Note added in proofs: After this commentary was accepted for publication, Ghoreschi et al. reported that even murine Th17 cells do not require TGF-b signalling for their differentiation, which occurs in response to the combined activity of IL-1b and IL-23, whereas TGF-b may play only an indirect role by suppressing T-bet expression and the development of Th1 cells. This study demolishes the still ruling dogma on the origin of Th17 cells and fully supports all our findings [14, 17, 19, 27, and this Commentary], as well as our concept that studies in humans have better depicted Th17 cells than initial studies in mice . Ghoreschi, K., Laurence, A., Yang, X. P., Tato, C. M., McGeachy, M. J., Konkel, J. E., Ramos, H. L., Wei, L.et al. Generation of pathogenic T(H)17 cells in the absence of TGF-b signalling. Nature. 2010. 467: 967-971.
The transient nature of the Th17 phenotype†
Article first published online: 25 NOV 2010
Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
European Journal of Immunology
Volume 40, Issue 12, pages 3312–3316, December 2010
How to Cite
Annunziato, F. and Romagnani, S. (2010), The transient nature of the Th17 phenotype. Eur. J. Immunol., 40: 3312–3316. doi: 10.1002/eji.201041145
- Issue published online: 25 NOV 2010
- Article first published online: 25 NOV 2010
- Manuscript Accepted: 26 OCT 2010
- Manuscript Received: 8 OCT 2010
- Manuscript Revised: 8 OCT 2010
- Ministero dell'Istruzione
- dell'Università e della Ricerca PRIN 2007
- FP6 European Union project INNOCHEM. Grant Number: FP6-LSHB-CT2005-518167
- Associazione Italiana Ricerca sul Cancro (AIRC)
- Ente Cassa Risparmio di Firenze
- Th cells
CD4+ Th lymphocytes represent a heterogeneous population of cells that play an essential role in adaptive immunity. In addition to type 1 (Th1) and type 2 (Th2) cells, a third subset of CD4+ Th effector cells has recently been discovered, named type 17 (Th17) because of its unique ability to produce interleukin (IL)-17. Initial studies in mice suggested that Th17 cells are the pathogenic cells in autoimmune disorders, whereas Th1 cells are protective. Studies in humans have demonstrated the plasticity of Th17 cells and their ability to convert to Th1 cells. This Th17 to Th1 cell plasticity has also been confirmed in mice and, furthermore, it was found that Th17 cells appear to be pathogenic only when they shift to Th1 cells. A study in this issue of the European Journal of Immunology uses an IL-17 fate mapping mouse strain, which permits the identification of the cells that have been IL-17 producers, to provide definitive evidence that Th17 cells, either generated in vitro or in vivo, represent a transient phenotype that tend to convert into IFN-γ-producing cells. Our Commentary discusses this interesting point in light of previous data suggesting the same concept.
Effector CD4+ Th lymphocytes are heterogeneous with regard to their protective function, enabling a type of response which differs depending on the nature of the invading microorganism. More than 20 years ago, two main subsets of CD4+ Th cells with different functions and patterns of cytokine secretion were identified in both mice and humans, and were named type 1 Th (Th1), for the cells able to produce IFN-γ, and type 2 Th (Th2) for the cells able to produce IL-4, IL-5 and IL-13 1, 2. In addition to their distinctive protective functions, it was suggested that Th1 and Th2 cells contribute to the development of human disorders, with Th1 cells being involved in the pathogenesis of organ-specific autoimmune diseases, Crohn's disease, sarcoidosis and atherosclerosis, and Th2 cells playing a central role in the development of allergic disorders 3. Both Th1 and Th2 cells were found to be stable for a long time, although it has been recently reported in both mice and humans that Th2 cells can be induced to produce IFN-γ in addition to IL-4 (Th0) after repeated stimulation with IFNs and IL-12 4, 5.
The validity of this Th1/Th2 paradigm has been maintained until a few years ago, when a third subset of CD4+ effector Th cells, named Th17 cells, was identified 6. Th17 cells express the transcription factor RORγt (RORC in humans) and produce IL-17A, a cytokine that is involved in the recruitment, activation and migration of neutrophils by inducing the production by fibroblasts, epithelial and endothelial cells of colony stimulatory factors and CXCL8 7. Other cytokines produced by Th17 cells include IL-17F, IL-21 and IL-22, which also contribute to the activation of mononuclear and/or resident cells and therefore may induce and/or maintain a chronic inflammatory process. The main protective function of Th17 cells appears to be the clearance of extracellular pathogens, including fungi 7. Nevertheless, the major research emphasis has been placed on the predominant or even exclusive pathogenic role of Th17 cells in some murine models of autoimmunity, such as EAE, collagen-induced arthritis and inflammatory bowel disorder, a role that has mainly been identified using gene-knockouts 6, 8, 9. Mainly based on these murine models, a pathogenic role for Th17 cells in human immunopathology was, we feel, prematurely inferred, whereas Th1 cells were considered as being protective 10.
The source of Th17 cells, as well as the cytokines responsible for Th17-cell development, seem to differ in mice and humans. Murine Th17 cells have been suggested to originate from naïve Th cells, as a result of the combined activity of IL-6 and TGF-β 11, 12, with IL-21 and IL-23 appearing to be involved only in Th17-cell expansion and/or viability 11–13. Our studies performed with human Th17 cells showed features clearly different from, or even incompatible with, the Th17 paradigm established in mice 11, 12, questioning the full validity of this paradigm. We found that many human clones, subsequently named Th17/Th1 cells, were able to produce both IL-17 and IFN-γ and this mixed cytokine profile was also seen in ex vivo studies performed in both peripheral blood and inflamed tissues 14, suggesting that IFN-γ and IL-17 can be produced by the same cell. Moreover, both Th17 and Th17/Th1 clones not only expressed IL-23R and RORC, but also the IL-12Rβ2 chain and the Th1-related transcription factor T-bet 14. Finally, and most importantly, we demonstrated that the stimulation of human Th17 clones in the presence of IL-12 down-regulates RORC and up-regulates T-bet, enabling these Th17 cells to produce IFN-γ in addition to IL-17A. In our hands, however, Th1 clones could never be induced to produce IL-17A. Our findings 14 provided clear evidence for the plasticity of human Th17 cells to shift to a Th1 profile, whereas Th1 cells seem to be unable to shift to Th17. It is important to note that a small number of the human Th17 clones also showed the ability to produce IL-4 (Th17/Th2 cells) and that, although very few of these Th17/Th2 cells were found in ex vivo studies performed in the circulation of healthy subjects, their number was increased in patients with severe chronic asthma. These Th17/Th2 cells could be generated from Th17 clones in the presence of IL-4, whereas Th2 clones could never be induced to produce IL-17A 15. Our data differ from those recently reported by Wang et al. 16, who showed that classical Th2 memory/effector cells have the potential to produce IL-17 after stimulation with the proinflammatory cytokines IL-1β, IL-6 and IL-21; however, in agreement with our results, Wang et al. 16 also found that circulating Th17/Th2-cell numbers were significantly increased in patients with atopic asthma (Fig. 1).
Another important finding regarding the physiology of human Th17 cells was our demonstration 17 that virtually all human Th17 cells are contained within the CD161+ fraction of either circulating or tissue-infiltrating CD4+ T cells. This has been confirmed by Kleinschek et al. 18, who examined the phenotype of CD4+ T cells in the gut of patients with Crohn's disease. We have also found that human Th17 cells can only originate from a small subset of CD161+ naïve CD4+ T cells present in umbilical cord blood and in newborn thymus in response to IL-1β and IL-23, without any need for TGF-β 17, 19. More recent studies in mice have reported that murine Th17 cells also originate from thymic precursors 20 and that the addition of TGF-β to the culture is dispensable 21, whereas IL-1 is critical 22 for the orchestration of Th17-development.
The demonstration of the plasticity of human Th17 cells to Th1 cells is of great relevance with regard to the controversy surrounding the respective role of these two effector cell types in the pathogenesis of autoimmune, as well as of other chronic inflammatory disorders. Indeed, after the initial assumption that Th17 cells are responsible for pathogenicity based on the results of gene-knockout models 6, 8, 9, a series of well-performed studies questioned this hypothesis 23–27. Some clarification of the issue came recently from at least four different and independent studies 28–31. First, the propagation of committed murine Th17 precursors in the presence of IL-23, but the absence of TGF-β, was shown to result in a progressive extinction of IL-17A and IL-17F production and to promote the emergence of IFN-γ-producing cells that lacked IL-17A expression; the stimulation of these Th17 precursors with IL-12 led to the induction of a rapid, STAT4- and T-bet-dependent transition marked by the extinction of RORγt, RORα, IL-17A and IL-17F and the induction of a Th1-like expression signature 28. Second, the potential plasticity of murine Th17 to Th1 cells was reported 29, thus supporting the substantial developmental plasticity of the Th17 lineage that had already been observed in humans 14. Third, epigenetic modifications across the clustered Il17a and Il17f loci and the Ifng loci before and after differential IL-12 or TGF-β cytokine signalling, which induce divergent fates of Th17-cell precursors, have been identified 30, revealing substantial chromatin instability in the cytokine genes of Th17 cells. Fourth, in the previous issue of the European Journal of Immunology, it was demonstrated that IFN-γ and IL-12 synergize to convert in vivo generated Th17 into Th17/Th1 cells 31. Altogether, these data provide some explanation for the controversy regarding whether Th17 or Th1 cells are the effector cells that mediate immunopathology. Indeed, it has also been found that Th17 cells can promote pancreatic inflammation, but that they only induce type 1 insulin-dependent diabetes mellitus (IDDM) efficiently in lymphopenic mice after conversion into Th1 cells 32. Accordingly, highly purified Th17 cells from BDC2.5NOD mice have been shown to shift into Th1-like cells in NOD/SCID recipient mice 33. The transferred Th17 cells, completely devoid of IFN-γ at the time of transfer, rapidly converted to IFN-γ secretors in the NOD/SCID recipients and, more importantly, the development of IDDM was prevented by treatment with an anti-IFN-γ-, but not an anti-IL17A-, specific antibody 33. These findings provide evidence that, in murine IDDM, the true pathogenic cells are not Th17, but rather Th17-derived Th1 cells. A similar conclusion was also obtained in humans; Nistala et al. 34 showed Th17 to Th1 plasticity driven by the inflammatory environment in patients with autoimmune polyarthritis. Likewise, we have found an accumulation of CD4+CD161+ T cells able to produce IFN-γ (Th1), or both IFN-γ and IL-17A (Th17/Th1), but not IL-17A alone (Th17), in the synovial fluid (SF) of children with oligoarticular juvenile idiopathic arthritis (JIA) (Cosmi et al., unpublished data). The Th17 cells shared clonal ancestry with CD161+ Th17/Th1 and CD161+ Th1 cells present in the SF, and circulating Th17 cells from healthy subjects demonstrated Th17 to Th1 plasticity in vitro in the presence of SF from JIA patients, which could be blocked by IL-12-neutralization. More importantly, the numbers of CD4+CD161+ cells, particularly the Th17/Th1 cells, in SF of JIA patients positively correlated with levels of erythrocyte sedimentation rate and C-reactive protein (Cosmi et al., unpublished data). Taken together, these findings support the hypothesis that Th17-derived Th17/Th1 and Th1 cells, rather than Th17 cells, play a critical role in disease activity.
In this issue, Kurschus et al. 35 use a recently generated IL-17F-CreEYFP Th17 reporter mouse line to demonstrate that both in vitro- and in vivo-generated Th17 cells convert into Th17/Th1, or even into pure Th1 cells. Briefly, in vitro-activated myelin oligodendrocyte glycoprotein-specific EYFP+ Th17 cells were transferred into RAG−/− mice. The EYFP+ cells were recovered from the central nervous system, lymph nodes and spleen of the mice at the peak of EAE and were analysed for IL-17A, IL-17F and IFN-γ-expression, as well as for the expression of Th17- and Th1-specific transcription factors. RT-PCR analysis of the EYFP+ cells recovered from the mice at the peak of EAE showed higher IFN-γ and T-bet, and lower il17a and il17f, mRNA expression than the EYFP+ cells before transfer, while no changes in the expression of RORC and IRF4 were found. In contrast with the above-mentioned studies, which strongly argued against the plasticity of Th1 cells towards the production of IL-17A 14, 15, 17, Kurschus et al. 35 found that a highly pure Th1-cell population can convert in vivo into IFN-γ/IL-17A double-producing T cells, even if this effect was only observed in the mesenteric lymph nodes, suggesting that the gut immune system can create a specific local milieu highly favourable to a Th17/Th1 dichotomous response. However, the most important point emerging from the study of Kurschus et al. 35 is the definitive demonstration at the genetic level that the Th17 response can be quickly lost, since Th17 cells unavoidably shift to the Th1 phenotype. Such a behaviour is different from that of Th1 and Th2 cells which are known to remain stable in their cytokine secretion pattern for a long time even in the context of inflamed tissues. A scheme illustrating the high flexibility of CD4+ T effector cells is shown in Figure 1. These findings are of particular importance in light of the current debate on the respective role of Th17, Th17/Th1 and Th1 cells in the pathogenesis of chronic inflammatory disorders and, therefore, in view of developing possible biological therapeutic strategies. With regard to the latter point, it is of note that ustekinumab, a humanized monoclonal antibody that inhibits receptor-binding of both IL-12 and IL-23, is effective in reducing the signs and symptoms of psoriatic arthritis 36, thus supporting the concept that Th1 and Th17 cells cooperate in the inflammatory processes.
Ministero dell'Istruzione, dell'Università e della Ricerca PRIN 2007, FP6 European Union project INNOCHEM (FP6-LSHB-CT2005-518167), Associazione Italiana Ricerca sul Cancro (AIRC), Ente Cassa Risparmio di Firenze.
Conflict of interest: The authors declare no financial or commercial conflict of interest.
See accompanying article: http://dx.doi.org/10.1002/eji.201040755
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