To protect or to kill: A persisting Darwinian immune dilemma.

The immune system, which evolved as a protective system, can paradoxically mediate lethal effects when it is over-activated. These effects can be traced back to infected insects and are mainly mediated by phylogenetically old cytokines that have been found already in starfishes and sponges. We hypothesize that these anti-homeostatic effects are important for restricting the cumulative risk of transmission of highly mutating environmental pathogens that may endanger species, particularly when they start to originate and expand. Considering the Darwinian view that evolution is a permanent process, this anti-homeostatic program is preserved and expressed even when there is no risk for the species. Here, we review these aspects and discuss how evolutionary-imposed anti-homeostatic immune programs are expressed during acute and chronic human diseases, which can be further aggravated in the absence of medical interventions. The relevance of early identification of ancestral biomarkers that predict a shift from protective to deleterious immune outcomes is emphasized.

Intranasal Methylprednisolone Effectively Reduces Neuroinflammation in Mice With Experimental Autoimmune Encephalitis.

Relapsing-remitting multiple sclerosis, the most common form, is characterized by acute neuroinflammatory episodes. In addition to continuous disease-modifying therapy, these relapses require treatment to prevent lesion accumulation and progression of disability. Intravenous methylprednisolone (1-2 g for 3-5 days) is the standard treatment for relapses. However, this treatment is invasive, requires hospitalization, leads to substantial systemic exposure of glucocorticoids, and can only reach modest concentrations in the central nervous system (CNS). Intranasal delivery may represent an alternative to deliver relapse treatment directly to the CNS with higher concentrations and reducing side effects. Histopathological analysis revealed that intranasal administration of methylprednisolone to mice with experimental autoimmune encephalomyelitis (EAE) suppressed the neuroinflammatory peak, and reduced immune cell infiltration and demyelination in the CNS similarly to intravenous administration. Treatment also downregulated Iba1 and GFAP expression. A similar significant reduction of IL-1ß, IL-6, IL-17, IFN-γ, and TNF-α levels in the spinal cord was attained in both intranasal and intravenously treated mice. No damage in the nasal cavity was found after intranasal administration. This study demonstrates that intranasal delivery of methylprednisolone is as efficient as the intravenous route to treat neuroinflammation in EAE.

The immune system as a sensorial system that can modulate brain functions and reset homeostasis.

Evidence indicates that activated immune cells release products, typically cytokines, that can convey information to the brain about the type of ongoing peripheral immune responses. This evidence led colleagues and me to categorize the immune system as another sensorial system that, upon receiving this information, can emit neuroendocrine signals with immunoregulatory functions that can also reset homeostatic mechanisms. Here, I discuss evidence and clues indicating (1) possible mechanisms by which cytokines, such as those of the interleukin 1 (IL-1) family, can reset energy homeostasis to balance the high fuel requirement of the immune system and the brain; and (2) the possibility that the tripartite synapse, which includes astrocytes as a third component, processes and integrates immune signals at brain levels with other sensorial signals that the central nervous system permanently receives.

Immune-Neuro-Endocrine Reflexes, Circuits, and Networks: Physiologic and Evolutionary Implications.

The existence of a network of interactions between the immune and nervous systems that influences host defenses and brain functions is now well-established. Here we discuss how immune and classical neuro/sensorial signals are processed in the brain and how neuro-endocrine immunoregulatory and behavioral responses are integrated. Considering the ability of brain cells to produce cytokines, originally described as immune cell products, we propose that the tripartite synapse plays a central role in the integration of neuro-endocrine-immune interactions. We also propose that the immune-neuro-endocrine responses that influence the course of transmissible and other diseases predisposing to infections can be relevant for evolution, either by restoring health or by mediating an active process of negative selection.

Mimicking disruption of brain-immune system-joint communication results in collagen type II-induced arthritis in non-susceptible PVG rats.

The brain-immune system-joint communication is disrupted during collagen type II (CII) arthritis in DA rats. Since PVG rats are not susceptible to arthritis induction, comparison of hypothalamic and peripheral neuro-endocrine and immune responses between immunized DA and PVG rats might help to explain their different susceptibility to develop the disease. PVG and DA rats were immunized with CII. Corticosterone, neurotransmitters, anti-CII antibodies, and cytokine concentrations in plasma, and hypothalamic neurotransmitters and cytokines were determined by ELISA, Luminex, HPLC and RT-qPCR. Adrenalectomy or sham-operation was performed in PVG and DA rats 14 days before immunization. Basal plasma corticosterone and adrenaline concentrations were significantly higher, and plasma cytokines and hypothalamic noradrenaline were lower in PVG rats than in DA rats. While DA rats developed severe arthritis upon immunization (maximum score 16), only 12 out of 28 PVG rats showed minimal symptoms (score 1-2). The density of sympathetic nerve fibers in arthritic joints of DA rats markedly decreased, but it remained stable in immunized PVG rats. The ratio corticosterone to IL-1ß levels in plasma was markedly higher in immunized PVG rats than in arthritic DA rats. Adrenalectomy resulted in severe arthritis in PVG rats upon immunization with CII. While DA rats show an altered immune-brain communication that favors the development of arthritis, PVG rats express a protective neuro-endocrine milieu, particularly linked to the basal tone of the HPA axis. Mimicking disruption of this axis elicits arthritis in non-susceptible PVG rats.

Gender-associated differential expression of cytokines in specific areas of the brain during helminth infection.

Intraperitoneal infection with Taenia crassiceps cysticerci in mice alters several behaviors, including sexual, aggressive, and cognitive function. Cytokines and their receptors are produced in the central nervous system (CNS) by specific neural cell lineages under physiological and pathological conditions, regulating such processes as neurotransmission. This study is aimed to determine the expression patterns of cytokines in various areas of the brain in normal and T. crassiceps-infected mice in both genders and correlate them with the pathology of the CNS and parasite counts. IL-4, IFN-γ, and TNF-α levels in the hippocampus and olfactory bulb increased significantly in infected male mice, but IL-6 was downregulated in these regions in female mice. IL-1ß expression in the hippocampus was unaffected by infection in either gender. Our novel findings demonstrate a clear gender-associated pattern of cytokine expression in specific areas of the brain in mammals that parasitic infection can alter. Thus, we hypothesize that intraperitoneal infection is sensed by the CNS of the host, wherein cytokines are important messengers in the host-parasite neuroimmunoendocrine network.

Physiologic versus diabetogenic effects of interleukin-1: a question of weight.

Pleiotropic effects, great potency, and the capacity to induce its own production are distinguishing characteristics of IL-1. Among the multiple physiological effects of this cytokine, we emphasize here its role in supporting immune processes by stimulating most immune cells, and in re-setting glucose homeostasis. These aspects are complementary because stimulatory actions of IL-1 may be due to its capacity to increase glucose uptake by immune cells in the periphery and to affect the control of glucose homeostasis at brain levels, so as to deviate this main fuel to immune cells during inflammatory and infectious diseases. Thus, IL-1 can contribute to maintain a lean phenotype, inhibit food intake, and exert hypoglycemic effects. However, these effects of IL-1 can be overridden particularly when it is overproduced ectopically in other tissues, as it occurs during the autoimmune process that destroys the pancreas and causes type 1 diabetes, or when obesity triggers its production in adipose tissue and influences the development of type 2 diabetes. During obesity, products of enlarged adipocytes, e.g. fatty acids, are sensed as danger signals by infiltrating immune cells and, together with hypoxia, results in an ectopic overproduction of IL-1 that is largely mediated by activation of the NLRP3-caspase-1 inflammasome. Insulin and leptin resistance develops by mutual IL-1ß-TNFα induction, and hyperglycemia causes ectopic production of IL-1 in the pancreas, which deregulates insulin production and favors the development of type 2 diabetes. In conclusion, whether IL-1 exerts physiologic or pathologic effects depends on its amount and on the spatial and temporal pattern of its production.

A cytokine network involving brain-borne IL-1ß, IL-1ra, IL-18, IL-6, and TNFα operates during long-term potentiation and learning.

We have previously shown that long-term potentiation (LTP) induces hippocampal IL-1ß and IL-6 over-expression, and interfering their signalling either inhibits or supports, respectively, LTP maintenance. Consistently, blockade of endogenous IL-1 or IL-6 restricts or favours hippocampal-dependent memory, effects that were confirmed in genetically manipulated mice. Since cytokines are known for their high degree of mutual crosstalk, here we studied whether a network of cytokines with known neuromodulatory actions is activated during LTP and learning. We found that, besides IL-1ß and IL-6, also IL-1 receptor antagonist (IL-1ra) and IL-18, but not TNFα are over-expressed during LTP maintenance in freely moving rats. The increased expression of these cytokines is causally related to an increase in synaptic strength since it was abrogated when LTP was interfered by blockade of NMDA-glutamate receptors. Likewise, IL-1 and IL-6 were found to be over-expressed in defined regions of the hippocampus during learning a hippocampus-dependent task. However, during learning, changes in IL-18 were restricted to the dorsal hippocampus, and no differences in TNFα and IL1-ra expression were noticed in the hippocampus. Noticeably, IL-1ra transcripts were significantly reduced in the prefrontal cortex. The relation between cytokine expression and learning was causal because such changes were not observed in animals from a pseudo-trained group that was subject to the same manipulation but could not learn the task. Taken together with previous studies, we conclude that activation of a cytokine network in the brain is a physiologic relevant phenomenon not only for LTP maintenance but also for certain types of learning.

T cells affect central and peripheral noradrenergic mechanisms and neurotrophin concentration in the spleen and hypothalamus.

Interactions between T cells and noradrenergic pathways were investigated using athymic nude mice as a model. Higher noradrenaline (NA) concentrations and increased density of noradrenergic fibers were found in the spleen and hypothalamus, but not in the kidney, of 21-day-old Foxn1(n) (athymic) mice, compared with Foxn1(n) /Foxn1(+) (heterozygous) littermates. Although no differences in nerve growth factor concentrations were detected, significantly higher brain-derived neurotrophic factor concentrations were found in the spleen and hypothalamus of athymic mice compared with the controls. All of these alterations were abrogated in Foxn1(n) mice reconstituted by thymus transplantation at birth. These results suggest that T lymphocytes or their products can induce (1) a decrease in the number and activity in splenic sympathetic nerve fibers; (2) a decrease in NA content in the hypothalamus, which, in turn, may influence the pituitary-adrenal axis and the descending neural pathways associated with the autonomic nervous system; and (3) changes in neurotrophin concentration in the spleen and hypothalamus.

Different peripheral neuroendocrine responses to Trypanosoma cruzi infection in mice lacking adaptive immunity.

Trypanosoma cruzi infection in mice triggers neuroendocrine responses that affect the course of the disease. To analyze the contribution of adaptive immunity to these responses, comparative studies between normal C57Bl/6J and recombinase activator gene 1 (RAG-1)-deficient mice, which lack mature B and T lymphocytes, were performed. There was no difference between both types of mice in basal body weight. Following infection, higher parasitemia, increased IL-1ß and IL-6 blood levels, less marked changes in lymphoid organs weight, no cardiomegaly, and earlier mortality were observed in RAG-1-deficient, compared with normal mice. The response of the hypothalamus-pituitary-adrenal axis after infection occurred earlier and was more intense in RAG-1-deficient mice than in normal mice. Noradrenaline concentration and serotonergic metabolism in the spleen, lymph nodes, and heart differed between RAG-1-deficient and normal mice. Our studies indicate that the absence of adaptive immunity to T. cruzi influences the neuroendocrine response to the infection with this parasite.

Chronic neuropathic pain-like behavior and brain-borne IL-1ß.

Neuropathic pain in animals results in increased IL-1ß expression in the damaged nerve, the dorsal root ganglia, and the spinal cord. Here, we discuss our results showing that this cytokine is also overexpressed at supraspinal brain regions, in particular in the contralateral side of the hippocampus and prefrontal cortex and in the brainstem, in rats with neuropathic pain-like behavior. We show that neuropathic pain degree and development depend on the specific nerve injury model and rat strain studied, and that there is a correlation between hippocampal IL-1ß expression and tactile sensitivity. Furthermore, the correlations between hippocampal IL-1ß and IL-1ra or IL-6 observed in control animals, are disrupted in rats with increased pain sensitivity. The lateralization of increased cytokine expression indicates that this alteration may reflect nociception. The potential functional consequences of increased IL-1ß expression in the brain during neuropathic pain are discussed.

Chronic neuropathic pain-like behavior correlates with IL-1ß expression and disrupts cytokine interactions in the hippocampus.

We have proposed that neuropathic pain engages emotional learning, suggesting the involvement of the hippocampus. Because cytokines in the periphery contribute to induction and maintenance of neuropathic pain but might also participate centrally, we used 2 neuropathic pain models, chronic constriction injury (CCI) and spared nerve injury (SNI), to investigate the temporal profile of hippocampal cytokine gene expression in 2 rat strains that show different postinjury behavioral threshold sensitivities. SNI induced long-lasting allodynia in both strains, while CCI induced allodynia with time-dependent recovery in Sprague Dawley (SD) and no allodynia in Wistar Kyoto (WK) rats. In WK rats, only SNI induced sustained upregulation of hippocampal interleukin (IL)-1ß, while IL-6 expression was transiently increased and no significant changes in IL-1ra expression were detected. Conversely, in SD rats, SNI resulted in sustained and robust increased hippocampal IL-1ß expression, which was only transient in rats with CCI. In this strain, IL-6 expression was not affected in any of the 2 injury models and IL-1ra expression was significantly increased in rats with SNI or CCI at late phases. We found that the degree and development of neuropathic pain depend on the specific nerve injury model and rat strain; that hippocampal IL-1ß mRNA levels correlate with neuropathic pain behavior; that, in contrast to sham-operated animals, there are no correlations between hippocampal IL-1ß and IL-1ra or IL-6 in neuropathic rats; and that alterations in cytokine expression are restricted to the hippocampus contralateral to the injury side, again implying that the observed changes reflect nociception.

The role of the sympathetic nervous system in the thymus in health and disease.

The existence of a network of immunoneuroendocrine interactions that results in the reciprocal modulation of the classical functions of each system is well established at present. Most of the evidence derives from studies on secondary lymphoid organs, such as the spleen and lymph nodes. In this article, several aspects relevant to understand the role of the sympathetic nervous system in the establishment of these interactions in the thymus are discussed. At present, the sympathetic innervation of the thymus, the expression of adrenergic receptors in thymic cells, particularly of ß-adrenergic receptors, and the effect of sympathetic neurotransmitters, although mainly derived from in vitro or pharmacological studies, seem to be relatively well studied. However, other aspects, such as the relevance that immune-sympathetic interactions at the thymic level may have for certain diseases, specially autoimmune or other diseases that primarily involve the activation of the immune system, as well as how the integration of sympathetic and hormonal signals at local levels may affect thymic functions, certainly deserve further investigation.

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.

Interleukin-1 resets glucose homeostasis at central and peripheral levels: relevance for immunoregulation.

We briefly discuss here evidence showing that the capacity of IL-1beta to mediate adjustments of glucose homeostasis can be added to the already well-known pleiotropic effects of this cytokine. Such adjustments, which are necessary for satisfying the high energetic demands of immune/inflammatory responses, can be mediated by effects of endogenous IL-1 exerted at peripheral and brain levels in a concerted action with other cytokines and neuroendocrine mechanisms.

When immune-neuro-endocrine interactions are disrupted: experimentally induced arthritis as an example.

We studied whether, in parallel to the activity of the hypothalamus-pituitary-adrenal axis and the sympathetic nervous system, hypothalamic cytokine expression and monoaminergic neurotransmitter concentrations are affected during the development and chronification of arthritis induced by immunization of rats with type II collagen. Corticosterone levels were increased only transiently, and were even below the normal range as the disease progressed. Increased adrenaline blood levels and hypothalamic IL-1beta and IL-6 overexpression were observed only during the induction phase of the disease. The increase in hypothalamic noradrenaline content during the symptomatic phase was paralleled by a gradual loss of sympathetic fibers in the joints. Depletion of hypothalamic noradrenergic neurons at this time did not affect the symptomatology. Contrary to observations in healthy animals, no correlation between hypothalamic IL-1beta expression and noradrenaline content was observed in rats with arthritis. The dissociation between hypothalamic cytokine gene expression and noradrenergic neuronal activity, the lack of sustained stimulation of the stress axes, and the loss of sympathetic signals in the joints indicate that the communication between afferent immune messages to the central nervous system and two main efferent anti-inflammatory pathways under control of the brain are disrupted during experimental arthritis.

New insights into cytokine gene expression in the rat hypothalamus following endotoxin challenge.

Peripheral injection of the endotoxin LPS in rats 3 weeks prior to a second injection of LPS derived from another bacterial strain results in elevated corticosterone and decreased pro-inflammatory cytokines in the blood. We further investigated this model by measuring cytokine expression in the hypothalamus and spleen. In LPS-pretreated rats, hypothalamic expression of a range of cytokines was attenuated in response to the second injection of LPS while splenic expression was elevated. This is the first demonstration that prior exposure to an endotoxin can differentially affect cytokine expression in the brain and peripheral tissues when a host is confronted with a second, acute, pro-inflammatory stimulus. Changes in hypothalamic cytokine expression in endotoxin pretreated rats may provide new evidence for the involvement of central cytokine pathways in modulating peripheral inflammation and mediating psychopathological alterations associated with inflammatory diseases.

Chemical destruction of brain noradrenergic neurons affects splenic cytokine production.

The neurotransmitter noradrenaline (NA) plays a pivotal role in immune regulation. Here we used the selective neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to investigate the impact of central NA depletion on cytokine production by splenic monocytes/macrophages and T cells. Intraperitoneal administration of DSP-4 in adult rats induced a substantial reduction of noradrenergic neurons in the locus coeruleus and the A5 cell group. The degeneration of brainstem noradrenergic neurons was accompanied by a significant decrease in the production of interleukin (IL)-1beta, IL-6, and tumor necrosis factor (TNF)-alpha by lipopolysaccharide-stimulated splenocytes. In addition, upon T cell receptor stimulation with anti-CD3, isolated splenocytes of DSP-4 treated animals produced significantly less interferon (IFN)-gamma but not IL-2 and IL-4. The proportion of monocytes/macrophages and T cells in the spleen remained unaffected by the neurotoxin treatment, however, the percentage of natural killer cells decreased significantly. The findings suggest that a certain level of central noradrenergic tone is required for normal functioning of peripheral immune cells.