Partial impairment of interleukin-12 (IL-12) and IL-18 signaling in Tyk2-deficient mice

Tyrosine kinase 2 modulates splenic B cells through type I IFN and TLR7 signaling

Tyrosine kinase 2 (TYK2) is involved in type I interferon (IFN-I) signaling through IFN receptor 1 (IFNAR1). This signaling pathway is crucial in the early antiviral response and remains incompletely understood on B cells. Therefore, to understand the role of TYK2 in B cells, we studied these cells under homeostatic conditions and following in vitro activation using Tyk2-deficient (Tyk2-/-) mice. Splenic B cell subpopulations were altered in Tyk2-/- compared to wild type (WT) mice. Marginal zone (MZ) cells were decreased and aged B cells (ABC) were increased, whereas follicular (FO) cells remained unchanged. Likewise, there was an imbalance in transitional B cells in juvenile Tyk2-/- mice. RNA sequencing analysis of adult MZ and FO cells isolated from Tyk2-/- and WT mice in homeostasis revealed altered expression of IFN-I and Toll-like receptor 7 (TLR7) signaling pathway genes. Flow cytometry assays corroborated a lower expression of TLR7 in MZ B cells from Tyk2-/- mice. Splenic B cell cultures showed reduced proliferation and differentiation responses after activation with TLR7 ligands in Tyk2-/- compared to WT mice, with a similar response to lipopolysaccharide (LPS) or anti-CD40 + IL-4. IgM, IgG, IL-10 and IL-6 secretion was also decreased in Tyk2-/- B cell cultures. This reduced response of the TLR7 pathway in Tyk2-/- mice was partially restored by IFNα addition. In conclusion, there is a crosstalk between TYK2 and TLR7 mediated by an IFN-I feedback loop, which contributes to the establishment of MZ B cells and to B cell proliferation and differentiation.

The preclinical discovery and development of deucravacitinib for the treatment of psoriasis

INTRODUCTION

Psoriasis is a chronic inflammatory skin disease that most commonly presents as plaque psoriasis. The understanding of the pivotal pathogenetic role of the IL-23/IL-17 axis has dramatically changed the therapeutic approach to the disease. The identification of intracellular signaling pathways mediating IL-23 activity provided the rationale for targeting TYK2.

AREAS COVERED

This review assesses the underlying rationale that led to development of deucravacitinib, a novel oral TYK2 inhibitor, as a therapeutic option for the treatment of moderate-to-severe psoriasis, primarily focusing on pre-clinical and early phase clinical studies.

EXPERT OPINION

Innovative therapies used in patients with moderate-to-severe psoriasis include biologic agents and small molecules, which are associated with less adverse events than traditional systemic agents. Deucravacitinib, which selectively targets TYK2, has demonstrated to be effective in treating psoriasis, preserving a more favorable safety profile compared to other JAK inhibitors approved for the treatment of other immune diseases that block the ATP-binding site. Because of its oral administration, deucravacitinib represents an intriguing option in the therapeutic armamentarium of psoriasis, though the evaluation of long-term efficacy and safety is necessary to establish its place-in-therapy.

TYK2 in Immune Responses and Treatment of Psoriasis

Tyrosine kinase 2 (TYK2), a key part of the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway, plays an integral role in the differentiation and immune responses of intrinsic immune cells and regulates the mediation of cytokines. TYK2 leads to inflammatory cascade responses in the pathogenesis of immune-mediated inflammatory diseases (IMIDs), especially psoriasis. Small-molecule TYK2 inhibitors are considered to be an effective strategy for modulating psoriasis. Here, we attempt to review the pro-inflammatory mechanisms of the JAK-STAT signaling pathway, the regulatory roles of TYK2 in the pathogenesis of psoriasis, and provide updates on ongoing and recently completed trials of TYK2 inhibitors.

Selective tyrosine kinase 2 inhibitors in inflammatory bowel disease

Recent significant advances have been made in the treatment of chronic inflammatory diseases with initiation of the era of biologics. However, an unmet medical need still exists for novel targeted therapies. Compared with biologics, Janus kinase inhibitors (JAKis) are a new drug class of orally administered small molecules that have been shown to efficiently modulate complex cytokine-driven inflammation in preclinical models and human studies. Unfortunately, serious adverse effects have been reported with the first introduced pan-JAKi, tofacitinib. Here, we review tyrosine kinase 2 (TYK2) signaling in the pathophysiology of inflammatory bowel disease (IBD), examine mechanisms of action of selective TYK2 inhibitors (TYK2is), and discuss the potential for these inhibitors in efforts to balance benefits and harms.

Borrelia burgdorferi is a poor inducer of interferon-gamma: amplification induced by interleukin-12

Background Laboratory diagnosis of Lyme borreliosis (LB) is mainly based on serology, which has limitations, particularly in the early stages of the disease. In recent years there have been conflicting reports concerning a new diagnostic tool using the cytokine interferon-gamma (IFN-γ). Previous studies have generally found low concentrations of IFN-γ in early LB infection. The goal of this study is to investigate IFN-γ regulation during early LB and provide insights into the host response to B. burgdorferi. Methods We performed in vitro experiments with whole blood assays and peripheral blood mononuclear cells (PBMCs) of LB patients and healthy volunteers exposed to B. burgdorferi and evaluated the IFN-γ response using ELISA and related interindividual variation in IFN-γ production to the presence of single nucleotide polymorphisms. Results IFN-γ production of B. burgdorferi-exposed PBMCs and whole blood was amplified by the addition of IL-12 to the stimulation system. This effect was observed after 24 hours of B. burgdorferi stimulation in both healthy individuals and LB patients. The effect was highly variable between individuals, but was significantly higher in LB patients six weeks since the start of antibiotic treatment compared to healthy individuals. IL-12 p40 and IL-18 mRNA was upregulated upon exposure to B. burgdorferi, whereas IL-12 p35 and IFN-γ mRNA expression remained relatively unchanged. SNP Rs280520 in the downstream IL-12 pathway, Tyrosine Kinase 2, was associated with increased IFN-γ production. Conclusions This study shows that IL-12 evokes an IFN-γ response in B. burgdorferi exposed cells, and LB patients and healthy controls respond differently to this stimulation.

Prognostic values and clinical relationship of TYK2 in laryngeal squamous cell cancer

ABSTRACT

Laryngeal squamous cell cancer (LSCC) is the second most common head and neck cancer with the increasing mortality. The tyrosine kinase 2 (TYK2) has previously been reported to play an important role in various cancers excepting LSCC. We used available data from the cancer genome atlas program (TCGA), gene expression omnibus, and gene expression profiling interactive analysis (GEPIA) to evaluate the role of TYK2 in LSCC.The difference of TYK2 expression level between normal and tumor samples was analyzed based on TCGA, gene expression omnibus, and GEPIA databases. The relationship between clinical features and TYK2 were analyzed using the Wilcoxon signed-rank test. We applied Cox regression and the Kaplan-Meier method to finding which clinical characteristics is associated with overall survival. Also, we used GEPIA database to validate the relationship between TYK2 and overall survival. At last, we performed gene set enrichment analysis based on TCGA data set.The expression level of TYK2 in LSCC was significantly associated with gender, lymph node status and metastasis (P-values <.05). Kaplan-Meier survival analysis, as same as GEPIA validation, demonstrated that LSCC with TYK2-low had a worse prognosis than that with TYK2-high. The univariate analysis showed that TYK2-high correlated significantly with a better overall survival (hazard ratio: 0.351, 95% confidence interval: 0.194-0.637, P < .001). The multivariate analysis revealed that TYK2 remained independently associated with overall survival (hazard ratio: 0.36, 95% confidence interval: 0.185-0.699, P = .003). Gene set enrichment analysis shows that Janus kinases-STAT signaling pathway, p53 signalling pathway and natural killer cell mediated cytotoxicity, etc are enriched in TYK2 high expression phenotype.Gene TYK2 may be a potential prognostic molecular marker for LSCC. Moreover, the Janus kinases-STAT signaling pathway and p53 signaling pathway are probably the key pathway associated with TYK2 in LC.

The mechanism of Tyk2 deficiency-induced immunosuppression in mice involves robust IL-10 production in macrophages

Macrophages are highly plastic in their pro-inflammatory/anti-inflammatory roles. Type I and II interferons (IFNs) are known to modulate macrophage activation. Tyrosine kinase 2 (Tyk2) has an intimate relationship with type I and II IFN signaling. Animal studies have shown that Tyk2 knock-out (KO) in mice is associated with reduced inflammatory responses in various mouse models of diseases. To investigate the role of Tyk2 in inflammation in more detail, we intraperitoneally injected heat-killed Propionibacterium acnes (P. acnes) to Tyk2 KO mice. P. acnes-induced acute peritoneal inflammation, assessed by neutrophil infiltration, was reduced in Tyk2 KO mice. The reduction was accompanied with diminished productions of inflammatory cytokines and an enhanced production of anti-inflammatory IL-10. Unexpectedly, pre-treatment of wild-type mice with the neutralizing antibodies for IFNs did not affect P. acnes-induced neutrophil infiltration. A neutralizing antibody for the IL-10 receptor in Tyk2 KO mice restored P. acnes-induced peritoneal inflammation. Enhanced production of IL-10 from Tyk2 KO peritoneal cells was suppressed by either the cyclooxygenase inhibitor diclofenac or protein kinase A inhibitor H-89. The level of prostaglandin E2 (PGE2) in the steady-state peritoneal cavity in Tyk2 KO mice was higher than that in wild-type mice. Tyk2 KO macrophages showed an enhanced CREB phosphorylation induced by P. acnes plus PGE2. Taken together, these results showed that Tyk2 deficiency potentiates the PGE2-protein kinase A-IL-10 pathway in macrophages, and thereby contributes to potentiation of the immunosuppressive phenotype.

JAK/STAT Cytokine Signaling at the Crossroad of NK Cell Development and Maturation

Natural Killer (NK) cells are cytotoxic lymphocytes of the innate immune system and play a critical role in anti-viral and anti-tumor responses. NK cells develop in the bone marrow from hematopoietic stem cells (HSCs) that differentiate through common lymphoid progenitors (CLPs) to NK lineage-restricted progenitors (NKPs). The orchestrated action of multiple cytokines is crucial for NK cell development and maturation. Many of these cytokines such as IL-2, IL-7, IL-12, IL-15, IL-21, IL-27, and interferons (IFNs) signal via the Janus Kinase / Signal Transducer and Activator of Transcription (JAK/STAT) pathway. We here review the current knowledge about these cytokines and the downstream signaling involved in the development and maturation of conventional NK cells and their close relatives, innate lymphoid cells type 1 (ILC1). We further discuss the role of suppressor of cytokine signaling (SOCS) proteins in NK cells and highlight their potential for therapeutic application.

Genetics of leprosy: today and beyond

Leprosy is a chronic infectious disease of the skin and peripheral nerves that presents a strong link with the host genetic background. Different approaches in genetic studies have been applied to leprosy and today leprosy is among the infectious diseases with the greatest number of genetic risk variants identified. Several leprosy genes have been implicated in host immune response to pathogens and point to specific pathways that are relevant for host defense to infection. In addition, host genetic factors are also involved in the heterogeneity of leprosy clinical manifestations and in excessive inflammatory responses that occur in some leprosy patients. Finally, genetic studies in leprosy have provided strong evidence of pleiotropic effects between leprosy and other complex diseases, such as immune-mediated or neurodegenerative diseases. These findings not only impact on the field of leprosy and infectious diseases but also make leprosy a good model for the study of complex immune-mediated diseases. Here, we summarize recent genetic findings in leprosy susceptibility and discuss the overlap of the genetic control in leprosy with Parkinson's disease and inflammatory bowel disease. Moreover, some limitations, challenges, and potential new avenues for future genetics studies of leprosy are also discussed in this review.

Control of Tyrosine Kinase Signalling by Small Adaptors in Colorectal Cancer

Tyrosine kinases (TKs) phosphorylate proteins on tyrosine residues as an intracellular signalling mechanism to coordinate intestinal epithelial cell communication and fate decision. Deregulation of their activity is ultimately connected with carcinogenesis. In colorectal cancer (CRC), it is still unclear how aberrant TK activities contribute to tumour formation because TK-encoding genes are not frequently mutated in this cancer. In vertebrates, several TKs are under the control of small adaptor proteins with potential important physiopathological roles. For instance, they can exert tumour suppressor functions in human cancer by targeting several components of the oncogenic TK signalling cascades. Here, we review how the Src-like adaptor protein (SLAP) and the suppressor of cytokine signalling (SOCS) adaptor proteins regulate the SRC and the Janus kinase (JAK) oncogenic pathways, respectively, and how their loss of function in the intestinal epithelium may influence tumour formation. We also discuss the potential therapeutic value of these adaptors in CRC.

Essential Kinases and Transcriptional Regulators and Their Roles in Autoimmunity

Kinases and transcriptional regulators are fundamental components of cell signaling that are expressed on many types of immune cells which are involved in secretion of cytokines, cell proliferation, differentiation, and apoptosis. Both play important roles in biological responses in health as well as in illnesses such as the autoimmune diseases which comprise at least 80 disorders. These diseases are caused by complex genetic and environmental interactions that lead to a breakage of immunologic tolerance and a disruption of the balance between self-reactive cells and regulatory cells. Kinases or transcriptional regulatory factors often have an abnormal expression in the autoimmune cells that participate in the pathogenesis of autoimmune disease. These abnormally expressed kinases or transcriptional regulators can over-activate the function of self-reactive cells to produce inflammatory cytokines or down-regulate the activity of regulatory cells, thus causing autoimmune diseases. In this review we introduce five kinds of kinase and transcriptional regulator related to autoimmune diseases, namely, members of the Janus kinase (JAK) family (JAK3 and/or tyrosine kinase 2 (TYK2)), fork head box protein 3 (Foxp3), the retinoic acid-related orphan receptor gamma t (RORγt), and T-box expressed in T cells (T-bet) factors. We also provide a mechanistic insight into how these kinases and transcriptional regulators affect the function of the immune cells related to autoimmune diseases, as well as a description of a current drug design targeting these kinases and transcriptional regulators. Understanding their exact role helps offer new therapies for control of the inflammatory responses that could lead to clinical improvement of the autoimmune diseases.

NK Cells Require Cell-Extrinsic and -Intrinsic TYK2 for Full Functionality in Tumor Surveillance and Antibacterial Immunity

Tyrosine kinase 2 (TYK2) is a widely expressed receptor-associated kinase that is involved in signaling by a variety of cytokines with important immune regulatory activities. Absence of TYK2 in mice results in impaired NK cell maturation and antitumor activity, although underlying mechanisms are largely unknown. Using conditional ablation of TYK2 in NK cells we show that TYK2 is required for IFN-γ production by NK cells in response to IL-12 and for an efficient immune defense against Listeria monocytogenes Deletion of TYK2 in NK cells did not impact NK cell maturation and IFN-γ production upon NK cell activating receptor (actR) stimulation. Similarly, NK cell-mediated tumor surveillance was unimpaired upon deletion of TYK2 in NK cells only. In line with the previously reported maturation-associated Ifng promoter demethylation, the less mature phenotype of Tyk2^-/- NK cells correlated with an increased CpG methylation at the Ifng locus. Treatment with the DNA hypomethylating agent 5-aza-2-deoxycytidine restored the ability of Tyk2^-/- NK cells to produce IFN-γ upon actR but not upon IL-12 stimulation. NK cell maturation was dependent on the presence of TYK2 in dendritic cells and could be rescued in Tyk2-deficient mice by treatment with exogenous IL-15/IL-15Rα complexes. IL-15 treatment also rescued the in vitro cytotoxicity defect and the impaired actR-induced IFN-γ production of Tyk2^-/- NK cells. Collectively, our findings provide the first evidence, to our knowledge, for a key role of TYK2 in the host environment in promoting NK cell maturation and antitumor activity.

JAK/STAT proteins and their biological impact on NK cell development and function

NK cells are important early effectors in the innate immune response to a variety of viral infections and for elimination of tumor cells. The JAK/STAT signaling cascade is critical for NK cell development, maturation, survival, and proliferation, therefore, it is important to understand the role of this pathway in NK cell biology. Many cytokines can activate multiple JAK/STAT protein family members, creating a severe phenotype when mutations impair their function or expression. Here we discuss the impact of defective JAK/STAT signaling pathways on NK cell development, activation and cytotoxicity.

Emerging IL-12 family cytokines in the fight against fungal infections

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IL-12 and IL-23 have established roles during anti-fungal immunity.

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IL-27 promotes regulatory effector responses during fungal infections.

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IL-35 drives T cell differentiation to produce anti-inflammatory responses.

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Increasing evidence for IL-12 family cytokines in maintaining anti-fungal immune homeostasis.

Invasive fungal infections cause approximately 1.5 million deaths per year worldwide and are a growing threat to human health. Current anti-fungal therapies are often insufficient, therefore studies into host-pathogen interactions are critical for the development of novel therapies to improve mortality rates. Myeloid cells, such as macrophages and dendritic cells, express pattern recognition receptor (PRRs), which are important for fungal recognition. Engagement of these PRRs by fungal pathogens induces multiple cytokines, which in turn activate T effector responses. Interleukin (IL)-12 family members (IL-12p70, IL-23, IL-27 and IL-35) link innate immunity with the development of adaptive immunity and are also important for regulating T cell responses. IL-12 and IL-23 have established roles during anti-fungal immunity, whereas emerging roles for IL-27 and IL-35 have recently been reported. Here, we discuss the IL-12 family, focusing on IL-27 and IL-35 during anti-fungal immune responses to pathogens such as Candida and Aspergillus.

Comprehensive analysis of three TYK2 gene variants in the susceptibility to Chagas disease infection and cardiomyopathy

Tyrosine kinase 2 (TYK2) is a member of the Janus kinases family implicated in the signal transduction of type I interferons and several interleukins. It has been described that genetic mutations within TYK2 lead to multiple deleterious effects in the immune response. In this work, we have analyzed three functional independent variants from the frequency spectrum on the TYK2 gene (common and low-frequency variants) suggested to reduce the function of the gene in mediating cytokine signaling and the susceptibility to infections by Trypanosoma cruzi and/or the development of Chagas cardiomyopathy in the Colombian population. A total of 1,323 individuals from a Colombian endemic region for Chagas disease were enrolled in the study. They were classified as seronegative (n = 445), seropositive asymptomatic (n = 336), and chronic Chagas Cardiomyopathy subjects (n = 542). DNA samples were genotyped using TaqMan probes. Our results showed no statistically significant differences between the allelic frequencies of the three analyzed variants when seropositive and seronegative individuals were compared, therefore these variants were not associated with susceptibility to Chagas disease. Moreover, when Chagas cardiomyopathy patients were compared to asymptomatic patients, no significant associations were found. Previous reports highlighted the association of this gene in immune-related disorders under an autoimmunity context, but not predisposing patients to infectious diseases, which is consistent with our findings. Therefore, according to our results, TYK2 gene variants do not seem to play an important role in Chagas disease susceptibility and/or chronic Chagas cardiomyopathy.

STATs in NK-Cells: The Good, the Bad, and the Ugly

Natural killer (NK)-cells are major players in the fight against viral infections and transformed cells, but there is increasing evidence attributing a disease-promoting role to NK-cells. Cytokines present in the tumor microenvironment shape NK-cell maturation, function, and effector responses. Many cytokines signal via the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway that is also frequently altered and constitutively active in a broad range of tumor cells. As a consequence, there are currently major efforts to develop therapeutic strategies to target this pathway. Therefore, it is of utmost importance to understand the role and contributions of JAK-STAT molecules in NK-cell biology-only this knowledge will allow us to predict effects of JAK-STAT inhibition for NK-cell functions and to successfully apply precision medicine. We will review the current knowledge on the role of JAK-STAT signaling for NK-cell functions and discuss conditions involved in the switch from NK-cell tumor surveillance to disease promotion.

Human Dendritic Cells Mitigate NK-Cell Dysfunction Mediated by Nonselective JAK1/2 Blockade

Janus kinase (JAK) inhibitors have achieved positive responses in myeloproliferative neoplasms, but at the expense of decreased natural killer (NK) cell numbers and compromised function. Selective JAK2 inhibition may also have a role in preventing and treating graft-versus-host disease after allogeneic hematopoietic stem cell transplantation. Although JAK inhibitors can impair monocyte-derived dendritic cell (moDC) activation and function and suppress effector T-cell responses, the effects on NK cells and the relevant mechanisms remain undefined. Using common γ_c cytokines and distinct human dendritic cell (DC) subtypes, we compared the effects of a JAK2-specific (TG101348) with a less selective JAK1/2 (ruxolitinib) inhibitor on NK-cell activation and function. Ruxolitinib treatment completely blocked IL2, IL15, and DC-mediated STAT5 phosphorylation, along with the capacity of NK cells to secrete IFNγ or lyse NK cell-sensitive targets. Only NK-cell proliferation stimulated by moDCs resisted ruxolitinib treatment. In contrast, TG101348 treatment of stimulated NK cells resulted in far less functional compromise. TG101348 completely inhibited only soluble IL15-mediated STAT5 phosphorylation, which Langerhans-type DCs (LCs), presenting membrane-bound IL15 in trans, could salvage. These results demonstrate that ruxolitinib's nonselective inhibition of JAK1/2 results in profound NK-cell dysfunction by blocking downstream pSTAT5, hence providing a persuasive rationale for the development of selective JAK2 inhibitors for immunotherapeutic applications. Cancer Immunol Res; 5(1); 52-60. ©2016 AACR.

Integrating Immunologic Signaling Networks: The JAK/STAT Pathway in Colitis and Colitis-Associated Cancer

Cytokines are believed to be crucial mediators of chronic intestinal inflammation in inflammatory bowel diseases (IBD) such as Crohn's disease (CD) and ulcerative colitis (UC). Many of these cytokines trigger cellular effects and functions through signaling via janus kinase (JAK) and signal transducer and activator of transcription (STAT) molecules. In this way, JAK/STAT signaling controls important events like cell differentiation, secretion of cytokines or proliferation and apoptosis in IBD in both adaptive and innate immune cells. Moreover, JAK/STAT signaling, especially via the IL-6/STAT3 axis, is believed to be involved in the transition of inflammatory lesions to tumors leading to colitis-associated cancer (CAC). In this review, we will introduce the main cellular players and cytokines that contribute to pathogenesis of IBD by JAK/STAT signaling, and will highlight the integrative function that JAK/STATs exert in this context as well as their divergent role in different cells and processes. Moreover, we will explain current concepts of the implication of JAK/STAT signaling in CAC and finally discuss present and future therapies for IBD that interfere with JAK/STAT signaling.

Human TYK2 deficiency: Mycobacterial and viral infections without hyper-IgE syndrome

Kreins et al. report the identification and immunological characterization of a group of TYK2-deficient patients.

Autosomal recessive, complete TYK2 deficiency was previously described in a patient (P1) with intracellular bacterial and viral infections and features of hyper-IgE syndrome (HIES), including atopic dermatitis, high serum IgE levels, and staphylococcal abscesses. We identified seven other TYK2-deficient patients from five families and four different ethnic groups. These patients were homozygous for one of five null mutations, different from that seen in P1. They displayed mycobacterial and/or viral infections, but no HIES. All eight TYK2-deficient patients displayed impaired but not abolished cellular responses to (a) IL-12 and IFN-α/β, accounting for mycobacterial and viral infections, respectively; (b) IL-23, with normal proportions of circulating IL-17⁺ T cells, accounting for their apparent lack of mucocutaneous candidiasis; and (c) IL-10, with no overt clinical consequences, including a lack of inflammatory bowel disease. Cellular responses to IL-21, IL-27, IFN-γ, IL-28/29 (IFN-λ), and leukemia inhibitory factor (LIF) were normal. The leukocytes and fibroblasts of all seven newly identified TYK2-deficient patients, unlike those of P1, responded normally to IL-6, possibly accounting for the lack of HIES in these patients. The expression of exogenous wild-type TYK2 or the silencing of endogenous TYK2 did not rescue IL-6 hyporesponsiveness, suggesting that this phenotype was not a consequence of the TYK2 genotype. The core clinical phenotype of TYK2 deficiency is mycobacterial and/or viral infections, caused by impaired responses to IL-12 and IFN-α/β. Moreover, impaired IL-6 responses and HIES do not appear to be intrinsic features of TYK2 deficiency in humans.

Human Pegivirus (HPgV; formerly known as GBV-C) inhibits IL-12 dependent natural killer cell function

Human Pegivirus (HPgV, formally GB virus C) infects lymphocytes and NK cells in vivo, and infection is associated with reduced T cell and NK cell activation in HIV-infected individuals. The mechanism by which HPgV inhibits NK cell activation has not been assessed. Following IL-12 stimulation, IFNγ expression was lower in HIV-HPgV co-infected subjects compared to HIV mono-infected subjects (p=0.02). In addition, HPgV positive human sera, extracellular vesicles containing E2 protein, recombinant E2 protein and synthetic E2 peptides containing a predicted Tyk2 interacting motif inhibited NK cell IL-12-mediated IFNγ release. E2 protein also inhibited Tyk2 activation following IL-12 stimulation. In contrast, cytolytic NK cell function was not altered by HPgV. Inhibition of NK cell-induced proinflammatory/antiviral cytokines may contribute to both HPgV persistence and reduced immune activation during HIV-coinfection. Understanding mechanisms by which HPgV alters immune activation may contribute towards novel immunomodulatory therapies to treat HIV and inflammatory diseases.

In vivo tumor surveillance by NK cells requires TYK2 but not TYK2 kinase activity

Tyrosine kinase 2 (TYK2) is a Janus kinase (JAK) that is crucially involved in inflammation, carcinogenesis and defense against infection. The cytotoxic activity of natural killer (NK) cells in TYK2-deficient (Tyk2^−/−) mice is severely reduced, although the underlying mechanisms are largely unknown. Using Tyk2^−/− mice and mice expressing a kinase-inactive version of TYK2 (Tyk2^K923E), we show that NK cell function is partly independent of the enzymatic activity of TYK2. Tyk2^−/− and Tyk2^K923E NK cells develop normally in the bone marrow, but the maturation of splenic Tyk2^−/− NK cells (and to a lesser extent of Tyk2^K923E NK cells) is impaired. In contrast, the production of interferon γ (IFNγ) in response to interleukin 12 (IL-12) or to stimulation through NK cell-activating receptors strictly depends on the presence of enzymatically active TYK2. The cytotoxic activity of Tyk2^K923E NK cells against a range of target cells in vitro is higher than that of Tyk2^−/− NK cells. Consistently, Tyk2^K923E mice control the growth of NK cell-targeted tumors significantly better than TYK2-deficient mice, showing the physiological relevance of the finding. Inhibitors of TYK2's kinase activity are being developed for the treatment of inflammatory diseases and cancers, but their effects on tumor immune surveillance have not been investigated. Our finding that TYK2 has kinase-independent functions in vivo suggests that such inhibitors will leave NK cell mediated tumor surveillance largely intact and that they will be suitable for use in cancer therapy.

Therapeutic potential of tyrosine kinase 2 in autoimmunity

INTRODUCTION

Tyrosine kinase 2 (Tyk2) is a Janus kinase family member that is crucial for signaling transduction in response to a wide variety of cytokines, including type I IFNs, IL-6, IL-10, IL-12 and IL-23. An appropriate expression of Tyk2-mediated signaling might be essential for maintaining normal immune responses.

AREAS COVERED

This review summarizes that Tyk2 is essential for the differentiation and function of a wide variety of immune cells, including natural killer cells, B cells, as well as T helper cells. In addition, Tyk2-mediated signaling promoted the production of autoimmune-associated components, which is implicated in the pathogenesis of autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis. Aberrant expression of Tyk2 was observed in many autoimmune conditions.

EXPERT OPINION

Until recently, no patent filings had claimed selective inhibitors of Tyk2. Both CP-690,500 and CMP6 failed to be used in clinical treatment due to the difficulties of finding suitable selective leads or due to detrimental toxicities. Although the result of Cmpd1 is promising, it remains to be seen how specific the Tyk2 inhibitor is and how they are working. Currently, structure-based drug design (SBDD) technology has provided us with a quite useful window for SBDD of Tyk2 inhibitors.

IL-18 in inflammatory and autoimmune disease

Inflammation serves as the first line of defense in response to tissue injury, guiding the immune system to ensure preservation of the host. The inflammatory response can be divided into a quick initial phase mediated mainly by innate immune cells including neutrophils and macrophages, followed by a late phase that is dominated by lymphocytes. Early in the new millennium, a key component of the inflammatory reaction was discovered with the identification of a number of cytosolic sensor proteins (Nod-like receptors) that assembled into a common structure, the 'inflammasome'. This structure includes an enzyme, caspase-1, which upon activation cleaves pro-forms of cytokines leading to subsequent release of active IL-1 and IL-18. This review focuses on the role of IL-18 in inflammatory responses with emphasis on autoimmune diseases.

Involvement of tyrosine kinase-2 in both the IL-12/Th1 and IL-23/Th17 axes in vivo

Tyrosine kinase-2 (Tyk2), a member of the Jak family of kinases, mediates the signals triggered by various cytokines, including type I IFNs, IL-12, and IL-23. In the current study, we investigated the in vivo involvement of Tyk2 in several IL-12/Th1- and IL-23/Th17-mediated models of experimental diseases, including methylated BSA injection-induced footpad thickness, imiquimod-induced psoriasis-like skin inflammation, and dextran sulfate sodium- or 2,4,6-trinitrobenzene sulfonic acid-induced colitis. In these disease models, Tyk2 deficiency influenced the phenotypes in immunity and/or inflammation. Our findings demonstrate a somewhat broader contribution of Tyk2 to immune systems than previously expected and suggest that Tyk2 may represent an important candidate for drug development by targeting both the IL-12/Th1 and IL-23/Th17 axes.

Cytokine regulation of natural killer cell effector functions

Initially described as effectors of natural cytotoxicity and critical players for the control of viral infections and tumor growth, recent investigations unraveled more widespread functions for the natural killer (NK) cells. Through the establishment of a crosstalk with dendritic cells, NK cells promote T helper-1- and cytotoxic T lymphocyte-mediated immunity, whereas through the establishment of a crosstalk with macrophages, NK cells contribute to the activation of their microbicidal functions. Recent evidence has shown that NK cells also display memory, a characteristic thought to be privative of T and B cells, and that NK cells acquire their mature phenotype during a complex ontogeny program which tunes their activation threshold. Cytokines play critical roles in regulating all aspects of immune responses, including lymphoid development, homeostasis, differentiation, tolerance, and memory. Cytokines such as interleukin (IL)-2, IL-12, IL-15, IL-18, IL-21, and type I interferons constitute pivotal factors involved in the maturation, activation, and survival of NK cells. In addition, the discovery of novel cytokines is increasing the spectrum of soluble mediators that regulate NK cell immunobiology. In this review, we summarize and integrate novel concepts about the role of different cytokines in the regulation of NK cell function. We believe that a full understanding of how NK cells become activated and develop their effector functions in response to cytokines and other stimuli may lead to the development of novel immunotherapeutic strategies for the treatment of different types of cancer, viral infections, and chronic autoimmune diseases.

Interleukin 18 in the CNS

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

A single nucleotide polymorphism in Tyk2 controls susceptibility to experimental allergic encephalomyelitis

Genes controlling immunopathologic diseases of differing etiopathology may also influence susceptibility to autoimmune disease. B10.D1-H2(q)/SgJ mice with a 2538 G-->A missense mutation in the tyrosine kinase-2 gene (Tyk2) are susceptible to Toxoplasma gondii yet resistant to autoimmune arthritis, unlike the wild-type B10.Q/Ai substrain. To understand whether Tyk2 is also important in a second autoimmune model, experimental allergic encephalomyelitis (EAE) was induced in B10.D1-H2(q)/SgJ (Tyk2(A)) and B10.Q/Ai (Tyk2(G)) mice with the myelin oligodendrocyte glycoprotein peptide 79-96. B10.D1-H2(q)/SgJ mice were resistant to EAE whereas B10.Q/Ai mice were susceptible, and a single copy of the Tyk2(G) allele conferred EAE susceptibility in F(1) hybrids. Furthermore, EAE resistance in B10.D1-H2(q)/SgJ mice was overridden when pertussis toxin (PTX) was used to mimic the effects of environmental factors derived from infectious agents. Numerous cytokines and chemokines were increased when PTX was included in the immunization protocol. However, only RANTES, IL-6, and IFN-gamma increased significantly with both genetic compensation and PTX treatment. These data indicate that Tyk2 is a shared autoimmune disease susceptibility gene whose genetic contribution to disease susceptibility can be modified by environmental factors. Single nucleotide polymorphisms like the one that distinguishes Tyk2 alleles are of considerable significance given the potential role of gene-by-environment interactions in autoimmune disease susceptibility.

Janus kinases in immune cell signaling

The Janus family kinases (Jaks), Jak1, Jak2, Jak3, and Tyk2, form one subgroup of the non-receptor protein tyrosine kinases. They are involved in cell growth, survival, development, and differentiation of a variety of cells but are critically important for immune cells and hematopoietic cells. Data from experimental mice and clinical observations have unraveled multiple signaling events mediated by Jaks in innate and adaptive immunity. Deficiency of Jak3 or Tyk2 results in defined clinical disorders, which are also evident in mouse models. A striking phenotype associated with inactivating Jak3 mutations is severe combined immunodeficiency syndrome, whereas mutation of Tyk2 results in another primary immunodeficiency termed autosomal recessive hyperimmunoglobulin E syndrome. By contrast, complete deletion of Jak1 or Jak2 in the mouse are not compatible with life and, unsurprisingly, do not have counterparts in human disease. However, activating mutations of each of the Jaks are found in association with malignant transformation, the most common being gain-of-function mutations of Jak2 in polycythemia vera and other myeloproliferative disorders. Our existing knowledge on Jak signaling pathways and fundamental work on their biochemical structure and intracellular interactions allow us to develop new strategies for controlling autoimmune diseases or malignancies by developing selective Jak inhibitors, which are now coming into clinical use. Despite the fact that Jaks were discovered only a little more than a decade ago, at the time of writing there are 20 clinical trials underway testing the safety and efficacy of Jak inhibitors.

Tyk2-signaling plays an important role in host defense against Escherichia coli through IL-23-induced IL-17 production by gammadelta T cells

Tyrosine kinase 2 (Tyk2), a member of the JAK-signal transducer family, is involved in intracellular signaling triggered by various cytokines, including IL-23. We have recently reported that resident gammadelta T cells in the peritoneal cavity of naive mice produced IL-17 in response to IL-23. In this study, we examined importance of Tyk2-mediated signaling in the IL-17 production by gammadelta T cells using Tyk2 deficient (-/-) mice. Gammadelta T cells in the peritoneal cavity of Tyk2(-/-) mice displayed effecter/memory phenotypes and TCR V repertoire similar to those in Tyk2(+/+) mice and produced comparable level of IL-17 to those in Tyk2(+/+) mice in response to PMA and ionomycin, indicating normal differentiation to IL-17-producing effectors in the absence of Tyk2-signaling. However, gammadelta T cells in Tyk2(-/-) mice produced less amount of IL-17 in response to IL-23 in vitro than those in Tyk2(+/+) mice. Similarly, gammadelta T cells in the peritoneal cavity of Tyk2(-/-) mice showed severely impaired IL-17 production after an i.p. infection with E. coli despite comparable level of IL-23 production to Tyk2(+/+) mice. As a consequence, Tyk2(-/-) mice showed a reduced infiltration of neutrophils and severely impaired bacterial clearance after Escherichia coli infection. These results indicate that Tyk2-signaling is critical for IL-23-induced IL-17 production by gammadelta T cells, which is involved in the first line of host defense by controlling neutrophil-mediated immune responses.

JAK and MPL mutations in myeloid malignancies

The Janus family of non-receptor tyrosine kinases (JAK1, JAK2, JAK3 and tyrosine kinase 2) transduces signals downstream of type I and II cytokine receptors via signal transducers and activators of transcription (STATs). JAK3 is important in lymphoid and JAK2 in myeloid cell proliferation and differentiation. The thrombopoietin receptor MPL is one of several JAK2 cognate receptors and is essential for myelopoiesis in general and megakaryopoiesis in particular. Germline loss-of-function (LOF) JAK3 and MPL mutations cause severe combined immunodeficiency and congenital amegakaryocytic thrombocytopenia, respectively. Germline gain-of-function (GOF) MPL mutation (MPLS505N) causes familial thrombocytosis. Somatic JAK3 (e.g. JAK3A572V, JAK3V722I, JAK3P132T) and fusion JAK2 (e.g. ETV6-JAK2, PCM1-JAK2, BCR-JAK2) mutations have respectively been described in acute megakaryocytic leukemia and acute leukemia/chronic myeloid malignancies. However, current attention is focused on JAK2 (e.g. JAK2V617F, JAK2 exon 12 mutations) and MPL (e.g. MPLW515L/K/S, MPLS505N) mutations associated with myeloproliferative neoplasms (MPNs). A JAK2 mutation, primarily JAK2V617F, is invariably associated with polycythemia vera (PV). The latter mutation also occurs in the majority of patients with essential thrombocythemia (ET) or primary myelofibrosis (PMF). MPL mutational frequency in MPNs is substantially less (<10%). In general, despite a certain degree of genotype - phenotype correlations, the prognostic relevance of harbouring one of these mutations, or their allele burden when present, remains dubious. Regardless, based on the logical assumption that amplified JAK-STAT signalling is central to the pathogenesis of PV, ET and PMF, several anti-JAK2 tyrosine kinase inhibitors have been developed and are currently being tested in humans with these disorders.

Immunological decision-making: how does the immune system decide to mount a helper T-cell response?

Aberrant T-cell responses underpin a range of diseases, including asthma and allergy and autoimmune diseases. Pivotal immune elements of these diseases are the development of antigen-specific effector T-helper type 2 (Th2) cells, Th1 cells, or the recently defined Th17 cells that are associated with the clinical features and disease progression. In order to identify crucial processes in the pathogenesis of these diseases it is critical to understand how the development of these T cells occurs. The phenotype of a polarized T-cell that differentiates from a naïve precursor is determined by the complex interaction of antigen-presenting cells with naïve T cells and involves a multitude of factors, including the dominant cytokine environment, costimulatory molecules, type and load of antigen presented and a plethora of signaling cascades. The decision to take the immune response in a certain direction is not made by one signal alone, instead many different elements act synergistically, antagonistically and through positive feedback loops to activate a Th1, Th2, or Th17 immune response. The elucidation of the mechanisms of selection of T-cell phenotype will facilitate the development of therapeutic strategies to intervene in the development of deleterious T-cell responses. This review will focus on the pathways and key factors responsible for the differentiation of the various subsets of effector CD4 T cells. We will primarily discuss what is known of the Th1 and Th2 differentiation pathways, while also reviewing the emerging research on Th17 differentiation.

Tyk2 Signaling in Host Environment Plays an Important Role in Contraction of Antigen-Specific CD8+T Cells following a Microbial Infection

Tyrosine kinase 2 (Tyk2), a member of JAK signal transducer family contributes to the signals triggered by IL-12 for IFN-gamma production. To elucidate potential roles of Tyk2 in generation and maintenance of Ag-specific CD8+ T cells, we followed the fate of OVA-specific CD8+ T cells in Tyk2-deficient (-/-) mice after infection with recombinant Listeria monocytogenes expressing OVA (rLM-OVA). Results showed that the numbers of OVA(257-264)/K(b) tetramer-positive CD8+ T cells in Tyk2(-/-) mice were almost the same as those in Tyk2(+/+) mice at the expansion phase on day 7 but were significantly larger in Tyk2(-/-) mice than those in Tyk2(+/+) mice at the contraction phase on day 10 and at the memory phase on day 60 after infection. The intracellular expression level of active caspase-3 was significantly decreased in the OVA-specific CD8+ T cells of Tyk2(-/-) mice on day 7 compared with those of Tyk2(+/+) mice. Adaptive transfer experiments revealed that Tyk2 signaling in other factors rather than CD8+ T cells played a regulatory role in CD8+ T cell contraction following infection. Administration of exogenous IFN-gamma from day 6 to day 9 restored the CD8+ T cell contraction in Tyk2(-/-) mice after infection with rLM-OVA. These results suggest that Tyk2 signaling for IFN-gamma production in host environment plays an important role in contraction of effector CD8+ T cells following a microbial infection.

Type 1 cytokine/chemokine production by mouse NK cells following activation of their TLR/MyD88-mediated pathways

It is well established that IL-18R- and toll-like receptor (TLR)-mediated signalings share a common signal pathway mediated by signal adaptor, MyD88, and that IL-18 synergizes with IL-12 for IFN-gamma production by NK cells. Here, we investigated whether TLR agonists can replace IL-18 for production of IFN-gamma by NK cells. Freshly isolated NK cells possessed functional LPS receptor composed of TLR4/MD2 complex and of CD14, and also expressed other various tlrs. Hepatic CD3(-)DX5(+) NK cells produced IFN-gamma in response to TLR2 or TLR7 agonists only when co-stimulated with IL-12, indicating that TLR agonists synergize with IL-12 for IFN-gamma. The tlr2(-/-) or tlr7(-/-) NK cells could not produce IFN-gamma in response to IL-12 plus TLR2 or TLR7 ligands, respectively, indicating requirement of the corresponding TLRs. Furthermore, upon stimulation with these combinations, wild-type NK cells produced type 1 chemokines, such as CCL3, CCL4 and CCL5 as well. NK cells from bacterium (e.g. Propionibacterium acnes)-inoculated rag2(-/-) mice, when compared with those from naive mice, exhibited significantly enhanced capacity to produce these CC chemokines and IFN-gamma, suggesting that microbial infection enhances responsiveness of NK cells to TLR agonists. These results indicate that upon microbial infection, macrophages produce IL-12 that renders NK cells highly responsive to TLR agonists to produce IFN-gamma and chemokines, which might in turn recruit and fully activate macrophages, leading to the development of inflammatory foci presumably necessary for efficient microbial eradication. Thus, NK cells, like T cells, induce orchestrated immune responses in collaboration with macrophages to show potent host defense effects during early infectious phase.

An important role of Tyk2 in APC function of dendritic cells for priming CD8+ T cells producing IFN-gamma

Tyrosine kinase 2 (Tyk2) contributes to the signals triggered by IL-12 for IFN-gamma production by NK cells and T cells. We found in this study that Tyk2-deficient (-/-) mice showed increased susceptibility at the early stage after an i.p. infection with Listeria monocytogenes, accompanied by impaired IFN-gamma production. The numbers of both MHC class Ib (H2-M3)- or MHC class Ia (Kb)-restricted CD8+ T cells producing IFN-gamma and exhibiting cytotoxicity were significantly decreased in Tyk2-/- mice after infection with L. monocytogenes. Using an adoptive transfer system of OT-I cells expressing OVA(257-264)/Kb-specific TCR into Tyk2-/- mice followed by challenge with recombinant L. monocytogenes expressing OVA, we found that the defective Tyk2 signaling in the host environment was at least partially responsible for the impaired CD8+ T cytotoxic-1 (Tc1) cell responses in Tyk2-/- mice following the infection. Adoptive transfer with MHC class Ib- or MHC class Ia-binding peptide-pulsed BM-derived DC from Tyk2-/- mice induced lower levels of the Ag-specific CD8+ Tc1 cells producing IFN-gamma. These results suggest that Tyk2 signaling is also important for DC function in the induction of MHC class Ia- and class Ib-restricted CD8+ Tc1 cells following L. monocytogenes infection.

Interleukins and STAT signaling

Metazoan cells secrete small proteins termed cytokines that execute a variety of biological functions essential for the survival of organisms. Binding of cytokines that belong to the hematopoietin- or interferon-family, to their cognate receptors on the surface of target cells, induces receptor aggregation, which in turn sequentially triggers tyrosine-phosphorylation-dependent activation of receptor-associated Janus-family tyrosine kinases (JAKs), receptors, and signal transducers and activators of transcription (STATs). Phosphorylated STATs form dimers that migrate to the nucleus, bind to cognate enhancer elements and activate transcription of target genes. Each cytokine activates a specific set of genes to execute its biological functions with a certain degree of redundancy. Cytokine signals are, in general, transient in nature. Therefore, under normal physiological conditions, initiation and attenuation of cytokine signals are tightly controlled via multiple cellular and molecular mechanisms. Aberrant activation of cytokine signaling pathways is, however, found under a variety of patho-physiological conditions including cancer and immune diseases.

Factors effecting the Th2-like immune response after gamma-irradiation: low production of IL-12 heterodimer in antigen-presenting cells and small expression of the IL-12 receptor in T cells

Ionizing radiation is known to reduce the helper T (Th) 1-like function, but not the Th2-like function, resulting in a Th1/Th2 imbalance. While this has been known for some time, the mechanism behind the preferential suppression of the Th1 cell activation has not yet been explained. The aim is to elucidate the mechanism in the Th cell imbalance after ionizing irradiation. C57BL/6 mice, 7 weeks old, received whole-body gamma-irradiation (WBI) of 5 Gy. In all instances, the spleen and peritoneal cells were obtained from mice 7 weeks after irradiation. To distinguish Th1 and Th2 cell function, interferon (IFN)-gamma and interleukin (IL)-4 produced by these cells were analysed by an enzyme-linked immunosorbent assay (ELISA). To isolate the primary T cells, the anti-CD90.2 microbead-conjugated antibody was used and the labelled cells were separated by magnetic cell sorting (MACS). To investigate the influence of the IL-12p70 secreted by the antigen-presenting cells, ovalbumin (OVA)-primed peritoneal adherent cells (PAC) were fixed by 1% paraformaldehyde and co-cultured with OVA-specific Th cells in the presence of supernatant of PAC culture with OVA for 16 h. IL-12 receptor, signal transducers and activators of transcription 4 (STAT4) and IFN-gamma expression in the T cells of the WBI mice were detected by reverse transcriptase-polymerase chain reaction (RT-PCR). The spleen lymphocytes of WBI mice showed a depression of IFN-gamma production against OVA, although the total IL-12 was highly secreted. However, the heterodimer IL-12, biologically active protein, was induced less in WBI mice. Although the OVA-specific Th cells were co-cultured with fixed OVA-primed PAC obtained from normal mice, the OVA-specific Th cells showed a decreased IFN-gamma secretion in the presence of the culture supernatant of the activated PAC from the WBI mice. In addition, recombinant IL-12p70 restored the cytokine balance of the OVA-specific Th cells. However the cytokine balance of primary T cells from WBI mice was not completely restored by the normal antigen-presenting cells that abundantly secrete IL-12p70. It was assumed that after WBI, the regenerated T cells also have some problems. It was then observed that the IL-12 receptor expression and intracellular levels of the STAT4 were much lower in the T cells of the WBI mice. The results suggest that the shifted response of the helper T cells after WBI exposure is due not only due to a significant suppression of the secretion of the IL-12p70 in the antigen-presenting cells, but also to the lower expression of the IL-12 receptor on T cells.

Pathophysiological roles for IL-18 in inflammatory arthritis

IL-18 is a unique cytokine with prominently wide spectrum biological actions. Among these, its IFN-gamma/TNF-alpha-inducing activity primarily contributes to the development of various inflammatory diseases including inflammatory arthritis. IL-18 levels correlate with the disease activity of rheumatoid arthritis (RA) and osteoarthritis (OA). IL-18 is spontaneously released from RA synovial cells and OA chondrocytes and seems to participate in the development of the inflammatory and destructive alterations of joints via induction of TNF-alpha, a potent effector molecule. TNF-alpha, in turn, increases IL-18 expression in RA synovial cells. Recent clinical trials have revealed the efficacy of TNF-alpha in RA with a reduction in circulatory IL-18 levels. These may implicate the positive circuit between IL-18 and TNF-alpha for development of RA. As IL-18-deficient mice evade collagen-induced arthritis in a mouse RA model, therapeutics targeting IL-18 may be beneficial against RA/OA. Here, the authors review the possible roles of IL-18 in inflammatory arthritis.

Induction of allergic inflammation by interleukin-18 in experimental animal models

Interleukin-18 (IL-18) has been regarded as a proinflammatory cytokine because of its potent interferon-gamma-inducing activity. However, mutant mice that release excess amounts of IL-18 spontaneously develop pruritic chronic dermatitis-like atopic dermatitis (AD), suggesting the importance of IL-18 for the development of AD. Intriguingly, depletion of il-18 but not stat6, an essential transcriptional factor for IL-4 signaling, rescues the mice from AD, indicating IL-18-dependent, T-helper 2 (Th2) cell-independent AD. This type of AD is classified as innate-type allergy in contrast to Th2 cell-dependent ordinary allergy. Consistent with the previous studies, mice transferred with antigen-specific Th1 cells exhibit no airway hyperresponsiveness and respiratory eosinophilic inflammation after challenge with antigen alone. However, they suffer from asthma upon challenge with antigen plus IL-18, with comparable levels of both the alterations as in those transferred with Th2 cells following challenge with antigen. The former type of asthma is categorized as Th1-associated allergy. Therefore, it is definitely necessary to evaluate whether individual allergic disorders involve either of these IL-18-mediated pathways or a Th2-mediated one.

Hapten-induced contact hypersensitivity is enhanced in Tyk2-deficient mice

BACKGROUND

Previous studies have shown that Tyk2, a member of the Janus family of protein tyrosine kinases, which are activated by a variety of cytokines, plays a crucial role in interleukin (IL)-12-mediated T-cell functions such as IFN-gamma production. On the other hand, hapten-induced contact hypersensitivity (CHS) is mediated by IFN-gamma producing CD8+ T cells and regulated by CD4+ T cells.

OBJECTIVE

This study hypothesized that the CHS response might be reduced in Tyk2-deficient mice because of a lack of IFN-gamma production from CD4+ and CD8+ T cells.

METHODS

The CHS reaction was evoked in wild-type and Tyk2-deficient mice and the ears of the mice were examined to measure for several cytokines.

RESULTS

Ear swelling during CHS was significantly enhanced in Tyk2-deficient mice compared with the controls. IL-12 and IFN-gamma levels at the reaction sites in Tyk2-deficient mice were significantly lower than in the controls, whereas IL-2 and IL-4 levels were elevated. Furthermore, STAT3- and STAT4-phosphorylation in the draining lymph node cells of Tyk2-deficient mice decreased.

CONCLUSION

These results suggest that the lack of Tyk2-mediated signal transduction enhances a compensative pathway during CHS.

TYK2 is a key regulator of the surveillance of B lymphoid tumors

Aberrant activation of the JAK-STAT pathway has been implicated in tumor formation; for example, constitutive activation of JAK2 kinase or the enforced expression of STAT5 induces leukemia in mice. We show here that the Janus kinase TYK2 serves an opposite function. Mice deficient in TYK2 developed Abelson-induced B lymphoid leukemia/lymphoma as well as TEL-JAK2-induced T lymphoid leukemia with a higher incidence and shortened latency compared with WT controls. The cell-autonomous properties of Abelson murine leukemia virus-transformed (A-MuLV-transformed) TYK2(-/-) cells were unaltered, but the high susceptibility of TYK2(-/-) mice resulted from an impaired tumor surveillance, and accordingly, TYK2(-/-) A-MuLV-induced lymphomas were easily rejected after transplantation into WT hosts. The increased rate of leukemia/lymphoma formation was linked to a decreased in vitro cytotoxic capacity of TYK2(-/-) NK and NKT cells toward tumor-derived cells. RAG2/TYK2 double-knockout mice succumbed to A-MuLV-induced leukemia/lymphoma faster than RAG2(-/-)TYK2(+/-) mice. This defines NK cells as key players in tumor surveillance in Abelson-induced malignancies. Our observations provide compelling evidence that TYK2 is an important regulator of lymphoid tumor surveillance.

TYK2 is a key regulator of the surveillance of B lymphoid tumors

Aberrant activation of the JAK-STAT pathway has been implicated in tumor formation; for example, constitutive activation of JAK2 kinase or the enforced expression of STAT5 induces leukemia in mice. We show here that the Janus kinase TYK2 serves an opposite function. Mice deficient in TYK2 developed Abelson-induced B lymphoid leukemia/lymphoma as well as TEL-JAK2-induced T lymphoid leukemia with a higher incidence and shortened latency compared with WT controls. The cell-autonomous properties of Abelson murine leukemia virus-transformed (A-MuLV-transformed) TYK2(-/-) cells were unaltered, but the high susceptibility of TYK2(-/-) mice resulted from an impaired tumor surveillance, and accordingly, TYK2(-/-) A-MuLV-induced lymphomas were easily rejected after transplantation into WT hosts. The increased rate of leukemia/lymphoma formation was linked to a decreased in vitro cytotoxic capacity of TYK2(-/-) NK and NKT cells toward tumor-derived cells. RAG2/TYK2 double-knockout mice succumbed to A-MuLV-induced leukemia/lymphoma faster than RAG2(-/-)TYK2(+/-) mice. This defines NK cells as key players in tumor surveillance in Abelson-induced malignancies. Our observations provide compelling evidence that TYK2 is an important regulator of lymphoid tumor surveillance.

Re-examination of the Role of Suppressor of Cytokine Signaling 1 (SOCS1) in the Regulation of Toll-like Receptor Signaling

Suppressor of cytokine signaling 1 (SOCS1) is an obligate negative regulator of cytokine signaling and most importantly in vivo, signaling via the interferon-gamma (IFN-gamma) receptor. SOCS1, via its Src homology 2 domain, binds to phosphotyrosine residues in its targets, reducing the amplitude of signaling from cytokine receptors. SOCS1 is also implicated in blocking Toll-like receptor (TLR) signaling in macrophages activated by TLR agonists such as lipopolysaccharide (LPS), thus regulating multiple steps in the activation of innate immune responses. To rigorously test this, we isolated macrophages from Socs1-/- mice on multiple genetic backgrounds. We found no evidence that SOCS1 blocked TLR-activated pathways, endotoxin tolerance, or nitric oxide production. However, Socs1-/-;IFN-gamma-/- mice were extremely susceptible to LPS challenge, confirming previous findings. Because LPS induces IFN-beta production from macrophages, we tested whether SOCS1 regulates IFN-alpha/beta receptor signaling. We find that SOCS1 is required to inhibit IFN-alpha/beta receptor signaling in vitro. Furthermore, the absence of a single allele encoding TYK2, a JAK (Janus kinase) family member essential IFN-alpha/beta receptor signaling, rescued Socs1-/- mice from early lethality, even in the presence of IFN-gamma. We conclude that previous reports linking SOCS1 to TLR signaling are most likely due to effects on IFN-alpha/beta receptor signaling.

Interleukin-18: recent advances

PURPOSE OF REVIEW

Interleukin-18 (IL-18) has potent immunomodulatory effects. It is the only cytokine with a unique capacity to induce T helper 1 or T helper 2 polarization, depending on the immunologic context. Serum levels of IL-18 are increased in many human diseases and it has been implicated in the pathogenesis of several immune-mediated processes. Some of the recent key advances in the immunobiology of IL-18 are discussed in this review.

RECENT FINDINGS

Recent data from several laboratories have shed light on the structure of IL-18; the signaling cascades that are initiated; and its role on modulating T cells, dendritic cells, and natural killer cell function. Several new reports have expanded and delineated the role of IL-18 in a multitude of diseases, but only recent advances in the role of IL-18 in three disease processes (acute graft-versus-host disease, insulin-dependent diabetes, and sepsis), where it appears to play paradoxic roles are discussed.

SUMMARY

Although emerging data shed more light on the complex role of IL-18 in immune reactions, they also pose more questions. Given the pleiotropic, complex, and at times paradoxic effects of IL-18 in various disease processes, better understanding of its immunobiology might lead to the development of IL-18 and/or its antagonists as therapeutic agents against immune-mediated diseases.

Differential Upregulation of Interleukin-18 Receptor α Chain Between CD4+ and CD8+ T Cells During Acute Graft-Versus-Host Disease in Mice

Interleukin-18 (IL-18), a unique cytokine that stimulates both T helper 1 (Th1) and Th2 responses, is associated with acute graft-versus-host disease (aGVHD), the major limiting toxicity following allogeneic stem cell transplantation. Here, we investigated the mechanism underlying the upregulation of IL-18 receptor (IL-18R) expression on T cells in murine aGVHD models. The induction of aGVHD elevated host serum IL-12 levels and increased expression of IL-18Ralpha chain (IL-18Ralpha) on engrafted T cells, in particular on CD8+ T cells. However, IL-18Ralpha expression did not increase on the CD4+ T cells of an IL-12-deficient host, indicating the IL-12-dependent upregulation of IL-18Ralpha expression on CD4+ T cells during aGVHD. Purified donor CD8+ T cells transferred in the host increased IL-18Ralpha expression. In vitro experiments showed that IL-18Ralpha expression upregulated on CD8+ T cells but not on CD4+ T cells on stimulation through the T cell receptor (TCR). These results suggest that IL-18Ralpha expression is differentially upregulated between CD4+ and CD8+ T cells during aGVHD, depending on endogenous IL-12 and TCR engagement, respectively.

Two waves of antigen-containing dendritic cells in vivo in experimental Leishmania major infection

Dendritic cells (DC) can induce Th1 cell differentiation by producing IL-12. In experimental infection with Leishmania major, DC could differently respond to infection and induce Th1 cells in C57BL/6 but not BALB/c mice, and thus determine the resistance or susceptibility of these mice. We characterized L. major antigen-containing DC in vivo in draining lymph nodes of both strains. Conventional experimental infection is shown to result in two waves of these DC and our data argue against a relevant genetic difference in the DC initiating the anti-parasite Th cell response in these mice. In both strains the first wave of DC presented L. major antigens but was not infected, produced IL-12 but induced disease-mediating Th2 cells upon adoptive transfer. In contrast to current belief, this response was therefore not initiated by infected DC, which were only detected in the second wave. The kinetics of the two waves suggests that DC turnover has an important impact on antigen presentation during infections with complex pathogens.