Protein kinase C-η controls CTLA-4-mediated regulatory T cell function.

Regulatory T (Treg) cells, which maintain immune homeostasis and self-tolerance, form an immunological synapse (IS) with antigen-presenting cells (APCs). However, signaling events at the Treg cell IS remain unknown. Here we show that the kinase PKC-η associated with CTLA-4 and was recruited to the Treg cell IS. PKC-η-deficient Treg cells displayed defective suppressive activity, including suppression of tumor immunity but not of autoimmune colitis. Phosphoproteomic and biochemical analysis revealed an association between CTLA-4-PKC-η and the GIT2-αPIX-PAK complex, an IS-localized focal adhesion complex. Defective activation of this complex in PKC-η-deficient Treg cells was associated with reduced depletion of CD86 from APCs by Treg cells. These results reveal a CTLA-4-PKC-η signaling axis required for contact-dependent suppression and implicate this pathway as a potential cancer immunotherapy target.

Transcriptional reprogramming of mature CD4⁺ helper T cells generates distinct MHC class II-restricted cytotoxic T lymphocytes.

TCRαß thymocytes differentiate into either CD8αß(+) cytotoxic T lymphocytes or CD4(+) helper T cells. This functional dichotomy is controlled by key transcription factors, including the helper T cell master regulator ThPOK, which suppresses the cytolytic program in major histocompatibility complex (MHC) class II-restricted CD4(+) thymocytes. ThPOK continues to repress genes of the CD8 lineage in mature CD4(+) T cells, even as they differentiate into effector helper T cell subsets. Here we found that the helper T cell fate was not fixed and that mature, antigen-stimulated CD4(+) T cells terminated expression of the gene encoding ThPOK and reactivated genes of the CD8 lineage. This unexpected plasticity resulted in the post-thymic termination of the helper T cell program and the functional differentiation of distinct MHC class II-restricted CD4(+) cytotoxic T lymphocytes.

Interleukin 10 acts on regulatory T cells to maintain expression of the transcription factor Foxp3 and suppressive function in mice with colitis.

Regulatory T cells (T(reg) cells) that express the transcription factor Foxp3 suppress the activity of other cells. Here we show that interleukin 10 (IL-10) produced by CD11b(+) myeloid cells in recombination-activating gene 1-deficient (Rag1(-/-)) recipient mice was needed to prevent the colitis induced by transferred CD4(+)CD45RB(hi) T cells. In Il10(-/-)Rag1(-/-) mice, T(reg) cells failed to maintain Foxp3 expression and regulatory activity. The loss of Foxp3 expression occurred only in recipients with colitis, which indicates that the requirement for IL-10 is manifested in the presence of inflammation. IL-10 receptor-deficient (Il10rb(-/-)) T(reg) cells also failed to maintain Foxp3 expression, which suggested that host IL-10 acted directly on the T(reg) cells. Our data indicate that IL-10 released from myeloid cells acts in a paracrine manner on T(reg) cells to maintain Foxp3 expression.

HVEM signalling at mucosal barriers provides host defence against pathogenic bacteria.

The herpes virus entry mediator (HVEM), a member of the tumour-necrosis factor receptor family, has diverse functions, augmenting or inhibiting the immune response. HVEM was recently reported as a colitis risk locus in patients, and in a mouse model of colitis we demonstrated an anti-inflammatory role for HVEM, but its mechanism of action in the mucosal immune system was unknown. Here we report an important role for epithelial HVEM in innate mucosal defence against pathogenic bacteria. HVEM enhances immune responses by NF-κB-inducing kinase-dependent Stat3 activation, which promotes the epithelial expression of genes important for immunity. During intestinal Citrobacter rodentium infection, a mouse model for enteropathogenic Escherichia coli infection, Hvem−/− mice showed decreased Stat3 activation, impaired responses in the colon, higher bacterial burdens and increased mortality. We identified the immunoglobulin superfamily molecule CD160 (refs 7 and 8), expressed predominantly by innate-like intraepithelial lymphocytes, as the ligand engaging epithelial HVEM for host protection. Likewise, in pulmonary Streptococcus pneumoniae infection, HVEM is also required for host defence. Our results pinpoint HVEM as an important orchestrator of mucosal immunity, integrating signals from innate lymphocytes to induce optimal epithelial Stat3 activation, which indicates that targeting HVEM with agonists could improve host defence.

Defective natural killer cell activity in a mouse model of eczema herpeticum.

BACKGROUND: Patients with atopic dermatitis (AD) are susceptible to several viruses, including herpes simplex virus (HSV). Some patients experience 1 or more episodes of a severe skin infection caused by HSV termed eczema herpeticum (EH). There are numerous mouse models of AD, but no established model exists for EH. OBJECTIVE: We sought to establish and characterize a mouse model of EH. METHODS: We infected AD-like skin lesions with HSV1 to induce severe skin lesions in a dermatitis-prone mouse strain of NC/Nga. Gene expression was investigated by using a microarray and quantitative PCR; antibody titers were measured by means of ELISA; and natural killer (NK) cell, cytotoxic T-cell, regulatory T-cell, and follicular helper T-cell populations were evaluated by using flow cytometry. The role of NK cells in HSV1-induced development of severe skin lesions was examined by means of depletion and adoptive transfer. RESULTS: Inoculation of HSV1 induced severe erosive skin lesions in eczematous mice, which had an impaired skin barrier, but milder lesions in small numbers of normal mice. Eczematous mice exhibited lower NK cell activity but similar cytotoxic T-cell activity and humoral immune responses compared with normal mice. The role of NK cells in controlling HSV1-induced skin lesions was demonstrated by experiments depleting or transferring NK cells. CONCLUSION: A murine model of EH with an impaired skin barrier was established in this study. We demonstrated a critical role of defective NK activities in the development of HSV1-induced severe skin lesions in eczematous mice.

SAMP1/YitFc mice develop ileitis via loss of CCL21 and defects in dendritic cell migration.

BACKGROUND & AIMS: The lymphatic chemokine CCL21 is required for dendritic cell (DC) migration from tissues to lymph nodes, which helps establish tolerance to foreign yet harmless antigens. We demonstrate that CCL21 is almost completely absent from SAMP1/YitFc (SAMP) mice, which spontaneously develop chronic ileitis that resembles Crohn's disease, and that DC migration is severely impaired in these mice compared with AKR mice (controls). Toll-like receptor agonists like the Toll-like receptor 7 agonist R848 induce DC maturation and mobilization. METHODS: We collected intestinal and other tissues and mesenteric lymph nodes (MLN) from SAMP mice. Expression of CCL21 was measured by quantitative reverse transcription polymerase chain reaction and immunofluorescence analyses; spontaneous and induced migration of DCs were assessed by flow cytometry. We analyzed production of retinoic acid by DCs and their ability to induce development of regulatory T cells. Mice were fed R848 to determine its effects on migration of DCs and development of ileitis in SAMP mice. RESULTS: SAMP mice expressed almost no CCL21 in any tissue tested. Their CD11b(+)CD103(+) DCs were defective in migration from the ileal lamina propria to the MLN. DCs from SAMP mice also had a greatly reduced ability to produce retinoic acid and induce development of regulatory T cells compared with control mice. Young SAMP mice had reduced CCL21 expression and decreased DC migration before developing ileitis. Administration of R848 to adult SAMP mice increased migration of DC to the MLN and development of regulatory T cells there, and reduced the severity of ileitis. CONCLUSIONS: Loss of CCL21 signaling and DC migration is required for development of ileitis in SAMP mice. Reagents such as R848, which activate DC migration to the MLN, may be developed as treatments for patients with Crohn's disease.

The tumor necrosis factor family member TNFSF14 (LIGHT) is required for resolution of intestinal inflammation in mice.

BACKGROUND & AIMS: The pathogenesis of inflammatory bowel disease (IBD) is associated with a dysregulated mucosal immune response. Expression of the tumor necrosis factor (TNF) superfamily member 14 (TNFSF14, also known as LIGHT [homologous to lymphotoxins, exhibits inducible expression, and competes with HSV glycoprotein D for HVEM, a receptor expressed by T lymphocytes]) on T cells is involved in their activation; transgenic expression of LIGHT on T cells in mice promotes inflammation in multiple organs, including intestine. We investigated the roles for LIGHT in recovery from intestinal inflammation in mice. METHODS: We studied the role of LIGHT in intestinal inflammation using Tnfsf14(-/-) and wild-type mice. Colitis was induced by transfer of CD4(+)CD45RB(high) T cells into Rag1(-/-) or Tnfsf14(-/-)Rag1(-/-) mice, or by administration of dextran sulfate sodium to Tnfsf14(-/-) or wild-type C57BL/6J mice. Mice were weighed, colon tissues were collected and measured, and histology analyses were performed. We measured infiltrating cell populations and expression of cytokines, chemokines, and LIGHT. RESULTS: After administration of dextran sulfate sodium, Tnfsf14(-/-) mice developed more severe colitis than controls, based on their reduced survival, accelerated loss of body weight, and histologic scores. LIGHT protected mice from colitis via the lymphotoxin ß receptor and was expressed mainly by myeloid cells in the colon. Colons of Tnfsf14(-/-) mice also had increased accumulation of innate immune cells and higher levels of cytokines than colons from control mice. LIGHT, therefore, appears to regulate inflammation in the colon. CONCLUSIONS: Tnfsf14(-/-) mice develop more severe colitis than control mice. LIGHT signals through the lymphotoxin ß receptor in the colon to regulate the innate immune response and mediate recovery from intestinal inflammation.

A novel role for IL-27 in mediating the survival of activated mouse CD4 T lymphocytes.

IL-27, an IL-12 family cytokine, has pleiotropic functions in the differentiation and expansion of CD4(+) T cell subsets. In this study, we discovered a novel function of IL-27. CD4(+)CD45RB(high) T cells from mice deficient for the α-chain of IL-27 receptor failed to induce colitis in Rag(-/-) recipients, because of an inability of activated donor cells to survive. Interestingly, IL-27 was indispensable for the prevention of colitis by regulatory T cells, also because of a defect in long-term cell survival. IL-27 affected the survival of activated T lymphocytes, rather than promoting cell proliferation, by inhibiting Fas-mediated activation-induced T cell death, acting through the STAT3 signaling pathway. The addition of IL-27 during activation resulted in an increased cell number, which was correlated with decreased activation of both caspases 3 and 8. This prosurvival effect was attributed to downregulation of FasL and to the induction of the antiapoptotic protein cFLIP. Although activation induced cell death is an important mechanism for the maintenance of immunological homeostasis, protection of lymphocytes from excessive cell death is essential for effective immunity. Our data indicate that IL-27 has a crucial role in the inhibition of activation-induced cell death, thereby permitting Ag-driven T cell expansion.

Interleukin-27 receptor limits atherosclerosis in Ldlr-/- mice.

RATIONALE: Atherosclerosis is a chronic inflammatory disease of the arterial wall. Several proinflammatory cytokines are known to promote atherosclerosis, but less is known about the physiological role of anti-inflammatory cytokines. Interleukin (IL)-27 is a recently discovered member of the IL-6/IL-12 family. The IL-27 receptor is composed of IL-27 receptor A (WSX-1) and gp130 and is required for all established IL-27 signaling pathways. The expression of the IL-27 subunit Ebi3 is elevated in human atheromas, yet its function in atherosclerosis remains unknown. OBJECTIVE: The aim of this study was to test the role of IL-27 receptor signaling in immune cells in atherosclerosis development. METHODS AND RESULTS: Atherosclerosis-prone Ldlr(-/-) mice transplanted with Il27ra(-/-) bone marrow and fed Western diet for 16 weeks developed significantly larger atherosclerotic lesions in aortic roots, aortic arches, and abdominal aortas. Augmented disease correlated with increased accumulation of CD45(+) leukocytes and CD4(+) T cells in the aorta, which produced increased amounts of IL-17A and tumor necrosis factor. Several chemokines, including CCL2, were upregulated in the aortas of Ldlr(-/-) mice receiving Il27ra(-/-) bone marrow, resulting in accumulation of CD11b(+) and CD11c(+) macrophages and dendritic cells in atherosclerotic aortas. CONCLUSIONS: The absence of anti-inflammatory IL-27 signaling skews immune responses toward T-helper 17, resulting in increased production of IL-17A and tumor necrosis factor, which in turn enhances chemokine expression and drives the accumulation of proatherogenic myeloid cells in atherosclerotic aortas. These findings establish a novel antiatherogenic role for IL-27 receptor signaling, which acts to suppress the production of proinflammatory cytokines and chemokines and to curb the recruitment of inflammatory myeloid cells into atherosclerotic aortas.

Recognition of bacterial glycosphingolipids by natural killer T cells.

Natural killer T (NKT) cells constitute a highly conserved T lymphocyte subpopulation that has the potential to regulate many types of immune responses through the rapid secretion of cytokines. NKT cells recognize glycolipids presented by CD1d, a class I-like antigen-presenting molecule. They have an invariant T-cell antigen receptor (TCR) alpha-chain, but whether this invariant TCR recognizes microbial antigens is still controversial. Here we show that most mouse and human NKT cells recognize glycosphingolipids from Sphingomonas, Gram-negative bacteria that do not contain lipopolysaccharide. NKT cells are activated in vivo after exposure to these bacterial antigens or bacteria, and mice that lack NKT cells have a marked defect in the clearance of Sphingomonas from the liver. These data suggest that NKT cells are T lymphocytes that provide an innate-type immune response to certain microorganisms through recognition by their antigen receptor, and that they might be useful in providing protection from bacteria that cannot be detected by pattern recognition receptors such as Toll-like receptor 4.

Spontaneous colitis occurrence in transgenic mice with altered B7-mediated costimulation.

The B7 costimulatory molecules govern many aspects of T cell immune responses by interacting with CD28 for costimulation, but also with CTLA-4 for immune suppression. Although blockade of CTLA-4 with Ab in humans undergoing cancer immune therapy has led to some cases of inflammatory bowel disease, spontaneous animal models of colitis that depend upon modulation of B7 interactions have not been previously described. In this study, we demonstrate that mice expressing a soluble B7-2 Ig Fc chimeric protein spontaneously develop colitis that is dependent on CD28-mediated costimulation of CD4(+) T cells. We show that the chimeric protein has mixed agonistic/antagonist properties, and that it acts in part by blocking the cell intrinsic effects on T cell activation of engagement of CTLA-4. Disease occurred in transgenic mice that lack expression of the endogenous B7 molecules (B7 double knock-out mice), because of the relatively weak costimulatory delivered by the chimeric protein. Surprisingly, colitis was more severe in this context, which was associated with the decreased number of Foxp3(+) regulatory T cells in transgenic B7 double knock-out mice. This model provides an important tool for examining how B7 molecules and their effects on CTLA-4 modulate T cell function and the development of inflammatory diseases.

Reduced expression of phosphatase PTPN2 promotes pathogenic conversion of Tregs in autoimmunity.

Genetic variants at the PTPN2 locus, which encodes the tyrosine phosphatase PTPN2, cause reduced gene expression and are linked to rheumatoid arthritis (RA) and other autoimmune diseases. PTPN2 inhibits signaling through the T cell and cytokine receptors, and loss of PTPN2 promotes T cell expansion and CD4- and CD8-driven autoimmunity. However, it remains unknown whether loss of PTPN2 in FoxP3+ regulatory T cells (Tregs) plays a role in autoimmunity. Here we aimed to model human autoimmune-predisposing PTPN2 variants, the presence of which results in a partial loss of PTPN2 expression, in mouse models of RA. We identified that reduced expression of Ptpn2 enhanced the severity of autoimmune arthritis in the T cell-dependent SKG mouse model and demonstrated that this phenotype was mediated through a Treg-intrinsic mechanism. Mechanistically, we found that through dephosphorylation of STAT3, PTPN2 inhibits IL-6-driven pathogenic loss of FoxP3 after Tregs have acquired RORγt expression, at a stage when chromatin accessibility for STAT3-targeted IL-17-associated transcription factors is maximized. We conclude that PTPN2 promotes FoxP3 stability in mouse RORγt+ Tregs and that loss of function of PTPN2 in Tregs contributes to the association between PTPN2 and autoimmunity.

Synergistic costimulation by both B7 molecules regulates colitis pathogenesis.

It has been reported that B7-1 and B7-2 play different roles in the pathogenesis of autoimmunity, but this issue is controversial. Here we analyzed colitis induced by transfer of CD45RB(high) CD4+ T cells to immune-deficient recipients that lack expression of either B7-1 or B7-2. Surprisingly, disease was greatly accelerated in Rag(-/-) recipients deficient for either B7 molecule. Antigen presenting cells (APCs) lacking B7-1 or B7-2 stimulated T cell proliferation in vitro, but caused suboptimal IL-2 production, leading to decreased induction of CTLA-4. The data suggest that regulatory T cells function relatively normally in B7 single-deficient recipients, but they cannot restrain the increased pathogenesis by naïve cells primed in B7 single-deficient mice. Therefore, the inhibitory effect of CTLA-4 on pathogenic T cells likely slows colitis, even in the absence of regulatory T cells. While a full block of costimulation may prevent autoimmunity, our data indicate, surprisingly, that a partial block may in some cases augment disease.

IL-10-producing intestinal macrophages prevent excessive antibacterial innate immunity by limiting IL-23 synthesis.

Innate immune responses are regulated in the intestine to prevent excessive inflammation. Here we show that a subset of mouse colonic macrophages constitutively produce the anti-inflammatory cytokine IL-10. In mice infected with Citrobacter rodentium, a model for enteropathogenic Escherichia coli infection in humans, these macrophages are required to prevent intestinal pathology. IL-23 is significantly increased in infected mice with a myeloid cell-specific deletion of IL-10, and the addition of IL-10 reduces IL-23 production by intestinal macrophages. Furthermore, blockade of IL-23 leads to reduced mortality in the context of macrophage IL-10 deficiency. Transcriptome and other analyses indicate that IL-10-expressing macrophages receive an autocrine IL-10 signal. Interestingly, only transfer of the IL-10 positive macrophages could rescue IL-10-deficient infected mice. Therefore, these data indicate a pivotal role for intestinal macrophages that constitutively produce IL-10, in controlling excessive innate immune activation and preventing tissue damage after an acute bacterial infection.

TSC1 regulates the balance between effector and regulatory T cells.

Mammalian target of rapamycin (mTOR) plays a crucial role in the control of T cell fate determination; however, the precise regulatory mechanism of the mTOR pathway is not fully understood. We found that T cell-specific deletion of the gene encoding tuberous sclerosis 1 (TSC1), an upstream negative regulator of mTOR, resulted in augmented Th1 and Th17 differentiation and led to severe intestinal inflammation in a colitis model. Conditional Tsc1 deletion in Tregs impaired their suppressive activity and expression of the Treg marker Foxp3 and resulted in increased IL-17 production under inflammatory conditions. A fate-mapping study revealed that Tsc1-null Tregs that lost Foxp3 expression gained a stronger effector-like phenotype compared with Tsc1-/- Foxp3+ Tregs. Elevated IL-17 production in Tsc1-/- Treg cells was reversed by in vivo knockdown of the mTOR target S6K1. Moreover, IL-17 production was enhanced by Treg-specific double deletion of Tsc1 and Foxo3a. Collectively, these studies suggest that TSC1 acts as an important checkpoint for maintaining immune homeostasis by regulating cell fate determination.

A crucial role for HVEM and BTLA in preventing intestinal inflammation.

The interaction between the tumor necrosis factor (TNF) family member LIGHT and the TNF family receptor herpes virus entry mediator (HVEM) co-stimulates T cells and promotes inflammation. However, HVEM also triggers inhibitory signals by acting as a ligand that binds to B and T lymphocyte attenuator (BTLA), an immunoglobulin super family member. The contribution of HVEM interacting with these two binding partners in inflammatory processes remains unknown. In this study, we investigated the role of HVEM in the development of colitis induced by the transfer of CD4(+)CD45RB(high) T cells into recombination activating gene (Rag)(-/-) mice. Although the absence of HVEM on the donor T cells led to a slight decrease in pathogenesis, surprisingly, the absence of HVEM in the Rag(-/-) recipients led to the opposite effect, a dramatic acceleration of intestinal inflammation. Furthermore, the critical role of HVEM in preventing colitis acceleration mainly involved HVEM expression by radioresistant cells in the Rag(-/-) recipients interacting with BTLA. Our experiments emphasize the antiinflammatory role of HVEM and the importance of HVEM expression by innate immune cells in preventing runaway inflammation in the intestine.

Reciprocal TH17 and regulatory T cell differentiation mediated by retinoic acid.

The cytokine transforming growth factor-beta (TGF-beta) converts naïve T cells into regulatory T (Treg) cells that prevent autoimmunity. However, in the presence of interleukin-6 (IL-6), TGF-beta has also been found to promote the differentiation of naïve T lymphocytes into proinflammatory IL-17 cytokine-producing T helper 17 (T(H)17) cells, which promote autoimmunity and inflammation. This raises the question of how TGF-beta can generate such distinct outcomes. We identified the vitamin A metabolite retinoic acid as a key regulator of TGF-beta-dependent immune responses, capable of inhibiting the IL-6-driven induction of proinflammatory T(H)17 cells and promoting anti-inflammatory Treg cell differentiation. These findings indicate that a common metabolite can regulate the balance between pro- and anti-inflammatory immunity.

Paradoxical effect of reduced costimulation in T cell-mediated colitis.

B7-1 and B7-2 play different roles in the pathogenesis of autoimmunity, but this is controversial. We analyzed colitis induced by transfer of CD45RB(high)CD4(+) T cells to RAG(-/-) recipients lacking B7-1 and/or B7-2. Surprisingly, disease was greatly accelerated in RAG(-/-) recipients deficient for either B7-1 or B7-2, especially in the B7-2(-/-) recipients. This accelerated colitis induction correlated with increased T cell division in vivo and production of Th1 cytokines. Although colitis pathogenesis following T cell transfer was inhibited in the absence of CD40L expression, CD40-CD40L interactions were not required in the B7-2(-/-) RAG(-/-) recipients. In vitro priming by APCs lacking either B7-1 or B7-2 caused decreased IL-2 production, which led to decreased CTLA-4 expression, although T cells primed in this way could respond vigorously upon restimulation by producing increased IL-2 and proinflammatory cytokines. Consistent with this mechanism, we demonstrate that blocking IL-2 early after T cell transfer accelerated colitis. Our data therefore outline a mechanism whereby synergistic costimulation by B7-1 and B7-2 molecules during priming is required for optimal IL-2 production. The consequent inhibitory effect of full CTLA-4 expression, induced by IL-2, may slow colitis, even in the absence of regulatory T cells.