Peripheral levels of caspase-1 mRNA correlate with disease activity in patients with multiple sclerosis; a preliminary study

Intracerebroventricular infusion of secretoneurin inhibits neuronal NLRP3-Apoptosis pathway and preserves learning and memory after cerebral ischemia

Transient global cerebral ischemia (GCI) results in delayed neuronal death, primarily apoptosis, in the hippocampal CA1 subregion, which leads to severe cognitive deficits. While therapeutic hypothermia is an approved treatment for patients following cardiac arrest, it is associated with various adverse effects. Secretoneurin (SN) is an evolutionarily conserved neuropeptide generated in the brain, adrenal medulla and other endocrine tissues. In this study, SN was infused into the rat brain by intracerebroventricular injection 1 day after GCI, and we demonstrated that SN could significantly preserve spatial learning and memory in the Barnes maze tasks examined on days 14-17 after GCI. To further investigate underlying pathways involved, we demonstrated that, on day 5 after GCI, SN could significantly inhibit GCI-induced expression levels of Apoptosis Inducing Factor (AIF) and cleaved-PARP1, as well as neuronal apoptosis and synaptic loss in the hippocampal CA1 region. Additionally, SN could attenuate GCI-induced activation of both caspase-1 and caspase-3, and the levels of pro-inflammatory cytokines IL-1β and IL-18 in the CA1 region. Mechanically, we observed that treatment with SN effectively inhibited NLRP3 protein elevation and the bindings of NLRP3-ASC and ASC-caspase-1 in hippocampal neurons after GCI. In summary, our data indicate that SN could effectively attenuate NLRP3 inflammasome formation, as well as the activation of caspase-1 and -3, the production of pro-inflammatory cytokines, and ultimately the neuronal apoptotic loss induced by GCI. Potential neuronal pyroptosis, or caspase-1-dependent cell death, could also be involved in ischemic neuronal death, which needs further investigation.

Central nervous system manifestations of monogenic autoinflammatory disorders and the neurotropic features of SARS-CoV-2: Drawing the parallels

Central nervous system (CNS) involvement in monogenic autoinflammatory disorders (AID) is increasingly recognized and can be life threatening. Therefore, a low threshold to consider CNS disease should be maintained in patients with systemic inflammation. Hyperinflammation is also a key feature of severe acute COVID-19 and post COVID-19 entities such as multisystem inflammatory syndrome in children. Like AID, COVID-19 patients can present with severe CNS involvement. The impact of COVID-19 on AID and CNS involvement in particular is still obscure, nevertheless dreaded. In the current review, we synthesize the spectrum of CNS manifestations in monogenic AID. We explore common pathophysiological and clinical features of AID and COVID-19. Moreover, we assess the impact of immune dysregulation associated with SARS-CoV-2 infections and post COVID-19 hyperinflammation in AID. The striking commonalities found between both disease entities warrant caution in the management of AID patients during the current pandemic.

Regulated cell death: discovery, features and implications for neurodegenerative diseases

Regulated cell death (RCD) is a ubiquitous process in living organisms that is essential for tissue homeostasis or to restore biological balance under stress. Over the decades, various forms of RCD have been reported and are increasingly being found to involve in human pathologies and clinical outcomes. We focus on five high-profile forms of RCD, including apoptosis, pyroptosis, autophagy-dependent cell death, necroptosis and ferroptosis. Cumulative evidence supports that not only they have different features and various pathways, but also there are extensive cross-talks between modes of cell death. As the understanding of RCD pathway in evolution, development, physiology and disease continues to improve. Here we review an updated classification of RCD on the discovery and features of processes. The prominent focus will be placed on key mechanisms of RCD and its critical role in neurodegenerative disease. Video abstract.

The Role of the Inflammasome in Neurodegenerative Diseases

Neurodegenerative diseases are chronic, progressive disorders that occur in the central nervous system (CNS). They are characterized by the loss of neuronal structure and function and are associated with inflammation. Inflammation of the CNS is called neuroinflammation, which has been implicated in most neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS). Much evidence indicates that these different conditions share a common inflammatory mechanism: the activation of the inflammasome complex in peripheral monocytes and in microglia, with the consequent production of high quantities of the pro-inflammatory cytokines IL-1β and IL-18. Inflammasomes are a group of multimeric signaling complexes that include a sensor Nod-like receptor (NLR) molecule, the adaptor protein ASC, and caspase-1. The NLRP3 inflammasome is currently the best-characterized inflammasome. Multiple signals, which are potentially provided in combination and include endogenous danger signals and pathogens, trigger the formation of an active inflammasome, which, in turn, will stimulate the cleavage and the release of bioactive cytokines including IL-1β and IL-18. In this review, we will summarize results implicating the inflammasome as a pivotal player in the pathogenesis of neurodegenerative diseases and discuss how compounds that hamper the activation of the NLRP3 inflammasome could offer novel therapeutic avenues for these diseases.

Association between familial Mediterranean fever and multiple sclerosis: A case series from the JIR cohort and systematic literature review

INTRODUCTION

Familial Mediterranean fever (FMF) is the most frequent monogenic autoinflammatory disorder; and leads to the uncontrolled production of interleukin (IL)-1β. Multiple sclerosis (MS) is an inflammatory disease of the central nervous system; and its development seems to be partly correlated with IL-1β levels. It is hypothesized that FMF could be associated with MS. We aim to describe the features of patients displaying both diseases and to investigate the MEFV mutation rate in MS patients.

METHODS

Patients with definite MS were retrieved from the cohort of FMF patients in the Reference Center for Rare Auto-inflammatory Diseases and Amyloidosis (CEREMAIA). We also performed a systematic literature review of articles from PubMed that were published from 1990 to 2020.

RESULTS

Twenty-four patients were included in the case series: five patients (1.3%) from our cohort of 364 and 19 patients from the literature. The sex ratio was 2:1. The mean age at diagnosis of FMF was 19 years old; and that for MS was 29 years old. Seven studies investigating the MEFV mutation rate in MS patients were included. Three studies found a higher mutation rate in MS patients than in the control group.

CONCLUSION

FMF and MS features were comparable to those of patients with unrelated diseases; and MEFV mutation carriage was not positively correlated with MS. However; MS prevalence in FMF patients was higher than was expected in a healthy population. To a lesser extent; FMF prevalence in MS patients was higher than expected in a healthy population and the difference might not be significant. These data suggest that FMF could be associated with MS; and further studies are needed to investigate a potential causal association.

An overview of disease models for NLRP3 inflammasome over-activation

Introduction: Inflammatory reactions, including those mediated by the NLRP3 inflammasome, maintain the body's homeostasis by removing pathogens, repairing damaged tissues, and adapting to stressed environments. However, uncontrolled activation of the NLRP3 inflammasome tends to cause various diseases using different mechanisms. Recently, many inhibitors of the NLRP3 inflammasome have been reported and many are being developed. In order to assess their efficacy, specificity, and mechanism of action, the screening process of inhibitors requires various types of cell and animal models of NLRP3-associated diseases.Areas covered: In the following review, the authors give an overview of the cell and animal models that have been used during the research and development of various inhibitors of the NLRP3 inflammasome.Expert opinion: There are many NLRP3 inflammasome inhibitors, but most of the inhibitors have poor specificity and often influence other inflammatory pathways. The potential risk for cross-reaction is high; therefore, the development of highly specific inhibitors is essential. The selection of appropriate cell and animal models, and combined use of different models for the evaluation of these inhibitors can help to clarify the target specificity and therapeutic effects, which is beneficial for the development and application of drugs targeting the NLRP3 inflammasome.

Mechanisms of NLRP3 priming in inflammaging and age related diseases

The NLRP3 inflammasome is a vital part of the innate immune response, whilst its aberrant activation drives the progression of a number of non-communicable diseases. Thus, NLRP3 inflammasome assembly must be tightly controlled at several checkpoints. The priming step of NLRP3 inflammasome activation is associated with increased NLRP3 gene expression, as well as post-translational modifications that control NLRP3 levels and licence the NLRP3 protein for inflammasome assembly. Increasing life expectancy in modern society is accompanied by a growing percentage of elderly individuals. The process of aging is associated with chronic inflammation that drives and/or worsens a range of age related non-communicable conditions. The NLRP3 inflammasome is known to contribute to pathological inflammation in many settings, but the mechanisms that prime NLRP3 for activation throughout aging and related co-morbidities have not been extensively reviewed. Here we dissect the biochemical changes that occur during aging and the pathogenesis of age related diseases and analyse the mechanisms by which they prime the NLRP3 inflammasome, thus exacerbating inflammation.

The PANoptosome: A Deadly Protein Complex Driving Pyroptosis, Apoptosis, and Necroptosis (PANoptosis)

Programmed cell death is regulated by evolutionarily conserved pathways that play critical roles in development and the immune response. A newly recognized pathway for proinflammatory programmed cell death called PANoptosis is controlled by a recently identified cytoplasmic multimeric protein complex named the PANoptosome. The PANoptosome can engage, in parallel, three key modes of programmed cell death—pyroptosis, apoptosis, and necroptosis. The PANoptosome components have been implicated in a wide array of human diseases including autoinflammatory diseases, neurodegenerative diseases, cancer, microbial infections, and metabolic diseases. Here, we review putative components of the PANoptosome and present a phylogenetic analysis of their molecular domains and interaction motifs that support complex assembly. We also discuss genetic data that suggest PANoptosis is coordinated by scaffolding and catalytic functions of the complex components and propose mechanistic models for PANoptosome assembly. Overall, this review presents potential mechanisms governing PANoptosis based on evolutionary analysis of the PANoptosome components.

'Prototypical' proinflammatory cytokine (IL-1) in multiple sclerosis: role in pathogenesis and therapeutic targeting

Introduction: It has been recognized for about 20 years that interleukin (IL)-1 signaling is implicated in Multiple Sclerosis (MS), a disabling, chronic, inflammatory and neurodegenerative disease of the central nervous system (CNS). Only recently, multifaceted roles of IL-1 emerged in MS pathophysiology as a result of both clinical and preclinical studies. Notably, drugs that directly target the IL-1 system have not been tested so far in MS.Areas covered: Recent studies in animal models, together with the development of ex vivo chimeric MS models, have disclosed a critical role for IL-1 not only at the peripheral level but also within the CNS. In the present review, we highlight the IL-1-dependent neuropathological aspects of MS, by providing an overview of the cells of the immune and CNS systems that respond to IL-1 signaling, and by emphasizing the subsequent effects on the CNS, from demyelinating processes, to synaptopathy, and excitotoxicity.Expert opinion: Drugs that act on the IL-1 system show a therapeutic potential in several autoinflammatory diseases and preclinical studies have highlighted the effects of these compounds in MS. We will discuss why anti-IL-1 therapies in MS have been neglected to date.

Fiery Cell Death: Pyroptosis in the Central Nervous System

Pyroptosis ('fiery death') is an inflammatory type of regulated cell death (RCD), which occurs downstream of inflammasome activation. Pyroptosis is mediated directly by the recently identified family of pore-forming proteins known as gasdermins, the best characterized of which is gasdermin D (GSDMD). Recent investigations implicate pyroptosis in the pathogenesis of multiple neurological diseases. In this review, we discuss molecular mechanisms that drive pyroptosis, evidence for pyroptosis within the CNS, and emerging therapeutic strategies for its inhibition in the context of neurological disease.

Inflammasome-Mediated Inflammation in Neurodegenerative Diseases

Inflammasomes are protein platforms consisting of multiple proteins. The biological function includes the activation of caspase-1, leading to the maturation of IL-1β and IL-18. These pro-inflammatory cytokines promote fundamental inflammatory processes in numerous infectious diseases. The inflammasome-mediated inflammation has become increasingly important in central nervous system disorders. In neurodegenerative disorders, significant contributors to disease progression include neuroinflammation and inflammatory cascades initiated by the inflammasome protein complex. This review discusses the recent progress of research on inflammasome associated with neurodegenerative disorders.

Monosodium Urate Crystals Activate the Inflammasome in Primary Progressive Multiple Sclerosis

Inflammasome-driven inflammation is postulated to play a role in multiple sclerosis (MS), but there is no direct evidence that the nod-like receptor protein 3 (NLRP3) inflammasome is involved in MS pathogenesis. Uric acid was shown to be one of the “danger” signals involved in the activation of NLRP3 inflammasome; notably, the concentration of uric acid is increased in the serum and in the cerebrospinal fluid of MS individuals. To better investigate the role of the NLRP3 inflammasome in MS-associated inflammation, we primed with lipopolysaccharide and stimulated with monosodium urate crystals PBMCs of 41 MS patients with different disease phenotypes. Eleven individuals with primary progressive MS (PPMS), 10 individuals with stable relapsing–remitting MS (SMS), 10 individuals with acute relapsing–remitting MS (AMS), 10 individuals with benign MS were analyzed; 10 healthy controls were enrolled as well in the study. The expression of the NLRP3, apoptosis-associated speck-like protein containing CARD (ASC), caspase-1, caspase-8, IL-1β, and IL-18 inflammasome genes was evaluated by RT-PCR. NLRP3 and ASC-speck protein expression was analyzed by FlowSight AMNIS, whereas production of the pro-inflammatory cytokines IL-1β and IL-18 and of caspase-1 and caspase-8 was measured by ELISA in supernatants. Results showed that uric acid serum concentration was significantly increased in PPMS; in these and in AMS patients, mRNA for NLRP3, ASC, and IL-18 was upregulated as well, but caspase-8 mRNA was upregulated only in PPMS. Expression of NLRP3 and ASC-speck protein was significantly increased in PPMS, SMS, and AMS patients, but IL-18 and caspase-8 production was significantly increased only in PPMS, in whom a direct correlation between hyperuricemia and caspase-8 was detected. The NLRP3/caspase-8 inflammasome pathway is activated in PPMS, possibly as a consequence of hyperuricemia. Therapeutic strategies reducing NLRP3 activation and/or lowering hyperuricemia could be useful in the therapy of PPMS.

Role of Inflammasomes in Neuroimmune and Neurodegenerative Diseases: A Systematic Review

Inflammasomes are multiprotein complexes that can sense pathogen-associated molecular patterns and damage-associated molecular signals. They are involved in the initiation and development of inflammation via activation of IL-1β and IL-18. Many recent studies suggest a strong correlation between inflammasomes and neurological diseases, such as multiple sclerosis (MS), Alzheimer's disease (AD), and Parkinson's disease (PD). Several components of inflammasomes, such as nucleotide-binding oligomerization domain- (NOD-) like receptor, absent in melanoma 2- (AIM2-) like receptors (ALRs), apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), and caspase-1, as well as the upstream factors and downstream effectors, are associated with the initiation and development of MS and its animal model, experimental autoimmune encephalomyelitis. Additionally, inflammasomes affect the efficacy of interferon-β therapy in patients with MS. Finally, the strong association of inflammasomes with AD and PD needs to be further studied. In this review of latest literatures, we comprehensively tease out diverse roles of different kinds of inflammasomes in neuroimmune and neurodegenerative diseases, especially in the perspective of double roles involved in pathogenesis, and identify future research priorities.

Inflammasomes in neurological diseases: emerging pathogenic and therapeutic concepts

Inflammasome activation in the central nervous system occurs in both health and disease. Inflammasomes are cytosolic protein complexes that sense specific infectious or host stimuli and initiate inflammatory responses through caspase activation. Assembly of inflammasomes results in caspase-1-mediated proteolytic cleavage and release of the pro-inflammatory cytokines, interleukin-1β and interleukin-18, with initiation of pyroptosis, an inflammatory programmed cell death. Recent developments in the inflammasome field have uncovered novel molecular mechanisms that contribute to a broad range of neurological disorders including those associated with specific mutations in inflammasome genes as well as diseases modulated by inflammasome activation. This update focuses on recent developments in the field of inflammasome biology highlighting different inflammasome activators and pathways discovered in the nervous system. We also discuss targeted therapies that regulate inflammasomes and improve neurological outcomes.

How Inflammasomes Inform Adaptive Immunity

An immune response consists of a finely orchestrated interplay between initial recognition of potential microbial threats by the innate immune system and subsequent licensed adaptive immune neutralization. The initial recognition integrates environmental cues derived from pathogen-associated molecular patterns and cell-intrinsic damage-associated molecular patterns to contextualize the insult and inform a tailored adaptive response via T and B lymphocytes. While there are much data to support the role of transcriptional responses downstream of pattern recognition receptors in informing the adaptive immune response, markedly less attention has been paid to the role of post-translational responses to pathogen-associated molecular pattern and damage-associated molecular pattern recognition by the innate immune system, and how this may influence adaptive immunity. A well-characterized post-translational consequence of pattern recognition receptor signaling is the assembly of a multimeric signaling platform, termed the inflammasome, by members of the nucleotide-binding oligomerization domain (Nod), leucine-rich repeat-containing receptors (NLRs), and pyrin and HIN domain (PYHIN) families. Inflammasomes assemble in response to cytosolic perturbations, such as mitochondrial dysfunction and aberrant ion fluxes in the case of the canonical NLRP3 inflammasome or the presence of bacterial lipopolysaccharides in the case of the non-canonical inflammasome. Assembly of the inflammasome allows for the cleavage and activation of inflammatory caspases. These activated inflammatory caspases in turn cleave pro-form inflammatory cytokines into their mature bioactive species and lead to unconventional protein secretion and lytic cell death. In this review, we discuss evidence for inflammasome-mediated instruction and contextualization of infectious and sterile agents to the adaptive immune system.

New Insights into the Role of IL-1β in Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis

Multiple sclerosis (MS), and its animal model experimental autoimmune encephalomyelitis, are neuroinflammatory diseases driven by autoreactive pathogenic T_H cells that elicit demyelination and axonal damage. How T_H cells acquire pathogenicity and communicate with myeloid cells and cells of the CNS remain unclear. IL-1β is recognized to play an important role in experimental autoimmune encephalomyelitis (EAE) and perhaps MS. Clinical EAE is significantly attenuated in IL-1R-deficient and IL-1β-deficient mice, and IL-1β is found in the blood, cerebrospinal fluid, and CNS lesions of MS patients. In this article, we focus on new reports that elucidate the cellular sources of IL-1β and its actions during EAE, in both lymphoid tissues and within the CNS. Several immune cell types serve as critical producers of IL-1β during EAE, with this cytokine inducing response in both hematopoietic and nonhematopoietic cells. These findings from the EAE model should inspire efforts toward investigating the therapeutic potential of IL-1 blockade in MS.

Involvement of the IL-1 system in experimental autoimmune encephalomyelitis and multiple sclerosis: Breaking the vicious cycle between IL-1β and GM-CSF

Multiple sclerosis (MS) is an autoimmune disease that affects hundreds of thousands of people worldwide. Given the autoimmune nature of the disease, a large part of the research has focused on autoreactive T and B cells. However, research on the involvement of myeloid cells in the pathophysiology of MS has received a strong and renewed attention over the recent years. Despite the multitude of inflammatory mediators involved in innate immunity, only a select group of cytokines are absolutely critical to the development of CNS autoimmunity, among which is interleukin (IL)-1. While the importance of the IL-1 system in experimental autoimmune encephalomyelitis (EAE) and MS has been recognized for about 20years, it is only recently that we have begun to understand that IL-1 plays multifaceted roles in disease initiation, development, amplification and chronicity. Here, we review the recent findings showing an implication of the IL-1 system in EAE and MS, and introduce a model that highlights how IL-1β and granulocyte-macrophage colony-stimulating factor (GM-CSF) are interacting together to create a vicious feedback cycle of CNS inflammation that ultimately leads to myelin and neuronal damage.

Meningeal mast cell-T cell crosstalk regulates T cell encephalitogenicity

GM-CSF is a cytokine produced by T helper (Th) cells that plays an essential role in orchestrating neuroinflammation in experimental autoimmune encephalomyelitis, a rodent model of multiple sclerosis. Yet where and how Th cells acquire GM-CSF expression is unknown. In this study we identify mast cells in the meninges, tripartite tissues surrounding the brain and spinal cord, as important contributors to antigen-specific Th cell accumulation and GM-CSF expression. In the absence of mast cells, Th cells do not accumulate in the meninges nor produce GM-CSF. Mast cell-T cell co-culture experiments and selective mast cell reconstitution of the meninges of mast cell-deficient mice reveal that resident meningeal mast cells are an early source of caspase-1-dependent IL-1β that licenses Th cells to produce GM-CSF and become encephalitogenic. We also provide evidence of mast cell-T cell co-localization in the meninges and CNS of recently diagnosed acute MS patients indicating similar interactions may occur in human demyelinating disease.

Complementing the inflammasome

The innate immune system is an ancient surveillance system able to sense microbial invaders as well as aberrations in normal cell function. No longer viewed as a static and non-specific part of immunity, the innate immune system employs a plethora of specialized pattern recognition sensors to monitor and achieve homeostasis; these include the Toll-like receptors, the retinoic acid-inducible gene-like receptors, the nucleotide-binding oligomerization domain receptors (NLRs), the C-type lectins and the complement system. In order to increase specificity and diversity, innate immunity uses homotypic and heterotypic associations among these different components. Multi-molecular assemblies are formed both on the cell surface and in the cytosol to respond to pathogen and danger signals. Diverse, but tailored, responses to a changing environment are orchestrated depending on the the nature of the challenge and the repertoire of interacting receptors and components available in the sensing cell. It is now emerging that innate immunity operates a system of 'checks and balances' where interaction among the sensors is key in maintaining normal cell function. Complement sits at the heart of this alarm system and it is becoming apparent that it is capable of interacting with all the other pathways to effect a tailored immune response. In this review, we will focus on complement interactions with NLRs, the so-called 'inflammasomes', describing the molecular mechanisms that have been revealed so far and discussing the circumstantial evidence that exists for these interactions in disease states.

Non-canonical manifestations of familial Mediterranean fever: a changing paradigm

Paroxysmal crises of fever and systemic inflammation herald familial Mediterranean fever (FMF), considered as the archetype of all inherited systemic autoinflammatory diseases. Inflammatory bouts are characterized by short-term and self-limited abdominal, thoracic, and/or articular symptoms which subside spontaneously. Erysipelas-like findings, orchitis, and different patterns of myalgia may appear in a minority of patients. In recent years, many non-classical manifestations have been reported in the clinical context of FMF, such as vasculitides and thrombotic manifestations, neurologic and sensory organ abnormalities, gastrointestinal diseases, and even macrophage activation syndrome. As FMF left unrecognized and untreated is ominously complicated by the occurrence of AA-amyloidosis, it is highly desirable that diagnosis of this autoinflammatory disorder with its multiple clinical faces can be contemplated at whatever age and brought forward.

Increased inflammasome related gene expression profile in PBMC may facilitate T helper 17 cell induction in multiple sclerosis

The NLRP3 inflammasome is a macromolecular complex importantly involved in IL-1β processing. A role for this has been described in multiple sclerosis (MS). One mechanism by which IL-1β might be involved in MS is by inducing pathogenic Th17 cells, i.e. GM-CSF+ Th17 cells. In the present study, we show that expression of the inflammasome related genes, NLRP3, caspase-1, IL-1β and the IL-1β/IL-1Ra ratio, was increased in PBMC from MS patients compared to healthy controls (HC). However, in an in vitro inflammasome activity assay with PBMC, IL-1β protein secretion and the IL-1β/IL-1Ra protein ratio were similar in MS patients and HC. Th cells cultured in the presence of supernatant derived from LPS/ATP inflammasome activated PBMC showed increased Th17 and GM-CSF+ Th17 cell frequencies in HC and MS patients and decreased anti-inflammatory IL-10+Th cell frequency in HC compared to Th cells cultured in the presence of control supernatant. Moreover, addition of the immune modulator calcitriol to the former condition resulted in reduced frequencies of Th17 and GM-CSF+Th17 cells, and also of IL-10+ Th cells. Evidently, our data indicate that inflammasome activity can skew the Th cell population toward a more pro-inflammatory composition, an effect that might be inhibited by vitamin D, and that might be importantly involved in inflammation within the central nervous system.

Licensing Adaptive Immunity by NOD-Like Receptors

The innate immune system is composed of a diverse set of host defense molecules, physical barriers, and specialized leukocytes and is the primary form of immune defense against environmental insults. Another crucial role of innate immunity is to shape the long-lived adaptive immune response mediated by T and B lymphocytes. The activation of pattern recognition receptors (PRRs) from the Toll-like receptor family is now a classic example of innate immune molecules influencing adaptive immunity, resulting in effective antigen presentation to naïve T cells. More recent work suggests that the activation of another family of PRRs, the NOD-like receptors (NLRs), induces a different set of innate immune responses and accordingly, drives different aspects of adaptive immunity. Yet how this unusually diverse family of molecules (some without canonical PRR function) regulates immunity remains incompletely understood. In this review, we discuss the evidence for and against NLR activity orchestrating adaptive immune responses during infectious as well as non-infectious challenges.

The role of interferon-β in the treatment of multiple sclerosis and experimental autoimmune encephalomyelitis - in the perspective of inflammasomes

Inflammasomes in innate immune cells mediate the induction of inflammation by sensing microbes and pathogen-associated/damage-associated molecular patterns. Inflammasomes are also known to be involved in the development of some human and animal autoimmune diseases. The Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is currently the most fully characterized inflammasome, although a limited number of studies have demonstrated its role in demyelinating autoimmune diseases in the central nervous system of humans and animals. Currently, the development of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), is known to be induced by the NLRP3 inflammasome through enhanced recruitment of inflammatory immune cells in the central nervous system. On the other hand, interferon-β (IFNβ), a first-line drug to treat MS, inhibits NLRP3 inflammasome activation, and ameliorates EAE. The NLRP3 inflammasome is indeed a factor capable of inducing EAE, but it is dispensable when EAE is induced by aggressive disease induction regimens. In such NLRP3 inflammasome-independent EAE, IFN-β treatment is generally not effective. This might therefore be one mechanism that leads to occasional failures of IFN-β treatment in EAE, and possibly, in MS as well. In the current review, we discuss inflammasomes and autoimmunity; in particular, the impact of the NLRP3 inflammasome on MS/EAE, and on IFN-β therapy.

NLRP3 Inflammasome and MS/EAE

Inflammasomes are cytosolic sensors that detect pathogens and danger signals in the innate immune system. The NLRP3 inflammasome is currently the most fully characterized inflammasome and is known to detect a wide array of microbes and endogenous damage-associated molecules. Possible involvement of the NLRP3 inflammasome (or inflammasomes) in the development of multiple sclerosis (MS) was suggested in a number of studies. Recent studies showed that the NLRP3 inflammasome exacerbates experimental autoimmune encephalomyelitis (EAE), an animal model of MS, although EAE can also develop without the NLRP3 inflammasome. In this paper, we discuss the NLRP3 inflammasome in MS and EAE development.

Interferon-β therapy against EAE is effective only when development of the disease depends on the NLRP3 inflammasome

Interferon-β (IFN-β) is widely used to treat multiple sclerosis (MS), and its efficacy was demonstrated in the setting of experimental autoimmune encephalomyelitis (EAE), an animal model of MS; however, IFN-β is not effective in treating all cases of MS. Here, we demonstrate that signaling by IFNAR (the shared receptor for IFN-α and IFN-β) on macrophages inhibits activation of Rac1 and the generation of reactive oxygen species (ROS) through suppressor of cytokine signaling 1 (SOCS1). The inhibition of Rac1 activation and ROS generation suppressed the activity of the Nod-like receptor (NLR) family, pyrin domain-containing 3 (NLRP3) inflammasome, which resulted in attenuated EAE pathogenicity. We further found that two subsets of EAE could be defined on the basis of their dependency on the NLRP3 inflammasome and that IFN-β was not an effective therapy when EAE was induced in an NLRP3 inflammasome-independent fashion. Thus, our study demonstrates a previously uncharacterized signaling pathway that is involved in the suppression of EAE by IFN-β and characterizes NLRP3-independent EAE, which cannot be treated with IFN-β.

Type I interferon inhibits interleukin-1 production and inflammasome activation

Type I interferon (IFN) is a common therapy for autoimmune and inflammatory disorders, yet the mechanisms of action are largely unknown. Here we showed that type I IFN inhibited interleukin-1 (IL-1) production through two distinct mechanisms. Type I IFN signaling, via the STAT1 transcription factor, repressed the activity of the NLRP1 and NLRP3 inflammasomes, thereby suppressing caspase-1-dependent IL-1β maturation. In addition, type I IFN induced IL-10 in a STAT1-dependent manner; autocrine IL-10 then signaled via STAT3 to reduce the abundance of pro-IL-1α and pro-IL-1β. In vivo, poly(I:C)-induced type I IFN diminished IL-1β production in response to alum and Candida albicans, thus increasing susceptibility to this fungal pathogen. Importantly, monocytes from multiple sclerosis patients undergoing IFN-β treatment produced substantially less IL-1β than monocytes derived from healthy donors. Our findings may thus explain the effectiveness of type I IFN in the treatment of inflammatory diseases but also the observed "weakening" of the immune system after viral infection.

Inflammasome signaling at the heart of central nervous system pathology

Neuroinflammation is a complex innate response of neural tissue against harmful effects of diverse stimuli viz., pathogens, damaged cells and irritants within the Central Nervous System (CNS). Studies show that multiple inflammatory mediators including cytokines, chemokines and prostaglandins are elevated in the Cerebrospinal Fluid (CSF) and in post-mortem brain tissues of patients with history of neuroinflammatory conditions as well as neurodegenerative disorders like Alzheimer's disease, Parkinson's disease and Multiple Sclerosis. The innate immunity mediators in the brain, namely microglia and astrocytes, express certain Pattern Recognition Receptors (PRRs), which are always on 'high-alert' for pathogens or other inflammatory triggers and participate in the assembly and activation of the inflammasome. The inflammasome orchestrates the activation of the precursors of proinflammatory caspases, which in turn, cleave the precursor forms of interleukin-1beta, IL-18 and IL-33 into their active forms; the secretion of which leads to a potent inflammatory response, and/or influences the release of toxins from glial and endothelial cells. Altered expression of inflammasome mediators can either promote or inhibit neurodegenerative processes. Therefore, modulating the inflammasome machinery seems a better combat strategy than summarily suppressing all inflammation in most neuroinflammatory conditions. In the current review we have surveyed the identified triggers and pathways of inflammasome activation and the following events which ultimately accomplish the innate inflammatory response in the CNS, with a goal to provide an analytical insight into disease pathogenesis that might provide cues for devising novel therapeutic strategies.

NLRP3 plays a critical role in the development of experimental autoimmune encephalomyelitis by mediating Th1 and Th17 responses

The interplay between innate and adaptive immunity is important in multiple sclerosis (MS). The inflammasome complex, which activates caspase-1 to process pro-IL-1beta and pro-IL-18, is rapidly emerging as a pivotal regulator of innate immunity, with nucleotide-binding domain, leucine-rich repeat containing protein family, pyrin domain containing 3 (NLRP3) (cryopyrin or NALP3) as a prominent player. Although the role of NLRP3 in host response to pathogen associated molecular patterns and danger associated molecular patterns is well documented, its role in autoimmune diseases is less well studied. To investigate the role of NLRP3 protein in MS, we used a mouse model of MS, experimental autoimmune encephalomyelitis (EAE). Nlrp3 expression was elevated in the spinal cords during EAE, and Nlrp3(-/-) mice had a dramatically delayed course and reduced severity of disease. This was accompanied by a significant reduction of the inflammatory infiltrate including macrophages, dendritic cells, CD4, and CD8(+) T cells in the spinal cords of the Nlrp3(-/-) mice, whereas microglial accumulation remained the same. Nlrp3(-/-) mice also displayed improved histology in the spinal cords with reduced destruction of myelin and astrogliosis. Nlrp3(-/-) mice with EAE produced less IL-18, and the disease course was similar to Il18(-/-) mice. Furthermore, Nlrp3(-/-) and Il18(-/-) mice had similarly reduced IFN-gamma and IL-17 production. Thus, NLRP3 plays a critical role in the induction of the EAE, likely through effects on capase-1-dependent cytokines which then influence Th1 and Th17.

Interleukin 18 in the CNS

Interleukin (IL)-18 is a cytokine isolated as an important modulator of immune responses and subsequently shown to be pleiotropic. IL-18 and its receptors are expressed in the central nervous system (CNS) where they participate in neuroinflammatory/neurodegenerative processes but also influence homeostasis and behavior. Work on IL-18 null mice, the localization of the IL-18 receptor complex in neurons and the neuronal expression of decoy isoforms of the receptor subunits are beginning to reveal the complexity and the significance of the IL-18 system in the CNS. This review summarizes current knowledge on the central role of IL-18 in health and disease.

The multiple sclerosis degradome: enzymatic cascades in development and progression of central nervous system inflammatory disease

An array of studies implicate different classes of protease and their endogenous inhibitors in multiple sclerosis (MS) pathogenesis based on expression patterns in MS lesions, sera, and/or cerebrospinal fluid (CSF). Growing evidence exists regarding their mechanistic roles in inflammatory and neurodegenerative aspects of this disease. Proteolytic events participate in demyelination, axon injury, apoptosis, and development of the inflammatory response including immune cell activation and extravasation, cytokine and chemokine activation/inactivation, complement activation, and epitope spreading. The potential significance of proteolytic activity to MS therefore relates not only to their potential use as important biomarkers of disease activity, but additionally as prospective therapeutic targets. Experimental data indicate that understanding the net physiological consequence of altered protease levels in MS development and progression necessitates understanding protease activity in the context of substrates, endogenous inhibitors, and proteolytic cascade interactions, which together make up the MS degradome. This review will focus on evidence regarding the potential physiologic role of those protease families already identified as markers of disease activity in MS; that is, the metallo-, serine, and cysteine proteases.

IL-18: a key player in neuroinflammation and neurodegeneration?

Interleukin (IL)-18 is a potent inflammatory cytokine of the IL-1 family. It is synthesized as an inactive precursor (pro-IL-18), which is cleaved into its functionally active form by caspase-1. Resident cells of the CNS express IL-18 and caspase-1 constitutively, thus providing a local IL-18-dependent immune response. Recent studies have highlighted a crucial role for IL-18 in mediating neuroinflammation and neurodegeneration in the CNS under pathological conditions, such as bacterial and viral infection, autoimmune demyelinating disease, and hypoxic-ischemic, hyperoxic and traumatic brain injuries. This review provides a synopsis of the current knowledge of IL-18-dependent mechanisms of action during acute neurodegeneration in immature and adult brains.

Increased expression of caspase-1 and interleukin-18 in peripheral blood mononuclear cells in patients with multiple sclerosis

Multiple sclerosis (MS) is supposedly a T-cell mediated autoimmune disorder of the central nervous system. Cytokines and other molecules involved in the regulation of apoptosis are thought to be of importance for the pathogenesis of MS. In this study, the mRNA levels of interleukin 18 (IL-18), IL-1beta and their processing enzyme caspase-1 were quantified by a competitive RT-PCR method in unstimulated peripheral blood mononuclear cells (PBMCs) in MS patients never treated with disease modifying drugs. Western blot was used to support the expression pattern at the protein level. We found that the expression of caspase-1 and IL-18 was significantly increased in MS patients compared with healthy controls. Analysis of clinical subgroups revealed that caspase-1 was increased in all subgroups, whereas IL-18 was upregulated in chronic progression (P=0.001) and relapsing MS patients in remission (P=0.002) but not significantly during relapses (P=0.12). mRNA levels of IL-1beta were not significantly altered in MS except for a possible decrease in chronic progression (P=0.03). An increased IL-18 expression, potentially augmented at the mature protein level, may indicate a pathway worth considering in future therapeutic strategies in MS.

Decreased levels of CD95 and caspase-8 mRNA in multiple sclerosis patients with gadolinium-enhancing lesions on MRI

Magnetic resonance imaging (MRI) allows examining inflammation and central nervous system (CNS) tissue damage in patients suffering from multiple sclerosis (MS), a demyelinating disease of the CNS. Using real-time PCR, we quantified mRNA levels of apoptosis regulators CD95, CD95 ligand, caspase-8, -10 and cellular FLICE-inhibitory protein (cFLIP), and cytokines IL-10 and TNF-alpha in blood mononuclear cells of MS patients at the time of MRI examination. Patients with detectable gadolinium-enhancing lesions had lower expression of CD95 and caspase-8 (P<0.05). Lesion load and brain atrophy did not correlate with expression levels of any of the target molecules studied. Disease duration correlated positively with both FLIP/caspase-8 and CD95/CD3 ratios (P<0.05). These results support the notion that the CD95-dependent pathway plays a complex role in the regulation of survival of activated immune cells in MS.

Caspase inhibitors as anti-inflammatory and antiapoptotic agents

The striking efficacy of Z-VAD-fmk in the various animal models presented above may reflect its ability to inhibit multiple enzymes including caspases. In accord with this, more selective, reversible inhibitors usually show low efficacy in multifactorial models such as ischaemia, but may offer some protection against NMDA-induced excitotoxicity and hepatitis. Importantly, caspase inhibitors may exhibit significant activity in vivo even when they are applied post insult. As far as the CNS is concerned, the first systemically active inhibitors have emerged. Functional recovery could be achieved in some ischaemia models, but long-term protection by caspase inhibitors is still being questioned. Recent developments in drug design enabled the first caspase inhibitors to enter the clinic. Although initially directed towards peripheral indications such as rheumatoid arthritis, caspase inhibitors will no doubt eventually be used to target CNS disorders. For this purpose the peptidic character of current inhibitors will have to be further reduced. Small molecule, nonpeptidic caspase inhibitors, which have appeared recently, indicate that this goal can be accomplished. Unfortunately, many fundamental questions still remain to be addressed. In particular, the necessary spectrum of inhibitory activity required to achieve the desired effect needs to be determined. There is also a safety aspect associated with prolonged administration. Therefore, the next therapeutic areas for broader-range caspase inhibitors are likely to involve acute treatment. Recent results with synergistic effects between MK-801 and caspase inhibitors in ischaemia suggest that caspase inhibitors may need to be used in conjunction with other drugs. It can be expected that, in the near future, research on caspases and their inhibitors will remain a rapidly developing area of biology and medicinal chemistry. More time, however, may be needed for the first caspase inhibitors to appear on the market.

Caspase inhibitors protect against neuronal apoptosis induced by cerebrospinal fluid from multiple sclerosis patients

Neuronal apoptosis has recently been implicated in multiple sclerosis (MS). Apoptotic cell death of neurons is induced in cultures exposed to cerebrospinal fluid (CSF) from MS patients. Since caspases are essential in the regulation of apoptosis, direct evidence was sought linking caspases to CSF-induced neuronal death. Caspase activity was measured in cell extracts from MS CSF-treated cultured neurons by the cleavage of caspase-1 and caspase-3 substrates. Caspase-3 activity, but not caspase-1, was induced in neuronal cultures in response to MS CSF treatment. This caspase-3 activity was inhibited in vitro by Ac-YVAD-cmk and Ac-DEVD-cmk caspase inhibitors. Treatment of MS CSF-incubated neuronal cells with these caspase inhibitors completely preserved neuronal survival and largely attenuated DNA fragmentation detected in situ. These findings show that neuronal cells are rescued from MS CSF-induced death by caspase inhibitors and suggest ways to treat MS.

Interleukin-18

Interleukin-18 (IL-18), a recently described member of the IL-1 cytokine superfamily, is now recognized as an important regulator of innate and acquired immune responses. IL-18 is expressed at sites of chronic inflammation, in autoimmune diseases, in a variety of cancers, and in the context of numerous infectious diseases. This short review will describe the basic biology of IL-18 and thereafter address its potential effector and regulatory role in several human disease states including autoimmunity and infection. IL-18, previously known as interferon-gamma (IFN-gamma)-inducing factor, was identified as an endotoxin-induced serum factor that stimulated IFN-gamma production by murine splenocytes [(1) ]. IL-18 was cloned from a murine liver cell cDNA library generated from animals primed with heat-killed Propionibacterium acnes and subsequently challenged with lipopolysaccharide [(2) ]. Nucleotide sequencing of murine IL-18 predicted a precursor polypeptide of 192 amino acids lacking a conventional signal peptide and a mature protein of 157 amino acids. Subsequent cloning of human IL-18 cDNA revealed 65% homology with murine IL-18 [(3) ] and showed that both contain an unusual leader sequence consisting of 35 amino acids at their N terminus.

Possible correlation between high levels of IL-18 in the cord blood of pre-term infants and neonatal development of periventricular leukomalacia and cerebral palsy

Periventricular leukomalacia (PVL) is a neonatal white matter damage of the brain of pre-term infants that often leads to cerebral palsy (CP). At present, diagnosis of PVL can be done by magnetic resonance imaging (MRI) and ultrasonography only when the infant is at least one week of age. No biochemical methods are available to identify high-risk infants at birth. Cytokines are usually not present in the cord blood but recently an elevation of IL-6 and TNF-alpha levels has been reported in amniotic fluid, cord blood and brain sections of infants with white matter damages. Levels of interleukin-18 (IL-18), a pleiotropic cytokines expressed in the brain and many other tissues, are highly sensitive to pathophysiological changes to raise the possibility that IL-18 may provide a useful indicator of PVL. The cord blood from 17 pre-term infants with PVL, 38 pre-term infants without PVL, and 30 normal full-term infants were retrospectively analysed for IL-18, IL-6, IL-1beta, and TNF-alpha. The possible factors involved in alteration of IL-18 concentration in relation to PVL and CP were examined. IL-18 is undetectable in the cord blood of normal full-term infants. However, high levels of IL-18 exist in the cord blood samples obtained from pre-term infants who neonatally developed PVL followed by CP. For pre-term infants under 35 weeks of gestation, seven out of eight showing more than 200 pg/ml of IL-18 (87.5%) developed PVL neonatally, with five of them subsequently developing CP. In contrast, only five out of 38 pre-term infants with less than 100 pg/ml of IL-18 (13.2%) developed PVL. For pre-term infants with less than 30 weeks of gestation, eight out of nine showing more than 100 pg/ml of IL-18 (88.9%) developed PVL, with six of these eight (75%) developing CP later. In conclusion, the presence of high levels of IL-18 in the cord blood of the pre-term infants is correlated with the incidence of PVL and CP and may provide a prognostic marker applicable at birth.

Anti-S-nitrosocysteine antibodies are a predictive marker for demyelination in experimental autoimmune encephalomyelitis: implications for multiple sclerosis

Multiple sclerosis (MS) is characterized by inflammation within the CNS. This inflammatory response is associated with production of nitric oxide (NO) and NO-related species that nitrosylate thiols. We postulated that MS patients would exhibit an antibody (Ab) response directed against proteins containing S-nitrosocysteine (SNO-cysteine) and showed that anti-NO-cysteine Abs of the IgM isotype are in fact present in the sera of some MS patients (Boullerne et al., 1995). We report here the presence of a seemingly identical Ab response directed against SNO-cysteine in an acute model of MS, experimental autoimmune encephalomyelitis (EAE) induced in Lewis rats with the 68-84 peptide of guinea pig myelin basic protein (MBP(68-84)). Serum levels of anti-SNO-cysteine Abs peaked 1 week before the onset of clinical signs and well before the appearance of anti-MBP(68-84) Abs. The anti-SNO-cysteine Ab peak titer correlated with the extent of subsequent CNS demyelination, suggesting a link between Ab level and CNS lesion formation. In relapsing-remitting MS patients, we found elevated anti-SNO-cysteine Ab at times of relapse and normal values in most patients judged to be in remission. Two-thirds of patients with secondary progressive MS had elevated anti-SNO-cysteine Ab levels, including those receiving interferon beta-1b. The data show that a rise in circulating anti-SNO-cysteine Ab levels precedes onset of EAE. Anti-SNO-cysteine Abs are also elevated at times of MS attacks and in progressive disease, suggesting a possible role for these Abs, measurable in blood, as a biological marker for clinical activity.

Anti-S-nitrosocysteine antibodies are a predictive marker for demyelination in experimental autoimmune encephalomyelitis: implications for multiple sclerosis

Multiple sclerosis (MS) is characterized by inflammation within the CNS. This inflammatory response is associated with production of nitric oxide (NO) and NO-related species that nitrosylate thiols. We postulated that MS patients would exhibit an antibody (Ab) response directed against proteins containing S-nitrosocysteine (SNO-cysteine) and showed that anti-NO-cysteine Abs of the IgM isotype are in fact present in the sera of some MS patients (Boullerne et al., 1995). We report here the presence of a seemingly identical Ab response directed against SNO-cysteine in an acute model of MS, experimental autoimmune encephalomyelitis (EAE) induced in Lewis rats with the 68-84 peptide of guinea pig myelin basic protein (MBP(68-84)). Serum levels of anti-SNO-cysteine Abs peaked 1 week before the onset of clinical signs and well before the appearance of anti-MBP(68-84) Abs. The anti-SNO-cysteine Ab peak titer correlated with the extent of subsequent CNS demyelination, suggesting a link between Ab level and CNS lesion formation. In relapsing-remitting MS patients, we found elevated anti-SNO-cysteine Ab at times of relapse and normal values in most patients judged to be in remission. Two-thirds of patients with secondary progressive MS had elevated anti-SNO-cysteine Ab levels, including those receiving interferon beta-1b. The data show that a rise in circulating anti-SNO-cysteine Ab levels precedes onset of EAE. Anti-SNO-cysteine Abs are also elevated at times of MS attacks and in progressive disease, suggesting a possible role for these Abs, measurable in blood, as a biological marker for clinical activity.

Gene expression profile of herpesvirus-infected T cells obtained using immunomicroarrays: induction of proinflammatory mechanisms

Herpesvirus infections can frequently lead to acute inflammation, yet the mechanisms regulating this event remain poorly understood. In order to determine some of the immunological mechanisms regulated by human herpesvirus infections, we studied the gene expression profile of lymphocytes infected with human herpesvirus 6 (HHV-6) by using a novel immunomicroarray. Our nylon-based immunomicroarray contained more than 1,150 immune response-related genes and was highly consistent between experiments. Experimentally, we found that independently of the HHV-6 strain used to infect T cells, multiple proinflammatory genes were increased and anti-inflammatory genes were decreased at the mRNA and protein levels. HHV-6 strains A and B increased expression of the genes for interleukin-18 (IL-18), the IL-2 receptor, members of the tumor necrosis factor alpha superfamily receptors, mitogen-activated protein kinase, and Janus kinase signaling proteins. As reported previously, CD4 protein levels were also increased significantly. Specific type 2 cytokines, including IL-10, its receptor, and IL-14, were downregulated by HHV-6 infection and, interestingly, amyloid precursor proteins and type 1 and 2 presenilins. Thus, T cells respond to HHV-6 infection by inducing a type 1 immune response that may play a significant role in the development and progression of diseases associated with HHV-6, including pediatric, hematologic, transplant, and neurologic disorders.

Short-term evolution of autoreactive T cell repertoire in multiple sclerosis

T cells reactive to self-antigens are present in the peripheral blood of patients with autoimmune diseases as well as in healthy subjects. Although T cell-response to the self-myelin antigen myelin basic protein (MBP) has been widely investigated in multiple sclerosis (MS) patients, very little is known about the evolution over time of this response and its correlation with the disease activity. In recent years magnetic resonance imaging (MRI) techniques have provided new tools for following the inflammatory activity in the central nervous system (CNS) of MS patients. In the present study the T cell response to MBP was longitudinally investigated in terms of frequency, epitope specificity, and cytokine production profile in four patients with relapsing-remitting MS enrolled in a gadolinium-enhanced MRI serial study. In spite of different profiles of inflammatory activity within the CNS, all the patients examined showed major changes in their reactivity to MBP during the follow-up period in terms of both frequency and epitope specificity. Episodic expansions of MBP-specific T cell populations were observed in each patient, and overall they did not correlate with disease activity. In these patients the expansions: 1) occurred in the context of a steady level of disease activity, 2) correlated with a burst of CNS inflammation, 3) followed the appearance of a new active lesion, and 4) were observed even in the absence of detectable signs of CNS inflammation during the entire follow-up period. These results suggest that the evolution over time of the T cell response to a self-antigen such as MBP is more complex than previously expected. The short-term repertoire dynamics of autoreactive T cells in MS underscore the importance of longitudinal studies for evaluating autoreactivity to myelin antigens and probably to any self-antigen in other autoimmune diseases.

Interleukin-18 regulates both Th1 and Th2 responses

Although interleukin-18 is structurally homologous to IL-1 and its receptor belongs to the IL-1R/Toll-like receptor (TLR) superfamily, its function is quite different from that of IL-1. IL-18 is produced not only by types of immune cells but also by non-immune cells. In collaboration with IL-12, IL-18 stimulates Th1-mediated immune responses, which play a critical role in the host defense against infection with intracellular microbes through the induction of IFN-gamma. However, the overproduction of IL-12 and IL-18 induces severe inflammatory disorders, suggesting that IL-18 is a potent proinflammatory cytokine that has pathophysiological roles in several inflammatory conditions. IL-18 mRNA is expressed in a wide range of cells including Kupffer cells, macrophages, T cells, B cells, dendritic cells, osteoblasts, keratinocytes, astrocytes, and microglia. Thus, the pathophysiological role of IL-18 has been extensively tested in the organs that contain these cells. Somewhat surprisingly, IL-18 alone can stimulate Th2 cytokine production as well as allergic inflammation. Therefore, the functions of IL-18 in vivo are very heterogeneous and complicated. In principle, IL-18 enhances the IL-12-driven Th1 immune responses, but it can also stimulate Th2 immune responses in the absence of IL-12.

Interleukin-18 is induced in acute inflammatory demyelinating polyneuropathy

T lymphocytes of the Th1 subset producing the proinflammatory cytokine interferon-gamma (IFN-gamma) have been implicated in the pathogenesis of immune-mediated diseases of the peripheral nervous system (PNS) such as the acute Guillain-Barré syndrome (GBS) and its animal model experimental autoimmune neuritis (EAN). Interleukin-18 (IL-18) is a potent IFN-gamma-inducing cytokine that is synthesized as an inactive precursor molecule and cleaved by caspase-1 into its mature active form. In our present study we analyzed the expression of IL-18 and caspase-1 in the nerve roots of EAN rats using reverse transcriptase-polymerase chain reaction and immunocytochemistry. Using an enzyme-linked immunosorbent assay, we furthermore determined IL-18 protein levels in paired serum and cerebrospinal fluid (CSF) samples from patients with GBS as well as from noninflammatory neurologic disease (NIND) controls. In EAN, IL-18 and caspase-1 mRNA levels in the nerve roots increased during the stage of active disease progression. Immunocytochemically, both perivascular and parenchymal IL-18 protein expression was increased in the roots of EAN rats and mainly associated with ED1+ macrophages stained on serial sections. IL-18 serum levels were significantly higher in GBS patients than in NIND controls (238+/-71 vs. 42+/-7 pg/ml, P<0.001). Our data implicate the Th1-inducing cytokine IL-18 in the pathogenesis of acute immune-mediated PNS demyelination.