miR-182 and miR-10a are key regulators of Treg specialisation and stability during Schistosome and Leishmania-associated inflammation

A diverse suite of effector immune responses provide protection against various pathogens. However, the array of effector responses must be immunologically regulated to limit pathogen- and immune-associated damage. CD4⁺Foxp3⁺ regulatory T cells (Treg) calibrate immune responses; however, how Treg cells adapt to control different effector responses is unclear. To investigate the molecular mechanism of Treg diversity we used whole genome expression profiling and next generation small RNA sequencing of Treg cells isolated from type-1 or type-2 inflamed tissue following Leishmania major or Schistosoma mansoni infection, respectively. In-silico analyses identified two miRNA “regulatory hubs” miR-10a and miR-182 as critical miRNAs in Th1- or Th2-associated Treg cells, respectively. Functionally and mechanistically, in-vitro and in-vivo systems identified that an IL-12/IFNγ axis regulated miR-10a and its putative transcription factor, Creb. Importantly, reduced miR-10a in Th1-associated Treg cells was critical for Treg function and controlled a suite of genes preventing IFNγ production. In contrast, IL-4 regulated miR-182 and cMaf in Th2-associed Treg cells, which mitigated IL-2 secretion, in part through repression of IL2-promoting genes. Together, this study indicates that CD4⁺Foxp3⁺ cells can be shaped by local environmental factors, which orchestrate distinct miRNA pathways preserving Treg stability and suppressor function.

The diversity of pathogens that the immune system encounters are controlled by a diverse suite of immunological effector responses. Preserving a well-controlled protective immune response is essential. Too vigorous an effector response can be as damaging as too little. Regulatory T cells (Treg) calibrate immune responses; however, how Treg cells adapt to control the diverse suite of effector responses is unclear. In this study we investigated the molecular identity of regulatory T cells that control distinct effector immune responses against two discrete pathogens, an intracellular parasitic protozoa, Leishmania major, and an extracellular helminth parasite, Schitsosoma mansoni. The two Treg populations studied were phenotypically and functionally different. We identified molecular pathways that influence this diversity and more specifically, we identified that two miRNAs (miR-182 and miR-10a) act as “regulatory hubs” critically controlling distinct properties within each Treg population. This is the first study identifying the upstream molecular pathways controlling Treg cell specialization and provides a new platform of Treg cell manipulation to fine-tune their function.

Effector and memory T cell responses to commensal bacteria

Barrier surfaces are home to a vast population of commensal organisms that together encode millions of proteins; each of them possessing several potential foreign antigens. Regulation of immune responses to this enormous antigenic load represents a tremendous challenge for the immune system. Tissues exposed to commensals have developed elaborate systems of regulation including specialized populations of resident lymphocytes that maintain barrier function and limit potential responses to commensal antigens. However, in settings of infection and inflammation these regulatory mechanisms are compromised and specific effector responses against commensal bacteria can develop. This review discusses the circumstances controlling the fate of commensal specific T cells and how dysregulation of these responses could lead to severe pathological outcomes.

Inflammatory monocytes regulate pathologic responses to commensals during acute gastrointestinal infection

Commensal flora can promote both immunity to pathogens and mucosal inflammation. How commensal driven inflammation is regulated in the context of infection remains poorly understood. Here, we show that during acute mucosal infection, Ly6C^hi inflammatory monocytes acquire a tissue specific regulatory phenotype associated with production of the lipid mediator prostaglandin E₂ (PGE₂). Notably, in response to commensals, Ly6C^hi monocytes can directly inhibit neutrophil activation in a PGE₂-dependent manner. Further, in the absence of inflammatory monocytes, mice develop severe neutrophil-mediated pathology that can be controlled by PGE₂ analog treatment. Complementing these findings, inhibition of PGE₂ led to enhanced neutrophil activation and host mortality. These data demonstrate a previously unappreciated dual action of inflammatory monocytes in controlling pathogen expansion while limiting commensal mediated damage to the gut. Collectively, our results place inflammatory monocyte derived PGE₂ at the center of a commensal driven regulatory loop required to control host-commensal dialogue during inflammation.

Evaluating the in vivo Th2 priming potential among common allergens

Exposure to allergens, both man-made and from our environment is increasingly associated with the development of significant human health issues such as allergy and asthma. Allergen induced production of the cytokine interleukin (IL-)4 by Th2 cells is central to the pathogenesis of allergic disease (Gavett et al., 1994). The development of the G4 mouse, that expresses green fluorescent protein (GFP) as a surrogate for IL-4 protein expression has made it possible to directly track the immune cells that produce IL-4. By combining a reliable intradermal immunisation technique with the transgenic G4 mouse we have been able to develop a novel & unique in vivo primary Th2 immune response model (PTh2). When allergens relevant to human disease are evaluated using the PTh2 assay a dose dependent hierarchy of allergenicity is revealed with environmental allergens (cockroach, house dust mite) the most potent and food allergens being the least. In addition, the PTh2 assay is extremely sensitive to the immunoregulatory effects of Mycobacterial extracts and immunosuppressive drugs on primary Th2 cell development. Taken together, this assay provides a standardised method for the identification of the structural and functional properties of proteins relevant to allergenicity, and is a powerful screening tool for novel lead compounds that are effective at inhibiting the primary Th2 response in allergic diseases.

Cross-talk between T cell receptor and cytokine signaling is mediated by Itk: implications for the balance between Th17 and regulatory T cells (P1141)

A proper balance between Th17 and T regulatory cells (Tregs) is critical for the generation of immune protective immune responses while minimizing autoimmunity. We show that the Tec family kinase Itk influences the balance between Th17 and T regulatory cells by regulating crosstalk between T Cell Receptor (TCR) and cytokine signaling. Under both Th17 and Tregs differentiation conditions, Itk-/- CD4 T cells develop higher percentages of functional Foxp3+ cells, associated with increased responsiveness to IL-2. Itk-deficient CD4 cells show reduced TCR-induced phosphorylation of mTOR targets, including ribosomal S6 and AKT, accompanied by changes in metabolic signatures that are affected by mTOR pathways. Surprisingly, iTreg Itk-/- cells also show reduced IL-2-induced mTOR pathways, despite increased STAT5 phosphorylation. We find that TCR stimulation in WT CD4+ T cells leads to a dose-dependent repression of expression of Pten; however, at low TCR stimulation or in the absence of Itk, Pten is not effectively repressed, thereby uncoupling STAT5 phosphorylation and PI3K/mTOR pathways and increasing Foxp3 expression. Functional studies show that naïve Itk-/- CD4 T cells also preferentially develop into Tregs in an in vivo colitis model. Furthermore Itk-deficient Tregs are capable to inhibit the development of colitis demonstrating that Treg function is independent of Itk. Our results suggest that Itk is a potential target for altering the balance between Th17 and Treg cells.

IL-20 receptor signaling inhibits cutaneous IL-1β and IL-17A production to promote methicillin-resistant Staphylococcus aureus infection (P6350)

Staphylococcus aureus causes the majority of human skin and soft tissue infections, and is a major infectious cause of mortality. However, host defense mechanisms against S. aureus are incompletely understood. Interleukin (IL)-19, -20 and -24 signal through type I and type II IL-20 receptors and have been associated with inflammatory skin diseases such as psoriasis and atopic dermatitis. We hypothesized these cytokines may play a role in S. aureus skin infections. We show here that this family of cytokines promotes cutaneous S. aureus infection in mice by downregulating IL-1β- and IL-17A-dependent inflammatory pathways required for control of infection. Our findings identify an immunosuppressive role for these cytokines during infection, and suggest these pathways may contribute to infectious susceptibility.

Distinct requirements for T-bet in gut innate lymphoid cells

Interleukin (IL)-22-producing innate lymphoid cells (ILCs; ILC22) comprise a heterogeneous population of cells that are dependent on the transcription factor retinoid-related orphan γt (RORγt) and are critical for barrier function of the intestinal mucosa. A distinct ILC22 subset expresses the natural cytotoxicity receptor NKp46 (NKp46+ ILC22); however, the factors that contribute to the generation of this population versus other subsets are largely unknown. Herein, we show that T-bet (encoded by Tbx21) was highly expressed in NKp46+ ILC22, a feature shared by all NKp46+ cells present in the intestine but not by other IL-22-producing populations. Accordingly, the absence of T-bet resulted in loss of NKp46+ ILC22 in the intestinal lamina propria. The residual NKp46+ ILC22 present in Tbx21(-/-) mice showed a marked reduction of Rorγt expression and impairment in IL-22 production. Generation and functions of gut NK1.1+ cells were also altered. Bone marrow chimera experiments revealed a cell-intrinsic requirement for T-bet in these subsets and competitive reconstitution experiments revealed roles for T-bet in multiple ILC subsets. Thus, T-bet has a general importance for ILC in the gut and plays a selective and critical role in the generation of NKp46+ ILC22.

Loss of mucosal CD103+ DCs and IL-17+ and IL-22+ lymphocytes is associated with mucosal damage in SIV infection

Human immunodeficiency virus (HIV) and Simian immunodeficiency virus (SIV) disease progression is associated with multifocal damage to the gastrointestinal tract epithelial barrier that correlates with microbial translocation and persistent pathological immune activation, but the underlying mechanisms remain unclear. Investigating alterations in mucosal immunity during SIV infection, we found that damage to the colonic epithelial barrier was associated with loss of multiple lineages of interleukin (IL)-17-producing lymphocytes, cells that microarray analysis showed expressed genes important for enterocyte homeostasis, including IL-22. IL-22-producing lymphocytes were also lost after SIV infection. Potentially explaining coordinate loss of these distinct populations, we also observed loss of CD103+ dendritic cells (DCs) after SIV infection, which associated with the loss of IL-17- and IL-22-producing lymphocytes. CD103+ DCs expressed genes associated with promotion of IL-17/IL-22+ cells, and coculture of CD103+ DCs and naïve T cells led to increased IL17A and RORc expression in differentiating T cells. These results reveal complex interactions between mucosal immune cell subsets providing potential mechanistic insights into mechanisms of mucosal immune dysregulation during HIV/SIV infection, and offer hints for development of novel therapeutic strategies to address this aspect of AIDS virus pathogenesis.

Acute gastrointestinal infection induces long-lived microbiota-specific T cell responses

The mammalian gastrointestinal tract contains a large and diverse population of commensal bacteria and is also one of the primary sites of exposure to pathogens. How the immune system perceives commensals in the context of mucosal infection is unclear. Here, we show that during a gastrointestinal infection, tolerance to commensals is lost, and microbiota-specific T cells are activated and differentiate to inflammatory effector cells. Furthermore, these T cells go on to form memory cells that are phenotypically and functionally consistent with pathogen-specific T cells. Our results suggest that during a gastrointestinal infection, the immune response to commensals parallels the immune response against pathogenic microbes and that adaptive responses against commensals are an integral component of mucosal immunity.

The cytokines interleukin 27 and interferon-γ promote distinct Treg cell populations required to limit infection-induced pathology

Interferon-γ (IFN-γ) promotes a population of T-bet(+) CXCR3(+) regulatory T (Treg) cells that limit T helper 1 (Th1) cell-mediated pathology. Our studies demonstrate that interleukin-27 (IL-27) also promoted expression of T-bet and CXCR3 in Treg cells. During infection with Toxoplasma gondii, a similar population emerged that limited T cell responses and was dependent on IFN-γ in the periphery but on IL-27 at mucosal sites. Transfer of Treg cells ameliorated the infection-induced pathology observed in Il27(-/-) mice, and this was dependent on their ability to produce IL-10. Microarray analysis revealed that Treg cells exposed to either IFN-γ or IL-27 have distinct transcriptional profiles. Thus, IFN-γ and IL-27 have different roles in Treg cell biology and IL-27 is a key cytokine that promotes the development of Treg cells specialized to control Th1 cell-mediated immunity at local sites of inflammation.

Compartmentalized control of skin immunity by resident commensals

Intestinal commensal bacteria induce protective and regulatory responses that maintain host-microbial mutualism. However, the contribution of tissue-resident commensals to immunity and inflammation at other barrier sites has not been addressed. We found that in mice, the skin microbiota have an autonomous role in controlling the local inflammatory milieu and tuning resident T lymphocyte function. Protective immunity to a cutaneous pathogen was found to be critically dependent on the skin microbiota but not the gut microbiota. Furthermore, skin commensals tuned the function of local T cells in a manner dependent on signaling downstream of the interleukin-1 receptor. These findings underscore the importance of the microbiota as a distinctive feature of tissue compartmentalization, and provide insight into mechanisms of immune system regulation by resident commensal niches in health and disease.

Dietary and commensal derived nutrients: shaping mucosal and systemic immunity

The intestine serves as the primary site of nutrient absorption in the body while also harboring the highest burden of commensal microflora and representing a major portal of pathogen exposure. As such, the immune network of the intestine relies on both dietary and commensal derived signals to guide appropriate function. Recent advances highlight the role of dietary derived nutrients and commensal derived metabolites in shaping gastrointestinal immunity. In particular, vitamin A has been shown to have dominant and pleiotropic effects in the intestine. In addition, dietary derived AHR ligands and commensal derived metabolites are now emerging as important players in mucosal immunity. Thus nutrition, commensal microflora and the mucosal immune system are all intimately connected.

Stromal-derived IL-6 alters the balance of myeloerythroid progenitors during Toxoplasma gondii infection

Inflammation alters hematopoiesis, often by decreasing erythropoiesis and enhancing myeloid output. The mechanisms behind these changes and how the BM stroma contributes to this process are active areas of research. In this study, we examine these questions in the setting of murine Toxoplasma gondii infection. Our data reveal that infection alters early myeloerythroid differentiation, blocking erythroid development beyond the Pre MegE stage, while expanding the GMP population. IL-6 was found to be a critical mediator of these differences, independent of hepcidin-induced iron restriction. Comparing the BM with the spleen showed that the hematopoietic response was driven by the local microenvironment, and BM chimeras demonstrated that radioresistant cells were the relevant source of IL-6 in vivo. Finally, direct ex vivo sorting revealed that VCAM(+)CD146(lo) BM stromal fibroblasts significantly increase IL-6 secretion after infection. These data suggest that BMSCs regulate the hematopoietic changes during inflammation via IL-6.

Intestinal microbiota: shaping local and systemic immune responses

Recent studies have highlighted the fundamental role of commensal microbes in the maintenance of host homeostasis. For instance, commensals can play a major role in the control of host defense, metabolism and tissue development. Over the past few years, abundant experimental data also support their central role in the induction and control of both innate and adaptive responses. It is now clearly established that commensals are not equal in their capacity to trigger control regulatory or effector responses, however, the molecular basis of these differences has only recently begun to be explored. This review will discuss recent findings evaluating how commensals shape both effector and regulatory responses at steady state and during infections and the consequence of this effect on local and systemic protective and inflammatory responses.

The role of retinoic acid in tolerance and immunity

Vitamin A elicits a broad array of immune responses through its metabolite, retinoic acid (RA). Recent evidence indicates that loss of RA leads to impaired immunity, whereas excess RA can potentially promote inflammatory disorders. In this review, we discuss recent advances showcasing the crucial contributions of RA to both immunological tolerance and the elicitation of adaptive immune responses. Further, we provide a comprehensive overview of the cell types and factors that control the production of RA and discuss how host perturbations may affect the ability of this metabolite to control tolerance and immunity or to instigate pathology.

GATA3 controls Foxp3⁺ regulatory T cell fate during inflammation in mice

Tregs not only keep immune responses to autoantigens in check, but also restrain those directed toward pathogens and the commensal microbiota. Control of peripheral immune homeostasis by Tregs relies on their capacity to accumulate at inflamed sites and appropriately adapt to their local environment. To date, the factors involved in the control of these aspects of Treg physiology remain poorly understood. Here, we show that the canonical Th2 transcription factor GATA3 is selectively expressed in Tregs residing in barrier sites including the gastrointestinal tract and the skin. GATA3 expression in both murine and human Tregs was induced upon TCR and IL-2 stimulation. Although GATA3 was not required to sustain Treg homeostasis and function at steady state, GATA3 played a cardinal role in Treg physiology during inflammation. Indeed, the intrinsic expression of GATA3 by Tregs was required for their ability to accumulate at inflamed sites and to maintain high levels of Foxp3 expression in various polarized or inflammatory settings. Furthermore, our data indicate that GATA3 limits Treg polarization toward an effector T cell phenotype and acquisition of effector cytokines in inflamed tissues. Overall, our work reveals what we believe to be a new facet in the complex role of GATA3 in T cells and highlights what may be a fundamental role in controlling Treg physiology during inflammation.

Regulatory T cells selectively control CD8+ T cell effector pool size via IL-2 restriction

Regulatory T cells (Treg) are key players in maintaining immune homeostasis but have also been shown to regulate immune responses against infectious pathogens. Therefore, Treg are a promising target for modulating immune responses to vaccines to improve their efficacy. Using a viral vector system, we found that Treg act on the developing immune response early postinfection by reducing the extent of dendritic cell costimulatory molecule expression. Due to this change and the lower IL-2 production that results, a substantial fraction of CD8(+) effector T cells lose CD25 expression several days after activation. Surprisingly, such Treg-dependent limitations in IL-2 signaling by Ag-activated CD8(+) T cells prevent effector differentiation without interfering with memory cell formation. In this way, Treg fine-tune the numbers of effector T cells generated while preserving the capacity for a rapid recall response upon pathogen re-exposure. This selective effect of Treg on a subpopulation of CD8(+) T cells indicates that although manipulation of the Treg compartment might not be optimal for prophylactic vaccinations, it can be potentially exploited to optimize vaccine efficacy for therapeutic interventions.

Essential role for retinoic acid in the promotion of CD4(+) T cell effector responses via retinoic acid receptor alpha

Vitamin A and its metabolite, retinoic acid (RA) are implicated in the regulation of immune homeostasis via the peripheral induction of regulatory T cells. Here we showed RA was also required to elicit proinflammatory CD4(+) helper T cell responses to infection and mucosal vaccination. Retinoic acid receptor alpha (RARα) was the critical mediator of these effects. Antagonism of RAR signaling and deficiency in RARα (Rara(-/-)) resulted in a cell-autonomous CD4(+) T cell activation defect, which impaired intermediate signaling events, including calcium mobilization. Altogether, these findings reveal a fundamental role for the RA-RARα axis in the development of both regulatory and inflammatory arms of adaptive immunity and establish nutritional status as a broad regulator of adaptive T cell responses.

Co-adjuvant effects of retinoic acid and IL-15 induce inflammatory immunity to dietary antigens

Under physiological conditions the gut-associated lymphoid tissues not only prevent the induction of a local inflammatory immune response, but also induce systemic tolerance to fed antigens. A notable exception is coeliac disease, where genetically susceptible individuals expressing human leukocyte antigen (HLA) HLA-DQ2 or HLA-DQ8 molecules develop inflammatory T-cell and antibody responses against dietary gluten, a protein present in wheat. The mechanisms underlying this dysregulated mucosal immune response to a soluble antigen have not been identified. Retinoic acid, a metabolite of vitamin A, has been shown to have a critical role in the induction of intestinal regulatory responses. Here we find in mice that in conjunction with IL-15, a cytokine greatly upregulated in the gut of coeliac disease patients, retinoic acid rapidly activates dendritic cells to induce JNK (also known as MAPK8) phosphorylation and release the proinflammatory cytokines IL-12p70 and IL-23. As a result, in a stressed intestinal environment, retinoic acid acted as an adjuvant that promoted rather than prevented inflammatory cellular and humoral responses to fed antigen. Altogether, these findings reveal an unexpected role for retinoic acid and IL-15 in the abrogation of tolerance to dietary antigens.

Generation of pathogenic T(H)17 cells in the absence of TGF-β signalling

CD4(+) T-helper cells that selectively produce interleukin (IL)-17 (T(H)17), are critical for host defence and autoimmunity. Although crucial for T(H)17 cells in vivo, IL-23 has been thought to be incapable of driving initial differentiation. Rather, IL-6 and transforming growth factor (TGF)-β1 have been proposed to be the factors responsible for initiating specification. Here we show that T(H)17 differentiation can occur in the absence of TGF-β signalling. Neither IL-6 nor IL-23 alone efficiently generated T(H)17 cells; however, these cytokines in combination with IL-1β effectively induced IL-17 production in naive precursors, independently of TGF-β. Epigenetic modification of the Il17a, Il17f and Rorc promoters proceeded without TGF-β1, allowing the generation of cells that co-expressed RORγt (encoded by Rorc) and T-bet. T-bet(+)RORγt(+) T(H)17 cells are generated in vivo during experimental allergic encephalomyelitis, and adoptively transferred T(H)17 cells generated with IL-23 without TGF-β1 were pathogenic in this disease model. These data indicate an alternative mode for T(H)17 differentiation. Consistent with genetic data linking IL23R with autoimmunity, our findings re-emphasize the importance of IL-23 and therefore may have therapeutic implications.

Helminth secretions induce de novo T cell Foxp3 expression and regulatory function through the TGF-β pathway

Foxp3-expressing regulatory T (T reg) cells have been implicated in parasite-driven inhibition of host immunity during chronic infection. We addressed whether parasites can directly induce T reg cells. Foxp3 expression was stimulated in naive Foxp3⁻ T cells in mice infected with the intestinal helminth Heligmosomoides polygyrus. In vitro, parasite-secreted proteins (termed H. polygyrus excretory-secretory antigen [HES]) induced de novo Foxp3 expression in fluorescence-sorted Foxp3⁻ splenocytes from Foxp3-green fluorescent protein reporter mice. HES-induced T reg cells suppressed both in vitro effector cell proliferation and in vivo allergic airway inflammation. HES ligated the transforming growth factor (TGF) β receptor and promoted Smad2/3 phosphorylation. Foxp3 induction by HES was lost in dominant-negative TGF-βRII cells and was abolished by the TGF-β signaling inhibitor SB431542. This inhibitor also reduced worm burdens in H. polygyrus-infected mice. HES induced IL-17 in the presence of IL-6 but did not promote Th1 or Th2 development under any conditions. Importantly, antibody to mammalian TGF-β did not recognize HES, whereas antisera that inhibited HES did not affect TGF-β. Foxp3 was also induced by secreted products of Teladorsagia circumcincta, a related nematode which is widespread in ruminant animals. We have therefore identified a novel pathway through which helminth parasites may stimulate T reg cells, which is likely to be a key part of the parasite's immunological relationship with the host.

99th Dahlem conference on infection, inflammation and chronic inflammatory disorders: induction and control of regulatory T cells in the gastrointestinal tract: consequences for local and peripheral immune responses

Regulatory T cells play a crucial role in normal gut homeostasis, as well as during infection with microbial or parasitic pathogens. Prior to infection, interactions with the commensal microflora are essential to differentiation of a healthy steady-state level of immunoregulation, mediated through both Toll-like receptor-dependent and -independent pathways. The ingress of pathogenic organisms may, according to the context, promote or reverse the regulatory environment, with onward consequences for inflammation in both the intestinal and extra-intestinal settings. Appropriate regulation of gut immunity thus depends upon a complex three-way interplay between host cells, commensals and pathogens, and can exert a major impact on systemic responses including allergy and autoimmunity.

Plasticity of T reg at infected sites

Regulatory T cells (T(reg)) control an array of immune responses both in the context of various polarized settings as well as in distinct microenvironments. This implies that maintenance of peripheral homeostasis relies on the capacity of T(reg) to appropriately adapt to these defined settings while sustaining a regulatory program in the face of inflammation. Adaptation of T(reg) is particularly critical in tissues constantly exposed to microbes, such as the gut or the skin, or in the context of exposure to pathogenic microbes. Recent evidence supports the idea that the capacity of T(reg) to control defined polarized settings can be associated with the acquisition of specific transcription factors previously associated with effector T-cell lineages. In this review we will discuss how such adaptation of T(reg) can have a major role in the control of host-microbe interaction.