IL-1 receptor-associated kinase 1 regulates susceptibility to organ-specific autoimmunity

Infections often precede the development of autoimmunity. Correlation between infection with a specific pathogen and a particular autoimmune disease ranges from moderately strong to quite weak. This lack of correspondence suggests that autoimmunity may result from microbial activation of a generic, as opposed to pathogen-specific host-defense response. The Toll-like receptors, essential to host recognition of microbial invasion, signal through a common, highly conserved pathway, activate innate immunity, and control adaptive immune responses. To determine the influence of Toll/IL-1 signaling on the development of autoimmunity, the responses of wild-type (WT) mice and IL-1R-associated kinase 1 (IRAK1)-deficient mice to induction of experimental autoimmune encephalomyelitis were compared. C57BL/6 and B6.IRAK1-deficient mice were immunized with MOG 35-55/CFA or MOG 35-55/CpG DNA/IFA. WT animals developed severe disease, whereas IRAK1-deficient mice were resistant to experimental autoimmune encephalomyelitis, exhibiting little or no CNS inflammation. IRAK1-deficient T cells also displayed impaired Th1 development, particularly during disease induction, despite normal TCR signaling. These results suggest that IRAK1 and the Toll/IL-1 pathway play an essential role in T cell priming, and demonstrate one means through which innate immunity can control subsequent development of autoimmunity. These findings may also help explain the association between antecedent infection and the development or exacerbations of some autoimmune diseases.     

Inflammasomes and autoimmunity

The NOD-like receptor (NLR) family members are cytosolic sensors of microbial components and danger signals. A subset of NLRs control inflammasome assembly that results in caspase-1 activation and, in turn, IL-1β and IL-18 production. Excessive inflammasome activation can cause autoinflammatory disorders, including the hereditary periodic fevers. Autoinflammatory and autoimmune diseases form a disease spectrum of aberrant, immune-mediated inflammation against self, through innate and adaptive immunity. However, the role of inflammasomes in autoimmune disease is less clear than in autoinflammation, despite the numerous effects IL-1β and IL-18 can have on shaping adaptive immunity. We summarize the role of inflammasomes in autoimmune disorders, highlight the need for a better understanding of inflammasomes in these conditions and offer suggestions for future research directions.     

Structural characterization of the inhibitory DNA motif for the type A (D)-CpG-induced cytokine secretion and NK-cell lytic activity in mouse spleen cells

Oligodeoxynucleotides (ODN) with the CpG motif have been shown to be potent stimulators of innate immunity. A theoretical concern is that uncontrolled stimulation of the innate immune system through the TLR-9 receptor could induce, or worsen, some autoimmune diseases such as adjuvant arthritis or systemic lupus erythematosus. Safe therapeutic use of such ODN could be enhanced if one could regulate some of their stimulatory activities. We have designed a group of synthetic ODNs, which were able to inhibit the induction of NK lytic activity, IL-12p40 and IFN-gamma cytokine secretion by type A (D)-CpG-ODNs. Inhibition occurred in both DNA-sequence and dose-dependent fashion. Fifty percent inhibition was achieved with ~10-nM concentration of the most potent inhibitory ODNs. Delayed addition of these ODNs for up to 2 h was still able to profoundly affect CpG-induced IL-12p40 production at 18 h. Inhibitory DNA motif consists of two nucleotide triplets, a proximal pyrimidine-rich CCT sequence and a more distal GGG triplet. Optimal distance between these blocks is between three to five nucleotides. The linker sequence between the CCT and GGG blocks can additionally modify the activity of inhibitory ODNs, in both a positive and in negative way. When the order of CCT and GGG blocks is reversed, inhibition is completely lost. These findings suggest that CpG regulation of innate immunity can itself be regulated by particular motifs, which could be of therapeutic benefit in autoimmune diseases.     

Interleukin-17 contributes to the pathogenesis of autoimmune hepatitis through inducing hepatic interleukin-6 expression

T helper cells that produce IL-17 (Th17 cells) have recently been identified as the third distinct subset of effector T cells. Emerging data suggests that Th17 cells play an important role in the pathogenesis of many liver diseases by regulating innate immunity, adaptive immunity, and autoimmunity. In this study, we examine the role and mechanism of Th17 cells in the pathogenesis of autoimmune hepatitis (AIH). The serum levels of IL-17 and IL-23, as well as the frequency of IL-17+ cells in the liver, were significantly elevated in patients with AIH, compared to other chronic hepatitis and healthy controls. The hepatic expressions of IL-17, IL-23, ROR-γt, IL-6 and IL-1β in patients with AIH were also significantly increased and were associated with increased inflammation and fibrosis. IL-17 induces IL-6 expression via the MAPK signaling pathway in hepatocytes, which, in turn, may further stimulate Th17 cells and forms a positive feedback loop. In conclusion, Th17 cells are key effector T cells that regulate the pathogenesis of AIH, via induction of MAPK dependent hepatic IL-6 expression. Blocking the signaling pathway and interrupting the positive feedback loop are potential therapeutic targets for autoimmune hepatitis.     

Caspase-1-processed cytokines IL-1beta and IL-18 promote IL-17 production by gammadelta and CD4 T cells that mediate autoimmunity

IL-1β plays a critical role in promoting IL-17 production by γδ and CD4 T cells. However, IL-1-targeted drugs, although effective against autoinflammatory diseases, are less effective against autoimmune diseases. Conversely, gain-of-function mutations in the NLRP3 inflammasome complex are associated with enhanced IL-1β and IL-18 production and Th17 responses. In this study, we examined the role of caspase-1-processed cytokines in IL-17 production and in induction of experimental autoimmune encephalomyelitis (EAE). Killed Mycobacterium tuberculosis, the immunostimulatory component in CFA used for inducing EAE, stimulated IL-1β and IL-18 production by dendritic cells through activation of the inflammasome complex and caspase-1. Dendritic cells stimulated with M. tuberculosis and myelin oligodendrocyte glycoprotein promoted IL-17 production by T cells and induced EAE following transfer to naive mice, and this was suppressed by a caspase-1 inhibitor and reversed by administration of IL-1β or IL-18. Direct injection of the caspase-1 inhibitor suppressed IL-17 production by CD4 T cells and γδ T cells in vivo and attenuated the clinical signs of EAE. γδ T cells expressed high levels of IL-18R and the combination of IL-18 and IL-23, as with IL-1β and IL-23, stimulated IL-17 production by γδ T cells, but also from CD4 T cells, in the absence of TCR engagement. Our findings demonstrate that caspase-1-processed cytokines IL-1β and IL-18 not only promote autoimmunity by stimulating innate IL-17 production by T cells but also reveal redundancy in the functions of IL-1β and IL-18, suggesting that caspase-1 or the inflammasome may be an important drug target for autoimmune diseases.     

IL-1 receptor-associated kinase 4 is essential for IL-18-mediated NK and Th1 cell responses

IL-18 is an important cytokine for both innate and adaptive immunity. NK T cells and Th1 cells depend on IL-18 for their divergent functions. The IL-18R, IL-1R, and mammalian Toll-like receptors (TLRs) share homologous intracellular domains known as the TLR/IL-1R/plant R domain. Previously, we reported that IL-1R-associated kinase (IRAK)-4 plays a critical role in IL-1R and TLR signaling cascades and is essential for the innate immune response. Because TLR/IL-1R/plant R-containing receptors mediate signal transduction in a similar fashion, we investigated the role of IRAK-4 in IL-18R signaling. In this study, we show that IL-18-induced responses such as NK cell activity, Th1 IFN-gamma production, and Th1 cell proliferation are severely impaired in IRAK-4-deficient mice. IRAK-4(-/-) Th1 cells also do not exhibit NF-kappaB activation or IkappaB degradation in response to IL-18. Moreover, AP-1 activation which is triggered by c-Jun N-terminal kinase activation is also completely inhibited in IRAK-4(-/-) Th1 cells. These results suggest that IRAK-4 is an essential component of the IL-18 signaling cascade.     

IL-18 bridges innate and adaptive immunity through IFN-gamma and the CD134 pathway

IL-18 induces inflammation resulting in either enhanced protection from pathogens or exacerbation of autoimmunity, and T cells are profoundly activated during these responses. How IL-18 influences T cell activation is unknown, but this study in mice shows that IL-18 boosted Ag-specific T cell clonal expansion of effector T cells and induced a subpopulation of IFN-gamma superproducing T cells. Commitment to IFN-gamma production through IL-18 was independent of NK cells and IL-12 but dependent on host-derived IFN-gamma. To determine how expansion of these effectors occurred, IL-18 was shown to induce OX40L on dendritic cells, whereas peptide stimulation induced CD134 (OX40) on specific T cells. CD134 blockade inhibited T cell effector expansion thereby reducing the number of IFN-gamma superproducers by 12-fold. Thus, independent of IL-12, IL-18 impacts T cell immunity throughout lymphoid and nonlymphoid tissue by bridging the innate and adaptive arms of the immune system through IFN-gamma and the CD134 costimulatory pathway.     

An emerging role for calcium signalling in innate and autoimmunity via the cGAS-STING axis

Type I interferons are effector cytokines essential for the regulation of the innate immunity. A key effector of the type I interferon response that is dysregulated in autoimmunity and cancer is the cGAS-STING signalling axis. Recent work suggests that calcium and associated signalling proteins can regulate both cGAS-STING and autoimmunity. How calcium regulates STING activation is complex and involves both stimulatory and inhibitory mechanisms. One of these is calmodulin-mediated signalling that is necessary for STING activation. The alterations in calcium flux that occur during STING activation can also regulate autophagy, which in turn plays a role in innate immunity through the clearance of intracellular pathogens. Also connected to calcium signalling pathways is the cGAS inhibitor TREX1, a cytoplasmic exonuclease linked to several autoimmune diseases including systemic lupus erythematosus (SLE). In this review, we summarize these and other findings that indicate a regulatory role for calcium signalling in innate and autoimmunity through the cGAS-STING pathway.     

Invariant NKT cells amplify the innate immune response to lipopolysaccharide

NKT cells are thought of as a bridge between innate and adaptive immunity. In this study, we demonstrate that mouse NKT cells are activated in response to Escherichia coli LPS, and produce IFN-gamma, but not IL-4, although activation through their TCR typically induces both IL-4 and IFN-gamma production. IFN-gamma production by NKT cells is dependent on LPS-induced IL-12 and IL-18 from APC. LPS induced IFN-gamma production by NKT cells does not require CD1d-mediated presentation of an endogenous Ag and exposure to a combination of IL-12 and IL-18 is sufficient to activate them. In mice that are deficient for NKT cells, innate immune cells are activated less efficiently in response to LPS, resulting in the reduced production of TNF and IFN-gamma. We propose that in addition to acting as a bridge to adaptive immunity, NKT cells act as an early amplification step in the innate immune response and that the rapid and complete initiation of this innate response depends on the early production of IFN-gamma by NKT cells.     

The dsRNA-mimetic poly (I:C) and IL-18 synergize for IFNγ and TNFα expression

Interleukin (IL)-18 bioactivity and dsRNA sensing by receptors of innate immunity are key components of anti-viral host defense. Despite extensive data on signal transduction activated by both pathways knowledge on cross-communication is incomplete. By using human PBMC and predendritic KG1 cells, as prototypic IL-18-responsive cellular models, we sought to assess cytokine production under the influence of IL-18 and the dsRNA-mimetic poly (I:C). Here, we report on potent synergy between both mediators concerning pro-inflammatory IFNγ and TNFα production. KG1 data revealed that synergistic induction likely relied on TLR3 and was associated with prolonged/increased activation of NF-κB, as detected by IκB analysis and luciferase reporter assays, respectively. Moreover, extended activation of JNK was mediated by IL-18/poly (I:C). Although vital for innate immunity, overwhelming induction of inflammatory cytokines during viral infections poses the threat of serious collateral tissue damage. The stunning synergism inherent to IL-18/dsRNA-induced TNFα/IFNγ detected herein may contribute to this pathological phenomenon.     

Spontaneous Autoimmunity in the Absence of IL-2 Is Driven by Uncontrolled Dendritic Cells

BALB/c IL-2-deficient (IL-2-KO) mice develop systemic autoimmunity, dying within 3 to 5 wk from complications of autoimmune hemolytic anemia. Disease in these mice is Th1 mediated, and IFN-γ production is required for early autoimmunity. In this study, we show that dendritic cells (DCs) are required for optimal IFN-γ production by T cells in the IL-2-KO mouse. Disease is marked by DC accumulation, activation, and elevated production of Th1-inducing cytokines. IL-2-KO DCs induce heightened proliferation and cytokine production by naive T cells compared with wild-type DCs. The depletion of either conventional or plasmacytoid DCs significantly prolongs the survival of IL-2-KO mice, demonstrating that DCs contribute to the progression of autoimmunity. Elimination of Th1-inducing cytokine signals (type 1 IFN and IL-12) reduces RBC-specific Ab production and augments survival, indicating that cytokines derived from both plasmacytoid DCs and conventional DCs contribute to disease severity. DC activation likely precedes T cell activation because DCs are functionally activated even in an environment lacking overt T cell activation. These data indicate that both conventional and plasmacytoid DCs are critical regulators in the development of this systemic Ab-mediated autoimmune disease, in large part through the production of IL-12 and type 1 IFNs.     

Activation of virus-specific CD8+ T cells by lipopolysaccharide-induced IL-12 and IL-18

Virus-specific T cells represent a hallmark of Ag-specific, adaptive immunity. However, some T cells also demonstrate innate functions, including non-Ag-specific IFN-gamma production in response to microbial products such as LPS or exposure to IL-12 and/or IL-18. In these studies we examined LPS-induced cytokine responses of CD8(+) T cells directly ex vivo. Following acute viral infection, 70-80% of virus-specific T cells will produce IFN-gamma after exposure to LPS-induced cytokines, and neutralization experiments indicate that this is mediated almost entirely through production of IL-12 and IL-18. Different combinations of these cytokines revealed that IL-12 decreases the threshold of T cell activation by IL-18, presenting a new perspective on IL-12/IL-18 synergy. Moreover, memory T cells demonstrate high IL-18R expression and respond effectively to the combination of IL-12 and IL-18, but cannot respond to IL-18 alone, even at high cytokine concentrations. This demonstrates that the synergy between IL-12 and IL-18 in triggering IFN-gamma production by memory T cells is not simply due to up-regulation of the surface receptor for IL-18, as shown previously with naive T cells. Together, these studies indicate how virus-specific T cells are able to bridge the gap between innate and adaptive immunity during unrelated microbial infections, while attempting to protect the host from cytokine-induced immunopathology and endotoxic shock.     

Activation of invariant NKT cells ameliorates experimental ocular autoimmunity by a mechanism involving innate IFN-gamma production and dampening of the adaptive Th1 and Th17 responses

Invariant NKT cells (iNKT cells) have been reported to play a role not only in innate immunity but also to regulate several models of autoimmunity. Furthermore, iNKT cells are necessary for the generation of the prototypic eye-related immune regulatory phenomenon, anterior chamber associated immune deviation (ACAID). In this study, we explore the role of iNKT cells in regulation of autoimmunity to retina, using a model of experimental autoimmune uveitis (EAU) in mice immunized with a uveitogenic regimen of the retinal Ag, interphotoreceptor retinoid-binding protein. Natural strain-specific variation in iNKT number or induced genetic deficiencies in iNKT did not alter baseline susceptibility to EAU. However, iNKT function seemed to correlate with susceptibility and its pharmacological enhancement in vivo by treatment with iNKT TCR ligands at the time of uveitogenic immunization reproducibly ameliorated disease scores. Use of different iNKT TCR ligands revealed dependence on the elicited cytokine profile. Surprisingly, superior protection against EAU was achieved with alpha-C-GalCer, which induces a strong IFN-gamma but only a weak IL-4 production by iNKT cells, in contrast to the ligands alpha-GalCer (both IFN-gamma and IL-4) and OCH (primarily IL-4). The protective effect of alpha-C-GalCer was associated with a reduction of adaptive Ag-specific IFN-gamma and IL-17 production and was negated by systemic neutralization of IFN-gamma. These data suggest that pharmacological activation of iNKT cells protects from EAU at least in part by a mechanism involving innate production of IFN-gamma and a consequent dampening of the Th1 as well as the Th17 effector responses.     

Targeting T cell-specific costimulators and growth factors in a model of autoimmune hemolytic anemia

Although it is established that failure of regulatory mechanisms underlies many autoimmune diseases, the stimuli that activate autoreactive lymphocytes remain poorly understood. Defining these stimuli will lead to therapeutic strategies for autoimmune diseases. IL-2-deficient mice develop spontaneous autoimmunity, because of a deficiency of regulatory T cells, and on the BALB/c background, they rapidly die from autoimmune hemolytic anemia. To define the importance of costimulatory pathways in various components of this autoimmune disorder, we first intercrossed IL-2-deficient mice with mice lacking CD28 or CD40L. Elimination of CD28 reduced the activation of autoreactive T cells and lymphoproliferation as well as production of autoantibodies, whereas elimination of CD40L reduced autoantibody production without affecting T cell expansion and accumulation. To examine the role of IL-7, we blocked IL-7R signaling with neutralizing Abs. This treatment inhibited the production of autoantibodies and the development of autoimmune hemolytic anemia. Together, these data indicate that specific costimulatory and cytokine signals are critical for the spontaneous autoantibody-mediated disease that develops in IL-2-deficient mice.     

Caspase-1-processed IL-1 family cytokines play a vital role in driving innate IL-17

The interleukin (IL)-1 cyokine family plays a vital role in inflammatory responses during infection and in autoimmune diseases. The pro-inflammatory cytokines, IL-1β and IL-18 are members of the IL-1 family that require cleavage by caspase-1 in the inflammasome to generate the mature active cytokines. Cells of the innate immune system, including γδ T cells and invariant natural killer T (iNKT) cells respond rapidly to invading pathogens by producing inflammatory cytokines, such as IFN-γ and IL-17. IL-1β or IL-18 in combination with IL-23 can induce IL-17 production by γδ T cells without T cell receptor (TCR) engagement. IL-1β and IL-23 can also synergize to induce IL-17 production by iNKT cells. Furthermore, CD4+ αβ effector memory T cells secrete IL-17 in response to IL-23 in combination with either IL-1β or IL-18, in the absence of any TCR stimulation. The early IL-17 produced by innate cells induces recruitment of neutrophils to the site of infection, stimulates local epithelial cells to secrete anti-microbial proteins, such as lipocalins and calgranulins, induces production of structural proteins important in tight junction stability, and promotes production of matrix metalloproteinases. Caspase-1 processed IL-1 family cytokines therefore play a vital role in the innate immune response and induction of IL-17 from innate immune cells which functions to fight infections and promote autoimmunity.     

Suppressor of cytokine signaling 1 DNA administration inhibits inflammatory and pathogenic responses in autoimmune myocarditis

Myocarditis and subsequent dilated cardiomyopathy are major causes of heart failure in young adults. Myocarditis in humans is highly heterogeneous in etiology. Recent studies have indicated that a subgroup of myocarditis patients may benefit from immune-targeted therapies, because autoimmunity plays an important role in myocarditis as well as contributing to the progression to cardiomyopathy and heart failure. Suppressor of cytokine signaling (SOCS) 1 plays a key role in the negative regulation of both TLR- and cytokine receptor-mediated signaling, which is involved in innate immunity and subsequent adaptive immunity. In this study, we investigated the therapeutic effect of SOCS1 DNA administration on experimental autoimmune myocarditis (EAM) in mice. EAM was induced by s.c. immunization with cardiac-specific peptides derived from α myosin H chain in BALB/c mice. In contrast to control myocarditis mice, SOCS1 DNA-injected mice were protected from development of EAM and heart failure. SOCS1 DNA administration was effective for reducing the activation of autoreactive CD4(+) T cells by inhibition of the function of Ag-presenting dendritic cells. Our findings suggest that SOCS1 DNA administration has considerable therapeutic potential in individuals with autoimmune myocarditis and dilated cardiomyopathy.     

Role of γδ T cells in α-galactosylceramide-mediated immunity

Attempts to harness mouse type I NKT cells in different therapeutic settings including cancer, infection, and autoimmunity have proven fruitful using the CD1d-binding glycolipid α-galactosylceramide (α-GalCer). In these different models, the effects of α-GalCer mainly relied on the establishment of a type I NKT cell-dependent immune cascade involving dendritic cell, NK cell, B cell, or conventional CD4(+) and CD8(+) T cell activation/regulation as well as immunomodulatory cytokine production. In this study, we showed that γδ T cells, another population of innate-like T lymphocytes, displayed a phenotype of activated cells (cytokine production and cytotoxic properties) and were required to achieve an optimal α-GalCer-induced immune response. Using gene-targeted mice and recombinant cytokines, a critical need for IL-12 and IL-18 has been shown in the α-GalCer-induced IFN-γ production by γδ T cells. Moreover, this cytokine production occurred downstream of type I NKT cell response, suggesting their bystander effect on γδ T cells. In line with this, γδ T cells failed to directly recognize the CD1d/α-GalCer complex. We also provided evidence that γδ T cells increase their cytotoxic properties after α-GalCer injection, resulting in an increase in killing of tumor cell targets. Moreover, using cancer models, we demonstrated that γδ T cells were required for an optimal α-GalCer-mediated anti-tumor activity. Finally, we reported that immunization of wild-type mice with α-GalCer enhanced the adaptive immune response elicited by OVA, and this effect was strongly mediated by γδ T cells. We conclude that γδ T cells amplify the innate and acquired response to α-GalCer, with possibly important outcomes for the therapeutic effects of this compound.     

Natural killer cells in human autoimmune disorders

Natural killer (NK) cells are innate lymphocytes that play a critical role in early host defense against viruses. Through their cytolytic capacity and generation of cytokines and chemokines, NK cells modulate the activity of other components of the innate and adaptive immune systems and have been implicated in the initiation or maintenance of autoimmune responses. This review focuses on recent research elucidating a potential immunoregulatory role for NK cells in T-cell and B-cell-mediated autoimmune disorders in humans, with a particular focus on multiple sclerosis, rheumatoid arthritis, and systemic lupus erythematous. A better understanding of the contributions of NK cells to the development of autoimmunity may lead to novel therapeutic targets in these diseases.