The cytokine milieu in the interplay of pathogenic Th1/Th17 cells and regulatory T cells in autoimmune disease

Abstract

The propagation and regulation of an immune response is driven by a network of effector and regulatory T (Treg) cells. The interplay of effector T and Treg cells determines the direction of the immune response towards inflammation or its resolution in an autoimmune disease setting. In autoimmune diseases, this interplay shifts the balance in favor of the development of autoreactive effector T cells, resulting in inflammatory pathology. The objective of an effective therapeutic approach for autoimmune disease is to restore this balance. In this review, we describe the characteristics and development of pathogenic T helper 1 (Th1) and Th17 cells and the beneficial Treg cells in autoimmune diseases and the crucial roles of the cytokine milieu in influencing the balance of these T-cell subsets. Given the importance of cytokines, we discuss current immunotherapeutic strategies using cytokine or cytokine receptor antibodies for the treatment of autoimmune diseases.

Introduction

CD4⁺ T helper (Th) cells are essential effectors of the immune response and play an important role in inflammation. For more than two decades, the Th1/Th2 paradigm, introduced by Mosmann and Coffman, has been used to explain most of the phenomena related to adaptive immunity.¹ This paradigm has been modified following the recent discovery of Th17 and regulatory T (Treg) cells, two newly characterized T-cell subsets that are thought to be critically involved in mediating and regulating autoimmune responses.^{2, 3, 4, 5} An in-depth understanding of the precise pathological mechanisms of these pathogenic T-cell subsets is urgently needed to identify novel targets and to develop specific new treatment approaches for autoimmune diseases, in which interplay or interaction of Th1/Th17 and Treg cells is controlled by the cytokine environment. Pathogenic Th1/Th17 cells secreting signature proinflammatory cytokines, i.e., interferon (IFN)-γ and IL-17, have been shown to promote inflammatory responses and to contribute to the pathogenesis of multiple human autoimmune disorders and their animal models, including experimental autoimmune encephalomyelitis (EAE) for multiple sclerosis (MS) and collagen-induced arthritis (CIA) for rheumatoid arthritis (RA).^{6, 7, 8} In contrast, Treg cells are crucial for the maintenance of localized immunosuppression and normally keep pathogenic Th1/Th17 cells in check.⁹ In this review, we discuss recent advances in this area with particular focus given to novel cytokine signaling pathways involved in the regulation of development of Th17 and Treg cells. We also highlight potential new therapeutic targets of these two T-cell subsets for the development of new treatments for MS and other autoimmune diseases.

Roles of Th1/Th17 and Treg cells in autoimmune diseases

Th1 and Th17 cells in autoimmune diseases

The pathogenesis of chronic autoimmune diseases, including MS and RA, arises from the break in tolerance of self-antigens and the development of autoaggressive effector T cells infiltrating the target tissues. Previously, Th1 cells have been regarded as the main pathogenic T cells driving autoimmune tissue damage, which is attributed to the activation of macrophages by the Th1-secreted cytokine IFN-γ.¹⁰ Paradoxically, IFN-γ^−/− mice were found to be highly susceptible to many organ-specific autoimmune diseases, including EAE, and blocking IFN-γ or its signaling pathway exacerbates autoimmune disease severity in EAE and CIA.^{11, 12} This dilemma was explained partially by the paradoxical properties of IFN-γ¹³ but mainly through the discovery of a new lineage of Th subset, IL-17-producing Th17 cells, which are susceptible to inhibition by IFN-γ.¹³ There is evidence that Th17 cells are activated during the disease process and are responsible for recruiting other inflammatory cell types, especially neutrophils, to mediate pathology in the target tissues.¹⁴ The observations accumulated thus far indicate that both Th1 and Th17 cells are important drivers of the inflammatory process in tissue-specific autoimmunity.¹⁵ Using EAE as an example, adoptive transfer of myelin oligodendrocyte glycoprotein-specific Th1 or Th17 cells, but not Th2 cells, to naive mice was able to induce EAE.^{16, 17} Interestingly, the two T-cell subsets caused different central nervous system (CNS) pathologies in the recipients, suggesting that they are functionally distinct. Both Th1 and Th17 cells were observed in the infiltrates of EAE-, CNS- and CIA-inflamed joints. Furthermore, in our study, analysis of myelin oligodendrocyte glycoprotein-specific T cells from the spinal cords of mice with EAE further revealed the presence of substantial numbers of T cells coexpressing IL-17 and IFN-γ,¹⁸ raising questions about the function of this double-positive population.

Treg cells in autoimmune diseases

Another lineage of T cells that coexpress CD4, CD25 and the transcription factor Forkhead box p3 (Foxp3) are Tregs.¹⁸ Treg cells have the capacity to actively suppress effector cells and to dampen a wide spectrum of immune responses, including those associated with autoimmune diseases.⁴ Thymic-derived naturally occurring Treg (nTreg) cells are critical for maintaining peripheral immune tolerance and for preventing autoimmunity and tissue injury by inhibiting the proliferation and effector functions of pathogenic Th1/Th17 cells.^{18, 19, 20} In addition, transforming growth factor (TGF)-β can promote the generation of Treg cells from naive T cells through the induction of Foxp3 expression; these are the so-called iTreg cells.²¹ Once generated, Treg cells can act as a negative regulator for the control of activation and effector functions of autopathogenic T cells in the immune system.²² Recent studies have suggested that CD4⁺CD25^hi Treg cells were functionally impaired in MS patients.²³ Interestingly, it has been shown that Treg cells isolated from the circulation of MS patients, though normal in number, poorly inhibited effector T-cell proliferation in an in vitro coculture system.^{24, 25} The functional impairment of Treg cells was also reported in a mouse EAE model, in which myelin-specific functional Treg cells accumulate in the CNS but fail to control autoimmune inflammation.²⁶ The situation in RA is different from that in MS, in which reduced number of CD4⁺CD25^hi Treg cells were observed in the peripheral blood of RA patients. It has been reported that Treg cells derived from RA patients can neither regulate proinflammatory cytokine secretion by pathogenic T cells and monocytes nor suppress autoreactive T cells.^{27, 28} It should be noted that the frequency of CD4⁺CD25^hi Treg cells is higher in rheumatoid synovium than in the peripheral blood of patients with RA, and there were considerable numbers of Treg cells in the CNS lesions of peak disease-stage EAE mice.^{26, 29, 30, 31, 32, 33} In parallel, our recent study also demonstrated that Treg cells accumulate in the rheumatoid synovium and coexist with pathogenic Th17 and Th1 cells that are highly concentrated at the site of inflammation (data unpublished). Taken together, these findings suggest that functionally impaired Treg cells and increased numbers of autopathogenic Th17 cells contribute to the break in tolerance and shift of the immune system towards a proinflammatory state.

Cytokine environment and signaling pathways for differentiation and maintenance of Th1/Th17 and Treg cells

Roles of cytokines in differentiation of T-cell subsets

Efficient host defense against pathogens is achieved through a complex signaling cross-talk between the innate and adaptive immune systems. In response to the type of pathogen presented by conventional antigen-presenting cells (APCs) in a cytokine milieu produced by activated APCs, naive CD4⁺ T cells can differentiate to specific lineages, Th1, Th2, Th17 and Treg cells, as categorized by the expression of specific transcription factors, and have distinct effector functions with specific cytokine profiles (Figure 1). In the presence of IFN-γ and IL-12, naive CD4 T cells differentiate towards Th1 in a process that is dependent on the activities of STAT1, the transcription factor T-bet and STAT4. Th1 cells produce large quantities of IFN-γ and play a critical role in protective immunity against intracellular pathogens through the activation of macrophages.^{1, 34, 35, 36} IL-4 promotes Th2 differentiation through the activation of STAT6 and the transcription factor GATA3. Th2 cells produce IL-4, IL-5, IL-13 and IL-25, which are important for the orchestration of humoral immune responses clearing extracellular pathogens and parasites through the induction of immunoglobulin class switching to immunoglobulin G1 and immunoglobulin E, respectively.^{34, 35, 37, 38} The newly identified Th17 cells differentiate from naive CD4⁺ T cells in response to IL-6 and TGF-β, can produce IL-17A, IL-17F and IL-22, play important roles in the clearance of extracellular bacteria and fungi, and have been linked to autoimmune disorders.^{3, 5} Differentiation of both Th17 and Treg cells from naive CD4⁺ cells requires TGF-β.³⁹ IL-6 activates STAT3 and, in combination with TGF-β signaling, increases the expression of the retinoid-related orphan receptor (ROR)γt and RORα transcription factors, resulting in the initiation of Th17 differentiation.^{40, 41, 42, 43} TGF-β also promotes the expression of the transcription factor Foxp3 that, in the absence of IL-6, blocks the activities of RORα and RORγt and allows CD4⁺ T cells to differentiate into Treg cells.⁴⁴

Figure 1.

Model for Th cell differentiation from naive CD4⁺ T cells. In the presence of IL-12, differentiation of naive CD4⁺ T cells into Th1 cells requires activation of the master regulator transcription factor T-bet through STAT1 and STAT4. Th1 cells produce IFN-γ and are involved in cell-mediated immunity against intracellular bacteria and viruses. IL-4 promotes the activation of STAT6 and GATA3, which are responsible for Th2 cell differentiation. Th2 cells are important in humoral immunity against parasites, an action that is mediated through their production of IL-4, IL-5 and IL-13. The combination of TGF-β and proinflammatory cytokines, such as IL-6 and IL-23, drives the differentiation of naive CD4⁺ T cells into IL-17-producing Th cells (Th17) through the regulation of STAT3 and RORγt. Th17 cells play a critical role in host protection against extracellular pathogens and in inflammatory autoimmune diseases. In addition, TGF-β can induce differentiation of naive CD4⁺ T cells into Foxp3⁺ Treg cells, which produce TGF-β and IL-10 and act as modulators of immune responses. APC, antigen-presenting cell; Foxp3⁺, forkhead box p3⁺; IFN, interferon; MHC–TCR, major histocompatibility complex–T-cell receptor; ROR, retinoid-related orphan receptor; TGF, transforming growth factor; Th, T helper; Treg, regulatory T.

Th17 differentiation and maintenance in autoimmune disease

We have recently provided new evidence suggesting that the development of Th17 cells is driven by a complex dichotomous process in autoimmune disease, as characterized by an initial differentiation phase and a latter phase of survival and expansion of committed Th17 cells. Several cytokines, such as TGF-β, IL-6, IL-1β, IL-21 and IL-23, have been demonstrated to induce Th17 cell differentiation, critically contributing to the clinical outcome of autoimmune diseases.^{45, 46, 47, 48} IL-6 and TGF-β have been reported as the minimal requirements for murine Th17 cell differentiation from naive CD4⁺ T cells.⁴⁸ IL-1 signaling in T cells was reported to be critically required for the early programming of Th17 cell lineage in vivo in a mouse EAE model.⁴⁹ In humans, IL-1β, a proinflammatory cytokine produced predominantly by macrophages, monocytes and dendritic cells, is essential for developing Th17 cells along with IL-6, TGF-β and IL-21.⁵⁰ IL-1 and IL-6 can induce IL-1 receptor expression in T cells.⁵¹ TGF-β and IL-6 also induce the expression of IL-23 receptor on differentiating Th17 cells.⁵² IL-23 is important to the amplification and stabilization of the Th17 phenotype.⁵³ IL-1 can synergize with IL-6 and IL-23 to regulate Th17 cell differentiation and to maintain cytokine expression in effector Th17 cells.⁴⁹ Another cytokine, IL-21, produced by Th17 cells, has been shown to provide an additional autocrine amplificatory signal.⁴⁷ The cytokines IL-6, IL-21 and IL-23 all utilize the JAK–STAT pathway and activate STAT3.^{40, 42, 54}

Our recent study revealed that IL-7 plays a crucial role in the in vivo maintenance of differentiated Th17 cells in EAE. This discovery was initially linked to a polymorphism in the IL-7 receptor (IL-7R) alpha subunit locus, which was identified as an important susceptibility factor for MS.^{55, 56, 57} It is known that IL-7 binds to IL-7R and activates the JAK/STAT5 and PI3K–AKT signaling pathways.⁵⁸ As a T-cell growth factor, IL-7 plays a role in the regulation of peripheral homeostasis of the CD4 T-cell pool.⁵⁹ Upon T-cell receptor activation of naive T cells, IL-7R is downregulated. Cua et al. recently indicated that IL-23 may play an important role in the terminal differentiation of Th17 cells, potentially through its effect on re-expression of IL-7R on Th17 cells.⁵³ Our study shows that IL-7 is essential for the survival and expansion of pathogenic Th17 cells in EAE.⁶⁰ IL-7 directly triggered expansion of effector Th17 cells in EAE and human Th17 cells in MS subjects, but it was not required for Th17 differentiation.⁶⁰ IL-7R antagonism rendered differentiated Th17 cells susceptible to apoptosis through altered expression levels of the prosurvival protein Bcl-2 and the proapoptotic protein Bax, leading to decreased EAE severity.⁶⁰ Our study leads to the new understanding that there are two distinct steps in the development of Th17 cells: induction/terminal differentiation and survival/expansion, in which several cytokines are involved (IL-6, IL-21, IL-23 and IL-7) and play different roles during the two phases of Th17 development (Figure 2).

Figure 2.

Critical role of IL-7/IL-7R signaling in survival and expansion of differentiated Th17 cells. Th17 cell development is a dichotomous process that is regulated through a complex cytokine network. The differentiation of Th17 cells is mainly mediated by STAT3 signaling through cytokines such as IL-6, IL-21 and IL-23. There is dynamic expression of IL-7R on Th17 cells in the course of T-cell activation/differentiation. In the latter phase, where IL-7R is re-expressed, IL-7 is critically required to sustain survival and expansion of differentiated Th17 cells through STAT5 signaling. IL-7R antagonism renders differentiated Th17 cells susceptible to apoptosis through altered expression of the proapoptotic protein Bax and the antiapoptotic molecule Bcl-2. APC, antigen-presenting cell; IL-7R, IL-7 receptor; MHC–TCR, major histocompatibility complex–T-cell receptor; TGF, transforming growth factor; Th, T helper.

Treg development and function in autoimmune disease

The cytokine milieu not only regulates the Th1 and Th17 response but also affects Treg induction and function. Treg cells express the transcription factor Foxp3, which is not only a lineage specification factor but also a functional marker of Treg cells.^{61, 62, 63} Genetic defects in the foxp3 gene, which affect the development and function of Treg cells, have been identified as being responsible for X-linked recessive inflammatory disease in Scurfy mice and immunodysregulation polyendocrinopathy enteropathy X-linked syndrome in humans.^{64, 65, 66, 67} Furthermore, ectopic expression of Foxp3 is sufficient to confer regulatory properties to conventional T cells.^{61, 68, 69}

Multiple signals, such as T-cell receptor and the costimulatory molecule CD28, are required for the thymic development of Treg cells.^{70, 71, 72} Common γ-chain cytokines, IL-2 and, to a lesser extent, IL-7 and IL-15, are required for Foxp3 expression and Treg cell development in the thymus.^{73, 74, 75, 76, 77} Gene knockout of γ-chains in mice leads to complete absence of Foxp3⁺ Treg cells in the thymus and the peripheral immune system.⁷⁸ In this process, the transcription factor STAT5, which is downstream signal of the activation of γ-chain cytokine receptors, is shown to bind to the Foxp3 promoter.^{79, 80}

There are two types of Foxp3-expressing Treg cells: thymic-derived nTreg cells that prevent autoimmunity and post-thymic-induced iTreg cells that maintain a non-inflammatory environment in the gut, to control immune responses to environmental and food allergens and to decrease chronic inflammation. However, iTreg cells that differentiate from naive CD4⁺ T cells in the peripheral immune system in response to TGF-β seem less stable than nTreg cells.^{44, 81} The T-cell receptor repertoire of nTreg cells that are selected by high-avidity interactions in the thymus is mostly against self-antigens, whereas that of iTreg cells that are differentiated from conventional CD4⁺ is similar to the naive conventional CD4⁺ T-cell repertoire.⁸² Cytokines, e.g., IL-4, IFN-γ and IL-6, which induce other Th cell differentiation, mostly dampen iTreg development.⁸³ Inhibition of TGF-β-induced Treg cells by these cytokines or by a high amount of costimulation could be suppressed by retinoic acid.^{84, 85, 86} CD103⁺ dendritic cells isolated from the small intestine and the mesenteric lymph node, which produce both TGF-β and retinoic acid, efficiently promote iTreg differentiation.^{87, 88}

Although Treg cells comprise a specific lineage of T-cell subset, a recent study has revealed that in vivo, Foxp3 was transiently expressed in a substantial portion of cells called ‘exFoxp3' T cells.⁸⁹ These ‘exFoxp3' T cells show a memory phenotype and produce proinflammatory cytokines such as IFN-γ and IL-17. Moreover, an inflammatory milieu in the context of self-antigen can induce ‘exFoxp3' T cells, which could differentiate from nTreg or iTreg cells.⁸⁹ Furthermore, recent studies have indicated that Foxp3⁺ Treg cells are not a homogeneous population; they can/cannot co-express T-bet, IFN regulatory factor 4 or STAT3 with Foxp3 to control various subsets of effector T cells.^{90, 91, 92} IFN-γ induces T-bet expression in Foxp3⁺ Treg cells, which could promote CXCR3 expression to facilitate migration of Treg cells to Th1 inflammatory sites. T-bet is also critical for Treg cell homeostasis and function during the Th1 response.⁹⁰ Similarly, Treg cells can also express IFN regulatory factor 4, which is an essential transcription factor for Th2 cell differentiation. IFN regulatory factor 4 deficiency in Treg cells resulted in selective dysregulation of Th2 response in vivo.⁹¹ Furthermore, suppression of the Th17 response by Treg cells was inhibited by specific ablation of Stat3 in Treg cells.⁹² Taken together, Foxp3⁺ Treg cells are more plastic in vivo; under inflammatory conditions, Treg cells could lose the expression of Foxp3 and become pathogenic T cells contributing to the inflammatory response or could acquire the transcriptional machinery of a particular type of CD4 effector T cells, efficiently regulating the corresponding type of immune response.

Foxp3⁺ Treg cells control the immune response through multiple mechanisms. Treg cells are potent suppressors of T cells. Many studies have demonstrated that Treg-mediated suppression occurs through inhibition of production of IL-2 mRNA in the responder T cells.⁹³ Although Treg cells exert their suppressive function in a cell–cell contact manner in vitro, it should not be ruled out that Tregs could secrete soluble factor(s). IL-35, a new inhibitory cytokine, contributes to Treg function by directly acting on responder T cells.⁹⁴ Another candidate factor is galectin-1, which induces cell cycle arrest of responder cells.⁹⁵ One other mechanism is cytolysis of target cells; Treg cells can express granzyme A or B and kill target cells.^{96, 97} Furthermore, Treg cells could downregulate or inhibit upregulation of CD80/86 in APCs through CTLA-4.⁹⁸ Catalytic inactivation of extracellular adenosine triphosphate by CD39 expressed by Treg cells represents a mechanism that prevents the deleterious effects of adenosine triphosphate on APC function.^{99, 100} Moreover, IL-10 and TGF-β have been shown to be important mediators of Treg-mediated suppression of colitis, type 1 diabetes and EAE.^{101, 102, 103, 104, 105}

Interplay between Th17 and Treg

Distinct populations of Th17 and Treg cells coexist and are reciprocally regulated during differentiation in healthy tissue.³⁹ Imbalances in the ratio of these lymphocytes have been implicated in a wide range of autoimmune disorders including MS and RA. Bettelli et al. have demonstrated a reciprocal developmental relationship between Foxp3⁺ Treg and Th17 cells; TGF-β triggers the expression of Foxp3 in naive T cells, whereas TGF-β drives Th17 differentiation from naive T cells in combination with IL-6.³⁹ It is well established that inflammatory cytokines such as IL-6 and IL-21 inhibit TGF-β-induced Treg generation.^{39, 85, 106} However, blocking IL-6 signaling in vivo can generate Foxp3⁺ Treg cells and can reduce EAE severity.¹⁰⁷ Furthermore, Infliximab therapy leads to the generation of CD4⁺CD25^hiFoxp3⁺ Treg cells that are CD62L^low and suppress effector T cells through TGF-β and IL-10.¹⁰⁸ Recent findings showed that Treg cells could be converted to IL-17-producing cells. Proinflammatory cytokines, such as IL-6, IL-1 and IL-21, were able to decrease the Foxp3 expression levels of committed iTreg or nTreg cells and induce IL-17 production by these cells in a TGF-β-dependent manner.⁸¹ In addition, the IL-17⁺/Foxp3⁺ T cells were functionally impaired.⁸¹ These findings indicate that the proinflammatory milieu could convert Treg cells to Th17 cells and shift the balance between immune regulation and inflammation towards inflammation.

In short, proinflammatory cytokines, e.g., IL-6, IL-1β and IL-21, not only support Th17 differentiation but also constrain Treg development by affecting the balance between RORγt and Foxp3.⁸¹ In contrast, IL-2 and retinoic acid, which support Treg differentiation, could perturb Th17 development.^{86, 109} Furthermore, cytokines secreted by Th17 cells, such as TNF-α and IL-21, could diminish Treg function, while IL-10 and IL-35 are able to suppress the proinflammatory response. Taken together, these findings show that the persistence of inflammation in lesion sites is significantly associated with reduced levels and impaired function of CD4⁺CD25⁺Foxp3⁺ Treg cells induced by proinflammatory cytokines. Targeting these cytokines, which restores the functions of Treg cells and induces new Treg cells, holds considerable potential as a treatment for autoimmune disease.

Therapeutic strategies for autoimmune diseases

The understanding of the interplay between pathogenic and regulatory T cells holds great promise in the development of effective therapeutic approaches to autoimmune diseases. Novel therapies are being designed to restore the balance of the two groups of T cells through inhibition of Th17 and promotion of Treg cells. Many companies are targeting cytokines to affect the development and function of Th17 and Treg cells. These approaches are summarized in Table 1.

Table 1. Cytokine therapeutic strategies for autoimmune diseases.

Cytokine	Therapeutic agent (company: drug name)	Stage of clinical trial	Therapeutic outcome	Reference
IL-6	Anti-IL-6 receptor (Roche: Actemra/Tocilizumab)		RA: efficacious	110
IL-12/IL-23	Anti-P40 (Centocor: Stelara/Ustekinumab)	MS: discontinued	MS: failed	116
		Crohn's: phase IIb	PS: efficacious
	Anti-P40 (Abbot: ABT-874)	Crohn's: phase II		117, 118
		PS: phase II
		MS: phase II
	Antagonist (SYNTA: Apilimod/STA5326)	RA: phase II		119, 120
		Crohn's: phase II
IL-23	Anti-P19 (Shering Plough)	Phase I
	Anti-IL-23 (Eli Lilly)	Phase I
IL-1β	IL-1β antagonist (Amgen: Kineret/Anakinra)		RA: approved	121
	Anti-IL-1 (Amgen: AMG108)	RA: phase II		121
IL-17	Anti-IL-17 (Eli Lilly: LY2439821)	RA: phase II		122, 123
		PS: phase II
	Anti-IL-17 (Norvatis: AIN457)	RA: phase II		122, 123
		Crohn's: phase II
	Anti-IL-17 (Amgen)	RA: phase Ib/IIa		122
TNF-α	Soluble receptor (Amgen: Enbrel/etanercept)		RA: approved	111, 124
			MS: failed
	Anti-TNF-α (Centocor: Remicade/Infliximab)	Crohn's: phase III	RA: approved	111, 125
			MS: failed
	Anti-TNF-α (Abbot: Humira/Adalimumab)	Crohn's: phase III	RA: approved	111, 126
		Completed	MS: failed
IL-2	Anti-IL-2 receptor (Biogen-Idec: Zenapax/Dacliqumab)	MS: phase II		127
		Completed
BAFF/April	Soluble receptor (Zymogenetics/Merck: Atacicept)		MS: failed	128

Abbreviations: Crohn's, Crohn's disease; MS, multiple sclerosis; PS, psoriasis; RA, rheumatoid arthritis.

In general, the various anticytokine approaches are relatively successful in the treatment of autoimmune diseases. Therapeutic agents against IL-6 and TNF-α pathways are efficacious in RA despite the fact that these drugs show side effects, such as the elevation of cholesterol levels in patients treated with anti-IL-6R¹¹⁰ and the development of CNS demyelination in anti-TNF-α treated RA subjects.¹¹¹ An antibody against the p40 subunit of IL-12 and IL-23 (ustekinumab) is very efficacious in the treatment of psoriasis. A few companies are developing anti-IL-17 antibodies based on knowledge of the pathological role of Th17 cells. Unfortunately, MS treatment is an exception. Ustekinumab and anti-TNF-α agents all failed in MS clinical trials. The possibility that therapeutic agents against the IL-6 and IL-23 pathways that target Th17 differentiation may not work in MS has been suggested by EAE experiments where anti-IL-6 or anti-IL-23 treatments were efficacious when they were applied at the time of immunization but not at the peak of disease.^{41, 112} Targeting mature instead of differentiating pathogenic Th cell subsets could be a viable alternative. An antibody against the IL-2R subunit CD25 is currently in phase II trials for MS.¹¹³ Interestingly, IL-2R and IL-7R were identified as genes associated with MS susceptibility.⁵⁶ These two cytokine receptors have roles in the survival and expansion of committed pathogenic Th1 and Th17 cells.^{60, 114} An additional advantage to the therapeutic use of an IL-7R-specific antibody is that Treg cells responsible for resolving inflammation express very low levels of IL-7R and will be unaffected.^{60, 115}

Conclusion

It is important to maintain an appropriate balance between Treg cells and effector Th17/Th1 cells that can ensure effective immunity while avoiding pathological autoimmunity. The proinflammatory cytokine milieu produced during an immune response plays an important role in driving the imbalance between these two T-cell subsets and the outcome of inflammatory autoimmune diseases. The dynamic nature of an immune response was recently highlighted by the demonstration of interconversion between Th17 and Treg cells under the influence of a particular cytokine microenvironment. A better understanding of the molecular mechanisms regulating Treg and Th17 cells in vitro and in vivo will create opportunities for the development of therapeutic approaches, including anticytokine therapies that could be used to treat human autoimmune diseases.