A functionally specialized population of mucosal CD103+ DCs induces Foxp3+ regulatory T cells via a TGF-beta and retinoic acid-dependent mechanism

Division of labor, plasticity, and crosstalk between dendritic cell subsets

For years, dendritic cell (DC) biologists have oscillated between two seemingly antagonistic ideas: functional specialization (division of labor) of DC subsets and plasticity (multitasking). More recently, a third hypothesis is gathering support: crosstalk between functionally distinct DC subsets. This reveals a previously unappreciated hierarchy of organization within the DC system, and provides a conceptual framework to understand how cooperation between functionally distinct, yet plastic, DC subsets can shape adaptive immunity and immunological memory. Here we review the recent advances in this area.

Starring stellate cells in liver immunology

Stellate cells are star-shaped cells located in the liver and mediate a multitude of primarily non-immunological functions. They play a pivotal role in the metabolism of vitamin A and store 80% of total body retinol. Upon activation, stellate cells differentiate to myofibroblasts for production of extracellular matrix, leading to liver fibrosis. Moreover, activated stellate cells regulate liver blood flow through vasoconstriction implicated in portal hypertension. Earlier work demonstrated stellate cell derived secretion of chemokines and cytokines such as transforming growth factor beta (TGF-beta), suggesting an association with immunological processes. Indeed, recent evidence indicated that hepatic stellate cells perform potent APC function for stimulation of NKT cells as well as CD8 and CD4 T cells. Additionally, stellate cell mediated antigen presentation induced protective immunity against bacterial infection. Current experiments reveal that the presenting ability of stellate cells is the key to antigen-dependent T cell instruction by vitamin A derived retinoic acid. Finally, future studies will show whether in the firmament of immunology stellate cells will represent fixed or falling stars.

Dendritic cells are specialized accessory cells along with TGF- for the differentiation of Foxp3+ CD4+ regulatory T cells from peripheral Foxp3 precursors

Foxp3(+)CD25(+)CD4(+) regulatory T cells are produced in the thymus (natural T regs) but can also differentiate from peripheral Foxp3(-)CD4(+) precursors (induced or adaptive T regs). We assessed antigen presenting cell (APC) requirements for the latter differentiation. With added transforming growth factor (TGF)-beta, both immature and mature populations of dendritic cells (DCs) induced antigen-specific Foxp3(+) T regs from Foxp3(-) precursors. Using endogenous TGF-beta, DCs from gut-associated mesenteric lymph nodes were capable of differentiating Foxp3(+)T regs. Spleen DCs were 100-fold more potent than DC-depleted APCs for the induction of T regs and required 10-fold lower doses of peptide antigen. Interleukin-2 (IL-2) was essential, but could be provided endogenously by T cells stimulated by DCs, but not other APCs. The required IL-2 was induced by DCs that expressed CD80/CD86 costimulatory molecules. The DC-induced Foxp3(+)T regs divided up to 6 times in 6 days and were comprised of CD62L and CD103 positive and negative forms. The induced Foxp3(+)T regs exerted suppression in vitro and blocked tumor immunity in vivo. These results indicate that DCs are specialized to differentiate functional peripheral Foxp3(+)T regs and help set the stage to use DCs to actively suppress the immune response in an antigen-specific manner.

Signal transduction pathways and transcriptional regulation in the control of Th17 differentiation

The discovery of a new lineage of helper T cells that selectively produces interleukin (IL)-17 has provided exciting new insights into immunoregulation, host defense and the pathogenesis of autoimmune diseases. Additionally, the discovery of this T cell subset has offered a fresh look at how the complexity of selective regulation of cytokine gene expression might relate to lineage commitment, terminal differentiation and immunologic memory. Information continues to accumulate on factors that regulate Th17 differentiation at a rapid pace and a few lessons have emerged. Like other lineages, Th17 cells preferentially express a transcription factor, retinoic acid-related orphan receptor (ROR)gammat, whose expression seems to be necessary for IL-17 production. In addition, signals from the T-cell receptor are a critical aspect of controlling IL-17 production and the transcription factor nuclear factor of activated T cells (NFATs) appears to be another important regulator. IL-6, IL-21 and IL-23 are all cytokines that activate the transcription factor STAT3, which has been established to be necessary for multiple aspects of the biology of Th17 cells. Similarly, TGFbeta-1 is important for the differentiation of murine Th17 cells and inducible regulatory T cells (iTregs), but how it exerts its effect on IL-17 gene transcription is unknown and there are data indicating TGFbeta-1 is not required for human Th17 differentiation. The extent to which Th17 cells represent terminally differentiated cells or whether they retain plasticity and can develop into another lineage such as IFNgamma secreting Th1 cells is also unclear. Precisely how cytokines produced by this lineage are selectively expressed and selectively extinguished through epigenetic modifications is an area of great importance, but considerable uncertainty.

Transcriptional regulation of Th17 cell differentiation

The paradigm of effector T helper cell differentiation into either Th1 or Th2 lineages has been profoundly shaken by the discovery of T cells that secrete IL-17 and other inflammatory cytokines. This subset, referred to as Th17, is centrally involved in autoimmune disease and is important in host defense at mucosal surfaces. In mouse, a series of cytokines, including IL-6, IL-21, IL-23, and TGF-beta, function sequentially or synergistically to induce the Th17 lineage. Other cytokines, including IL-2, IL-4, IFNgamma, and IL-27, inhibit differentiation of this lineage. Here we review how the nuclear orphan receptor RORgammat functions to coordinate the diverse cytokine-induced signals and thus controls Th17 cell differentiation.

Th17 T cells: linking innate and adaptive immunity

While the cytokine IL-17 has been cloned and described more than 10 years ago [Yao Z, Fanslow WC, Seldin MF, Rousseau AM, Painter SL, Comeau MR, et al. Herpesvirus Saimiri encodes a new cytokine, IL-17, which binds to a novel cytokine receptor. Immunity 1995;3(6):811-21; Kennedy J, Rossi DL, Zurawski SM, Vega Jr F, Kastelein RA, Wagner JL, et al. Mouse IL-17: a cytokine preferentially expressed by alpha beta TCR+CD4-CD8-T cells. J Interferon Cytokine Res 1996;16(8):611-7], it was only 2 years ago that IL-17 producing T cells have been classified as a new distinct CD4 T cell subset [Harrington LE, Hatton RD, Mangan PR, Turner H, Murphy TL, Murphy KM, et al. Interleukin 17-producing CD4+ effector T cells develop via a lineage distinct from the T helper type 1 and 2 lineages. Nat Immunol 2005;6(11):1123-32] and only in 2006 the molecular mechanisms underlying their differentiation were identified [Veldhoen M, Hocking RJ, Atkins CJ, Locksley RM, Stockinger B. TGFbeta in the context of an inflammatory cytokine milieu supports de novo differentiation of IL-17-producing T cells. Immunity 2006;24(2):179-89; Bettelli E, Carrier Y, Gao W, Korn T, Strom TB, Oukka M, et al. Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells. Nature 2006;441(7090):235-8; Mangan PR, Harrington LE, O'Quinn DB, Helms WS, Bullard DC, Elson CO, et al. Transforming growth factor-beta induces development of the T(H)17 lineage. Nature 2006;441(7090):231-4]. Since then the literature on IL-17 producing cells has grown steadily and many reviews of the field are already outdated by the time they are published, a fate that no doubt will affect this review as well. In order to avoid too many repetitions we focus this review mainly on publications in 2006 and 2007 and refer to a number of reviews, which cover earlier aspects of Th17/IL-17 biology.

Taking dendritic cells into medicine

Dendritic cells (DCs) orchestrate a repertoire of immune responses that bring about resistance to infection and silencing or tolerance to self. In the settings of infection and cancer, microbes and tumours can exploit DCs to evade immunity, but DCs also can generate resistance, a capacity that is readily enhanced with DC-targeted vaccines. During allergy, autoimmunity and transplant rejection, DCs instigate unwanted responses that cause disease, but, again, DCs can be harnessed to silence these conditions with novel therapies. Here we present some medical implications of DC biology that account for illness and provide opportunities for prevention and therapy.

TGFbeta1 and Treg cells: alliance for tolerance

Transforming growth factor beta1 (TGFbeta1), an important pleiotropic, immunoregulatory cytokine, uses distinct signaling mechanisms in lymphocytes to affect T-cell homeostasis, regulatory T (Treg)-cell and effector-cell function and tumorigenesis. Defects in TGFbeta1 expression or its signaling in T cells correlate with the onset of several autoimmune diseases. TGFbeta1 prevents abnormal T-cell activation through the modulation of Ca2+-calcineurin signaling in a Caenorhabditis elegans Sma and Drosophila Mad proteins (SMAD)3 and SMAD4-independent manner; however, in Treg cells, its effects are mediated, at least in part, through SMAD signaling. TGFbeta1 also acts as a pro-inflammatory cytokine and induces interleukin (IL)-17-producing pathogenic T-helper cells (Th IL-17 cells) synergistically during an inflammatory response in which IL-6 is produced. Here, we will review TGFbeta1 and its signaling in T cells with an emphasis on the regulatory arm of immune tolerance.

Regulatory-T-cell inhibition versus depletion: the right choice in cancer immunotherapy

Tumour-induced expansion of regulatory T (T(Reg)) cells is an obstacle to successful cancer immunotherapy. The potential benefit of T(Reg)-cell depletion through the interleukin-2 receptor is lost by the concurrent elimination of activated effector lymphocytes and possibly by the de novo induction of T(Reg)-cell replenishment. In theory, the functional inactivation of T(Reg) cells will maintain them at high numbers in tumours and avoid their replenishment from the peripheral lymphocyte pool, which has the capacity to further suppress the effector lymphocyte anti-tumour response.

The yin and yang of intestinal epithelial cells in controlling dendritic cell function

Recent work suggests that dendritic cells (DCs) in mucosal tissues are "educated" by intestinal epithelial cells (IECs) to suppress inflammation and promote immunological tolerance. After attack by pathogenic microorganisms, however, "non-educated" DCs are recruited from nearby areas, such as the dome of Peyer's patches (PPs) and the blood, to initiate inflammation and the ensuing immune response to the invader. Differential epithelial cell (EC) responses to commensals and pathogens may control these two tolorogenic and immunogenic functions of DCs.

Lamina propria macrophages and dendritic cells differentially induce regulatory and interleukin 17-producing T cell responses

The intestinal immune system must elicit robust immunity against harmful pathogens but must also restrain immune responses directed against commensal microbes and dietary antigens. The mechanisms that maintain this dichotomy are poorly understood. Here we describe a population of CD11b+F4/80+CD11c- macrophages in the lamina propria that expressed several anti-inflammatory molecules, including interleukin 10 (IL-10), but little or no proinflammatory cytokines, even after stimulation with Toll-like receptor ligands. These macrophages induced, by a mechanism dependent on IL-10, retinoic acid and exogenous transforming growth factor-beta, the differentiation of Foxp3+ regulatory T cells. In contrast, lamina propria CD11b+ dendritic cells elicited IL-17 production. This IL-17 production was suppressed by lamina propria macrophages, indicating that a dynamic interaction between these subsets may influence the balance between immune activation and tolerance.

TGFbeta and retinoic acid intersect in immune-regulation

Transforming growth factor (TGFbeta) prevents T(H)1 and T(H)2 differentiation and converts naïve CD4 cells into Foxp3-expressing T regulatory (Treg) cell.(1,2) In sharp contrast, in the presence of pro-inflammatory cytokines, including IL-6, TGFbeta not only inhibits Foxp3 expression but also promotes the differentiation of pro-inflammatory IL17-producing CD4 effector T (T(H)17) cells.(3-5) This reciprocal TGFbeta-dependent differentiation imposes a critical dilemma between pro- and anti-inflammatory immunity and suggests that a sensitive regulatory mechanism must exist to control TGFbeta-driven T(H)17 effector and Treg differentiation. A vitamin A metabolite, retinoic acid (RA), was recently identified as a key modulator of TGFbeta-driven- immune deviation capable of suppressing T(H)17 differentiation while promoting Foxp3(+)Treg generation.(6-10).

Oral tolerance: is it all retinoic acid?

Oral tolerance has been argued to depend on "special" presentation of antigen in the gut. New studies support this idea by showing that the catalysis of vitamin A into retinoic acid (RA) in gut-associated dendritic cells (DCs) enhances the transforming growth factor (TGF)-beta-dependent conversion of naive T cells into regulatory T (T reg) cells and also directs T reg cell homing to the gut. These results reveal new tolerance mechanisms that will aid the use of T reg cells in the clinic.

Human T cell cytokine responses are dependent on multidrug resistance protein-1

Multidrug resistance protein-1 (MRP1) belongs to subfamily C of the ATP-binding cassette transporters, and exports leukotriene C(4) and organic anions including the fluorescent calcium indicator indo-1. The observation that leukocytes from patients with an autoimmune disease exported indo-1 at a higher rate than controls prompted the hypothesis that MRP1 contributes to the function of activated cells. To test this, we defined the expression of MRP1 on resting and activated human T cells, and determined whether T cell activation is dependent upon MRP1 function. MRP1 is expressed on resting memory but not on naive CD4 and CD8 T cells. After activation through the TCR, cord blood CD4 T cells express high levels of MRP1. Blockade of MRP1 with the specific inhibitor MK-571 abrogated superantigen-induced expression of IFN-gamma, tumor necrosis factor-alpha, IL-10, IL-2, IL-4 and CD69 by T cells without affecting their viability, and was reversible upon removal of MK-571 from the culture media. Electrophoretic mobility shift assays demonstrate that MRP1 blockade with MK-571 induces activation of the transcriptional repressor peroxisome proliferator-activated receptor-gamma in CD4 T cells, thus providing insight into the potential mechanism by which their responses are abrogated.

Oral tolerance

Multiple mechanisms of tolerance are induced by oral antigen. Low doses favor active suppression, whereas higher doses favor clonal anergy/deletion. Oral antigen induces T-helper 2 [interleukin (IL)-4/IL-10] and Th3 [transforming growth factor (TGF)-beta] T cells plus CD4+CD25+ regulatory cells and latency-associated peptide+ T cells. Induction of oral tolerance is enhanced by IL-4, IL-10, anti-IL-12, TGF-beta, cholera toxin B subunit, Flt-3 ligand, and anti-CD40 ligand. Oral (and nasal) antigen administration suppresses animal models of autoimmune diseases including experimental autoimmune encephalitis, uveitis, thyroiditis, myasthenia, arthritis, and diabetes in the non-obese diabetic (NOD) mouse, plus non-autoimmune diseases such as asthma, atherosclerosis, graft rejection, allergy, colitis, stroke, and models of Alzheimer's disease. Oral tolerance has been tested in human autoimmune diseases including multiple sclerosis (MS), arthritis, uveitis, and diabetes and in allergy, contact sensitivity to dinitrochlorobenzene (DNCB), and nickel allergy. Although positive results have been observed in phase II trials, no effect was observed in phase III trials of CII in rheumatoid arthritis or oral myelin and glatiramer acetate (GA) in MS. Large placebo effects were observed, and new trials of oral GA are underway. Oral insulin has recently been shown to delay onset of diabetes in at-risk populations, and confirmatory trials of oral insulin are being planned. Mucosal tolerance is an attractive approach for treatment of autoimmune and inflammatory diseases because of lack of toxicity, ease of administration over time, and antigen-specific mechanisms of action. The successful application of oral tolerance for the treatment of human diseases will depend on dose, developing immune markers to assess immunologic effects, route (nasal versus oral), formulation, mucosal adjuvants, combination therapy, and early therapy.