Th1 and Th17 cells are distinct lineages of effector/memory cells, imprinted for reexpression of , by upregulated expression of T-bet and retinoic acid-related orphan receptor ct (RORct) , respectively. Apparently, Th1 and Th17 cells share tasks in the control of inflammatory immune responses. Th cells coexpressing IFN-c and IL-17 have been observed in vivo, but it remained elusive, how these cells had been generated and whether they represent a distinct lineage of Th differentiation. It has been shown that ex vivo isolated Th1 and Th17 cells are not interconvertable by TGF-b/IL-6 and IL-12, respectively. Here, we show that ex vivo isolated Th17 cells can be converted into Th1/Th17 cells by combined IFN-c and IL-12 signaling. IFN-c is required to upregulate expression of the IL-12Rb2 chain, and IL-12 for Th1 polarization. These Th1/Th17 cells stably coexpress RORct and T-bet at the single-cell level. Our results suggest a molecular pathway for the generation of Th1/Th17 cells in vivo, which combine the pro-inflammatory potential of Th1 and Th17 cells.
IntroductionTh1 cells, with a memory for IFN-g expression and determined by the master transcription factor T-bet, are considered to be essential for protection against intracellular pathogens, and had been viewed as the major pathogenic drivers of chronic autoimmune inflammation, e.g. EAE [1][2][3], uveitis [4] or colitis [5]. Recently, Th17 cells, with a memory for expression of IL-17 and determined by the transcription factor retinoic acid related orphan receptor gt (RORgt), have been identified as another pathogenic Th-cell lineage driving pathogenesis in these autoimmune models [6,7]. Th17 cells contribute to inflammation through the recruitment of neutrophils and the induction of secretion of pro-inflammatory mediators such as IL-6, IL-8, TNF-a, IL-1b, CXCL1, CXCL10 and matrix metalloproteinases from tissue cells (reviewed in [8]). Th1 cells contribute to inflammation by activation of macrophages [9]. The concerted action of IFN-g and IL-17 has been shown to be essential in the effective induction and maintenance of autoimmunity [10,11], e.g. Th1 cells being required for the recruitment of Th17 cells into the central nervous system in EAE.In inflamed tissue of autoimmune patients, Th cells coexpressing IFN-g and IL-17 have been identified [12][13][14].Ã These authors have contributed equally to this study.
ResultsIn vivo, Th17 cells do not express IL-12Rb2 and do not respond to IL-12Although IL-17-expressing cells isolated from cultures stimulated in vitro respond to subsequent stimulation with IL-12 with gain of IFN-g expression and loss of IL-17 expression (Fig. 1A, upper panel), IL-17 expressing cells directly isolated ex vivo maintained IL-17 expression and could not be induced to express IFN-g in the presence of IL-12 (Fig. 1A, lower panel) [17,19]. To identify the molecular mechanism of refraction of in vivo generated Th17 cells to conversion by IL-12, we here compared the expression of genes relevant for IL-12 signaling by Th17 cells generated in vit...