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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References

T helper 17 (TH17) cells have well-described roles in autoimmune diseases. The immune modulations of development and function of TH17 have become a key issue. In this review, we summarize the recent findings regarding the direct and indirect signalling regulatory mechanisms of TH17 cells in the general mouse model of autoimmune diseases and other human diseases.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References

CD4+ T helper (TH) cells now include different types based on their cytokine and transcription factor signatures: the cytokines interleukin (IL)-12 and interferon (IFN)-γ and the transcription factors STAT1, STAT4 and T-bet promote the development of TH1 cells, which produce IFN-γ as their ‘signature’ cytokine. The cytokine IL-4, together with the transcription factors STAT6 and GATA3, promotes the development of TH2 cells, which produce IL-4, IL-5 and IL-13[1, 2]. Natural regulatory T (nTreg) cells, which develop in the thymus, gained acceptance as a bona fide CD4+ T cell lineage with the discovery that the transcription factor Foxp3 directs their development. Induced Treg (iTreg) cells are generated by transforming growth factor (TGF)-β and retinoic acid in the periphery and, like nTreg cells, can produce TGF-β, IL-10 and IL-35[3]. Follicular helper T (TFH) cells can develop under the influence of IL-6 by the induction of the transcription factor Bcl-6 [4]. Largely through their capacity to induce expression of the transcription factors related orphan receptor-γt (RORγt), RORα and STAT3, the cytokines TGF-β, IL-6 and IL-23 promote the development of TH17 cells, which produce IL-17A, IL-17F and IL-22. IL-21 is principally a product of TH17 and TFH cells and further promotes, through an autocrine feedback loop, the development of these cells [5–7]. The differentiation of TH17 cells from naïve CD4+ T cells is regulated directly by cytokines and transcriptional factors and indirectly by other immune cells [5, 7]. TGF-β and IL-6, broadly expressed by many cell types, including dendritic cells (DCs) and epithelial cells, are dominant in the initiation of TH17 cell differentiation. IL-23, IL-1β and IL-21, which are products of activated DCs, macrophages, activated T cells or inflamed epithelial cells, possibly expand and maintain the differentiated TH17 cells in the presence of IL-6 and TGF-β1. TH17 cells appear to be key effector T cells in a variety of human inflammatory and autoimmune diseases as well as in experimental animal models. Therefore, to totally understand the accurate immunoregulatory mechanism of TH17 cells, this should become an important research issue.

Direct regulatory signalling pathway of TH 17 cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References

Cytokine-dependent modulation of TH17 differentiation and function

The functional differentiation of TH cells is instructed by the innate immune system, which provides co-stimulatory molecules to allow for optimal T cell activation and proliferation and cytokine production that mediates the development of TH cell lineages in response to antigen [8, 9]. The co-stimulatory receptors CD28 and inducible T cell co-stimulator (ICOS) have both been shown to be important for the generation of TH17 cells, similar to TH2 cells, whereas TH1 cell differentiation might only require one of these co-stimulatory molecules [10]. Now, significant progress has been made in understanding the unique cytokine requirements for the development of TH17 cells in mice and humans.

TGF-β and IL-6

Research results showed that although TGF-β induces the expression of Foxp3, IL-6 inhibits its expression [11]. Moreover, IL-6 together with TGF-β induced the optimal expression of IL-17 by activated T cells (Fig. 1). So, inducible regulatory T cells and TH17 cells are reciprocally regulated during differentiation and share TGF-β as a common inducer [11, 12]. Further studies reported that over-expression of TGF-β in T cells resulted in increased differentiation of TH17 cells and in TH17 cell–mediated experimental autoimmune encephalomyelitis (EAE) [11, 13]. More clearly, from T cell–specific TGF-β1 gene knockout mouse, results showed that T cell–derived TGF-β was indispensable for TH17 cell differentiation and induction of EAE in mice [13–15]. Basically, researchers have believed that TGF-β orchestrates in vitro TH17 and Treg cell differentiation programmes in a concentration-dependent manner. IL-6 was believed to be necessary for the induction and maintenance of TH17 cells. And the loss of IL-6 during autoimmune responses resulted in significantly increased numbers of Foxp3+ Treg [16–18]. Most of these studies used IL-17 expression as a marker of TH17 cell differentiation, but it is becoming evident that although TGF-β and IL-6 synergistically regulate transcription of the genes encoding IL-17 and IL-17F, many other TH17 cell–specific genes are regulated by either exogenous TGF-β or IL-6. Moreover, results showed TGF-β deficiency resulted in decreased TH17 cell differentiation but greatly increased IFN-γ production [14, 19, 20]. It indicates that the requirement for TGF-β in TH17 cell differentiation is attributable to the inhibition of TH1 cell differentiation, which otherwise would strongly inhibit the differentiation of TH17 cells through IFN-γ production or the activity of transcription factors associated with TH1 cells. To understand the downstream mediators of TGF-β signalling in TH17 cell differentiation is a crucial next step.

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Figure 1.  Cytokine-dependent regulation of TH17 cells. Naïve CD4+ T cells, after activation by signalling through the T cell receptor and co-stimulatory molecules such as CD28 and inducible T cell co-stimulator (ICOS), can differentiate into TH17 cells. Transforming growth factor-β, IL-6, IL-23 and IL-1 contribute to the differentiation of TH17 cells, which can produce effective cytokine IL-17, IL-17F, IL-21, IL-22 and CCL20.

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IL-21

IL-21 has been shown to regulate the function of T cells, B cells, natural killer cells and DCs in immune responses [21]. Although activated CD4+ T cells produce IL-21, recent data suggest that TH17 cells are the major producers in immune responses. IL-21 was reported not only to be expressed by TH17 cells but also to regulate the differentiation of TH17 cells in vitro and in vivo [22]. IL-6-deficient mice have greatly decreased IL-21 expression in vivo [23]. In vitro, in conjunction with T cell antigen receptor (TCR) activation, the pro-inflammatory cytokine IL-6 induces the expression of IL-21 in naïve CD4+ T cells [24]. IL-21, in turn, induces its own expression in an autocrine manner, acting through the IL-21 receptor that is induced by TCR stimulation (Fig. 1). Intriguingly, the absence of IL-21 signalling appears to have little effect on TH17 cell differentiation in vivo [24, 25], suggesting that some factors (e.g. IL-6) may compensate for the loss of IL-21 signalling. In humans, results showed that CCR6+ CD4+ T cells produce high levels of both IL-21 and IL-17. Similar to mouse T cells, IL-21 autoregulates its own production in human CD4+ T cells. IL-21 potently enhances TH17 proliferation and suppresses Foxp3 expression [26]. IL-21 is therefore an autocrine cytokine that regulates human TH17 and serves as a good target for treating this autoimmune disease.

IL-23

IL-23 is a heterodimeric cytokine composed of the IL-23-specific subunit p19 and the p40 subunit, which also constitutes part of IL-12. Our current limited understanding of the roles of IL-23 and other cytokines in the differentiation of TH17 cells is largely derived from in vitro studies. IL-21 (but not IL-6) also induces the expression of the IL-23R. Together with TGF-β, IL-21, independently of IL-6, can induce IL-17 in mice and in humans [11, 27]. In line with these findings, IL-21R-deficient cells expressed about threefold less IL-17 upon stimulation of IL-6 plus TGF-β, and IL-6-induced IL-23R expression was markedly reduced in IL-21R-deficient cells [9, 28, 29]. These results suggested that IL-21 contributes to TH17 cell differentiation and serves as an intermediary mediator for IL-6-initiated signalling in the induction of IL-23R. Once IL-23R is up-regulated by cytokines such as IL-21, then IL-23 plus TGF-β are capable of inducing IL-17 expression. IL-23 further up-regulates IL-23R expression, therefore imposing another amplifying loop and contributing to induction of TH17 cells. To unravel the role of IL-23 in subgroup polarization of TH17 and/or TH1 cells, using the prone autoimmune DBA-1 mice with and without collagen-induced arthritis, it was shown that in splenic CD4+ T cells from naïve DBA-1 mice, IL-23 alone hardly induced TH17 polarization and TH17 cytokines but inhibited T-bet expression [28]. In contrast, TGF-β/IL-6 was a potent inducer of IL-17A, IL-17F, IL-21 and Foxp3 in these cells. IL-23 in contrast to TGF-β/IL-6 was critical for the induction of IL-22 in splenic CD4+ T cells from both naïve and collagen-immunized DBA-1 mice. In line with these findings, IL-23 was more pronounced in inducing the IL-17A+ IFN-γ subset in CD4+ T cells from collagen-immunized mice. However, in CD4+ T cells from naïve DBA-1 mice, IL-23 significantly increased the TGF-β-/IL-6-induced TH17 polarization including elevated IL-17A and IL-17F levels and decreased T-bet and Foxp3 expression [30, 31]. This indicates that IL-23 promotes TH17 differentiation by inhibiting T-bet and Foxp3 and is required for the elevation of IL-22 but not IL-21 in autoimmune arthritis. Altogether, these data suggest that IL-23 may function at a late stage of TH17 cell differentiation after initial induction by other pro-inflammatory cytokines (e.g. IL-6 and IL-21; Fig. 1). Further, two recent studies showed that IL-23 could induce the production of the cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) and contribute to the TH17 cells, whereas IL-12, IFN-γ and IL-27 acted as negative regulators [32, 33]. Such cross-regulation of IL-23 and GM-CSF explains the similar pattern of resistance to autoimmunity when either of the two cytokines is absent and identifies TH17 cells as a crucial source of GM-CSF in autoimmune inflammation.

Transcription factor signalling in regulating TH17 differentiation and function

RORγt

Results showed that the orphan nuclear receptor retinoic acid receptor–RORγt, one subtype of RORγ, is the transcription factor that directs the differentiation of inflammatory TH17. TH17 development in the gut requires RORγt expression in CD4+ T cells [31, 34–37]. RORγt expression results from the action of IL-6 and TGF-β (but not IL-23) produced by activated DCs and other cells in the lamina propria. DCs can be activated by signals derived from the luminal flora or infectious agents and TLR ligands that gain access to the lamina propria [35]. IL-6 may also inhibit TGF-β-induced differentiation of iTregs, thus further promoting TH17 development. RORγt+ T cells up-regulate IL-23R and thus become responsive to IL-23 [35, 38]. IL-23 reinforces the TH17 phenotype by possibly helping in maintenance, expansion or further differentiation of the cells. In vitro, IL-6 plus TGF-β treatment-induced IL-17 expression requires induction of RORγt, and forced expression of RORγt is sufficient to induce IL-17 expression in the absence of any exogenous cytokines [31]. Interestingly, RORγt-deficient mice develop less severe autoimmune diseases and specifically lack TH17 cells in the inflammatory tissues [34, 38, 39]. RORα, another ROR family member, is also up-regulated during in vitro TH17 differentiation. Although the over-expression of RORα is sufficient to induce IL-17, lack of RORα has only a minor effect on TH17 cell differentiation [30, 40]. These two closely related transcription factors, which presumably share the same DNA-binding sequence, may have similar function in TH17 cell differentiation (Fig. 2).

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Figure 2.  Transcriptional factor signalling in regulating the TH17 cells. Differentiation of TH17 cells is initiated by signal transducer and activator of transcription 3 (STAT3), downstream of IL-6 and IL-21 induced signalling. Activation of STAT3 induces the expression of related orphan receptor-γt (RORγt) and RORα. STAT5, which is downstream of IL-2 signalling, is a negative regulator of TH17 cell differentiation.

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JAK-STAT pathway

IL-6, IL-21 and IL-23 signalling all utilize the JAK-STAT pathway and activate signal transducer and activator of transcription 3 (STAT3). STAT3 binds to the IL-17A promoter directly as shown by ChIP [41]. STAT3 is also required for the induction of RORγt by cytokines [35, 42]. Over-expression of RORγt can partially rescue IL-17 expression in STAT3-deficient cells, suggesting that RORγt may function downstream of STAT3 to induce IL-17 expression [42, 43] (Fig. 2). Now, STAT3 signalling has been regarded as a critical component of TH17-dependent autoimmune processes. Genome-wide association studies (GWAS) have revealed the role of the STAT3 gene in inflammatory bowel disease (IBD) susceptibility, although confirmation in clinical subphenotypes is warranted [44]. Mice with targeted deletion of STAT3 in T cells are resistant to EAE, which is a multiple sclerosis (MS) model [28]. In human, increased phosphorylated STAT3 was reported in T cells of patients evolving from clinically isolated syndrome to defined MS and in relapsing patients [45]. Further evidence showed the role of STAT3 in Crohn’s disease (CD), ulcerative colitis (UC) and MS risk. Polymorphisms in the STAT3 region were genotyped, and the inferred haplotypes were subsequently analysed in patients with IBD and MS. The haplotype conformed by the risk alleles of each polymorphism was significantly associated with both clinical phenotypes of IBD [46–48]. No evidence of association was detected for MS. The originally described association of IBD with STAT3 polymorphisms is corroborated for the two clinical phenotypes, CD and UC, in an independent population. A major role of this gene in MS seems unlikely [46–48]. Moreover, direct and indirect inhibitions of the JAKs, with small molecule inhibitors like CP-690550 and INCB018424, have demonstrated rapid and sustained improvement in clinical measures of disease, consistent with their respective preclinical experiments [48, 49]. Furthermore, to maximize therapeutic opportunities, some studies were done to identify optimized JAK inhibitors with unique profiles. INCB028050 is a selective orally bioavailable JAK1/JAK2 inhibitor with nanomolar potency against JAK1 and JAK2. Significant efficacy was achieved in the rat adjuvant arthritis model with doses of INCB028050 providing partial and/or periodic inhibition of JAK1/JAK2 and no inhibition of JAK3 [50, 51]. Diminution of inflammatory TH17-associated cytokine mRNA levels was observed in the draining lymph nodes of treated rats. INCB028050 was also effective in multiple murine models of arthritis [51, 52]. This suggests that fractional inhibition of JAK1 and JAK2 is sufficient for significant activity in autoimmune disease models. A newly published evidence shows that Zn suppresses TH17-mediated autoimmune diseases at least in part by inhibiting the development of TH17 cells via attenuating STAT3 activation. In mice injected with collagen II to induce arthritis, Zn treatment inhibited TH17 cell development. IL-6-mediated activation of STAT3 and in vitro TH17 cell development were all suppressed by Zn [53]. Importantly, Zn binding changed the alpha-helical secondary structure of STAT3, disrupting the association of STAT3 with JAK2 kinase and with a phospho-peptide that included a STAT3-binding motif from the IL-6 signal transducer gp130 [53]. Thus, it suggests that Zn suppresses STAT3 activation, which is a critical step for TH17 development. Therefore, the JAK-STAT pathway and activated STAT3 can be involved in the TH17 development and function.

Other factors

IRF4-deficient mice were protected from EAE, and T cells from these animals failed to differentiate into TH17 cells [54]. RORγt and RORα induction was impaired in IRF4-deficient T cells, but over-expression of IRF4 could partially restore induction of IL-17, suggesting that IRF-4 may function upstream of the nuclear receptors [29] (Fig. 2).

AhR, a mediator of the effects of environmental toxins (e.g. dioxin, a polycyclic aromatic hydrocarbon xenobiotic compound), is a ligand-dependent transcription factor that is structurally distinct from the nuclear receptor superfamily (Fig. 2). Upon binding with its ligand, cytosolic AhR translocates into the nucleus, heterodimerizes with its partner aryl hydrocarbon receptor nuclear translocator (ARNT) and turns on transcription of its target genes [55] (Fig. 2). Analysis of AhR-deficient cells has shown that it is required for IL-22 and, to a lesser extent, IL-17 expression in TH17-polarizing conditions in the presence of either dioxin or FICZ [56, 57].

Transforming growth factor-β has an essential role in the generation of iTreg and TH17 cells. Recent studies showed that CD4+ T cells lacking the molecular adaptor TNF receptor–associated factor 6 (TRAF6) exhibit a specific increase in TH17 differentiation in vivo and in vitro [58]. Especially, this is related to driving the early differentiation of these cell populations. TRAF6 deficiency renders T cells more sensitive to TGF-β-induced Smad2/3 activation and proliferation arrest. Consistent with this, in TRAF6-deficient T cells, TGF-β more effectively down-regulates IL-2, a known inhibitor of TH17 differentiation [59, 60]. Remarkably, TRAF6-deficient cells generate normal numbers of Foxp3-expressing cells in iTreg differentiation conditions where exogenous IL-2 is supplied [58, 61]. Thus, the data also suggest that a major function of TGF-β in early TH17 differentiation may be the inhibition of autocrine and paracrine IL-2-mediated suppression of TH17 cell generation (Fig. 2).

Chemokines and their receptors

Chemokines and receptors also have definitive regulatory effects on TH17 cell function. CCR6 is expressed on an IL-10-producing, autoreactive memory T cell population with context-dependent regulatory function [62]. T cells recruited to the kidney contribute to tissue damage in crescentic and proliferative glomerulonephritides. As we known, chemokines and their receptors regulate T cell trafficking, but the recent study indicates the expression profile and functional importance of chemokine receptors for renal CD4+ T cell subsets. Results showed that renal Treg and TH17 cells express the chemokine receptor CCR6, whereas IFN-γ-producing TH1 cells are CCR6 negative. Induction of experimental glomerulonephritis (nephrotoxic nephritis) in mice resulted in up-regulation of the only CCR6 ligand, CCL20, followed by T cell recruitment, renal tissue injury, albuminuria and loss of renal function [62, 63]. CCR6 deficiency aggravated renal injury and increased mortality (from uraemia) among nephritic mice [64]. Compared with wild-type mice, CCR6 deficiency reduced infiltration of Tregs and TH17 cells but did not affect recruitment of TH1 cells in the setting of glomerulonephritis [65]. Adoptive transfer of wild-type but not CCR6-deficient Tregs attenuated morphologic and functional renal injury in nephritic mice. Furthermore, reconstitution with wild-type Tregs protected CCR6-negative mice from aggravated nephritis [66]. Thus, CCR6 mediates renal recruitment of both Tregs and TH17 cells and that the reduction in anti-inflammatory Tregs in the presence of a fully functional TH17 response aggravates experimental glomerulonephrit. Recently, other results also show that, although IL-17 neutralization results in amelioration of murine ovalbumin (OVA)-induced arthritis, in vitro-primed TH17 cells cannot exacerbate arthritic symptoms after adoptive transfer [67]. Furthermore, TH17 cells cannot induce an inflammatory delayed-type hypersensitivity reaction because they fail to migrate into inflamed sites, possibly due to the lack of CXCR3 expression. Also, re-isolated TH17 cells acquired IFN-γ expression, indicating instability of the TH17 phenotype [68]. Taken together, we propose that IL-6, TGF-β and IL-23 might not provide sufficient signals to induce ‘fully qualified’ TH17 cells, and the chemokine receptor signalling also can be involved in the modulation of TH17 cells.

microRNA

microRNAs (miRNA) are a novel group of small, conserved, non-coding RNA molecules that are present in all species. These molecules post-transcriptionally regulate gene expression by targeting mRNAs for degradation or by repressing the translation of the mRNAs. A good understanding of miRNA-mediated gene regulation is critical to gain a comprehensive view of many physiological processes and disease states. Emerging evidence demonstrates that miRNA participates in the regulation of TH17 differentiation and autoimmune disease [69–71]. Results showed the amount of the miR-326 is positively associated with active disease relapse in patients with MS. And, use the over-expression and interference tools to manipulate the amount of miR-326 in the EAE animal model of MS, in which TH17 cells have a critical pathologic role. Forced expression of miR-326 worsens EAE symptoms and enhances TH17 differentiation, whereas neutralization of miR-326 improves clinical outcome and impairs TH17 differentiation [72]. Further, miR-326 binds to and prevents translation of Ets-1 mRNA, a known inhibitor of TH17 differentiation [72]. The finding that miRNAs can directly regulate TH17 differentiation and contribute to autoimmune pathology is a substantial advance; however, several outstanding questions remain. Including the therapeutic potential of this target is less clear and further effective delivery method for therapy, and the precise function of miRNA on the other T cell differentiation and function. It would be much more clinically relevant to see the outcome of treating animals with the miR-326 sponge vector after they present with clinical symptoms of EAE [73]. If miRNAs are to be seriously considered as therapeutic targets in autoimmune disorders, the development of an efficient non-viral delivery method is of paramount importance, particularly if activation of T cells to spread epitopes before disease relapse occurs mainly in the CNS.

To summarize the current research progresses, TH17 cell differentiation is directly regulated by different cytokines and transcriptional factors. This also depends on the TGF-β concentrations. At lower concentrations, together with IL-6 or IL-21, TGF-β synergistically induces IL-23R and thus promotes TH17 cell differentiation in the presence of IL-23. However, at higher concentrations, TGF-β inhibits IL-23R, IL-22 and IL-17 expression and favours induction of Foxp3 and thus Treg lineage differentiation. In spite of the absolute requirement of TGF-β, little is known about its precise signalling pathways in TH17 and Treg cell differentiation. Smad4 appears not to be required for TH17 cell differentiation [74]. But recent research report that neither Smad2 nor Smad3 gene deficiency abrogates TGF-β-dependent iTreg induction by a deacetylase inhibitor trichostatin A in vivo, although the loss of the Smad2 or Smad3 gene partially reduces iTreg induction in vitro. Similarly, Smad2 and Smad3 have a redundant role in the development of TH17 in vitro and in EAE [74–76]. So, the involvement of Smad-dependent or Smad-independent pathways in TH17 and Treg cell differentiation still needs to be carefully examined.

Indirect regulatory signalling pathway of TH 17 cells

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References

Other than cytokine receptor signalling, transcriptional factor regulation and miRNA that direct regulatory effects on TH17, the indirect regulatory signalling pathway also plays a great role in T cell immunity. This always involves that the antigen-presenting cells (APCs) initiate some definitive signalling pathways to participate in the function and differentiation of TH17 cell. T cell polarization depends on concentration of the danger signal used to activate APCs.

MAPK signalling

DCs are professional APC that instruct T cells during the inflammatory course of EAE. As we know, MAPK3 (Erk1) is important for the induction of T cell anergy. Another result showed that DC from MAPK3−/− mice has a significantly higher membrane expression of CD86 and MHC-II and, when loaded with the myelin oligodendrocyte glycoprotein, shows a superior capacity to prime naïve T cells towards an inflammatory phenotype than wild-type DC [77]. MAPK3−/− mice were only slightly but not significantly more susceptible to myelin oligodendrocyte glycoprotein-induced EAE than wild-type littermate mice. However, wild-type mice engrafted with MAPK3−/− BM developed a severe form of EAE. Infiltration of DC and accumulation of TH17 cells were also observed in the CNS [77–79]. Therefore, triggering of MAPK3 in the periphery might be an initiation or effect of TH17 cell–dominant neuron inflammation because absence of this kinase in the immune system leads to severe EAE. Other studies showed that Bordetella pertussis commits human DCs to promote a TH1/TH17 response through the activity of adenylate cyclase toxin and MAPK pathways. The complex pathology of B. pertussis infection is attributable to multiple virulence factors having disparate effects on different cell types. Researcher used human monocyte-derived DCs (MDDC), an ex vivo model useful for the evaluation of the regulatory potential of DC on T cell immune responses [80, 81]. The work compared MDDC functions after encounter with wild-type B. pertussis (BpWT) or a mutant lacking CyaA (BpCyaA), or the BpCyaA strain supplemented with either the fully functional CyaA or a derivative, CyaA, lacking adenylate cyclase activity. Results showed that CyaA expressed by B. pertussis strongly interferes with DC functions, by reducing the expression of phenotypic markers and immunomodulatory cytokines and blocking IL-12p70 production. B. pertussis-treated MDDC promoted a mixed TH1/TH17 polarization, and the activity of CyaA altered the TH1/TH17 balance, enhancing TH17 and limiting TH1 expansion. Also, TH1 effectors are induced by B. pertussis MDDC in the absence of IL-12p70 through an ErK1/2-dependent mechanism, and p38 MAPK is essential for MDDC-driven TH17 expansion. This suggests that CyaA mediates an escape strategy for the bacterium, as it reduces TH1 immunity and increases TH17 responses thought to be responsible, when the response is exacerbated, for enhanced lung inflammation and injury (Fig. 3).

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Figure 3.  Indirect regulatory signalling pathway of TH17 cells through antigen-presenting cells (APCs). Different danger signals can initiate APC and contribute to the TH17 cell differentiation or expansion through MAPK, JAGGD1 or BCL-6 pathway.

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JAGGED1 signalling

TH17-inducing activity is carried by certain polysaccharides such as β-glucan derived from Candia albicans. Using human MDDCs and some leukemic cell lines such as THP-1, TH1- and TH2-inducing activities can be qualitatively evaluated by the expression patterns of Notch ligand isoforms. So the association of TH17-inducing activities with Notch ligand expression patterns has been detected. MDDCs from healthy volunteers were co-cultured with HLA-DR-non-shared allogeneic CD4+ naive T cells to induce a mixed lymphocyte reaction, in the presence of adjuvants, such as curdlan. Results showed that curdlan, one of the β-glucans, has the ability to induce DC-mediated TH17 differentiation. It is also interesting to note that Jagged1 mRNA in MDDCs and THP-1 cells is up-regulated by curdlan [82]. Furthermore, polyclonal anti-Jagged1 antibody inhibited such DC-mediated TH17 differentiation [83]. This suggests that curdlan induces human DC-mediated TH17 polarization via Jagged1 activation in DCs, and JAGGED signalling has effects in the DC-mediated TH17 cell differentiation (Fig. 3).

CD40 ligands

CD40 ligand-triggered human DCs mount IL-23 responses, and it is further enhanced by danger signals. Recent studies indicate that there is also a role for IL-23 and IL-17 in tumourigenesis, promoting tumour growth and vascularization and affecting tumour incidence. Results show that human CD14+ peripheral blood DCs, as used for clinical applications in anti-tumour immunization strategies, produce high amounts of IL-23 [84, 85]. CD40 triggering of immature and mature DC but not of primary monocytes induced a rapid expression of high levels of IL-23p19, free p40 and minor levels of IL-12. Upon stimulation of DC subsets with a variety of different danger signals such as single-stranded and double-stranded RNA, bacterial components or viral infections, IL-23 expression pattern was analysed. Interestingly, co-stimulation with CD40L enabled IL-23 expression by DC subsets towards danger signals to which they have been unresponsive upon single stimulation [86]. Furthermore, results showed two novel splice variants of the IL-23-specific subunit p19 that could be associated with the regulation of IL-23 expression [87]. This suggests DC-based cancer vaccination strategies might contribute to a better understanding of the complex regulation of the heterodimeric cytokine IL-23 and evaluate DC regulatory effects on TH17 cell differentiation [84, 88].

LPS

Lipopolysaccharide (LPS) is a potent, natural, adjuvant, TLR4 ligand, commonly used to amplify TH1 responses. However, results also showed that systemic immunization using LPS generates large numbers of specific TH17 cells in murine small intestinal lamina propria. The priming of these TH17 cells required IL-23p19 production by bone marrow-derived cells. In contrast, IL-23 had no impact on TH1 differentiation or overall numbers of Ag-specific regulatory T cells [89]. Experiments using T cell adoptive transfers revealed a previously unappreciated mechanism for how TH17 responses are amplified in vivo through APCs: stimulation through LPS expanded precommitted TH17 cells rather than causing TH17 differentiation. Second, LPS drove TH17 cell expansion independently of IL-23, demonstrating that this cytokine is not necessary for expansion and possibly functions at an earlier stage in TH17 priming [90–93]. This indicates that using LPS-based peripheral vaccination can augment specific TH17 cell–mediated immunity in the gut mucosa, which probably related through APC.

c-kit

The receptor tyrosine kinase c-kit could bind with its ligand stem cell factor (SCF). Similarly, activation of granulocytes, mast cells and eosinophils in particular by c-kit ligation has long been known to result in degranulation with concomitant release of pro-inflammatory mediators, including cytokines. Signalling through the c-kit-SCF axis could have a significant impact on the pathogenesis of diseases associated with an immunological component. c-kit up-regulation on DCs via TH2- and TH17-inducing stimuli led to c-kit activation and immune skewing towards these T helper subsets and away from TH1 responses [94]. Others have shown that DCs’ treatment with inhibitors of c-kit activation, such as imatinib mesylate (Gleevec), favoured breaking of T cell tolerance, skewing of responses towards production of TH1 cytokines and activation of NK cells [95].

Bcl-6

The transcriptional repressor protein Bcl-6 regulates T cell differentiation by repressing TH2 responses and promoting follicular TH cell responses. Now, researcher found that memory T cells from Bcl-6-deficient mice had increased IL-17 production. Additionally, Bcl-6 expression is up-regulated in CD4+ T cells cultured under TH17 conditions [69]. T cells from Bcl-6-deficient mice showed defective TH17 differentiation and enhanced IL-4 production in vitro; however, normal TH17 differentiation was obtained with Bcl-6-deficient T cells under culture conditions when highly pure naive CD4+ T cells were used, when IL-4 production was inhibited or when TGF-β levels were increased. Retrovirus-mediated expression of Bcl-6 in CD4+ T cells repressed IL-4 and augmented basal IL-17 mRNA expression [69, 96]. These data support the idea that Bcl-6 promotes TH17 differentiation through suppression of TH2 differentiation. Bcl-6-deficient T cells transplanted into Rag1−/− mice produced WT levels of IL-17, indicating that, in vivo, Bcl-6-deficient T cells develop relatively normal TH17 responses [69]. Macrophages from Bcl-6-deficient mice showed strikingly increased expression of the TH17-promoting cytokines IL-6, IL-23 and TGF-β, and conditioned media from Bcl-6-deficient macrophages promoted augmented IL-17 expression by T cells [69, 96]. This indicates that the increased TH17 activity in Bcl-6-deficient mice is due, in part, to Bcl-6-deficient macrophages promoting increased TH17 differentiation in vivo. T cells may require Bcl-6 for optimal TH17 differentiation; however, Bcl-6 function in macrophages critically regulates TH17 differentiation in vivo. Probably, the increased TH17 differentiation aggravates the severe TH2-type inflammatory disease in Bcl-6-deficient mice (Fig. 3).

Moreover, different APC subpopulation also can be studied in the indirect regulation of differentiation and function of TH17 cells. Enteric flora expands gut lamina propria CX3CR1+ DCs supporting inflammatory immune responses under normal and inflammatory conditions [97]. CD103 or CX3CR1 surface expression defines distinct DCs and macrophages in the murine lamina propria of the colon (cLP). cLP CD11c+ cells were isolated from specific pathogen-free or germ-free mice to elucidate the role of the commensal flora in their development. The cLP CD11c+ cells are a heterogeneous cell population that includes 16% CX3CR1+, 34% CD103+, 30% CD103 CX3CR1 DCs and 17% CD68+ F4/80+ CX3CR1+ CD11c+ macrophages. All DCs expressed high levels of MHC II but low levels of co-stimulatory (CD40, CD86 and CD80) and co-inhibitory (programmed death ligand-1, PDL1) molecules. Ex vivo confocal microscopy demonstrated that CX3CR1+ CD11c+ cells, but not CD103+ DCs, were reduced in the cLP of germ-free CX3CR1-GFP mice. The absence of the enteric flora prevents the formation of transepithelial processes by the CX3CR1+ DCs. CX3CR1+ DCs preferentially supported TH1/TH17 CD4+ T cell differentiation. CD103+ DCs preferentially induced the differentiation of Foxp3-expressing Tregs [60]. Stimulation of cLP DCs with fractalkine/CX3CL1 increased the release of IL-6 and TNF-α. In the absence of CX3CR1, the CD45RBhigh CD4 transfer colitis was suppressed and associated with reduced numbers of DCs in the mesenteric lymph nodes and a reduction in serum IFN-γ and IL-17. The local bacteria-driven accumulation of CX3CR1+ DCs seems to support inflammatory immune responses.

Conclusive remark

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References

Despite the recent identification of TH17 cells, over the past few years we have made rapid and large advances in our understanding of regulatory mechanism of the development, regulation and function of these cells. This has been particularly true in the context of autoimmune diseases, where the pathogenic role of TH17 cells has been well documented. However, the exact nature and detailed regulatory pathway of TH17 cells in immune-related diseases are still need to be further explored. A better understanding of these research key questions could be used to develop and refine new immunological therapies.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References

This work was supported by the National Natural Science Foundation of China (Contract No. 30801042 and 30930001); Key Laboratory of Pathogenic Microbiology and Immunology, Chinese Academy of Sciences (Contract No. 2009CASPMI-001).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Direct regulatory signalling pathway of TH 17 cells
  5. Indirect regulatory signalling pathway of TH 17 cells
  6. Conclusive remark
  7. Acknowledgment
  8. References