IL-22 induces lipopolysaccharide-binding protein in hepatocytes: a potential systemic role of IL-22 in Crohn's disease

Regulation and function of proinflammatory TH17 cells

Naïve CD4(+) helper T (TH) cells, upon activation by antigen-presenting cells (APC), differentiate into different types of effector cells that are characterized by their distinct cytokine production profiles and immune regulatory functions. In addition to TH1 and TH2 cells, a third subset of effector TH cells has recently been described and termed TH17. Since their identification, TH17 cells have emerged as crucial players in infectious, inflammatory, and autoimmune diseases, and cancer. In this review, we summarize the latest discoveries on the cytokine-mediated regulation and transcriptional programming of TH17 cells and their roles in different immune responses and diseases.

Structure of IL-22 bound to its high-affinity IL-22R1 chain

IL-22 is an IL-10 family cytokine that initiates innate immune responses against bacterial pathogens and contributes to immune disease. IL-22 biological activity is initiated by binding to a cell-surface complex composed of IL-22R1 and IL-10R2 receptor chains and further regulated by interactions with a soluble binding protein, IL-22BP, which shares sequence similarity with an extracellular region of IL-22R1 (sIL-22R1). IL-22R1 also pairs with the IL-20R2 chain to induce IL-20 and IL-24 signaling. To define the molecular basis of these diverse interactions, we have determined the structure of the IL-22/sIL-22R1 complex. The structure, combined with homology modeling and surface plasmon resonance studies, defines the molecular basis for the distinct affinities and specificities of IL-22 and IL-10 receptor chains that regulate cellular targeting and signal transduction to elicit effective immune responses.

IL-23 and Th17 cytokines in intestinal homeostasis

The discovery of the Th1/Th2 paradigm of CD4(+) T-cell subsets redefined our understanding of immunity by highlighting the essential roles of cytokine networks in the induction and regulation of immune responses. Most recently, the identification of an additional subset, known as Th17 cells, has further illustrated the complexity and diversity of effector CD4(+) T cells. Th17 responses have been closely associated with the cytokine interleukin (IL)-23 and, although originally pinpointed as having a deleterious role in autoimmune tissue pathology, the IL-23/Th17 axis has also been associated with protective immunity at mucosal surfaces. Recent progress has highlighted the heterogeneous nature of Th17 responses, has demonstrated diverse cellular sources for Th17-associated cytokines, and has begun to dissect the individual roles of these cytokines in different disease processes. Here, we will review the evidence linking the IL-23/Th17 axis to chronic intestinal inflammation and also will discuss its beneficial roles in intestinal protection and homeostasis.

Gene expression study of IL10 family genes in vitiligo skin biopsies, peripheral blood mononuclear cells and sera

BACKGROUND

Vitiligo is a pigmentation disorder, the cause of which is complex and not yet fully understood. There is a significant change of epidermal cytokines in involved skin of patients with vitiligo compared with uninvolved skin and skin of healthy controls, thus suggesting a possible involvement of cytokines in the pathogenesis of vitiligo.

OBJECTIVES

To evaluate potential roles of IL10 family cytokines (IL10, IL19, IL20, IL22 and IL24) in vitiligo. Along with the selected cytokines, we investigated subunits of the receptors (IL10RA, IL10RB, IL20RA and IL22RA1) which are involved in the signalling pathway of the cytokines.

METHODS

Quantitative real-time polymerase chain reaction was used to detect mRNA expression levels in samples extracted from skin biopsies and peripheral blood mononuclear cells and an enzyme-linked immunosorbent assay was used to measure protein concentrations in serum from patients with vitiligo and healthy controls.

RESULTS

IL22 is significantly associated with vitiligo, especially with the active stage of vitiligo, as shown by results of mRNA expression and supported by results of protein level in sera. IL22 may provoke inflammation which leads to destruction of melanocytes.

CONCLUSIONS

The actual role of IL22 during pathogenesis of vitiligo remains to be better characterized. Signal transductions of other investigated cytokines seem to be regulated on the expression level of their receptor complex subunits.

Osteopontin as two-sided mediator of intestinal inflammation

Osteopontin (OPN) is characterized as a major amplifier of Th1-immune responses. However, its role in intestinal inflammation is currently unknown. We found considerably raised OPN levels in blood of wild-type (WT) mice with dextran sodium sulfate (DSS)-induced colitis. To identify the role of this mediator in intestinal inflammation, we analysed experimental colitis in OPN-deficient (OPN(-/-)) mice. In the acute phase of colitis these mice showed more extensive colonic ulcerations and mucosal destruction than WT mice, which was abrogated by application of soluble OPN. Within the OPN(-/-) mice, infiltrating macrophages were not activated and showed impaired phagocytosis. Reduced mRNA expression of interleukin (IL)-1 beta and matrix metalloproteinases was found in acute colitis of OPN(-/-) mice. This was associated with decreased blood levels of IL-22, a Th17 cytokine that may mediate epithelial regeneration. However, OPN-(/-) mice showed increased serum levels of tumour necrosis factor (TNF)-alpha, which could be due to systemically present lipopolysaccharide translocated to the gut. In contrast to acute colitis, during chronic DSS-colitis, which is driven by a Th1 response of the lamina propria infiltrates, OPN(-/-) mice were protected from mucosal inflammation and demonstrated lower serum levels of IL-12 than WT mice. Furthermore, neutralization of OPN in WT mice abrogated colitis. Lastly, we demonstrate that in patients with active Crohn's disease OPN serum concentration correlated significantly with disease activity. Taken together, we postulate a dual function of OPN in intestinal inflammation: During acute inflammation OPN seems to activate innate immunity, reduces tissue damage and initiates mucosal repair whereas during chronic inflammation it promotes the Th1 response and strengthens inflammation.

Altered balance between Th17 and Th1 cells at mucosal sites predicts AIDS progression in simian immunodeficiency virus-infected macaques

Loss of CD4(+) T cells in the gut is necessary but not sufficient to cause AIDS in animal models, raising the possibility that a differential loss of CD4(+) T-cell subtypes may be important. We found that CD4(+) T cells that produce interleukin (IL)-17, a recently identified lineage of effector CD4(+) T-helper cells, are infected by SIV(mac251)in vitro and in vivo, and are found at lower frequency at mucosal and systemic sites within a few weeks from infection. In highly viremic animals, Th1 cells predominates over Th17 T cells and the frequency of Th17 cells at mucosal sites is negatively correlated with plasma virus level. Because Th17 cells play a central role in innate and adaptive immune response to extracellular bacteria, our finding may explain the chronic enteropathy in human immunodeficiency virus (HIV) infection. Thus, therapeutic approaches that reconstitute an adequate balance between Th1 and Th17 may be beneficial in the treatment of HIV infection.

The genetics and immunopathogenesis of inflammatory bowel disease

Genome-wide association studies efficiently and powerfully assay common genetic variation. The application of these studies to Crohn's disease has provided insight into the immunopathogenesis of this disease, implicating a role for genes of the innate and adaptive immune systems. In this Review, I discuss our current understanding of the genetics and immunopathogenesis of Crohn's disease and ulcerative colitis. Crohn's disease, but not ulcerative colitis, is associated with genetic variation in NOD2 and an autophagy gene, ATG16L1, both of which affect the intracellular processing of bacterial components. By contrast, variation in the gene encoding the interleukin-23 (IL-23) receptor subunit, as well as in the IL12B, STAT3 and NKX2-3 gene regions, is associated with both Crohn's disease and ulcerative colitis. Comparative analyses of gene associations between these two inflammatory bowel diseases reveal common and unique mechanisms of their immunopathogenesis.

Intestinal epithelial cell up-regulation of LY6 molecules during colitis results in enhanced chemokine secretion

In the healthy colon, intestinal epithelial cells (IEC) form a physical barrier separating the myriad of gut Ags from the cells of the immune system. Simultaneously, IEC use several mechanisms to actively maintain immunologic tolerance to nonpathogenic Ags, including commensal bacteria. However, during inflammatory bowel disease (IBD), the line of defense provided by IEC is breached, resulting in uncontrolled immune responses. As IEC are a principal mediator of immune responses in the gut, we were interested in discerning the gene expression pattern of IEC during development and progression of IBD. Laser capture microdissection and microarray analysis were combined to identify the LY6 superfamily as strongly up-regulated genes in inflamed IEC of the colon in two models of murine colitis. Surface expression of LY6A and LY6C on IEC is induced by several cytokines present within the colitic gut, including IL-22 and IFN-gamma. Furthermore, cross-linking of LY6C results in production of a number of chemokines which are known to be involved in the immunopathogenesis of IBD. Increased chemokine production was cholesterol dependent, suggesting a role for lipid raft structures in the mechanism. As such, LY6 molecules represent novel targets to down-regulate chemokine expression in the colon and limit subsequent inflammation associated with IBD.

Innate and adaptive immune responses related to IBD pathogenesis

Although the adaptive immune system traditionally has been the primary focus of investigations into the pathogenesis of inflammatory bowel disease (IBD), it is now clear that innate immune responses play an equally important, or perhaps even primary, role in disease initiation. Intestinal barrier function defects and genetic associations with the nucleotide-binding oligomerization domain and Toll-like receptor pathways suggest that the innate immune system has failed to protect the host against the vast array of commensal bacteria in the gut. This hypothesis is supported further by the observation that probiotic agents exert anti-inflammatory effects in the intestine through stimulation, rather than suppression, of the mucosal innate immune system. Moreover, it is now clear that adaptive immune responses involved in IBD pathogenesis are more complex than the traditionally dichotomous Th1/Th2 paradigm. Finally, mounting evidence suggests that the Th17 effector pathway may contribute to Crohn's disease pathogenesis.

Inflammatory bowel disease, past, present and future: lessons from animal models

Accumulating data from animal models indicate that Inflammatory bowel disease (IBD) is mediated by a much more complicated mechanism than previously predicted. For example, the role of an individual molecule in the pathogenesis of IBD distinctly differs depending on several factors, including the fundamental mechanism of induction of the disease, the target cell type, the phase of disease, and the environment. Therefore, it has been difficult in the past to fully explain the complicated mechanism. Novel concepts have recently been proposed to further explain the complicated mechanism of IBD. In this review, we introduce past, current, and possible future concepts for IBD models regarding T helper (Th) 1, Th2, and Th17, antigen sampling and presentation, regulatory cell networks, NOD2, Toll-like receptors, bacteria/epithelia interaction, stem cells, autophagy, microRNAs, and glycoimmunology, and we also discuss the relevance of these new concepts, developed at the bench (in animal models), to the bedside.

The extended IL-10 superfamily: IL-10, IL-19, IL-20, IL-22, IL-24, IL-26, IL-28, and IL-29

Cytokines are involved in virtually every aspect of immunity and inflammation. A cascade of responses evolves after cytokine activation, although optimal function might ultimately involve several complementary cytokines. Understanding the function of individual cytokines is complicated because their role can vary depending on the cellular source, target, and phase of the immune response. In fact, numerous cytokines have both proinflammatory and anti-inflammatory potential, with the contrasting outcome observed being determined by the immune cells present and their state of responsiveness to the cytokine. These issues make the study of cytokine biology daunting, particularly so for IL-10 and IL-10-related genes. The IL-10 superfamily is highly pleiotropic. These genes are linked together through genetic similarity and intron-exon gene structure. Significant commonality exists not only through shared receptors but also through conserved signaling cascades. However, its members mediate diverse activities, including immune suppression, enhanced antibacterial and antiviral immunity, antitumor activity, and promotion of self-tolerance in autoimmune diseases.

Maturing dendritic cells are an important source of IL-29 and IL-20 that may cooperatively increase the innate immunity of keratinocytes

IL-19, IL-20, IL-22, IL-24, IL-26, IL-28, and IL-29 are new members of the IL-10 interferon family. Monocytes are well-known sources of IL-19, IL-20, and IL-24. We demonstrated here that monocytes also expressed IL-29, and monocyte differentiation into macrophages (Mphi) or dendritic cells (DCs) strongly changed their production capacity of these cytokines. Maturation of DCs with bacterial stimuli induced high expression of IL-28/IL-29 and IL-20. Simulated T cell interaction and inflammatory cytokines induced IL-29 and IL-20 in maturing DCs, respectively. Compared with monocytes, DCs expressed only minimal IL-19 levels and no IL-24. The differentiation of monocytes into Mphi reduced their IL-19 and terminated their IL-20, IL-24, and IL-29 production capacity. Like monocytes, neither Mphi nor DCs expressed IL-22 or IL-26. The importance of maturing DCs as a source of IL-28/IL-29 was supported by the much higher mRNA levels of these mediators in maturing DCs compared with those in CMV-infected fibroblasts, and the presence of IL-28 in lymph nodes but not in liver of lipopolysaccharide-injected mice. IL-19, IL-20, IL-22, IL-24, and IL-26 do not seem to affect Mphi or DCs as deduced from the lack of corresponding receptor chains. The significance of IL-20 and IL-28/IL-29 coexpression in maturing DCs may lie in the broadly amplified innate immunity in neighboring tissue cells like keratinocytes. In fact, IL-20 induced the expression of antimicrobial proteins, whereas IL-28/IL-29 enhanced the expression of toll-like receptors (TLRs) and the response to TLR ligands. However, the strongest response to TLR2 and TLR3 activation showed keratinocytes in the simultaneous presence of IL-20 and IL-29.

Regulation of FoxP3 regulatory T cells and Th17 cells by retinoids

Vitamin A has both positive and negative regulatory functions in the immune system. While vitamin A is required for normal formation of immune cells and epithelial cell barriers, vitamin A deficiency can lead to increased inflammatory responses and tissue damage. The mechanism with which vitamin A and its metabolites such as retinoids negatively regulate inflammatory responses has not been clearly defined. Recently, it has been established that retinoids promote the generation of immune-suppressive FoxP3⁺ regulatory T cells while they suppress the T cell differentiation into inflammatory Th17 cells in the periphery such as intestine. These novel functions of retinoids provide a potentially important immune regulatory mechanism. In this review, we discuss the functions of retinoids in the development of the FoxP3⁺ cells and Th17 cells, the phenotype and functions of retinoid-induced FoxP3⁺ T cells, and the impact of retinoid-induced FoxP3⁺ T cells on the immune tolerance.

The biological functions of T helper 17 cell effector cytokines in inflammation

T helper 17 (Th17) cells belong to a recently identified T helper subset, in addition to the traditional Th1 and Th2 subsets. These cells are characterized as preferential producers of interleukin-17A (IL-17A), IL-17F, IL-21, and IL-22. Th17 cells and their effector cytokines mediate host defensive mechanisms to various infections, especially extracellular bacteria infections, and are involved in the pathogenesis of many autoimmune diseases. The receptors for IL-17 and IL-22 are broadly expressed on various epithelial tissues. The effector cytokines of Th17 cells, therefore, mediate the crucial crosstalk between immune system and tissues, and play indispensable roles in tissue immunity.

The role of interleukin-22 in hepatitis C virus infection

In this study, we analyzed if IL-22 displays, similar to other IL-10 like cytokines such as IL-28A, antiviral properties in hepatic cells. Using RT-PCR and immunoblotting, we demonstrated that hepatic cell lines and primary hepatocytes express the functional IL-22 receptor complex consisting of IL-22R1 and IL-10R2. Hepatic IL-22 mRNA expression as measured by quantitative PCR was up-regulated in autoimmune and viral hepatitis compared to cholestatic liver diseases, while IL-22 serum levels did not differ significantly between patients with viral hepatitis and normal controls. IL-22 did not significantly change the expression levels of IFN-alpha/-beta and of the antiviral proteins MxA and 2',5'-OAS. Consequently, it had in comparison to IFN-alpha no relevant antiviral activity in in vitro models of HCV replication and infection. Taken together, hepatic IL-22 expression is up-regulated in viral hepatitis but IL-22 does not directly regulate antiviral proteins and has, in contrast to IFN-alpha, no effect on HCV replication.

Linking genetic susceptibility to Crohn's disease with Th17 cell function: IL-22 serum levels are increased in Crohn's disease and correlate with disease activity and IL23R genotype status

BACKGROUND

We analyzed the influence of Crohn's disease (CD)-associated IL23R gene variants on IL-22 that is expressed in IL-23R+ Th17 cells.

METHODS

IL-22 serum levels were measured in 242 CD patients and in 31 healthy controls. Subanalyses included serum levels of IL-6, TNF-alpha, IL-17A, IL-17F, C-reactive protein (CRP), and leukocyte count. In all patients, genotyping for 10 CD-associated IL23R single nucleotide polymorphisms (SNPs) and the 3 main CD-associated CARD15 variants was performed.

RESULTS

There was a highly significant increase in IL-22 serum expression in CD patients compared to healthy controls (P = 2.53 x 10(-9)). IL-22 serum levels correlated with disease activity: IL-22 levels in patients with a Crohn's disease activity index (CDAI) >150 were significantly higher than in patients with a CDAI <150 (P = 0.001), while TNF-alpha and IL-6 were not significantly different between these 2 groups. Analyzing the effect of 10 IL23R variants on IL-22 serum levels, we demonstrated that the quotients of mean IL-22 serum levels of carriers of the minor allele to the mean serum IL-22 in wildtype carriers correlated highly with the corresponding CD susceptibility risk for each gene variant (r = 0.807). The IL-22 levels in carriers of CD risk-increasing IL23R variants were significantly higher than in carriers of CD risk-decreasing IL23R variants (P = 0.008).

CONCLUSIONS

The Th17 cytokine IL-22 is expressed at high levels in CD and correlates with disease activity, offering a better separation between active and inactive CD than IL-6 and TNF-alpha. IL23R genotypes influence IL-22 serum expression, linking genetic CD susceptibility to Th17 cell function for the first time.

IL-22 ameliorates intestinal inflammation in a mouse model of ulcerative colitis

Expression of IL-22 is induced in several human inflammatory conditions, including inflammatory bowel disease (IBD). Expression of the IL-22 receptor is restricted to innate immune cells; however, the role of IL-22 in colitis has not yet been defined. We developed what we believe to be a novel microinjection-based local gene-delivery system that is capable of targeting the inflamed intestine. Using this approach, we demonstrated a therapeutic potency for IL-22-mediated activation of the innate immune pathway in a mouse model of Th2-mediated colitis that induces disease with characteristics similar to that of IBD ulcerative colitis (UC). IL-22 gene delivery enhanced STAT3 activation specifically within colonic epithelial cells and induced both STAT3-dependent expression of mucus-associated molecules and restitution of mucus-producing goblet cells. Importantly, IL-22 gene delivery led to rapid amelioration of local intestinal inflammation. The amelioration of disease by IL-22 was mediated by enhanced mucus production. In addition, local gene delivery was used to inhibit IL-22 activity through overexpression of IL-22-binding protein. Treatment with IL-22-binding protein suppressed goblet cell restitution during the recovery phase of a dextran sulfate sodium-induced model of acute colitis. These data demonstrate what we believe to be a novel function for IL-22 in the intestine and suggest the potency of a local IL-22 gene-delivery system for treating UC.

Targeting the development and effector functions of TH17 cells

T helper (TH) cells can assume different phenotypes characterized by the secretion of distinct effector molecules. Interferon-gamma producing TH1 and IL-4 producing TH2 cells have long been recognized as important mediators of host defense, whereas regulatory T cells are known to suppress T cell responses. Recently, TH17 cells were characterized as a novel CD4(+) subset that preferentially produces IL-17, IL-17F, and IL-22 as the signature cytokines. TH17 cells appear to play a critical role in sustaining the inflammatory response and their presence is closely associated with autoimmune disease, which makes them an attractive therapeutic target. In this review, we focus on the mechanisms that regulate the differentiation of naive T cells into TH17 cells and on TH17 effector cytokines, as they represent opportunities for therapeutic intervention.

Langerhans cell histiocytosis reveals a new IL-17A-dependent pathway of dendritic cell fusion

IL-17A is a T cell-specific cytokine that is involved in chronic inflammations, such as Mycobacterium infection, Crohn's disease, rheumatoid arthritis and multiple sclerosis. Mouse models have explained the molecular basis of IL-17A production and have shown that IL-17A has a positive effect not only on granuloma formation and neurodegeneration through unknown mechanisms, but also on bone resorption through Receptor activator of NF-kappaB ligand (RANKL) induction in osteoblasts. Langerhans cell histiocytosis (LCH) is a rare disease of unknown etiology, lacking an animal model, that cumulates symptoms that are found separately in various IL-17A-related diseases, such as aggressive chronic granuloma formation, bone resorption and soft tissue lesions with occasional neurodegeneration. We examined IL-17A in the context of LCH and found that there were high serum levels of IL-17A during active LCH and unexpected IL-17A synthesis by dendritic cells (DCs), the major cell type in LCH lesions. We also found an IL-17A-dependent pathway for DC fusion, which was highly potentiated by IFN-gamma and led to giant cells expressing three major tissue-destructive enzymes: tartrate resistant acidic phosphatase and matrix metalloproteinases 9 and 12. IFN-gamma expression has been previously documented in LCH and observed in IL-17A-related diseases. Notably, serum IL-17A-dependent fusion activity correlates with LCH activity. Thus, IL-17A and IL-17A-stimulated DCs represent targets that may have clinical value in the treatment of LCH and other IL-17A-related inflammatory disorders.

Th17 T cells: linking innate and adaptive immunity

While the cytokine IL-17 has been cloned and described more than 10 years ago [Yao Z, Fanslow WC, Seldin MF, Rousseau AM, Painter SL, Comeau MR, et al. Herpesvirus Saimiri encodes a new cytokine, IL-17, which binds to a novel cytokine receptor. Immunity 1995;3(6):811-21; Kennedy J, Rossi DL, Zurawski SM, Vega Jr F, Kastelein RA, Wagner JL, et al. Mouse IL-17: a cytokine preferentially expressed by alpha beta TCR+CD4-CD8-T cells. J Interferon Cytokine Res 1996;16(8):611-7], it was only 2 years ago that IL-17 producing T cells have been classified as a new distinct CD4 T cell subset [Harrington LE, Hatton RD, Mangan PR, Turner H, Murphy TL, Murphy KM, et al. Interleukin 17-producing CD4+ effector T cells develop via a lineage distinct from the T helper type 1 and 2 lineages. Nat Immunol 2005;6(11):1123-32] and only in 2006 the molecular mechanisms underlying their differentiation were identified [Veldhoen M, Hocking RJ, Atkins CJ, Locksley RM, Stockinger B. TGFbeta in the context of an inflammatory cytokine milieu supports de novo differentiation of IL-17-producing T cells. Immunity 2006;24(2):179-89; Bettelli E, Carrier Y, Gao W, Korn T, Strom TB, Oukka M, et al. Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells. Nature 2006;441(7090):235-8; Mangan PR, Harrington LE, O'Quinn DB, Helms WS, Bullard DC, Elson CO, et al. Transforming growth factor-beta induces development of the T(H)17 lineage. Nature 2006;441(7090):231-4]. Since then the literature on IL-17 producing cells has grown steadily and many reviews of the field are already outdated by the time they are published, a fate that no doubt will affect this review as well. In order to avoid too many repetitions we focus this review mainly on publications in 2006 and 2007 and refer to a number of reviews, which cover earlier aspects of Th17/IL-17 biology.

Cytokine inhibitors in rheumatoid arthritis and other autoimmune diseases

The clinical success of TNFalpha blocking biologics in a growing number of immune-mediated pathologies, including rheumatoid arthritis, Crohn's disease, ankylosing spondylitis and psoriasis, confirms the importance of TNFalpha in driving chronic inflammation and represents an important step forward in the treatment of these conditions. TNFalpha blockade, however, is a treatment, rather than a cure, and is not effective in all patients or in all autoimmune diseases and further research is needed to get closer to a cure. Recently, the identification of a novel, IL-17 producing, T helper cell subset, that plays a dominant pathogenic role in animal models of autoimmunity, is a major advance on existing knowledge, although the role of these cells in human disease remains to be established. Cytokines driving angiogenesis are also important in disease chronicity and thus might be valid therapeutic targets.