T helper (Th) cell differentiation and the dynamic shifts between Th1, Th2, and Th17 phenotypes represent fundamental mechanisms by which the adaptive immune system coordinates distinct immunological responses tailored to specific threats. Since the seminal work of Mosmann and Coffman in 1986 establishing the Th1/Th2 paradigm [1], our understanding of these processes has evolved substantially. While initially conceptualised as relatively fixed developmental pathways driven by distinct master regulators, contemporary research reveals that T helper cell specification involves remarkable plasticity, heterogeneity, and functional flexibility. These shifts between lineages are not merely academic curiosities but have profound implications for infectious disease susceptibility, autoimmunity, cancer immunity, and allergic diseases.
Table of contents
- Master transcription factors and lineage specification
- Cytokine-mediated signals in Th cell differentiation
- The Th1/Th2 balance: Mechanisms and clinical implications
- Th17 cells: Lineage specification, heterogeneity, and plasticity
- Environmental and systemic factors driving Th shifts
- Disease-associated Th shifts: From infection to autoimmunity
- Mechanisms of Th cell plasticity and transdifferentiation
- Therapeutic implications and targeting Th shifts
- Conclusion
- References:
Master transcription factors and lineage specification
The differentiation of naive CD4+ T cells into distinct Th lineages is orchestrated by specific lineage-determining transcription factors (LDTFs) that serve as master regulators of cell fate. The transcription factor T-bet (T-box expressed in T cells) drives Th1 cell differentiation and maintains Th1 identity by activating interferon-gamma (IFN-?) production and other Th1-associated genes [2]. Conversely, GATA3 (GATA binding protein 3) functions as the master regulator for Th2 cell differentiation, promoting interleukin-4 (IL-4) and IL-5 production, and is essential for the expression of Th2-characteristic cytokines and transcription factors [3]. More recently, ROR?t (retinoid-related orphan receptor-gamma-t) has been identified as the critical transcription factor driving Th17 differentiation, controlling IL-17 production and maintaining the Th17 phenotype [4].
Importantly, these master transcription factors do not function in isolation. Rather, they interact within complex regulatory networks involving other transcription factors, signalling pathways, and epigenetic modifications. T-bet and GATA3 actively suppress each other’s expression, creating a mutually antagonistic relationship that ensures commitment to one lineage over another [2]. However, this antagonism is not absolute, as research has revealed that T-bet can physically sequester GATA3 at its DNA binding sites, thereby preventing GATA3 from activating Th2 genes while simultaneously being unable to completely eliminate GATA3 expression [2]. This mechanism allows Th1 cells to maintain their identity even when GATA3 is present, explaining how differentiated T helper cells can function despite the co-expression of lineage-specifying transcription factors.
Cytokine-mediated signals in Th cell differentiation
The differentiation of naive CD4+ T cells into specific Th lineages depends critically on the cytokine microenvironment present during antigen presentation. Interleukin-12 (IL-12) and interleukin-18 (IL-18) promote Th1 differentiation through STAT4 activation and T-bet induction [5]. IL-4, on the other hand, drives Th2 differentiation via STAT6 phosphorylation and GATA3 induction. The signals promoting Th17 differentiation are more complex, involving IL-6 and IL-23 through STAT3 activation, in combination with transforming growth factor-beta (TGF-?) and IL-1 for initial IL-17 production [6]. These cytokine signals engage specific intracellular signalling cascades that ultimately converge on the activation of lineage-specific transcription factors.
Critically, the same naive T cell, depending on which cytokine combinations it encounters, can differentiate into dramatically different effector phenotypes. This demonstrates that T cell fate is not intrinsically predetermined but rather is dynamically shaped by extrinsic environmental signals. Furthermore, the absence of polarising cytokines or the presence of alternative signals can direct CD4+ T cells toward regulatory T cell (Treg) differentiation via Foxp3 expression, highlighting the flexibility of the immune system in responding to contextual cues [6].
The Th1/Th2 balance: Mechanisms and clinical implications
The relative balance between Th1 and Th2 responses has profound consequences for immune function. Th1 cells, characterised by IFN-? production and expression of the chemokine receptor CXCR3, provide cellular immunity essential for protection against intracellular pathogens and are critical for cell-mediated immunity [7]. In contrast, Th2 cells produce IL-4, IL-5, and IL-13, promoting humoral immunity and antibody production, particularly beneficial against extracellular pathogens and parasites [7]. A balanced Th1/Th2 response is crucial for immune homeostasis; dysregulation of this balance is associated with numerous pathological conditions.
In autoimmune and inflammatory diseases, an excessive Th1 response can drive pathogenic cellular immunity. Studies in multiple sclerosis patients demonstrate significantly elevated T-bet expression and diminished GATA-3 expression, indicating a skewed Th1/Th2 balance [8]. Similarly, in various autoimmune diseases, increased Th1-to-Th2 ratios correlate with disease severity. Conversely, in allergic diseases and asthma, excessive Th2 responses mediated by elevated GATA3 expression and IL-4 production drive pathogenic inflammation and tissue remodelling [9].
The mechanisms regulating Th1/Th2 balance extend beyond transcription factor expression to include epigenetic modifications and chromatin remodelling. Different histone modifications at the IFN-? locus (Ifng) and IL-4 locus (Il4) distinguish Th1 and Th2 cells, reflecting the specialised epigenetic landscapes of each lineage [10]. These epigenetic differences provide a layer of stability to the differentiated state while also creating vulnerability points where changes in the epigenetic landscape could potentially allow lineage switching.
Th17 cells: Lineage specification, heterogeneity, and plasticity
The identification of Th17 cells as a distinct T helper lineage approximately two decades ago fundamentally altered our understanding of helper T cell biology. Th17 cells are defined by the production of interleukin-17 (IL-17), particularly IL-17A and IL-17F, and are critical mediators of protective immunity against extracellular bacteria and fungi at mucosal barrier sites [6]. The differentiation toward Th17 cells requires IL-6 and IL-23, along with TGF-?, and is controlled primarily by ROR?t expression [11]. This lineage represents a significant shift from the classical Th1/Th2 paradigm and has been implicated in the pathogenesis of numerous autoimmune and inflammatory diseases, from rheumatoid arthritis to inflammatory bowel disease.
However, contemporary understanding emphasises that Th17 cells exhibit remarkable heterogeneity and plasticity. Rather than representing a uniform population, Th17 cells comprise multiple subsets with distinct developmental origins, phenotypic characteristics, and functional capabilities [6]. Th17 cells can differentiate under conditions promoting either pathogenic or tissue-protective functions, with the inflammatory milieu determining their ultimate phenotype. Some Th17 cells remain relatively stable producers of IL-17, while others develop the capacity to produce IFN-? alongside IL-17, creating Th17.1 cells that bridge Th17 and Th1 immunity [12].
The plasticity of Th17 cells extends to transdifferentiation into other lineages. Th17 cells can convert into IFN-?-producing Th1-like cells, a process termed Th17-to-Th1 transdifferentiation [13]. This conversion is regulated by alterations in the cytokine environment, particularly shifts away from IL-23 signalling and toward IL-12/IL-18 signalling, which promote STAT4 activation and T-bet upregulation in Th17 cells [14]. Notably, metabolic reprogramming underlies this plasticity; Th17 cells with higher mTORC1 activity and increased anabolic metabolism are more prone to transdifferentiate into Th1-like cells, whereas those with lower metabolic activity retain stemness-like features and greater self-renewal capacity [15]. This metabolic heterogeneity within the Th17 compartment explains how individual cells respond differently to the same environmental signals.
Environmental and systemic factors driving Th shifts
The shift between Th1, Th2, and Th17 phenotypes in vivo is influenced by multiple environmental and systemic factors beyond the immediate cytokine milieu. The composition of the gut microbiota plays a critical regulatory role in shaping the Th1/Th2/Th17 balance, with specific bacterial species promoting or suppressing particular lineages [16]. Metabolic byproducts from bacterial fermentation, such as short-chain fatty acids, modulate Th cell differentiation and promote regulatory T cell development [17]. Tissue-resident factors including hypoxia, nutrient availability, and the presence of damage-associated molecular patterns further influence Th differentiation and plasticity.
In pregnancy, dynamic shifts in the Th1/Th2 balance are essential for successful foetal tolerance and implantation. Early in pregnancy, controlled Th1 immunity facilitates trophoblast invasion, but rapid shifting toward Th2 and regulatory T cell responses follows placental implantation [7]. Dysregulation of these shifts has been implicated in recurrent pregnancy loss and preeclampsia, demonstrating the clinical importance of temporal control over Th cell differentiation [18]. Vitamin D has been identified as an important modulator of this balance, shifting responses away from Th1 and Th17 cells toward Th2 and Treg phenotypes [19].
Disease-associated Th shifts: From infection to autoimmunity
The pathogenesis of numerous diseases involves characteristic shifts in Th cell populations. In amyotrophic lateral sclerosis (ALS), peripheral T cell populations show a marked shift toward pro-inflammatory Th1 and Th17 cells, with decreased anti-inflammatory Th2 and Treg cells, a pattern that correlates with disease severity and progression [20]. This Th1/Th17 skewing appears to drive neuroinflammation and contribute to motor neuron degeneration, suggesting that modulating these shifts might offer therapeutic opportunities [20].
In early rheumatoid arthritis, in vivo-derived Th1 and Th2 cells demonstrate enhanced plasticity toward IL-17-expressing phenotypes compared with healthy controls, while simultaneously, Th17 cells from RA patients show reduced capacity to transdifferentiate into Th1 or Th2 cells [12]. This altered plasticity, where cells preferentially shift toward Th17 but resist opposing shifts, creates a pro-inflammatory state locked into Th17 production. The serum/glucocorticoid-regulated kinase 1-forkhead box protein O1-IL-23 receptor (SGK1-FOXO1-IL-23R) signalling axis, together with increased RORC expression, maintains this pathogenic Th17 predominance[12].
In COVID-19, evidence suggests that Th17 cells play a crucial pathogenic role, not only through their direct pro-inflammatory effects but also by inhibiting Th1 differentiation and suppressing Treg cells [21]. The elevated IL-17 levels observed in severely ill COVID-19 patients may represent a maladaptive immune response where Th17-mediated inflammation exacerbates tissue damage rather than providing protective immunity [21].
Mechanisms of Th cell plasticity and transdifferentiation
Th cell plasticity occurs through both reversible phenotypic changes and more committed transdifferentiation events. At the molecular level, plasticity is facilitated by the relatively fluid nature of transcription factor expression in recently activated T cells. While differentiated effector cells tend to maintain stable expression of lineage-specifying transcription factors through positive feedback loops and epigenetic locking, these cells retain the potential to shift transcription factor expression in response to strong environmental perturbations [13].
Transdifferentiation involves more substantial reprogramming, where cells must downregulate the transcription factors defining their current lineage while simultaneously upregulating factors driving an alternative lineage. For instance, Th17-to-Th1 transdifferentiation requires not only upregulation of T-bet but also downregulation of ROR?t expression, a transition that typically requires several cell divisions and sustained exposure to Th1-promoting cytokines [14]. The epigenetic landscape must be substantially remodelled to enable this transition, involving histone acetylation/deacetylation at key loci and chromatin accessibility changes [22].
TGF-? signalling occupies a critical position in governing Th cell plasticity. TGF-? can direct differentiation toward Th17 or Treg cells depending on the presence or absence of pro-inflammatory cytokines [23]. Remarkably, TGF-? signaling is also required for maintaining certain Th17 populations and enabling their transdifferentiation, illustrating how the same cytokine can orchestrate distinct functional outcomes depending on the broader cytokine context [24]. The intricacies of TGF-? signaling underscore the importance of understanding immune responses within their full inflammatory landscape rather than in isolation.
Therapeutic implications and targeting Th shifts
Understanding Th cell plasticity and the mechanisms driving shifts between lineages has significant therapeutic implications. In autoimmune diseases driven by pathogenic Th1 or Th17 cells, therapeutic interventions have historically focused on blocking these lineages entirely. However, emerging evidence suggests that inducing plasticity toward alternative lineages—rather than complete lineage elimination—might offer superior outcomes with fewer adverse effects [6]. For instance, in Neisseria gonorrhoeae infections, blocking TGF-? with antibodies during primary infection suppresses Th17 responses while promoting Th1 and Th2 responses, leading to the development of protective immunity that controls secondary infection [25].
In inflammatory bowel disease and other Th17-driven pathologies, simultaneously targeting multiple upstream signals that control Th17 plasticity—such as IL-23 signalling, metabolic pathways, or calcium signalling through CRAC channels—may prevent pathogenic Th17 cells from acquiring stability while enabling their conversion toward regulatory phenotypes [26]. The identification of metabolic checkpoints, particularly mTOR signalling, as critical regulators of Th17 plasticity opens new therapeutic avenues [15]. Targeting mTOR in pathogenic Th17 cells prevents their transdifferentiation into more effector-like phenotypes while preserving tissue-protective functions of Th17 cells.
Conclusion
The Th1/Th2/Th17 shift represents a fundamental mechanism by which the adaptive immune system maintains flexibility in responding to diverse immunological challenges while maintaining the ability to mount appropriate protective responses. Rather than deterministic developmental pathways, these shifts reflect the complex interplay of transcriptional, epigenetic, metabolic, and environmental signals that collectively determine T helper cell identity and function. Master transcription factors T-bet, GATA3, and ROR?t provide the core regulatory framework, but their function is modulated by extensive networks of co-regulators, signalling pathways, and contextual factors that enable remarkable plasticity and heterogeneity within each lineage [1]. Understanding these mechanisms at the molecular, cellular, and systems levels provides crucial insights into immune-mediated diseases and creates opportunities for more precise therapeutic interventions that exploit rather than simply block the inherent plasticity of the immune system.
Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice.
References:
[5] E. Ylikoski et al., “IL-12 up-regulates T-bet independently of IFN- in human CD4+ T cells,” Wiley, Oct. 2005, doi: https://onlinelibrary.wiley.com/doi/10.1002/eji.200526101
[6] S. Cerboni, U. Gehrmann, S. Preite, and S. Mitra, “Cytokineregulated Th17 plasticity in human health and diseases,” Wiley, Oct. 2020, doi:https://onlinelibrary.wiley.com/doi/10.1111/imm.13280
[7] W. Wang, N. Sung, A. GilmanSachs, and J. KwakKim, “T Helper (Th) Cell Profiles in Pregnancy and Recurrent Pregnancy Losses: Th1/Th2/Th9/Th17/Th22/Tfh Cells,” Frontiers in Immunology, Aug. 2020, doi: 10.3389/fimmu.2020.02025.
