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DOI: 10.1002/eji.201444465
Eur. J. Immunol. 2014. 44: 646–649
Commentary
Better safe than sorry: TOB1 employs multiple parallel regulatory pathways to keep Th17 cells quiet Fiamma Salerno, Ren´e A. W. van Lier and Monika C. Wolkers Department of Hematopoiesis, Sanquin Research/Landsteiner laboratory AMC, Amsterdam, The Netherlands Th17 cells are key players in antibacterial and antifungal immunity, but have also been implicated in autoimmunity. Interestingly, Th17 cells poorly proliferate upon stimulation, a phenotype that was attributed to a decreased sensitivity to T-cell receptor (TCR) stimulation, and to low IL-2 production by Th17 cells. In this issue of the European Journal of Immunology, Santarlasci et al. [Eur. J. Immunol. 2014. 44: 654–661] shed further light on the molecular mechanism that keeps Th17 cells at bay. They identify the transcriptional regulator TOB1, which not only impairs IL-2 production in Th17 cells, but also blocks the expression of cell cycle genes. Strikingly, TOB1 suppresses Th17-cell proliferation through several pathways, including impaired signal transduction, transcription, and possibly also post-transcriptional regulation.
Keywords: Gene regulation r Proliferation r Th17 cells
r
TOB1
See accompanying article by Santarlasci et al.
Th17 cells are located at mucosal and epithelial barriers where they respond to bacterial and fungal intruders. They do so by producing the proinflammatory cytokine interleukin-17 (IL-17). IL-17 potentiates chemokine production and thereby enhances the attraction of monocytes and neutrophils, which in turn help to clear the infection [1]. The importance of IL-17 in infection is evident from mouse studies and from human deficiency disorders where deficiency in IL-17, or the IL-17 receptor, results in systemic Candida albicans infection [2–4]. Conversely, Th17-cell responses are also associated with increased pathology in several infection models, such as Candida albicans, Aspergillus fumigatus, and Chlamydia muridarum [5–7], stressing the importance of controlling the magnitude of Th17 responses during infection. Furthermore, Th17 cells have been implicated in the pathogenesis of several autoimmune diseases, such as multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis, and psoriasis [8–11]. Because the IL-17 receptor is broadly expressed and many different
Correspondence: Dr. Monika C. Wolkers e-mail: [email protected] C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
cell types can respond to this cytokine, massive tissue responses can occur upon Th17-cell activation, which can compromise the delicate balance between immunity and immunopathology [12]. Interestingly, despite their well-described role in the pathophysiology of immune-mediated inflammatory diseases, Th17 cells appear to comprise only a small fraction of infiltrates in some autoimmune diseases, as evidenced in the synovial fluid of juvenile arthritis patients and the intestine of Crohn’s disease patients [13, 14]. Th17 cells can shift to a Th1 phenotype during inflammation, losing their capacity to produce IL-17 and turning on IFN-γ production [15–17]. However, this high plasticity and functional change can only partially explain the low frequency of IL-17producing cells at the site of inflammation. An alternative explanation was recently provided by Annunziato and colleagues [18], who demonstrated that Th17 cells intrinsically exhibit a low capacity to proliferate upon activation. This is at least in part governed by their inability to produce the proliferation-inducing cytokine IL-2 [18]. Given that Th17 cells also display reduced expression levels of the T-cell receptor (TCR) subunits CD3ζ and CD3ε when compared with those in classical Th1 cells, T-cell activation is suboptimal, as is the activation of the downstream transcription www.eji-journal.eu
Eur. J. Immunol. 2014. 44: 646–649
factors. This reduction in sensitivity is controlled by RORCdependent induction of the IL-4 inducing factor 1 (IL4I1) [18]. In this issue, Santarlasci et al. [19] show that in addition to impaired IL-2 gene expression, human Th17 cells also display low expression levels of several cell cycle genes including cyclin A, B, D, E, and Cdk2, which is mediated through IL4I1-mediated TOB1 expression. TOB1 (also known as transducer of ERBB2,1) is a member of the TOB/BTG antiproliferative protein family and is constitutively expressed in naive primary human CD4+ and CD8+ T cells [20]. Upon T-cell activation, TOB1 is rapidly downregulated, allowing for proliferation and cytokine production to occur [20]. Of note, TOB1 downregulation requires full T-cell activation with TCR triggering and costimulation. Na¨ıve T cells that have solely been activated through the TCR — a condition that generates T cells defined as anergic — fail to downmodulate TOB1 levels, which results in a lack of proliferation and cytokine production [20]. Importantly, T cells that lack TOB1 due to gene silencing are fully activated by TCR triggering alone, both in terms of proliferation and effector function [20], suggesting that TOB1 acts as a rheostat and increases the threshold for na¨ıve T cells to become activated. Santarlasci et al. [19] report elevated expression of TOB1 in differentiated Th17 cells when compared with classical Th1 cells, and find that TOB1 continues to exert its immunoregulatory function in this cell type. The importance of maintaining the threshold for T-cell activation high became evident from mouse models [21]. In line with the TOB1 gene silencing studies performed in vitro, Tob1-deficient mice develop severe CNS autoimmunity and display an increased infiltration of Th1 and Th17 cells in the central nervous system, while the proportional number of infiltrating regulatory T cells decreases [21]. Data from human studies also imply that TOB1 is required to dampen T-cell activity, because downregulation of TOB1 gene expression is correlated with the progression of multiple sclerosis [22]. Interestingly, TOB1 interferes with several gene regulatory pathways to block proliferation and cytokine production. While TOB1 was originally described as a co-transcriptional regulator, accumulating data point to its role in post-transcriptional regulation. TOB1 is not only localized in the nucleus, but is also found in the cytosol where it blocks translation and RNA stability of IL-2 in T cells. First, TOB1 interacts with poly(A) binding protein (PABP). When PABP is induced upon T-cell activation it binds to the poly(A) tail of mRNAs at the 3 untranslated region (3 UTR) [23]. This binding allows for the interaction of PABP with the translational initiation complex that is bound to the 5 UTR. Due to this interaction, the mRNA circularizes and the ribosomal subunit 60S can dock to the translational initiation complex, a critical step for translation to commence [24]. Binding of TOB1 to PABP interferes with this activity, therefore TOB1 blocks translational initiation (Fig. 1), a process shown to affect IL-2 protein levels in T cells [23, 25]. Second, TOB1 interacts with CAF1 and recruits the CCR4-CAF1 complex at the 3 UTR of specific mRNAs that enhances the deadenylation, and hence the degradation of the target mRNA (Fig. 1). This mechanism has also been implicated in the regu C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
HIGHLIGHTS
Figure 1. Schematic overview of how TOB1 blocks T-cell proliferation and cytokine production. (I) TOB1 inhibits transcription of genes encoding for IL-2, cyclins A, B, D, E, and CDK2. (II) TOB1 binding to PABP prevents the circularization of mRNA in the cytosol and hence the entry of the ribosomal 60S subunit at the 5 end of the mRNA target, which blocks translation initiation. (III) TOB1 mediates the recruitment of the CAF1-CCR4 complex at the 3 end of the mRNA target in the cytosol, which enhances mRNA deadenylation, a process that results in mRNA degradation.
lation of IL-2 production in T cells [26]. The post-transcriptional regulation by TOB1 is also likely to occur in Th17 cells, but experimental evidence for this has yet to be obtained. Taken together, these data show that TOB1 interferes not only with transcription, but also with translation and the stability of target mRNA to keep protein levels low. Of note, TOB1 not only blocks IL-2 production, but also affects the protein levels of IFN-γ, IL-4, and IL-17 in activated T cells [20, 21]. Provided that almost all mRNAs contain a poly(A) tail, including cyclins and CDK2, it is tempting to speculate that TOB1 also regulates the translational block and the RNA breakdown of these genes. Possibly, the combination of TOB1 activity on transcription and on post-transcriptional regulation allows for a strong safeguard to keep gene expression in Th17 cells in check. The importance of post-transcriptional regulation to maintain and modulate immune cell function has only recently been appreciated [27, 28]. Our own data that focus on CD8+ T-cell responses provide evidence that post-transcriptional regulation is crucial to keep IFN-γ production, and that of other effector cytokines, in check (Salerno et al. unpublished data). Specifically, disrupting the regulatory sequences within the 3 UTR of IFN-γ results in hyper-responsiveness of memory CD8+ T cells and in leakiness www.eji-journal.eu
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of IFN-γ production in situ. With that in mind, it is tempting to speculate whether the TOB1-mediated block in cellular proliferation and cytokine production is specific to differentiated Th17 cells, or whether it also applies to other differentiated T-cell subsets. Interestingly, not only CD4+ T cells, but also CD8+ T cells express TOB1, and TOB1 expression levels are downregulated upon T-cell activation in both cell types [20]. While TOB1 is expressed in memory CD8+ T cells, and in tissueresident memory CD8+ or CD4+ T cells, we do not find indications in gene expression data that TOB1 is differentially expressed when compared with other T-cell subsets. Interestingly, its paralogue, TOB2, is significantly increased in tissue-resident memory T cells when compared with peripheral blood-derived memory T cells, and in CMV-specific memory CD8+ T cells compared to na¨ıve and effector CMV-specific T cells (Hombrink and van Lier, unpublished data). Provided that TOB1 and TOB2 are interchangeable in their interaction with PABP and CCR4-CAF1 to block IL-2 production [29], TOB2 activity may be more prominent in tissue-resident memory T cells to reduce their proliferative capacity and excessive cytokine production to prevent tissue damage. As Th17 cells are also mainly found in tissues, it is tempting to speculate whether a block in cellular proliferation through the TOB proteins represents a general feature of tissue-resident T cells, and would also account for, as an example, Th9 cells, which are mainly skin-tropic. In conclusion, TOB1 possibly blocks Th17-cell proliferation through many pathways including transcription, translation initiation, and mRNA decay, thereby providing “extra security” in keeping Th17 cells in check. Whether other annotated transcriptional regulators also have the capacity to modulate gene expression not only through transcription but also by modulating posttranscriptional events is hardly explored and may lead to novel insights in cellular processes.
Eur. J. Immunol. 2014. 44: 646–649
for mucosal host defense against oral candidiasis. J. Exp. Med. 2009. 206: 299–311. 3 Huang, W., Na, L., Fidel, P. L. and Schwarzenberger, P., Requirement of interleukin-17A for systemic anti-Candida albicans host defense in mice. J. Infect. Dis. 2004. 190: 624–631. 4 Puel, A., Cypowyj, S., Bustamante, J., Wright, J. F., Liu, L., Lim, H. K., Migaud, M. et al., Chronic mucocutaneous candidiasis in humans with inborn errors of interleukin-17 immunity. Science 2011. 332: 65–68. 5 Moore-Connors, J. M., Fraser, R., Halperin, S. A. and Wang, J., CD4(+)CD25(+)Foxp3(+) regulatory T cells promote Th17 responses and genital tract inflammation upon intracellular Chlamydia muridarum infection. J. Immunol. 2013. 191: 3430–3439. 6 Whibley, N., Maccallum, D. M., Vickers, M. A., Zafreen, S., Waldmann, H., Shohei, H., Gaffen, S. L. et al., Expansion of Foxp3 T-cell populations by Candida albicans enhances both Th17-cell responses and fungal dissemination after intravenous challenge. Eur. J. Immunol. 2014. In press. doi: 10.1002/eji.201343604. 7 Zelante, T., De Luca, A., Bonifazi, P., Montagnoli, C., Bozza, S., Moretti, S., Belladonna, M. L. et al., IL-23 and the Th17 pathway promote inflammation and impair antifungal immune resistance. Eur. J. Immunol. 2007. 37: 2695–2706. 8 Kebir, H., Kreymborg, K., Ifergan, I., Dodelet-Devillers, A., Cayrol, R., Bernard, M., Giuliani, F. et al., Human TH17 lymphocytes promote bloodbrain barrier disruption and central nervous system inflammation. Nat. Med. 2007. 13: 1173–1175. 9 Dubin, P. J. and Kolls, J. K., Th17 cytokines and mucosal immunity. Immunol. Rev. 2008. 226: 160–171. 10 Murphy, C. A., Langrish, C. L., Chen, Y., Blumenschein, W., McClanahan, T., Kastelein, R. A., Sedgwick, J. D. et al., Divergent pro- and antiinflammatory roles for IL-23 and IL-12 in joint autoimmune inflammation. J. Exp. Med. 2003. 198: 1951–1957. 11 Nishimoto, S., Kotani, H., Tsuruta, S., Shimizu, N., Ito, M., Shichita, T., Morita, R. et al., Th17 cells carrying TCR recognizing epidermal autoantigen induce psoriasis-like skin inflammation. J. Immunol. 2013. 191: 3065– 3072. 12 Korn, T., Bettelli, E., Oukka, M. and Kuchroo, V. K., IL-17 and Th17 Cells. Annu. Rev. Immunol. 2009. 27: 485–517. 13 Annunziato, F., Cosmi, L., Santarlasci, V., Maggi, L., Liotta, F., Mazzinghi, B., Parente, E. et al., Phenotypic and functional features of human Th17
Acknowledgments: The authors thank P. Hombrink for critical reading of the manuscript. This work was supported by a fellowship from the Landsteiner Foundation for Blood Research (LSBR 1373) to M.C.W.
cells. J. Exp. Med. 2007. 204: 1849–1861. 14 Cosmi, L., Cimaz, R., Maggi, L., Santarlasci, V., Capone, M., Borriello, F., Frosali, F. et al., Evidence of the transient nature of the Th17 phenotype of CD4+CD161+ T cells in the synovial fluid of patients with juvenile idiopathic arthritis. Arthritis. Rheum. 2011. 63: 2504–2515. 15 Kurschus, F. C., Croxford, A. L., Heinen, A. P., Wortge, S., Ielo, D. and
Conflict of interest: The authors declare no financial or commercial conflict of interest.
Waisman, A., Genetic proof for the transient nature of the Th17 phenotype. Eur. J. Immunol. 2010. 40: 3336–3346. 16 Murphy, K. M. and Stockinger, B., Effector T cell plasticity: flexibility in the face of changing circumstances. Nat. Immunol. 2010. 11:
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674–680. 17 Peck, A. and Mellins, E. D., Plasticity of T-cell phenotype and function: the T helper type 17 example. Immunology. 2009. 129: 147–153.
Hernandez-Santos, N., Edgerton, M. et al., CD4(+)CD25(+)Foxp3(+) reg-
18 Santarlasci, V., Maggi, L., Capone, M., Querci, V., Beltrame, L., Cavalieri,
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2 Conti, H. R., Shen, F., Nayyar, N., Stocum, E., Sun, J. N., Lindemann, M.
19 Santarlasci, V., Maggi, L., Mazzoni, A., Capone, M., Querci, V., Rossi,
J., Ho, A. W. et al., Th17 cells and IL-17 receptor signaling are essential
M. C., Beltrame, L. et al., IL-4-induced gene 1 maintains high Tob1
C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eji-journal.eu
HIGHLIGHTS
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expression that contributes to TCR unresponsiveness in human T helper 17 cells. Eur. J. Immunol. 2014. 654–661. 20 Tzachanis, D., Freeman, G. J., Hirano, N., van Puijenbroek, A. A., Delfs, M. W., Berezovskaya, A., Nadler, L. M. et al., Tob is a negative regulator of activation that is expressed in anergic and quiescent T cells. Nat. Immunol. 2001. 2: 1174–1182. 21 Schulze-Topphoff, U., Casazza, S., Varrin-Doyer, M., Michel, K., Sobel, R. A., Hauser, S. L., Oksenberg, J. R. et al., Tob1 plays a critical role in the activation of encephalitogenic T cells in CNS autoimmunity. J. Exp. Med. 2013. 210: 1301–1309.
factor, is a poly(A)-binding protein-dependent positive regulator of cytoplasmic mRNA deadenylation. Mol. Cell. Biol. 2007. 27: 7791–7801. 27 Anderson, P., Post-transcriptional regulons coordinate the initiation and resolution of inflammation. Nat. Rev. Immunol. 2010. 10: 24–35. 28 Turner, M. and Hodson, D., Regulation of lymphocyte development and function by RNA-binding proteins. Curr. Opin. Immunol. 2012. 24: 160–165. 29 Ezzeddine, N., Chen, C. Y. and Shyu, A. B., Evidence providing new insights into TOB-promoted deadenylation and supporting a link between TOB’s deadenylation-enhancing and antiproliferative activities. Mol. Cell. Biol. 2012. 32: 1089–1098.
22 Corvol, J. C., Pelletier, D., Henry, R. G., Caillier, S. J., Wang, J., Pappas, D., Casazza, S. et al., Abrogation of T cell quiescence characterizes patients
Abbreviations: PABP: poly(A) binding protein · 3 UTR: 3 untranslated
at high risk for multiple sclerosis after the initial neurological event. Proc.
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Natl. Acad. Sci. U. S. A. 2008. 105: 11839–11844. 23 Okochi, K., Suzuki, T., Inoue, J., Matsuda, S. and Yamamoto, T., Interaction of anti-proliferative protein Tob with poly(A)-binding protein and inducible poly(A)-binding protein: implication of Tob in translational control. Genes Cells 2005. 10: 151–163. 24 Mangus, D. A., Evans, M. C. and Jacobson, A., Poly(A)-binding proteins: multifunctional scaffolds for the post-transcriptional control of gene expression. Genome. Biol. 2003. 4: 223.
Full correspondence: Dr. Monika. Wolkers. Department of Hematopoiesis, Sanquin Research/Landsteiner laboratory AMC. Plesmanlaan 125, 1066CX Amsterdam, The Netherlands Fax: +31-20-5123474 e-mail: [email protected] See accompanying article: http://dx.doi.org/10.1002/eji.201344047
25 Yang, H., Duckett, C. S. and Lindsten, T., iPABP, an inducible poly(A)binding protein detected in activated human T cells. Mol. Cell. Biol. 1995. 15: 6770–6776. 26 Ezzeddine, N., Chang, T. C., Zhu, W., Yamashita, A., Chen, C. Y., Zhong, Z., Yamashita, Y. et al., Human TOB, an antiproliferative transcription
C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: 28/1/2014 Revised: 28/1/2014 Accepted: 30/1/2014 Accepted article online: 4/2/2014
www.eji-journal.eu
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DOI: 10.1002/eji.201444465
Eur. J. Immunol. 2014. 44: 646–649
Commentary
Better safe than sorry: TOB1 employs multiple parallel regulatory pathways to keep Th17 cells quiet Fiamma Salerno, Ren´e A. W. van Lier and Monika C. Wolkers Department of Hematopoiesis, Sanquin Research/Landsteiner laboratory AMC, Amsterdam, The Netherlands Th17 cells are key players in antibacterial and antifungal immunity, but have also been implicated in autoimmunity. Interestingly, Th17 cells poorly proliferate upon stimulation, a phenotype that was attributed to a decreased sensitivity to T-cell receptor (TCR) stimulation, and to low IL-2 production by Th17 cells. In this issue of the European Journal of Immunology, Santarlasci et al. [Eur. J. Immunol. 2014. 44: 654–661] shed further light on the molecular mechanism that keeps Th17 cells at bay. They identify the transcriptional regulator TOB1, which not only impairs IL-2 production in Th17 cells, but also blocks the expression of cell cycle genes. Strikingly, TOB1 suppresses Th17-cell proliferation through several pathways, including impaired signal transduction, transcription, and possibly also post-transcriptional regulation.
Keywords: Gene regulation r Proliferation r Th17 cells
r
TOB1
See accompanying article by Santarlasci et al.
Th17 cells are located at mucosal and epithelial barriers where they respond to bacterial and fungal intruders. They do so by producing the proinflammatory cytokine interleukin-17 (IL-17). IL-17 potentiates chemokine production and thereby enhances the attraction of monocytes and neutrophils, which in turn help to clear the infection [1]. The importance of IL-17 in infection is evident from mouse studies and from human deficiency disorders where deficiency in IL-17, or the IL-17 receptor, results in systemic Candida albicans infection [2–4]. Conversely, Th17-cell responses are also associated with increased pathology in several infection models, such as Candida albicans, Aspergillus fumigatus, and Chlamydia muridarum [5–7], stressing the importance of controlling the magnitude of Th17 responses during infection. Furthermore, Th17 cells have been implicated in the pathogenesis of several autoimmune diseases, such as multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis, and psoriasis [8–11]. Because the IL-17 receptor is broadly expressed and many different
Correspondence: Dr. Monika C. Wolkers e-mail: [email protected] C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
cell types can respond to this cytokine, massive tissue responses can occur upon Th17-cell activation, which can compromise the delicate balance between immunity and immunopathology [12]. Interestingly, despite their well-described role in the pathophysiology of immune-mediated inflammatory diseases, Th17 cells appear to comprise only a small fraction of infiltrates in some autoimmune diseases, as evidenced in the synovial fluid of juvenile arthritis patients and the intestine of Crohn’s disease patients [13, 14]. Th17 cells can shift to a Th1 phenotype during inflammation, losing their capacity to produce IL-17 and turning on IFN-γ production [15–17]. However, this high plasticity and functional change can only partially explain the low frequency of IL-17producing cells at the site of inflammation. An alternative explanation was recently provided by Annunziato and colleagues [18], who demonstrated that Th17 cells intrinsically exhibit a low capacity to proliferate upon activation. This is at least in part governed by their inability to produce the proliferation-inducing cytokine IL-2 [18]. Given that Th17 cells also display reduced expression levels of the T-cell receptor (TCR) subunits CD3ζ and CD3ε when compared with those in classical Th1 cells, T-cell activation is suboptimal, as is the activation of the downstream transcription www.eji-journal.eu
Eur. J. Immunol. 2014. 44: 646–649
factors. This reduction in sensitivity is controlled by RORCdependent induction of the IL-4 inducing factor 1 (IL4I1) [18]. In this issue, Santarlasci et al. [19] show that in addition to impaired IL-2 gene expression, human Th17 cells also display low expression levels of several cell cycle genes including cyclin A, B, D, E, and Cdk2, which is mediated through IL4I1-mediated TOB1 expression. TOB1 (also known as transducer of ERBB2,1) is a member of the TOB/BTG antiproliferative protein family and is constitutively expressed in naive primary human CD4+ and CD8+ T cells [20]. Upon T-cell activation, TOB1 is rapidly downregulated, allowing for proliferation and cytokine production to occur [20]. Of note, TOB1 downregulation requires full T-cell activation with TCR triggering and costimulation. Na¨ıve T cells that have solely been activated through the TCR — a condition that generates T cells defined as anergic — fail to downmodulate TOB1 levels, which results in a lack of proliferation and cytokine production [20]. Importantly, T cells that lack TOB1 due to gene silencing are fully activated by TCR triggering alone, both in terms of proliferation and effector function [20], suggesting that TOB1 acts as a rheostat and increases the threshold for na¨ıve T cells to become activated. Santarlasci et al. [19] report elevated expression of TOB1 in differentiated Th17 cells when compared with classical Th1 cells, and find that TOB1 continues to exert its immunoregulatory function in this cell type. The importance of maintaining the threshold for T-cell activation high became evident from mouse models [21]. In line with the TOB1 gene silencing studies performed in vitro, Tob1-deficient mice develop severe CNS autoimmunity and display an increased infiltration of Th1 and Th17 cells in the central nervous system, while the proportional number of infiltrating regulatory T cells decreases [21]. Data from human studies also imply that TOB1 is required to dampen T-cell activity, because downregulation of TOB1 gene expression is correlated with the progression of multiple sclerosis [22]. Interestingly, TOB1 interferes with several gene regulatory pathways to block proliferation and cytokine production. While TOB1 was originally described as a co-transcriptional regulator, accumulating data point to its role in post-transcriptional regulation. TOB1 is not only localized in the nucleus, but is also found in the cytosol where it blocks translation and RNA stability of IL-2 in T cells. First, TOB1 interacts with poly(A) binding protein (PABP). When PABP is induced upon T-cell activation it binds to the poly(A) tail of mRNAs at the 3 untranslated region (3 UTR) [23]. This binding allows for the interaction of PABP with the translational initiation complex that is bound to the 5 UTR. Due to this interaction, the mRNA circularizes and the ribosomal subunit 60S can dock to the translational initiation complex, a critical step for translation to commence [24]. Binding of TOB1 to PABP interferes with this activity, therefore TOB1 blocks translational initiation (Fig. 1), a process shown to affect IL-2 protein levels in T cells [23, 25]. Second, TOB1 interacts with CAF1 and recruits the CCR4-CAF1 complex at the 3 UTR of specific mRNAs that enhances the deadenylation, and hence the degradation of the target mRNA (Fig. 1). This mechanism has also been implicated in the regu C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
HIGHLIGHTS
Figure 1. Schematic overview of how TOB1 blocks T-cell proliferation and cytokine production. (I) TOB1 inhibits transcription of genes encoding for IL-2, cyclins A, B, D, E, and CDK2. (II) TOB1 binding to PABP prevents the circularization of mRNA in the cytosol and hence the entry of the ribosomal 60S subunit at the 5 end of the mRNA target, which blocks translation initiation. (III) TOB1 mediates the recruitment of the CAF1-CCR4 complex at the 3 end of the mRNA target in the cytosol, which enhances mRNA deadenylation, a process that results in mRNA degradation.
lation of IL-2 production in T cells [26]. The post-transcriptional regulation by TOB1 is also likely to occur in Th17 cells, but experimental evidence for this has yet to be obtained. Taken together, these data show that TOB1 interferes not only with transcription, but also with translation and the stability of target mRNA to keep protein levels low. Of note, TOB1 not only blocks IL-2 production, but also affects the protein levels of IFN-γ, IL-4, and IL-17 in activated T cells [20, 21]. Provided that almost all mRNAs contain a poly(A) tail, including cyclins and CDK2, it is tempting to speculate that TOB1 also regulates the translational block and the RNA breakdown of these genes. Possibly, the combination of TOB1 activity on transcription and on post-transcriptional regulation allows for a strong safeguard to keep gene expression in Th17 cells in check. The importance of post-transcriptional regulation to maintain and modulate immune cell function has only recently been appreciated [27, 28]. Our own data that focus on CD8+ T-cell responses provide evidence that post-transcriptional regulation is crucial to keep IFN-γ production, and that of other effector cytokines, in check (Salerno et al. unpublished data). Specifically, disrupting the regulatory sequences within the 3 UTR of IFN-γ results in hyper-responsiveness of memory CD8+ T cells and in leakiness www.eji-journal.eu
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of IFN-γ production in situ. With that in mind, it is tempting to speculate whether the TOB1-mediated block in cellular proliferation and cytokine production is specific to differentiated Th17 cells, or whether it also applies to other differentiated T-cell subsets. Interestingly, not only CD4+ T cells, but also CD8+ T cells express TOB1, and TOB1 expression levels are downregulated upon T-cell activation in both cell types [20]. While TOB1 is expressed in memory CD8+ T cells, and in tissueresident memory CD8+ or CD4+ T cells, we do not find indications in gene expression data that TOB1 is differentially expressed when compared with other T-cell subsets. Interestingly, its paralogue, TOB2, is significantly increased in tissue-resident memory T cells when compared with peripheral blood-derived memory T cells, and in CMV-specific memory CD8+ T cells compared to na¨ıve and effector CMV-specific T cells (Hombrink and van Lier, unpublished data). Provided that TOB1 and TOB2 are interchangeable in their interaction with PABP and CCR4-CAF1 to block IL-2 production [29], TOB2 activity may be more prominent in tissue-resident memory T cells to reduce their proliferative capacity and excessive cytokine production to prevent tissue damage. As Th17 cells are also mainly found in tissues, it is tempting to speculate whether a block in cellular proliferation through the TOB proteins represents a general feature of tissue-resident T cells, and would also account for, as an example, Th9 cells, which are mainly skin-tropic. In conclusion, TOB1 possibly blocks Th17-cell proliferation through many pathways including transcription, translation initiation, and mRNA decay, thereby providing “extra security” in keeping Th17 cells in check. Whether other annotated transcriptional regulators also have the capacity to modulate gene expression not only through transcription but also by modulating posttranscriptional events is hardly explored and may lead to novel insights in cellular processes.
Eur. J. Immunol. 2014. 44: 646–649
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Acknowledgments: The authors thank P. Hombrink for critical reading of the manuscript. This work was supported by a fellowship from the Landsteiner Foundation for Blood Research (LSBR 1373) to M.C.W.
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Conflict of interest: The authors declare no financial or commercial conflict of interest.
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Received: 28/1/2014 Revised: 28/1/2014 Accepted: 30/1/2014 Accepted article online: 4/2/2014
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