Juan J. Lafaille

THE ORPHAN NUCLEAR RECEPTOR RORGAMMAT DIRECTS THE DIFFERENTIATION PROGRAM OF PROINFLAMMATORY IL-17+ T HELPER CELLS

IL-17-producing T lymphocytes have been recently shown to comprise a distinct lineage of proinflammatory T helper cells, termed Th17 cells, that are major contributors to autoimmune disease. We show here that the orphan nuclear receptor RORgammat is the key transcription factor that orchestrates the differentiation of this effector cell lineage. RORgammat induces transcription of the genes encoding IL-17 and the related cytokine IL-17F in naive CD4(+) T helper cells and is required for their expression in response to IL-6 and TGF-beta, the cytokines known to induce IL-17. Th17 cells are constitutively present throughout the intestinal lamina propria, express RORgammat, and are absent in mice deficient for RORgammat or IL-6. Mice with RORgammat-deficient T cells have attenuated autoimmune disease and lack tissue-infiltrating Th17 cells. Together, these studies suggest that RORgammat is a key regulator of immune homeostasis and highlight its potential as a therapeutic target in inflammatory diseases.

Natural and Adaptive Foxp3+ Regulatory T Cells: More of the Same or a Division of Labor?

Adaptive Foxp3(+)CD4(+) regulatory T (iTreg) cells develop outside the thymus under subimmunogenic antigen presentation, during chronic inflammation, and during normal homeostasis of the gut. iTreg cells are essential in mucosal immune tolerance and in the control of severe chronic allergic inflammation, and most likely are one of the main barriers to the eradication of tumors. The Foxp3(+) iTreg cell repertoire is drawn from naive conventional CD4(+) T cells, whereas natural Treg (nTreg) cells are selected by high-avidity interactions in the thymus. The full extent of differences and similarities between iTreg and nTreg cells is yet to be defined. We speculate that iTreg cell development is driven by the need to maintain a noninflammatory environment in the gut, to suppress immune responses to environmental and food allergens, and to decrease chronic inflammation, whereas nTreg cells prevent autoimmunity and raise the activation threshold for all immune responses.

Interleukin 2 signaling is required for CD4(+) regulatory T cell function.

Mice deficient in interleukin (IL)-2 production or the IL-2 receptor α or β chains develop a lethal autoimmune syndrome. CD4+ regulatory T cells have been shown to prevent autoimmune diseases, allograft rejection, and to down-regulate antibody responses against foreign antigens. To assess the role of IL-2 in the generation and function of regulatory T cells, we transferred CD4+ T cells from mice genetically deficient in IL-2 or IL-2Rα (CD25) expression. A small number of splenic or thymic CD4+ T cells from IL-2 knockout mice can protect mice from spontaneous experimental autoimmune encephalomyelitis (EAE). In contrast, splenic or thymic CD4+ T cells from CD25 knockout donor mice conferred little or no protection. We conclude that T cells with regulatory potential can develop, undergo thymic selection, and migrate to the peripheral lymphoid organs in the absence of IL-2, and do not protect from disease by means of IL-2 secretion. However, IL-2 signaling in regulatory T cells is essential for their protective function. Altogether, our results favor a model whereby IL-2 induces regulatory T cell activity.

Oral tolerance in the absence of naturally occurring Tregs

Mucosal tolerance prevents pathological reactions against environmental and food antigens, and its failure results in exacerbated inflammation typical of allergies and asthma. One of the proposed mechanisms of oral tolerance is the induction of Tregs. Using a mouse model of hyper-IgE and asthma, we found that oral tolerance could be effectively induced in the absence of naturally occurring thymus-derived Tregs. Oral antigen administration prior to i.p. immunization prevented effector/memory Th2 cell development, germinal center formation, class switching to IgE, and lung inflammation. Oral exposure to antigen induced development of antigen-specific CD4CD25Foxp3CD45RB cells that were anergic and displayed suppressive activity in vivo and in vitro. Oral tolerance to the Th2 allergic response was in large part dependent on TGF-beta and independent of IL-10. Interestingly, Tregs were also induced by single i.p. immunization with antigen and adjuvant. However, unlike oral administration of antigen, which induced Tregs but not effector T cells, i.p. immunization led to the simultaneous induction of Tregs and effector Th2 cells displaying the same antigen specificity.

CD25− T Cells Generate CD25+Foxp3+ Regulatory T Cells by Peripheral Expansion

Naturally occurring CD4+ regulatory T cells are generally identified through their expression of CD25. However, in several experimental systems considerable Treg activity has been observed in the CD4+CD25− fraction. Upon adoptive transfer, the expression of CD25 in donor-derived cells is not stable, with CD4+CD25+ cells appearing in CD4+CD25− T cell-injected animals and vice versa. We show in this study that CD25+ cells arising from donor CD25− cells upon homeostatic proliferation in recipient mice express markers of freshly isolated Treg cells, display an anergic state, and suppress the proliferation of other cells in vitro. The maintenance of CD25 expression by CD4+CD25+ cells depends on IL-2 secreted by cotransferred CD4+CD25− or by Ag-stimulated T cells in peripheral lymphoid organs.

Digoxin and its derivatives suppress TH17 cell differentiation by antagonizing RORγt activity.

CD4+ T helper lymphocytes that express interleukin-17 (TH17 cells) have critical roles in mouse models of autoimmunity, and there is mounting evidence that they also influence inflammatory processes in humans. Genome-wide association studies in humans have linked genes involved in TH17 cell differentiation and function with susceptibility to Crohn’s disease, rheumatoid arthritis and psoriasis. Thus, the pathway towards differentiation of TH17 cells and, perhaps, of related innate lymphoid cells with similar effector functions, is an attractive target for therapeutic applications. Mouse and human TH17 cells are distinguished by expression of the retinoic acid receptor-related orphan nuclear receptor RORγt, which is required for induction of IL-17 transcription and for the manifestation of TH17-dependent autoimmune disease in mice. By performing a chemical screen with an insect cell-based reporter system, we identified the cardiac glycoside digoxin as a specific inhibitor of RORγt transcriptional activity. Digoxin inhibited murine TH17 cell differentiation without affecting differentiation of other T cell lineages and was effective in delaying the onset and reducing the severity of autoimmune disease in mice. At high concentrations, digoxin is toxic for human cells, but non-toxic synthetic derivatives 20,22-dihydrodigoxin-21,23-diol and digoxin-21-salicylidene specifically inhibited induction of IL-17 in human CD4+ T cells. Using these small-molecule compounds, we demonstrate that RORγt is important for the maintenance of IL-17 expression in mouse and human effector T cells. These data indicate that derivatives of digoxin can be used as chemical templates for the development of RORγt-targeted therapeutic agents that attenuate inflammatory lymphocyte function and autoimmune disease.

Adaptive Foxp3+ Regulatory T Cell-Dependent and -Independent Control of Allergic Inflammation

Adaptive Foxp3(+) regulatory T (Treg) cells develop during induction of mucosal tolerance and after immunization. Large numbers of Foxp3(+) T cells have been found in inflamed tissues. We investigated the role of adaptive Foxp3(+) Treg cells in mucosal tolerance and in chronic allergic lung inflammation. We used two strains of mice that are devoid of naturally occurring Treg cells; one is capable of generating adaptive Foxp3(+) Treg cells upon exposure to antigen, whereas the other is deficient in both naturally occurring and adaptive Foxp3(+) Treg cells. We found that adaptive Foxp3(+) Treg cells were essential for establishing mucosal tolerance and for suppressing IL-4 production and lymphoid neogenesis in chronic inflammation, whereas IL-5 production and eosinophilia could be controlled by Foxp3-independent, IFN-gamma-dependent mechanisms. Thus, whereas adaptive Foxp3(+) Treg cells regulate sensitization to allergens and the severity of chronic inflammation, IFN-gamma-producing cells can play a beneficial role in inflammatory conditions involving eosinophils.

Focused specificity of intestinal TH17 cells towards commensal bacterial antigens.

T-helper-17 (TH17) cells have critical roles in mucosal defence and in autoimmune disease pathogenesis. They are most abundant in the small intestine lamina propria, where their presence requires colonization of mice with microbiota. Segmented filamentous bacteria (SFB) are sufficient to induce TH17 cells and to promote TH17-dependent autoimmune disease in animal models. However, the specificity of TH17 cells, the mechanism of their induction by distinct bacteria, and the means by which they foster tissue-specific inflammation remain unknown. Here we show that the T-cell antigen receptor (TCR) repertoire of intestinal TH17 cells in SFB-colonized mice has minimal overlap with that of other intestinal CD4+ T cells and that most TH17 cells, but not other T cells, recognize antigens encoded by SFB. T cells with antigen receptors specific for SFB-encoded peptides differentiated into RORγt-expressing TH17 cells, even if SFB-colonized mice also harboured a strong TH1 cell inducer, Listeria monocytogenes, in their intestine. The match of T-cell effector function with antigen specificity is thus determined by the type of bacteria that produce the antigen. These findings have significant implications for understanding how commensal microbiota contribute to organ-specific autoimmunity and for developing novel mucosal vaccines.

Protein Kinase C-θ Mediates Negative Feedback on Regulatory T Cell Function

Yin-Yang T Cell Signaling Immune responses are kept in check by CD4+ regulatory T cells (Treg) that suppress other immune cells, including CD4+ effector T cells (Teff). Treg and Teff cells have many signaling components in common, yet triggering through their T cell receptors (TCRs) leads to very different outcomes. Zanin-Zhorov et al. (p. 372, published online 25 March) compared the recruitment of signaling molecules to the immunological synapse after TCR triggering in Treg and Teff cells. Although Treg cells do form synapses, signaling molecules that promote Teff activation, such as protein kinase C-θ (PKC-θ), were not recruited. Inhibition or depletion of PKC-θ in Treg cells led to suppressive activity against Teff cells, whereas costimulation enhanced PKC-θ recruitment and less suppression. Together, this suggests that PKC-θ is inflammatory in both Treg and Teff cells; however, by excluding it from the synapse, Treg cells are able to maintain suppression in the face of TCR signaling. Suppressive T cells repurpose inflammatory signaling pathways to promote their suppressive functions. T cell receptor (TCR)–dependent regulatory T cell (Treg) activity controls effector T cell (Teff) function and is inhibited by the inflammatory cytokine tumor necrosis factor–α (TNF-α). Protein kinase C-θ (PKC-θ) recruitment to the immunological synapse is required for full Teff activation. In contrast, PKC-θ was sequestered away from the Treg immunological synapse. Furthermore, PKC-θ blockade enhanced Treg function, demonstrating PKC-θ inhibits Treg-mediated suppression. Inhibition of PKC-θ protected Treg from inactivation by TNF-α, restored activity of defective Treg from rheumatoid arthritis patients, and enhanced protection of mice from inflammatory colitis. Treg freed of PKC-θ–mediated inhibition can function in the presence of inflammatory cytokines and thus have therapeutic potential in control of inflammatory diseases.

Suppression of experimental autoimmune encephalomyelitis by selective blockade of encephalitogenic T-cell infiltration of the central nervous system

Multiple sclerosis (MS) is a devastating neuroinflammatory disorder of the central nervous system (CNS) in which T cells that are reactive with major components of myelin sheaths have a central role. The receptor for advanced glycation end products (RAGE) is present on T cells, mononuclear phagocytes and endothelium. Its pro-inflammatory ligands, S100-calgranulins, are upregulated in MS and in the related rodent model, experimental autoimmune encephalomyelitis (EAE). Blockade of RAGE suppressed EAE when disease was induced by myelin basic protein (MBP) peptide or encephalitogenic T cells, or when EAE occurred spontaneously in T-cell receptor (TCR)-transgenic mice devoid of endogenous TCR-α and TCR-β chains. Inhibition of RAGE markedly decreased infiltration of the CNS by immune and inflammatory cells. Transgenic mice with targeted overexpression of dominant-negative RAGE in CD4+ T cells were resistant to MBP-induced EAE. These data reinforce the importance of RAGE-ligand interactions in modulating properties of CD4+ T cells that infiltrate the CNS.