CD4+ Th cells are responsible for orchestrating diverse, pathogen-specific immune responses through their differentiation into a number of subsets, including TH1, TH2, TH9, T follicular helper, T follicular regulatory, and regulatory T cells. The differentiation of each subset is guided by distinct regulatory requirements, including those derived from extracellular cytokine signals. IL-2 has emerged as a critical immunomodulatory cytokine that both positively and negatively affects the differentiation of individual Th cell subsets. IL-2 signals are propagated, in part, via activation of STAT5, which functions as a key regulator of CD4+ T cell gene programs. In this review, we discuss current understanding of the mechanisms that allow IL-2–STAT5 signaling to exert divergent effects across CD4+ T cell subsets and highlight specific roles for this pathway in the regulation of individual Th cell differentiation programs.

The cytokine IL-2 was first discovered in 1976 as a T cell growth factor (1). Subsequently, a large body of literature has identified IL-2 as a pivotal cytokine responsible for the formation and function of lymphocyte populations, including CD4+ Th cells. IL-2 exhibits a broad range of functions across Th cell populations and is an essential regulator of numerous signaling pathways, including those underlying cell survival, proliferation, differentiation, and effector functions. Thus, predictably, dysregulation of IL-2 signaling has been identified as a key factor in the genesis of autoimmune disorders and immunodeficiency. In this study, we discuss basic aspects of the IL-2 signaling pathway and its pleiotropic effects on the differentiation of effector and regulatory CD4+ T cell populations.

IL-2 signals through a heterotrimeric receptor composed of IL-2Rα, IL-2Rβ, and the γ common (γc) chain subunits (28). Expression of individual subunits varies across immune cell populations, and expression levels are dependent upon both immune cell activation state and cytokine signals. In CD4+ T cells, although the γc chain is expressed constitutively, IL-2Rα and IL-2Rβ expression are upregulated following TCR stimulation and further induced by STAT5 activity downstream of IL-2 signaling (2, 912). In the presence of environmental IL-2, assembly of the trimeric form of IL-2R occurs sequentially, beginning with IL-2/IL-2Rα binding, followed by association with IL-2Rβ and subsequent γc chain recruitment. Formation of this heterotrimeric receptor results in high-affinity IL-2 signaling (12, 13). Although the trimeric receptor combination predominates, weaker signals may be relayed through IL-2Rβ/γc, although downstream mechanisms are altered because of diminished IL-2R binding affinity (14, 15).

Highlighting the importance of effective IL-2 signaling in immune function, mutations in IL-2R and downstream signaling molecules have been implicated in the generation of disrupted immune cell responses in humans. For example, mutations in the γc receptor subunit or JAK3 result in SCID diseases (1619). Conversely, patients with IL-2Rα or IL-2Rβ mutations develop autoimmune disease because of disrupted immune tolerance while paradoxically also exhibiting immunodeficiency (2, 2023). Given its role in such disease states, the IL-2 signaling pathway is an attractive target for a number of immunotherapies, which focus on modulation of IL-2 responses to target individual T cell populations for the treatment of not only autoimmune disease but also graft-versus-host disease and cancer. Different strategies to manipulate IL-2 signaling for therapeutic benefit will be briefly discussed below and have also been extensively reviewed elsewhere (2, 12, 24).

IL-2 signaling is propagated through a number of signaling cascades, including the JAK/STAT pathway (12, 13). Upon high-affinity IL-2 signaling, JAK1 and JAK3 associate with IL-2Rβ and γc, respectively, resulting in cross-phosphorylation and JAK activation (18, 2530). Next, JAK1 and JAK3 phosphorylate specific tyrosine resides on IL-2Rβ, allowing for recruitment of members of the STAT family via their conserved SH2 domain (27, 31) (Fig. 1). Whereas IL-2 has been shown to activate several STAT family members, including STAT1, STAT3, and STAT5, STAT5 is the predominant IL-2 signaling molecule (32, 33). Upon recruitment, two STAT5 isoforms are activated via JAK-mediated phosphorylation of the tyrosine 694 (STAT5A) and 699 (STAT5B) residues (34, 35) (Fig. 1). This results in STAT5 dimerization, nuclear translocation, and downstream transcriptional activities via direct DNA binding and cofactor recruitment (30). It is important to note that in addition to IL-2, signals from other cytokines, including IL-7, IL-9, and IL-15, are also propagated through STAT5. Whereas this review focuses on the role of STAT5 downstream of signals from IL-2, these other STAT5-dependent pathways have been expertly discussed elsewhere (3640).

FIGURE 1.

Schematic depicting the key domains and phosphorylation sites of STAT5A and STAT5B. IL-2 signaling through the high-affinity, trimeric IL-2R complex results in STAT5 activation via tyrosine phosphorylation at Y694 (STAT5A) and Y699 (STAT5B). Tyrosine-phosphorylated STAT5 dimerizes and translocates to the nucleus where it performs diverse functions to directly regulate the expression of individual target genes. STAT5 activity can also been modulated via phosphorylation of specific serine residues, detailed above. In addition to dimerization, STAT5 is also capable of forming tetramers mediated via STAT5 N-terminal domain interactions. Tetramerization requires specific isoleucine, tryptophan, lysine, phenylalanine, and leucine residues, as noted, as well as DNA binding at sequential γ-activated sequence motifs (sequence provided above). Other domains involved in STAT5 function are also highlighted.

FIGURE 1.

Schematic depicting the key domains and phosphorylation sites of STAT5A and STAT5B. IL-2 signaling through the high-affinity, trimeric IL-2R complex results in STAT5 activation via tyrosine phosphorylation at Y694 (STAT5A) and Y699 (STAT5B). Tyrosine-phosphorylated STAT5 dimerizes and translocates to the nucleus where it performs diverse functions to directly regulate the expression of individual target genes. STAT5 activity can also been modulated via phosphorylation of specific serine residues, detailed above. In addition to dimerization, STAT5 is also capable of forming tetramers mediated via STAT5 N-terminal domain interactions. Tetramerization requires specific isoleucine, tryptophan, lysine, phenylalanine, and leucine residues, as noted, as well as DNA binding at sequential γ-activated sequence motifs (sequence provided above). Other domains involved in STAT5 function are also highlighted.

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The expression and function of both STAT5 isoforms are critical for immune cell development and function as their loss, similar to γc and JAK3, has been shown to result in SCID in mice (41). Curiously, in humans, individual mutations in STAT5A and STAT5B have highlighted their distinct roles in immune cell development and function as STAT5B defects alone are sufficient to drive autoimmunity (42). Subsequent studies have since determined that these differential functions can be attributed to distinct gene targets for each isoform, discussed further below (43, 44).

Although STAT5 is a critical downstream mediator of IL-2 signaling, studies using expression of constitutively active STAT5 have demonstrated that its activity alone is insufficient to reproduce the full biological effects of IL-2 (45). Indeed, IL-2 has also been shown to signal via the MAPK pathway via ERK as well as the PI3K pathway (12, 14, 46). IL-2–dependent activation of PI3K results in the subsequent activation of mammalian target of rapamycin and protein kinase B (46). Collectively, these pathways have been implicated in regulating aspects of CD4+ T cell differentiation, effector function, metabolism, and survival.

STAT5 broadly influences the development of a number of immune cell populations through a diverse set of regulatory mechanisms. Such functional diversity is in turn mediated via mechanisms that control STAT5 activities. This includes alterations in STAT5 activation status mediated by posttranslational modifications, including phosphorylation. First, as discussed briefly above, phosphorylation at Y694/9 (STAT5A/B) by JAK1/3 downstream of IL-2 signaling leads to its dimerization and subsequent transcriptional activity (35, 46). In addition to Y694/9, a number of serine residues have also been identified as phosphorylation sites within STAT5A and B, including S127/8 (STAT5A), S725 (STAT5A), S730 (STAT5B), and S780 (STAT5A) (Fig. 1) (4649). Although IL-2–dependent phosphorylation of these sites has been suggested to modulate STAT5 transcriptional activity, their precise functions and roles in Th cell regulation are areas of active investigation (50, 51).

Given the requirement for STAT5 phosphorylation in propagating its function, predictably, dephosphorylation of STAT5 also regulates its activity by diminishing phosphorylation-dependent mechanisms. Enzymes identified as mediators of this process include the SH2 domain-containing protein and low m.w. protein tyrosine phosphatases, which have both been shown to dephosphorylate STAT5 tyrosine residues (52, 53). Additionally, the dual-specificity phosphatase family member dual-specificity phosphatase 4 has been shown to dephosphorylate both tyrosine and serine residues to regulate STAT5 activity (54, 55).

Beyond phosphorylation, STAT5 activity is also modulated via differential oligomerization states. Following dimerization, STAT5 is capable of forming tetramers at specific tandemly linked γ-activated sequence DNA binding motifs (5659). Tetramer formation is dependent upon conserved isoleucine 28, tryptophan 37, lysine 70, phenylalanine 81, and leucine 82 residues in the N-terminal domain of both STAT5A and STAT5B (57, 58, 60) (Fig. 1). Functionally, it has been established that mice incapable of forming STAT5 tetramers exhibit a reduced ability to generate a number of immune cell populations, including CD4+CD25+ regulatory T (TREG) cells (57). Mechanistically, STAT5 tetramerization is essential for the direct regulation of a specific subset of genes activated in response to IL-2 signals, including the IL-2R subunit Il2ra (CD25) and effector cytokines Ifng and Tnf (57). The mechanisms underlying dimeric and tetrameric STAT5 function also appear to be distinct as tetrameric STAT5 has been observed predominantly within intronic regions, whereas dimers are found proximal to the transcriptional start sites of target gene loci (57). To date, much of the mechanism underlying STAT5 tetramer function, including the identity of recruited coregulators in Th cells, remains unclear. However, in B cells, tetrameric STAT5 has been shown to recruit the histone methyltransferase Ezh2, which works in concert with the polycomb repressive complex 2 to regulate histone modifications (61). Although this role has not been directly demonstrated in CD4+ T cells, Ezh2 is known to regulate TH1, TH2, and TREG cell programs, suggesting a potential mechanism by which STAT5 tetramerization may regulate gene expression patterns in these populations (62).

In addition to Ezh2, interactions between STAT5 and other transcriptional regulators are known to influence STAT5 transcriptional activity and downstream functions. In TREG cell populations, STAT5 has been shown to recruit the 10–11 translocation enzymes Tet1 and Tet2, which function to demethylate target gene loci (63). Additionally, STAT5 has been shown to interact with a number of other coregulators, including the transcriptional coactivators p300/CREB-binding protein and nuclear receptor coactivator 1/steroid receptor coactivator 1 as well as corepressors, such as silencing mediator for retinoic acid receptor and thyroid hormone receptor, Sac3 domain-containing protein, and suppressor of cytokine signaling family member SOCS7 (64, 65). Ultimately, such interactions result in alterations to gene expression via changes in cytokine signaling and the accessibility of target gene loci. It is important to note that although much of the work defining STAT5 interaction with these factors was performed outside of the context of CD4+ T cells, it is possible that these interactions are conserved within Th cell subsets. Thus, they may represent mechanisms underlying differential STAT5 activities across T helper gene programs.

CD4+ T cells are essential to the adaptive immune response as they coordinate the pathogen-specific effector functions of an array of immune cell populations. Central to this process is the ability of naive CD4+ T cells to differentiate into distinct subsets that perform individual functions during the course of immune responses to allergens, infection, and cancer. Key determinants of CD4+ T cell differentiation include signals from environmental cytokines, which drive the differentiation and function of individual subsets. Of these, IL-2 has been found to promote or repress the expression of specific Th cell gene programs (12, 66, 67). The dynamic nature of this regulation is dependent upon diverse mechanisms including modulation of both cytokine and cytokine receptor expression as well as positive and negative regulation of cell type–specific transcription factor expression and metabolic programs. The collective effects of IL-2–STAT5 signaling are summarized in Fig. 2.

FIGURE 2.

Functional regulation of individual Th cell gene programs by IL-2–STAT5 signaling. IL-2–STAT5 signaling has been shown to modulate CD4+ Th cell differentiation by both activating and repressing the expression of subset-specific gene programs. Notable genes regulated by STAT5, including those encoding cytokines, cytokine receptors, and key transcription factors, are highlighted above. Starred genes are those induced/repressed in response to IL-2 signaling, but, to our knowledge, STAT5 has not been found to directly associate with these target gene loci.

FIGURE 2.

Functional regulation of individual Th cell gene programs by IL-2–STAT5 signaling. IL-2–STAT5 signaling has been shown to modulate CD4+ Th cell differentiation by both activating and repressing the expression of subset-specific gene programs. Notable genes regulated by STAT5, including those encoding cytokines, cytokine receptors, and key transcription factors, are highlighted above. Starred genes are those induced/repressed in response to IL-2 signaling, but, to our knowledge, STAT5 has not been found to directly associate with these target gene loci.

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TH1 cells.

TH1 cells coordinate immune responses to intracellular pathogens and promote anti-cancer immunity through the production of effector molecules, including the inflammatory cytokine IFN-γ (68, 69). TH1 differentiation is driven by signals from the cytokine IL-12, which are propagated through a heterodimeric receptor consisting of IL-12Rβ1 and IL-12Rβ2. This results in the activation of STAT4 and the subsequent induction of the transcriptional activator T-bet (7072). T-bet, the lineage-defining transcription factor for the TH1 cell type, drives expression of IFN-γ, which further promotes T-bet expression via STAT1 activation in a feed-forward fashion (7376). TH1 differentiation also relies on expression of the transcriptional repressor Blimp-1, which promotes a terminal effector state and suppresses alternative T cell fates (77).

In addition to the above, IL-2 has well-characterized roles in the positive regulation of TH1 differentiation. This has been demonstrated by studies illustrating that CD4+ T cells lacking either IL-2 or IL-2Rα display marked deficiencies in TH1 development (12, 78, 79). Mechanistically, STAT5 activated downstream of IL-2 binds to gene-specific regulatory elements to directly induce the expression of IL-12Rβ2, Blimp-1, and IFN-γ to support both TH1 cell differentiation and effector cytokine production as well as cytotoxic function in a subset of cells (78, 80, 81). In this context, IL-2–STAT5 signals are amplified through a feed-forward mechanism as STAT5 directly induces expression of IL-2Rα. Conversely, IL-2–activated STAT5 directly represses expression of Bcl-6, which drives the differentiation of the alternative T follicular helper (TFH) cell gene program (66, 8285).

IL-2 signaling has also been implicated in regulating Th cell differentiation via modulation of subset-specific metabolic pathways. In TH1 cells, IL-2 signals promote glycolytic metabolism by both governing the activity of the transcriptional regulators c-Myc and HIF-1α and by repressing the glycolytic pathway antagonist Bcl-6 (86). Increased glycolysis further supports TH1 effector function by enhancing the production of IFN-γ (87, 88). Additionally, IL-2 signaling in TH1 cells promotes activation of the serine–threonine kinase protein kinase B and the metabolic regulator mammalian target of rapamycin. These factors drive the expression of T-bet and Blimp-1, clonal expansion, nutrient uptake, cellular growth, and alterations to epigenetic marks and chromatin accessibility to support effector differentiation in TH1 cells (8991). Finally, IL-2 drives the accumulation of α-ketoglutarate, a metabolite generated during glutaminolysis, which supports TH1 differentiation via CCCTC-binding factor–mediated restructuring of the chromatin landscape (92). Thus, IL-2 signals drive TH1 differentiation by supporting TH1 gene expression patterns and metabolic function while also repressing alterative gene programs.

TH2 cells.

TH2 cell populations are critical for immune responses to extracellular parasites and have also been implicated in the development of asthma and other allergic disorders (93). TH2 cells secrete a number of effector cytokines, including IL-4, IL-5, IL-9, and IL-13, which exhibit various functions, including the modulation of B cell immunoglobin class switching, activation of eosinophils and basophils, and macrophage polarization (94). Differentiation of TH2 cells requires signals from IL-4, which are propagated via activation of STAT6 (95, 96). IL-4–STAT6 signaling drives expression of the TH2 lineage-defining transcription factor GATA binding protein 3, which stabilizes the TH2 program by directly inducing the expression of key TH2 target genes, including those encoding the effector cytokines discussed above (97102).

IL-2–STAT5 signaling positively regulates TH2 differentiation as studies have shown that IL-2 neutralization disrupts expression of the TH2 gene program, whereas conversely, expression of constitutively active STAT5 is sufficient to induce a subset of TH2 genes (93, 103). Functionally, IL-2–STAT5 signaling promotes TH2 differentiation in part by inducing expression of the IL-4R subunit, IL-4Rα, thereby increasing TH2 responsiveness to IL-4 signals (104). Additionally, STAT5 has been shown to bind to and modulate accessibility of the Il4 cytokine locus (105). STAT5 also regulates Il4 expression by promoting expression of the transcription factor cMAF and NLR family pyrin domain-containing 3, which are both direct inducers of IL-4 expression (93, 105108). Beyond cytokine regulation, IL-2 signaling supports the TH2 program by promoting the expression of Blimp-1, which represses key aspects of alternative Th cell gene programs, including expression of the TFH lineage-defining transcription factor Bcl-6 (77, 109, 110). Collectively, IL-2–STAT5 signaling promotes TH2 differentiation by modulating both effector cytokine and key transcription factor expression.

TH9 cells.

IL-9–producing TH9 cells have been implicated in the clearance of extracellular pathogens, including parasites, inflammatory allergic responses, and anti-tumor immunity (111115). Although the mechanisms underlying their formation are still being characterized, their differentiation is induced at least in part via signals received from the cytokines TGF-β1 and IL-4, which promote the expression of downstream TH9-associated transcription factors PU.1 and IRF4 (115117). TH9 cells share developmental requirements with TH2 cells, including a dependence on both IL-4–STAT6 signaling and the subsequent induction of GATA binding protein 3.

A number of studies have identified IL-2 as a positive regulator of TH9 effector function as IL-9 production is disrupted upon the addition of IL-2– and/or IL-2Rα–neutralizing Abs (118, 119). A recent study by Warren Leonard’s laboratory expanded upon this understanding to define the mechanisms underlying IL-2 function in TH9 cells (120). Briefly, this study established that the addition of IL-2 to TH9 cell cultures augments expression of IRF4, which directly associates with the Il9 promoter to induce its expression. Further, IL-2–dependent STAT5 activation is required for mediating IL-9 production as deletion of STAT5A, STAT5B, or both resulted in a marked decrease in IL-9 expression (120). Mechanistically, in the presence of IL-2 signaling, both STAT5B and STAT6 were found to directly associate with the Il9 locus. Curiously, however, association of both STAT factors was reduced in the absence of IL-2 (120). As such, it has been suggested that IL-2 signals may also support IL-4–STAT6 signaling via positive regulation of IL-4R expression as previously observed in other Th cell populations (104). Finally, opposition between STAT5 and Bcl-6 is also a key aspect of TH9 differentiation (as with TH1 and TH2 populations) as STAT5 induces, whereas Bcl-6 represses, IL-9 production. These factors exhibit adjacent binding sites not only at the Il9 locus but also within additional gene loci differentially expressed in TH9 populations exposed to IL-2 versus the Bcl-6–promoting cytokine IL-21. These findings suggest that STAT5 and Bcl-6 may regulate the TH9 program more broadly (120). Together, these data support roles for IL-2 in promoting TH9 differentiation and function via cytokine receptor, transcription factor, and effector cytokine-mediated mechanisms.

TH17 cells.

TH17 cells secrete IL-17 and promote inflammatory responses that are key for the clearance of extracellular pathogens and mucosal immunity. Their differentiation is dependent upon combined signals from TGF-β and IL-6, which result in expression of the lineage-defining transcription factor RORγt and production of the cytokine IL-23, which further augments TH17 differentiation (121123). Both IL-6 and IL-23 signal via activation of STAT3, which is a key transcriptional activator of both RORγt and IL-17 expression (124, 125).

A number of studies have demonstrated that the IL-2–STAT5 pathway negatively regulates TH17 differentiation and effector functions (124, 126, 127). For example, IL-2 signaling represses expression of the IL-6R subunit IL-6Rα, thus diminishing the sensitivity of activated CD4+ T cells to signals received from IL-6 (78). IL-2–STAT5 signaling also results in reduced expression of the TH17 lineage-defining transcription factor RORγt, which promotes expression of a number of TH17 genes including the Il17a/f cytokine locus (124, 126). Mechanistically, STAT5 has also been shown to repress expression of IL-17 by competing with STAT3 for binding sites located within the Il17a/f locus (124, 126, 128).

Although the role for IL-2–STAT5 in TH17 differentiation seems relatively straightforward, a recent study determined that ablation of IL-2Rα or STAT5 reduced TH17 generation in vivo (128). This may be explained by another study demonstrating that IL-2 signaling, while repressing the differentiation of TH17 populations, may also support their expansion (129). Collectively, the current literature suggests that IL-2 may exert nuanced, stage-specific control over TH17 differentiation, function, and proliferation.

TFH cells.

TFH cells support humoral immunity by both interacting directly with B cells and producing cytokines, including IL-21, to promote B cell activation, germinal center (GC) formation, and high-affinity Ab production (130). TFH cell differentiation from naive CD4+ T cell progenitors is driven by a number of cytokines, including IL-6 and autocrine signals from IL-21, which are propagated via STAT3 activation to promote expression of the TFH lineage-defining transcription factor Bcl-6. Bcl-6 broadly supports expression of the TFH gene program by repressing expression of the TFH antagonist Blimp-1 as well as other transcription factors that promote the differentiation of additional Th cell subsets (77, 131133).

The IL-2–STAT5 signaling axis is a well-established negative regulator of TFH cell differentiation and function (8285). In vivo, IL-2–deficient mice exhibit enhanced TFH cell generation and GC formation, even in the absence of infection (134). Conversely, systemic administration of IL-2 during influenza infection results in deficient TFH cell responses and GC formation (82). Similarly, the absence of TREG cells, which function to deplete environmental IL-2, has been shown to reduce the number of TFH cells during influenza infection (135). These phenotypes have been attributed to repression of the TFH gene program via STAT5-dependent mechanisms. First, IL-2–STAT5 signaling has been shown to repress expression of the IL-6R subunits IL-6Rα and gp130, leading to a reduction in IL-6 responsiveness and STAT3 activation (78, 83, 136). STAT5 also competes directly with STAT3 for binding at the Bcl6 promoter to repress its expression. In the absence of Bcl-6, the TFH antagonist Blimp-1 is released from Bcl6-mediated repression and directly suppresses expression of TFH target genes, including Cxcr5 (77, 83). In the absence of Cxcr5, TFH cells are incapable of homing to the B cell follicle to interact with B cells and support GC formation. Finally, loss of Bcl-6 expression also permits both STAT5 binding and reduced DNA methylation within gene loci associated with non-TFH gene programs, allowing for their expression (137). Thus, IL-2–STAT5 signals repress TFH cell differentiation and activity by suppressing both TFH cell–associated cytokine receptor and transcription factor expression.

TREG cells.

TREG cell populations comprise a unique compartment of Th cells that modulate immune responses, reduce inflammation, and prevent potential autoimmunity mediated by effector T cell populations (138, 139). These cells are subdivided into thymically derived natural TREG cells and those induced in mature CD4+ T cells in the periphery (induced TREG cells) (140, 141). Differentiation of TREG cell populations is dependent upon signals from TGF-β1 and expression of the lineage-defining transcription factor Foxp3, which performs both activating and repressive functions to support expression of the TREG gene program (142). A hallmark of TREG cell populations is their constitutive expression of IL-2Rα, which allows TREG cells, but not other T cell populations, to respond to low doses of IL-2 via constitutive high-affinity IL-2 signaling (143). Unlike effector Th cell subsets, TREG cell populations do not produce IL-2 and, indeed, one function of Foxp3 is to repress Il2 expression (2, 13, 144). Instead, TREG cells depend upon paracrine IL-2 signals from effector Th cells (13, 139).

It is well-established that TREG cell deficiency results in the development of severe autoimmune phenotypes. Similarly, early studies using germline knockouts for IL-2, IL-2Rα, or IL-2Rβ revealed that germline disruptions in the IL-2 signaling pathway result in an absence of TREG cell generation and suppressive function, leading to the development of autoimmune disorders (139, 145150). However, conditional deletion of IL-2Rα exclusively in TREG cell populations results in a comparatively more severe phenotype because of the aforementioned IL-2 requirements for specific effector populations as well (151). Functionally, IL-2 signaling is required for the early induction of Foxp3 via activation of STAT5, which directly binds to both promoter and enhancer elements to induce its expression (139, 146, 152156). Thus, unsurprisingly, loss of STAT5A/B has been shown to result in a significant reduction in murine Foxp3+ Th cells in vivo (157). In humans, this effect may be STAT5B specific, as STAT5B defects, in the presence of normal STAT5A expression, result in reduced Foxp3 expression and TREG cell suppressive function and are consequently sufficient to induce autoimmune disease (44).

Mechanistically, Foxp3 induction has been attributed to collaborative STAT5 and TGF-β–Smad3–dependent recruitment of the 10–11 translocation (Tet) methylcytosine dioxygenases Tet1 and Tet2 to the Foxp3 locus, which results in decreased DNA methylation and stable Foxp3 expression (158). Further emphasizing the importance of STAT5 activity in TREG cell development, a recent study found that inhibition of the cyclin-dependent kinase 8 and its paralog cyclin-dependent kinase 19 prevented repressive serine phosphorylation of STAT5 (Fig. 1). This resulted in enhanced STAT5 tyrosine phosphorylation and allowed for conversion of CD4+ T cells into Foxp3+TREG cells (48). Furthermore, overexpression of a STAT5B mutant incapable of such phosphorylation promoted Foxp3+TREG cell differentiation to a much higher degree than wild-type STAT5B. Together, these findings support the need for enhanced STAT5 activation signals for the differentiation of TREG cell populations (48). Finally, much like TH1 populations, IL-2 signaling has been found to support TREG cell differentiation via STAT5 activation and Blimp-1 expression as well as by supporting glycolytic metabolism, which is induced by IL-2–induced PI3K signaling in this population (159163).

Expanding upon early global knockout studies, recent work has begun to define dichotomous roles for IL-2 signaling in early TREG cell development versus the stability and suppressive capabilities of mature TREG cell populations. First, in contrast to germline knockout studies, inducible deletion of IL-2Rα after thymic development or exclusively in mature peripheral TREG cells revealed that mature TREG cell populations require IL-2 for long-term survival and homeostasis but not to maintain Foxp3 expression (151, 160). Additionally, these cells exhibit reduced expression of the coinhibitory receptor CTLA4 as well as disrupted metabolic and biosynthetic pathways, further supporting a role for IL-2 in TREG cell differentiation via metabolic modulation (151, 160). Although the precise roles of IL-2 during each stage of TREG cell development are still being elucidated, these findings collectively suggest that IL-2 signaling is required for differentiation and functional maturation in early TREG cell stages and helps to maintain survival, homeostatic proliferation, and metabolic function in mature TREG cell populations.

Because of their central role in regulating immune tolerance and their dependence upon IL-2 signaling, TREG cells have been the target of a number of IL-2–based immunotherapeutic strategies to treat both autoimmune disorders and cancer (2, 164, 165). These include the use of IL-2 targeting Abs, such as JES6-1, which selectively induce TREG cell proliferation in efforts to treat a number of autoimmune diseases (166, 167). Conversely, Abs that target IL-2 and/or modulate IL-2R subunit interactions have been used to limit TREG cell responses and thus promote inflammatory environments that are beneficial in tumor immunotherapy approaches (166, 168, 169). Furthermore, the elevated expression of IL-2Rα on TREG cells has been leveraged in therapies using low doses of the IL-2 cytokine itself to preferentially enhance TREG cell populations (170173). Finally, efforts have been made to engineer altered versions of IL-2 or IL-2Rα that exhibit differential binding affinities and thus preferentially induce or limit TREG cell responses in treatments of autoimmune disease and cancer, respectively (174176).

T follicular regulatory cells.

In addition to their modulation of proinflammatory effector T cell responses, Foxp3+TREG cells have also been shown to suppress GC responses and B cell Ab production via differentiation into the T follicular regulatory (TFR) cell type (159, 177180). These cells can also arise directly from naive CD4+ T cell precursors in response to either self- or foreign Ag (181). TFR cells exhibit both TREG cell– and TFH cell–associated traits, including expression of both Foxp3 and Bcl-6 as well as Cxcr5 (159, 177, 179181). Although IL-2 is needed for initial TREG cell development as discussed above, it is perhaps unsurprising that strong signals from IL-2 have been shown to repress the differentiation of the Bcl-6–dependent TFR cell population (159). Specifically, IL-2 signaling results in activation of STAT5, upregulation of Blimp-1, and subsequent repression of Bcl-6 (159). Thus, as IL-2 production wanes during the resolution of infection, loss of IL-2 signaling allows for the downregulation of IL-2Rα, upregulation of Bcl-6, and the subsequent differentiation of the Foxp3+Treg cell population into TFR cells (159, 182, 183). This is consistent with studies demonstrating that, although IL-2 is required for the initiation of Foxp3 in TREG cell populations, its expression can be maintained in mature TREG cells even in the absence of further IL-2 signals (151, 160). Thus, it would seem that this mechanism allows for concurrent Foxp3 and Bcl-6 expression in the TFR cell compartment. This stage-specific, IL-2–dependent generation of TFR cell populations thus functions to protect against autoimmunity as humoral immune responses are directly suppressed during the resolution of infection (184).

IL-2 signaling has emerged as an essential and multifunctional regulator of an array of immune cell populations, including effector and regulatory CD4+ T cell subsets. As such, it is no surprise that this pathway has long been the target of therapeutic strategies to treat diseases ranging from cancer to autoimmunity. Although the many, and sometimes counterintuitive, roles of IL-2 signaling have made this a challenging endeavor, these efforts have been aided by decades of work that have provided insight into the IL-2–dependent regulatory mechanisms that govern these critical immune responses. Continued research into the mechanisms that govern the IL-2–STAT5 signaling pathway and its subsequent effects on subset-specific cytokine pathways and transcription factors are thus an important avenue for further study moving forward. Such understanding will provide routes for more specific manipulation of IL-2 signals to augment Th cell responses in immunotherapy approaches to treat human disease.

This work was supported in part by National Institutes of Health Grant AI134972 (to K.J.O.).

Abbreviations used in this article:

γc

γ common

GC

germinal center

TFH

T follicular helper

TFR

T follicular regulatory

TREG

regulatory T.

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The authors have no financial conflicts of interest.