shown in red and blue, respectively, with the width of the arrows being proportionate to the strength of the impact. (b) Communication pathways between the immune and central nervous systems. Glucocorticoids released from the adrenal glands suppress inflammation, completing the regulatory feedback loop between the immune and central nervous systems. Leukemia inhibitory factor (LIF), an IL-6-related cytokine, was shown to be required for the hypothalamic (HPA) response to stress. Low adrenocorticotrophic (ACTH) responses to acute psychological stress or low corticosterone responses to chronic inflammatory stress were observed in LIF-deficient animals32. A low corticosterone response was also observed in IL-6-deficient mice during local inflammation2. Abbreviations: CNS, central nervous system; IL, interleukin; LPS, lipopolysaccharide; SNS, sympathetic nervous system; TNF, tumor necrosis factor.
systemic responses to localized inflammation. IL-6 and several other cytokines are known to trigger rapid phosphorylation and activation of signal transducer and activator of transcription 3 (STAT3). In line with the absence of an acute inflammatory response, the activation of STAT3 was abolished in IL-6-deficient mice treated with turpentine. Although tumor necrosis factor α (TNF-α) is assumed to be involved in the response to influenza virus or LPS, neither the absence of TNF-α nor the lack of TNF type I and II receptors (TNFRI and TNFRII) appeared to affect the anorexia and body-weight loss observed in response to LPS (Ref. 11). It was reported recently that mice lacking IL-1β converting enzyme (ICE), which produces the mature, biologically active 17 kDa form of IL-1β, were resistant to anorexia due to intracerebroventricular, but not systemic, administration of LPS (Ref. 12). The typical anorexic response seen in http://immunology.trends.com
wild-type mice after the administration of LPS was restored in ICE-deficient mice by the central administration of the ICE analog cathepsin G. These results indicate that IL-1β is a crucial neurocytokine for the regulation of food intake, particularly when inflammatory stimuli are localized in the CNS. …injection of IL-6 into the CNS was ‘… required to restore the febrile response, indicating the importance of a central source of this cytokine.’
A unique, naturally occurring antagonist of IL-1R (IL-1ra) is known to be expressed by cells of the CNS and to function as an inhibitor of IL-1 by binding to the IL-1R without triggering any biological response. IL-1ra-deficient mice had lower body weight than control littermates13. This weight difference was apparent by six weeks of age and continued into adult life. Probable mechanisms underlying this phenomenon
include the lack of antagonism of IL-1mediated suppression of food intake and altered catabolism. Cafeteria feeding (i.e. the feeding of palatable, energy-rich foods) was demonstrated to induce the transcription of mRNA encoding IL-1β in the hypothalamus, indicating the involvement of physiological, rather than pathological, processes. Recent studies demonstrate that, in terms of signaling pathways, neuronal IL-1 is situated downstream of leptin, a 16 kDa protein product of the ob gene, which is an important regulator of energy balance through its hypothalamic action on appetite and energy expenditure8,14. This conclusion is based on the findings that: LPS stimulates the expression of the ob gene in adipose tissue and therefore, increased serum protein levels of leptin; leptin increases levels of IL-1β in the hypothalamus; IL-1ra inhibits the decreased food intake caused by leptin; and leptin has no anorexic effect in mutant mice lacking IL-1RI (Ref. 14). It has also been demonstrated that IL-1 induces the expression of leptin, and the resistance of IL-1β-deficient mice to turpentine-induced anorexia is associated with the decreased induction of expression of leptin. These results indicate that IL-1 might be able to act both in the presence and absence of a specific pathogenic stimulus. Fever
Body temperature is controlled by the thermoregulatory center of the brain, the preoptic anterior hypothalamus (POAH). The pathogenesis of fever involves the activation of macrophages by various exogenous pyrogens, such as LPS and turpentine, which results in the synthesis and release of endogenous pyrogens, including IL-1β and IL-6 (Ref. 4). Also, the production of cytokines in the brain is increased during, for example, LPS-induced fever. IL-1β-deficient mice are completely resistant to turpentineinduced fever, as are IL-6-deficient mice7,9,10. Neither IL-1β- nor IL-6deficient mice demonstrated any increase in the level of circulating prostaglandin E2 (PGE2) compared with wild-type mice. This result suggests that these pyrogenic cytokines are essential for the induction of expression of PGE2, which might act as a final signal for the initiation of fever10. The turpentineinduced febrile response was enhanced in
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Box 1. Examples of behavioral paradigms Spatial learning is often studied using the Morris water maze taska–c. In the spatial form of this task, a mouse or a rat is placed in a circular pool of opaque water with a platform located slightly below the water level. The platform is always in the same location, but the starting location of the animal is varied from trial to trial. Normal animals learn to escape the water quite readily by finding the hidden platform. Because the platform is not visible and the animal begins different trials at different locations in the maze, the animal can only learn the location of the platform using spatial cues provided by the surrounding environment. There is also a nonspatial form of this task, in which the platform is above the water and visible. Here, the subject does not have to use spatial cues to learn to escape efficiently. A general result from this set-up is that manipulation of the hippocampus alters performance on the spatial, but not the nonspatial, form of the taskb. It has been reported that mice infected with Legionella pneumophilia exhibited impaired performance on the spatial, but not nonspatial, form of the taskb,c. Peripheral stimulation of the immune system during infection or inflammation can interfere with the cognitive process particularly, and interleukin-1β has been assumed to mediate this interference by its effect on the hippocampusb,c. Exposure to stressors results in decreased aggressiveness, depressed mood and increased sensitivity and reactivity to pain, as well as reductions in the intake of food and waterb,c. Anxiety is one of the core symptoms of major depressive illness. The elevated plus maze has been validated for the detection of anxiolytic and anxiogenic drug effectsa,d. This test is based on the conflict between the exploratory drive of the animal and its fear of exposed, open spaces. Two opposing branches of the maze are surrounded by black walls (closed arms), whereas the other
IL-1ra-deficient mice, in association with the enhanced expression of IL-1β and cycloxygenase 2 (COX2) in the brain. However, although IL-1β does contribute, it was demonstrated to be nonessential for the febrile response to LPS or influenza pneumonitis; systemic injection of LPS triggered a febrile response in transgenic mice overexpressing IL-1ra in the CNS or lacking TNFRI and TNFRII (Ref. 11). However, IL-6 appears to be crucial in LPS-, IL-1β- and TNF-αinduced fever, because none of these substances (with the exception of large doses of LPS) triggered fever in IL-6deficient mice15. The intraperitoneal injection of IL-6 was not sufficient to rescue the febrile response in IL-6deficient mice. Rather, injection of IL-6 into the CNS was required to restore the febrile response, indicating the importance of a central source of this cytokine15. Although ciliary neurotrophic factor (CNTF), an IL-6-related cytokine, has been proposed as an endogenous pyrogen, it failed to elicit a febrile http://immunology.trends.com
branches are devoid of walls (open arms). Wild-type animals display a preference for the safe, enclosed arms, and this preference is increased by anxiogenic drugs and decreased by anxiolytic drugs. The aggression of male mice towards other males is a form of social behavior in which adult males fight to establish dominance relationshipse,f. A resident–intruder test has been employed to evaluate intermale aggression. An intruder is transferred to the home cage of a resident and the behavioral responses of the resident, such as wrestling and biting, are determined. The intermale aggression test is a sensitive method, and has been shown to be affected specifically by the exogenous administration of cytokines5. Nociception is another biological function found to be affected by exposure to stressors. Stressinduced analgesia is normally displayed in animals that are stressed by restraint, forced swimming, or hot or cold water, in an opioid-dependent or -independent mannere,f. References a Petitto, J.M. et al. (1999) Impaired learning and memory and altered hippocampal neurodevelopment resulting from interleukin-2 gene deletion. J. Neurosci. Res. 56, 441–446 b Maier, S.F. and Watkins, L.R. (1998) Cytokines for psychologists: implications of bidirectional immune-to-brain communication for understanding behavior, mood and cognition. Psychol. Rev. 105, 83–107 c Dantzer, R. (2001) Cytokine-induced sickness behavior: where do we stand? Brain Behav. Immun. 15, 7–24 d Asakawa, A. et al. A role of ghrelin in neuroendocrine and behavioral response to stress. Neuroendocrinology (in press) e Alleva, E. et al. (1998) Behavioural characterization of interleukin-6 overexpressing or deficient mice during agonistic encounters. Eur. J. Neurosci. 10, 3664–3672 f De Felipe, C. et al. (1998) Altered nociception, analgesia and aggression in mice lacking the receptor for substance P. Nature 392, 394–397
response in the absence of IL-6. None of the mutant mice discussed was found to have any alteration in circadian variations of body temperature. IL-10 inhibits the synthesis of pro-inflammatory cytokines known to be involved in fever, including IL-1β, IL-6 and TNF-α. IL-10-deficient mice exhibited an exacerbated and prolonged fever in response to LPS, indicating that endogenously produced IL-10 has antipyretic properties7. Sleep and other behaviors
The systemic administration of LPS is associated with alterations in sleep patterns and electroencephalogram readings, as is Gram-negative bacterial infection. Available data suggest that alterations in sleep pattern are likely to be triggered by cytokines. Mice lacking TNFRI did not exhibit nonrapid eye movement sleep (NREMS) responses after the administration of exogenous TNF-α, although they retained the ability to express excess levels of NREMS if given
IL-1β, another well characterized NREMS-promoting cytokine16. The finding that TNFR1 mutant mice had less sleep than wild-type controls, primarily during the light period, indicates that endogenous TNF is involved in the physiological regulation of sleep patterns and a deficiency in the function of TNF is not, or at least not completely, compensated for developmentally by other sleep-promoting substances. IL-1RI-deficient mice had less NREMS during the dark period and more REMS during the light period than wildtype mice. They did not exhibit the wildtype sleep responses to IL-1β (increased NREMS), but exhibited increased NREMS after the administration of TNF-α. It was demonstrated recently that LPS upregulates brain levels of RGS7 protein, a regulator of G-protein signaling, in wildtype mice17. Endotoxin failed to induce the accumulation of RGS7 in TNFRI-deficient mice, indicating that the TNF-mediated upregulation of RGS7 might contribute to sepsis-induced changes in the function of the CNS (Ref. 17).
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IL-2 has been implicated as a brain neurotrophic factor and neuromodulator. IL-2-deficient mice have a specific defect in spatial learning and memory in the Morris water maze (Box 1)18. Simple association learning and memory performance were normal when the escape platform was visible, and fearfulness and locomotor activity were also normal in response to novelty using an elevated plus maze. IL-6-deficient mice exhibited a higher degree of aggressive behavior in the intermale aggression paradigm19. IL-6-deficient mice also showed an absence of stress-induced analgesia, with specific alterations in the brain opioid system. The chemokine family
The chemokines are a large superfamily of chemotactic cytokines, which are used to direct the trafficking and migration of leukocytes within the immune system20–23. Chemokines might regulate other physiological functions also, including hematopoiesis, angiogenesis, differentiation and development. They have been divided into four distinct subfamilies according to the number and spacing of the conserved N-terminal cysteines: the CXC or α subfamily [e.g. IL-8, growth-regulated oncogene (GRO), stromal-cell-derived factor 1 (SDF1) and interferon (IFN)-inducible protein 10 (IP10)]; the CC or β subfamily [e.g. macrophage inflammatory protein 1α (MIP1α) and MIP1β, monocyte chemotactic protein 1 (MCP1) and regulated on activation, normal T-cell expressed and secreted (RANTES)]; the C or γ subfamily (e.g. lymphotactin); and the CX3C or δ subfamily (e.g. fractalkine and neurotactin)20–23. Many chemokines have an expression pattern that strongly suggests a role in inflammation. Their expression is induced typically in either monocytes or macrophages, or in epithelial, endothelial or fibroblast cells by pro-inflammatory cytokines (e.g. IL-1, TNF-α and IFN-γ) or stimuli (e.g. LPS)20. There are also several chemokines that are expressed in normal organs or tissues in the apparent absence of inflammatory stimuli. An example is fractalkine, which is highly expressed in the brain, where it might have a homeostatic function; but its expression is also induced by TNF-α in epithelial cells and thus, it might participate in inflammatory reactions20,24. http://immunology.trends.com
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It is thought that the CNS might have its own chemokine ligand–receptor network, the function of which could extend well beyond the regulation of leukocyte trafficking in immunoinflammatory disease22. Previous studies have demonstrated that some members of the α [e.g. IL-8, platelet factor 4 (PF4) and IP10] and β (e.g. MCP1 and RANTES) chemokine subfamilies decrease the intake of food following central administration25. RANTES and other β chemokines, such as MIP1α and MIP1β, produce fever through direct actions on the POAH also26. An increase in the number of chemokine and/or chemokine receptor transgenic and knockout mice has helped to define the functions of chemokines in vivo21–23,27. For example, MIP1α-deficient mice are resistant to pathologies associated with viral infections, such as myocarditis caused by coxsackievirus and pneumonitis caused by influenza virus, indicating that in vivo, MIP1α is an important mediator of virally induced inflammation in the mouse28. However, to date, little effort has been directed at understanding the roles of chemokines in normal as well as sickness behavior. Such studies are awaited in view of the broader physiological functions of chemokines and their impact on the CNS. …the CNS might have its own chemokine ‘… ligand–receptor network, the function of which could extend well beyond the regulation of leukocyte trafficking in immunoinflammatory disease.’ Concluding remarks
We are now beginning to understand the intricacies by which cytokines regulate the functions of the CNS. Knockout mouse models have been useful in establishing the functions of cytokines and their receptors in the immune system3. The power of knockout mouse models has made it possible to elucidate the causative relationships between cytokines and their central manifestations, such as anorexia, fever and disturbed sleep patterns, which are part of the acute-phase response (Fig. 1). Despite the low concentrations of cytokines in the CNS, cytokines seem to be involved in the physiological mechanisms regulating appetite and sleep, and part of the cytokine-induced changes seen in sickness behavior might result from an amplification of these physiological mechanisms29,30.
Sickness behavior is a normal response of the host to pathogens that reorganizes the organism’s priorities to cope with infections29,30. However, there is evidence that the activation of the brain cytokine system is associated with psychopathological states such as depression30. The involvement of IL-1 in neurodegeneration might share mechanisms with its effects on fever, appetite or activation of the hypothalamus–pituitary–adrenal (HPA) axis31. It appears that the peripheral overexpression of cytokines contributes little to CNS pathology, whereas overexpression in neural cells might be detrimental almost uniformly6. Thus, future experiments should include determining the exact relationship between cytokines, sickness behavior and CNS pathology. It is necessary to use more-refined genetic techniques, such as tissue-specific or inducible gene knockouts, to not only rescue the animals from their developmental defects or death, but also, address the issues of compensation and redundancy more directly2,8 and determine the exact roles of neurocytokines and chemokines in sickness behavior. Acknowledgements
I am indebted to Masato Kasuga and Shigeaki Baba (Kobe University, Japan) for many stimulating discussions. The work was supported by grants from the Ministry of Education, Science, Sports and Culture of Japan. References 1 Plata-Salamán, C.R. (1999) Brain mechanisms in cytokine-induced anorexia. Psychoneuroendocrinology 24, 25–41 2 Turnbull, A.V. and Rivier, C.L. (1999) Regulation of the hypothalamic–pituitary–adrenal axis by cytokines: actions and mechanisms of action. Physiol. Rev. 79, 1–71 3 Durum, S.K. and Muegge, K., eds (1998) Preface. In Cytokine Knockouts, pp. VII–XVI, Humana Press 4 Elmquist, J.K. et al. (1997) Mechanisms of CNS response to systemic immune challenge: the febrile response. Trends Neurosci. 20, 565–570 5 Sternberg, E.M. (1997) Neural–immune interactions in health and disease. J. Clin. Invest. 100, 2641–2647 6 Campbell, I.L. (1998) Transgenic mice and cytokine actions in the brain: bridging the gap between structural and functional neuropathology. Brain Res. Rev. 26, 327–336 7 Kluger, M.J. et al. (1998) The use of knockout mice to understand the role of cytokines in fever. Clin. Exp. Pharmacol. Physiol. 25, 141–144 8 Inui, A. (2000) Transgenic approach to the study of body-weight regulation. Pharmacol. Rev. 52, 35–61
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9 Zheng, H. et al. (1995) Resistance to fever induction and impaired acute-phase response in interleukin-1β-deficient mice. Immunity 3, 9–19 10 Kozak, W. et al. (1998) IL-6 and IL-1β in fever. Studies using cytokine-deficient (knockout) mice. Ann. New York Acad. Sci. 856, 33–47 11 Leon, L.R. et al. (1997) Exacerbated febrile responses to LPS, but not turpentine, in TNF double receptor-knockout mice. Am. J. Physiol. 272, R563–R569 12 Yao, J-H. et al. (1999) Mice deficient in interleukin-1β converting enzyme resist anorexia induced by central lipopolysaccharide. Am. J. Physiol. 277, R1435–R1443 13 Hirsch, E. et al. (1996) Functions of interleukin-1 receptor antagonist in gene knockout and overproducing mice. Proc. Natl. Acad. Sci. U. S. A. 93, 11008–11013 14 Luheshi, G.N. et al. (1999) Leptin actions on food intake and body temperature are mediated by IL-1. Proc. Natl. Acad. Sci. U. S. A. 96, 7047–7052 15 Sundgren-Andersson, A.K. et al. (1998) IL-6 is essential in TNF-α-induced fever. Am. J. Physiol. 275, R2028–R2034 16 Fang, J. et al. (1997) Mice lacking the TNF 55 kDa receptor fail to sleep more after TNF-α treatment. J. Neurosci. 17, 5949–5955 17 Benzing, T. et al. (1999) Upregulation of RGS7 may contribute to tumor necrosis factor-induced changes in central nervous function. Nat. Med. 5, 913–918
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18 Petitto, J.M. et al. (1999) Impaired learning and memory and altered hippocampal neurodevelopment resulting from interleukin-2 gene deletion. J. Neurosci. Res. 56, 441–446 19 Alleva, E. et al. (1998) Behavioural characterization of interleukin-6 overexpressing or deficient mice during agonistic encounters. Eur. J. Neurosci. 10, 3664–3672 20 Rossi, D. and Zlotnik, A. (2000) The biology of chemokines and their receptors. Annu. Rev. Immunol. 18, 217–242 21 Cascieri, M.A. and Springer, M.S. (2000) The chemokine/chemokine-receptor family: potential and progress for therapeutic intervention. Curr. Opin. Chem. Biol. 4, 420–427 22 Asensio, V.C. and Campbell, I.L. (1999) Chemokines in the CNS: plurifunctional mediators in diverse states. Trends Neurosci. 22, 504–512 23 Wells, T.N. et al. (1998) Definition, function and pathophysiological significance of chemokine receptors. Trends Pharmacol. Sci. 19, 376–380 24 Bazan, J.F. et al. (1997) A new class of membranebound chemokine with a CX3C motif. Nature 385, 640–644 25 Plata-Salaman, C.R. and Borkoski, J.P. (1994) Chemokines/intercrines and central regulation of feeding. Am. J. Physiol. 266, R1711–R1715 26 Tavares, E. and Minano, F.J. (2000) RANTES: a new prostaglandin-dependent endogenous pyrogen in the rat. Neuropharmacology 39, 2505–2513
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27 Slattery, D.M. et al. (2000) Gene targeting of chemokines and their receptors. Springer Semin. Immunopathol. 22, 417–432 28 Cook, D.N. et al. (1995) Requirement of MIP-1α for an inflammatory response to viral infection. Science 269, 1583–1585 29 Maier, S.F. and Watkins, L.R. (1998) Cytokines for psychologists: implications of bidirectional immune-to-brain communication for understanding behavior, mood and cognition. Psychol. Rev. 105, 83–107 30 Dantzer, R. (2001) Cytokine-induced sickness behavior: where do we stand? Brain Behav. Immun. 15, 7–24 31 Rothwell, N.J. and Luheshi, G.N. (2000) Interleukin-1 in the brain: biology, pathology and therapeutic target. Trends Neurosci. 23, 618–625 32 Chesnokova, V. et al. (1998) Murine leukemia inhibitory factor gene disruption attenuates the hypothalamo–pituitary–adrenal axis stress response. Endocrinology 139, 2209–2216
Akio Inui Division of Diabetes, Digestive and Kidney Diseases, Dept of Clinical Molecular Medicine, Kobe University Graduate School of Medicine, Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. e-mail: [email protected]
Meeting Report
Exercise-induced immune changes – an influence on metabolism? Bente Klarlund Pedersen, Jeffrey A. Woods and David C. Nieman The International Society of Exercise and Immunology Fifth Convention was held in Baltimore, USA, on 29–30 May 2001.
It is clear that exercise-induced changes in the immune system might explain the increased susceptibility to infections in athletes. However, results presented at the convention indicate that the cytokine response to exercise might mediate important metabolic effects also. Heat shock proteins
The primary role of heat shock proteins (HSPs) is to act as molecular chaperones by binding to denatured proteins and catalyzing the assembly of protein complexes within cells. A recent study demonstrated that exogenous HSP72 binds specifically to the cell surface of human monocytes in vitro1. Importantly, the resultant activation of the gene http://immunology.trends.com
encoding interleukin-6 (IL-6) occurs by a CD14-dependent pathway. These data suggest that to induce the expression of IL-6 by monocytes, HSP72 must act by binding to the plasma membrane. …increased extracellular levels of HSPs, ‘… following necrotic cell death, facilitate the functions of innate immunity.’
Data presented at the Convention suggest that exercise-induced increases in the levels of HSPs could play a role in modulating immune function. M. Febbraio (Melbourne, Australia) found that HSP72 is released into the peripheral circulation during exercise, suggesting that HSPs might indeed provide a ‘danger signal’ to the immune system. Furthermore, A. Niess (Tuebingen, Germany) and colleagues found that exercise increased the level of HSP72 within monocytes
themselves. By contrast, very stressful exercise decreased the intracellular production of IL-6 by monocytes in vivo (M. Febbraio). Therefore, it appears that for HSP72 to activate an IL-6 response within monocytes, it must be released by other cells first, before adhering to the surface of the monocyte to signal through the CD14-dependent pathway. Increased intracellular levels of HSPs are protective against cellular stress. By contrast, increased extracellular levels of HSPs, following necrotic cell death, facilitate the functions of innate immunity. M. Fleshner (Boulder, CO, USA) demonstrated that physically active, stressed rats had increased levels of HSP70 in every tissue tested, whereas sedentary, stressed animals had increased levels of HSP70 in the blood, spleen, liver and adrenal glands only. Also, the increase in the level of HSP70 in these organs was
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References 1 Tisch, R. and McDevitt, H. (1996) Insulindependent diabetes mellitus. Cell 85, 291–297 2 Tian, J. et al. (1997) Determinant spreading of T helper cell 2 (Th2) responses to pancreatic islet autoantigens. J. Exp. Med. 186, 2039–2043 3 Bradley, L.M. et al. (1999) Islet-specific Th1, but not Th2, cells secrete multiple chemokines and promote rapid induction of autoimmune diabetes. J. Immunol. 162, 2511–2520 4 Katz, J.D. et al. (1995) T helper cell subsets in insulin-dependent diabetes. Science 268, 1185–1188 5 Poulin, M. and Haskins, K. (2000) Induction of diabetes in nonobese diabetic mice by Th2 T-cell clones from a TCR-transgenic mouse. J. Immunol. 164, 3072–3078 6 Concannon, P. et al. (1998) A second-generation screen of the human genome for susceptibility to insulin-dependent diabetes mellitus. Nat. Genet. 19, 292–296 7 Mein, C.A. et al. (1998) A search for type 1 diabetes susceptibility genes in families from the United Kingdom. Nat. Genet. 19, 297–300 8 She, J.X. (1996) Susceptibility to type 1 diabetes: HLA-DQ and DR revisited. Immunol. Today 17, 323–329
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9 Undlien, D.E. et al. (2001) HLA complex genes in type 1 diabetes and other autoimmune diseases. Which genes are involved? Trends Genet. 17, 93–100 10 Thorsby, E. (1997) HLA-associated diseases. Anniversary review article. Hum. Immunol. 53, 1–11 11 Sheehy, M.J. et al. (1989) A diabetes-susceptible HLA haplotype is best defined by a combination of HLA-DR and -DQ alleles. J. Clin. Invest. 83, 830–835 12 Caillat-Zucman, S. et al. (1992) Age-dependent HLA genetic heterogeneity of type 1 insulindependent diabetes mellitus. J. Clin. Invest. 90, 2242–2250 13 Cucca, F. et al. (1995) The distribution of DR4 haplotypes in Sardinia suggests a primary association of type 1 diabetes with DRB1 and DQB1 loci. Hum. Immunol. 43, 301–308 14 Undlien, D.E. et al. (1997) HLA-encoded genetic predisposition in insulin-dependent diabetes mellitus (IDDM). DR4 subtypes may be associated with different degrees of protection. Diabetes 46, 143–149 15 Sheehy, M.J. (1992) HLA and insulin-dependent diabetes. A protective perspective. Diabetes 41, 123–129
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16 Wen, L. et al. (2001) The regulatory role of DR4 in a spontaneous diabetes DQ8 transgenic model. J. Clin. Invest. 107, 871–880 17 Wen, L. et al. (2000) In vivo evidence for the contribution of human histocompatibility leukocyte antigen (HLA)-DQ molecules to the development of diabetes. J. Exp. Med. 191, 97–104 18 Abraham, R.S. et al. (2000) Coexpression of HLA DR3 and DQ8 results in the development of spontaneous insulitis and loss of tolerance to GAD65 in transgenic mice. Diabetes 49, 548–554 19 Schmidt, D. et al. (1997) A mechanism for the major histocompatibility complex-linked resistance to autoimmunity. J. Exp. Med. 186, 1059–1075 20 Sonderstrup, G. and McDevitt, H.O. (2001) DR, DQ and you: MHC alleles and autoimmunity. J. Clin. Invest. 107, 795–796
Dag E. Undlien* Erik Thorsby Institute of Immunology, Rikshospitalet University Hospital, N-0027 Oslo, Norway. *e-mail: [email protected]
Cytokines and sickness behavior: implications from knockout animal models Akio Inui Cytokines are key regulators of host defenses mediated by the central nervous system and are important in the development of signs of sickness, such as anorexia and fever. Despite the welldocumented redundancy in the cytokine cascade, recent studies with knockout animals have demonstrated successfully considerable functional specificity of cytokines in normal, as well as sickness, behavior.
Cytokines are a heterogeneous group of polypeptide mediators that have been associated classically with the activation of the immune system and inflammatory responses1–3. An increasing number of related mediators are now included in this category, and most of them have been shown to act on a variety of tissues, including the central nervous system (CNS). The CNS mediates a coordinated set of autonomic, endocrine and behavioral responses, which constitute the cerebral component of the acute-phase reaction1,4. Virtually all known cytokines and their receptors have been found in many different types of CNS cell, http://immunology.trends.com
including those in the hypothalamus5, as well as in cells of organs of the gastrointestinal tract. …IL-1β β is a crucial neurocytokine for the ‘… regulation of food intake, particularly when inflammatory stimuli are localized in the CNS.’
Novel techniques for genetic manipulation now permit researchers to generate a precise and long-lasting alteration in the expression level of a specific cytokine, so that its functions can be deduced in vivo3,6–8. The analysis of such mutants has increased our understanding of the roles of cytokines in defense mechanisms substantially, not only in the immune system but also in the CNS, where sickness behavior is evoked during infection and inflammation to regulate the pathological processes and maintain body homeostasis (Fig. 1). Anorexia and body-weight loss
Interleukin-1 (IL-1) plays a crucial role in the development of the pathophysiological responses to infection and inflammation.
Biologically active IL-1 consists of two distinct forms, IL-1α and IL-1β, the latter of which is the major form of IL-1 released from cells. Following treatment with turpentine, which causes localized inflammation and tissue injury, knockout mice lacking IL-1β or its type I receptor (IL-1RI) lost the elevation of levels of plasma IL-6 seen in wild-type mice and had defective production of acute-phase proteins in the liver9. The control mice lost a significant amount of body weight following turpentine treatment, and decreased body weight correlated well with a decreased intake of food and water. However, IL-1β-deficient mice were completely resistant to turpentineinduced anorexia and body-weight loss, although they did develop anorexia and weight loss in response to influenza virus or systemic inflammation induced by injection of the bacterial endotoxin lipopolysaccharide (LPS)9. Similar results were obtained from IL-6-deficient mice10, indicating that IL-1β and IL-6 are interrelated in the cytokine network, with IL-1β being a potent inducer of IL-6, and that both cytokines are involved in
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(a) Sickness behavior Fever
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IL-1β
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Aggression
IL-10 Sleep
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HPA axis
Immune system Glucocorticoids
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SNS (vagus) Pituitary ACTH
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Fig. 1. (a) Sickness behavior induced by cytokines. The stimulatory or inhibitory effects of cytokines on sickness behavior are shown in red and blue, respectively, with the width of the arrows being proportionate to the strength of the impact. (b) Communication pathways between the immune and central nervous systems. Glucocorticoids released from the adrenal glands suppress inflammation, completing the regulatory feedback loop between the immune and central nervous systems. Leukemia inhibitory factor (LIF), an IL-6-related cytokine, was shown to be required for the hypothalamic (HPA) response to stress. Low adrenocorticotrophic (ACTH) responses to acute psychological stress or low corticosterone responses to chronic inflammatory stress were observed in LIF-deficient animals32. A low corticosterone response was also observed in IL-6-deficient mice during local inflammation2. Abbreviations: CNS, central nervous system; IL, interleukin; LPS, lipopolysaccharide; SNS, sympathetic nervous system; TNF, tumor necrosis factor.
systemic responses to localized inflammation. IL-6 and several other cytokines are known to trigger rapid phosphorylation and activation of signal transducer and activator of transcription 3 (STAT3). In line with the absence of an acute inflammatory response, the activation of STAT3 was abolished in IL-6-deficient mice treated with turpentine. Although tumor necrosis factor α (TNF-α) is assumed to be involved in the response to influenza virus or LPS, neither the absence of TNF-α nor the lack of TNF type I and II receptors (TNFRI and TNFRII) appeared to affect the anorexia and body-weight loss observed in response to LPS (Ref. 11). It was reported recently that mice lacking IL-1β converting enzyme (ICE), which produces the mature, biologically active 17 kDa form of IL-1β, were resistant to anorexia due to intracerebroventricular, but not systemic, administration of LPS (Ref. 12). The typical anorexic response seen in http://immunology.trends.com
wild-type mice after the administration of LPS was restored in ICE-deficient mice by the central administration of the ICE analog cathepsin G. These results indicate that IL-1β is a crucial neurocytokine for the regulation of food intake, particularly when inflammatory stimuli are localized in the CNS. …injection of IL-6 into the CNS was ‘… required to restore the febrile response, indicating the importance of a central source of this cytokine.’
A unique, naturally occurring antagonist of IL-1R (IL-1ra) is known to be expressed by cells of the CNS and to function as an inhibitor of IL-1 by binding to the IL-1R without triggering any biological response. IL-1ra-deficient mice had lower body weight than control littermates13. This weight difference was apparent by six weeks of age and continued into adult life. Probable mechanisms underlying this phenomenon
include the lack of antagonism of IL-1mediated suppression of food intake and altered catabolism. Cafeteria feeding (i.e. the feeding of palatable, energy-rich foods) was demonstrated to induce the transcription of mRNA encoding IL-1β in the hypothalamus, indicating the involvement of physiological, rather than pathological, processes. Recent studies demonstrate that, in terms of signaling pathways, neuronal IL-1 is situated downstream of leptin, a 16 kDa protein product of the ob gene, which is an important regulator of energy balance through its hypothalamic action on appetite and energy expenditure8,14. This conclusion is based on the findings that: LPS stimulates the expression of the ob gene in adipose tissue and therefore, increased serum protein levels of leptin; leptin increases levels of IL-1β in the hypothalamus; IL-1ra inhibits the decreased food intake caused by leptin; and leptin has no anorexic effect in mutant mice lacking IL-1RI (Ref. 14). It has also been demonstrated that IL-1 induces the expression of leptin, and the resistance of IL-1β-deficient mice to turpentine-induced anorexia is associated with the decreased induction of expression of leptin. These results indicate that IL-1 might be able to act both in the presence and absence of a specific pathogenic stimulus. Fever
Body temperature is controlled by the thermoregulatory center of the brain, the preoptic anterior hypothalamus (POAH). The pathogenesis of fever involves the activation of macrophages by various exogenous pyrogens, such as LPS and turpentine, which results in the synthesis and release of endogenous pyrogens, including IL-1β and IL-6 (Ref. 4). Also, the production of cytokines in the brain is increased during, for example, LPS-induced fever. IL-1β-deficient mice are completely resistant to turpentineinduced fever, as are IL-6-deficient mice7,9,10. Neither IL-1β- nor IL-6deficient mice demonstrated any increase in the level of circulating prostaglandin E2 (PGE2) compared with wild-type mice. This result suggests that these pyrogenic cytokines are essential for the induction of expression of PGE2, which might act as a final signal for the initiation of fever10. The turpentineinduced febrile response was enhanced in
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Box 1. Examples of behavioral paradigms Spatial learning is often studied using the Morris water maze taska–c. In the spatial form of this task, a mouse or a rat is placed in a circular pool of opaque water with a platform located slightly below the water level. The platform is always in the same location, but the starting location of the animal is varied from trial to trial. Normal animals learn to escape the water quite readily by finding the hidden platform. Because the platform is not visible and the animal begins different trials at different locations in the maze, the animal can only learn the location of the platform using spatial cues provided by the surrounding environment. There is also a nonspatial form of this task, in which the platform is above the water and visible. Here, the subject does not have to use spatial cues to learn to escape efficiently. A general result from this set-up is that manipulation of the hippocampus alters performance on the spatial, but not the nonspatial, form of the taskb. It has been reported that mice infected with Legionella pneumophilia exhibited impaired performance on the spatial, but not nonspatial, form of the taskb,c. Peripheral stimulation of the immune system during infection or inflammation can interfere with the cognitive process particularly, and interleukin-1β has been assumed to mediate this interference by its effect on the hippocampusb,c. Exposure to stressors results in decreased aggressiveness, depressed mood and increased sensitivity and reactivity to pain, as well as reductions in the intake of food and waterb,c. Anxiety is one of the core symptoms of major depressive illness. The elevated plus maze has been validated for the detection of anxiolytic and anxiogenic drug effectsa,d. This test is based on the conflict between the exploratory drive of the animal and its fear of exposed, open spaces. Two opposing branches of the maze are surrounded by black walls (closed arms), whereas the other
IL-1ra-deficient mice, in association with the enhanced expression of IL-1β and cycloxygenase 2 (COX2) in the brain. However, although IL-1β does contribute, it was demonstrated to be nonessential for the febrile response to LPS or influenza pneumonitis; systemic injection of LPS triggered a febrile response in transgenic mice overexpressing IL-1ra in the CNS or lacking TNFRI and TNFRII (Ref. 11). However, IL-6 appears to be crucial in LPS-, IL-1β- and TNF-αinduced fever, because none of these substances (with the exception of large doses of LPS) triggered fever in IL-6deficient mice15. The intraperitoneal injection of IL-6 was not sufficient to rescue the febrile response in IL-6deficient mice. Rather, injection of IL-6 into the CNS was required to restore the febrile response, indicating the importance of a central source of this cytokine15. Although ciliary neurotrophic factor (CNTF), an IL-6-related cytokine, has been proposed as an endogenous pyrogen, it failed to elicit a febrile http://immunology.trends.com
branches are devoid of walls (open arms). Wild-type animals display a preference for the safe, enclosed arms, and this preference is increased by anxiogenic drugs and decreased by anxiolytic drugs. The aggression of male mice towards other males is a form of social behavior in which adult males fight to establish dominance relationshipse,f. A resident–intruder test has been employed to evaluate intermale aggression. An intruder is transferred to the home cage of a resident and the behavioral responses of the resident, such as wrestling and biting, are determined. The intermale aggression test is a sensitive method, and has been shown to be affected specifically by the exogenous administration of cytokines5. Nociception is another biological function found to be affected by exposure to stressors. Stressinduced analgesia is normally displayed in animals that are stressed by restraint, forced swimming, or hot or cold water, in an opioid-dependent or -independent mannere,f. References a Petitto, J.M. et al. (1999) Impaired learning and memory and altered hippocampal neurodevelopment resulting from interleukin-2 gene deletion. J. Neurosci. Res. 56, 441–446 b Maier, S.F. and Watkins, L.R. (1998) Cytokines for psychologists: implications of bidirectional immune-to-brain communication for understanding behavior, mood and cognition. Psychol. Rev. 105, 83–107 c Dantzer, R. (2001) Cytokine-induced sickness behavior: where do we stand? Brain Behav. Immun. 15, 7–24 d Asakawa, A. et al. A role of ghrelin in neuroendocrine and behavioral response to stress. Neuroendocrinology (in press) e Alleva, E. et al. (1998) Behavioural characterization of interleukin-6 overexpressing or deficient mice during agonistic encounters. Eur. J. Neurosci. 10, 3664–3672 f De Felipe, C. et al. (1998) Altered nociception, analgesia and aggression in mice lacking the receptor for substance P. Nature 392, 394–397
response in the absence of IL-6. None of the mutant mice discussed was found to have any alteration in circadian variations of body temperature. IL-10 inhibits the synthesis of pro-inflammatory cytokines known to be involved in fever, including IL-1β, IL-6 and TNF-α. IL-10-deficient mice exhibited an exacerbated and prolonged fever in response to LPS, indicating that endogenously produced IL-10 has antipyretic properties7. Sleep and other behaviors
The systemic administration of LPS is associated with alterations in sleep patterns and electroencephalogram readings, as is Gram-negative bacterial infection. Available data suggest that alterations in sleep pattern are likely to be triggered by cytokines. Mice lacking TNFRI did not exhibit nonrapid eye movement sleep (NREMS) responses after the administration of exogenous TNF-α, although they retained the ability to express excess levels of NREMS if given
IL-1β, another well characterized NREMS-promoting cytokine16. The finding that TNFR1 mutant mice had less sleep than wild-type controls, primarily during the light period, indicates that endogenous TNF is involved in the physiological regulation of sleep patterns and a deficiency in the function of TNF is not, or at least not completely, compensated for developmentally by other sleep-promoting substances. IL-1RI-deficient mice had less NREMS during the dark period and more REMS during the light period than wildtype mice. They did not exhibit the wildtype sleep responses to IL-1β (increased NREMS), but exhibited increased NREMS after the administration of TNF-α. It was demonstrated recently that LPS upregulates brain levels of RGS7 protein, a regulator of G-protein signaling, in wildtype mice17. Endotoxin failed to induce the accumulation of RGS7 in TNFRI-deficient mice, indicating that the TNF-mediated upregulation of RGS7 might contribute to sepsis-induced changes in the function of the CNS (Ref. 17).
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IL-2 has been implicated as a brain neurotrophic factor and neuromodulator. IL-2-deficient mice have a specific defect in spatial learning and memory in the Morris water maze (Box 1)18. Simple association learning and memory performance were normal when the escape platform was visible, and fearfulness and locomotor activity were also normal in response to novelty using an elevated plus maze. IL-6-deficient mice exhibited a higher degree of aggressive behavior in the intermale aggression paradigm19. IL-6-deficient mice also showed an absence of stress-induced analgesia, with specific alterations in the brain opioid system. The chemokine family
The chemokines are a large superfamily of chemotactic cytokines, which are used to direct the trafficking and migration of leukocytes within the immune system20–23. Chemokines might regulate other physiological functions also, including hematopoiesis, angiogenesis, differentiation and development. They have been divided into four distinct subfamilies according to the number and spacing of the conserved N-terminal cysteines: the CXC or α subfamily [e.g. IL-8, growth-regulated oncogene (GRO), stromal-cell-derived factor 1 (SDF1) and interferon (IFN)-inducible protein 10 (IP10)]; the CC or β subfamily [e.g. macrophage inflammatory protein 1α (MIP1α) and MIP1β, monocyte chemotactic protein 1 (MCP1) and regulated on activation, normal T-cell expressed and secreted (RANTES)]; the C or γ subfamily (e.g. lymphotactin); and the CX3C or δ subfamily (e.g. fractalkine and neurotactin)20–23. Many chemokines have an expression pattern that strongly suggests a role in inflammation. Their expression is induced typically in either monocytes or macrophages, or in epithelial, endothelial or fibroblast cells by pro-inflammatory cytokines (e.g. IL-1, TNF-α and IFN-γ) or stimuli (e.g. LPS)20. There are also several chemokines that are expressed in normal organs or tissues in the apparent absence of inflammatory stimuli. An example is fractalkine, which is highly expressed in the brain, where it might have a homeostatic function; but its expression is also induced by TNF-α in epithelial cells and thus, it might participate in inflammatory reactions20,24. http://immunology.trends.com
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It is thought that the CNS might have its own chemokine ligand–receptor network, the function of which could extend well beyond the regulation of leukocyte trafficking in immunoinflammatory disease22. Previous studies have demonstrated that some members of the α [e.g. IL-8, platelet factor 4 (PF4) and IP10] and β (e.g. MCP1 and RANTES) chemokine subfamilies decrease the intake of food following central administration25. RANTES and other β chemokines, such as MIP1α and MIP1β, produce fever through direct actions on the POAH also26. An increase in the number of chemokine and/or chemokine receptor transgenic and knockout mice has helped to define the functions of chemokines in vivo21–23,27. For example, MIP1α-deficient mice are resistant to pathologies associated with viral infections, such as myocarditis caused by coxsackievirus and pneumonitis caused by influenza virus, indicating that in vivo, MIP1α is an important mediator of virally induced inflammation in the mouse28. However, to date, little effort has been directed at understanding the roles of chemokines in normal as well as sickness behavior. Such studies are awaited in view of the broader physiological functions of chemokines and their impact on the CNS. …the CNS might have its own chemokine ‘… ligand–receptor network, the function of which could extend well beyond the regulation of leukocyte trafficking in immunoinflammatory disease.’ Concluding remarks
We are now beginning to understand the intricacies by which cytokines regulate the functions of the CNS. Knockout mouse models have been useful in establishing the functions of cytokines and their receptors in the immune system3. The power of knockout mouse models has made it possible to elucidate the causative relationships between cytokines and their central manifestations, such as anorexia, fever and disturbed sleep patterns, which are part of the acute-phase response (Fig. 1). Despite the low concentrations of cytokines in the CNS, cytokines seem to be involved in the physiological mechanisms regulating appetite and sleep, and part of the cytokine-induced changes seen in sickness behavior might result from an amplification of these physiological mechanisms29,30.
Sickness behavior is a normal response of the host to pathogens that reorganizes the organism’s priorities to cope with infections29,30. However, there is evidence that the activation of the brain cytokine system is associated with psychopathological states such as depression30. The involvement of IL-1 in neurodegeneration might share mechanisms with its effects on fever, appetite or activation of the hypothalamus–pituitary–adrenal (HPA) axis31. It appears that the peripheral overexpression of cytokines contributes little to CNS pathology, whereas overexpression in neural cells might be detrimental almost uniformly6. Thus, future experiments should include determining the exact relationship between cytokines, sickness behavior and CNS pathology. It is necessary to use more-refined genetic techniques, such as tissue-specific or inducible gene knockouts, to not only rescue the animals from their developmental defects or death, but also, address the issues of compensation and redundancy more directly2,8 and determine the exact roles of neurocytokines and chemokines in sickness behavior. Acknowledgements
I am indebted to Masato Kasuga and Shigeaki Baba (Kobe University, Japan) for many stimulating discussions. The work was supported by grants from the Ministry of Education, Science, Sports and Culture of Japan. References 1 Plata-Salamán, C.R. (1999) Brain mechanisms in cytokine-induced anorexia. Psychoneuroendocrinology 24, 25–41 2 Turnbull, A.V. and Rivier, C.L. (1999) Regulation of the hypothalamic–pituitary–adrenal axis by cytokines: actions and mechanisms of action. Physiol. Rev. 79, 1–71 3 Durum, S.K. and Muegge, K., eds (1998) Preface. In Cytokine Knockouts, pp. VII–XVI, Humana Press 4 Elmquist, J.K. et al. (1997) Mechanisms of CNS response to systemic immune challenge: the febrile response. Trends Neurosci. 20, 565–570 5 Sternberg, E.M. (1997) Neural–immune interactions in health and disease. J. Clin. Invest. 100, 2641–2647 6 Campbell, I.L. (1998) Transgenic mice and cytokine actions in the brain: bridging the gap between structural and functional neuropathology. Brain Res. Rev. 26, 327–336 7 Kluger, M.J. et al. (1998) The use of knockout mice to understand the role of cytokines in fever. Clin. Exp. Pharmacol. Physiol. 25, 141–144 8 Inui, A. (2000) Transgenic approach to the study of body-weight regulation. Pharmacol. Rev. 52, 35–61
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9 Zheng, H. et al. (1995) Resistance to fever induction and impaired acute-phase response in interleukin-1β-deficient mice. Immunity 3, 9–19 10 Kozak, W. et al. (1998) IL-6 and IL-1β in fever. Studies using cytokine-deficient (knockout) mice. Ann. New York Acad. Sci. 856, 33–47 11 Leon, L.R. et al. (1997) Exacerbated febrile responses to LPS, but not turpentine, in TNF double receptor-knockout mice. Am. J. Physiol. 272, R563–R569 12 Yao, J-H. et al. (1999) Mice deficient in interleukin-1β converting enzyme resist anorexia induced by central lipopolysaccharide. Am. J. Physiol. 277, R1435–R1443 13 Hirsch, E. et al. (1996) Functions of interleukin-1 receptor antagonist in gene knockout and overproducing mice. Proc. Natl. Acad. Sci. U. S. A. 93, 11008–11013 14 Luheshi, G.N. et al. (1999) Leptin actions on food intake and body temperature are mediated by IL-1. Proc. Natl. Acad. Sci. U. S. A. 96, 7047–7052 15 Sundgren-Andersson, A.K. et al. (1998) IL-6 is essential in TNF-α-induced fever. Am. J. Physiol. 275, R2028–R2034 16 Fang, J. et al. (1997) Mice lacking the TNF 55 kDa receptor fail to sleep more after TNF-α treatment. J. Neurosci. 17, 5949–5955 17 Benzing, T. et al. (1999) Upregulation of RGS7 may contribute to tumor necrosis factor-induced changes in central nervous function. Nat. Med. 5, 913–918
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18 Petitto, J.M. et al. (1999) Impaired learning and memory and altered hippocampal neurodevelopment resulting from interleukin-2 gene deletion. J. Neurosci. Res. 56, 441–446 19 Alleva, E. et al. (1998) Behavioural characterization of interleukin-6 overexpressing or deficient mice during agonistic encounters. Eur. J. Neurosci. 10, 3664–3672 20 Rossi, D. and Zlotnik, A. (2000) The biology of chemokines and their receptors. Annu. Rev. Immunol. 18, 217–242 21 Cascieri, M.A. and Springer, M.S. (2000) The chemokine/chemokine-receptor family: potential and progress for therapeutic intervention. Curr. Opin. Chem. Biol. 4, 420–427 22 Asensio, V.C. and Campbell, I.L. (1999) Chemokines in the CNS: plurifunctional mediators in diverse states. Trends Neurosci. 22, 504–512 23 Wells, T.N. et al. (1998) Definition, function and pathophysiological significance of chemokine receptors. Trends Pharmacol. Sci. 19, 376–380 24 Bazan, J.F. et al. (1997) A new class of membranebound chemokine with a CX3C motif. Nature 385, 640–644 25 Plata-Salaman, C.R. and Borkoski, J.P. (1994) Chemokines/intercrines and central regulation of feeding. Am. J. Physiol. 266, R1711–R1715 26 Tavares, E. and Minano, F.J. (2000) RANTES: a new prostaglandin-dependent endogenous pyrogen in the rat. Neuropharmacology 39, 2505–2513
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27 Slattery, D.M. et al. (2000) Gene targeting of chemokines and their receptors. Springer Semin. Immunopathol. 22, 417–432 28 Cook, D.N. et al. (1995) Requirement of MIP-1α for an inflammatory response to viral infection. Science 269, 1583–1585 29 Maier, S.F. and Watkins, L.R. (1998) Cytokines for psychologists: implications of bidirectional immune-to-brain communication for understanding behavior, mood and cognition. Psychol. Rev. 105, 83–107 30 Dantzer, R. (2001) Cytokine-induced sickness behavior: where do we stand? Brain Behav. Immun. 15, 7–24 31 Rothwell, N.J. and Luheshi, G.N. (2000) Interleukin-1 in the brain: biology, pathology and therapeutic target. Trends Neurosci. 23, 618–625 32 Chesnokova, V. et al. (1998) Murine leukemia inhibitory factor gene disruption attenuates the hypothalamo–pituitary–adrenal axis stress response. Endocrinology 139, 2209–2216
Akio Inui Division of Diabetes, Digestive and Kidney Diseases, Dept of Clinical Molecular Medicine, Kobe University Graduate School of Medicine, Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. e-mail: [email protected]
Meeting Report
Exercise-induced immune changes – an influence on metabolism? Bente Klarlund Pedersen, Jeffrey A. Woods and David C. Nieman The International Society of Exercise and Immunology Fifth Convention was held in Baltimore, USA, on 29–30 May 2001.
It is clear that exercise-induced changes in the immune system might explain the increased susceptibility to infections in athletes. However, results presented at the convention indicate that the cytokine response to exercise might mediate important metabolic effects also. Heat shock proteins
The primary role of heat shock proteins (HSPs) is to act as molecular chaperones by binding to denatured proteins and catalyzing the assembly of protein complexes within cells. A recent study demonstrated that exogenous HSP72 binds specifically to the cell surface of human monocytes in vitro1. Importantly, the resultant activation of the gene http://immunology.trends.com
encoding interleukin-6 (IL-6) occurs by a CD14-dependent pathway. These data suggest that to induce the expression of IL-6 by monocytes, HSP72 must act by binding to the plasma membrane. …increased extracellular levels of HSPs, ‘… following necrotic cell death, facilitate the functions of innate immunity.’
Data presented at the Convention suggest that exercise-induced increases in the levels of HSPs could play a role in modulating immune function. M. Febbraio (Melbourne, Australia) found that HSP72 is released into the peripheral circulation during exercise, suggesting that HSPs might indeed provide a ‘danger signal’ to the immune system. Furthermore, A. Niess (Tuebingen, Germany) and colleagues found that exercise increased the level of HSP72 within monocytes
themselves. By contrast, very stressful exercise decreased the intracellular production of IL-6 by monocytes in vivo (M. Febbraio). Therefore, it appears that for HSP72 to activate an IL-6 response within monocytes, it must be released by other cells first, before adhering to the surface of the monocyte to signal through the CD14-dependent pathway. Increased intracellular levels of HSPs are protective against cellular stress. By contrast, increased extracellular levels of HSPs, following necrotic cell death, facilitate the functions of innate immunity. M. Fleshner (Boulder, CO, USA) demonstrated that physically active, stressed rats had increased levels of HSP70 in every tissue tested, whereas sedentary, stressed animals had increased levels of HSP70 in the blood, spleen, liver and adrenal glands only. Also, the increase in the level of HSP70 in these organs was
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