IL-21 limits peripheral lymphocyte numbers through T cell homeostatic mechanisms

Atopic dermatitis: pathomechanisms and lessons learned from novel systemic therapeutic options

Atopic dermatitis (AD) is a chronic, heterogenous, inflammatory skin disorder associated with a high skin-related health burden, typically starting in childhood and often persisting into adulthood. AD is characterized by a wide range of clinical phenotypes, reflecting multiple underlying pathophysiological mechanisms and interactions between genetics, immune system dysregulation, and environmental factors. In this review, we describe the diverse cellular and molecular mechanisms involved in AD, including the critical role of T cell-driven inflammation, primarily via T helper (Th) 2- and Th17-derived cytokines, many of which are mediated by the Janus kinase (JAK) signaling pathway. These local inflammatory processes interact with sensory neuronal pathways, contributing to the clinical manifestations of AD, including itch, pain, and sleep disturbance. The recent elucidation of the molecular pathways involved in AD has allowed treatment strategies to evolve from broad-acting systemic immunosuppressive therapies to more targeted agents, including JAK inhibitors and cytokine-specific biologic agents. Evidence from the clinical development of these targeted therapies has reinforced and expanded our understanding of the pathophysiological mechanisms underlying AD and holds promise for individualized treatment strategies tailored to specific AD subtypes.

Lymphopenia, Lymphopenia-Induced Proliferation, and Autoimmunity

Immune homeostasis is a tightly regulated system that is critical for defense against invasion by foreign pathogens and protection from self-reactivity for the survival of an individual. How the defects in this system might result in autoimmunity is discussed in this review. Reduced lymphocyte number, termed lymphopenia, can mediate lymphopenia-induced proliferation (LIP) to maintain peripheral lymphocyte numbers. LIP not only occurs in normal physiological conditions but also correlates with autoimmunity. Of note, lymphopenia is also a typical marker of immune aging, consistent with the fact that not only the autoimmunity increases in the elderly, but also autoimmune diseases (ADs) show characteristics of immune aging. Here, we discuss the types and rates of LIP in normal and autoimmune conditions, as well as the coronavirus disease 2019 in the context of LIP. Importantly, although the causative role of LIP has been demonstrated in the development of type 1 diabetes and rheumatoid arthritis, a two-hit model has suggested that the factors other than lymphopenia are required to mediate the loss of control over homeostasis to result in ADs. Interestingly, these factors may be, if not totally, related to the function/number of regulatory T cells which are key modulators to protect from self-reactivity. In this review, we summarize the important roles of lymphopenia/LIP and the Treg cells in various autoimmune conditions, thereby highlighting them as key therapeutic targets for autoimmunity treatments.

The Effects of an IL-21 Receptor Antagonist on the Alloimmune Response in a Humanized Mouse Skin Transplant Model

BACKGROUND

Interleukin 21 (IL-21) is involved in regulating the expansion and effector function of a broad range of leukocytes, including T cells and B cells. In transplantation, the exact role of IL-21 in the process of allograft rejection is unknown. To further explore this, the aim of this study is to test the effect of an IL-21 receptor (IL-21R) blocking antibody on the early phase of allograft rejection in a humanized skin transplantation model in mice reconstituted with human T and B cells.

METHODS

Immunodeficient Balb/c IL2rγRag2 mice were transplanted with human skin followed by adoptive transfer of human allogeneic splenocytes. Control animals were treated with a phosphate buffered saline vehicle while the other group was treated with a humanized anti-IL-21R antibody (αIL-21R).

RESULTS

In the phosphate buffered saline-treated animals, human skin allografts were infiltrated with lymphocytes and developed a thickened epidermis with increased expression of the inflammatory markers Keratin 17 (Ker17) and Ki67. In mice treated with αIL-21R, these signs of allograft reactivity were significantly reduced. Concordantly, STAT3 phosphorylation was inhibited in this group. Of note, treatment with αIL-21R attenuated the process of T and B cell reconstitution after adoptive cellular transfer.

CONCLUSIONS

These findings demonstrate that blockade of IL-21 signaling can delay allograft rejection in a humanized skin transplantation model.

IL-21 regulates SOCS1 expression in autoreactive CD8+ T cells but is not required for acquisition of CTL activity in the islets of non-obese diabetic mice

In type 1 diabetes, maturation of activated autoreactive CD8⁺ T cells to fully armed effector cytotoxic T lymphocytes (CTL) occurs within the islet. At present the signals required for the maturation process are poorly defined. Cytokines could potentially provide the necessary "third signal" required to generate fully mature CTL capable of killing insulin-producing β-cells. To determine whether autoreactive CTL within islets respond to cytokines we generated non-obese diabetic (NOD) mice with a reporter for cytokine signalling. These mice express a reporter gene, hCD4, under the control of the endogenous regulatory elements for suppressor of cytokine signalling (SOCS)1, which is itself regulated by pro-inflammatory cytokines. In NOD mice, the hCD4 reporter was expressed in infiltrated islets and the expression level was positively correlated with the frequency of infiltrating CD45⁺ cells. SOCS1 reporter expression was induced in transferred β-cell-specific CD8⁺ 8.3T cells upon migration from pancreatic draining lymph nodes into islets. To determine which cytokines induced SOCS1 promoter activity in islets, we examined hCD4 reporter expression and CTL maturation in the absence of the cytokine receptors IFNAR1 or IL-21R. We show that IFNAR1 deficiency does not confer protection from diabetes in 8.3 TCR transgenic mice, nor is IFNAR1 signalling required for SOCS1 reporter upregulation or CTL maturation in islets. In contrast, IL-21R-deficient 8.3 mice have reduced diabetes incidence and reduced SOCS1 reporter activity in islet CTLs. However IL-21R deficiency did not affect islet CD8⁺ T cell proliferation or expression of granzyme B or IFNγ. Together these data indicate that autoreactive CD8⁺ T cells respond to IL-21 and not type I IFNs in the islets of NOD mice, but neither IFNAR1 nor IL-21R are required for islet intrinsic CTL maturation.

PTPN2 attenuates T-cell lymphopenia-induced proliferation

When the peripheral T-cell pool is depleted, T cells undergo homoeostatic expansion. This expansion is reliant on the recognition of self-antigens and/or cytokines, in particular interleukin-7. The T cell-intrinsic mechanisms that prevent excessive homoeostatic T-cell responses and consequent overt autoreactivity remain poorly defined. Here we show that protein tyrosine phosphatase N2 (PTPN2) is elevated in naive T cells leaving the thymus to restrict homoeostatic T-cell proliferation and prevent excess responses to self-antigens in the periphery. PTPN2-deficient CD8(+) T cells undergo rapid lymphopenia-induced proliferation (LIP) when transferred into lymphopenic hosts and acquire the characteristics of antigen-experienced effector T cells. The enhanced LIP is attributed to elevated T-cell receptor-dependent, but not interleukin-7-dependent responses, results in a skewed T-cell receptor repertoire and the development of autoimmunity. Our results identify a major mechanism by which homoeostatic T-cell responses are tuned to prevent the development of autoimmune and inflammatory disorders.

IL-21R signaling is critical for induction of spontaneous experimental autoimmune encephalomyelitis

IL-17-producing CD4+ T cells (Th17 cells) have well-described pathogenic roles in tissue inflammation and autoimmune diseases, such as experimental autoimmune encephalomyelitis (EAE); however, the involvement of IL-21 in these processes has remained controversial. While IL-21 is an essential autocrine amplification factor for differentiation of Th17 cells, the loss of IL-21 or IL-21 receptor (IL-21R) does not protect mice from actively induced EAE. Here, we utilized a transgenic EAE mouse model, in which T and B cells overexpress receptors for myelin oligodendrocyte glycoprotein (MOG) (referred to as 2D2xTH mice), and demonstrated that IL-21 is critical for the development of a variant form of spontaneous EAE in these animals. Il21r deletion in 2D2xTH mice reduced the incidence and severity of spontaneous EAE, which was associated with a defect in Th17 cell generation. Moreover, IL-21R deficiency limited IL-23R expression on Th17 cells and inhibited expression of key molecules involved in the generation of pathogenic Th17 cells. Conversely, loss of IL-23R in 2D2xTH mice resulted in complete resistance to the development of spontaneous EAE. Our data identify a previously unappreciated role for IL-21 in EAE and reveal that IL-21-mediated signaling supports generation and stabilization of pathogenic Th17 cells and development of spontaneous autoimmunity.

Pouring fuel on the fire: Th17 cells, the environment, and autoimmunity

Cytokines play a critical role in controlling the differentiation of CD4 Th cells into distinct subsets, including IL-17-producing Th17 cells. Unfortunately, the incidence of a number of autoimmune diseases, particularly those in which the IL-23/IL-17 axis has been implicated, has risen in the last several decades, suggesting that environmental factors can promote autoimmunity. Here we review the role of cytokines in Th17 differentiation, particularly the role of IL-23 in promoting the differentiation of a pathogenic subset of Th17 cells that potently induce autoimmune tissue inflammation. Moreover, we highlight emerging data that indicate that environmental factors, including the intestinal microbiota and changes in diet, can alter normal cytokine regulation with potent effects on Th17 differentiation and thus promote autoimmunity, which has strong implications for human disease.

Premature CD4+ T cell aging and its contribution to lymphopenia-induced proliferation of memory cells in autoimmune-prone non-obese diabetic mice

Lymphopenia-induced proliferation (LIP), a mechanism to maintain a constant number of T cells in circulation, occurs in both normal aging and autoimmune disease. The incidence of most autoimmune diseases increases with age, and premature CD4(+) T cell aging has been reported in several autoimmune diseases. In this study, we tested the hypothesis that premature CD4(+) T cell aging can cause autoimmune disease by examining whether premature CD4(+) T cell aging exists and causes LIP in our mouse model. Non-obese diabetic (NOD) mice were used because, in addition to Treg defects, the LIP of T cells has been shown to plays a causative role in the development of insulin-dependent diabetes mellitus (IDDM) in these mice. We found that with advancing age, NOD mice exhibited an accelerated decrease in the number of CD4(+) T cells due to the loss of naïve cells. This was accompanied by an increase in the percentage of memory cells, leading to a reduced naïve/memory ratio. In addition, both the percentage of CD28(+) cells in CD4(+) T cells and IL-2 production decreased, while the percentage of FAS(+)CD44(+) increased, suggesting that NOD mice exhibit premature CD4(+) T cell aging. This process preferentially contributed to LIP of memory cells. Therefore, our results suggest that premature CD4(+) T cell aging underlies the development of IDDM in NOD mice. Given that CD28 and IL-2 play important roles in Treg function, the relationships between premature CD4(+) T cell aging and lymphopenia as well as Treg defects in autoimmune-prone NOD mice are proposed.

Interleukin-21 up-regulates interleukin-21R expression and interferon gamma production by CD8+ cells in SHIV-infected macaques

Interleukin-21 (IL-21) is produced primarily by CD4+ T cells and regulates immunity against human/simian immunodeficiency virus (HIV/SIV) infection. Activated CD8+ cells and their secreted interferon-gamma (IFN-γ) are crucial for the control of acute HIV/SIV infection. However, whether IL-21 can regulate IFN-γ production by CD8+ cells remains controversial. Rhesus macaques (RMs, n = 8) were infected with SHIV and the levels of plasma IL-21, IFN-γ and the frequency of peripheral blood activated T cells were measured longitudinally. Following infection with SHIV, the levels of plasma IL-21 and IFN-γ increased, peaked at 17 days postinfection and declined later. Furthermore, IL-21 induced IL-21 receptor (IL-21R) and IFN-γ, perforin, but not granmyze B, expression in CD8+ cells from four selected SHIV-infected RMs. The regulatory effect of IL-21 on CD8+ cell function appeared to be associated with increased levels of STAT3, but not STAT5, phosphorylation in CD8+ cells from SHIV-infected RMs. In parallel, treatment with soluble IL-21R/Fc, an inhibitor of IL-21-induced activation of JAK1/3 and STAT3, abrogated IL-21-induced STAT3 activation and IFN-γ production in CD8+ cells from SHIV-infected RMs in vitro. Our data indicated that IL-21 was a positive regulator of IFN-γ-secreting CD8+ cells and increased the STAT3 phosphorylation, regulating T-cell immunity against acute SHIV infection in RMs. Our findings may provide a new basis for the development of immunotherapies for the control of SHIV/HIV infection.

The PTPN22 1858T allele but not variants in the proximal promoter region of IL-21 gene is associated with the susceptibility to type 1 diabetes and the presence of autoantibodies in a Brazilian cohort

Interleukin (IL)-21 and protein tyrosine phosphatase non-receptor 22 (PTPN22) regulate lymphocyte function and have been implicated in the pathogenesis of autoimmune diabetes. We sequenced the proximal promoter of the IL-21 gene for the first time and analysed the PTPN22 1858T polymorphism in type 1A diabetes (T1AD) patients and healthy controls (HC). We correlated the frequencies of islet and extra-pancreatic autoantibodies with genotypes from both loci. The case series comprised 612 T1AD patients and 792 HC. Genotyping of PTPN22 C1858T was performed on 434 T1AD patients and 689 HC. The -448 to +83 base pairs (bp) region of the IL-21 gene was sequenced in 309 Brazilian T1AD and 189 HC subjects. We also evaluated human leucocyte antigen (HLA) DR3/DR4 alleles. The frequencies of glutamic acid decarboxylase (GAD65), tyrosine phosphatase-like protein (IA)-2, anti-nuclear antibody (ANA), thyroid peroxidase (TPO), thyroglobulin (TG), thyrotrophin receptor autoantibody (TRAb), anti-smooth muscle (ASM) and 21-hydroxylase (21-OH) autoantibodies were higher in T1AD patients than in HC. The PTPN22 1858T allele was associated with an increased risk for developing T1AD [odds ratio (OR) = 1·94; P < 0·001], particularly in patients of European ancestry, and with a higher frequency of GAD65 and TG autoantibodies. HLA-DR3/DR4 alleles predominated in T1AD patients. A heterozygous allelic IL-21 gene variant (g.-241 T > A) was found in only one patient. In conclusion, only PTPN22 C1858T polymorphism and HLA-DR3 and/or DR4 alleles, but not allelic variants in the 5'-proximal region of the IL-21 gene were associated with T1AD risk. Patients with T1AD had increased frequencies of anti-islet-cell, anti-thyroid, anti-nuclear, anti-smooth muscle and anti-21-OH autoantibodies. The C1858T PTPN22 polymorphism was also associated with a higher frequency of GAD65 and TG autoantibodies.

Interleukin-21 receptor-mediated signals control autoreactive T cell infiltration in pancreatic islets

It remains unclear how interleukin-21 receptor (IL-21R) contributes to type 1 diabetes. Here we have shown that dendritic cells (DCs) in the pancreas required IL-21R not for antigen uptake, but to acquire the chemokine receptor CCR7 and migrate into the draining lymph node. Consequently, less antigen, major histocompatibility complex (MHC) class II, and CD86 was provided to autoreactive effector cells in Il21r(-/-) mice, impairing CD4(+) T cell activation, CD40:CD40L interactions, and pancreatic infiltration by autoreactive T cells. CD40 crosslinking restored defective CD4(+) cell expansion and CD4 independently expanded autoreactive CD8(+) cells, but CD8(+) cells still required CD4(+) cells to reach the pancreas and induce diabetes. Diabetes induction by transferred T cells required IL-21R-sufficient host antigen-presenting cells. Transferring IL-21R-sufficient DCs broke diabetes resistance in Il21r(-/-) mice. We conclude that IL-21R controls both antigen transport by DCs and the crucial beacon function of CD4(+) cells for autoreactive CD8(+) cells to reach the islets.

Differential IL-21 signaling in APCs leads to disparate Th17 differentiation in diabetes-susceptible NOD and diabetes-resistant NOD.Idd3 mice

Type 1 diabetes (T1D) is an autoimmune disease that shows familial aggregation in humans and likely has genetic determinants. Disease linkage studies have revealed many susceptibility loci for T1D in mice and humans. The mouse T1D susceptibility locus insulin-dependent diabetes susceptibility 3 (Idd3), which has a homologous genetic interval in humans, encodes cytokine genes Il2 and Il21 and regulates diabetes and other autoimmune diseases; however, the cellular and molecular mechanisms of this regulation are still being elucidated. Here we show that T cells from NOD mice produce more Il21 and less Il2 and exhibit enhanced Th17 cell generation compared with T cells from NOD.Idd3 congenic mice, which carry the protective Idd3 allele from a diabetes-resistant mouse strain. Further, APCs from NOD and NOD.Idd3 mice played a central role in this differential Th17 cell development, and IL-21 signaling in APCs was pivotal to this process. Specifically, NOD-derived APCs showed increased production of pro-Th17 mediators and dysregulation of the retinoic acid (RA) signaling pathway compared with APCs from NOD.Idd3 and NOD.Il21r-deficient mice. These data suggest that the protective effect of the Idd3 locus is due, in part, to differential RA signaling in APCs and that IL-21 likely plays a role in this process. Thus, we believe APCs provide a new candidate for therapeutic intervention in autoimmune diseases.

Immunomodulation and regeneration of islet Beta cells by cytokines in autoimmune type 1 diabetes

Juvenile or type 1 diabetes (T1D) involves autoimmune-mediated destruction of insulin-producing β cells in the islets of Langerhans in the pancreas. Lack of insulin prevents the absorption and metabolism of glucose throughout the body by interfering with cell signaling. Cytokines have been shown to play a key role in β cell destruction and regulation of autoimmunity in T1D. The multiple roles of cytokines in T1D pathogenesis, regulation, and regeneration of β cells presents both promise and challenge for their use in immunotherapy. We found that mycobacterial adjuvants induce various regulatory T cells in the non-obese diabetic (NOD) mouse model of T1D. Cytokines produced by these cells not only regulate innate and adaptive immunity but also prevent the development of diabetes and partially restored normoglycemia in diabetic NOD mice. We discovered that adjuvant immunotherapy upregulated Regenerating (Reg) genes in the islets and induced interleukin 22 (IL-22)-producing Th17 cells. IL-22 is known to upregulate Reg gene expression in islets and could potentially induce regeneration of β cells and prevent their apoptosis. Therefore, cytokines both induce and regulate T1D and have the potential to regenerate and preserve insulin-producing β cells in the islets.

Mapping quantitative trait loci for T lymphocyte subpopulations in peripheral blood in swine

Background

Increased disease resistance through improved general immune capacity would be beneficial for the welfare and productivity of farm animals. T lymphocyte subpopulations in peripheral blood play an important role in immune capacity and disease resistance in animals. However, very little research to date has focused on quantitative trait loci (QTL) for T lymphocyte subpopulations in peripheral blood in swine.

Results

In the study, experimental animals consist of 446 piglets from three different breed populations. To identify QTL for T lymphocyte subpopulations in peripheral blood in swine, the proportions of CD4+, CD8+, CD4+CD8+, CD4+CD8-, CD4-CD8+, and CD4-CD8- T cells and the ratio of CD4+:CD8+ T cells were measured for all individuals before and after challenge with modified live CSF (classical swine fever) vaccine. Based on the combined data of individuals from three breed populations, genome-wide scanning of QTL for these traits was performed based on a variance component model, and the genome wide significance level for declaring QTL was determined via permutation tests as well as FDR (false discovery rate) correction. A total of 27 QTL (two for CD4+CD8+, one for CD4+CD8-, three for CD4-CD8+, two for CD4-CD8-, nine for CD4+, two for CD8+, and eight for CD4+:CD8+ ratio) were identified with significance level of FDR < 0.10, of which 11 were significant at the level of FDR < 0.05, including the five significant at FDR < 0.01.

Conclusions

Within these QTL regions, a number of known genes having potential relationships with the studied traits may serve as candidate genes for these traits. Our findings herein are helpful for identification of the causal genes underlying these immune-related trait and selection for immune capacity of individuals in swine breeding in the future.

No association observed between schizophrenia and non-HLA coeliac disease genes: integration with the initial MYO9B association with coeliac disease

Schizophrenia is a severe psychotic illness with a heterogeneous presentation and a devastating impact on social and occupational function. Worldwide variations in schizophrenia incidence rates suggest that local conditions may modify disease risk. The human leukocyte antigen (HLA) region has been confirmed to be associated with schizophrenia by genome-wide association studies in populations across the world. While the presence of autoimmune processes in a subgroup of schizophrenia cases is contentious, the immune system could allow environmental exposures to lead to schizophrenia by generating improper immune response. To investigate this topic, we reviewed the current evidence of the relationship between schizophrenia and coeliac disease. Based on this review, we performed genetic analysis of the MYO9B gene and the IL-2/IL-21 locus by genotyping SNPs that have been previously associated with coeliac disease or schizophrenia in 223 families, 108 unrelated individuals with schizophrenia and 120 controls. Finding no evidence for association with these two loci in our study samples, we applied meta-analytic techniques to combine our findings with previous reports. This synthesis, in light of our review of previous reports, suggests a differing developmental trajectory for schizophrenia and coeliac disease. It is possible that these two conditions do not share any functional overlap.

Post-operative infection and sepsis in humans is associated with deficient gene expression of γc cytokines and their apoptosis mediators

Introduction

Lymphocyte homeostasis is dependent on the γ_c cytokines. We hypothesised that sepsis in humans is associated with differential gene expression of the γ_c cytokines and their associated apoptosis mediators.

Methods

The study population consisted of a total of 60 patients with severe sepsis, 15 with gram negative bacteraemia, 10 healthy controls and 60 patients undergoing elective lung resection surgery. Pneumonia was diagnosed by CDC NNIC criteria. Gene expression in peripheral blood leukocytes (PBLs) of interleukin (IL)-2, 7, 15 and interferon (IFN)-γ, Bax, Bim, Bcl-2 was determined by qRT-PCR and IL-2 and IL-7 serum protein levels by ELISA. Gene expression of IL-2, 7 and IFN-γ was measured in peripheral blood leukocytes (PBL), cultured in the presence of lipopolysacharide (LPS) and CD3 binding antibody (CD3ab)

Results

IL-2 gene expression was lower in the bacteraemia group compared with controls, and lower still in the sepsis group (P < 0.0001). IL-7 gene expression was similar in controls and bacteraemia, but lower in sepsis (P < 0.0001). IL-15 gene expression was similar in the three groups. Bcl-2 gene expression was less (P < 0.0001) and Bim gene expression was greater (P = 0.0003) in severe sepsis compared to bacteraemic and healthy controls. Bax gene expression was similar in the three groups.

In lung resection surgery patients, post-operative pneumonia was associated with a perioperative decrease in IL-2 mRNA (P < 0.0001) and IL-7 mRNA (P = 0.003). IL-2 protein levels were reduced in sepsis and bacteraemia compared to controls (P = 0.02) but similar in pneumonia and non-pneumonia groups. IL-7 protein levels were similar in all groups.

In cultured PBLs, IFN-γ gene expression was decreased in response to LPS and increased in response to CD3ab with sepsis: IL-7 gene expression increased in response to LPS in controls and to CD3ab with sepsis; Bcl-2 gene expression decreased in response to combined CD3ab and IL-2 with sepsis.

Conclusions

Patients with infection and sepsis have deficient IL-2 and IL-7 gene expression in PBLs. Aberrant cytokine gene expression may precede the onset of infection.

A subset of interleukin-21+ chemokine receptor CCR9+ T helper cells target accessory organs of the digestive system in autoimmunity

This study describes a CD4+ T helper (Th) cell subset marked by coexpression of the cytokine interleukin 21 (IL-21) and the gut-homing chemokine receptor CCR9. Although CCR9+ Th cells were observed in healthy mice and humans, they were enriched in the inflamed pancreas and salivary glands of NOD mice and in the circulation of Sjögren's syndrome patients. CCR9+ Th cells expressed large amounts of IL-21, inducible T cell costimulator (ICOS), and the transcription factors Bcl6 and Maf, and also supported antibody production from B cells, thereby resembling T follicular B helper (Tfh) cells. However, in contrast to Tfh cells, CCR9+ Th cells displayed limited expression of CXCR5 and the targets of CCR9+ Th cells were CD8+ T cells whose responsiveness to IL-21 was necessary for the development of diabetes. Thus, CCR9+ Th cells are a subset of IL-21-producing T helper cells that influence regional specification of autoimmune diseases that affect accessory organs of the digestive system.

An update on the use of NOD mice to study autoimmune (Type 1) diabetes

The widely used nonobese diabetic (NOD) mouse model of autoimmune (Type 1) diabetes mellitus shares multiple characteristics with the human disease, and studies employing this model continue to yield clinically relevant and important information. Here, we review some of the recent key findings obtained from NOD mouse investigations that have both advanced our understanding of disease pathogenesis and suggested new therapeutic targets and approaches. Areas discussed include antigen discovery, identification of genes and pathways contributing to disease susceptibility, development of strategies to image islet inflammation and the testing of therapeutics. We also review recent technical advances that, combined with an improved understanding of the NOD mouse model's limitations, should work to ensure its popularity, utility and relevance in the years ahead.

Abrogation of donor T-cell IL-21 signaling leads to tissue-specific modulation of immunity and separation of GVHD from GVL

IL-21 is a proinflammatory cytokine produced by Th17 cells. Abrogation of IL-21 signaling has recently been shown to reduce GVHD while retaining graft-versus-leukemia/lymphoma (GVL) responses. However, the mechanisms by which IL-21 may lead to a separation of GVHD and GVL remain incompletely understood. In a murine MHC-mismatched BM transplantation model, we observed that IL-21 receptor knockout (IL-21R KO) donor T cells mediate decreased systemic and gastrointestinal GVHD in recipients of a transplant. This reduction in GVHD was associated with expansion of transplanted donor regulatory T cells and with tissue-specific modulation of Th-cell function. IL-21R KO and wild-type donor T cells showed equivalent alloactivation, but IL-21R KO T cells showed decreased infiltration and inflammatory cytokine production within the mesenteric lymph nodes. However, Th-cell cytokine production was maintained peripherally, and IL-21R KO T cells mediated equivalent immunity against A20 and P815 hematopoietic tumors. In summary, abrogation of IL-21 signaling in donor T cells leads to tissue-specific modulation of immunity, such that gastrointestinal GVHD is reduced, but peripheral T-cell function and GVL capacity are retained. IL-21 is thus an exciting target for therapeutic intervention and improvement of clinical transplantation outcomes.

Reduced IL-2 expression in NOD mice leads to a temporal increase in CD62Llo FoxP3+ CD4+ T cells with limited suppressor activity

IL-2 plays a critical role in the induction and maintenance of FoxP3-expressing regulatory T cells (FoxP3(+) Tregs). Reduced expression of IL-2 is linked to T-cell-mediated autoimmune diseases such as type 1 diabetes (T1D), in which an imbalance between FoxP3(+) Tregs and pathogenic T effectors exists. We investigated the contribution of IL-2 to dysregulation of FoxP3(+) Tregs by comparing wildtype NOD mice with animals congenic for a C57BL/6-derived disease-resistant Il2 allele and in which T-cell secretion of IL-2 is increased (NOD.B6Idd3). Although NOD mice exhibited a progressive decline in the frequency of CD62L(hi) FoxP3(+) Tregs due to an increase in CD62L(lo) FoxP3(+) Tregs, CD62L(hi) FoxP3(+) Tregs were maintained in the pancreatic lymph nodes and islets of NOD.B6Idd3 mice. Notably, the frequency of proliferating CD62L(hi) FoxP3(+) Tregs was elevated in the islets of NOD.B6Idd3 versus NOD mice. Increasing levels of IL-2 in vivo also resulted in larger numbers of CD62L(hi) FoxP3(+) Tregs in NOD mice. These results demonstrate that IL-2 influences the suppressor activity of the FoxP3(+) Tregs pool by regulating the balance between CD62L(lo) and CD62L(hi) FoxP3(+) Tregs. In NOD mice, reduced IL-2 expression leads to an increase in nonsuppressive CD62L(lo) FoxP3(+) Tregs, which in turn correlates with a pool of CD62L(hi) FoxP3(+) Tregs with limited proliferation.

Exposure to IL-15 and IL-21 enables autoreactive CD8 T cells to respond to weak antigens and cause disease in a mouse model of autoimmune diabetes

Autoreactive CD8(+) T lymphocytes play a key role in the pathogenesis of several autoimmune diseases. It is not yet well understood how autoreactive CD8(+) T cells, which express TCRs with low reactivity toward self-Ags, gain the ability to respond to autoantigens to cause disease. Previously, we have shown that prior stimulation of CD8(+) T cells with synergistic combinations of cytokines produced by the innate immune response, such as IL-21 and IL-15, induces Ag-independent proliferation. Such "cytokine-primed" CD8 T cells displayed increased responsiveness to limiting quantities of the cognate Ag. In this paper, we report that prior stimulation with IL-15 and IL-21 also enables CD8(+) T cells to respond to weakly agonistic TCR ligands, resulting in proliferation, cytokine secretion, and cytolytic activity. Using a transgenic mouse model of autoimmune diabetes, we show that cytokine-primed autoreactive CD8(+) T cells induce disease following stimulation by weak TCR ligands, but their diabetogenic potential is dependent on continuous availability of IL-15 in vivo. These findings suggest that inflammatory cytokines could facilitate the triggering of autoreactive CD8(+) T cells by weak autoantigens, and this mechanism may have important implications for autoimmune diseases associated with microbial infections and chronic inflammation.

Targeting interleukin-21 in inflammatory diseases

INTRODUCTION

IL-21, a new member of the type 1 cytokine superfamily, is produced by various subsets of CD4(+) T cells and binds to a composite receptor that consists of a specific receptor, termed IL-21 receptor and the common γ-chain subunit. Initially considered to be a critical regulator of T and B cell function, IL-21 is now known to regulate the activity of many other cell types, including both immune and non-immune cells.

AREAS COVERED

In this review, we discuss the biological features of IL-21 and summarize recent advances in the pathogenic role of IL-21 in chronic inflammatory diseases. Moreover, we discuss why IL-21 blockers can have a place in the therapeutic armamentarium for patients with immune-mediated diseases and the potential risks of such treatments.

EXPERT OPINION

Data emerging from studies in human and experimental models of autoimmunity suggest that IL-21 is critically involved in the initiation and/or progression of inflammatory reactions where self-reactive immune cells or antibodies cause damage in tissue. Thus, theoretically, targeting IL-21 could help attenuate the activation of inflammatory pathways and facilitate the resolution of tissue damaging immune responses. However, one should also take into consideration some potential risks that could derive from the blockade of IL-21.

Interleukin-21 is critically required in autoimmune and allogeneic responses to islet tissue in murine models

OBJECTIVE

Type 1 diabetes is an incurable chronic autoimmune disease. Although transplantation of pancreatic islets may serve as a surrogate source of insulin, recipients are subjected to a life of immunosuppression. Interleukin (IL)-21 is necessary for type 1 diabetes in NOD mice. We examined the efficacy of an IL-21-targeted therapy on prevention of diabetes in NOD mice, in combination with syngeneic islet transplantation. In addition, we assessed the role of IL-21 responsiveness in islet allograft rejection in mouse animal models.

RESEARCH DESIGN AND METHODS

NOD mice were treated with IL-21R/Fc, an IL-21-neutralizing chimeric protein. This procedure was combined with syngeneic islet transplantation to treat diabetic NOD mice. Survival of allogeneic islet grafts in IL-21R-deficient mice was also assessed.

RESULTS

Evidence is provided that IL-21 is continually required by the autoimmune infiltrate, such that insulitis was reduced and reversed and diabetes inhibited by neutralization of IL-21 at a late preclinical stage. Recovery from autoimmune diabetes was achieved by combining neutralization of IL-21 with islet transplantation. Furthermore, IL-21-responsiveness by CD8+ T-cells was sufficient to mediate islet allograft rejection.

CONCLUSIONS

Neutralization of IL-21 in NOD mice can inhibit diabetes, and when paired with islet transplantation, this therapeutic approach restored normoglycemia. The influence of IL-21 on a graft-mounted immune response was robust, since the absence of IL-21 signaling prevented islet allograft rejection. These findings suggest that therapeutic manipulation of IL-21 may serve as a suitable treatment for patients with type 1 diabetes.

Aberrant ROCK activation promotes the development of type I diabetes in NOD mice

Aberrant production of IL-21 by T cells is critical for the development of type 1 diabetes (T1D) in NOD mice. The pathogenic effects of IL-21 are partly due to its ability to promote the generation of T(H)-17 cells. Interferon Regulatory Factor (IRF4) is a crucial regulator of IL-17 and IL-21 production. We recently found that the serine-threonine kinase ROCK2 phosphorylates IRF4 and regulates its ability to control IL-17 and IL-21 production. Here we show that NOD T cells aberrantly activate ROCK2. We furthermore demonstrate that ROCK inhibition corrects the abnormal IRF4 function in NOD T cells and diminishes their production of IL-17 and IL-21. Importantly, administration of a ROCK inhibitor to NOD mice protects against diabetes development. These studies thus support the idea that ROCK2 is inappropriately activated in NOD T cells and that ROCK kinases could represent important therapeutic targets for the treatment of T1D.

Phosphorylation of IRF4 by ROCK2 regulates IL-17 and IL-21 production and the development of autoimmunity in mice

Deregulated production of IL-17 and IL-21 plays a key pathogenic role in many autoimmune disorders. A delineation of the mechanisms that underlie the inappropriate synthesis of IL-17 and IL-21 in autoimmune diseases can thus provide important insights into potential therapies for these disorders. Here we have shown that the serine-threonine kinase Rho-associated, coiled-coil-containing protein kinase 2 (ROCK2) becomes activated in mouse T cells under Th17 skewing conditions and phosphorylates interferon regulatory factor 4 (IRF4), a transcription factor that is absolutely required for the production of IL-17 and IL-21. We furthermore demonstrated that ROCK2-mediated phosphorylation of IRF4 regulated the synthesis of IL-17 and IL-21 and the differentiation of Th17 cells. Whereas CD4+ T cells from WT mice activated ROCK2 physiologically under Th17 conditions, CD4+ T cells from 2 different mouse models of spontaneous autoimmunity aberrantly activated ROCK2 under neutral conditions. Moreover, administration of ROCK inhibitors ameliorated the deregulated production of IL-17 and IL-21 and the inflammatory and autoantibody responses observed in these autoimmune mice. Our findings thus uncover a crucial link among ROCK2, IRF4, and the production of IL-17 and IL-21 and support the idea that selective inhibition of ROCK2 could represent an important therapeutic regimen for the treatment of autoimmune disorders.

Confirmation of an association between rs6822844 at the Il2-Il21 region and multiple autoimmune diseases: evidence of a general susceptibility locus

OBJECTIVE

Autoimmune diseases often have susceptibility genes in common, indicating similar molecular mechanisms. Increasing evidence suggests that rs6822844 at the IL2-IL21 region is strongly associated with multiple autoimmune diseases in individuals of European descent. This study was undertaken to attempt to replicate the association between rs6822844 and 6 different immune-mediated diseases in non-European populations, and to perform disease-specific and overall meta-analyses using data from previously published studies.

METHODS

We evaluated case-control associations between rs6822844 and celiac disease (CD) in subjects from Argentina; rheumatoid arthritis (RA), type 1 diabetes mellitus (DM), primary Sjögren's syndrome (SS), and systemic lupus erythematosus (SLE) in subjects from Colombia; and Behçet's disease (BD) in subjects from Turkey. Allele and gene distributions were compared between cases and controls. Meta-analyses were performed using data from the present study and previous studies.

RESULTS

We detected significant associations of rs6822844 with SLE (P = 0.008), type 1 DM (P = 0.014), RA (P = 0.019), and primary SS (P = 0.033) but not with BD (P = 0.34) or CD (P = 0.98). We identified little evidence of population differentiation (F(ST) = 0.01) within cases and controls from Argentina and Colombia, suggesting that association was not influenced by population substructure. Disease-specific meta-analysis indicated significant association for RA (P(meta) = 3.61 x 10(-6)), inflammatory bowel disease (IBD; Crohn's disease and ulcerative colitis) (P(meta) = 3.48 x 10(-12)), type 1 DM (P(meta) = 5.33 x 10(-5)), and CD (P(meta) = 5.30 x 10(-3)). Overall meta-analysis across all autoimmune diseases reinforced association with rs6822844 (23 data sets; P(meta) = 2.61 x 10(-25), odds ratio 0.73 [95% confidence interval 0.69-0.78]).

CONCLUSION

Our results indicate that there is an association between rs6822844 and multiple autoimmune diseases in non-European populations. Meta-analysis results strongly reinforce this robust association across multiple autoimmune diseases in both European-derived and non-European populations.

Lineage specification and heterogeneity of T follicular helper cells

T follicular helper (Tfh) cells were originally described as a non-polarized CD4(+) T cell subset with follicular homing capacity and a potent ability to induce antibody production from B cells. However, a number of studies published in the past year have revealed a degree of heterogeneity within the germinal center CD4(+) T cell population, which suggests additional complexity. The overzealous activities of Tfh cells, or inappropriate expression of certain cytokines, represent new pathways for the development of autoimmune diseases. This review focuses on current progress in unraveling the biology of Tfh cells in health and disease, and understanding the relationship of Tfh cells to other CD4(+) T cell lineages.

Loss of parity between IL-2 and IL-21 in the NOD Idd3 locus

IL-2 and IL-21 are two cytokines with great potential to affect autoimmune infiltration of nonlymphoid tissue, and are contained within the strongest non-MHC-linked locus for type 1 diabetes (T1D) susceptibility on the nonobese diabetic (NOD) mouse (Idd3). IL-21 is necessary for the development of diabetes in the NOD mouse, but a number of important studies argue that decreased expression of IL-2 explains Idd3. In this study, we demonstrate that the amount of IL-21, but not IL-2, correlated with T1D incidence. During our analyses of the IL-2/IL-21 interval, we found that mice segregate into one of two distinct expression profiles. In the first group, which includes the C57BL/6 strain, both Il2 and Il21 were expressed at low levels. In the other group, which includes the NOD strain, Il2 and Il21 were both highly expressed. However, because NOD IL-2 mRNA was relatively unstable, IL-2 production was remarkably similar between strains. The increased production of IL-21 in NOD mice was found to result from two single nucleotide polymorphisms within the distal promoter region that conferred increased binding affinity for the transcription factor Sp1. Our findings indicate that a loss of locus parity after decreased IL-2 mRNA stability ensures that the high-expressing IL-21 allele persists in nature and provides a basis for autoimmunity.

Lymphocyte proliferation in immune-mediated diseases

Defects in T cell homeostatic mechanisms can result in T cell lymphopenia, defined as decreased numbers of lymphocytes. Lymphopenia results in homeostatic proliferation in order to maintain T cell homeostasis. It has been proposed that homeostatic proliferation can expand the pool of autoreactive T cells that promote autoimmunity, and indeed recent studies have further substantiated this observation in both animal models and humans. Conversely, homeostatic proliferation can promote tumor immunity by allowing tumor-specific T cells to accumulate. In this review, we discuss how the outcome of homeostatic proliferation can function both in a deleterious manner in autoimmunity and a beneficial way in tumor immunity. We also discuss the roles of various cytokines and T regulatory cells that control homeostatic proliferation.

New insights into the regulation of T cells by gamma(c) family cytokines

Common cytokine receptor gamma-chain (gamma(c)) family cytokines have crucial roles in the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immune systems. In this Review, we focus on our current understanding of the distinct and overlapping effects of interleukin-2 (IL-2), IL-7, IL-9, IL-15 and IL-21, as well as the IL-7-related cytokine thymic stromal lymphopoietin (TSLP), on the survival and proliferation of conventional alphabeta T cells, gammadelta T cells and regulatory T cells. This knowledge potentially allows for the therapeutic manipulation of immune responses for the treatment of cancer, autoimmunity, allergic diseases and immunodeficiency, as well as for vaccine development.

Cutting edge: IL-27 induces the transcription factor c-Maf, cytokine IL-21, and the costimulatory receptor ICOS that coordinately act together to promote differentiation of IL-10-producing Tr1 cells

IL-27 has recently been identified as a differentiation factor for the generation of IL-10-producing regulatory type 1 (Tr1) T cells. However, how IL-27 induces the expansion of Tr1 cells has not been elucidated. In this study we demonstrate that IL-27 drives the expansion and differentiation of IL-10-producing murine Tr1 cells by inducing three key elements: the transcription factor c-Maf, the cytokine IL-21, and the costimulatory receptor ICOS. IL-27-driven c-Maf expression transactivates IL-21 production, which acts as an autocrine growth factor for the expansion and/or maintenance of IL-27-induced Tr1 cells. ICOS further promotes IL-27-driven Tr1 cells. Each of those elements is essential, because loss of c-Maf, IL-21-signaling, or ICOS decreases the frequency of IL-27-induced differentiation of IL-10-producing Tr1 cells.

Interleukin-21 is required for the development of type 1 diabetes in NOD mice

OBJECTIVE

Interleukin (IL)-21 is a type 1 cytokine that has been implicated in the pathogenesis of type 1 diabetes via the unique biology of the nonobese diabetic (NOD) mouse strain. The aim of this study was to investigate a causal role for IL-21 in type 1 diabetes.

RESEARCH DESIGN AND METHODS

We generated IL-21R-deficient NOD mice and C57Bl/6 mice expressing IL-21 in pancreatic beta-cells, allowing the determination of the role of insufficient and excessive IL-21 signaling in type 1 diabetes.

RESULTS

Deficiency in IL-21R expression renders NOD mice resistant to insulitis, production of insulin autoantibodies, and onset of type 1 diabetes. The lymphoid compartment in IL-21R-/- NOD is normal and does not contain an increased regulatory T-cell fraction or diminished effector cytokine responses. However, we observed a clear defect in autoreactive effector T-cells in IL-21R-/- NOD by transfer experiments. Conversely, overexpression of IL-21 in pancreatic beta-cells induced inflammatory cytokine and chemokines, including IL-17A, IL17F, IFN-gamma, monocyte chemoattractant protein (MCP)-1, MCP-2, and interferon-inducible protein-10 in the pancreas. The ensuing leukocytic infiltration in the islets resulted in destruction of beta-cells and spontaneous type 1 diabetes in the normally diabetes-resistant C57Bl/6 and NOD x C57Bl/6 backgrounds.

CONCLUSIONS

This work provides demonstration of the essential prodiabetogenic activities of IL-21 on diverse genetic backgrounds (NOD and C57BL/6) and indicates that IL-21 blockade could be a promising strategy for interventions in human type 1 diabetes.

Interleukin-21 (IL-21)-mediated pathways in T cell-mediated disease

Interleukin-21 (IL-21) is produced mostly by activated CD4+ T cells and controls the differentiation and functional activity of effector T helper cells, counteracts the suppressive effects of regulatory T cells, and stimulates non-immune cells to make inflammatory mediators. IL-21-driven tissue damage has been demonstrated in a number of organs, such as the gut, pancreas, and brain. Therefore new treatment modalities to neutralise IL-21 in vivo would be a valuable addition to the therapeutic armamentarium to combat immune-mediated inflammation. Here we describe the emerging role of IL-21 in the initiation and progress of the tissue-damaging inflammatory response in immune-mediated pathologies.