Mohamed Oukka

Reciprocal developmental pathways for the generation of pathogenic effector TH17 and regulatory T cells

On activation, T cells undergo distinct developmental pathways, attaining specialized properties and effector functions. T-helper (TH) cells are traditionally thought to differentiate into TH1 and TH2 cell subsets. TH1 cells are necessary to clear intracellular pathogens and TH2 cells are important for clearing extracellular organisms. Recently, a subset of interleukin (IL)-17-producing T (TH17) cells distinct from TH1 or TH2 cells has been described and shown to have a crucial role in the induction of autoimmune tissue injury. In contrast, CD4+CD25+Foxp3+ regulatory T (Treg) cells inhibit autoimmunity and protect against tissue injury. Transforming growth factor-β (TGF-β) is a critical differentiation factor for the generation of Treg cells. Here we show, using mice with a reporter introduced into the endogenous Foxp3 locus, that IL-6, an acute phase protein induced during inflammation, completely inhibits the generation of Foxp3+ Treg cells induced by TGF-β. We also demonstrate that IL-23 is not the differentiation factor for the generation of TH17 cells. Instead, IL-6 and TGF-β together induce the differentiation of pathogenic TH17 cells from naive T cells. Our data demonstrate a dichotomy in the generation of pathogenic (TH17) T cells that induce autoimmunity and regulatory (Foxp3+) T cells that inhibit autoimmune tissue injury.

IL-17 and Th17 Cells

CD4+ T cells, upon activation and expansion, develop into different T helper cell subsets with different cytokine profiles and distinct effector functions. Until recently, T cells were divided into Th1 or Th2 cells, depending on the cytokines they produce. A third subset of IL-17-producing effector T helper cells, called Th17 cells, has now been discovered and characterized. Here, we summarize the current information on the differentiation and effector functions of the Th17 lineage. Th17 cells produce IL-17, IL-17F, and IL-22, thereby inducing a massive tissue reaction owing to the broad distribution of the IL-17 and IL-22 receptors. Th17 cells also secrete IL-21 to communicate with the cells of the immune system. The differentiation factors (TGF-beta plus IL-6 or IL-21), the growth and stabilization factor (IL-23), and the transcription factors (STAT3, RORgammat, and RORalpha) involved in the development of Th17 cells have just been identified. The participation of TGF-beta in the differentiation of Th17 cells places the Th17 lineage in close relationship with CD4+CD25+Foxp3+ regulatory T cells (Tregs), as TGF-beta also induces differentiation of naive T cells into Foxp3+ Tregs in the peripheral immune compartment. The investigation of the differentiation, effector function, and regulation of Th17 cells has opened up a new framework for understanding T cell differentiation. Furthermore, we now appreciate the importance of Th17 cells in clearing pathogens during host defense reactions and in inducing tissue inflammation in autoimmune disease.

IL-21 initiates an alternative pathway to induce proinflammatory T H 17 cells

On activation, naive T cells differentiate into effector T-cell subsets with specific cytokine phenotypes and specialized effector functions. Recently a subset of T cells, distinct from T helper (TH)1 and TH2 cells, producing interleukin (IL)-17 (TH17) was defined and seems to have a crucial role in mediating autoimmunity and inducing tissue inflammation. We and others have shown that transforming growth factor (TGF)-β and IL-6 together induce the differentiation of TH17 cells, in which IL-6 has a pivotal function in dictating whether T cells differentiate into Foxp3+ regulatory T cells (Treg cells) or TH17 cells. Whereas TGF-β induces Foxp3 and generates Treg cells, IL-6 inhibits the generation of Treg cells and induces the production of IL-17, suggesting a reciprocal developmental pathway for TH17 and Treg cells. Here we show that IL-6-deficient (Il6-/-) mice do not develop a TH17 response and their peripheral repertoire is dominated by Foxp3+ Treg cells. However, deletion of Treg cells leads to the reappearance of TH17 cells in Il6-/- mice, suggesting an additional pathway by which TH17 cells might be generated in vivo. We show that an IL-2 cytokine family member, IL-21, cooperates with TGF-β to induce TH17 cells in naive Il6-/- T cells and that IL-21-receptor-deficient T cells are defective in generating a TH17 response.

Small intestine lamina propria dendritic cells promote de novo generation of Foxp3 T reg cells via retinoic acid

To maintain immune homeostasis, the intestinal immune system has evolved redundant regulatory strategies. In this regard, the gut is home to a large number of regulatory T (T reg) cells, including the Foxp3+ T reg cell. Therefore, we hypothesized that the gut environment preferentially supports extrathymic T reg cell development. We show that peripheral conversion of CD4+ T cells to T reg cells occurs primarily in gut-associated lymphoid tissue (GALT) after oral exposure to antigen and in a lymphopenic environment. Dendritic cells (DCs) purified from the lamina propria (Lp; LpDCs) of the small intestine were found to promote a high level of T reg cell conversion relative to lymphoid organ–derived DCs. This enhanced conversion by LpDCs was dependent on TGF-β and retinoic acid (RA), which is a vitamin A metabolite highly expressed in GALT. Together, these data demonstrate that the intestinal immune system has evolved a self-contained strategy to promote T reg cell neoconversion.

Adenosine generation catalyzed by CD39 and CD73 expressed on regulatory T cells mediates immune suppression

The study of T regulatory cells (T reg cells) has been limited by the lack of specific surface markers and an inability to define mechanisms of suppression. We show that the expression of CD39/ENTPD1 in concert with CD73/ecto-5′-nucleotidase distinguishes CD4+/CD25+/Foxp3+ T reg cells from other T cells. These ectoenzymes generate pericellular adenosine from extracellular nucleotides. The coordinated expression of CD39/CD73 on T reg cells and the adenosine A2A receptor on activated T effector cells generates immunosuppressive loops, indicating roles in the inhibitory function of T reg cells. Consequently, T reg cells from Cd39-null mice show impaired suppressive properties in vitro and fail to block allograft rejection in vivo. We conclude that CD39 and CD73 are surface markers of T reg cells that impart a specific biochemical signature characterized by adenosine generation that has functional relevance for cellular immunoregulation.

Control of T(reg) and T(H)17 cell differentiation by the aryl hydrocarbon receptor.

Regulatory T cells (Treg) expressing the transcription factor Foxp3 control the autoreactive components of the immune system. The development of Treg cells is reciprocally related to that of pro-inflammatory T cells producing interleukin-17 (TH17). Although Treg cell dysfunction and/or TH17 cell dysregulation are thought to contribute to the development of autoimmune disorders, little is known about the physiological pathways that control the generation of these cell lineages. Here we report the identification of the ligand-activated transcription factor aryl hydrocarbon receptor (AHR) as a regulator of Treg and TH17 cell differentiation in mice. AHR activation by its ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin induced functional Treg cells that suppressed experimental autoimmune encephalomyelitis. On the other hand, AHR activation by 6-formylindolo[3,2-b]carbazole interfered with Treg cell development, boosted TH17 cell differentiation and increased the severity of experimental autoimmune encephalomyelitis in mice. Thus, AHR regulates both Treg and TH17 cell differentiation in a ligand-specific fashion, constituting a unique target for therapeutic immunomodulation.

Induction and effector functions of T H 17 cells

T helper (TH) cells constitute an important arm of the adaptive immune system because they coordinate defence against specific pathogens, and their unique cytokines and effector functions mediate different types of tissue inflammation. The recently discovered TH17 cells, the third subset of effector T helper cells, have been the subject of intense research aimed at understanding their role in immunity and disease. Here we review emerging data suggesting that TH17 cells have an important role in host defence against specific pathogens and are potent inducers of autoimmunity and tissue inflammation. In addition, the differentiation factors responsible for their generation have revealed an interesting reciprocal relationship with regulatory T (Treg) cells, which prevent tissue inflammation and mediate self-tolerance.

IL-21 and TGF-beta are required for differentiation of human T(H)17 cells.

The recent discovery of CD4+ T cells characterized by secretion of interleukin (IL)-17 (TH17 cells) and the naturally occurring regulatory FOXP3+ CD4 T cell (nTreg) has had a major impact on our understanding of immune processes not readily explained by the TH1/TH2 paradigm. TH17 and nTreg cells have been implicated in the pathogenesis of human autoimmune diseases, including multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease and psoriasis. Our recent data and the work of others demonstrated that transforming growth factor-β (TGF-β) and IL-6 are responsible for the differentiation of naive mouse T cells into TH17 cells, and it has been proposed that IL-23 may have a critical role in stabilization of the TH17 phenotype. A second pathway has been discovered in which a combination of TGF-β and IL-21 is capable of inducing differentiation of mouse TH17 cells in the absence of IL-6 (refs 6–8). However, TGF-β and IL-6 are not capable of differentiating human TH17 cells and it has been suggested that TGF-β may in fact suppress the generation of human TH17 cells. Instead, it has been recently shown that the cytokines IL-1β, IL-6 and IL-23 are capable of driving IL-17 secretion in short-term CD4+ T cell lines isolated from human peripheral blood, although the factors required for differentiation of naive human CD4 to TH17 cells are still unknown. Here we confirm that whereas IL-1β and IL-6 induce IL-17A secretion from human central memory CD4+ T cells, TGF-β and IL-21 uniquely promote the differentiation of human naive CD4+ T cells into TH17 cells accompanied by expression of the transcription factor RORC2. These data will allow the investigation of this new population of TH17 cells in human inflammatory disease.

Myelin-specific regulatory T cells accumulate in the CNS but fail to control autoimmune inflammation.

Treatment with ex vivo–generated regulatory T cells (T-reg) has been regarded as a potentially attractive therapeutic approach for autoimmune diseases. However, the dynamics and function of T-reg in autoimmunity are not well understood. Thus, we developed Foxp3gfp knock-in (Foxp3gfp.KI) mice and myelin oligodendrocyte glycoprotein (MOG)35–55/IAb (MHC class II) tetramers to track autoantigen-specific effector T cells (T-eff) and T-reg in vivo during experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. MOG tetramer–reactive, Foxp3+ T-reg expanded in the peripheral lymphoid compartment and readily accumulated in the central nervous system (CNS), but did not prevent the onset of disease. Foxp3+ T cells isolated from the CNS were effective in suppressing naive MOG-specific T cells, but failed to control CNS-derived encephalitogenic T-eff that secreted interleukin (IL)-6 and tumor necrosis factor (TNF). Our data suggest that in order for CD4+Foxp3+ T-reg to effectively control autoimmune reactions in the target organ, it may also be necessary to control tissue inflammation.

A dominant function for interleukin 27 in generating interleukin 10-producing anti-inflammatory T cells

Regulatory T cells (Treg cells) expressing the transcription factor Foxp3 are key in maintaining the balance of immune homeostasis. However, distinct induced T regulatory type 1 (Tr1) cells that lack Foxp3 expression also regulate T cell function, mainly by producing the immunosuppressive cytokine interleukin 10 (IL-10). However, the factors required for the induction of IL-10-producing suppressive T cells are not fully understood. Here we demonstrate that dendritic cells modified by Treg cells induced the generation of IL-10-producing Tr1 cells. The differentiation of naive CD4+ T cells into IL-10-producing cells was mediated by IL-27 produced by the Treg cell–modified dendritic cells, and transforming growth factor-β amplified the generation of induced IL-10+ Tr1 cells by IL-27. Thus, IL-27 and transforming growth factor-β promote the generation of IL-10-producing Tr1 cells.