Interleukin-36-receptor antagonist deficiency and generalized pustular psoriasis

Unexplained recurrent fever: when is autoinflammation the explanation?

Recurrent fever can be the sole or leading manifestation of a variety of diseases including malignancies, autoimmune diseases and infections. Because the differential diagnoses are manifold, no formal guidelines for the approach of patients with recurrent fever exists. The newly recognized group of autoinflammatory diseases are often accompanied by repetitive fever attacks. As these episodes are frequently associated by a variety of divergent presentations, the differentiation of other causes for febrile illnesses can be difficult. In this article, we first review disease entities, which frequently present with the symptom of recurrent fever. In a next step, we summarize their characteristic pattern of disease presentation. Finally, we analyse key features of autoinflammatory diseases, which are helpful to distinguish this group of diseases from the other causes of recurrent fever. Recognizing these symptom patterns can provide the crucial clues and, thus, lead to the initiation of targeted specific diagnostic tests and therapies.

Autoinflammation: From monogenic syndromes to common skin diseases

Autoinflammation is characterized by aberrant regulation of the innate immune system and often manifests as periodic fevers and systemic inflammation involving multiple organs, including the skin. Mutations leading to abnormal behavior or activity of the interleukin 1 beta (IL-1ß)-processing inflammasome complex have been found in several rare autoinflammatory syndromes, for which anticytokine therapy such as IL-1 or tumor necrosis factor-alfa inhibition may be effective. It is becoming clear that features of autoinflammation also affect common dermatoses, some of which were previously thought to be solely autoimmune in origin (eg, vitiligo, systemic lupus erythematosus). Recognizing the pathogenetic role of autoinflammation can open up new avenues for the targeted treatment of complex, inflammatory dermatoses.

Putting together the psoriasis puzzle: an update on developing targeted therapies

Psoriasis vulgaris is a chronic, debilitating skin disease that affects millions of people worldwide. There is no mouse model that accurately reproduces all facets of the disease, but the accessibility of skin tissue from patients has facilitated the elucidation of many pathways involved in the pathogenesis of psoriasis and highlighted the importance of the immune system in the disease. The pathophysiological relevance of these findings has been supported by genetic studies that identified polymorphisms in genes associated with NFκB activation, IL-23 signaling and T helper 17 (Th17)-cell adaptive immune responses, and in genes associated with the epidermal barrier. Recently developed biologic agents that selectively target specific components of the immune system are highly effective for treating psoriasis. In particular, emerging therapeutics are focused on targeting the IL-23–Th17-cell axis, and several agents that block IL-17 signaling have shown promising results in early-phase clinical trials. This review discusses lessons learned about the pathogenesis of psoriasis from mouse-and patient-based studies, emphasizing how the outcomes of clinical trials with T-cell-targeted and cytokine-blocking therapies have clarified our understanding of the disease.

Monogenic causes of inflammatory disease in rheumatology

PURPOSE OF REVIEW

To review the single-gene defects that can mimic rheumatologic diseases.

RECENT FINDINGS

Monogenic disorders can cause a variety of diseases that may be seen by a rheumatologist. Many of these illnesses present with recurrent episodes of arthritis, rash, fever and inflammation, and serositis. Recent discoveries have defined inflammatory diseases due to mutations in the IL-1 and IL-36 receptor antagonists, as well as the immunoproteosome. Further study of well defined monogenic causes of inflammatory diseases, such as FMF, PAPA, TRAPS, and HIDS, has elucidated the pathophysiology of these diseases leading to targeted immunotherapy with anticytokine biological medications.

SUMMARY

A rheumatologist should be aware of the genetic causes of inflammatory disease mimics. This will not only help with the prognosis of these diseases, but also help to guide therapy to prevent long-term complications associated with these disorders.

The IL-23/T17 pathogenic axis in psoriasis is amplified by keratinocyte responses

Psoriasis is a complex inflammatory process resulting from activation of the well-defined interleukin (IL)-23/T17 cytokine axis. We review the role of key cytokines IL-17 and IL-23 in psoriasis, as well as tumor necrosis factor (TNF)α, focusing on therapeutic cytokine interventions and what they reveal about psoriatic inflammation. The potential role of recently described epidermal IL-36RN and CARD14 genetic mutations in psoriasis pathogenesis is also explored, because they augment keratinocyte responses to proinflammatory cytokines. The discovery of these genetic mutations in familial and pustular psoriasis suggests new links between cytokine-induced gene products and IL-1 family members from keratinocytes, which may regulate features of the disease, including epidermal hyperplasia and neutrophil infiltrating responses.

The novel cytokine interleukin-36α is expressed in psoriatic and rheumatoid arthritis synovium

BACKGROUND

Interleukin (IL)-36α is a recently described member of the IL-1 cytokine family with pro-inflammatory and clearly pathogenic properties in psoriasis.

OBJECTIVE

To determine the IL-36α expression in psoriatic arthritis (PsA) compared to rheumatoid arthritis (RA) and osteoarthritis (OA).

METHODS

Synovial tissues obtained from arthritis patients were stained for IL-36α, IL-36 receptor (IL-36R) and IL-36R antagonist (IL-36Ra) by immunohistochemistry and immunofluorescence. Lysates were examined for IL-36α by western blot analysis. Synovial fibroblasts (FLS) cultured in the presence of IL-36α were assayed for cytokine expression by quantitative real time PCR and multiplex assay. IL-36α-induced signal transduction in FLS was analysed by immunoblotting.

RESULTS

Expression of IL-36R and its ligands IL-36α and IL-36Ra was detected in the synovial lining layer and cellular infiltrates of patients with inflammatory arthritis. IL-36α was expressed significantly higher in PsA and RA than in OA synovium. CD138-positive plasma cells were identified as the main cellular source of IL-36α. No differences were observed for the expression of IL-36R and IL-36Ra between PsA, RA and OA. Functionally, IL-36α induced the expression of IL-6 and IL-8 in FLS through p38/NFkB activation.

CONCLUSIONS

IL-36α is up-regulated in PsA and RA synovium, expressed by tissue plasma cells and leads to IL-6 and IL-8 production by synovial fibroblasts. Hence, IL-36α links plasma cells to inflammatory cytokine production by FLS and may represent a key link between autoimmunity and the induction of synovitis.

The IL-36 receptor pathway regulates Aspergillus fumigatus-induced Th1 and Th17 responses

IL-1 drives Th responses, particularly Th17, in host defense. Sharing the same co-receptor, the IL-1 family member IL-36 exhibits properties similar to those of IL-1. In the present study, we investigated the role of IL-36 in Aspergillus fumigatus-induced human Th responses. We observed that different morphological forms of A. fumigatus variably increase steady-state mRNA of IL-36 subfamily members. IL-36α is not significantly induced by any morphological form of Aspergillus. Most strikingly, IL-36γ is significantly induced by live A. fumigatus conidia and heat-killed hyphae, whereas IL-36Ra (IL-36 receptor antagonist) is significantly induced by heat-killed conidia, hyphae, and live conidia. We also observed that IL-36γ expression is dependent on the dectin-1/Syk and TLR4 signaling pathway. In contrast, TLR2 and CR3 inhibit IL-36γ expression. The biological relevance of IL-36 induction by Aspergillus is demonstrated by experiments showing that inhibition of the IL-36 receptor by IL-36Ra reduces Aspergillus-induced IL-17 and IFN-γ. These data describe that IL-36-dependent signals are a novel cytokine pathway that regulates Th responses induced by A. fumigatus, and demonstrate a role for TLR4 and dectin-1 in the induction of IL-36γ.

Psoriasiform dermatitis is driven by IL-36-mediated DC-keratinocyte crosstalk

Psoriasis is a chronic inflammatory disorder of the skin affecting approximately 2% of the world's population. Accumulating evidence has revealed that the IL-23/IL-17/IL-22 pathway is key for development of skin immunopathology. However, the role of keratinocytes and their crosstalk with immune cells at the onset of disease remains poorly understood. Here, we show that IL-36R-deficient (Il36r-/-) mice were protected from imiquimod-induced expansion of dermal IL-17-producing γδ T cells and psoriasiform dermatitis. Furthermore, IL-36R antagonist-deficient (Il36rn-/-) mice showed exacerbated pathology. TLR7 ligation on DCs induced IL-36-mediated crosstalk with keratinocytes and dermal mesenchymal cells that was crucial for control of the pathological IL-23/IL-17/IL-22 axis and disease development. Notably, mice lacking IL-23, IL-17, or IL-22 were less well protected from disease compared with Il36r-/- mice, indicating an additional distinct activity of IL-36 beyond induction of the pathological IL-23 axis. Moreover, while the absence of IL-1R1 prevented neutrophil infiltration, it did not protect from acanthosis and hyperkeratosis, demonstrating that neutrophils are dispensable for disease manifestation. These results highlight a central and unique IL-1-independent role for IL-36 in control of the IL-23/IL-17/IL-22 pathway and development of psoriasiform dermatitis.

IL-36γ/IL-1F9, an innate T-bet target in myeloid cells

By concerted action in dendritic (DC) and T cells, T-box expressed in T cells (T-bet, Tbx21) is pivotal for initiation and perpetuation of Th1 immunity. Identification of novel T-bet-regulated genes is crucial for further understanding the biology of this transcription factor. By combining siRNA technology with genome-wide mRNA expression analysis, we sought to identify new T-bet-regulated genes in predendritic KG1 cells activated by IL-18. One gene robustly dependent on T-bet was IL-36γ, a recently described novel IL-1 family member. Promoter analysis revealed a T-bet binding site that, along with a κB site, enables efficient IL-36γ induction. Using knock-out animals, IL-36γ reliance on T-bet was extended to murine DC. IL-36γ expression by human myeloid cells was confirmed using monocyte-derived DC and M1 macrophages. The latter model was employed to substantiate dependence of IL-36γ on endogenous T-bet in human primary cells. Ectopic expression of T-bet likewise mediated IL-36γ production in HaCaT keratinocytes that otherwise lack this transcription factor. Additional experiments furthermore revealed that mature IL-36γ has the capability to establish an inflammatory gene expression profile in human primary keratinocytes that displays enhanced mRNA levels for TNFα, CCL20, S100A7, inducible NOS, and IL-36γ itself. Data presented herein shed further light on involvement of T-bet in innate immunity and suggest that IL-36γ, besides IFNγ, may contribute to functions of this transcription factor in immunopathology.

Mutation analysis of the IL36RN gene in 14 Japanese patients with generalized pustular psoriasis

Generalized pustular psoriasis (GPP) is a rare, potentially life threatening, and aggressive form of psoriasis, which is characterized by sudden onset with repeated episodic skin inflammation leading to pustule formation. Familial GPP is known to be caused by recessively inherited mutations in the IL36RN gene, which encodes interleukin 36 receptor antagonist (IL-36Ra). In this article, we performed mutation analysis of the IL36RN gene in 14 Japanese patients with GPP, and identified mutations in two of these patients analyzed. One patient was compound heterozygous for mutations c.115+6T>C and c.368C>G (p.Thr123Arg), whereas the other carried compound heterozygous mutations c.28C>T (p.Arg10*) and c.115+6T>C in the IL36RN gene. Expression studies using total RNA from the patients' skin revealed that the mutation c.115+6T>C resulted in skipping of exon 3, leading to a frameshift and a premature termination codon (p.Arg10Argfs*1). The protein structure analysis suggested that the missense mutation p.Thr123Arg caused misfolding and instability of IL-36Ra protein. In vitro studies in cultured cells showed impaired expression of the p.Thr123Arg mutant IL-36Ra protein, which failed to antagonize the IL-36 signaling pathway. Our data further underscore the critical role of IL36RN in pathogenesis of GPP.

IL-36α exerts pro-inflammatory effects in the lungs of mice

Interleukin (IL-) 36 cytokines (previously designated as novel IL-1 family member cytokines; IL-1F5- IL-1F10) constitute a novel cluster of cytokines structurally and functionally similar to members of the IL-1 cytokine cluster. The effects of IL-36 cytokines in inflammatory lung disorders remains poorly understood. The current study sought to investigate the effects of IL-36α (IL-1F6) and test the hypothesis that IL-36α acts as a pro-inflammatory cytokine in the lung in vivo. Intratracheal instillation of recombinant mouse IL-36α induced neutrophil influx in the lungs of wild-type C57BL/6 mice and IL-1αβ(-/-) mice in vivo. IL-36α induced neutrophil influx was also associated with increased mRNA expression of neutrophil-specific chemokines CXCL1 and CXCL2 in the lungs of C57BL/6 and IL-1αβ(-/-) mice in vivo. In addition, intratracheal instillation of IL-36α enhanced mRNA expression of its receptor IL-36R in the lungs of C57BL/6 as well as IL-1αβ(-/-) mice in vivo. Furthermore, in vitro incubation of CD11c(+) cells with IL-36α resulted in the generation of neutrophil-specific chemokines CXCL1, CXCL2 as well as TNFα. IL-36α increased the expression of the co-stimulatory molecule CD40 and enhanced the ability of CD11c(+) cells to induce CD4(+) T cell proliferation in vitro. Furthermore, stimulation with IL-36α activated NF-κB in a mouse macrophage cell line. These results demonstrate that IL-36α acts as a pro-inflammatory cytokine in the lung without the contribution of IL-1α and IL-1β. The current study describes the pro-inflammatory effects of IL-36α in the lung, demonstrates the functional redundancy of IL-36α with other agonist cytokines in the IL-1 and IL-36 cytokine cluster, and suggests that therapeutic targeting of IL-36 cytokines could be beneficial in inflammatory lung diseases.

IL-36 signaling amplifies Th1 responses by enhancing proliferation and Th1 polarization of naive CD4+ T cells

The interleukin-1 (IL-1) superfamily of cytokines comprises a set of pivotal mediators of inflammation. Among them, the action of IL-36 cytokines in immune responses has remained elusive. In a recent study, we demonstrated a direct effect of IL-36 on immune cells. Here we show that, among T cells, the IL-36 receptor is predominantly expressed on naive CD4(+) T cells and that IL-36 cytokines act directly on naive T cells by enhancing both cell proliferation and IL-2 secretion. IL-36β acts in synergy with IL-12 to promote Th1 polarization and IL-36 signaling is also involved in mediating Th1 immune responses to Bacillus Calmette-Guerin infection in vivo. Our findings point toward a critical function of IL-36 in the priming of Th1 cell responses in vitro, and in adaptive immunity in a model of mycobacterial infection in vivo.

Identification, cloning and characterisation of interleukin-1F5 (IL-36RN) in the chicken

The human IL-1 family contains eleven genes encoded at three separate loci. Nine, including IL-36 receptor antagonist (IL-36RN), also known as IL-1F5, are present at a single locus on chromosome 2, whereas IL-18 and IL-33 lie on chromosomes 11 and 9 respectively. There are currently only three known orthologues in the chicken - IL-1β, IL-18 and IL-1RN - which are encoded on chromosomes 22, 24 and unplaced, respectively. A novel chicken IL-1 family sequence representing IL-36RN (IL-1F5) was initially identified from an expressed sequence tag (EST) library by its similarity to both chicken IL-1RN and chicken IL-1β. Following isolation of the cDNA from the liver of an uninfected bird, a number of unique sequence features were identified. The predicted protein has a longer NH(2)-terminus than the human protein; however, as in mammals, this region contains neither a prodomain nor a signal peptide. A putative nuclear export sequence is also apparent, yet a similar motif is absent in mammalian IL-36RN. Although chIL-36RN exhibits low homology with its mammalian orthologues, it encodes a predicted β-trefoil structure whose β-strands are conserved with those of the mouse sequence. Unlike in mammals, chIL-36RN expression was constitutive in all tissues and cell subsets examined. In response to viral infection, expression was significantly downregulated in a line of birds which are susceptible to the virus. Chicken IL-36RN, like chIL-1RN, is not encoded at the chIL-1β locus, further emphasising the genomic fragmentation of the large IL-1 gene cluster found in mammals. This suggests differential evolution of this cytokine family since the divergence of birds and mammals from a common ancestor, and underlines the difficulty of determining the full repertoire of chIL-1 family members.

[Pustular psoriasis]

The pustular forms o f psoriasis make up a heterogeneous entity from a clinical point of view. However, the existence of clinically detectable aseptic pustular lesions is common to all these forms. How they are related to plaque psoriasis, also called psoriasis vulgaris, the most frequent form of psoriasis, and the genetic and molecular mechanisms recently updated in some forms, have renewed interest in these pustular forms, resulting in reconsideration of their place within cutaneous and systemic inflammatory diseases.

Lighting the fires within: the cell biology of autoinflammatory diseases

Autoinflammatory diseases are characterized by seemingly unprovoked pathological activation of the innate immune system in the absence of autoantibodies or autoreactive T cells. Discovery of the causative mutations underlying several monogenic autoinflammatory diseases has identified key regulators of innate immune responses. Recent studies have highlighted the role of misfolding, oligomerization and abnormal trafficking of pathogenic mutant proteins in triggering autoinflammation, and suggest that more common rheumatic diseases may have an autoinflammatory component. This coincides with recent discoveries of new links between endoplasmic reticulum stress and inflammatory signalling pathways, which support the emerging view that autoinflammatory diseases may be due to pathological dysregulation of stress-sensing pathways that normally function in host defence.

Current understanding of the pathogenesis and management of chronic recurrent multifocal osteomyelitis

Chronic recurrent multifocal osteomyelitis (CRMO) is an inflammatory disorder that primarily affects children. Its hallmark is recurring episodes of sterile osteomyelitis. The clinical presentation is insidious onset of bone pain with or without fever. Laboratory studies typically reveal nonspecific evidence of inflammation. Radiologic imaging and histologic appearance resemble those of infectious osteomyelitis. There is a strong association with inflammatory disorders of the skin and intestinal tract in affected individuals and their close relatives, suggesting a shared pathophysiology and supporting a genetic component to disease susceptibility. Two genetic syndromes have CRMO as a prominent phenotype-Majeed syndrome and deficiency of the interleukin-1 receptor antagonist-and suggest that interleukin-1 may be a key cytokine in disease pathogenesis. This review briefly summarizes the main clinical and radiologic aspects of the disease and then focuses on genetics and pathophysiology and provides an update on treatment.

Epidermal ADAM17 maintains the skin barrier by regulating EGFR ligand-dependent terminal keratinocyte differentiation

ADAM17 (a disintegrin and metalloproteinase 17) is ubiquitously expressed and cleaves membrane proteins, such as epidermal growth factor receptor (EGFR) ligands, l-selectin, and TNF, from the cell surface, thus regulating responses to tissue injury and inflammation. However, little is currently known about its role in skin homeostasis. We show that mice lacking ADAM17 in keratinocytes (A17(ΔKC)) have a normal epidermal barrier and skin architecture at birth but develop pronounced defects in epidermal barrier integrity soon after birth and develop chronic dermatitis as adults. The dysregulated expression of epidermal differentiation proteins becomes evident 2 d after birth, followed by reduced transglutaminase (TGM) activity, transepidermal water loss, up-regulation of the proinflammatory cytokine IL-36α, and inflammatory immune cell infiltration. Activation of the EGFR was strongly reduced in A17(ΔKC) skin, and topical treatment of A17(ΔKC) mice with recombinant TGF-α significantly improved TGM activity and decreased skin inflammation. Finally, we show that mice lacking the EGFR in keratinocytes (Egfr(ΔKC)) closely resembled A17(ΔKC) mice. Collectively, these results identify a previously unappreciated critical role of the ADAM17-EGFR signaling axis in maintaining the homeostasis of the postnatal epidermal barrier and suggest that this pathway could represent a good target for treatment of epidermal barrier defects.

PSORS2 is due to mutations in CARD14

Psoriasis is a common, immune-mediated genetic disorder of the skin and is associated with arthritis in approximately 30% of cases. Previously, we localized PSORS2 (psoriasis susceptibility locus 2) to chromosomal region 17q25.3-qter after a genome-wide linkage scan in a family of European ancestry with multiple cases of psoriasis and psoriatic arthritis. Linkage to PSORS2 was also observed in a Taiwanese family with multiple psoriasis-affected members. In caspase recruitment domain family, member 14 (CARD14), we identified unique gain-of-function mutations that segregated with psoriasis by using genomic capture and DNA sequencing. The mutations c.349G>A (p.Gly117Ser) (in the family of European descent) and c.349+5G>A (in the Taiwanese family) altered splicing between CARD14 exons 3 and 4. A de novo CARD14 mutation, c.413A>C (p.Glu138Ala), was detected in a child with sporadic, early-onset, generalized pustular psoriasis. CARD14 activates nuclear factor kappa B (NF-kB), and compared with wild-type CARD14, the p.Gly117Ser and p.Glu138Ala substitutions were shown to lead to enhanced NF-kB activation and upregulation of a subset of psoriasis-associated genes in keratinocytes. These genes included chemokine (C-C motif) ligand 20 (CCL20) and interleukin 8 (IL8). CARD14 is localized mainly in the basal and suprabasal layers of healthy skin epidermis, whereas in lesional psoriatic skin, it is reduced in the basal layer and more diffusely upregulated in the suprabasal layers of the epidermis. We propose that, after a triggering event that can include epidermal injury, rare gain-of-function mutations in CARD14 initiate a process that includes inflammatory cell recruitment by keratinocytes. This perpetuates a vicious cycle of epidermal inflammation and regeneration, a cycle which is the hallmark of psoriasis.

[What's new in dermatological research?]

Dermatological research has been very active this year. Most of the numerous fields investigated involve the mechanisms of cutaneous regeneration and barrier function. A novel target of early ultraviolet-induced skin photodamage, the Syk kinase, has been recently identified. Synergistic relationship between telomere damage and cutaneous progerin production during cell senescence may also participate in the natural skin aging process. Interestingly, ultraviolet radiation induces an inhibitory effect on subcutaneous lipogenesis. Androgenetic alopecia or common baldness is not characterized by loss of hair follicle stem cells but by a defect in the conversion of hair follicle stem cells into active progenitor cells. It has been shown that the cornified envelope functions not only as a physicomechanical barrier, but also as both a biochemical line of antoxidant defense and an immunological line of defense. Like human papillomaviruses, Merckel cell polyomavirus belongs to the skin microbiome and different studies have demonstrated the protective role of epidermal resident microflora through the activation of innate immunity. Production of antimicrobial peptides and the activation of inflammasome and plasmacytoid dendritic cells are involved in the modulation of the cutaneous barrier function. Results from different studies suggest that IL-22 and IL-36 may be common mediators of both innate and adaptive immune responses. All these pathways interact not only to maintain cutaneous homeostasis and integrity (wound healing) but also to regulate autoinflammatory and autoimmune dermatoses (psoriasis, lupus, rosacea, atopic dermatitis, etc...). In addition, molecular mechanisms that regulate T helper type 2 differentiation and the retention at the site of inflammation of Th2 cells have been identified. New promising therapeutic targets for different chronic dermatosis are thus suggested. Mechanobiology and mechanotransduction are also emerging fields that investigate mechanical interactions between living cells and their environment and the conversion of mechanical cues into biochemical signals. Electronic second skin is now a current concept through bio-integrated epidermal electronics platforms used for different monitoring and stimulations of body functions.

Autoinflammatory syndromes

There has been an expansion of the autoinflammatory syndromes due to the discovery of new diseases related to mutations in genes regulating the innate immune system and the knowledge gained from these diseases as applied to more common nongenetic inflammatory conditions. Autoinflammatory syndromes are characterized by unprovoked (or triggered by minor events) recurrent episodes of systemic inflammation involving various body systems, which are often accompanied by fever. Inflammation is mediated by polymorphonuclear and macrophage cells through cytokines, particularly interleukin-1. This article reviews the clinical approach to patients with suspected autoinflammatory syndromes, several of the main and new (mostly genetics) syndromes, advances in treatment, and prognosis.

Immunology in clinic review series; focus on autoinflammatory diseases: update on monogenic autoinflammatory diseases: the role of interleukin (IL)-1 and an emerging role for cytokines beyond IL-1

OTHER THEMES PUBLISHED IN THIS IMMUNOLOGY IN THE CLINIC REVIEW SERIES Allergy, Host Responses, Cancer, Type 1 diabetes and viruses, Metabolic diseases.

SUMMARY

The disease-based discovery of the molecular basis for autoinflammatory diseases has led not only to a rapidly growing number of clinically and genetically identifiable disorders, but has unmantled key inflammatory pathways such as the potent role of the alarm cytokine interleukin (IL)-1 in human disease. Following its initial failures in the treatment of sepsis and the moderate success in the treatment of rheumatoid arthritis, IL-1 blocking therapies had a renaissance in the treatment of a number of autoinflammatory conditions, and IL-1 blocking therapies have been Food and Drug Administration (FDA)-approved for the treatment of the autoinflammatory conditions: cryopyrin-associated periodic syndromes (CAPS). CAPS and deficiency of the IL-1 receptor antagonist (DIRA), both genetic conditions with molecular defects in the IL-1 pathway, have provided a pathogenic rationale to IL-1 blocking therapies, and the impressive clinical results confirmed the pivotal role of IL-1 in human disease. Furthermore, IL-1 blocking strategies have shown clinical benefit in a number of other genetically defined autoinflammatory conditions, and diseases with clinical similarities to the monogenic disorders and not yet identified genetic causes. The discovery that IL-1 is not only triggered by infectious danger signals but also by danger signals released from metabolically 'stressed' or even dying cells has extended the concept of autoinflammation to disorders such as gout, and those that were previously not considered inflammatory, such as type 2 diabetes, coronary artery disease, obesity and some degenerative diseases, and provided the conceptual framework to target IL-1 in these diseases. Despite the tremendous success of IL-1 blocking therapy, the use of these agents in a wider spectrum of autoinflammatory conditions has uncovered disease subsets that are not responsive to IL-1 blockade, including the recently discovered proteasome-associated autoinflammatory syndromes such as chronic atypical neutrophilic dermatitis with lipodystrophy and elevated temperatures (CANDLE), Japanese autoinflammatory syndrome with lipodystrophy (JASL), Nakajo-Nishimura syndrome (NNS) and joint contractures, muscle atrophy, panniculitis induced lipodystrophy (JMP), and urge the continued quest to characterize additional dysregulated innate immune pathways that cause autoinflammatory conditions.

IL-36 in psoriasis

Psoriasis is a common but severe skin disease with significant health consequences, both physical and psychological. Evidence has emerged during the past several years pointing to a key role for IL-36 in psoriasis. Overexpression of IL-36 in mouse skin leads to a disease quite similar to human plaque psoriasis, and inhibition of IL-36 in human psoriatic skin ameliorates the inflammation. Loss of the natural antagonist of IL-36, IL-36Ra, results in a different, more severe skin disease known as pustular psoriasis. These effects are likely a consequence of the actions of IL-36 both on cells of the immune system as well as on components of skin including fibroblasts and keratinocytes.

Psoriasis and other complex trait dermatoses: from Loci to functional pathways

Driven by advances in molecular genetic technologies and statistical analysis methodologies, there have been huge strides taken in dissecting the complex genetic basis of many inflammatory dermatoses. One example is psoriasis, for which application of classical linkage analysis and genome-wide association investigation has identified genetic loci of major and minor effect. Although most loci independently have modest genetic effects, they identify important biological pathways potentially relevant to disease pathogenesis and therapeutic intervention. In the case of psoriasis, these appear to involve the epidermal barrier, NF-κB mechanisms, and T helper type 17 adaptive immune responses. The advent of next-generation sequencing methods will permit a more detailed and complete map of disease genetic architecture, a key step in developing personalized medicine strategies in the clinical management of the complex inflammatory dermatoses.

IL-38 binds to the IL-36 receptor and has biological effects on immune cells similar to IL-36 receptor antagonist

The functional role of IL-1 family member 10, recently renamed IL-38, remains unknown. In the present study we aimed to elucidate the biological function of IL-38 and to identify its receptor. Heat-killed Candida albicans was used to stimulate memory T-lymphocyte cytokine production in freshly obtained human peripheral blood mononuclear cells from healthy subjects. The addition of recombinant IL-38 (152 amino acids) inhibited the production of T-cell cytokines IL-22 (37% decrease) and IL-17 (39% decrease). The reduction in IL-22 and IL-17 caused by IL-38 was similar to that caused by the naturally occurring IL-36 receptor antagonist (IL-36Ra) in the same peripheral blood mononuclear cells cultures. IL-8 production induced by IL-36γ was reduced by IL-38 (42% decrease) and also was reduced by IL-36Ra (73% decrease). When human blood monocyte-derived dendritic cells were used, IL-38 as well as IL-36Ra increased LPS-induced IL-6 by twofold. We screened immobilized extracellular domains of each member of the IL-1 receptor family, including the IL-36 receptor (also known as "IL-1 receptor-related protein 2") and observed that IL-38 bound only to the IL-36 receptor, as did IL-36Ra. The dose-response suppression of IL-38 as well as that of IL-36Ra of Candida-induced IL-22 and IL-17 was not that of the classic IL-1 receptor antagonist (anakinra), because low concentrations were optimal for inhibiting IL-22 production, whereas higher concentrations modestly increased IL-22. These data provide evidence that IL-38 binds to the IL-36R, as does IL-36Ra, and that IL-38 and IL-36Ra have similar biological effects on immune cells by engaging the IL-36 receptor.

Rare hereditary autoinflammatory disorders: towards an understanding of critical in vivo inflammatory pathways

Hereditary autoinflammatory syndromes are monogenic disorders with an inborn error of innate immunity, and include periodic fever syndromes such as familial Mediterranean fever (FMF), tumor necrosis factor receptor-associated periodic syndrome and cryopyrin-associated periodic syndromes (CAPS), pyogenic diseases such as pyogenic arthritis, pyoderma gangrenosum and acne syndrome (PAPAS), and granulomatous diseases such as Blau syndrome. By identifying the genetic abnormalities and subsequent analyses of the molecular mechanisms underlying these disorders, several critical in vivo pathways for inflammatory processes have been discovered. In this review, three categories of autoinflammatory disorders are discussed: inflammasomopathies, receptor antagonist deficiencies and proteasome disability syndromes. Inflammasomopathies are diseases with dysregulated NLRP3 inflammasome activation, and include CAPS with NLRP3, FMF with MEFV, and PAPAS with PSTPIP1 mutations. Analyses of these diseases have clarified some critical pathways regulating NLRP3 inflammasome signaling. Receptor antagonist deficiencies include the newly defined deficiency for interleukin-1 receptor antagonist resulting in sterile multifocal osteomyelitis with periostosis and pustulosis, and deficiency for interleukin-36 receptor antagonist resulting in generalized pustular psoriasis. The identification of these genetic abnormalities has revealed a critical role for receptor antagonists of IL-1 family cytokines in regulating neutrophil activation/recruitment. Finally, proteasome disability syndromes with PSMB8 mutations include Nakajo-Nishimura syndrome and related disorders distributed globally. Analyses of these diseases have unexpectedly shown a critical role of the ubiquitin-proteasome system in the regulation or homeostasis of inflammation/metabolism. Since there still remain a number of predicted but undefined hereditary autoinflammatory syndromes, further clinical and genetic approaches are required to discover novel in vivo critical inflammatory pathways.

Interleukin-36 (IL-36) ligands require processing for full agonist (IL-36α, IL-36β, and IL-36γ) or antagonist (IL-36Ra) activity

IL-36α, IL-36β, and IL-36γ (formerly IL-1F6, IL-1F8, and IL-1F9) are IL-1 family members that signal through the IL-1 receptor family members IL-1Rrp2 (IL-1RL2) and IL-1RAcP. IL-36Ra (formerly IL-1F5) has been reported to antagonize IL-36γ. However, our previous attempts to demonstrate IL-36Ra antagonism were unsuccessful. Here, we demonstrate that IL-36Ra antagonist activity is dependent upon removal of its N-terminal methionine. IL-36Ra starting at Val-2 is fully active and capable of inhibiting not only IL-36γ but also IL-36α and IL-36β. Val-2 of IL-36Ra lies 9 amino acids N-terminal to an A-X-Asp motif conserved in all IL-1 family members. In further experiments, we show that truncation of IL-36α, IL-36β, and IL-36γ to this same point increased their specific activity by ∼10(3)-10(4)-fold (from EC(50) 1 μg/ml to EC(50) 1 ng/ml). Inhibition of truncated IL-36β activity required ∼10(2)-10(3)-fold excess IL-36Ra, similar to the ratio required for IL-1Ra to inhibit IL-1β. Chimeric receptor experiments demonstrated that the extracellular (but not cytoplasmic) domain of IL-1Rrp2 or IL-1R1 is required for inhibition by their respective natural antagonists. IL-36Ra bound to IL-1Rrp2, and pretreatment of IL-1Rrp2-expressing cells with IL-36Ra prevented IL-36β-mediated co-immunoprecipitation of IL-1Rrp2 with IL-1RAcP. Taken together, these results suggest that the mechanism of IL-36Ra antagonism is analogous to that of IL-1Ra, such that IL-36Ra binds to IL-1Rrp2 and prevents IL-1RAcP recruitment and the formation of a functional signaling complex. In addition, truncation of IL-36α, IL-36β, and IL-36γ dramatically enhances their activity, suggesting that post-translational processing is required for full activity.