Regulatory T cells suppress development of colitis, blocking differentiation of T-helper 17 into alternative T-helper 1 cells

Innate and Adaptive Immune Responses in Intestinal Transplant Rejection: Through the Lens of Inflammatory Bowel and Intestinal Graft-Versus-Host Diseases

Intestinal transplantation is a life-saving procedure utilized for patients failing total parenteral nutrition. However, intestinal transplantattion remains plagued with low survival rates and high risk of allograft rejection. The authors explore roles of innate (macrophages, natural killer cells, innate lymphoid cells) and adaptive immune cells (Th1, Th2, Th17, Tregs) in inflammatory responses, particularly inflammatory bowel disease and graft versus host disease, and correlate these findings to intestinal allograft rejection, highlighting which effectors exacerbate or suppress intestinal rejection. Better understanding of this immunology can open further investigation into potential biomolecular targets to develop improved therapeutic treatment options and immunomonitoring techniques to combat allograft rejection and enhance patient lives.

D-amino Acids Ameliorate Experimental Colitis and Cholangitis by Inhibiting Growth of Proteobacteria: Potential Therapeutic Role in Inflammatory Bowel Disease

BACKGROUND & AIMS

D-amino acids, the chiral counterparts of protein L-amino acids, were primarily produced and utilized by microbes, including those in the human gut. However, little was known about how orally administered or microbe-derived D-amino acids affected the gut microbial community or gut disease progression.

METHODS

The ratio of D- to L-amino acids was analyzed in feces and blood from patients with ulcerative colitis (UC) and healthy controls. Also, composition of microbe was analyzed from patients with UC. Mice were treated with D-amino acid in dextran sulfate sodium colitis model and liver cholangitis model.

RESULTS

The ratio of D- to L-amino acids was lower in the feces of patients with UC than that of healthy controls. Supplementation of D-amino acids ameliorated UC-related experimental colitis and liver cholangitis by inhibiting growth of Proteobacteria. Addition of D-alanine, a major building block for bacterial cell wall formation, to culture medium inhibited expression of the ftsZ gene required for cell fission in the Proteobacteria Escherichia coli and Klebsiella pneumoniae, thereby inhibiting growth. Overexpression of ftsZ restored growth of E. coli even when D-alanine was present. We found that D-alanine not only inhibited invasion of pathological K. pneumoniae into the host via pore formation in intestinal epithelial cells but also inhibited growth of E. coli and generation of antibiotic-resistant strains.

CONCLUSIONS

D-amino acids might have potential for use in novel therapeutic approaches targeting Proteobacteria-associated dysbiosis and antibiotic-resistant bacterial diseases by means of their effects on the intestinal microbiota community.

Cathepsin W restrains peripheral regulatory T cells for mucosal immune quiescence

Peripheral regulatory T (pT_reg) cells are a key T cell lineage for mucosal immune tolerance and anti-inflammatory responses, and interleukin-2 receptor (IL-2R) signaling is critical for T_reg cell generation, expansion, and maintenance. The expression of IL-2R on pT_reg cells is tightly regulated to ensure proper induction and function of pT_reg cells without a clear molecular mechanism. We here demonstrate that Cathepsin W (CTSW), a cysteine proteinase highly induced in pT_reg cells under transforming growth factor-β stimulation is essential for the restraint of pT_reg cell differentiation in an intrinsic manner. Loss of CTSW results in elevated pT_reg cell generation, protecting the animals from intestinal inflammation. Mechanistically, CTSW inhibits IL-2R signaling in pT_reg cells by cytosolic interaction with and process of CD25, repressing signal transducer and activator of transcription 5 activation to restrain pT_reg cell generation and maintenance. Hence, our data indicate that CTSW acts as a gatekeeper to calibrate pT_reg cell differentiation and function for mucosal immune quiescence.

Localization and movement of Tregs in gastrointestinal tract: a systematic review

BACKGROUND

The intestine is rich in food-derived and microbe-derived antigens. Regulatory T cells (Tregs) are an essential T-cell population that prevents systemic autoimmune diseases and inhibits inflammation by encountering antigens. Previously, it was reported that the functional loss of Tregs induces systemic inflammation, including inflammatory bowel disease and graft-versus-host disease in human and murine models. However, there is a dearth of information about how Tregs localize in different tissues and suppress effector cells.

MAIN BODY

The development of Tregs and their molecular mechanism in the digestive tract have been elucidated earlier using murine genetic models, infectious models, and human samples. Tregs suppress immune and other nonimmune cells through direct effect and cytokine production. The recent development of in vivo imaging technology allows us to visualize how Tregs localize and move in the settings of inflammation and homeostasis. This is important because, according to a recent report, Treg characterization and function are regulated by their location. Tregs located in the proximal intestine and its draining lymph nodes induce tolerance against food antigens, and those located in the distal intestine suppress the inflammation induced by microbial antigens. Taken together, various Tregs are induced in a location-specific manner in the gastrointestinal tract and influence the homeostasis of the gut.

CONCLUSION

In this review, we summarize how Tregs are induced in the digestive tract and the application of in vivo Treg imaging to elucidate immune homeostasis in the digestive tract.

Aryl hydrocarbon receptor signals in epithelial cells govern the recruitment and location of Helios+ Tregs in the gut

CD4⁺Foxp3⁺ regulatory T cells (Tregs) are essential for homeostasis in the colon, but the mechanism by which local environmental cues determine the localization of colonic Tregs is unclear. Here, we administer indigo naturalis (IN), a nontoxic phytochemical aryl hydrocarbon receptor (AhR) agonist used for treating patients with ulcerative colitis (UC) in Asia, and we show that IN increases Helios⁺ Tregs and MHC class II⁺ epithelial cells (ECs) in the colon. Interactions between Tregs and MHC class II⁺ ECs occur mainly near the crypt bottom in the steady state, whereas Tregs dramatically increase and shift toward the crypt top following IN treatment. Moreover, the number of CD25⁺ T cells is increased near the surface of ECs in IN-treated UC patients compared with that in patients treated with other therapies. We also highlight additional AhR-signaling mechanisms in intestinal ECs that determine the accumulation and localization of Helios⁺ Tregs in the colon.

Th17 cells in the liver: balancing autoimmunity and pathogen defense

In addition to carcinogenesis, T helper 17 (Th17) cells (a subtype of CD4 + T lymphocytes) are involved in the acute, chronic, and cirrhotic phases of liver diseases; however, their role in the development and progression of liver diseases remains unclear. It is difficult to elucidate the role of Th17 cells in liver diseases due to their dichotomous nature, i.e., plasticity in terms of pathogenic or host protective function depending on environmental and time phase factors. Moreover, insufficient depletion of Th17 cells by inhibiting the cytokines and transcription factors involved in their production causes difficulties in analyzing their specific role in vitro and in vivo murine models, partially due to complex interaction. This review summarizes the recent progress in understanding the plasticity and function of hepatic Th17 cells and type 3 cytokines.

A Retinoid X Receptor Agonist Directed to the Large Intestine Ameliorates T-Cell-Mediated Colitis in Mice

Retinoid X receptor (RXR) is a nuclear receptor that heterodimerizes with several nuclear receptors, integrating ligand-mediated signals across the heterodimers. Synthetic RXR agonists have been developed to cure certain inflammatory diseases, including inflammatory bowel diseases (IBDs). However, pre-existing RXR agonists, which are lipophilic and readily absorbed in the upper intestine, cause considerable adverse effects such as hepatomegaly, hyperlipidemia, and hypothyroidism. To minimize these adverse effects, we have developed an RXR agonist, NEt-3IB, which has lipophilic and thus poorly absorptive properties. In this study, we evaluated the effects of NEt-3IB in an experimental murine colitis model induced through the adoptive transfer of CD45RB^highCD4⁺ T cells. Pharmacokinetic studies demonstrated that the major portion of NEt-3IB was successfully delivered to the large intestine after oral administration. Notably, NEt-3IB treatment suppressed the development of T cell-mediated chronic colitis, as indicated by improvement of wasting symptoms, inflammatory infiltration, and mucosal hyperplasia. The protective effect of NEt-3IB was mediated by the suppression of IFN-γ-producing Th1 cell expansion in the colon. In conclusion, NEt-3IB, a large intestine-directed RXR agonist, is a promising drug candidate for IBDs.

Ablation of IL-17A leads to severe colitis in IL-10-deficient mice: implications of myeloid-derived suppressor cells and NO production

IL-10 is an immune regulatory cytokine and its genetic defect leads to gastrointestinal inflammation in humans and mice. Moreover, the IL-23/T_h17 axis is known to be involved in these inflammatory disorders. IL-17A, a representative cytokine produced by T_h17 cells, has an important role for the pathological process of inflammatory diseases. However, the precise function of IL-17A in inflammatory bowel disease (IBD) remains controversial. In this study, we evaluated the effect of IL-17A on colitis in IL-10-deficient (Il10^−/−) mice. Mice lacking both IL-10 and IL-17A (Il10^−/−Il17a^−/−) suffered from fatal wasting and manifested more severe colitis compared with Il10^−/−Il17a^+/− mice. Moreover, we found that CD11b⁺Gr-1⁺ myeloid-derived suppressor cells (MDSCs) accumulated in the bone marrow, spleen and peripheral blood of Il10^−/−Il17a^−/− mice. These MDSCs highly expressed inducible nitric oxide synthase (iNOS) (Nos2) and suppressed the T-cell response in vitro in a NOS-dependent manner. In correlation with these effects, the concentration of nitric oxide was elevated in the serum of Il10^−/−Il17a^−/− mice. Surprisingly, the severe colitis observed in Il10^−/−Il17a^−/− mice was ameliorated in Il10^−/−Il17a^−/−Nos2^−/− mice. Our findings suggest that IL-17A plays suppressive roles against spontaneous colitis in Il10^−/− mice in an iNOS-dependent manner and inhibits MDSC differentiation and/or proliferation.

Biological and clinical significance of T helper 17 cell plasticity

Mature T helper (Th) effector cells originate following antigen recognition by naive T precursors. The maturation process is accompanied by the acquisition of specific effector functions that distinguish at least three different T helper subsets: Th1, Th2 and Th17. In general, maturation of somatic cells is accompanied by terminal differentiation. However, accumulating evidence shows that effector T cells retain a certain degree of plasticity. This is especially true for Th17 cells, which have been shown to converge towards other phenotypes in response to specific microenvironmental pressure. In this review we will discuss the experimental evidence that supports the hypothesis of Th17 plasticity, with particular emphasis on the generation of Th17-derived 'non-classic' Th1 cells, and the molecular networks that control it. Moreover, we will consider why Th17 plasticity is important for host protection, but also why it can have pathogenic functions during chronic inflammation. Regarding the last point, we will discuss a possible role for biological drugs in the control of Th17 plasticity and disease course.

Intestinal Organoids as a Novel Complementary Model to Dissect Inflammatory Bowel Disease

Inflammatory bowel diseases (IBDs) include colitis ulcerosa and Crohn's disease, besides the rare microscopic colitis. Both diseases show a long-lasting, relapsing-remitting, or even chronic active course with tremendous impact on quality of life. IBDs frequently cause disability, surgical interventions, and high costs; as in other autoimmune diseases, their prevalent occurrence at an early phase of life raises the burden on health care systems. Unfortunately, our understanding of the pathogenesis is still incomplete and treatment therefore largely focuses on suppressing the resulting excessive inflammation. One obstacle for deciphering the causative processes is the scarcity of models that parallel the development of the disease, since intestinal inflammation is mostly induced artificially; moreover, the intestinal epithelium, which strongly contributes to IBD pathogenesis, is difficult to assess. Recently, the development of intestinal epithelial organoids has overcome many of those problems. Here, we give an overview on the current understanding of the pathogenesis of IBDs with reference to the limitations of previous well-established experimental models. We highlight the advantages and detriments of recent organoid-based experimental setups within the IBD field and suggest possible future applications.

IL-18Rα-deficient CD4(+) T cells induce intestinal inflammation in the CD45RB(hi) transfer model of colitis despite impaired innate responsiveness

IL-18 has been implicated in inflammatory bowel disease (IBD), however its role in the regulation of intestinal CD4(+) T-cell function remains unclear. Here we show that murine intestinal CD4(+) T cells express high levels of IL-18Rα and provide evidence that IL-18Rα expression is induced on these cells subsequent to their entry into the intestinal mucosa. Using the CD45RB(hi) T-cell transfer colitis model, we show that IL-18Rα is expressed on IFN-γ(+) , IL-17(+) , and IL-17(+) IFN-γ(+) effector CD4(+) T cells in the inflamed colonic lamina propria (cLP) and mesenteric lymph node (MLN) and is required for the optimal generation and/or maintenance of IFN-γ-producing cells in the cLP. In the steady state and during colitis, TCR-independent cytokine-induced IFN-γ and IL-17 production by intestinal CD4(+) T cells was largely IL-18Rα-dependent. Despite these findings however, IL-18Rα-deficient CD4(+) T cells induced comparable intestinal pathology to WT CD4(+) T cells. These findings suggest that IL-18-dependent cytokine induced activation of CD4(+) T cells is not critical for the development of T-cell-mediated colitis.

The Role of T-Cell Subsets in Chronic Inflammation in Celiac Disease and Inflammatory Bowel Disease Patients: More Common Mechanisms or More Differences?

Background

Chronic intestinal inflammation due to noninfectious causes represents a growing health issue all over the world. Celiac disease as well as inflammatory bowel diseases (IBD) like Crohn's disease and ulcerative and microscopic colitis involve uncontrolled T-cell activation and T-cell-mediated damage as common denominators. Therefore, diagnosis and treatment decisions clearly benefit from the knowledge of the intricacies of the systemic and the local T-cell activity.

Summary

Depending on the cytokine milieu, CD4⁺ T cells can differentiate into proinflammatory T helper 1 (Th1), anti-inflammatory Th2, antimicrobial Th17, pleiotropic Th9, tissue-instructing Th22 cells, and in the regulatory compartment forkhead box protein 3⁺ Treg, suppressive Tr1 or Th3 cells. Additionally, follicular Th cells provide B-cell help in antibody class switching; cytotoxic CD8⁺ T cells target virus-infected or tumor cells. This review discusses our current knowledge on the contribution of defined T-cell subpopulations to establishing and maintaining chronic intestinal inflammation in either of the above entities. It also puts emphasis on the differences in the prevalence of these diseases between Eastern and Western countries.

Key Messages

In celiac disease, the driving role of T cells in the lamina propria and in the epithelium mainly specific for two defined antigens is well established. Differences in genetics and lifestyle between Western and Eastern countries were instrumental in understanding underlying mechanisms. In IBD, the vast amount of potential antigens and the corresponding antigen-specific T cells makes it unlikely to find universal triggers. Increased mucosal CD4⁺ regulatory T cells in all four entities fail to control or abrogate local inflammatory processes. Thus, prevailing differences in the functional T-cell subtypes driving chronic intestinal inflammation in celiac disease and IBD at best allow some overlap in the treatment options for either disease.

CRTAM determines the CD4+ cytotoxic T lymphocyte lineage

Naive T cells differentiate into various effector T cells, including CD4(+) helper T cell subsets and CD8(+) cytotoxic T cells (CTL). Although cytotoxic CD4(+) T cells (CD4 +: CTL) also develop from naive T cells, the mechanism of development is elusive. We found that a small fraction of CD4(+) T cells that express class I-restricted T cell-associated molecule (CRTAM) upon activation possesses the characteristics of both CD4(+) and CD8(+) T cells. CRTAM(+) CD4(+) T cells secrete IFN-γ, express CTL-related genes, such as eomesodermin (Eomes), Granzyme B, and perforin, after cultivation, and exhibit cytotoxic function, suggesting that CRTAM(+) T cells are the precursor of CD4(+)CTL. Indeed, ectopic expression of CRTAM in T cells induced the production of IFN-γ, expression of CTL-related genes, and cytotoxic activity. The induction of CD4(+)CTL and IFN-γ production requires CRTAM-mediated intracellular signaling. CRTAM(+) T cells traffic to mucosal tissues and inflammatory sites and developed into CD4(+)CTL, which are involved in mediating protection against infection as well as inducing inflammatory response, depending on the circumstances, through IFN-γ secretion and cytotoxic activity. These results reveal that CRTAM is critical to instruct the differentiation of CD4(+)CTL through the induction of Eomes and CTL-related gene.

Galectin-3 suppresses mucosal inflammation and reduces disease severity in experimental colitis

Galectin-3, a member of the β-galactoside-binding lectin family, expresses in many different immune cells and modulates broad biological functions including cell adhesion, cell activation, cell growth, apoptosis, and inflammation. However, the role of galectin-3 in mucosal immunity or inflammatory bowel diseases is still not clear. We demonstrate here that galectin-3 knockout mice have more severe disease activity in the dextran sulfate sodium (DSS)-induced colitis model, indicating that galectin-3 may protect from inflammation in DSS-induced colitis. Furthermore, treating with galectin-3 reduced body weight loss, shortened colonic length, and ameliorated mucosal inflammation in mice having DSS-induced colitis. However, the protective effects of galectin-3 were eliminated by the administration of anti-CD25 mAb. In addition, primary T cells treated with galectin-3 ex vivo induced the expression of FOXP3, ICOS, and PD-1 with a Treg cell phenotype having a suppression function. Moreover, adoptive transfer of galectin-3-treated T cells reduced bowel inflammation and colitis in the T cell transfer colitis model. In conclusion, our results indicate that galectin-3 inhibited colonic mucosa inflammation and reduced disease severity by inducing regulatory T cells, suggesting that it is a potential therapeutic approach in inflammatory bowel disease.

KEY MESSAGES

Galectin-3 offers protection from inflammation in experimental colitis. Galectin-3 knockout mice have more severe disease activity in DSS-induced colitis. Adoptive transfer of galectin-3-treated T cells reduced bowel inflammation. Galectin-3 inhibited colonic mucosa inflammation by inducing regulatory T cells. Galectin-3 is a potential therapeutic approach in inflammatory bowel disease.

A breakthrough in probiotics: Clostridium butyricum regulates gut homeostasis and anti-inflammatory response in inflammatory bowel disease

Intestinal immune homeostasis is regulated by gut microbiota, including beneficial and pathogenic microorganisms. Imbalance in gut bacterial constituents provokes host proinflammatory responses causing diseases such as inflammatory bowel disease (IBD). The development of next-generation sequencing technology allows the identification of microbiota alterations in IBD. Several studies have shown reduced diversity in the gut microbiota of patients with IBD. Advances in gnotobiotic technology have made possible analysis of the role of specific bacterial strains in immune cells in the intestine. Using these techniques, we have shown that Clostridium butyricum as a probiotic induces interleukin-10-producing macrophages in inflamed mucosa via the Toll-like receptor 2/myeloid differentiation primary response gene 88 pathway to prevent acute experimental colitis. In this review, we focus on the new approaches for the role of specific bacterial strains in immunological responses, as well as the potential of bacterial therapy for IBD treatments.

The Special Relationship in the Development and Function of T Helper 17 and Regulatory T Cells

T helper 17 (Th17) cells play an essential role in the clearance of extracellular pathogenic bacteria and fungi. However, this subset is critically involved in the pathology of many autoimmune diseases, e.g., psoriasis, multiple sclerosis, allergy, rheumatoid arthritis, and inflammatory bowel diseases in humans. Therefore, Th17 responses need to be tightly regulated in vivo to mediate effective host defenses against pathogens without causing excessive host tissue damage. Foxp3(+) regulatory T (Treg) cells play an important role in maintaining peripheral tolerance to self-antigens and in counteracting the inflammatory activity of effector T helper cell subsets. Although Th17 and Treg cells represent two CD4(+) T cell subsets with opposing principal functions, these cell types are functionally connected. In this review, we will first give an overview on the biology of Th17 cells and describe their development and in vivo function, followed by an account on the special developmental relationship between Th17 and Treg cells. We will describe the identification of Treg/Th17 intermediates and consider their lineage stability and function in vivo. Finally, we will discuss how Treg cells may regulate the Th17 cell response in the context of infection and inflammation, and elude on findings demonstrating that Treg cells can also have a prominent function in promoting the differentiation of Th17 cells.

Regulatory T-cell Response to Enterotoxigenic Bacteroides fragilis Colonization Triggers IL17-Dependent Colon Carcinogenesis

Many epithelial cancers are associated with chronic inflammation. However, the features of inflammation that are procarcinogenic are not fully understood. Regulatory T cells (Treg) typically restrain overt inflammatory responses and maintain intestinal immune homeostasis. Their immune-suppressive activity can inhibit inflammation-associated cancers. Paradoxically, we show that colonic Tregs initiate IL17-mediated carcinogenesis in multiple intestinal neoplasia mice colonized with the human symbiote enterotoxigenic Bacteroides fragilis (ETBF). Depletion of Tregs in ETBF-colonized C57BL/6 FOXP3(DTR) mice enhanced colitis but diminished tumorigenesis associated with shifting of mucosal cytokine profile from IL17 to IFNγ; inhibition of ETBF-induced colon tumorigenesis was dependent on reduced IL17 inflammation and was independent of IFNγ. Treg enhancement of IL17 production is cell-extrinsic. IL2 blockade restored Th17 responses and tumor formation in Treg-depleted animals. Our findings demonstrate that Tregs limit the availability of IL2 in the local microenvironment, allowing the Th17 development necessary to promote ETBF-triggered neoplasia, and thus unveil a new mechanism whereby Treg responses to intestinal bacterial infection can promote tumorigenesis.

SIGNIFICANCE

Tregs promote an oncogenic immune response to a common human symbiote associated with inflammatory bowel disease and colorectal cancer. Our data define mechanisms by which mucosal Tregs, despite suppressing excessive inflammation, promote the earliest stages of immune procarcinogenesis via enhancement of IL17 production at the expense of IFNγ production.

Harnessing regulatory T cells for the treatment of inflammatory bowel disease

Regulatory CD4 T (Treg) cells are comprised of a heterogeneous population of cells that play a vital role in suppressing inflammation and maintaining immune tolerance. The immunoregulatory function of Treg cells is especially important in the intestine where the mucosa is exposed to a diverse array of foreign antigens-including those derived from food and commensal bacteria. Treg cells are enriched in the intestinal lamina propria and provide a crucial function in promoting tolerance to enteric antigens while modulating tissue inflammation. Correspondingly, Treg cell dysfunction is associated with a breakdown in intestinal tolerance and the induction of aberrant immune responses that may contribute to the pathogenesis of inflammatory bowel disease. This review will provide a brief overview of Treg cell biology with a focus on Foxp3 Treg and type 1 regulatory (Tr1) cells and summarize the evidence for defective Treg cells in experimental and human inflammatory bowel disease. The potential application of Treg cells as a treatment for inflammatory bowel disease will also be discussed in the context of Treg infusion therapy and the in vivo induction/expansion of intestinal Treg cells.

Experimental Models of Inflammatory Bowel Diseases

The understanding of the intestinal inflammation occurring in the inflammatory bowel diseases (IBD) has been immeasurably advanced by the development of the now numerous murine models of intestinal inflammation. The usefulness of this research tool in IBD studies has been enabled by our improved knowledge of mucosal immunity and thus our improved ability to interpret the complex responses of mice with various causes of colitis; in addition, it has been powered by the availability of models in which the mice have specific genetic and/or immunologic defects that can be related to the origin of the inflammation. Finally, and more recently, it has been enhanced by our newly acquired ability to define the intestinal microbiome under various conditions and thus to understand how intestinal microorganisms impact on inflammation. In this brief review of murine models of intestinal inflammation we focus mainly on the most often used models that are, not incidentally, also the models that have yielded major insights into IBD pathogenesis.

Classical Th1 cells obtain colitogenicity by co-existence of RORγt-expressing T cells in experimental colitis

BACKGROUND

Both Th1 and Th17 cell types are involved in the pathogenesis of chronic intestinal inflammation. We recently demonstrated that retinoid-related orphan receptor gamma t (RORγt)-expressing Th17 cells are progenitor cells for alternative Th1 cells, which have the potential to induce colitis. However, the involvement of classical Th1 (cTh1) cells generated directly from naive T cells without RORγt expression in the pathogenesis of colitis remains poorly understood.

METHODS

We performed a series of in vivo experiments using a murine chronic colitis model induced by adoptive transfer of splenic CD4CD45RB(high) T cells obtained from wild-type, RORγt(gfp/gfp), or RORγt(gfp/gfp) mice into RAG-2(-/-) mice.

RESULTS

RAG-2(-/-) mice receiving transfer of in vitro-manipulated RORγt(gfp/gfp) Th1 cells developed colitis. RAG-2(-/-) mice co-transferred with splenic CD4CD45RB(high) T cells obtained from wild-type mice and RORγt(gfp/gfp) mice developed colitis with a significant increase in RORγt cTh1 cell numbers when compared with noncolitic mice transferred with splenic CD4CD45RB(high) T cells obtained from RORγt(gfp/gfp) mice. Furthermore, RAG-2(-/-) mice transferred with in vivo-manipulated RORγt(gfp/gfp) cTh1 cells developed colitis with a significant increase in RORγt(gfp/gfp) cTh1 cell numbers.

CONCLUSIONS

These findings indicate that both alternative Th1 cells and cTh1 cells have the potential to be colitogenic in an adaptive transfer model. The development of cTh1 cells was dependent on the co-existence of RORγt-expressing T cells, suggesting a critical role for the interactions of these cell types in the development of chronic intestinal inflammation.

Use of animal models in elucidating disease pathogenesis in IBD

Inflammatory bowel diseases (IBD) are a collection of diseases characterized by chronic gastrointestinal inflammation resulting from an exuberant immune response to commensal flora in genetically susceptible individuals. Rapid advances in the field of genomics have resulted in the identification of at least 163 loci that contribute susceptibility to both Crohn's disease (CD) and ulcerative colitis (UC). Similar to other complex diseases, however, the "curse of missing heritability" remains a significant concern in understanding the mechanisms underlying IBD. While genetic discoveries, to date, only account for 7-14% of disease variance for IBD, studies have increasingly demonstrated a role for environmental factors in disease pathogenesis. Furthermore, the use of animal models of IBD has led to a greater understanding of disease pathogenesis implicating various aspects of the innate immune response including the bacterial, fungal, and viral microbiome and adaptive immune response such as the interleukin (IL)-23/IL-17 pathway.

Cross-talk between RORγt+ innate lymphoid cells and intestinal macrophages induces mucosal IL-22 production in Crohn's disease

BACKGROUND

Interleukin (IL)-22-producing RORγt innate lymphoid cells (ILCs) play a pivotal role in intestinal immunity. Recent reports demonstrated that ILCs contribute to mucosal protection and intestinal inflammation in mice. In humans, numbers of RORγt ILCs are significantly increased in the intestine of patients with Crohn's disease (CD), suggesting that ILCs may be associated with intestinal inflammation in CD. However, the mechanism by which ILCs are regulated in the intestine of patients with CD is poorly understood. This study aimed to determine the activation mechanism of intestinal ILCs in patients with CD.

METHODS

CD45 lineage marker ILCs were isolated from intestinal lamina propria of patients with CD. ILCs were then subdivided into 4 distinct populations based on the expression of CD56 and CD127. Purified ILC subsets were cocultured with intestinal CD14 macrophages, and IL-22 production was evaluated.

RESULTS

CD127CD56 and CD127CD56 ILC, but not CD127CD56 or CD127CD56 ILC, subsets expressed RORγt and produced IL-22. IL-22 production by these ILC subsets was enhanced when ILCs were cocultured with intestinal macrophages. IL-23 or cell-to-cell contact was required for macrophage-mediated activation of ILCs. IL-22 production by ILCs was perturbed in inflamed mucosa compared with noninflamed mucosa. IL-22 induced the expression of Reg1α and Claudin-1 in human intestinal epithelial organoids.

CONCLUSIONS

RORγt ILCs might enhance mucosal barrier function through the upregulation of Reg1α through production of IL-22. Although CD14 macrophages augment intestinal inflammation in patients with CD, macrophages also promote a negative feedback pathway through the activation of IL-22 production by RORγt ILCs.

Cytokines in inflammatory bowel disease

Cytokines have a crucial role in the pathogenesis of inflammatory bowel diseases (IBDs), such as Crohn's disease and ulcerative colitis, where they control multiple aspects of the inflammatory response. In particular, the imbalance between pro-inflammatory and anti-inflammatory cytokines that occurs in IBD impedes the resolution of inflammation and instead leads to disease perpetuation and tissue destruction. Recent studies suggest the existence of a network of regulatory cytokines that has important implications for disease progression. In this Review, we discuss the role of cytokines produced by innate and adaptive immune cells, as well as their relevance to the future therapy of IBD.

Interleukin 23 in Crohn's disease

: Crohn's disease (CD) is a lifelong inflammatory condition with underlying environmental and genetic components. CD affects multiple parts of the gastrointestinal tract, and it has a growing incidence in Western societies. IL-23 receptor variants have been identified as susceptibility or resistance factors for CD in genome-wide association studies. Accordingly, IL-23 is required for the development of experimental inflammatory bowel disease in many murine models. IL-23 receptor is expressed by both innate and adaptive immune cells, which include Th17, natural killer T, γδ T cells, and RORγt innate lymphoid cells all of which are capable of secreting IL-17A, IL-17F, IL-22, and interferon-γ upon IL-23 stimulation. During the past decade, pathogenic and protective roles have been described for these cytokines in the inflammatory bowel disease pathogenesis. More recently, innate lymphoid cells have been implicated in disease development. In this review, we have summarized and discussed these findings with an emphasis not only on the contribution of Th17 but also on innate lymphoid cells to disease etiology.

Th17 cells and Tregs: unlikely allies

Identification of CD4⁺Foxp3⁺ T_regs and Th17 modified the historical Th1-Th2 paradigm. Currently, the Th17-T_regs dichotomy provides a dominant conceptual framework for the comprehension of immunity/inflammation and tolerance/immunosuppression in an increasing number of diseases. Targeting proinflammatory Th17 cells or immunosuppressive T_regs has been widely considered as a promising therapeutic strategy in the treatment of major human diseases, including autoimmunity and cancer. The efficacy and safety of such therapy rely on a thorough understanding of immunobiology and interaction of these two subsets of Th cells. In this article, we review recent progress concerning complicated interplay of Th17 cells and T_regs There is compelling evidence that T_regs potently inhibit Th1 and Th2 responses; however, the inhibitory effect of T_regs on Th17 responses is a controversial subject. There is increasing evidence showing that T_regs actually promote the differentiation of Th17 cells in vitro and in vivo and consequently, enhanced the functional consequences of Th17 cells, including the protective effect in host defense, as well as detrimental effect in inflammation and in the support of tumor growth. On the other hand, Th17 cells were also the most potent Th subset in the stimulation and support of expansion and phenotypic stability of T_regs in vivo. These results indicate that these two subsets of Th cells reciprocally stimulate each other. This bidirectional crosstalk is largely dependent on the TNF-TNFR2 pathway. These mutual stimulatory effects should be considered in devising future Th17 cell- and T_reg-targeting therapy.

Immunopathology of inflammatory bowel disease

Inflammatory bowel disease (IBD) results from a complex series of interactions between susceptibility genes, the environment, and the immune system. The host microbiome, as well as viruses and fungi, play important roles in the development of IBD either by causing inflammation directly or indirectly through an altered immune system. New technologies have allowed researchers to be able to quantify the various components of the microbiome, which will allow for future developments in the etiology of IBD. Various components of the mucosal immune system are implicated in the pathogenesis of IBD and include intestinal epithelial cells, innate lymphoid cells, cells of the innate (macrophages/monocytes, neutrophils, and dendritic cells) and adaptive (T-cells and B-cells) immune system, and their secreted mediators (cytokines and chemokines). Either a mucosal susceptibility or defect in sampling of gut luminal antigen, possibly through the process of autophagy, leads to activation of innate immune response that may be mediated by enhanced toll-like receptor activity. The antigen presenting cells then mediate the differentiation of naïve T-cells into effector T helper (Th) cells, including Th1, Th2, and Th17, which alter gut homeostasis and lead to IBD. In this review, the effects of these components in the immunopathogenesis of IBD will be discussed.

In vitro generated Th17 cells support the expansion and phenotypic stability of CD4(+)Foxp3(+) regulatory T cells in vivo

CD4(+) T cells stimulate immune responses through distinct patterns of cytokine produced by Th1, Th2 or Th17 cells, or inhibit immune responses through Foxp3-expressing regulatory T cells (Tregs). Paradoxically, effector T cells were recently shown to activate Tregs, however, it remains unclear which Th subset is responsible for this effect. In this study, we found that Th17 cells expressed the highest levels of TNF among in vitro generated Th subsets, and most potently promoted expansion and stabilized Foxp3 expression by Tregs when co-transferred into Rag1(-/-) mice. Both TNF and IL-2 produced by Th17 cells contributed to this effect. The stimulatory effect of Th17 cells on Tregs was largely abolished when co-transferred with TNFR2-deficient Tregs. Furthermore, Tregs deficient in TNFR2 also supported a much lower production of IL-17A and TNF expression by co-transferred Th17 cells. Thus, our data indicate that the TNF-TNFR2 pathway plays a crucial role in the reciprocal stimulatory effect of Th17 cells and Tregs. This bidirectional interaction should be taken into account when designing therapy targeting Th17 cells, Tregs, TNF and TNFR2.

Revisiting the old link between infection and autoimmune disease with commensals and T helper 17 cells

Genetic composition and major histocompatibility complex polymorphisms unequivocally predispose to autoimmune disease, but environmental factors also play a critical role in precipitating disease in susceptible individuals. Notorious among these has been microbial infection. Older studies describing associations between microbial infection and autoimmune disease are now followed by new studies demonstrating correlations between susceptibility to autoimmune disease and commensal colonization of the intestinal tract. T helper 17 (T(H)17) cells have gained a prominent role in autoimmune disease, and notably, their development within the intestine has been linked to colonization with specific commensal bacteria. Here, we consider current views on how microbes, T(H)17 cells, and autoimmunity are connected. We speculate on how the intricate relationships among commensal, pathogen, and the host might ultimately determine susceptibility to autoimmune disease.

Immune aspects of the pathogenesis of inflammatory bowel disease

Although the precise etiologies of inflammatory bowel disease (IBD) (ulcerative colitis and Crohn's disease) remain obscure, several reports have indicated that dysfunction of the mucosal immune system plays an important role in its pathogenesis. Recent progress with genome-wide association studies has identified many IBD susceptibility genes. In individuals with genetic risk, abnormal interactions between the host immune system and gut flora, and dysregulation of cellular responses such as autophagy and ER stress, induce an abnormal host immune response in the gut resulting in intestinal inflammation. Research progress animal models in IBD, and in human IBD, has identified several key molecules in IBD pathogenesis such as TNFα and adhesion molecules, and molecular targeting therapies based on these molecules have been developed. Here, we review immunological aspects in IBD pathogenesis and the development of immunoregulatory therapy.

Role of the gut microbiota in the development and function of lymphoid cells

Mammals are colonized by large numbers of microorganisms, including trillions of bacteria, most of which live in the intestinal tract. These indigenous microorganisms that inhabit the body of humans and animals are referred collectively to as the microbiota. Accumulating evidence indicates that the microbiota regulates the development and/or function of different types of immune cells in the intestine. For example, the microbiota drives homeostatic, pathogenic, and regulatory T cell immune responses that contribute to tissue homeostasis, but also can promote disease. The gut microbes also facilitate IgA responses, which in turn regulate the composition and function of the gut microbiota. Thus, the reciprocal regulation of the gut microbiota and the host immune system may influence the balance between homeostasis and disease in the intestine.

Probiotics increase T regulatory cells and reduce severity of experimental colitis in mice

AIM: To investigate the effect of probiotics on regulating T regulatory cells and reducing the severity of experimental colitis in mice.

METHODS: Forty C57/BL mice were randomly divided into four groups. Colitis was induced in the mice using 2,4,6-trinitrobenzene sulfonic acid (TNBS). After 10-d treatment with Bifico capsules (combined bifidobacterium, lactobacillus and enterococcus), body weight, colonic weight, colonic weight index, length of colon, and histological scores were evaluated. CD4⁺CD25⁺Foxp3⁺T cell in mesenteric lymph nodes were measured by flow cytometry, and cytokines in colonic tissue homogenates were analyzed by a cytometric bead array.

RESULTS: The colonic weight index and the colonic weight of colitis mice treated with Bifico were lower than that of TNBS-induced mice without treatment. However, colonic length and percent of body weight amplification were higher than in TNBS-induced mice without treatment. Compared with TNBS-induced mice without treatment, the level of CD4⁺CD25⁺Foxp3⁺T cells in mesenteric lymph nodes, the expression of interleukin (IL)-2, IL-4 and IL-10 in colonic tissues from colitis mice treated with Bifico were upregulated, and tumor necrosis factor-α and interferon-γ were downregulated.

CONCLUSION: Probiotics effectively treat experimental colitis by increasing CD4⁺CD25⁺Foxp3⁺T cell and regulating the balance of Th1 and Th2 cytokines in the colonic mucosa.

TNFR2 is critical for the stabilization of the CD4+Foxp3+ regulatory T. cell phenotype in the inflammatory environment

Several lines of evidence indicate the instability of CD4(+)Foxp3(+) regulatory T cells (Tregs). We have therefore investigated means of promoting the stability of Tregs. In this study, we found that the proportion of Tregs in mouse strains deficient in TNFR2 or its ligands was reduced in the thymus and peripheral lymphoid tissues, suggesting a potential role of TNFR2 in promoting the sustained expression of Foxp3. We observed that upon in vitro activation with plate-bound anti-CD3 Ab and soluble anti-CD28 Ab, Foxp3 expression by highly purified mouse Tregs was markedly downregulated. Importantly, TNF partially abrogated this effect of TCR stimulation and stabilized Foxp3 expression. This effect of TNF was blocked by anti-TNFR2 Ab, but not by anti-TNFR1 Ab. Furthermore, TNF was not able to maintain Foxp3 expression by TNFR2-deficient Tregs. In a mouse colitis model induced by transfer of naive CD4 cells into Rag1(-/-) mice, the disease could be inhibited by cotransfer of wild-type Tregs, but not by cotransfer of TNFR2-deficient Tregs. Furthermore, in the lamina propria of the colitis model, most wild-type Tregs maintained Foxp3 expression. In contrast, an increased number of TNFR2-deficient Tregs lost Foxp3 expression. Thus, our data clearly show that TNFR2 is critical for the phenotypic and functional stability of Tregs in the inflammatory environment. This effect of TNF should be taken into account when designing future therapy of autoimmunity and graft-versus-host disease by using TNF inhibitors.

Dendritic cells administered intrarectally penetrate the intestinal barrier to break intestinal tolerance via Th2-medeiated colitis in mice

Intestinal lamina propria dendritic cells (LPDCs) in mice are known to extend dendrites between the intestinal epithelia and the luminal side when processing luminal antigens. We conducted intrarectal cell transfer experiments of bone marrow-derived dendritic cells (BMDCs) in mice to assess dendritic cell penetration of the intestine. Intrarectally administered GFP(+) BMDCs localized in the colonic LP within 3h and the spleen within 12h after administration. 72h after administration, recipient C57BL/6 mice showed acute diarrhea, and administration of BMDCs (once weekly for 3 weeks) induced intestinal inflammation with increased numbers of recipient macrophages and CD4(+) T cells exhibiting a Th2-mediated immune response. These results demonstrate that DCs actively communicate across the intestinal barrier, and highlight a potential technique for controlling colonic immune tolerance.

Mutual expression of the transcription factors Runx3 and ThPOK regulates intestinal CD4⁺ T cell immunity

The gut mucosa hosts large numbers of activated lymphocytes that are exposed to stimuli from the diet, microbiota and pathogens. Although CD4(+) T cells are crucial for defense, intestinal homeostasis precludes exaggerated responses to luminal contents, whether they are harmful or not. We investigated mechanisms used by CD4(+) T cells to avoid excessive activation in the intestine. Using genetic tools to label and interfere with T cell-development transcription factors, we found that CD4(+) T cells acquired the CD8-lineage transcription factor Runx3 and lost the CD4-lineage transcription factor ThPOK and their differentiation into the T(H)17 subset of helper T cells and colitogenic potential, in a manner dependent on transforming growth factor-β (TGF-β) and retinoic acid. Our results demonstrate considerable plasticity in the CD4(+) T cell lineage that allows chronic exposure to luminal antigens without pathological inflammation.

Dysregulated balance of retinoid-related orphan receptor γt-dependent innate lymphoid cells is involved in the pathogenesis of chronic DSS-induced colitis

Retinoid-related orphan receptor (ROR) γt-expressing and IL-22-producing NKp46(+) innate lymphoid (ILC22) cells reside in the lamina propria of the intestine in mice, suggesting that ILC22 cells contribute to host defense during intestinal damage in models of colitis in mice. Nevertheless, another set of pathological interferon (IFN)-γ and/or IL-17A-producing innate lymphoid cells (ILC1 and ICL17) may participate in the pathogenesis in different models of colitis. We here showed that RORγt(+)IL-22(+) ILC22 cells were localized in Thy-1(high)SCA-1(high) and/or Thy-1(high)SCA-1(low) subpopulations of the intestine in normal and dextran sodium sulfate (DSS)-induced colitic RORγt-sufficient Rag-2(-/-) mice. RORγt-deficient Rag-2(-/-) mice developed more severe DSS-induced colitis accompanied with lower expression of REG3β and REG3γ in the colon, but with a lower ratio and absolute number of IFN-γ-producing ILC1 cells as compared to control RORγt-sufficient Rag-2(-/-) mice. Collectively, not only the presence of ILC22 cells but also the balance of protective and pathogenic ILCs may be involved in the prevention of colitis.

T-helper 17 and interleukin-17-producing lymphoid tissue inducer-like cells make different contributions to colitis in mice

BACKGROUND & AIMS

T helper (Th) 17 cells that express the retinoid-related orphan receptor (ROR) γt contribute to the development of colitis in mice, yet are found in normal and inflamed intestine. We investigated their development and functions in intestines of mice.

METHODS

We analyzed intestinal Th17 cells in healthy and inflamed intestinal tissues of mice. We analyzed expression of lymphotoxin (LT)α by Th17 cells and lymphoid tissue inducer-like cells.

RESULTS

LTα(-/-) and RORγt(-/-) mice had significantly lower percentages of naturally occurring Th17 cells in the small intestine than wild-type mice. Numbers of CD3(-)CD4(+/-)interleukin-7Rα(+)c-kit(+)CCR6(+)NKp46(-) lymphoid tissue inducer-like cells that produce interleukin-17A were increased in LTα(-/-) and LTα(-/-) × recombination activating gene (RAG)-2(-/-) mice, compared with wild-type mice, but were absent from RORγt(-/-) mice. Parabiosis of wild-type and LTα(-/-) mice and bone marrow transplant experiments revealed that LTα-dependent gut-associated lymphoid tissue structures are required for generation of naturally occurring Th17 cells. However, when wild-type or LTα(-/-) CD4(+)CD45RB(high) T cells were transferred to RAG-2(-/-) or LTα(-/-)×RAG-2(-/-) mice, all groups, irrespective of the presence or absence of LTα on the donor or recipient cells, developed colitis and generated Th1, Th17, and Th17/Th1 cells. RAG-2(-/-) mice that received a second round of transplantation, with colitogenic but not naturally occurring Th17 cells, developed intestinal inflammation. The presence of naturally occurring Th17 cells in the colons of mice inhibited development of colitis after transfer of CD4(+)CD45RB(high) T cells and increased the numbers of Foxp3(+) cells derived from CD4(+)CD45RB(high) T cells.

CONCLUSIONS

Gut-associated lymphoid tissue structures are required to generate naturally occurring Th17 cells that have regulatory activities in normal intestines of mice, but not for colitogenic Th17 and Th17/Th1 cells during inflammation.

IgA plasma cells express the negative regulatory co-stimulatory molecule programmed cell death 1 ligand and have a potential tolerogenic role in the intestine

To maintain immune homeostasis in the intestine, the intestinal immune system has evolved several tolerogenic mechanisms toward intestinal microflora and food antigens. Although programmed cell death-1 (PD-1) protein has been implicated in immunological tolerance in the intestine and gut-associated lymphoid tissues (GALTs), distribution of its ligands PD-L1 and PD-L2 in the small intestine lamina propria (LP) are unknown. We investigated PD-L1 expression in intestinal LP and found that IgA plasma cells (PCs) were major PD-L1 expressing cells. PD-L1 expression levels on IgA PCs were higher than that on IgG PCs in peripheral lymphoid tissues. IgA PCs expressed antigen-presenting molecule MHC class II and co-stimulatory molecules CD80, CD86, and PD-L2. IgA PCs isolated from intestinal LP exhibited antigen presentation activity, and in the presence of TGF-β induced FoxP3(+) regulatory T cells, but not IFN-γ(+) Th1 cells, from naïve T cells. Thus, IgA PCs in the intestine may be involved in an immune regulatory role in the intestinal immune system.

Excessive CD4+ T cells co-expressing interleukin-17 and interferon-γ in patients with Behçet's disease

Excessive T helper type 1 (Th1) cell activity has been reported in Behçet's disease (BD). Recently, association of Th17 cells with certain autoimmune diseases was reported, and we thus investigated circulating Th17 cells in BD. CD4(+) CD45RO(-) (naive) T cells were cultured with Th0-, Th1-, Th2- and Th17-related cytokines and antibodies, and their mRNA was studied by real-time polymerase chain reaction (PCR). When naive CD4(+) T cells were cultured with Th1- and Th17-related cytokines, interferon (IFN)-γ mRNA and interleukin (IL)-17 mRNA were up-regulated, respectively, in BD patients. Naive CD4(+) T cells cultured in a Th17 cell-inducing condition expressed IL-23 receptor (IL-23R) mRNA excessively. IL-17 mRNA expression was induced only when naive CD4(+) T cells were cultured in the presence of IL-23. CD4(+) T cells cultured with Th17 cytokines expressed excessive RAR-related orphan receptor C (RORC) mRNA. Using intracellular cytokine staining, we found that CD45RO(+) (memory) CD4(+) T cells producing IL-17 and IFN-γ simultaneously were increased significantly. Memory CD4(+) T cells producing IFN-γ but not IL-17 decreased profoundly in BD patients. CD4(+) T cells producing IL-17 and IFN-γ simultaneously were found in BD skin lesions. Collectively, we found excessive CD4(+) T cells producing IL-17 and IFN-γ (Th1/Th17) cells in patients with BD, and possible involvement of IL-23/IL-23R pathway for the appearance of excessive Th1/Th17 cells.

Integrative continuum: accelerating therapeutic advances in rare autoimmune diseases

Autoimmune diseases are chronic, life threatening, and of burgeoning public health concern. They rank among the 10 most common causes of death in women, and some have incidence rates surpassing those of heart disease and cancer. Emerging information regarding molecular and cellular mechanisms affords opportunities for the discovery of novel therapeutic strategies or the repurposing of FDA-approved pharmacologic agents. Yet, obstacles to drug development amplify as an inverse function of the incidence of rare autoimmune disease; challenges include heterogeneous clinical presentation, paucity of definitive biomarkers, and poorly validated measures of therapeutic response. An integrative continuum model to address these challenges is being applied to neuromyelitis optica (NMO)-a potentially devastating neurodegenerative process that has had limited therapeutic options. This model links target discovery with pharmacologic application to accelerate improved clinical efficacy. The application of such innovative strategies may help researchers overcome barriers to therapeutic advances in NMO and other rare autoimmune diseases.

RORγt-dependent IL-17A-producing cells in the pathogenesis of intestinal inflammation

The hypothesis of helper T (T(h))1/T(h)2 cytokine balance proposed by Mosmann and Coffman is often invoked to explain the development of inflammatory diseases, including inflammatory bowel diseases (IBD). Recently, however, a newly identified class of T(h) cells-T(h)17 cells, which produce T(h)17 family cytokines-has been recognized as an essential subpopulation in the development of almost all kinds of human and animal inflammatory diseases, rather than T(h)1 and T(h)2 cells. A representative T(h)17 family cytokine, interleukin (IL)-17A, is produced by not only T(h)17 cells, but also by other types of cells, such as T-cell receptor γδ T cells, natural killer (NK) T cells, NK cells, myeloid cells, and innate lymphoid cells, which may also be critically involved in the initiation and persistence of IBD. Here we review recent advances in the study of such IL-17A-producing cells in the pathogenesis of IBD.