Th17 cells and T_regs: unlikely allies

Review on the reciprocal stimulatory effects of proinflammatory Th17 cells, wth immunosuppressive T_reg cells.

Abstract

Identification of CD4⁺Foxp3⁺ T_regs and Th17 modified the historical Th1–Th2 paradigm. Currently, the Th17–T_regs dichotomy provides a dominant conceptual framework for the comprehension of immunity/inflammation and tolerance/immunosuppression in an increasing number of diseases. Targeting proinflammatory Th17 cells or immunosuppressive T_regs has been widely considered as a promising therapeutic strategy in the treatment of major human diseases, including autoimmunity and cancer. The efficacy and safety of such therapy rely on a thorough understanding of immunobiology and interaction of these two subsets of Th cells. In this article, we review recent progress concerning complicated interplay of Th17 cells and T_regs. There is compelling evidence that T_regs potently inhibit Th1 and Th2 responses; however, the inhibitory effect of T_regs on Th17 responses is a controversial subject. There is increasing evidence showing that T_regs actually promote the differentiation of Th17 cells in vitro and in vivo and consequently, enhanced the functional consequences of Th17 cells, including the protective effect in host defense, as well as detrimental effect in inflammation and in the support of tumor growth. On the other hand, Th17 cells were also the most potent Th subset in the stimulation and support of expansion and phenotypic stability of T_regs in vivo. These results indicate that these two subsets of Th cells reciprocally stimulate each other. This bidirectional crosstalk is largely dependent on the TNF–TNFR2 pathway. These mutual stimulatory effects should be considered in devising future Th17 cell- and T_reg-targeting therapy.

Introduction

Th cells play central roles in orchestrating innate as well as adaptive immune responses [1]. The Th1–Th2 paradigm, proposed more than 25 years ago [2], has evolved with the identification of distinct additional lineages of immunosuppressive CD4⁺Foxp3⁺ T_regs and proinflammatory Th17 cells. T_regs are crucial for immunological homeostasis and play an important role in preventing immune responses to autoantigens [3], while they also dampen host immune responses against pathogens [4] and tumor antigens [5]. Up- or down-regulation of T_reg activity has clear, beneficial effects in various animal models, including autoimmune disorder [6] and cancer [7], suggesting a considerable potential for therapeutically targeting T_regs in the treatment of major human diseases. Indeed, ex vivo-expanded human T_regs have the capacity to inhibit the rejection of skin and islet allografts in humanized mouse models [8–10]. Ex vivo-expanded antigen-specific human T_regs hold the promise to prevent GvHD in the treatment of leukemia, while maintaining graft-versus-malignancy capacity [11–14]. On the other hand, immunotherapy by targeting T_regs, including depletion of T_regs, is currently being tested in tumor patients [15].

In contrast to T_regs, Th17 cells have been found to be major stimulatory participants in the pathogenesis of autoimmune disease [16, 17]. Increasing numbers of chronic inflammatory disorders, which were previously attributed to Th1 cells, were found to be caused by Th17 cells [18]. Although Th17 cytokines may provide proinflammatory support of tumor development [19], Th17 cells also contribute to anti-tumor immune responses [19, 20]. Consequently, Th17 cells have been considered to be a promising target in the treatment of autoimmune disorders [21, 22], as well as cancer [19, 20]. For example, treatment with secukinumab, a highly selective human anti-IL-17A mAb, resulted in rapid and sustained reductions of symptoms in patients with psoriasis, rheumatoid arthritis, psoriatic arthritis, and ankylosing spondylitis [23]. In contrast to its pathogenic role in autoimmune disorders, the contribution of Th17 cells to anti-tumor immunity was recapitulated by a recent report showing that the anti-tumor effect of cyclophosphamide critically depended on Th17 cells, which were induced by intestinal microbiota [24]. In mouse tumor models, infusion of in vitro-differentiated Th17 cells achieved a marked anti-tumor effect, which was even superior to the infusion of in vitro-differentiated Th1 cells [25, 26]. Thus, the targeting of Th17 cells holds promise for the treatment of human cancers.

The differentiation programs of T_regs and Th17 cells are reciprocally interconnected and are probably competitive in using naive CD4 T cells as precursors and TGF-β as a differentiation factor [27–29]. Currently, the Th17–T_reg dichotomy tends to become a dominant conceptual framework for comprehending the relationship of immunity/inflammation and tolerance/immunosuppression in a wide spectrum of diseases [30–36]. However, recent studies reveal that Th17 cells and T_regs do not always antagonize each other. Abundant numbers of Th17 cells and T_regs frequently colocalize in the same compartments. Importantly, these two populations of Th cells, although functionally opposite, can actually positively stimulate and support each other. The interplay of Th17 cells and T_regs can be complicated further by their phenotypic plasticity, which has been well-reviewed previously [37] and will not be discussed further in this article. In addition to Foxp3-expressing CD4 T_regs, it is known that other subsets of T cells with immunosuppressive capacity are differentiated from naive CD4 cells during immune responses, including IL-10-producing type 1 T_regs [38] and TGF-β-producing Th3 cells [39]. We will focus our discussion in this article on naturally occurring, thymic-derived, suppressive CD4⁺Foxp3⁺ T_regs.

Th17 CELLS AND T_regs FREQUENTLY COLOCALIZE IN THE SAME ANATOMIC COMPARTMENTS

Most Th17 cells in the body reside in the barrier tissues, including intestinal tracts and skin [18]. Interestingly, T_regs are also abundant in these interfaces of host and microbiota [40, 41]. It has been reported that the members of commensal microbiota can induce the generation of T_regs and Th17 cells [42–46]. Interestingly, a recent study showed that the up-regulation of Foxp3 expression by T_regs and increase of Th17 cells simultaneously occurred in NOD mice fed acidic water, which was associated with an alteration in the gut microbiome and a decreased risk of diabetes [47]. However, Th17 cells and T_regs are also frequently most abundant in other anatomical compartments, such as inflamed synovial fluids [48], and in cancerous tissues, such as aggressively growing breast cancer [49], uterine cervical cancer [50], colorectal cancer [51], gastric cancer [52], and malignant pleural effusion [53] in humans, as well as a number of mouse tumor models, including gliomas [54]. Therefore, the physiological relevance of colocalization of these two subsets of CD4 cells likely does more than maintain defense against invading pathogen and tolerance to self-tissues.

Th17 and T_regs intriguingly share many similarities, which may contribute to their colocalization. Th cells, in addition to their distinct cytokine profiles, express characteristic chemokine receptors that direct them to traffic to lymphoid or peripheral tissues. For example, naive Th cells express CCR7, whereas Th1 cells express CXCR3 and CCR5, and Th2 cells express CCR3 and CCR8 [55]. CCR6 is the predominant chemokine receptor shared by Th17 cells and T_regs [56]. Expression of this chemokine receptor was reported to recruit Th17 cells and T_regs simultaneously to inflammatory sites [57] or barrier tissues [58].

It was also clearly demonstrated that the differentiation of Th17 and T_regs requires TGF-β [27–29]. Th17 and T_regs highly express the AHR, which can promote the generation of both cell types by integrating environmental stimuli [59, 60]. Thus, it is likely that Th17 and T_regs could be generated at the sites enriched in TGF-β and respective AHR ligands. However, emerging evidence also favors the idea that these two subsets of Th cells mutually promote each other's generation and expansion, which would contribute to the colocalization of abundant Th17 and T_regs in the same anatomic compartments.

T_regs PROMOTE THE DIFFERENTIATION OF Th17 CELLS

T_regs have clearly been shown to inhibit the activation of Th1 and Th2 cells [61, 62]. Although a certain subset of CD39⁺Foxp3⁺ T_regs was reported to inhibit Th17 cells [63], the susceptibility of Th17 cells to T_reg-mediated inhibition is still controversial. For example, in experimental mouse gastritis, T_regs markedly inhibited Th1 cell- and Th2 cell-mediated pathogenesis but had no effect on Th17 cell-mediated pathogenesis [64]. In contrast, the majority of studies shows that T_regs actually promote the differentiation as well as function of Th17 cells, as discussed below.

T_regs promote differentiation of Th17 cells in vitro

The first evidence showing the stimulatory effect of T_regs on Th17 cells was actually a study revealing the biological signals for de novo generation of Th17 cells. In this study, Veldhoen and colleagues [29] showed that IL-17-producing T cells were differentiated from naive CD4 cells when cocultured with T_regs in the presence of TLR3, TLR4, or TLR9 stimuli. This led to the finding that TGF-β1, which could be a substitute for T_regs, and IL-6, which was produced by TLR stimuli, were able to induce the differentiation of Th17 cells. Subsequently, Xu and colleagues [65] reported that in the presence of IL-6, T_regs not only induced IL-17 expression by T_effs but themselves also expressed IL-17. Coculturing of T_effs with T_regs inhibited production of the Th1 cytokine (IFN-γ) and Th2 cytokine (IL-4), whereas IL-17 family cytokines, including IL-17A, IL-17F, IL-21, and IL-22, were markedly enhanced [66, 67]. This in vitro effect of T_regs in promoting Th17 differentiation was reportedly mediated by providing TGF-β [65] and consumption of IL-2 [67].

Cotransfer of T_regs promotes differentiation of Th17 cells and inhibits colitis in lymphopenic mice

The critical role of functional T_regs in immune homeostasis can be demonstrated in a mouse colitis model induced by transfer of naive CD4 T cells into Rag1^−/− mice [68]. After transfer into Rag^−/− mice, naive CD4 cells can be differentiated into Th1 and Th17 cells in the colon [69]. Thus, this model provides a system to evaluate the in vivo effect of T_regs on the differentiation of Th17 and Th1 cells. Sujino and colleagues [70] found three types of Th1 (IL-17A⁻IFN-γ⁺) cells to be generated in this system: RORγt⁻ classic Th1 cells that were differentiated directly from naive cells; RORγt⁺ Th1-like cells and RORγt⁻ alternative Th1 cells that were terminally differentiated from RORγt⁺ cells via Th17 (IL-17A⁺IFN-γ⁻); and Th17/Th1 (IL-17A⁺IFN-γ⁺) or Th1-like (IL-17A⁻IFN-γ⁺) cells. T_regs not only suppressed Th1 cells but also suppressed the transition of Th17/Th1 cells into alternative Th1 cells, resulting in an accumulation of Th17 and Th17/Th1 cells in mice with inhibition of colitis [70].

Systemic autoimmune disease can be generated in this model by transferring CD4 T cells from the DO11.10 TCR transgenic mouse, specific for the OVA_323–339 peptide, into a Rag^−/− host, expressing OVA as a secreted antigen [71]. With the use of this model, Lohr and colleagues [71] showed that transfer of OVA-specific T_regs prevented weight loss and skin inflammation, which were associated with the inhibition of T cell accumulation, as well as complete suppression of the proportion of Th1 cells. In contrast, T_regs increased the proportion of IL-17-expressing cells.

In the polyclonal system, e.g., transfer of WT naive CD4 T cells and WT T_regs into Rag^−/− mice, we also found that T_regs markedly inhibited development of colitis, accompanied by an inhibition of IFN-γ-producing cells, while increasing IL-17A-producing cells [69]. Th17 cells exhibited a tissue-protective and immunosuppressive effect and consequently, suppressed colon inflammation by multiple mechanisms, including inhibition of pathogenic Th1 responses [72], increase of barrier function of intestinal epithelial cells [73], up-regulation of polymeric IgR and intestinal IgA [74, 75], and production of protective IL-22 [76]. Thus, the Th17-promoting effect of T_regs may contribute to the inhibitory effect on the development of colitis and other inflammatory responses in this model.

Depletion of T_regs inhibits generation of antigen-specific Th17 cells

Chen and colleagues [77] examined the role of T_regs in the in vivo development of antigen-specific Th17 cells. They transferred OT-II T cells into Foxp3.luciDTR4 mice, which were immunized with OVA peptide and CFA. T_regs were depleted by administering diphtheria toxin. Depletion of T_regs at the time of immunization, but not at later time-points, reduced the number of antigen-specific IL-17-producing cells and consequently, reduced inflammatory skin responses [77]. This Th17-promoting effect of T_regs was found to be mediated by consumption of IL-2 [77], a cytokine critical for the maintenance and expansion of T_regs [78] and for the inhibition of differentiation of Th17 cells [79].

T_regs promote beneficial host defense function of Th17 cells against fungal infections

Th17 cells and IL-17 cytokines are critical for oral fungicidal immune responses, based on the recruitment of neutrophils to the oral mucosa and induction of salivary antimicrobial factors [80]. Patients with impaired Th17 responses [81] or lacking T_regs [82] were more susceptible to Candida albicans infection. Pandiyan and colleagues [67] reported that T_regs potently promoted the differentiation of naive CD4 cells into Th17 cells capable of producing the full suite of characteristic cytokines in vitro and in vivo. T_regs did not suppress but actually promoted IL-17A-dependent clearance of fungi during acute C. albicans infection. This is demonstrated by the fact that depletion of T_regs in WT B6 mice resulted in a reduced level of Th17 cells and increased the fungal burden. In addition, in the Rag^[−/−] mice cotransfer of T_regs with T_effs resulted in an increase in Th17 cells and enhanced fungal clearance and recovery from C. albicans infection [67]. Therefore, in addition to maintaining immune homeostasis and preventing autoimmunity, T_regs play a positive role in host defense and in clearance of fungal infections, by promoting Th17 responses. T_regs have also been shown to confer protection against viral infections [83, 84]. Whether this effect of T_regs was achieved by collaboration with Th17 cells should be clarified further.

T_regs enhance Th17 cell-mediated immunopathogenesis during intracellular bacterial injections

More recently, it has been shown that upon intracellular Chlamydia muridarum infection, T_regs not only promoted Th17 differentiation from conventional CD4⁺ T cells but also themselves converted into proinflammatory Th17 cells in in vitro and in vivo settings [66]. Intriguingly, partial depletion of T_regs markedly reduced the Th17 responses, as shown by the attenuated neutrophil infiltration and reduced severity of oviduct inflammation after C. muridarum genital infection [66]. Thus, T_regs play a critical role in the immunopathogenesis in this model, which is completely contradictory to their well-documented immunosuppressive activity. It is worth noting that Th17 responses, enhanced by T_regs, strengthen host resistance to C. albicans infection [67], whereas the same action causes the immunopathology in C. muridarum infection [66], suggesting that the biological outcome of interplay of T_regs and Th17 may be dependent on the specific pathogen.

Allograft rejection triggered by Th17 cells is fueled by T_regs

T_regs are considered as a therapy to induce immune tolerance in clinical transplantation [3]; thus, their interaction with rejection-inducing Th cells should be clarified. Vokaer and colleagues [85] reported that T cell-derived IL-17 was critical for the neutrophil infiltration and rejection of minor antigen-mismatched skin grafts. In this model, depletion of T_regs resulted in a marked reduction of IL-17A mRNA within the grafts and draining LNs, with a marginal increase of IFN-γ mRNA, consistent with the results of a study on silica-induced lung fibrosis [86]. Furthermore, cotransfer of T_regs together with anti-donor naive T cells into Rag^−/− mice not only enhanced Th17 differentiation by T_effs, but a considerable number of T_regs by themselves also became IL-17 producers [85]. Thus, the potential of T_regs to promote Th17-mediated, neutrophil-dependent rejection of graft should be considered in T_reg-based therapy in bone marrow transplantation and solid organ transplantation.

T_regs increase inflammatory support of tumor growth by Th17 cells

Th17 cells have been reported to play dual roles in tumors: they promote inflammatory support of tumor growth and contribute to the immune surveillance against tumor [19]. In the mouse glioma model, IL-10-producing Th17 cells appeared to support tumor growth [54]. In this model, an elevated number of T_regs promoted the generation of IL-10-producing Th17 cells, while inhibiting IFN-γ-producing Th17 cells [54]. Therefore, multiple mechanisms may be attributed to T_regs in promoting cancer immune evasion, including direct inhibition of anti-tumor Th1 responses and stimulation of tumor-supporting Th17 responses.

Taken together, recent studies do not support the view that Th17 cells are a cellular target of T_reg-mediated inhibition. Instead, there is clear in vitro and in vivo evidence that T_regs actually promote the differentiation of Th17 cells and consequently, enhance the beneficial as well as detrimental functions of Th17 cells.

Th17 CELLS PROMOTE THE EXPANSION AND PHENOTYPE STABILITY OF T_regs

In the past decade, extensive study of T_regs greatly improved our understanding of the effect of T_regs on T_effs; however, much less is known about the effect of T_effs on T_regs. Accumulating evidence indicates that in addition to being the cellular target for suppression by T_regs, T_effs can have a marked impact on T_regs. More specifically, T_effs are important in support of sustained suppressive function of T_regs. Among Th subsets, Th17 cells are the most potent stimulators of T_regs, as shown by our recent results. The interactions of Th17 cells and T_regs are bidirectional, and Th17 cells have a considerable effect on T_regs.

Activation of T_regs requires stimulation by T_effs

T_regs constitutively express high levels of functional cytokine receptors, such as CD25 [87, 88] and TNFR2 [89–91], but do not have the capacity to produce ligands for these receptors. This suggests that T_regs may rely on the cytokine producers, such as activated T_effs, for the maintenance of their function.

Thornton and colleagues [92] found that T_regs did not suppress the initial activation of T_effs but exerted their suppressive effects only after production of IL-2 by T_effs, resulting in the expansion of T_regs and the induction of their suppressor function. Early evidence of the in vivo-supportive effect of T_effs for T_regs was shown by Curotto de Lafaille and colleagues [93]. They found that CD25 expression on donor T_regs, defined by CD4⁺CD25⁺, was not stable after transfer into recipient mice. However, cotransferred T_effs, by producing IL-2, could maintain the expression of CD25 on T_regs [93]. Furthermore, the same group also showed that IL-2 from T_effs was required for the function of T_regs in the inhibition of EAE [94].

We found previously that the number of T_regs was reduced in mice with EAE, and PTX, contained in the immunogen used to induce EAE, was responsible for the reduction of T_regs [95]. Injection of PTX into a WT mouse resulted in a marked reduction of T_regs [95, 96]. To our surprise, in the absence of IL-6, PTX treatment had the opposite effect and expanded T_regs in vitro [97] and in vivo (data not shown). We therefore tested the effect of proinflammatory cytokines elicited by PTX on T_regs. This led us to find that TNF, though TNFR2, preferentially activated, expanded, and promoted the phenotypic stability of T_regs [69, 89, 98]. Although counterintuitive and contradictory to a previous report [99], our observation is supported by increasing evidence from other investigators [100–103].

TNF–TNFR2 pathways actually also enable T_effs to activate T_regs in vivo, as shown by an elegant study performed by Grinberg-Bleyer and colleagues [104]. They initially found that T_regs proliferated significantly more when coinfused into mice with activated T cells. When islet-specific T_effs were transferred alone, they induced diabetes, whereas mice injected with T_regs alone or T_regs plus T_effs did not develop diabetes. However, upon a second injection of activated T_effs, 3 weeks after the initial injection, mice that had received a first injection of T_regs alone developed diabetes, whereas mice that originally had been injected simultaneously with T_regs and T_effs were protected from diabetes. Thus, the T_effs were indirectly protective by stimulating T_reg expansion and suppressive activity. The results of the RNA microarray indicated that TNF/TNFR2 but not IL-2/CD25 was the pathway responsible for the T_reg-stimulatory effect of T_effs [104].

Th17 cells produce high levels of TNF: perhaps they should be termed Th-_TNF

We determined the identity of the subset of T_effs that was a major TNF producer. We compared Th0, Th1, Th2, and Th17 subsets in vitro, differentiated from naive CD4 cells under standard respective polarized culture conditions. Th17 cells expressed the highest level of TNF (46%), followed by Th1 cells (26%) and Th0 (14%), whereas Th2 cells (6%) expressed the lowest levels of TNF. The TNF expression by Th subsets was stable and remained largely unchanged 5 weeks after transfer into Rag^−/− mice [105]. Interestingly, although named for the production of IL-17, Th17 cells also expressed similar or even higher levels of TNF than IL-17A in vitro and in vivo [105]. In addition to being expressed by in vitro-differentiated mouse Th17 cells [106], TNF is expressed by natural Th17 cells present in human patients [107, 108]. Recently, it was reported that the pathogenic effect of Th17 cells was actually mediated by its TNF expression [109]. Thus, this subset of Th cells could also be renamed “Th-_TNF”.

Th17 cells stimulate expansion and promote stability of the T_reg phenotype, mainly depending on the TNF–TNFR2 pathway

As Th17 cells expressed the highest levels of TNF among Th subsets, they have the potential to stimulate T_regs in vivo. To test this idea, freshly isolated, highly purified T_regs (CD4⁺Foxp3/gfp⁺ cells) were transferred alone or cotransferred with Th subsets into Rag^−/− mice. After 5 weeks, the number and level of Foxp3 expression by T_regs were determined. The results showed that, when transferred alone, T_regs did not proliferate well in a lymphopenic environment and a majority of them lost Foxp3 expression, which is consistent with our previous report [69]. All types of Th subsets, including Th0 cells, were able to support expansion and phenotypic stability of T_regs in vivo. Among tested Th subsets, Th17 cells were the most potent stimulators of T_regs, resulting in markedly more T_regs with the highest levels of Foxp3 expression. This was followed, in order, by Th1, Th2, and Th0 cells [105]. The potency of the T_reg-stimulatory effect of Th subsets (Th17>Th1>Th0) correlated with their capacity to express TNF (Th17>Th1>Th0) but not with their capacity to express IL-2, as Th17 cells are poor producers of IL-2 [110]. Nevertheless, IL-2 also played a role, as the neutralizing antibody against IL-2 partially blocked the stimulatory effect of Th17 cells on T_regs. Other cytokines, in addition to TNF and IL-2, may also contribute to this effect, as Th2 cells expressed relatively lower levels of TNF and IL-2, while having more potent T_reg-stimulatory activity than Th0 cells. However, the TNF–TNFR2 pathway appears to be dominant, as the effect of Th17 cells on T_regs was largely abolished when cotransferred with TNFR2-deficient T_regs. Intriguingly, expression of IL-17A and TNF by WT Th17 cells was reduced by up to 80% when cotransferred with T_regs deficient in TNFR2 [105]. Based on these observations, we favor the idea that the TNF–TNFR2 signal is crucial for the reciprocal stimulatory effect of proinflammatory Th17 cells and immunosuppressive T_regs.

Why are Th17 cells most able to stimulate T_regs?

Distinct effector functions of Th subsets have evolved presumably to protect the host efficiently against myriad offending pathogens by mounting a robust immune response. However, excessive and prolonged activation of Th cells can result in the severe inflammation and collateral damage to normal tissues. The capacity of activated T_effs to stimulate T_regs is likely to represent a major mechanism by which a dynamic equilibrium is established between T_effs and T_regs in an ongoing immune or inflammatory response to avoid damage to self. Th17 cells have stem cell-like features [26, 111] and can mediate sustained autoimmune inflammation [112]. It is reasonable to hypothesize that the potent T_reg-stimulatory activity of Th17 cells coevolved to counter their robust and prolonged proinflammatory effector function. Th17 cells not only produce TNF by themselves, but the IL-17 cytokine is also able to stimulate the production of TNF from macrophages [113], which can further amplify the T_reg-stimulatory effect. Therefore, Th17 cells exert critical functions in host defense, immune responses, and inflammation and together with T_regs, orchestrate the maintenance of immunological homeostasis. Presumably, expanded and activated T_regs are responsible to attenuate and eventually quench the inflammatory cascade induced by Th17 cytokines. For example, activation of neutrophils by Th17 cells [114] could be inhibited by T_regs [115].

CONCLUDING REMARKS

Extensive study of T_regs and Th17 cells has established a critical role of these two subsets of Th cells in the pathogenesis of major human diseases. Consequently, targeting Th17 cells and T_regs, by up-regulating or down-regulating their respective function, can be considered a promising means of treating autoimmune disorders and tumors [21, 22, 116]. However, recent evidence shows that Th17 cells and T_regs do not simply antagonize each other, but instead, they stimulate each other reciprocally (Fig. 1). It is likely that therapeutic manipulation of one of these subsets would have an impact on the other. The complexity of their interactions should be considered carefully when designing therapeutic strategies. For example, increasing T_reg activity, by stimulating in vivo expansion of T_regs or adoptive transfer of ex vivo-expanded T_regs, has become a strategy in the treatment of autoimmunity, allograft rejection, and GvHD [116]. Although current data from studies on humanized mice and human patients suggest that such an approach is effective and safe [117, 118], the potential risk of developing acute and chronic Th17-mediated inflammation by such T_reg-based treatment, as has occurred in mouse models [85, 86], should be closely monitored. Another possibility—that Th17 cells generated by T_regs can actually promote T_reg activity further and that Th17 and T_regs may collaboratively inhibit autoimmunity, allograft rejection, and GvHD—should also be investigated. On the other hand, caution should be used when targeting Th17 cells in the treatment of autoimmunity, as it may reduce immunosuppressive T_reg activity as a side-effect. However, the interplay of Th17 and T_regs may also provide new avenues to up- or down-regulate their function therapeutically. For example, although the beneficial effect of T_reg depletion in experimental tumor models has been clearly shown [119], complete depletion of T_regs is difficult and risks the development of autoimmune responses in cancer patients. Tumor-infiltrating T_regs expressed markedly higher levels of TNFR2 [90], and abundant T_regs and Th17 cells frequently colocalized in tumors [49–54]; thus, therapeutically targeting Th17 cells may reduce the suppressive activity of tumor-associated T_regs and mitigate the autoimmune risk. Furthermore, Th17 cell-derived TNF may be considered a means to activate and expand T_regs. For example, T_reg expansion is usually achieved through agents, such as IL-2 and rapamycin [120–123]. Recently, Okubo and colleagues [100] reported that a TNFR2 agonist had a superb effect on causing homogenous expansion of human T_regs with potent suppressive capacity. Thus, it is worthwhile—using a TNF–TNFR2 pathway-based Th17–T_reg interaction or in conjunction with additional T_reg-activating agents—in the treatment of autoimmunity, GvHD, and allograft rejection by up-regulation of T_reg activity. It is known that Th17 cells and T_regs are not homogenous. For example, a subset of Th17 cells is immunosuppressive [124, 125], whereas a subset of T_regs has the capacity to produce proinflammatory cytokines [126]. Therefore, the interactions between Th17 subsets and T_reg subsets merit future investigation. Furthermore, the role of antigenic DCs, which have a major impact on the differentiation, expansion, and function of T_regs [127, 128] and Th17 cells [129, 130] in the reciprocal stimulation of these two Th subsets, also needs to be defined further.

Figure 1. Reciprocal stimulation of T_regs and Th17 cells.

T_regs are able to promote the differentiation and function of Th17 cells, through a mechanism involving the supply of TGF-β, which is required for Th17 differentiation, or inhibition of Th1 and Th2 cytokines and IL-2, which are known to block Th17 differentiation. Induced and natural Th17 cells also stimulate the activation and expansion and promote Foxp3 expression of T_regs by producing TNF, IL-2, or other mediators required for the survival and activation of T_regs. Therefore, the immunosuppressive T_regs and proinflammatory Th17 cells reciprocally stimulate and temper each other, resulting in a dynamic equilibrium in an ongoing immune/inflammatory response.

AUTHORSHIP

X.C. and J.J.O. contributed to the writing of this review.

DISCLOSURES

The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. government. The authors declare that there are no conflicts of interest.