IL-17-producing CD4+ T helper cells (TH17) have been extensively investigated in mouse models of autoimmunity. However, the requirements for differentiation and the properties of pathogen-induced human TH17 cells remain poorly defined. Using an approach that combines the in vitro priming of naive T cells with the ex vivo analysis of memory T cells, we describe here two types of human TH17 cells with distinct effector function and differentiation requirements. Candida albicans-specific TH17 cells produced IL-17 and IFN-γ, but no IL-10, whereas Staphylococcus aureus-specific TH17 cells produced IL-17 and could produce IL-10 upon restimulation. IL-6, IL-23 and IL-1β contributed to TH17 differentiation induced by both pathogens, but IL-1β was essential in C. albicans-induced TH17 differentiation to counteract the inhibitory activity of IL-12 and to prime IL-17/IFN-γ double-producing cells. In addition, IL-1β inhibited IL-10 production in differentiating and in memory TH17 cells, whereas blockade of IL-1β in vivo led to increased IL-10 production by memory TH17 cells. We also show that, after restimulation, TH17 cells transiently downregulated IL-17 production through a mechanism that involved IL-2-induced activation of STAT5 and decreased expression of ROR-γt. Taken together these findings demonstrate that by eliciting different cytokines C. albicans and S. aureus prime TH17 cells that produce either IFN-γ or IL-10, and identify IL-1β and IL-2 as pro- and anti-inflammatory regulators of TH17 cells both at priming and in the effector phase.

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Although T helper 17 (TH17) cells have been acknowledged as crucial mediators of autoimmune tissue damage, the effector cytokines responsible for their pathogenicity still remain poorly defined, particularly in humans. In mouse models of autoimmunity, the pathogenicity of TH17 cells has recently been associated with their production of granulocyte-macrophage colony-stimulating factor (GM-CSF). We analyzed the regulation of GM-CSF expression by human TH cell subsets. Surprisingly, the induction of GM-CSF expression by human TH cells is constrained by the interleukin-23 (IL-23)/ROR-γt/TH17 cell axis but promoted by the IL-12/T-bet/TH1 cell axis. IL-2-mediated signal transducer and activator of transcription 5 (STAT5) signaling induced GM-CSF expression in naïve and memory TH cells, whereas STAT3 signaling blocked it. The opposite effect was observed for IL-17 expression. Ex vivo, GM-CSF(+) TH cells that coexpress interferon-γ and T-bet could be distinguished by differential chemokine receptor expression from a previously uncharacterized subset of GM-CSF-only-producing TH cells that did not express TH1, TH2, and TH17 signature cytokines or master transcription factors. Our findings demonstrate distinct and counterregulatory pathways for the generation of IL-17- and GM-CSF-producing cells and also suggest a pathogenic role for GM-CSF(+) T cells in the inflamed brain of multiple sclerosis (MS) patients. This provides not only a scientific rationale for depleting T cell-derived GM-CSF in MS patients but also multiple new molecular checkpoints for therapeutic GM-CSF suppression, which, unlike in mice, do not associate with the TH17 but instead with the TH1 axis.

Since their discovery as a distinct T helper (Th) cell lineage, Th17 cells have been extensively investigated both in mice and in humans. These studies have identified factors involved in their differentiation and effector functions and have also revealed a high degree of flexibility that seems to be a characteristic of the Th17-cell lineage. In this review, we discuss recent studies addressing the heterogeneity of human Th17 cells, their differentiation requirements, their migratory capacities, and their role in defense against fungi and extracellular bacteria. Keywords: Cytokines r Chemokine receptors r Human r Th17 IntroductionHuman T cells producing IL-17 were described as early as the late 1990s in the context of chronic inflammatory conditions such as rheumatoid arthritis and airway inflammation [1,2], but it was only in 2005 that they were recognized as a new lineage of effector T cells [3]. Three lines of evidence obtained in the mouse system supported this notion. First, pathogenic inflammatory T cells produced high levels of IL-17A, IL-17F, and TNF and were dependent on IL-23 rather than IL-12 for their expansion [3]. Second, naïve CD4 + T cells acquired IL-17-, but not IFN-γ-or IL-4-producing capacity, when activated in vitro in the presence of TGF-β and IL-6 or IL-23 [4][5][6]. Third, overexpression of the orphan nuclear receptor RORγt was sufficient to induce differentiation of Th17 cells, while deficiency of RORγt in T cells attenuated autoimmune disease due to lack of tissue-infiltrating Th17 cells [7]. From these groundbreaking studies, the field has progressed at an astonishing pace, taking advantage of new and powerful technologies that have become accessible in recent years. As in many other areas of immunology, discoveries from studies performed in both experimental animal models and in human systems have contributed to our current understanding of the Th17 system and the role these cells play in physiology and pathology. Here, we will review, in particular, studies that address the heterogeneity of human Th17Correspondence: Dr. Federica Sallusto e-mail: federica.sallusto@irb.usi.ch cells, their differentiation requirements, their migratory capacities, and their role in defense against pathogens. Human Th17: heterogeneity, function, and migrationTo perform their function, effector, and memory T cells have to migrate to specific tissue sites, which are marked by the presence of constitutive or inflammatory chemokines [8]. Several studies have shown that chemokine receptors are differentially expressed on effector and memory T-cell subsets. For example, CCR7 identifies central memory T (T CM ) cells that home to secondary lymphoid organs, and distinguish them from effector memory T (T EM ) cells that home to peripheral nonlymphoid tissues [9]. CXCR3 and CCR5 are preferentially expressed on IFN-γ-producing Th1 cells, while CCR3, CCR4, and CRTH2 are preferentially expressed on subsets of Th2 cells that produce IL-4, . The Th1-and Th2-specifying transcription factors T-bet and GATA3 directly contr...