Coordinated induction by IL15 of a TCR-independent NKG2D signaling pathway converts CTL into lymphokine-activated killer cells in celiac disease

Human intestinal dendritic cell and macrophage subsets in coeliac disease

Dendritic cells (DC) and macrophages (Mϕ) constitute the most abundant antigen presenting cells in the human intestinal mucosa. In resting conditions, they are essential to maintain the mechanisms of immune tolerance toward food antigens and commensals, at the time that they keep the capacity to initiate and maintain antigen-specific pro-inflammatory immune responses toward invading pathogens. Nevertheless, this delicate equilibrium between immunity and tolerance is not perfect, like in coeliac disease (CD), where DC and Mϕ drive the development of antigen-specific immune responses toward dietary gluten peptides. In this review, we provide therefore a comprehensive discussion about CD pathogenesis, the human intestinal immune system and the biology of intestinal DC and Mϕ both in resting conditions and in CD. Last, but not least, we discuss about all the remaining issues pending to be studied regarding DC and Mϕ contribution toward CD pathogenesis. This may allow the identification of unique and specific factors which may be useful in the clinical practice, as well as identify new therapeutic targets in order to reestablish the loss intestinal homeostasis in CD.

IL-33 Alarmin and Its Active Proinflammatory Fragments Are Released in Small Intestine in Celiac Disease

Initially described as Th2 promoter cytokine, more recently, IL-33 has been recognized as an alarmin, mainly in epithelial and endothelial cells. While localized in the nucleus acting as a gene regulator, it can be also released after injury, stress or inflammatory cell death. As proinflammatory signal, IL-33 binds to the surface receptor ST2, which enhances mast cell, Th2, regulatory T cell, and innate lymphoid cell type 2 functions. Besides these Th2 roles, free IL-33 can activate CD8⁺ T cells during ongoing Th1 immune responses to potentiate its cytotoxic function. Celiac Disease (CD) is a chronic inflammatory disorder characterized by a predominant Th1 response leading to multiple pathways of mucosal damage in the proximal small intestine. By immunofluorescence and western blot analysis of duodenal tissues, we found an increased expression of IL-33 in duodenal mucosa of active CD (ACD) patients. Particularly, locally digested IL-33 releases active 18/21kDa fragments which can contribute to expand the proinflammatory signal. Endothelial (CD31⁺) and mesenchymal, myofibroblast and pericyte cells from microvascular structures in villi and crypts, showed IL-33 nuclear location; while B cells (CD20⁺) showed a strong cytoplasmic staining. Both ST2 forms, ST2L and sST2, were also upregulated in duodenal mucosa of CD patients. This was accompanied by increased number of CD8⁺ST2⁺ T cells and the expression of T-bet in some ST2⁺ intraepithelial lymphocytes and lamina propria cells. IL-33 and sST2 mRNA levels correlated with IRF1, an IFN induced factor relevant in responses to viral infections and interferon mediated proinflammatory responses highly represented in duodenal tissues in ACD. These findings highlight the potential contribution of IL-33 and its fragments to exacerbate the proinflammatory circuit and potentiate the cytotoxic activity of CD8⁺ T cells in CD pathology.

Interleukin-15 in autoimmunity

Interleukin-15 (IL-15) is a member of the IL-2 family of cytokines, which use receptor complexes containing the common gamma (γ_c) chain for signaling. IL-15 plays important roles in innate and adaptative immune responses and is implicated in the pathogenesis of several immune diseases. The IL-15 receptor consists of 3 subunits namely, the ligand-binding IL-15Rα chain, the β chain (also used by IL-2) and the γ_c chain. IL-15 uses a unique signaling pathway whereby IL-15 associates with IL-15Rα during biosynthesis, and this complex is 'trans-presented' to responder cells that expresses the IL-2/15Rβγ_c receptor complex. IL-15 is subject to post-transcriptional and post-translational regulation, and evidence also suggests that IL-15 cis-signaling can occur under certain conditions. IL-15 has been implicated in the pathology of various autoimmune diseases such as rheumatoid arthritis, autoimmune diabetes, inflammatory bowel disease, coeliac disease and psoriasis. Studies with pre-clinical models have shown the beneficial effects of targeting IL-15 signaling in autoimmunity. Unlike therapies targeting other cytokines, anti-IL-15 therapies have not yet been successful in humans. We discuss the complexities of IL-15 signaling in autoimmunity and explore potential immunotherapeutic approaches to target the IL-15 signaling pathway.

Mechanisms of activation of innate-like intraepithelial T lymphocytes

Intraepithelial T lymphocytes (T-IEL) contain subsets of innate-like T cells that evoke innate and adaptive immune responses to provide rapid protection at epithelial barrier sites. In the intestine, T-IEL express variable T cell antigen receptors (TCR), with unknown antigen specificities. Intriguingly, they also express multiple inhibitory receptors, many of which are normally found on exhausted or antigen-experienced T cells. This pattern suggests that T-IEL are antigen-experienced, yet it is not clear where, and in what context, T-IEL encounter TCR ligands. We review recent evidence indicating TCR antigens for intestinal innate-like T-IEL are found on thymic or intestinal epithelium, driving agonist selection of T-IEL. We explore the contributions of the TCR and various co-stimulatory and co-inhibitory receptors in activating T-IEL effector functions. The balance between inhibitory and activating signals may be key to keeping these highly cytotoxic, rapidly activated cells in check, and key to harnessing their immune surveillance potential.

Human liver CD8+ MAIT cells exert TCR/MR1-independent innate-like cytotoxicity in response to IL-15

BACKGROUND & AIMS

Mucosal-associated invariant T (MAIT) cells, the most abundant innate-like T cells in the human liver, can be activated by cytokines during viral infection without TCR stimulation. Here, we examined the mechanisms underlying TCR/MR1-independent innate-like cytotoxicity of cytokine-activated liver MAIT cells. We also examined the phenotype and function of MAIT cells from patients with acute viral hepatitis.

METHODS

We obtained liver sinusoidal mononuclear cells from donor liver perfusate during liver transplantation and examined the effect of various cytokines on liver MAIT cells using flow cytometry and in vitro cytotoxicity assays. We also obtained peripheral blood and liver-infiltrating T cells from patients with acute hepatitis A (AHA) and examined the phenotype and function of MAIT cells using flow cytometry.

RESULTS

IL-15-stimulated MAIT cells exerted granzyme B-dependent innate-like cytotoxicity in the absence of TCR/MR1 interaction. PI3K-mTOR signaling, NKG2D ligation, and CD2-mediated conjugate formation were critically required for this IL-15-induced innate-like cytotoxicity. MAIT cells from patients with AHA exhibited activated and cytotoxic phenotypes with higher NKG2D expression. The innate-like cytotoxicity of MAIT cells was significantly increased in patients with AHA and correlated with serum alanine aminotransferase levels.

CONCLUSIONS

Taken together, the results demonstrate that liver MAIT cells activated by IL-15 exert NKG2D-dependent innate-like cytotoxicity in the absence of TCR/MR1 engagement. Furthermore, the innate-like cytotoxicity of MAIT cells is associated with liver injury in patients with AHA, suggesting that MAIT cells contribute to immune-mediated liver injury.

LAY SUMMARY

Immune-mediated liver injury commonly occurs during viral infections of the liver. Mucosal-associated invariant T (MAIT) cells are the most abundant innate-like T cells in the human liver. Herein, we have identified a mechanism by which MAIT cells circumvent conventional T cell receptor interactions to exert cytotoxicity. We show that this innate-like cytotoxicity is increased during acute hepatitis A virus infection and correlates with the degree of hepatocyte injury.

Arming cytotoxic lymphocytes for cancer immunotherapy by means of the NKG2D/NKG2D-ligand system

INTRODUCTION

The activating NKG2D receptor plays a central role in the immune recognition and elimination of abnormal self-cells by cytotoxic lymphocytes. NKG2D binding to cell stress-inducible ligands (NKG2DL) up-regulated on cancer cells facilitates their immunorecognition. Yet tumor cells utilize various escape mechanisms to avert NKG2D-based immunosurveillance. Hence, therapeutic strategies targeting the potent NKG2D/NKG2DL axis and such immune escape mechanisms become increasingly attractive in cancer therapy.

AREAS COVERED

This perspective provides a brief introduction into the NKG2D/NKG2DL axis and its relevance for cancer immune surveillance. Subsequently, the most advanced therapeutic approaches targeting the NKG2D system are presented focusing on NKG2D-CAR engineered immune cells and antibody-mediated strategies to inhibit NKG2DL shedding by tumors.

EXPERT OPINION

Thus far, NKG2D-CAR engineered lymphocytes represent the most advanced therapeutic approach utilizing the NKG2D system. Similarly to other tumor-targeting CAR approaches, NKG2D-CAR cells demonstrate powerful on-target activity, but may also cause off-tumor toxicities or lose efficacy, if NKG2DL expression by tumors is reduced. However, NKG2D-CAR cells also act on the tumor microenvironment curtailing its immunosuppressive properties, thus providing an independent therapeutic benefit. The potency of tumoricidal NKG2D-expressing lymphocytes can be further boosted by enhancing NKG2DL expression through small molecules and therapeutic antibodies inhibiting tumor-associated shedding of NKG2DL.

NKG2D Signaling Within the Pancreatic Islets Reduces NOD Diabetes and Increases Protective Central Memory CD8+ T-Cell Numbers

NKG2D is implicated in autoimmune diabetes. However, the role of this receptor in diabetes pathogenesis is unclear owing to conflicting results with studies involving global inhibition of NKG2D signaling. We found that NKG2D and its ligands are present in human pancreata, with expression of NKG2D and its ligands increased in the islets of patients with type 1 diabetes. To directly assess the role of NKG2D in the pancreas, we generated NOD mice that express an NKG2D ligand in β-islet cells. Diabetes was reduced in these mice. The reduction corresponded with a decrease in the effector to central memory CD8⁺ T-cell ratio. Further, NKG2D signaling during in vitro activation of both mouse and human CD8⁺ T cells resulted in an increased number of central memory CD8⁺ T cells and diabetes protection by central memory CD8⁺ T cells in vivo. Taken together, these studies demonstrate that there is a protective role for central memory CD8⁺ T cells in autoimmune diabetes and that this protection is enhanced with NKG2D signaling. These findings stress the importance of anatomical location when determining the role NKG2D signaling plays, as well as when developing therapeutic strategies targeting this pathway, in type 1 diabetes development.

Granzyme B Inhibition by Tofacitinib Blocks the Pathology Induced by CD8 T Cells in Cutaneous Leishmaniasis

In cutaneous leishmaniasis, the immune response is not only protective but also mediates immunopathology. We previously found that cytolytic CD8 T cells promote inflammatory responses that are difficult to treat with conventional therapies that target the parasite. Therefore, we hypothesized that inhibiting CD8 T-cell cytotoxicity would reduce disease severity in patients. IL-15 is a potential target for such a treatment because it is highly expressed in human patients with cutaneous leishmaniasis lesions and promotes granzyme B‒dependent CD8 T-cell cytotoxicity. Here we tested whether tofacitinib, which inhibits IL-15 signaling by blocking Jak3, might decrease CD8-dependent pathology. We found that tofacitinib reduced the expression of granzyme B by CD8 T cells in vitro and in vivo systemic and topical treatment, with tofacitinib protecting mice from developing severe cutaneous leishmaniasis lesions. Importantly, tofacitinib treatment did not alter T helper type 1 responses or parasite control. Collectively, our results suggest that host-directed therapies do not need to be limited to autoimmune disorders and that topical tofacitinib application should be considered a strategy for the treatment of cutaneous leishmaniasis disease in combination with antiparasitic drugs.

Research Progress on NK Cell Receptors and Their Signaling Pathways

Natural killer cells (NK cells) play an important role in innate immunity. NK cells recognize self and nonself depending on the balance of activating receptors and inhibitory receptors. After binding to their ligands, NK cell receptors trigger subsequent signaling conduction and then determine whether NK is activated or inhibited. Furthermore, NK cell response includes cytotoxicity and cytokine release, which is tightly related to the activation of NK cell-activating receptors and the inhibition of inhibitory receptors on the surfaces of NK cells. The expression and function of NK cell surface receptors also alter in virus infection, tumor, and autoimmune diseases and influence the occurrence and development of diseases. So, it is important to understand the mechanism of recognition between NK receptors and their ligands in pathological conditions and the signaling pathways of NK cell receptors. This review mainly summarizes the research progress on NK cell surface receptors and their signal pathways.

Deciphering human γδ T cell response in cancer: Lessons from tumor-infiltrating γδ T cells

The finding that γδ T cells are present among tumor-infiltrating lymphocytes in humans suggests they participate in tumor immune surveillance, but their relevance is unclear because the relative abundance of tumor-infiltrating γδ T cells correlates with positive or negative, or even do not correlate with prognosis. This likely depends on the fact that tumor-infiltrating γδ T cells may play substantially different effector or regulatory functions, and correlation with patient's prognosis relies on distinct γδ T cell subsets in the context of the tumor. There is interest to exploit γδ T cells in tumor immunotherapy, but to make this approach successful there is urgent need to fully understand the biological functions of γδ T cells and of how they can be manipulated in vivo and ex vivo to safely provide benefit to the host. This review focuses on our previous and ongoing studies of tumor-infiltrating γδ T lymphocytes in different types of human cancer. Moreover, we discuss the interaction of tumor-infiltrating γδ T cells with other cells and molecules present in the tumor microenvironment, and their clinical relevance on the ground, that deep knowledge in this field can be used further for better immunotherapeutic intervention in cancer.

Intestinal intraepithelial lymphocytes: Maintainers of intestinal immune tolerance and regulators of intestinal immunity

Intestinal immune tolerance is essential for the immune system, as it prevents abnormal immune responses to large quantities of antigens from the intestinal lumen, such as antigens from commensal microorganisms, and avoids self‐injury. Intestinal intraepithelial lymphocytes (IELs), a special group of mucosal T lymphocytes, play a significant role in intestinal immune tolerance. To accomplish this, IELs exhibit a high threshold of activation and low reactivity to most antigens from the intestinal lumen. In particular, CD8αα⁺TCRαβ⁺ IELs, TCRγδ⁺ IELs, and CD4⁺CD8αα⁺ IELs show great potential for maintaining intestinal immune tolerance and regulating intestinal immunity. However, if the intestinal microenvironment becomes abnormal or intestinal tolerance is broken, IELs may be activated abnormally and become pathogenic.

Coeliac Disease Pathogenesis: The Uncertainties of a Well-Known Immune Mediated Disorder

Coeliac disease is a common small bowel enteropathy arising in genetically predisposed individuals and caused by ingestion of gluten in the diet. Great advances have been made in understanding the role of the adaptive immune system in response to gluten peptides. Despite detailed knowledge of these adaptive immune mechanisms, the complete series of pathogenic events responsible for development of the tissue lesion remains less certain. This review contributes to the field by discussing additional mechanisms which may also contribute to pathogenesis. These include the production of cytokines such as interleukin-15 by intestinal epithelial cells and local antigen presenting cells as a pivotal event in the disease process. A subset of unconventional T cells called gamma/delta T cells are also persistently expanded in the coeliac disease (CD) small intestinal epithelium and recent analysis has shown that these cells contribute to pathogenic inflammation. Other unconventional T cell subsets may play a local immunoregulatory role and require further study. It has also been suggested that, in addition to activation of pathogenic T helper cells by gluten peptides, other peptides may directly interact with the intestinal mucosa, further contributing to the disease process. We also discuss how myofibroblasts, a major source of tissue transglutaminase and metalloproteases, may play a key role in intestinal tissue remodeling. Contribution of each of these factors to pathogenesis is discussed to enhance our view of this complex disorder and to contribute to a wider understanding of chronic immune-mediated disease.

mTOR sustains inflammatory response in celiac disease

Celiac disease (CD) is an enteropathy triggered by the ingestion of gluten proteins in genetically predisposed individuals and characterized by excessive activation of effector immune cells and enhanced production of inflammatory cytokines. However, factors/mechanisms that amplify the ongoing mucosal inflammation in CD are not fully understood. In this study, we assessed whether mammalian target of Rapamycin (mTOR), a pathway that combines intra- and extra-cellular signals and acts as a central regulator for the metabolism, growth, and function of immune and non-immune cells, sustains CD-associated immune response. Our findings indicate that expression of phosphorylated (p)/active form of mTOR is increased in protein lysates of duodenal biopsy samples taken from patients with active CD (ACD) as compared to normal controls. In ACD, activation of mTOR occurs mainly in the epithelial compartment and associates with enhanced expression of p-4EBP, a downstream target of mTOR complex (mTORC)1, while expression of p-Rictor, a component of mTORC2, is not increased. Stimulation of mucosal explants of inactive CD patients with pepsin-trypsin-digested (PT)-gliadin or IFN-γ/IL-21, two cytokines produced in CD by gluten-specific T cells, increases p-4EBP expression. Consistently, blockade of such cytokines in cultures of ACD mucosal explants reduces p-4EBP. Finally, we show that inhibition of mTORC1 with rapamycin in ACD mucosal explants reduces p-4EBP and production of IL-15, a master cytokine produced by epithelial cells in this disorder. Our data suggest that ACD inflammation is marked by activation of mTORC1 in the epithelial compartment.

Intestinal Anti-tissue Transglutaminase2 Autoantibodies: Pathogenic and Clinical Implications for Celiac Disease

Celiac disease (CD) is a systemic disease that primarily affects the small intestine. The presence of anti-tissue transglutaminase 2 (anti-TG2) antibodies in the serum, as well as the presence of autoimmune phenomena, account for the inclusion of CD among autoimmune diseases. Anti-TG2 autoantibodies are produced at intestinal level, where they are deposited even before they appear in circulation. The pathogenic events that lead to their production are still not completely defined, but a central role seems to be played by gliadin-specific T cells. Interestingly, limited somatic mutations have been observed in VH and VL genes in TG2-specific plasma cells, another important aspect being the biased use of a heavy chain encoded by the VH5 gene. Conflicting data have been produced over the years on the effect of anti-TG2 antibodies on TG2 function. Although the presence of anti-TG2 antibodies in serum is considered a hallmark of CD and relevant from a clinical viewpoint, the role of these autoantibodies in the development of the celiac lesion remains to be defined. In the years, different technical approaches have been implemented to detect and measure intestinal CD-associated autoantibody production. Two aspects can make intestinal anti-TG2 antibodies relevant: from a clinical viewpoint: the first is their proposed ability in potential coeliac patients to predict the development of a full-blown enteropathy; the second is their possible role in revealing a condition of reactivity to gluten in patients with no circulating CD-associated autoantibodies. In fact, the detection of CD-specific autoantibodies production in the intestine, in the absence of serum positivity for the same antibodies, could be suggestive of a very early condition of gluten reactivity; alternatively, it could be not specific for CD and merely attributable to intestinal inflammation. In conclusion, the role of mucosal anti-TG2 antibodies in pathogenesis of CD is unknown. Their presence, the modalities of their production, their gluten dependence render them a unique model to study autoimmunity.

It Takes Two to Tango

In the study by Jacquemin et al., the authors reported that ligands for NKG2D are upregulated in vitiligo perilesional skin and especially in patients with active disease. The reasons for the elevated expression of NKG2D ligands are unknown. This study, however, provides a framework for understanding vitiligo: Skin resident CD8 T cells recognize and kill melanocytes through NKG2D signaling. This event results in the increased production and release of cyto/chemokines and the development of long-lasting CD8 T cells, which in turn causes the recruitment of new T cells, thus perpetuating and disseminating the disease.

Gut Microbiota in Celiac Disease: Is There Any Role for Probiotics?

Celiac disease (CD) is an immune-mediated disorder initiated by the ingestion of gluten in genetically predisposed individuals. Recent data shows that changes in the gut microbiome composition and function are linked with chronic inflammatory diseases; this might also be the case for CD. The main aim of this manuscript is to discuss our present knowledge of the relationships between gut microbiota alterations and CD and to understand if there is any role for probiotics in CD therapy. PubMed was used to search for all of the studies published from November 2009 to November 2019 using key words such as “Celiac Disease” and “Microbiota” (306 articles), “Celiac Disease” and “Gastrointestinal Microbiome” (139), and “Probiotics” and “Celiac Disease” (97 articles). The search was limited to articles published in English that provided evidence-based data. Literature analysis showed that the gut microbiota has a well-established role in gluten metabolism, in modulating the immune response and in regulating the permeability of the intestinal barrier. Promising studies suggest a possible role of probiotics in treating and/or preventing CD. Nevertheless, human trials on the subject are still scarce and lack homogeneity. A possible role was documented for probiotics in improving CD-related symptoms, modulating the peripheral immune response and altering the fecal microbiota, although the results were not consistent in all of the studies. No evidence was found that probiotic administration might prevent CD onset. Knowledge of the role of intestinal bacteria in the development of CD opens new possibilities for its treatment through probiotic administration, even though further studies are needed to better clarify whether probiotics can help treat or prevent the disease and to define which probiotics to use, at what dose and for how long.

Iron Absorption in Celiac Disease and Nutraceutical Effect of 7-Hydroxymatairesinol. Mini-Review

Anemia is the main extra-gastrointestinal symptom in inflammatory bowel diseases (IBDs). Interleukin-6 (IL-6) and other cytokines are secreted and act in the microenvironment of the small intestine mucous membrane of IBD patients. Iron is essential for multiple cell functions and its homeostasis is regulated by the hepcidin–ferroportin axis. Hepcidin (HEPC) is mainly produced by the liver in response to iron needs but is also an acute phase protein. During inflammation, hepcidin is upregulated by IL-6 and is responsible for iron compartmentalization within cells, in turn causing anemia of inflammation. Tissues other than liver can produce hepcidin in response to inflammatory stimuli, in order to decrease iron efflux at a local level, then acting in an autocrine–paracrine manner. In IBDs and, in particular, in celiac disease (CeD), IL-6 might trigger the expression, upregulation and secretion of hepcidin in the small intestine, reducing iron efflux and exacerbating defective iron absorption. 7-Hydroxymatairesinol (7-HMR) belongs to the family of lignans, polyphenolic compounds produced by plants, and has nutraceutical antioxidant, anti-inflammatory and estrogenic properties. In this mini-review we revise the role of inflammation in IBDs and in particular in CeD, focusing our attention on the close link among inflammation, anemia and iron metabolism. We also briefly describe the anti-inflammatory and estrogenic activity of 7-HMR contained in foods that are often consumed by CeD patients. Finally, considering that HEPC expression is regulated by iron needs, inflammation and estrogens, we explored the hypothesis that 7-HMR consumption could ameliorate anemia in CeD using Caco-2 cells as bowel model. Further studies are needed to verify the regulation pathway through which 7-HMR may interfere with the local production of HEPC in bowel.

Infection History Determines Susceptibility to Unrelated Diseases

Epidemiological data suggest that previous infections can alter an individual's susceptibility to unrelated diseases. Nevertheless, the underlying mechanisms are not completely understood. Substantial research efforts have expanded the classical concept of immune memory to also include long-lasting changes in innate immunity and antigen-independent reactivation of adaptive immunity. Collectively, these processes provide possible explanations on how acute infections might induce long-term changes that also affect immunity to unrelated diseases. Here, we review lasting changes the immune compartment undergoes upon infection and how infection experience alters the responsiveness of immune cells towards universal signals. This heightened state of alert enhances the ability of the immune system to combat even unrelated infections but may also increase susceptibility to autoimmunity. At the same time, infection-induced changes in the regulatory compartment may dampen subsequent immune responses and promote pathogen persistence. The concepts presented here outline how infection-induced changes in the immune system may affect human health.

IL-15, gluten and HLA-DQ8 drive tissue destruction in coeliac disease

Coeliac disease is a complex, polygenic inflammatory enteropathy caused by exposure to dietary gluten that occurs in a subset of genetically susceptible individuals who express either the HLA-DQ8 or HLA-DQ2 haplotypes1,2. The need to develop non-dietary treatments is now widely recognized3, but no pathophysiologically relevant gluten- and HLA-dependent preclinical model exists. Furthermore, although studies in humans have led to major advances in our understanding of the pathogenesis of coeliac disease4, the respective roles of disease-predisposing HLA molecules, and of adaptive and innate immunity in the development of tissue damage, have not been directly demonstrated. Here we describe a mouse model that reproduces the overexpression of interleukin-15 (IL-15) in the gut epithelium and lamina propria that is characteristic of active coeliac disease, expresses the predisposing HLA-DQ8 molecule, and develops villous atrophy after ingestion of gluten. Overexpression of IL-15 in both the epithelium and the lamina propria is required for the development of villous atrophy, which demonstrates the location-dependent central role of IL-15 in the pathogenesis of coeliac disease. In addition, CD4+ T cells and HLA-DQ8 have a crucial role in the licensing of cytotoxic T cells to mediate intestinal epithelial cell lysis. We also demonstrate a role for the cytokine interferon-γ (IFNγ) and the enzyme transglutaminase 2 (TG2) in tissue destruction. By reflecting the complex interaction between gluten, genetics and IL-15-driven tissue inflammation, this mouse model provides the opportunity to both increase our understanding of coeliac disease, and develop new therapeutic strategies.

Phenotypic shift of small intestinal intra-epithelial type 1 innate lymphoid cells in celiac disease is associated with enhanced cytotoxic potential

The small intestinal (SI) epithelium harbors a heterogeneous population of lymphocytes that mediate mucosal damage and repair in celiac disease (CD). The composition and roles of human proximal SI intra-epithelial innate lymphoid cells (ILCs), and their alterations in CD, are not well understood. We report that duodenal intra-epithelial ILCs predominantly consist of natural killer (NK)p44⁺ CD127^- cytotoxic ILC1s and NKp44^- CD127⁺ helper ILC1s, while ILC3s only represent a minor population. In patients with newly diagnosed or active CD (ACD) and refractory CD type 1 (RCD I), the frequency of SI NKp44⁺ ILCs is decreased, with restoration of NKp44⁺ ILC frequency observed in patients adhering to a gluten-free diet who show evidence of mucosal healing. Moreover, the frequency of SI NKp44^- ILCs is increased in ACD and RCD I patients and correlates with the severity of villous atrophy and epithelial damage, as assessed by serum levels of fatty acid binding protein 2 (FABP2). We show that the ILC alterations in CD represent a phenotypic shift of cytotoxic ILC1s rather than an increase in helper ILC1s or transdifferentiation of ILC1s to ILC3s, and activation-induced loss of NKp44 by cytotoxic ILC1s is associated with increased interferon (IFN)-γ expression and release of lytic granules. These findings suggest that intra-epithelial NKp44^- CD127^- cytotoxic ILC1s may contribute to mucosal damage in CD.

NKp46-expressing human gut-resident intraepithelial Vδ1 T cell subpopulation exhibits high antitumor activity against colorectal cancer

γδ T cells account for a large fraction of human intestinal intraepithelial lymphocytes (IELs) endowed with potent antitumor activities. However, little is known about their origin, phenotype, and clinical relevance in colorectal cancer (CRC). To determine γδ IEL gut specificity, homing, and functions, γδ T cells were purified from human healthy blood, lymph nodes, liver, skin, and intestine, either disease-free, affected by CRC, or generated from thymic precursors. The constitutive expression of NKp46 specifically identifies a subset of cytotoxic Vδ1 T cells representing the largest fraction of gut-resident IELs. The ontogeny and gut-tropism of NKp46+/Vδ1 IELs depends both on distinctive features of Vδ1 thymic precursors and gut-environmental factors. Either the constitutive presence of NKp46 on tissue-resident Vδ1 intestinal IELs or its induced expression on IL-2/IL-15-activated Vδ1 thymocytes are associated with antitumor functions. Higher frequencies of NKp46+/Vδ1 IELs in tumor-free specimens from CRC patients correlate with a lower risk of developing metastatic III/IV disease stages. Additionally, our in vitro settings reproducing CRC tumor microenvironment inhibited the expansion of NKp46+/Vδ1 cells from activated thymic precursors. These results parallel the very low frequencies of NKp46+/Vδ1 IELs able to infiltrate CRC, thus providing insights to either follow-up cancer progression or to develop adoptive cellular therapies.

The activation of bystander CD8+ T cells and their roles in viral infection

During viral infections, significant numbers of T cells are activated in a T cell receptor-independent and cytokine-dependent manner, a phenomenon referred to as "bystander activation." Cytokines, including type I interferons, interleukin-18, and interleukin-15, are the most important factors that induce bystander activation of T cells, each of which plays a somewhat different role. Bystander T cells lack specificity for the pathogen, but can nevertheless impact the course of the immune response to the infection. For example, bystander-activated CD8⁺ T cells can participate in protective immunity by secreting cytokines, such as interferon-γ. They also mediate host injury by exerting cytotoxicity that is facilitated by natural killer cell-activating receptors, such as NKG2D, and cytolytic molecules, such as granzyme B. Interestingly, it has been recently reported that there is a strong association between the cytolytic function of bystander-activated CD8⁺ T cells and host tissue injury in patients with acute hepatitis A virus infection. The current review addresses the induction of bystander CD8⁺ T cells, their effector functions, and their potential roles in immunity to infection, immunopathology, and autoimmunity.

Prevalance of Celiac Disease in Patients with Inflammatory Bowel Disease in Turkish Population

Background

Celiac disease (CD) and inflammatory bowel disease (IBD) involve inflammation of the gastrointestinal lumen, which environmental, genetic, and immunological factors have a role in their pathogenesis. The prevalence of celiac disease in IBD ranges from 0% to 14%. In this study, our aim was to determine the prevalence of CD in IBD patients followed by us who are attending the hospital or outpatient clinic over a period of time of seven years.

Methods

Seven hundred and fifty nine patients (425 M, 334 F, mean age: 46.75, 396 ulcerative colitis (UC), 363 Crohn's disease (CrD)) diagnosed and followed up for IBD between January 2009 and July 2016 were evaluated retrospectively, and clinical, demographic, laboratory, and endoscopic data were collected.

Results

CD was investigated in 79 (%10.4) inflammatory bowel disease patients according to symptoms, and in 5.06% (n = 4) of them, we diagnosed CD. The most common indication for investigating for CD was iron deficiency anemia unreponsive to iron supplementation.

Conclusions

We did not find an increased prevalance of celiac disease in Turkish IBD patients in this study. In the presence of refractory iron deficiency anemia without any other cause in IBD patients, investigations for celiac disease should be considered.

CXCR3 enables recruitment and site-specific bystander activation of memory CD8+ T cells

Bystander activation of memory T cells occurs in the absence of cognate antigen during infections that elicit strong systemic inflammatory responses, which subsequently affect host immune responses. Here we report that memory T cell bystander activation is not limited to induction by systemic inflammation. We initially observe potential T cell bystander activation in a cohort of human vaccine recipients. Using a mouse model system, we then find that memory CD8⁺ T cells are specifically recruited to sites with activated antigen-presenting cells (APCs) in a CXCR3-dependent manner. In addition, CXCR3 is also necessary for T cell clustering around APCs and T cell bystander activation, which temporospatially overlaps with the subsequent antigen-specific T cell response. Our data thus suggest that bystander activation is part of the initial localized immune response, and is mediated by a site-specific recruitment process of memory T cells.

CD4CD8αα IELs: They Have Something to Say

The intraepithelial lymphocytes (IELs) that reside within the epithelium of the intestine play a critical role in maintaining the immune balance of the gut. CD4CD8αα IELs are one of the most important types of IELs, and they play an irreplaceable role in maintaining the balance of the intestinal immune system. CD4CD8αα IELs are often regarded as a special subtype of CD4⁺ IELs that can express CD8αα on their cytomembrane. Hence, CD4CD8αα IELs not only have the ability to modulate the functions of immune cells but also are regarded as cytotoxic T lymphocytes (CTLs). Transcription factors, microbes, and dietary factors have a substantial effect on the development of CD4CD8αα IELs, which make them exert immunosuppression and cytotoxicity activities. In addition, there is an intimate relationship between CD4CD8αα IELs and inflammatory bowel disease (IBD), whereas it is still unclear how CD4CD8αα IELs influence IBD. As such, this review will focus on the unparalleled differentiation of CD4CD8αα IELs and discuss how these cells might be devoted to tolerance and immunopathological responses in the intestinal tract. In addition, the role of CD4CD8αα IELs in IBD would also be discussed.

Results From a First-in-Human Study of BNZ-1, a Selective Multicytokine Inhibitor Targeting Members of the Common Gamma (γc) Family of Cytokines

Pathologic roles of interleukin (IL)-2, IL-9, and IL-15, have been implicated in multiple T-cell malignancies and autoimmune diseases. BNZ-1 is a selective and simultaneous inhibitor of IL-2, IL-9, and IL-15, which targets the common gamma chain signaling receptor subunit. In this first-in-human study, 18 healthy adults (n = 3/cohort) received an intravenous dose of 0.2, 0.4, 0.8, 1.6, 3.2, or 6.4 mg/kg infused over ≤5 minutes on day 1 and were followed for 30 days for safety and pharmacokinetic/pharmacodynamic sample collection. No dose-limiting toxicities, infusion reactions, or serious or severe treatment-emergent adverse events were observed. Headache was the only treatment-emergent adverse event in >1 subject (n = 3). Peak and total BNZ-1 exposure was generally dose proportional, with a terminal elimination half-life of ∼5 days. Pharmacodynamic effects of BNZ-1 on regulatory T cells (Tregs, IL-2), natural killer (NK) cells (IL-15) and CD8 central memory T cells (Tcm, IL-15) were measured by flow cytometry and used to demonstrate target engagement. For Tregs, 0.2 mg/kg was an inactive dose, while a maximum ∼50% to 60% decrease from baseline was observed on day 4 after doses of 0.4 to 1.6 mg/kg, and higher doses produced an 80% to 93% decrease from baseline on day 15. Similar pharmacodynamic trends were observed for natural killer cells and CD8 Tcm, although decreases in CD8 Tcm were more prolonged. These subpopulations returned to/toward baseline by day 31. T cells (total, CD4, and CD8), B cells, and monocytes were unchanged throughout. These preliminary results suggest that BNZ-1 safely and selectively inhibits IL-2 and IL-15, which results in robust, reversible immunomodulation.

Diverse developmental pathways of intestinal intraepithelial lymphocytes

The intestinal epithelial barrier is patrolled by resident intraepithelial lymphocytes (IELs) that are involved in host defence against pathogens, wound repair and homeostatic interactions with the epithelium, microbiota and nutrients. Intestinal IELs are one of the largest populations of lymphocytes in the body and comprise several distinct subsets, the identity and lineage relationships of which have long remained elusive. Here, we review advances in unravelling the complexity of intestinal IEL populations, which comprise conventional αβ T cell receptor (TCRαβ)⁺ subsets, unconventional TCRαβ⁺ and TCRγδ⁺ subsets, group 1 innate lymphoid cells (ILC1s) and ILC1-like cells. Although these intestinal IEL lineages have partially overlapping effector programmes and recognition properties, they have strikingly different developmental pathways. We suggest that evolutionary pressure has driven the recurrent generation of cytolytic effector lymphocytes to protect the intestinal epithelial layer, but they may also precipitate intestinal inflammatory disorders, such as coeliac disease.

Transcriptional profiling of human intestinal plasma cells reveals effector functions beyond antibody production

Background

Plasma cells (PCs) are terminally differentiated B-lymphocytes producing antibodies. In coeliac disease (CeD) there is increased density of PCs in the small-intestinal lesion. Many of these PCs produce disease-specific autoantibodies targeting transglutaminase 2 (TG2).

Objective

The plasmacytosis of CeD motivated us to study the transcriptional programme of PCs from coeliac gut lesions.

Methods

RNA-seq was performed on the PCs of CeD patients and disease controls, being specific or non-specific for TG2.

Results

Being antibody-producing cells, 67% of the PCs' transcript was aligned to immunoglobulin genes. Strikingly, genes encoding ligands and receptors of chemokines and cytokines were abundant. Higher transcript levels of genes associated with cell activation and immune responses were observed in PCs of CeD patients compared to controls. TG2-specific compared to non-TG2 specific PCs expressed increased levels of CXCR3, CXCL10 and interleukin-15; factors that have been implicated in the pathogenesis of CeD yet with production attributed to other cells than PCs. The presence of transcripts of HLA class II and T-cell co-stimulatory molecules suggests that PCs may serve as antigen-presenting cells for CD4 + helper T cells.

Conclusions

Our findings shed new light on the biology of intestinal PCs, implicating functions that go beyond the production of immunoglobulins.

NKG2D-ligands: Putting everything under the same umbrella can be misleading

NKG2D is a key receptor for the activation of immune effector cells, mainly Natural Killer cells and T lymphocytes, in infection, cancer and autoimmune diseases. Since the detection of ligands for NKG2D in sera of cancer patients is, in many human models, indicative of prognosis, a large number of studies have been undertaken to improve understanding of the biology regulating this receptor and its ligands, with the aim of translating this knowledge into clinical practice. Although it is becoming clear that the NKG2D system can be used as a tool for diagnosis and manipulated for therapy, some questions remain open due to the complexity associated with the existence of a large number of ligands, each one of them displaying distinct biological properties. In this review, we have highlighted some key aspects of this system that differ between humans and mice, including the properties of NKG2D, as well as the genetic and biochemical complexity of NKG2D-ligands. All of these features affect the characteristics of the immune response exerted by NKG2D-expressing cells and are likely to be important factors in the clearance of a tumour or the development of autoimmunity. Implementation of more global analyses, including information on genotype, transcription and protein properties (cellular vs released to the blood stream) of NKG2D-ligands expressed in patients will be necessary to fully understand the links between this system and disease progression.

The role of CXCR3 and its ligands CXCL10 and CXCL11 in the pathogenesis of celiac disease

The chemokine receptor CXCR3 and its ligands CXCL10 and CXCL11 have been suggested to give rise to the most relevant chemokine axis able to facilitate the entrance of immune cells into inflamed tissues and be activated in different inflammatory disorders, such as celiac disease (CD).The aim of this study was to investigate the expression level of CXCR3, CXCL10, and CXCL11 genes in celiac patients compared to healthy controls. Both cohorts have been recruited from the Iranian population.In this case-control study, biopsy specimens were collected from 71 celiac patients (60.5% female) and 90 control subjects (57% female) during 2016. Total RNA was extracted and mRNA expression levels of CXCR3, CXCL10, and CXCL11 genes were investigated by SYBR green qPCR.Based on qPCR and relative quantification method, the mRNA expression levels of CXCR3, CXCL10, and CXCL11 were significantly higher in duodenal biopsies of celiac patients compared to healthy controls in the study population (P = .038, P = .021, and P = .012 respectively).The result of this study showed that CXCR3/CXCL10/CXCL11 signaling axis is overexpressed in the small intestinal mucosa of CD patients compared to controls. This finding might explain the specific enrollment of the main cell populations that infiltrate the epithelium.

IL-10 signaling prevents gluten-dependent intraepithelial CD4+ cytotoxic T lymphocyte infiltration and epithelial damage in the small intestine

Breach of tolerance to gluten leads to the chronic small intestinal enteropathy celiac disease. A key event in celiac disease development is gluten-dependent infiltration of activated cytotoxic intraepithelial lymphocytes (IELs), which cytolyze epithelial cells causing crypt hyperplasia and villous atrophy. The mechanisms leading to gluten-dependent small intestinal IEL infiltration and activation remain elusive. We have demonstrated that under homeostatic conditions in mice, gluten drives the differentiation of anti-inflammatory T cells producing large amounts of the immunosuppressive cytokine interleukin-10 (IL-10). Here we addressed whether this dominant IL-10 axis prevents gluten-dependent infiltration of activated cytotoxic IEL and subsequent small intestinal enteropathy. We demonstrate that IL-10 regulation prevents gluten-induced cytotoxic inflammatory IEL infiltration. In particular, IL-10 suppresses gluten-induced accumulation of a specialized population of cytotoxic CD4⁺CD8αα⁺ IEL (CD4⁺ CTL) expressing Tbx21, Ifng, and Il21, and a disparate non-cytolytic CD4⁺CD8α^- IEL population expressing Il17a, Il21, and Il10. Concomitantly, IL-10 suppresses gluten-dependent small intestinal epithelial hyperproliferation and upregulation of stress-induced molecules on epithelial cells. Remarkably, frequencies of granzyme B⁺CD4⁺CD8α⁺ IEL are increased in pediatric celiac disease patient biopsies. These findings demonstrate that IL-10 is pivotal to prevent gluten-induced small intestinal inflammation and epithelial damage, and imply that CD4⁺ CTL are potential new players into these processes.

Gluten Free Wheat: Are We There?

Gluten proteins, major determinants of the bread-making quality of wheat, are related to several digestive disorders. Advances in plant genetic breeding have allowed the production of wheat lines with very low gliadin content through the use of RNAi and gene editing technologies. In this review, we carried out a comprehensive study of the application of these cutting-edge technologies towards the development of wheat lines devoid of immunogenic gluten, and their genetic, nutritional and clinical characterization. One line, named E82, showed outstanding nutritional properties, with very low immunogenic gluten and a low stimulation capacity of T-cells from celiac patients. Moreover, a clinical trial with non-celiac wheat sensitivity (NCWS) patients showed that the consumption of bread made with this E82 low gliadin line induced positive changes in the gut microbiota composition.

Protective Effects of Aryl Hydrocarbon Receptor Signaling in Celiac Disease Mucosa and in Poly I:C-Induced Small Intestinal Atrophy Mouse Model

Aryl hydrocarbon receptor (AhR), a transcription factor activated by a large number of natural and synthetic agents, modulates the activity of immune cells in the gut and represents an important link between the environment and immune-mediated pathologies. In this study, we investigated the role of AhR in celiac disease (CD), a gluten-driven enteropathy. AhR expression was evaluated in intestinal biopsies taken from patients with CD and controls by real-time polymerase chain reaction (PCR), immunohistochemistry and flow cytometry. AhR was also analyzed in ex vivo organ cultures of duodenal biopsies taken from inactive CD patients incubated in presence or absence of peptic-tryptic digest of gliadin. IFN-γ, TNF-α, granzyme B, and perforin expression was evaluated in anti-CD3/CD28-activated intestinal lamina propria mononuclear cells (LPMC) and intestinal intra-epithelial cells (IEL) of active CD patients cultured in the presence or absence of the AhR agonist 6-formylindolo(3, 2-b)carbazole (Ficz). Finally, the protective role of AhR was evaluated in a mouse model of poly I:C-driven small intestine damage. AhR RNA transcripts were reduced in active CD samples as compared to inactive CD and normal controls. Flow cytometry confirmed such results and showed a reduction of AhR in both IEL and LPMC of active CD patients. The addition of a peptic-tryptic digest of gliadin to ex vivo organ cultures of duodenal biopsies taken from inactive CD patients reduced AhR expression. Treatment of CD IEL and LPMC with Ficz reduced the levels of inflammatory cytokines, granzyme B and perforin. Mice injected with Ficz were protected against poly I:C-induced intestinal lesions. Our findings suggest that defective AhR-driven signals could contribute to amplify pathogenic responses in the gut of CD patients.

Coeliac disease

Coeliac disease is an immune-mediated enteropathy against dietary gluten present in wheat, rye and barley and is one of the most common lifelong food-related disorders worldwide. Coeliac disease is also considered to be a systemic disorder characterized by a variable combination of gluten-related signs and symptoms and disease-specific antibodies in addition to enteropathy. The ingestion of gluten leads to the generation of harmful gluten peptides, which, in predisposed individuals, can induce adaptive and innate immune responses. The clinical presentation is extremely variable; patients may have severe gastrointestinal symptoms and malabsorption, extraintestinal symptoms or have no symptoms at all. Owing to the multifaceted clinical presentation, diagnosis remains a challenge and coeliac disease is heavily underdiagnosed. The diagnosis of coeliac disease is achieved by combining coeliac disease serology and small intestinal mucosal histology during a gluten-containing diet. Currently, the only effective treatment for coeliac disease is a lifelong strict gluten-free diet; however, the diet is restrictive and gluten is difficult to avoid. Optimizing diagnosis and care in coeliac disease requires continuous research and education of both patients and health-care professionals.

Pathogenesis of Enteropathy-Associated T Cell Lymphoma

PURPOSE OF REVIEW

To provide an update on the pathogenesis of enteropathy-associated T cell lymphoma (EATL) and its relationship with refractory celiac disease (RCD), in light of current knowledge of immune, genetic, and environmental factors that promote neoplastic transformation of intraepithelial lymphocytes (IELs).

RECENT FINDINGS

EATL frequently evolves from RCD type II (RCD II) but can occur "de novo" in individuals with celiac disease. Recurrent activating mutations in members of the JAK/STAT pathway have been recently described in EATL and RCD II, which suggests deregulation of cytokine signaling to be an early event in lymphomagenesis. Intraepithelial T cells are presumed to be the cell of origin of EATL (and RCD II). Recent in vitro molecular and phenotypic analyses and in vivo murine studies, however, suggest an origin of RCD II from innate IELs (NK/T cell precursors), which could also be the cell of origin of RCD II-derived EATL. The immune microenvironment of the small intestinal mucosa in celiac disease fosters the development of EATL, often in a multistep pathway.

Bystander T Cells: A Balancing Act of Friends and Foes

T cell responses are essential for appropriate protection against pathogens. T cell immunity is achieved through the ability to discriminate between foreign and self-molecules, and this relies heavily on stringent T cell receptor (TCR) specificity. Recently, bystander activated T lymphocytes, that are specific for unrelated epitopes during an antigen-specific response, have been implicated in diverse diseases. Numerous infection models have challenged the classic dogma of T cell activation as being solely dependent on TCR and major histocompatibility complex (MHC) interactions, indicating an unappreciated role for pathogen-associated receptors on T cells. We discuss here the specific roles of bystander activated T cells in pathogenesis, shedding light on the ability of these cells to modulate disease severity independently from TCR recognition.

Alopecia areata: a review of disease pathogenesis

BACKGROUND

Alopecia areata is a disorder that results in nonscarring hair loss. The psychological impact can be significant, leading to feelings of depression and social isolation. Objectives In this article, we seek to review the pathophysiological mechanisms proposed in recent years in a narrative fashion.

METHODS

We searched MEDLINE and Scopus for articles related to alopecia areata, with a particular emphasis on its pathogenesis.

RESULTS

The main theory of alopecia areata pathogenesis is that it is an autoimmune phenomenon resulting from a disruption in hair follicle immune privilege. What causes this breakdown is an issue of debate. Some believe that a stressed hair follicle environment triggers antigen presentation, while others blame a dysregulation in the central immune system entangling the follicles. Evidence for the latter theory is provided by animal studies, as well investigations around the AIRE gene. Different immune-cell lines including plasmacytoid dendritic cells, natural killer cells and T cells, along with key molecules such as interferon-γ, interleukin-15, MICA and NKG2D, have been identified as contributing to the autoimmune process.

CONCLUSIONS

Alopecia areata remains incurable, although it has been studied for years. Available treatment options at best are beneficial for milder cases, and the rate of relapse is high. Understanding the exact mechanisms of hair loss in alopecia areata is therefore of utmost importance to help identify potential therapeutic targets.

Natural killer cells in lung transplantation

Natural killer (NK) cells are innate lymphoid cells that have been increasingly recognised as important in lung allograft tolerance and immune defence. These cells evolved to recognise alterations in self through a diverse set of germline-encoded activating and inhibitory receptors and display a broad range of effector functions that play important roles in responding to infections, malignancies and allogeneic tissue. Here, we review NK cells, their diverse receptors and the mechanisms through which NK cells are postulated to mediate important lung transplant clinical outcomes. NK cells can promote tolerance, such as through the depletion of donor antigen-presenting cells. Alternatively, these cells can drive rejection through cytotoxic effects on allograft tissue recognised as 'non-self' or 'stressed', via killer cell immunoglobulin-like receptor (KIR) or NKG2D receptor ligation, respectively. NK cells likely mediate complement-independent antibody-mediated rejection of allografts though CD16A Fc receptor-dependent activation induced by graft-specific antibodies. Finally, NK cells play an important role in response to infections, particularly by mediating cytomegalovirus infection through the CD94/NKG2C receptor. Despite these sometimes-conflicting effects on allograft function, enumeration of NK cells may have an important role in diagnosing allograft dysfunction. While the effects of immunosuppression agents on NK cells may currently be largely unintentional, further understanding of NK cell biology in lung allograft recipients may allow these cells to serve as biomarkers of graft injury and as therapeutic targets.

iNKT Cells Suppress Pathogenic NK1.1+CD8+ T Cells in DSS-Induced Colitis

T cells producing IFNγ play a pathogenic role in the development of inflammatory bowel disease (IBD). To investigate the functions of CD1d-dependent invariant natural killer T (iNKT) cells in experimental colitis induced in Yeti mice with dysregulated expression of IFNγ, we generated iNKT cell-deficient Yeti/CD1d KO mice and compared colitis among WT, CD1d KO, Yeti, and Yeti/CD1d KO mice following DSS treatment. We found that deficiency of iNKT cells exacerbated colitis and disease pathogenesis was mainly mediated by NK1.1⁺CD8⁺ T cells. Furthermore, the protective effects of iNKT cells correlated with up-regulation of regulatory T cells. Taken together, our results have demonstrated that CD1d-dependent iNKT cells and CD1d-independent NK1.1⁺CD8⁺ T cells reciprocally regulate the development of intestinal inflammatory responses mediated by IFNγ-dysregulation. These findings also identify NK1.1⁺CD8⁺ T cells as novel target cells for the development of therapeutics for human IBD.

The elusive case of human intraepithelial T cells in gut homeostasis and inflammation

The epithelial barrier of the gastrointestinal tract is home to numerous intraepithelial T cells (IETs). IETs are functionally adapted to the mucosal environment and are among the first adaptive immune cells to encounter microbial and dietary antigens. They possess hallmark features of tissue-resident T cells: they are long-lived nonmigratory cells capable of rapidly responding to antigen challenges independent of T cell recruitment from the periphery. Gut-resident T cells have been implicated in the relapsing and remitting course and persisting low-grade inflammation of chronic gastrointestinal diseases, including IBD and coeliac disease. So far, most data IETs have been derived from experimental animal models; however, IETs and the environmental makeup differ between mice and humans. With advances in techniques, the number of human studies has grown exponentially in the past 5 years. Here, we review the literature on the involvement of human IETs in gut homeostasis and inflammation, and how these cells are influenced by the microbiota and dietary antigens. Finally, targeting of IETs in therapeutic interventions is discussed. Broad insight into the function and role of human IETs in gut homeostasis and inflammation is essential to identify future diagnostic, prognostic and therapeutic strategies.

Human intraepithelial lymphocytes

The location of intraepithelial lymphocytes (IEL) between epithelial cells, their effector memory, cytolytic and inflammatory phenotype positions them to kill infected epithelial cells and protect the intestine against pathogens. Human TCRαβ+CD8αβ+ IEL have the dual capacity to recognize modified self via natural killer (NK) receptors (autoreactivity) as well as foreign antigen via the T cell receptor (TCR), which is accomplished in mouse by two cell subsets, the naturally occurring TCRαβ+CD8αα+ and adaptively induced TCRαβ+CD8αβ+ IEL subsets, respectively. The private/oligoclonal nature of the TCR repertoire of both human and mouse IEL suggests local environmental factors dictate the specificity of IEL responses. The line between sensing of foreign antigens and autoreactivity is blurred for IEL in celiac disease, where recognition of stress ligands by induced activating NK receptors in conjunction with inflammatory signals such as IL-15 can result in low-affinity TCR/non-cognate antigen and NK receptor/stress ligand interactions triggering destruction of intestinal epithelial cells.

The Paradoxical Role of NKG2D in Cancer Immunity

The activating receptor NKG2D and its ligands are recognized as a potent immune axis that controls tumor growth and microbial infections. With regards to cancer surveillance, various studies have demonstrated the antitumor function mediated by NKG2D on natural killer cells and on conventional and unconventional T cells. The use of NKG2D-deficient mice established the importance of NKG2D in delaying tumor development in transgenic mouse models of cancer. However, we recently demonstrated an unexpected, flip side to this coin, the ability for NKG2D to contribute to tumor growth in a model of inflammation-driven liver cancer. With a focus on the liver, here, we review current knowledge of NKG2D-mediated tumor surveillance and discuss evidence supporting a dual role for NKG2D in cancer immunity. We postulate that in certain advanced cancers, expression of ligands for NKG2D can drive cancer progression rather than rejection. We propose that the nature of the microenvironment within and surrounding tumors impacts the outcome of NKG2D activation. In a form of autoimmune attack, NKG2D promotes tissue damage, mostly in the inflamed tissue adjacent to the tumor, facilitating tumor progression while being ineffective at rejecting transformed cells in the tumor bed.

The Innate Immune System in the Gastrointestinal Tract: Role of Intraepithelial Lymphocytes and Lamina Propria Innate Lymphoid Cells in Intestinal Inflammation

BACKGROUND

The gastrointestinal tract harbors the largest microbiota load in the human body, hence maintaining a delicate balance between immunity against invading pathogens and tolerance toward commensal. Such immune equilibrium, or intestinal homeostasis, is conducted by a tight regulation and cooperation of the different branches of the immune system, including the innate and the adaptive immune system. However, several factors affect this delicate equilibrium, ultimately leading to gastrointestinal disorders including inflammatory bowel disease. Therefore, here we decided to review the currently available information about innate immunity lymphocyte subsets playing a role in intestinal inflammation.

RESULTS

Intestinal innate lymphocytes are composed of intraepithelial lymphocytes (IELs) and lamina propria innate lymphoid cells (ILCs). While IELs can be divided into natural or induced, ILCs can be classified into type 1, 2, or 3, resembling, respectively, the properties of TH1, TH2, or TH17 adaptive lymphocytes. Noteworthy, the phenotype and function of both IELs and ILCs are disrupted under inflammatory conditions, where they help to exacerbate intestinal immune responses.

CONCLUSIONS

The modulation of both IELs and ILCs to control intestinal inflammatory responses represents a major challenge, as they provide tight regulation among the epithelium, the microbiota, and the adaptive immune system. An improved understanding of the innate immunity mechanisms involved in gastrointestinal inflammation would therefore aid in the diagnosis and further treatment of gastrointestinal inflammatory disorders.

Beneficial Effects of Human Anti-Interleukin-15 Antibody in Gluten-Sensitive Rhesus Macaques with Celiac Disease

Overexpression of interleukin-15 (IL-15) is linked with immunopathology of several autoimmune disorders including celiac disease. Here, we utilized an anti-human IL-15 antibody 04H04 (anti-IL-15) to reverse immunopathogenesis of celiac disease. Anti-IL-15 was administered to six gluten-sensitive rhesus macaques with celiac disease characteristics including gluten-sensitive enteropathy (GSE), and the following celiac-related metrics were evaluated: morphology (villous height/crypt depth ratio) of small intestine, counts of intestinal intraepithelial lymphocytes, IFN-γ-producing CD8+ and CD4+ T cells, plasma levels of anti-gliadin and anti-intestinal tissue transglutaminase IgG antibodies, as well as peripheral effector memory (CD3+CD28-CD95+) T cells. Anti-IL-15 treatment reversed the clinically relevant disease endpoints, intraepithelial lymphocyte counts, and villous height/crypt depth ratios within jejunal biopsies to normal levels (P < 0.001). Additionally, intestinal CD8+ and CD4+ T cell IFN-γ production was reduced (P < 0.05). Extra-intestinally, anti-IL-15 treatment reduced peripheral NK cell counts (P < 0.001), but otherwise, non-NK peripheral lymphocytes including effector memory T cells and serum blood chemistry were unaffected. Overall, providing the beneficial disease-modulatory and immunomodulatory effects observed, anti-IL-15 treatment might be considered as a novel therapy to normalize intestinal lymphocyte function in celiac disease patients with GSE.

Regulatory NK1.1-CD4+NKG2D+ subset induced by NKG2DL+ cells promotes tumor evasion in mice

Regulatory T cells play critical roles in self-tolerance and tumor evasion. CD4+NKG2D+ cells with regulatory activity are present in patients with NKG2DL+ tumors and juvenile systemic lupus erythematosus. We previously showed that TGF-β-producing CD4+NKG2D+ T cells are present in pCD86-Rae-1ε transgenic mice. Here, we performed both ex vivo and in vivo studies on pCD86-Rae-1ε transgenic mice and an MC38 tumor-bearing mouse model and show that NK1.1-CD4+NKG2D+ T cells have regulatory activity in pCD86-Rae-1ε transgenic mice. Furthermore, this T-cell subset was induced in mice transplanted with NKG2DL+ tumor cells and produced TGF-β and FasL, and secreted low amounts of IFN-γ. This T-cell subset downregulated the function of effector T cells and dendritic cells, which were abolished by anti-TGF-β antibody. In vivo, adoptive transfer of NK1.1-CD4+NKG2D+ T cells promoted TGF-β-dependent tumor growth in mice. We further found that ex vivo induction of NK1.1-CD4+NKG2D+ T cells was dependent on both anti-CD3 and NKG2DL stimulation. Furthermore, regulatory NK1.1-CD4+NKG2D+ T cells did not express Foxp3 or CD25 and expressed intermediate levels of T-bet. Western-blotting showed that STAT3 signaling was activated in NK1.1-CD4+NKG2D+ T cells of MC38 tumor-bearing and pCD86-Rae-1ε transgenic mice. In conclusion, we describe a regulatory NK1.1-CD4+NKG2D+ T-cell population, different from other regulatory T cells and abnormally elevated in pCD86-Rae-1ε transgenic and MC38 tumor-bearing mice.

Distinct human circulating NKp30+FcεRIγ+CD8+ T cell population exhibiting high natural killer-like antitumor potential

CD8⁺ T cells are considered prototypical cells of adaptive immunity. Here, we uncovered a distinct CD8⁺ T cell population expressing the activating natural killer (NK) receptor NKp30 in the peripheral blood of healthy individuals. We revealed that IL-15 could de novo induce NKp30 expression in a population of CD8⁺ T cells and drive their differentiation toward a broad innate transcriptional landscape. The adaptor FcεRIγ was concomitantly induced and was shown to be crucial to enable NKp30 cell-surface expression and function in CD8⁺ T cells. FcεRIγ de novo expression required promoter demethylation and was accompanied by acquisition of the signaling molecule Syk and the "innate" transcription factor PLZF. IL-15-induced NKp30⁺CD8⁺ T cells exhibited high NK-like antitumor activity in vitro and were able to synergize with T cell receptor signaling. Importantly, this population potently controlled tumor growth in a preclinical xenograft mouse model. Our study, while blurring the borders between innate and adaptive immunity, reveals a unique NKp30⁺FcεRIγ⁺CD8⁺ T cell population with high antitumor therapeutic potential.