The proinflammatory cytokine interleukin 1 (<em>IL</em>-1) activates the transcription of many genes encoding acute phase and proinflammatory proteins, a function mediated primarily by the transcription factor NF-kappaB. An early <em>IL</em>-1 signaling event is the recruitment of the Ser/Thr kinase IRAK to the type I <em>IL</em>-1 receptor (<em>IL</em>-1RI). Here we describe the function of a previously identified <em>IL</em>-1 receptor subunit designated <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>). <em>IL</em>-1 treatment of cells induces the formation of a complex containing both <em>IL</em>-1RI and <em>IL</em>-<em>1RAcP</em>. IRAK is recruited to this complex through its association with <em>IL</em>-<em>1RAcP</em>. Overexpression of an <em>IL</em>-<em>1RAcP</em> mutant lacking its intracellular domain, the IRAK-binding domain, prevented the recruitment of IRAK to the receptor complex and blocked <em>IL</em>-1-induced NF-kappaB activation.

The cytokine interleukin-1beta (<em>IL</em>-1beta) contributes to ischemic, excitotoxic, and traumatic brain injury. <em>IL</em>-1beta actions depend on interaction with a single receptor (<em>IL</em>-1RI), which associates with an accessory protein (<em>IL</em>-<em>1RAcP</em>), and is blocked by <em>IL</em>-1 receptor antagonist (<em>IL</em>-1ra). Here we show that in normal mice [wild-type (WT)], intracerebroventricular injection of <em>IL</em>-1ra markedly reduces (-50%; p < 0.01) ischemic brain damage caused by reversible occlusion of the middle cerebral artery, whereas injection of <em>IL</em>-1beta exacerbates damage (+45%; p < 0.05). Mice lacking <em>IL</em>-1RI [<em>IL</em>-1RI knock-out (KO)] exhibited ischemic brain damage that is almost identical to that of the WT (infarct volume 43.7 +/- 6.1 and 46.2 +/- 6.2 mm3, respectively), but failed to respond to injection of <em>IL</em>-1ra. However, injection of <em>IL</em>-1beta (intracerebroventricularly) exacerbated ischemic brain damage in <em>IL</em>-1RI KO (+61%; p < 0.001) and in WT mice (+45%). This effect of <em>IL</em>-1beta was abolished by heat denaturation in all animals, and was reversed by <em>IL</em>-1ra in WT, but not <em>IL</em>-1RI KO mice. In contrast, <em>IL</em>-1RI KO mice were completely resistant to effects of <em>IL</em>-1beta on food intake or body weight. <em>IL</em>-<em>1RAcP</em> mRNA was increased by stroke in WT, but reduced in <em>IL</em>-1RI KO mice compared with sham-operated mice. Type II <em>IL</em>-1 receptor mRNA was significantly increased 4 hr after ischemia in WT and <em>IL</em>-1RI KO (+20%) animals. These data show that <em>IL</em>-1beta can exacerbate ischemic brain damage independently of <em>IL</em>-1RI and suggest the existence of additional signaling receptor or receptors for <em>IL</em>-1 in the brain.

Interleukin-1β (<em>IL</em>-1β) is a pro-inflammatory cytokine that contributes to neuronal injury in various degenerative diseases, and is therefore a potential therapeutic target. It exerts its biological effect by activating the interleukin-1 receptor type I (<em>IL</em>-1RI) and recruiting a signalling core complex consisting of the myeloid differentiation primary response protein 88 (MyD88) and the <em>IL</em>-1R accessory protein (<em>IL</em>-<em>1RAcP</em>). This pathway has been clearly described in the peripheral immune system, but only scattered information is available concerning the molecular composition and distribution of its members in neuronal cells. The findings of this study show that <em>IL</em>-1RI and its accessory proteins MyD88 and <em>IL</em>-<em>1RAcP</em> are differently distributed in the hippocampus and in the subcellular compartments of primary hippocampal neurons. In particular, only <em>IL</em>-1RI is enriched at synaptic sites, where it co-localises with, and binds to the GluN2B subunit of NMDA receptors. Furthermore, treatment with NMDA increases <em>IL</em>-1RI interaction with NMDA receptors, as well as the surface expression and localization of <em>IL</em>-1RI at synaptic membranes. <em>IL</em>-1β also increases <em>IL</em>-1RI levels at synaptic sites, without affecting the total amount of the receptor in the plasma membrane. Our results reveal for the first time the existence of a dynamic and functional interaction between NMDA receptor and <em>IL</em>-1RI systems that could provide a molecular basis for <em>IL</em>-1β as a neuromodulator in physiological and pathological events relying on NMDA receptor activation.

Interleukin-1 (<em>IL</em>-1)-family cytokines are mediators of innate and adaptive immunity. They exert proinflammatory effects by binding a primary receptor that recruits a receptor accessory protein to form a signaling-competent heterotrimeric complex. Here we present the crystal structure of <em>IL</em>-1β bound to its primary receptor <em>IL</em>-1RI and its receptor accessory protein <em>IL</em>-<em>1RAcP</em>, providing insight into how <em>IL</em>-1-type cytokines initiate signaling and revealing an evolutionary relationship with the fibroblast growth factor receptor family.

Interleukin (<em>IL</em>)-1 is an important mediator of acute brain injury and inflammation, and has been implicated in chronic neurodegeneration. The main source of <em>IL</em>-1 in the CNS is microglial cells, which have also been suggested as targets for its action. However, no data exist demonstrating expression of <em>IL</em>-1 receptors [<em>IL</em>-1 type-I receptor (<em>IL</em>-1RI), <em>IL</em>-1 type-II receptor (<em>IL</em>-1RII) and <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>)] on microglia. In the present study we investigated whether microglia express <em>IL</em>-1 receptors and whether they present target or modulatory properties for <em>IL</em>-1 actions. RT-PCR analysis demonstrated lower expression of <em>IL</em>-1RI and higher expression of <em>IL</em>-1RII mRNAs in mouse microglial cultures compared with mixed glial or pure astrocyte cultures. Bacterial lipopolysaccharide (LPS) caused increased expression of <em>IL</em>-1RI, <em>IL</em>-1RII and <em>IL</em>-<em>1RAcP</em> mRNAs, induced the release of <em>IL</em>-1beta, <em>IL</em>-6 and prostaglandin-E2 (PGE2), and activated nuclear factor kappaB (NF-kappaB) and the mitogen-activated protein kinases (MAPKs) p38, and extracellular signal-regulated protein kinase (ERK1/2), but not c-Jun N-terminal kinase (JNK) in microglial cultures. In comparison, <em>IL</em>-1beta induced the release of PGE2, <em>IL</em>-6 and activated NF-kappaB, p38, JNK and ERK1/2 in mixed glial cultures, but failed to induce any of these responses in microglial cell cultures. <em>IL</em>-1beta also failed to affect LPS-primed microglial cells. Interestingly, a neutralizing antibody to <em>IL</em>-1RII significantly increased the concentration of <em>IL</em>-1beta in the medium of LPS-treated microglia and exacerbated the <em>IL</em>-1beta-induced <em>IL</em>-6 release in mixed glia, providing the first evidence that microglial <em>IL</em>-1RII regulates <em>IL</em>-1beta actions by binding excess levels of this cytokine during brain inflammation.

<em>IL</em>-1 binds to two types of receptors on the cell membrane, of which only type I (<em>IL</em>-1RI) transduces signals in concert with the coreceptor <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>) while type II (<em>IL</em>-1RII) allegedly functions solely as ligand sink and decoy receptor without participating in <em>IL</em>-1 signaling. To investigate the regulatory role of <em>IL</em>-1RII on <em>IL</em>-1 responsiveness, a chimeric receptor encompassing the extracellular and transmembrane portions of <em>IL</em>-1RII and the cytoplasmic signal-transducing domain of <em>IL</em>-1RI was transfected into two murine EL-4-derived sublines that do or do not express <em>IL</em>-<em>1RAcP</em>, respectively. The chimeric receptor was able to transduce the <em>IL</em>-1 signal and induce <em>IL</em>-2 production only in the cell line which expressed <em>IL</em>-<em>1RAcP</em>, suggesting effective interaction between the extracellular domains of <em>IL</em>-1RII and <em>IL</em>-<em>1RAcP</em> in the presence of <em>IL</em>-1. The physical association of ligated <em>IL</em>-1RII with <em>IL</em>-<em>1RAcP</em> was proven by crosslinking experiments with radio-iodinated <em>IL</em>-1 and subsequent immunoprecipitations in normal human B cells and in EL-4 D6/76 cells transiently cotransfected with <em>IL</em>-1RII and <em>IL</em>-<em>1RAcP</em>, respectively. Based on these findings, it is proposed that upon <em>IL</em>-1 binding <em>IL</em>-1RII can recruit <em>IL</em>-<em>1RAcP</em> into a nonfunctional trimeric complex and thus modulate <em>IL</em>-1 signaling by subtracting the coreceptor molecule from the signaling <em>IL</em>-1RI. In this novel mechanism of coreceptor competition, the ratio between <em>IL</em>-1RII and <em>IL</em>-1RI becomes the central factor in determining the <em>IL</em>-1 responsiveness of a cell and the availability of <em>IL</em>-<em>1RAcP</em> becomes limiting for effective <em>IL</em>-1 signaling.

Members of the Toll/interleukin-1 receptor (TIR) family are of importance for host defense and inflammation. Here we report that the TIR-family member interleukin-33R (<em>IL</em>-33R) cross-activates the receptor tyrosine kinase c-Kit in human and murine mast cells. The <em>IL</em>-33R-induced activation of signal transducer and activator of transcription 3 (STAT3), extracellular signal-regulated kinase 1/2 (Erk1/2), protein kinase B (PKB), and Jun NH(2)-terminal kinase 1 (JNK1) depends on c-Kit and is required to elicit optimal effector functions. Costimulation with the c-Kit ligand stem cell factor (SCF) is necessary for <em>IL</em>-33-induced cytokine production in primary mast cells. The structural basis for this cross-activation is the complex formation between c-Kit, <em>IL</em>-33R, and <em>IL</em>-1R accessory protein (<em>IL</em>-<em>1RAcP</em>). We found that c-Kit and <em>IL</em>-<em>1RAcP</em> interact constitutively and that <em>IL</em>-33R joins this complex upon ligand binding. Our findings support a model in which signals from seemingly disparate receptors are integrated for full cellular responses.

Interleukin-1 (<em>IL</em>-1) is a central molecule in inflammation and immune responses whose pleiotropic activities are mediated by the type I <em>IL</em>-1 receptor (<em>IL</em>-1RI). The <em>IL</em>-1RI alone on the cell surface is silent after binding of the ligand. We show that the recently identified <em>IL</em>-1RI accessory protein (<em>IL</em>-<em>1RAcP</em>) converts the silent into a fully functional <em>IL</em>-1RI complex. Although transfection of <em>IL</em>-<em>1RAcP</em> into <em>IL</em>-<em>1RAcP</em>-deficient EL4D6/76 cells did not alter the binding kinetics or dissociation constants of the 125I-labeled <em>IL</em>-1alpha/<em>IL</em>-1RI complex, a very early event, internalization of the activated receptor complex, and a late event, <em>IL</em>-1-stimulated <em>IL</em>-2 production, were successfully restored. Therefore, recruitment of <em>IL</em>-<em>1RAcP</em> is a critical early step in the signaling cascade mediated by the <em>IL</em>-1RI activation complex.

In the CNS, interleukin-1β (<em>IL</em>-1β) is synthesized and released during injury, infection, and disease, mediating inflammatory responses. However, <em>IL</em>-1β is also present in the brain under physiological conditions, and can influence hippocampal neuronal function. Several cell-specific <em>IL</em>-1-mediated signaling pathways and functions have been identified in neurons and astrocytes, but their mechanisms have not been fully defined. In astrocytes, <em>IL</em>-1β induced both the p38 MAPK and NF-κB (nuclear factor κB) pathways regulating inflammatory responses, however in hippocampal neurons <em>IL</em>-1β activated p38 but not NF-κB. Additionally, <em>IL</em>-1β induced Src phosphorylation at 0.01 ng/ml in hippocampal neurons, a dose 1000-fold lower than that used to stimulate inflammatory responses. <em>IL</em>-1 signaling requires the type 1 <em>IL</em>-1 receptor and the <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>) as a receptor partner. We previously reported a novel isoform of the <em>IL</em>-<em>1RAcP</em>, <em>IL</em>-<em>1RAcP</em>b, found exclusively in CNS neurons. In this study, we demonstrate that AcPb specifically mediates <em>IL</em>-1β activation of p-Src and potentiation of NMDA-induced calcium influx in mouse hippocampal neurons in a dose-dependent manner. Mice lacking the AcPb, but retaining the AcP, isoform were deficient in <em>IL</em>-1β regulation of p-Src in neurons. AcPb also played a modulatory role in the activation of p38 MAPK, but had no effect on NF-κB signaling. The restricted expression of AcPb in CNS neurons, therefore, governs specific neuronal signaling and functional responses to <em>IL</em>-1β.

Activated pancreatic stellate cells (PSCs) play a pivotal role in pancreatic fibrosis in chronic pancreatitis and pancreatic cancer. Recent studies have suggested a role of <em>IL</em>-33, a newly identified <em>IL</em>-1 family member, in fibrosis. We here examined the expression of <em>IL</em>-33 and the <em>IL</em>-33-mediated regulation of cell functions in PSCs. PSCs were isolated from human and rat pancreas tissues. The expression of <em>IL</em>-33 was examined by Western blotting, PCR, ELISA, and immunostaining. The roles of <em>IL</em>-33 in the regulation of PSC functions were examined by using recombinant <em>IL</em>-33 and small interfering RNA. Activated PSCs expressed <em>IL</em>-33 in the nucleus, and the expression was increased by <em>IL</em>-1β, TNF-α, PDGF-BB, and IFN-γ, but not TGF-β1. Nuclear <em>IL</em>-33 expression was also observed in the pancreatic acinar and ductal cells. <em>IL</em>-1β induced <em>IL</em>-33 expression mainly through the activation of NF-κB and ERK pathways and partially through that of p38 MAP kinase, whereas PDGF-BB induced <em>IL</em>-33 expression mainly through the activation of ERK pathway. PSCs expressed soluble ST2, ST2L, and <em>IL</em>-<em>1RAcP</em>, but the expression level of ST2L was relatively low. Recombinant <em>IL</em>-33 did not stimulate key cell functions of PSCs. Decreased <em>IL</em>-33 expression by small interfering RNA resulted in decreased proliferation in response to PDGF-BB. In conclusion, activated PSCs expressed <em>IL</em>-33 in the nucleus. <em>IL</em>-33 might regulate the PDGF-induced proliferation in PSCs.

<em>IL</em>-33/<em>IL</em>-1F11 is a new member of the <em>IL</em>-1 family ligand and provokes T helper-type immune responses. <em>IL</em>-33 is the ligand of ST2 and <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>) that triggers nuclear factor-κ light chain enhancer of activated B cells (NF-κB) and MAPK signaling. We discovered a novel short splice variant of <em>IL</em>-33 that was termed sp<em>IL</em>-33. The new sp<em>IL</em>-33 lacks exon 3 containing a proposed caspase-1 cleavage site. We isolated sp<em>IL</em>-33 cDNA from the Huh7 human hepatocarcinoma cell line and expressed the recombinant sp<em>IL</em>-33 protein in Escherichia coli. The recombinant sp<em>IL</em>-33 and pro-<em>IL</em>-33 were not cleaved by caspase-1, unlike <em>IL</em>-18 (<em>IL</em>-1F4). The recombinant sp<em>IL</em>-33 was constitutively active, and sp<em>IL</em>-33-induced inflammatory cytokine production was caspase-1-independent in HMC-1 and Raw 264.7 cells. The recombinant sp<em>IL</em>-33 induced the phosphorylation of <em>IL</em>-1 receptor-associated kinase (IRAK1), NF-κB, p38 MAPK, p44/42 MAPK, and JNK in a time- and dose-dependent manner. Anti-ST2 monoclonal antibody specifically blocked the sp<em>IL</em>-33-induced cytokine production. In this study, we identified and characterized a new <em>IL</em>-33 splice variant, which was a constitutively active <em>IL</em>-33 isoform. The existence of constitutively active sp<em>IL</em>-33 suggests that the biological activity of <em>IL</em>-33 could be triggered by diverse stimulations during immune responses. Further investigation of the sp<em>IL</em>-33 expression pattern may contribute to understanding the involvement of <em>IL</em>-33 in inflammatory disorders.

Human pancreatic cancer is one of the most fatal of all solid tissue malignancies. Pancreatic inflammation plays a key role in the development of pancreatic malignancy mediated by pro-inflammatory signalling cascades. Despite advances in surgery and radiation oncology, no significant improvements in overall survival have yet been achieved. Recent investigations suggest a crucial role of interleukin-33 (<em>IL</em>-33), a novel <em>IL</em>-1 family cytokine, in the pathogenesis of chronic pancreatitis and possibly pancreatic cancer. However, the precise role of <em>IL</em>-33 in pancreatic carcinogenesis is poorly understood. As <em>IL</em>-33 mediates its effects via the heterodimeric ST2L/<em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>) receptor complex, we investigated the influence of <em>IL</em>-33 alone, <em>IL</em>-33 combined with <em>IL</em>-1 and other inflammatory cytokines on <em>IL</em>-33 receptor/ligand mRNA expression and production of tumorigenic factors in the highly metastatic human pancreatic adenocarcinoma cell line Colo357. Our results demonstrated that <em>IL</em>-1 and <em>IL</em>-3 up-regulated <em>IL</em>-33 mRNA while <em>IL</em>-12 showed the opposite effect. We also detected a counter-regulatory effect of <em>IL</em>-33 and <em>IL</em>-1 on the mRNA expression of soluble <em>IL</em>-33 receptor ST2 and membrane-bound receptor ST2L. Furthermore, <em>IL</em>-33 and <em>IL</em>-1 acted synergistically in up-regulating secretion of pro-inflammatory <em>IL</em>-6. <em>IL</em>-33 alone stimulated spontaneous release of pro-angiogenic <em>IL</em>-8, but it did not affect <em>IL</em>-1-induced <em>IL</em>-8 secretion. <em>IL</em>-33/<em>IL</em>-1 effects on cytokine production appear to be mediated via NF-κB activation. These data argue for the pro-inflammatory role of <em>IL</em>-33 in Colo357 cells implying that <em>IL</em>-33 might act as a crucial mediator in inflammation-associated pancreatic carcinogenesis.

Interleukin-33 (<em>IL</em>-33) is a member of the <em>IL</em>-1 cytokine family, which includes <em>IL</em>-1 and <em>IL</em>-18, and is considered to be important for host defense against nematodes by inducing Th2 cytokine production via the <em>IL</em>-33 receptor. <em>IL</em>-33 receptor is a heterodimer of <em>IL</em>-1 receptor-like 1 (<em>IL</em>-1RL1; also called ST2, T1, Der4, and fit-1) and <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>). On the other hand, excessive and/or inappropriate production of <em>IL</em>-33 is considered to be involved in the development of various disorders, such as allergic and autoimmune diseases. Unlike <em>IL</em>-1β and <em>IL</em>-18, <em>IL</em>-33 does not seem to be secreted through the activation of inflammasomes in events such as apoptosis. However, <em>IL</em>-33 is localized in the nucleus of cells and is released during tissue injury associated with necrosis. This suggests that it acts as an alarmin, like <em>IL</em>-1α and high-mobility group box chromosomal protein-1 (HMGB-1). This review summarizes current knowledge regarding the roles of <em>IL</em>-33 in the functions of various cell types and the pathogenesis of allergy.

We investigated the composition of the endogenous ligand-bound type I interleukin-1 (<em>IL</em>-1) receptor (<em>IL</em>-1RI) signaling complex using immunoprecipitation and tandem mass spectrometry. Three proteins with approximate molecular masses of 60 (p60), 36 (p36), and 90 kDa (p90) became phosphorylated after treatment with <em>IL</em>-1. Phosphorylation in vitro of p60 has been reported previously, but its identity was unknown. We showed using tandem mass spectrometry that p60 is identical to interleukin-1 receptor-associated kinase (IRAK)-4. MS also enabled detection of <em>IL</em>-1, <em>IL</em>-1RI, <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>), and myeloid differentiation primary response protein 88 (MyD88) in the complex. The p60 protein (IRAK-4) was the earliest component of the complex to be phosphorylated. Phosphorylated IRAK-4 from the receptor complex migrated more slowly in SDS-PAGE than its unphosphorylated form as did recombinant IRAK-4 autophosphorylated in vitro. Phosphorylation was restricted to serine and threonine residues. IRAK-4, p36, <em>IL</em>-<em>1RAcP</em>, and MyD88 bound to the liganded receptor within 15 s of activation by <em>IL</em>-1 and remained associated upon prolonged activation, suggesting that the signaling complex is very stable. The p90 phosphoprotein was only transiently associated with the receptor. This behavior and its size were consistent with it being IRAK-1. Our work revealed that liganding of <em>IL</em>-1RI causes its strong and stable association with <em>IL</em>-<em>1RAcP</em>, MyD88, and the previously unidentified protein p60 (IRAK-4). The only component of the <em>IL</em>-1RI signaling complex that dissociated is IRAK-1. Our study is therefore the first detailed description of the endogenous <em>IL</em>-1RI complex.

Interleukin-33 (<em>IL</em>-33) is a recently discovered cytokine that belongs to the <em>IL</em>-1 superfamily and acts as an important regulator in several allergic disorders. It is considered to function as an alarmin, or danger cytokine, that is released upon structural cell damage. <em>IL</em>-33 activates several immune cells, including Th2 cells, mast cells and basophils, following its interaction with a cell surface heterodimer consisting of an <em>IL</em>-1 receptor-related protein ST2 (<em>IL</em>-1RL1) and <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>). This activation leads to the production of a variety of Th2-like cytokines that mediate allergic-type immune responses. Thus, <em>IL</em>-33 appears to be a double-edged sword because, in addition to its important contribution to host defence, it exacerbates allergic responses, such as allergic rhinitis and asthma. A major purported mechanism of <em>IL</em>-33 in allergy is the activation of mast cells to produce a variety of pro-inflammatory cytokines and chemokines. In this review, we summarize the current knowledge regarding the genetics and physiology of <em>IL</em>-33 and <em>IL</em>-1RL1 and its association with different allergic diseases by focusing on its effects on mast cells and basophils.

Males of many species are more susceptible than females to infections caused by parasites, bacteria, fungi, and viruses. Following inoculation with Seoul virus, male rats have more virus present in target organs and shed virus longer than females. The goal of this study was to test the hypothesis that variation in the expression of genes associated with immune function mediates sex differences in hantavirus infection. Using DNA microarrays, we examined changes in gene expression in lung tissue during the early (when animals are viremic and shedding virus; Day 15 post-inoculation (p.i.)) and late (animals have low levels of infectious virus, but high antibody titers; Day 40 p.i.) phases of infection in adult male and female rats. After normalizing the gene expression levels from infected animals to the gene expression levels from same-sex uninfected controls, our data revealed that 1,813 genes were differentially expressed between the sexes during infection. The expression of key transcriptional factors (e.g., eIF-2 alpha, NF-kappa B, IRF-1, NF-<em>IL</em>-6, and STAT6) and genes that encode for proinflammatory (e.g., TNF alpha R, <em>IL</em>-1R, and <em>IL</em>-<em>1RAcP</em>), antiviral (e.g., IFN gamma R and Mx proteins), T cell (e.g., CD3 and TCR), and Ig superfamily (e.g., IgM, IgG, and MHC class I and II) proteins was higher in females than males. Conversely, males had higher expression of heat shock protein genes (e.g., hsp70) suggesting that cellular stress is elevated in males. These data provide candidate genes and cellular pathways that may underlie sex differences in responses to Seoul virus and possibly other hemorrhagic fever viruses.

Interleukin-1 (<em>IL</em>-1) family cytokines are key signaling molecules in both the innate and adaptive immune systems, mediating inflammation in response to a wide range of stimuli. The basic mechanism of signal initiation is a stepwise process in which an agonist cytokine binds its cognate receptor. Together, this cytokine-receptor complex recruits an often-common secondary receptor. Intracellularly, the Toll/<em>IL</em>-1 Receptor (TIR) domains of the two receptors are brought into close proximity, initiating an NF-κB signal transduction cascade. Due to the potent inflammatory response invoked by <em>IL</em>-1 family cytokines, several physiological mechanisms exist to inhibit <em>IL</em>-1 family signaling, including antagonist cytokines and decoy receptors. The numerous cytokines and receptors in the <em>IL</em>-1 superfamily are further classified into four subfamilies, dependent on their distinct cognate receptors-the <em>IL</em>-1, <em>IL</em>-33, and <em>IL</em>-36 subfamilies share <em>IL</em>-<em>1RAcP</em> as their secondary receptor, while <em>IL</em>-18 subfamily utilizes a distinct secondary receptor. Here, we describe how structural biology has informed our understanding of <em>IL</em>-1 family cytokine signaling, with a particular focus on molecular mechanisms of signaling complex formation and antagonism at the atomic level, as well as how these findings have advanced therapeutics to treat some chronic inflammatory diseases that are the result of dysregulated <em>IL</em>-1 signaling.

The proinflammatory cytokine <em>IL</em>-1 induces the biosynthesis of a number of immunologically important proteins during infection, tissue damage, and/or stress, in part through the activation of the transcription factor NF-kappaB. Signal transduction is initiated at the cell membrane by complex formation between extracellular <em>IL</em>-1 and the transmembrane <em>IL</em>-1R type I (<em>IL</em>-1RI) and <em>IL</em>-1R accessory protein (<em>IL</em>-<em>1RAcP</em>). The intracellular signaling cascade involves recruitment of two <em>IL</em>-1R-associated kinases, IRAK1 and IRAK2, and the adapter protein MyD88, events which are dependent on the intracellular domain of membrane-bound <em>IL</em>-<em>1RAcP</em> (m<em>IL</em>-<em>1RAcP</em>). In mouse liver, <em>IL</em>-<em>1RAcP</em> is expressed as a soluble protein (s<em>IL</em>-<em>1RAcP</em>), the function of which is unknown. We have cloned the human s<em>IL</em>-<em>1RAcP</em> and established by sequence analysis that the human s<em>IL</em>-<em>1RAcP</em> mRNA arises from alternative splicing of the <em>IL</em>-<em>1RAcP</em> gene (shown here to encompass 12 exons spanning more than 56 kb). Furthermore, we demonstrate that human HepG2 hepatoma cells express both m<em>IL</em>-<em>1RAcP</em> and s<em>IL</em>-<em>1RAcP</em> and that signal transduction in these cells is mediated through IRAK1, IRAK2, and MyD88. We show that phorbol esters induce a change in the pre-mRNA splice pattern such that s<em>IL</em>-<em>1RAcP</em> mRNA becomes the dominant form. Overexpression of a membrane-anchored fusion protein of s<em>IL</em>-<em>1RAcP</em> and MHC in HepG2 cells inhibits <em>IL</em>-1-mediated NF-kappaB activation, whereas coexpression of <em>IL</em>-1RI with membrane-anchored s<em>IL</em>-<em>1RAcP</em> restores the capacity of the cells to respond to <em>IL</em>-1. This suggests that s<em>IL</em>-<em>1RAcP</em> may act as an inhibitor of <em>IL</em>-1 by directly interacting with <em>IL</em>-1RI to abolish its capacity to transduce signal.

Interleukin-33 (<em>IL</em>-33) is an unconventional member of the <em>IL</em>-1 family: it is a dual function cytokine. Many different cell types, tissue cells and leukocytes, produce <em>IL</em>-33 either constitutively or after stimulation and release it by a poorly defined molecular mechanism. Free <em>IL</em>-33 acts as a classical cytokine by binding to target cells expressing receptors for <em>IL</em>-33 minimally consisting of ST2 and <em>IL</em>-<em>1RAcP</em>. Depending on the target cell type <em>IL</em>-33 will stimulate cell-type specific signal transduction mechanisms and thereby change the biosynthetic profile of the respective cell. In addition, it is stored in the nucleus of cells and may be released after cell stress, death by injury or necrosis, acting as an alarmin by orchestrating a sterile inflammation. Furthermore, <em>IL</em>-33 has intracrine functions in the cell producing it, which are independent of <em>IL</em>-33 receptors. Intracellular <em>IL</em>-33 is predominantly found in the nucleus associated to the chromatin and may exert gene regulatory function by yet poorly defined mechanisms. It is the aim of this review to address two basic biological aspects of the <em>IL</em>-33/<em>IL</em>-33 receptor system. First, to summarize the current understanding of the fate and function of intracellular <em>IL</em>-33, and second, to discuss recent advances in the knowledge of the molecular composition, function and regulation of the <em>IL</em>-33 receptor complex, including initial signaling mechanisms.

Members of the interleukin-1 (<em>IL</em>-1) family of cytokines are key mediators in the regulation of host defence responses and the development of inflammation in response to acute and chronic injury to the brain. Two major agonists, <em>IL</em>-1alpha and <em>IL</em>-1beta, bind to a membrane receptor complex composed of the type-1 <em>IL</em>-1 receptor (<em>IL</em>-1RI) and the accessory protein (<em>IL</em>-<em>1RAcP</em>). The discovery of new orphan members of the <em>IL</em>-1 receptor superfamily (including ST2/T1, <em>IL</em>-1Rrp2, TIGIRR1 and -2, SIGGIR, <em>IL</em>-18Ralpha and <em>IL</em>-18Rbeta) has increased speculation that alternative <em>IL</em>-1 ligands signalling pathways exist in the brain. We demonstrate here that all the <em>IL</em>-1R-like orphan receptors are expressed by many brain cell types including astrocytes, microglia, oligodendrocytic progenitor cells and neurons. <em>IL</em>-18Rbeta expression was significantly increased in response to treatment of mixed glia with bacterial lipopolysaccharide (LPS) in vitro, whereas expression of <em>IL</em>-1Rrp2 and TIGIRR1 was reduced. Furthermore, <em>IL</em>-18Rbeta, <em>IL</em>-1Rrp2, but not TIGIRR1 expression, was increased in the brain in vivo in response to peripheral administration of LPS or middle cerebral artery occlusion (MCA). These results suggest possible roles for newly identified members of the <em>IL</em>-1 receptor family in CNS diseases.

Mitogen-activated protein kinase (MAPK) activation controls diverse cellular functions including cellular survival, proliferation, and apoptosis. Tuning of MAPK activation is counter-regulated by a family of dual-specificity phosphatases (DUSPs). <em>IL</em>-33 is a recently described cytokine that initiates Th2 immune responses through binding to a heterodimeric <em>IL</em>-33Rα (ST2L)/<em>IL</em>-1α accessory protein (<em>IL</em>-<em>1RAcP</em>) receptor that coordinates activation of ERK and NF-κB pathways. We demonstrate here that DUSP5 is expressed in eosinophils, is upregulated following <em>IL</em>-33 stimulation and regulates <em>IL</em>-33 signaling. Dusp5(-/-) mice have prolonged eosinophil survival and enhanced eosinophil effector functions following infection with the helminth Nippostrongylus brasiliensis. <em>IL</em>-33-activated Dusp5(-/-) eosinophils exhibit increased cellular ERK1/2 activation and BCL-XL expression that results in enhanced eosinophil survival. In addition, Dusp5(-/-) eosinophils demonstrate enhanced <em>IL</em>-33-mediated activation and effector functions. Together, these data support a role for DUSP5 as a novel negative regulator of <em>IL</em>-33-dependent eosinophil function and survival.

BACKGROUND

<em>IL</em>-33, a member of the <em>IL</em>-1 family of cytokines, provokes Th2-type inflammation accompanied by accumulation of eosinophils through <em>IL</em>-33R, which consists of ST2 and <em>IL</em>-<em>1RAcP</em>. We previously demonstrated that macrophages produce <em>IL</em>-33 in response to LPS. Some immune responses were shown to differ between ST2-deficient mice and soluble ST2-Fc fusion protein-treated mice. Even in anti-ST2 antibody (Ab)-treated mice, the phenotypes differed between distinct Ab clones, because the characterization of such Abs (i.e., depletion, agonistic or blocking Abs) was unclear in some cases.

RESULTS

To elucidate the precise role of IL-33, we newly generated neutralizing monoclonal Abs for IL-33. Exogenous IL-33 potentiated LPS-mediated cytokine production by macrophages. That LPS-mediated cytokine production by macrophages was suppressed by inhibition of endogenous IL-33 by the anti-IL-33 neutralizing mAbs.

CONCLUSIONS

Our findings suggest that LPS-mediated macrophage activation is accelerated by macrophage-derived paracrine IL-33 stimulation.

The interleukin (<em>IL</em>)-36 cytokines include 3 agonists, <em>IL</em>-36α, <em>IL</em>-36β, and <em>IL</em>-36γ that bind to a common receptor composed of <em>IL</em>-36R and <em>IL</em>-<em>1RAcP</em> to stimulate inflammatory responses. <em>IL</em>-36Ra is a natural antagonist that binds to <em>IL</em>-36R, but does not recruit the co-receptor <em>IL</em>-<em>1RAcP</em> and does not stimulate any intracellular responses. The <em>IL</em>-36 cytokines are expressed predominantly by epithelial cells and act on a number of cells including immune cells, epithelial cells, and fibroblasts. Processing of the N-terminus is required for full agonist or antagonist activity for all <em>IL</em>-36 members. The role of <em>IL</em>-36 has been extensively demonstrated in the skin where it can act on keratinocytes and immune cells to induce a robust inflammatory response that has been implicated in psoriatic disorders. Emerging data also suggest a role for this cytokine family in pulmonary and intestinal physiology and pathology.

Inflammatory cytokine interleukin-1 (<em>IL</em>-1) performs multiple functions in the central nervous system. The type 1 <em>IL</em>-1 receptor (<em>IL</em>-1R1) and the <em>IL</em>-1 receptor accessory protein (<em>IL</em>-<em>1RAcP</em>) form a functional <em>IL</em>-1 receptor complex that is thought to mediate most, if not all, <em>IL</em>-1-induced effects. Several recent studies, however, suggest the existence of a heretofore-unidentified receptor for <em>IL</em>-1. In this study, we report that the <em>IL</em>-1R1 gene contains an internal promoter that drives the transcription of a shortened <em>IL</em>-1R1 mRNA. This mRNA is the template for a unique <em>IL</em>-1R protein that is identical to <em>IL</em>-1R1 at the C terminus, but with a shorter extracellular domain at the N terminus. We have termed this molecule <em>IL</em>-1R3. The mRNA and protein for <em>IL</em>-1R3 are expressed in normal and two strains of commercially available <em>IL</em>-1R1 knockout mice. Western blot analysis shows <em>IL</em>-1R3 is preferentially expressed in neural tissues. Furthermore, <em>IL</em>-1β binds specifically to <em>IL</em>-1R3 when it is complexed with the newly discovered alternative <em>IL</em>-1 receptor accessory protein, <em>IL</em>-<em>1RAcP</em>b. Stimulation of neurons expressing both <em>IL</em>-1R3 and <em>IL</em>-<em>1RAcP</em>b with <em>IL</em>-1β causes fast activation of the Akt kinase, which leads to an increase in voltage-gated potassium current. These results demonstrate that <em>IL</em>-1R3/<em>IL</em>-<em>1RAcP</em>b complex mediates a unique subset of <em>IL</em>-1 activity that accounts for many previously unexplained <em>IL</em>-1 effects in the central nervous system.