Regulatory T cells in neurological disorders and tissue regeneration: Mechanisms of action and therapeutic potentials

Abstract

Regulatory T cells, a subset of CD4⁺ T cells, play a critical role in maintaining immune tolerance and tissue homeostasis due to their potent immunosuppressive properties. Recent advances in research have highlighted the important therapeutic potential of Tregs in neurological diseases and tissue repair, emphasizing their multifaceted roles in immune regulation. This review aims to summarize and analyze the mechanisms of action and therapeutic potential of Tregs in relation to neurological diseases and neural regeneration. Beyond their classical immune-regulatory functions, emerging evidence points to non-immune mechanisms of regulatory T cells, particularly their interactions with stem cells and other non-immune cells. These interactions contribute to optimizing the repair microenvironment and promoting tissue repair and nerve regeneration, positioning non-immune pathways as a promising direction for future research. By modulating immune and non-immune cells, including neurons and glia within neural tissues, Tregs have demonstrated remarkable efficacy in enhancing regeneration in the central and peripheral nervous systems. Preclinical studies have revealed that Treg cells interact with neurons, glial cells, and other neural components to mitigate inflammatory damage and support functional recovery. Current mechanistic studies show that Tregs can significantly promote neural repair and functional recovery by regulating inflammatory responses and the local immune microenvironment. However, research on the mechanistic roles of regulatory T cells in other diseases remains limited, highlighting substantial gaps and opportunities for exploration in this field. Laboratory and clinical studies have further advanced the application of regulatory T cells. Technical advances have enabled efficient isolation, ex vivo expansion and functionalization, and adoptive transfer of regulatory T cells, with efficacy validated in animal models. Innovative strategies, including gene editing, cell-free technologies, biomaterial-based recruitment, and in situ delivery have expanded the therapeutic potential of regulatory T cells. Gene editing enables precise functional optimization, while biomaterial and in situ delivery technologies enhance their accumulation and efficacy at target sites. These advancements not only improve the immune-regulatory capacity of regulatory T cells but also significantly enhance their role in tissue repair. By leveraging the pivotal and diverse functions of Tregs in immune modulation and tissue repair, regulatory T cells–based therapies may lead to transformative breakthroughs in the treatment of neurological diseases.

Introduction

The global population affected by neurological dysfunctions due to acute injuries, chronic diseases, or aging is steadily increasing, posing a great burden on healthcare systems, families, and societies (Li et al., 2018; Yang et al., 2023c; Gao et al., 2024). Historically, the nervous and immune systems were considered distinct domains, primarily due to the presence of the blood–brain barrier (BBB) and the blood–spinal cord barrier (BSCB) (Ning et al., 2024; Wang and Bai, 2025). However, in the last decades numerous studies have challenged this century-old concept of central nervous system (CNS) immune privilege, revealing a significant overlap between the CNS and the peripheral immune system (Castellani et al., 2023; Peng et al., 2024). Despite the restriction imposed by the BBB and BSCB to the passage of toxins, large molecules, and cells into the CNS (Yin et al., 2025), neuroimmune interactions are becoming increasingly evident at the tissue, cellular, and molecular levels (Louveau et al., 2015; Hu et al., 2018). It is now clear that the CNS operates under continuous immune surveillance and regulation, and that a close intertwining exists between lymphatic vessels and peripheral nerves (Alves de Lima et al., 2020; Badimon et al., 2020).

The complexity of neurological disorders and the limited regenerative capacity of the nervous tissue present central challenges in medicine. Researchers widely acknowledge that precisely regulating the immune system can effectively resolve inflammation and restore the structure and homeostasis of neural tissue (Feng et al., 2022; Yang et al., 2023a; Holl et al., 2024). Conversely, excessive or unregulated immune responses may exacerbate damage, leading to fibrosis and scar formation, which hinder the self-repair capacity of injured sites (Asboth et al., 2018; Adusei et al., 2021; Wang et al., 2023b). Therefore, there is an urgent need to deepen our understanding of the immune mechanisms underlying neurological disorders and tissue repair. This knowledge is essential for designing effective therapies aimed at counteracting the pathological processes that lead to organ dysfunction, promoting nerve regeneration, and enhance tissue repair.

Immune sentinel cells, such as dendritic cells, macrophages, fibroblasts, and peripherally derived T cells in peripheral tissues, and microglia in the CNS, play a dual role in maintaining immune balance and disrupting immune homeostasis, offering new possibilities for mitigating tissue damage and promoting the repair of the neurovascular unit (Ousman and Kubes, 2012; McGettrick, 2021). In this context, CD4⁺Foxp3⁺ regulatory T cells (Tregs), as key immune regulators, are indispensable for maintaining immune homeostasis through the precise modulation of immune responses (Savage et al., 2020; Dikiy and Rudensky, 2023). Developmental defects in Tregs can lead to severe and potentially fatal inflammatory disorders, such as immunodeficient endocrinopathies, enteropathy, and X-linked syndrome (Li et al., 2018; Attias et al., 2019; Becker et al., 2024; de la Fuente et al., 2024).

Tregs not only alleviate inflammation and facilitate debris clearance following neurological diseases and injuries but also exhibit unique neuroregenerative functions, as revealed by recent studies. These functions include myelin repair, induction of stem cell proliferation, neurovascular remodeling, and the restoration of regenerated neural circuits (Korn, 2023; Liston et al., 2023; Ho et al., 2024). Importantly, these regenerative roles are distinct from the immunosuppressive mechanisms typically associated with these cells, indicating that Tregs regulate neurological disorders and tissue repair through multiple pathways. In most scenarios of injury repair, immunosuppressive and regenerative mechanisms likely operate concurrently (Cohen et al., 2024; Ho et al., 2024; Loffredo et al., 2024).

This review comprehensively explores the multifaceted roles of Tregs in diseases and tissue injury, focusing particularly on their immunological and non-immunological mechanisms in neurological disorders and tissue repair. Additionally, we discuss recent preclinical research advances in adoptive Treg therapy for treating neurological diseases and nerve injury, highlighting their immense potential as therapeutic tools. Furthermore, we summarize various strategies aimed at enhancing the reparative properties of Tregs, including genetic engineering, cell-free technologies, biomaterial-based recruitment and conversion, and in situ localized delivery. These innovative approaches hold promise for effectively halting or reversing the progression of neurological disorders and promoting neural tissue regeneration and functional recovery. These strategies lay in turn a critical foundation for the development of next-generation cell therapies and therapeutic agents, providing novel directions for basic research and clinical applications.

Literature Search Strategy

A comprehensive literature search was conducted using the PubMed database, focusing on publications from 2000 to 2024. The search terms included “Treg cells,” “regulatory T cells,” “immune regulation,” “neural repair,” “neuroinflammation,” “tissue regeneration,” “cell therapy,” “gene engineering,” “biomaterials,” and “local delivery,” as well as specific terms related to both the immunological and non-immunological mechanisms of Treg cells in disease and tissue injury. These terms were carefully selected to include the broad spectrum of Treg cell functions, their roles in neurological disorders and tissue repair, and the development of innovative therapeutic strategies. Data extracted from the selected articles included study design, experimental models, therapeutic approaches, and key findings relevant to the roles of Treg cells in immune regulation and tissue regeneration. A detailed narrative synthesis approach was used to integrate findings from experimental studies, preclinical research, and previous review articles, providing a comprehensive overview of Treg cell functions, their therapeutic potential, and emerging engineering strategies. Particular attention was given to synthesizing information on the immunological and non-immunological pathways of Treg cells, highlighting their complex roles in the progression and recovery from neurological diseases, as well as innovative methods such as gene engineering, biomaterial-based recruitment, and local delivery systems.

Origin, Characteristics, and Mechanisms of Action of Regulatory T Cells

Origin and characteristics of Tregs

Tregs are a critical subset of T cells within the immune system, primarily responsible for maintaining immune homeostasis (Tiemessen et al., 2007; Zhang et al., 2017; Zhu et al., 2022). They play a broad role in various physiological and pathological processes, including infection, inflammation, tissue repair, and transplantation rejection. Additionally, Tregs are involved in regulating responses to bacterial and viral infections, autoimmune diseases, and cancer (Shanley et al., 2021; Zhong et al., 2022; Gao et al., 2024). Based on their developmental pathways and characteristics, Tregs can be divided into two categories: natural Tregs (nTregs) and induced Tregs (iTregs) (Dittmer et al., 2018; Attias et al., 2019). nTregs develop in the thymus and are characterized by the expression of CD4, CD25, and forkhead box p3 (Foxp3). These cells acquire their immunoregulatory functions upon exposure to specific antigens during thymic development, and exert their effects by secreting cytokines such as interleukin (IL)-10, IL-35, and transforming growth factor-β (TGF-β) (Burns et al., 2020). The IL-2/signal transducer and activator of transcription-5 (STAT5) signaling axis plays a pivotal role in the induction of Foxp3 and the stability of Tregs (Mao et al., 2019; Xie et al., 2023). Other critical molecules, such as Helios and GATA binding protein 3 (GATA3), also contribute to the maintenance of the suppressive functions of nTregs, particularly under inflammatory conditions (Ramanan et al., 2023). The interaction of thymic stromal cells and antigen-presenting cells with developing T cells provides essential signals, including costimulatory molecules and cytokines, that are vital for Treg lineage commitment (Yi et al., 2019). In contrast, iTregs are derived from conventional CD4⁺ T cells in peripheral tissues upon antigen stimulation and in the presence of specific cytokines such as TGF-β and IL-10 (Joetham et al., 2020). iTregs can be further subdivided into T regulatory-1 (TR1) and T-helper-3 (TH3) cells, depending on the inducing factors. TR1 cells are CD4⁺Foxp3^– and primarily mediate immunosuppression through the secretion of IL-10, while TH3 cells are CD4⁺Foxp3⁺ and primarily regulate immune responses via TGF-β secretion (Adusei et al., 2021; Hu et al., 2021b). The differentiation of iTregs is notably influenced by the local microenvironment, including the presence of retinoic acid and microbial-derived metabolites such as short-chain fatty acids (Smith et al., 2013; Jang et al., 2023). These factors enhance the stability and function of iTregs by epigenetically modifying Foxp3 expression and maintaining chromatin accessibility in regulatory regions (Wang et al., 2018).

Tregs are characterized by high expression of CD25 (the IL-2 receptor α chain) and the transcription factor Foxp3, while exhibiting low or undetectable levels of CD127 (the IL-7 receptor α chain). These markers are essential for the identification and characterization of Tregs (Tiemessen et al., 2007; Hu et al., 2018; Korn, 2023). Among them, Foxp3 is considered a key regulatory factor for maintaining the function and stability of Tregs, with its expression levels being closely correlated with the immunosuppressive capabilities of these cells (Attias et al., 2019; Honaker et al., 2020; Ho et al., 2024). Additionally, activated Tregs express CD134, which further enhances their functional stability (Queiroz-Glauss et al., 2022).

Tregs also express various inhibitory molecules and receptors, including cytotoxic T lymphocyte-associated antigen-4 (CTLA-4), programmed cell death protein 1 (PD-1), lymphocyte activation gene-3 (LAG-3), and Helios, all of which play major roles in regulating their immunosuppressive functions. Furthermore, surface expression of the ectonucleotidases CD39 and CD73, which metabolize extracellular ATP into adenosine, further strengthens the immunosuppressive effects of Tregs (Wang et al., 2022; Liston et al., 2023; Becker et al., 2024). The dynamic regulation of Tregs by external factors such as cytokines, tissue-derived signals, and microbial components significantly affects their phenotypic plasticity and functional specialization. For instance, IL-33 has been shown to enhance Treg-mediated tissue repair functions, while pro-inflammatory cytokines such as IL-6 can destabilize Tregs and impair their immunosuppressive abilities (Arpaia et al., 2015; Yan et al., 2021). Understanding these regulatory mechanisms is crucial for developing therapeutic applications that target Tregs.

Roles of regulatory T cells in disease

The roles of Tregs in autoimmune diseases, infectious diseases, and chronic inflammation have been extensively studied (Sakaguchi et al., 2020). Tregs primarily function to regulate immune homeostasis and maintain self-tolerance (Zhong et al., 2022; Ho et al., 2024). Dysfunction and reduced numbers of Tregs are closely associated with the onset and progression of autoimmune diseases such as multiple sclerosis (MS), rheumatoid arthritis, and systemic lupus erythematosus (Dombrowski et al., 2017; Li et al., 2018; Honaker et al., 2020). Clinical studies indicate that in patients with MS, decreased Treg numbers and impaired suppressive functions contribute to disease relapse and progression (Raphael et al., 2015; Shi et al., 2023a). Similarly, in rheumatoid arthritis, synovial fluid from inflamed joints has been found to contain Tregs with reduced Foxp3 expression, suggesting that an inflammatory microenvironment drives their dysfunction (Jiang et al., 2021). Research has demonstrated that the immunosuppressive function of Tregs relies on sustained expression of Foxp3. Accordingly, Foxp3 loss of function or depletion of Tregs can lead to severe autoimmune diseases (Hu et al., 2018; Korn, 2023). Notably, Hu et al. (2021b) reported that restoring Foxp3 expression in mice can rebalance immune activation, suppress tissue inflammation, and reverse fatal autoimmune disease. Previously, Worthington et al. (2015) had reported that effector Tregs inhibit pathogenic T-cell responses by expressing integrin αvβ8, which activates latent TGF-β. Under conditions of active inflammation, the lack of integrin αvβ8, significantly weakens Tregs’ suppressive effects.

Tregs play a crucial role in regulating the intensity of immune responses in infectious diseases, helping to prevent tissue damage caused by excessive immune reactions (Amaya-Uribe et al., 2019). For instance, during viral infections such as from human immunodeficiency virus, Tregs modulate immune responses to control inflammation and limit tissue damage. However, excessive Treg activity can suppress antiviral immunity, potentially leading to chronic infection (Méndez-Lagares et al., 2012; López-Abente et al., 2016). In cases of infectious lung injury, Tregs can respond to inflammatory cytokines such as IL-18 or IL-33 by secreting the tissue repair factor amphiregulin (AREG), thereby directly contributing to tissue repair. This function operates independently of their classical T-cell receptor-mediated immunosuppressive effects (Arpaia et al., 2015). A clinical study in patients with sepsis-induced acute respiratory distress syndrome showed that elevated levels of Tregs expressing IL-10 and AREG correlated with improved lung function recovery and reduced fibrosis (Halter et al., 2020). In the context of chronic inflammation, Tregs modulate cytokine networks to prevent persistent inflammation from damaging tissues. Langston et al. (2023) demonstrated that in a chronic inflammation model induced by exercise, Tregs protect from metabolic disorders and mitochondrial dysfunction by suppressing excessive interferon-γ production. Their study showed also that mice lacking Tregs exhibit significantly reduced benefits from exercise. Additional studies on obesity-related chronic inflammation have highlighted the tissue-specific heterogeneity of Tregs. For example, Tregs residing in visceral adipose tissue exhibit unique transcriptomic profiles compared to their counterparts in lymphoid tissues, enabling them to adapt to the metabolic and inflammatory signals of their microenvironment (Cipolletta et al., 2015; Zhu et al., 2017). Although the critical role of Tregs in various diseases is well recognized, precisely regulating their numbers and functions under pathological conditions remains a major challenge and a focus of current research. For instance, in cancer, tumor-resident Tregs can promote immune evasion, while in autoimmune diseases, restoring Treg numbers and functions is essential for maintaining immune tolerance.

Studies employing single-cell RNA sequencing (scRNA-seq) technology revealed the functional and phenotypic heterogeneity of Tregs across different tissues and disease states, highlighting their context-specific roles (Zemmour et al., 2018; Miragaia et al., 2019; Luo et al., 2021). However, existing research on the regulatory mechanisms of Tregs primarily focuses on their immunological functions, while their roles in tissue repair and regeneration remain largely unexamined. This gap may stem from the overlap and intersection of immunoregulatory and tissue repair/regeneration pathways. Therefore, studies aimed at distinguishing immunological from non-immunological pathways are crucial for a deeper understanding of the mechanisms by which Tregs contribute to tissue repair and regeneration.

Roles of regulatory T cells in tissue regeneration

Since the first report in 2009 suggesting the participation of Tregs in tissue repair, research on their regenerative functions has steadily grown (Loffredo et al., 2024). These cells include lymphoid tissue Tregs and tissue-resident Tregs found in non-lymphoid tissues. The latter play critical roles in the repair and regeneration of various tissues, such as muscle, heart, hair follicles, and nerves (Li et al., 2018; Ito et al., 2019; Loffredo et al., 2024). Tissue-resident Tregs exhibit high expression of tissue-specific transcription factors, which equip them with effective tissue-protective and reparative functions. For instance, in the visceral adipose tissue of obese individuals, resident Tregs are gradually depleted, contributing to chronic inflammation and metabolic dysfunction (Lee and Lee, 2014). In contrast, Tregs in muscle tissue express high levels of IL-6 receptor (IL-6R), which is critical for modulating local inflammation and promoting repair. However, the use of IL-6R antibodies in systemic inflammatory diseases often leads to side effects such as muscle weakness, highlighting the tissue-specific regulatory mechanisms of Tregs (Wang et al., 2022). Although the brain has traditionally been considered an immune-privileged site, studies have confirmed in mice and humans the presence of T cells, including Tregs, in the meninges and within the brain parenchyma (Pasciuto et al., 2020; Marin-Rodero et al., 2025). Genetic analyses showed that compared to peripheral Tregs, brain-resident Tregs express genes that are more closely associated with the nervous system. For example, they exhibit high levels of AREG, which directly contributes to the repair of damaged neural tissues (Korn, 2023; Liston et al., 2023). This area of research is a key focus of this review and will be discussed in detail in subsequent sections.

Beyond their role in regulating immune responses, Tregs can protect stem cell niches from inflammatory activity or directly signal to non-immune cells and/or stem cells (Fujisaki et al., 2011; Cohen et al., 2024). This signaling represents a new function and regenerative mechanism of Tregs. In the liver, for example, Tregs interact with hepatic stellate cells to regulate fibrosis, while in the lungs, they promote alveolar epithelial repair during injury through TGF-β and AREG signaling pathways (Dial et al., 2017; Wen et al., 2017).

Stem cells, with their capabilities for self-renewal and multipotent differentiation, are essential drivers of tissue regeneration and repair. By differentiating into specific cell types or secreting reparative bioactive factors, stem cells can repair damaged tissues, replace injured cells, and promote tissue regeneration (Xuan et al., 2018; Li et al., 2022). In recent years, the interaction between stem cells and specific microenvironments has been the focus of intense research. Among these interactions, the regulation of stem cells by Tregs has been recognized as a novel mechanism of regeneration, distinct from traditional immune responses. For instance, Tregs can enhance the secretion of IL-10 by macrophages in the bone marrow, promoting the recovery of hematopoietic stem cells following irradiation (Fujisaki et al., 2011). Similarly, during cardiac injury, Tregs have been shown to support the survival and differentiation of cardiac stem cells through paracrine effects mediated by IGF-1 and other factors (Wang et al., 2022). Although research in this area is still in its early stages, substantial progress has been achieved in certain tissues, providing new insights and promising implications for exploring regeneration in others, including neural tissue. For example, studies by M.D. Rosenblum’s research team demonstrated the critical role of Tregs in epidermal regeneration. They showed that skin-resident Tregs localize preferentially around hair follicles, sustaining their growth and renewal by promoting the activation and differentiation of hair follicle stem cells (HFSCs) through the Notch ligand JAG1 signaling pathway (Ali et al., 2017). In a subsequent study, they investigated the regulatory role of Tregs in the repair of skin injury. Their findings indicated that Tregs suppress the secretion of chemokine C–X–C motif ligand-5 (CXCL5) by keratinocytes, limiting the recruitment of T-helper 17 (Th17) cells and neutrophils. This negative modulation of the local inflammatory environment facilitates the differentiation of HFSCs into epithelial cells, thereby restoring the skin barrier. Accordingly, they showed that depletion of Tregs prevents skin wound healing by inhibiting HFSC differentiation (Mathur et al., 2019). More recently, Luan et al. (2024) revealed the mechanism by which HFSCs operate under inflammatory conditions during wound healing. Their research showed that HFSCs that migrate to the wound site promote the local expansion of Tregs by activating CD80, forming a protective immunoregulatory network that prevents excessive neutrophil accumulation and supports epithelialization and wound healing.

At the same time, by modulating Treg cell functions, stem cells exhibit broad application prospects in the regeneration of various tissues, including nerves, blood vessels, muscles, and bones (Hu et al., 2018; Li et al., 2018; Shanley et al., 2021). Research has shown that during tissue repair, stem cells induce the generation of Tregs, partly relying on their ability to promote the production of cytokines such as IL-10 and TGF-β by macrophages (Qi et al., 2018). In the nervous system, Tregs support neural repair by regulating the balance between pro-inflammatory and anti-inflammatory factors. Wang et al. (2020) reported that human Wharton’s jelly-derived mesenchymal stem cells (MSCs) promote sciatic nerve regeneration and functional recovery by inducing Treg expansion. Depletion of Tregs significantly impaired this repair effect, while supplementation with Tregs accelerated neural functional recovery (Wang et al., 2020). Dombrowski et al. (2017) demonstrated that Tregs directly promote the differentiation of oligodendrocyte precursor cells (OPCs) in demyelinating diseases, highlighting their crucial role in remyelination and neural repair (Dombrowski et al., 2017). The pivotal role of Tregs in regulating neurological diseases and promoting tissue regeneration is summarized in Figure 1.

Figure 1.

Mechanisms and milestones of Treg cells in neurological diseases and regeneration.

Treg cells exert their immunoregulatory functions through multiple pathways. First, they directly interact with DCs to mediate cell-to-cell interactions. Second, Treg cells release inhibitory cytokines that suppress the functions of various immune cells. Additionally, they inhibit effector T cell activity through competitive suppression. Furthermore, Treg cells secrete perforin and granzyme, which further suppress effector T cell functions and help maintain the balance of the immune microenvironment. Simultaneously, Treg cells promote the proliferation and differentiation of stem cells, creating a favorable microenvironment for stem cell activity. Created with BioRender.com. CNS: Central nervous system; CTLA-4: cytotoxic T lymphocyte associate protein-4; DC: dendritic cells; IL-10: interleukin-10; IL-2: interleukin-2; PD-1: programmed death 1; PNS: peripheral nervous system; Tconv: conventional T cells; TGF-β: transforming growth factor-β; Tregs/Treg cells: regulatory T cells.

In bone and vascular regeneration, Tregs regulate stem cell functions and promote osteogenesis and angiogenesis through their exosomes. Chen et al. (2022a) demonstrated that Treg-derived exosomes enriched with miR-142-3p accelerate fracture healing and bone remodeling via the TGFBR1/SMAD2 pathway. Another study showed that Tregs modulate the angiogenic properties of endothelial progenitor cells during vascular repair through the production of vascular endothelial growth factor (VEGF) (Loffredo et al., 2024). In summary, Tregs show significant potential in regenerative medicine through different mechanisms of action operating in diverse tissue types. This capability offers a promising, flexible basis for the development of novel therapeutic strategies based on Tregs.

Molecular mechanisms underlying regulatory T cell function

The immunosuppressive functions of Tregs are mediated by diverse and complex mechanisms, which include classical immunoregulatory pathways, tissue-specific mechanisms, and emerging interactions with stem cells (Georgiev et al., 2019). Classical mechanisms of Treg-mediated immunosuppression involve contact-dependent inhibition, secretion of inhibitory cytokines, metabolic competition, and direct cytotoxic effects (Tiemessen et al., 2007; Dombrowski et al., 2017; Attias et al., 2019). One of the most significant mechanisms is direct cell-to-cell contact, where inhibitory receptors on the surface of Tregs (such as CTLA-4 and PD-1) bind to CD80/86 on antigen-presenting cells such as dendritic cells, B cells, and macrophages. This interaction suppresses the maturation of these cells and prevents the activation of effector T cells (Yang et al., 2018, 2019; Jie et al., 2022). For instance, CTLA-4 not only competes with CD28 for binding to CD80/86 but also actively removes CD80/86 from the surface of dendritic cells through trans-endocytosis. This process prevents full dendritic cell activation, thereby reducing T-cell activation in the microenvironment (Marangoni et al., 2021). Additionally, Tregs express Lag-3, which binds to major histocompatibility complex class II molecules on antigen-presenting cells, further inhibiting their activity. As mentioned, Tregs also secrete inhibitory cytokines, such as IL-10, TGF-β, and IL-35, to regulate the functions of effector T cells, natural killer cells, and dendritic cells, thereby diminishing their activity (Muscella et al., 2019; Almanan et al., 2020; Yang et al., 2024). These cytokines not only suppress inflammatory responses but also promote the differentiation of induced Tregs in inflamed tissues, enhancing their regulatory capacity. Metabolic competition operates primarily in two ways. First, Tregs, which express high levels of CD25, compete with effector T cells for IL-2, limiting their proliferation. IL-2 is critical for T-cell survival, and its depletion creates an unfavorable microenvironment for effector T cells while supporting Treg expansion. Second, Tregs express ectonucleotidases CD39 and CD73 on their surface, which produce adenosine from ATP. ADP binds to A2A receptors on effector T cells, inhibiting their activation, proliferation, and cytokine production. Furthermore, Tregs modulate the metabolic environment by altering glucose and lipid metabolism, effectively depriving effector cells of essential nutrients and further limiting their function. Adding another layer of complexity to Treg-mediated regulation, emerging studies have highlighted the role of Treg-derived extracellular vesicles (EVs) in modulating metabolic pathways in target cells. Moreover, Tregs enhance their immunosuppressive effects by secreting perforin and granzyme, which can directly kill effector T cells (Silva et al., 2019; Yang et al., 2021a, b; Jung et al., 2022; Jang et al., 2024). In summary, Tregs use a combination of mechanisms to maintain immune homeostasis and prevent excessive immune activation, thereby playing a central role in immune regulation (Becker et al., 2024; Loffredo et al., 2024).

In addition to classical suppressive mechanisms, Tregs exhibit tissue-specific regulatory characteristics. For example, in skin tissue, Tregs protect HFSCs from immune damage through non-CTLA-4-dependent mechanisms, such as the IL-2 signaling pathway (Cohen et al., 2024b). Furthermore, as discussed previously, Tregs optimize the local inflammatory microenvironment by suppressing the secretion of CXCL5, which reduces the recruitment of neutrophils and Th17 cells, thereby promoting hair regeneration and skin barrier repair (Mathur et al., 2019). In neural tissue, Tregs alleviate neuroinflammation by regulating the balance between pro-inflammatory and anti-inflammatory factors. They also enhance neuronal repair capabilities, facilitating axon regeneration (Dombrowski et al., 2017). In cardiac tissue, Tregs secrete cytokines such as IL-10 and promote macrophage-mediated clearance of necrotic cells, which reduces inflammation and fibrosis during myocardial injury (Xia et al., 2024). A growing understanding of the mechanisms governing Treg functions, along with emerging evidence of substantial crosstalk between Tregs and stem cells, offers new perspectives on the therapeutic potential of Tregs in tissue repair (Figure 1).

Roles of Tregs in Neurological Diseases and Neural Regeneration

The human nervous system is anatomically divided into two parts: the CNS, which includes the brain and spinal cord, and the peripheral nervous system (PNS), encompassing all neural structures outside the CNS (Louveau et al., 2015; Wang et al., 2019; Varadarajan et al., 2022). The CNS integrates sensory input from the body, generates coordinated motor outputs, and serves as the neural substrate for learning and memory (Hong et al., 2017; Zhang et al., 2020). While the brain was long considered an immune-privileged organ, it is now evident that immune cells, including Tregs, can access the brain, particularly during disease and injury (Adusei et al., 2021; Korn, 2023). The BBB and the BSCB are critical for maintaining the stability of the CNS microenvironment. These barriers function not only as physical obstructions but also as dynamic regulatory interfaces that finely control the passage of molecules and cells. Composed of tightly connected endothelial cells, pericytes, astrocytes, and the basement membrane, the BBB and BSCB are endowed with tight junctions and adherens junctions between endothelial cells that serve to strictly regulate barrier permeability. This ensures that only specific nutrients, oxygen, and metabolic waste products can pass through, maintaining homeostasis within the microenvironment of the nervous system (Badimon et al., 2020; Zhang et al., 2020; Mahmoudi et al., 2024). This barrier function prevents the entry of blood-borne pathogens, toxins, and inflammatory mediators into the brain and spinal cord while regulating the transport of neurotransmitters, hormones, and other signaling molecules to ensure proper neural function. Under healthy conditions, the BBB and BSCB effectively isolate the CNS from the peripheral immune system; however, this isolation is not absolute. During disease and injury, immune cells can traverse these barriers and infiltrate the CNS.

In the brain microenvironment, Tregs exhibit transient residency characteristics, displaying specific phenotypes adapted to the neural environment during their brief stay in brain tissue. The Tregs population in the brain is dynamically renewed, with cells continuously replaced by newly recruited Tregs. Although their presence typically lasts only a few weeks, this transient residency can be prolonged through secondary in situ antigen recognition, enhancing their capacity for immunoregulation and tissue repair (Liston et al., 2023). Following neural injury, mechanical forces can directly damage the vasculature at the injury site, leading to the disruption of the BBB/BSCB, even in areas distant from the primary lesion (primary injury). Vascular contents, including macrophages and lymphocytes, infiltrate neural tissues through the compromised barrier, interacting with inflammatory mediators and triggering inflammatory responses. This exacerbates barrier permeability, further destabilizing the local microenvironment (secondary injury) (Feng et al., 2022; Liston et al., 2023; Yang et al., 2023a; Jerome et al., 2024; Lu et al., 2024; Zhao et al., 2024). The critical role of Tregs in neural injury repair makes them essential for understanding and promoting neural regeneration. The actions of Tregs on the nervous system are summarized in Figure 2.

Figure 2.

Role of Treg cells in neurological disorders and tissue regeneration.

Treg cells play critical roles in various nerve injury environments. In the CNS, Treg cells support nerve injury recovery and regeneration by releasing cytokines and proteins that modulate the phenotypes of glial cells and immune cells. In the peripheral nervous system, Treg cells can be combined with biomaterials or cytokines and delivered via in situ injection to treat peripheral nerve injuries. These approaches help regulate inflammatory responses and promote axonal regeneration, significantly contributing to injury repair. Created with BioRender.com. AREG: Amphiregulin; CNS: central nervous system; IL-2: interleukin-2; OPC: oligodendrocyte precursor cells; PNS: peripheral nervous system; Treg cell: regulatory T cell.

Roles of regulatory T cells in stroke and neurodegenerative disorders

Stroke

Ischemic stroke, commonly referred to as cerebral infarction, is a cerebrovascular disease primarily caused by the interruption or insufficiency of blood supply to the brain. This results in damage or necrosis of brain tissue due to a lack of oxygen and nutrients. As a prevalent type of stroke, ischemic stroke can rapidly worsen, causing direct and severe damage to brain tissue and leading to neurological deficits that pose a serious threat to patient survival and quality of life. Prompt treatment to restore blood flow is critical for preventing further damage and reducing mortality and disability rates in ischemic stroke patients. The role of Tregs in the brain has been extensively studied using the middle cerebral artery occlusion (MCAO) ischemic stroke model. This model is typically established through the intraluminal suture method, which involves a surgical procedure to expose the carotid artery system in mice. A suture is inserted into the common or external carotid artery and advanced to the origin of the middle cerebral artery, mechanically blocking blood flow to induce transient cerebral ischemia. After a defined duration of ischemia (e.g., 60 minutes), the suture is slowly removed to restore blood flow and achieve reperfusion. Using the MCAO model, Wang et al. (2015) uncovered the direct effects of Treg-derived IL-10 on neurogenesis in the subventricular zone. Their research revealed that following MCAO surgery, injection of activated Tregs into the left lateral ventricle promoted the proliferation of the Mash1⁺ neural stem cell population, an effect negated by administration of anti-IL-10 antibodies. In a seminal study, Ito et al. (2019) first reported that Tregs massively accumulate in the brain during the later stages of MCAO, promoting neural recovery in chronic ischemic brain injury. Their study found that although brain-resident Tregs share similarities with Tregs from other tissues, they exhibit unique gene expression profiles related to the nervous system. On further analysis, the proliferation of brain-resident Tregs was shown to depend on IL-2, IL-33, serotonin, and T-cell receptor recognition, while their migration into the brain is driven by the chemokines CCL1 and CCL20. It was further demonstrated that Tregs contribute to neural recovery by suppressing the proliferation of neurotoxic astrocytes and microglia by producing AREG, a low-affinity ligand for EGFR, which blocks the IL-6/STAT3 signaling pathway in those cells (Ito et al., 2019). Using also the MCAO stroke model, Shi et al. (2021) further demonstrated that Tregs infiltrate brain tissue between 1 to 5 weeks from stroke onset. Depletion of Tregs reduced oligodendrocyte generation, impaired white matter repair, and hindered post-stroke functional recovery. Mechanistically, Treg-derived osteopontin interacted with integrin receptors on microglia, enhancing their reparative activity and promoting oligodendrocyte generation and white matter repair. Increasing Treg cell numbers through IL-2 antibody complexes improved white matter integrity and facilitated long-term neurological recovery (Shi et al., 2021). These findings highlight Tregs as critical targets for neural repair during stroke recovery.

Alzheimer’s disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder characterized by memory decline, cognitive impairment, and behavioral changes. Hallmark pathological features of the disease include the abnormal deposition of amyloid-beta (Aβ) plaques in the brain, neurofibrillary tangles within neurons, and chronic neuroinflammation mediated by activated microglia and reactive astrocytes. These pathological changes lead to the gradual loss of neurons, particularly in the hippocampus and cerebral cortex, which are critical regions for memory and cognition. Using APPPS1 mice to model AD pathology, Dansokho et al. (2016) performed intraperitoneal injection of anti-CD25 monoclonal antibodies and low-dose IL-2 treatment to deplete and expand, respectively, Tregs. Their study found that compared to a control group with non-depleted Tregs, the Treg-depleted group exhibited a significant reduction in the number of activated microglia migrating toward Aβ deposits and demonstrated early cognitive impairment. In contrast, low-dose IL-2 treatment significantly increased the number of Iba1⁺ and CD68⁺ microglia associated with plaques, promoting their aggregation toward Aβ deposits and thereby restoring cognitive function in the mice. These findings suggest that Tregs play a crucial role in AD pathogenesis and can significantly influence pathology and cognitive function by modulating microglial reactivity. In another study, Stym-Popper et al. (2023) found that Tregs can limit the functional differentiation of plaque-associated reactive astrocytes into the neurotoxic C3+A1 subpopulation, thereby alleviating harmful neuroinflammation in AD-like pathology. This finding suggests the key role of Tregs in regulating neuroinflammation, indicating that they not only influence microglial reactivity but also regulate the functional differentiation of astrocytes, further mitigating neuroinflammation. Yang et al. (2022) intravenously administered Aβ-specific Tregs (Aβ⁺ Tregs) into 3×Tg-AD mice to investigate their effect on the pathological process of AD. The study found that following the adoptive transfer of Aβ⁺ Tregs, the accumulation of Aβ plaques in the hippocampus of AD mice was significantly reduced. Notably, upon early treatment, Aβ⁺ Tregs significantly inhibited the phosphorylation levels of tau protein. Furthermore, adoptive transfer of Aβ⁺ Tregs markedly reduced the upregulation of Iba1 in AD mice, indicating effective suppression of reactive microglia (Yang et al., 2022). These results demonstrate that Tregs alleviate neuroinflammation in AD and highlight their therapeutic potential to treat this prevalent neurodegenerative disease.

Parkinson’s disease

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by the gradual loss of dopamine-producing neurons in the brain. A recent study reported that following repetitive transcranial magnetic stimulation, improvements in motor dysfunction in patients with PD are closely associated with increased numbers of peripheral Tregs (Xie et al., 2024a). Further animal experiments indicated that repetitive transcranial magnetic stimulation alleviates neuroinflammation by increasing the number of peripheral Tregs, thereby helping to improve motor function (Xie et al., 2024a). In the 6-hydroxy-dopamine mouse model of PD, Xie et al. (2024b) found that calcitriol inhibited T-cell infiltration in the midbrain by promoting the expansion of Tregs, which reduced the release of pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6). This process improved the inflammatory microenvironment in the brain, suppressed the polarization of microglia toward the pro-inflammatory M1 phenotype, and promoted the survival of dopaminergic neurons. In research addressing cell therapy for PD, Park et al. (2023) reported that co-transplantation of autologous Tregs significantly improved the survival rate of transplanted midbrain dopaminergic precursor cells. By inhibiting acute neuroinflammation and immune cell infiltration caused by needle injury, Treg co-transplantation alleviated early post-transplantation death of dopaminergic neurons. Furthermore, Treg co-transplantation not only reduced graft volume, but also increased the proportion of dopaminergic neurons within the graft. Addressing this phenomenon, the research team concluded that Tregs inhibit excessive proliferation of transplanted cells by secreting immune-regulatory factors such as TGF-β1, thereby optimizing transplantation outcomes.

Amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that primarily affects motor neurons. Sheean et al. (2018) reported a negative correlation between Treg levels in peripheral blood and ALS progression rate in ALS patients. Experimenting on SOD1G93A transgenic mice, they further showed that treatment with an IL-2/IL-2 monoclonal antibody complex (IL-2c) increased the proportion of circulating Tregs and induced the expansion of effector (GITR⁺, CTLA4⁺, ICOS⁺) Tregs with immunosuppressive functions. This was correlated with a significant reduction in the activation of astrocytes and microglia, slowed disease progression, and prolonged survival. Using the same ALS mouse model, Beers et al. (2011) had previously reported that in the early stages of ALS, Tregs exert a neuroprotective effect by secreting IL-4, which inhibits the neurotoxic phenotype of microglia (characterized by NOX2 expression) while promoting a protective M2 phenotype. These findings indicate that Tregs play an important regulatory role in the pathological process of ALS, particularly in its early stages. By modulating the phenotype of microglia, Tregs can slow the progression of ALS, highlighting new potential targets for treatment.

Demyelinating diseases

Myelin is a protective structure that ensheaths neuronal axons, playing a crucial role in safeguarding neurons and accelerating nerve signal transmission. In the CNS, myelin is formed by oligodendrocytes, while in the PNS, it is produced by Schwann cells. Loss or damage to myelin disrupts nerve signal conduction, leading to neurological dysfunction and the development of demyelinating diseases. Tregs have been shown to play a pivotal role in myelin regeneration, which is essential for preventing neurodegeneration associated with conditions such as MS. Using demyelination models induced by lysophosphatidylcholine (in the spinal cord) and cuprizone (in the corpus callosum), D.C. Fitzgerald’s research team investigated the effects of Tregs on neuronal myelin regeneration. In both models, depletion of Tregs severely impaired the differentiation of OPCs and subsequent myelin regeneration. Conversely, myelin regeneration was restored after adoptive transfer of Tregs. The study also identified cellular communication network factor 3 (CCN3) as a potential mediator through which Tregs promote OPC differentiation in several co-culture models (Dombrowski et al., 2017). However, subsequent research suggested that Treg-derived CCN3 might not directly signal to OPCs (Dittmer et al., 2018).

The efficiency of myelin regeneration declines with age. In a recent study, the same research group demonstrated that aged Tregs exhibit an inherent decline in their ability to promote oligodendrocyte differentiation and myelin formation (de la Fuente et al., 2024). However, this regenerative capacity can be restored by exposure to a young systemic environment. Additionally, the referred study identified melanoma cell adhesion molecule 1 (MCAM1) and integrin α2 (ITGA2) as candidate factors mediating Treg-driven oligodendrocyte differentiation. The expression levels of these factors also decline with age, highlighting their potential as therapeutic targets for enhancing myelin regeneration associated with aging and neurodegenerative conditions.

Roles of regulatory T cells in peripheral nerve injury

The PNS includes all neural structures outside the spinal cord and cranial meninges, excluding the olfactory and optic nerves, which connect directly to the brain. In the PNS, somatic nerves are distributed throughout the body surface, skeleton, and skeletal muscles, whereas visceral nerves are associated with internal organs, blood vessels, smooth muscles, and glands. Most peripheral nerves are mixed, containing sensory, motor, sympathetic, and parasympathetic fibers, all of which are encased in connective tissue membranes along with blood vessels and lymphatic vessels (Cattin et al., 2015; Wang et al., 2019; Li et al., 2022; Zhao et al., 2024). Compared to the CNS, the PNS exhibits fewer restrictions on immune responses. Consequently, T-cell accumulation in injured sciatic nerves is significantly greater than in injured optic nerves, and T lymphocyte infiltration closely correlates with increased expression of major histocompatibility complex class II antigens (Adusei et al., 2021). The protective role of Tregs in the PNS is primarily attributed to their immunosuppressive functions. In mouse experiments, Hu et al. (2021a) demonstrated that low-dose IL-2 significantly increased the proportion of Tregs in injured sciatic nerves. Moreover, the combined administration of low-dose IL-2 and adoptive Treg transfer effectively alleviated punctate hyperalgesia and dynamic allodynia following sciatic nerve injury.

For peripheral nerve injuries exceeding the critical gap length, bridging grafts are required to promote nerve regeneration. While autologous graft transplantation provides the best results, allogeneic grafts from immunologically incompatible donors also offer an efficient bridging method; however, the high costs and health risks associated with systemic immunosuppression therapy pose major challenges. Santos Roballo et al. (2019) developed a novel strategy that encapsulates Tregs in biodegradable polyethylene glycol (PEG)-based norbornene hydrogels, serving as localized immunosuppressive agents. When injected around allogeneic nerve grafts, Tregs are gradually released from the hydrogel over 14 days, infiltrating the graft, suppressing host immune responses, and promoting nerve regeneration. Notably, the outcomes were comparable to those achieved with autografts. This approach to localized immunosuppression circumvents the risks associated with systemic immunosuppression, providing an effective new method for treating segmental peripheral nerve defects (Santos Roballo et al., 2019). Although current research on Tregs in peripheral nerve injury is relatively limited, their demonstrated role in promoting neural repair and regeneration in CNS injuries suggests that these cells may also play a similarly critical role in the PNS.

Clinical Application Potential of Regulatory T Cells

In vitro regulatory T cell culture and expansion

Tregs are crucial for establishing fundamental tolerance mechanisms that mitigate immune responses, offering promising therapeutic potential for treating neurological autoimmune diseases and promoting the regeneration of damaged tissues through adoptive therapy. Tregs are typically generated by the combined action of TGF-β and IL-2, along with stimulation signals from CD3 and CD28 (Shi et al., 2021; Zhong et al., 2022; Jang et al., 2024). Obtaining a large number of high-purity Tregs is essential for initiating adoptive transfer therapies. This process involves selecting appropriate Treg sources and determining effective isolation and culture methods. In murine experiments, Tregs are commonly derived from splenic tissue, although inguinal or axillary lymph nodes are also used in some cases. For clinical applications, human Tregs are typically isolated from autologous peripheral blood, while ongoing research is exploring non-autologous sources such as umbilical cord blood or pediatric thymus tissue (Ferreira et al., 2019). Following isolation, Tregs are purified using common methods, including cell sorting via flow cytometry or magnetic bead separation. Fluorescence-activated cell sorting offers higher cell purity and reduced sorting time, making it suitable for large-scale isolation. Magnetic-activated cell sorting yields higher cell viability, is simpler to operate, and is more appropriate for sorting smaller quantities of cells (Azimi et al., 2016). Both methods produce cells that are effective for downstream applications, with comparable outcomes. Isolated Tregs are typically identified using flow cytometry to assess cell surface markers, with CD4, CD25, and Foxp3 being the most commonly used. Table 1 summarizes the characteristics of these isolation methods.

Table 1.

Methods for the separation of regulatory T cells

Item	Magnetic bead sorting	Flow separation
Separation purity	Lower	Higher
Sorting activity	Higher	Lower
Technical difficulty	Lower, easy to operate	Higher, complexities in machine programming

Current research aims to develop innovative therapies that enhance the function of endogenous Tregs in vivo through the use of cytokines and small-molecule drugs. It also investigates the adoptive transfer of ex vivo cultured Tregs in autoimmune environments to promote immune regulation. However, adoptive cell therapies employing ex vivo cultured Tregs encounter significant challenges, including insufficient purity, unstable activity, and potential shifts in phenotype or functional loss following expansion (Li et al., 2018; Attias et al., 2019; Jie et al., 2022). In this regard, a study found that the molecular weight of hyaluronic acid specifically upregulates certain transcription factors that are crucial for maintaining the Treg phenotype (Bollyky et al., 2007). More recently, Shi et al. (2023b) demonstrated that the induction of human Tregs is influenced by the ability of T cells to sense the mechanical stiffness of the substrate. On polyacrylamide gels, Treg induction was highly sensitive to the elastic modulus of the substrate, with higher levels of induction observed as the material’s stiffness increased. ScRNA-seq analysis revealed that Treg induction on stiffer substrates was associated with upregulation of oxidative phosphorylation. Inhibiting ATP synthase significantly reduced Treg induction rates and eliminated the differences observed between gels of varying stiffness. Conversely, activating AMP-activated protein kinase enhanced Treg induction on softer substrates but had no effect on stiffer substrates. These findings thus revealed that Treg induction is mechanosensitive and reliant on oxidative phosphorylation (Shi et al., 2023b), suggesting new strategies to enhance the therapeutic potential of Tregs in cellular immunotherapy, especially in the fields of biomedical engineering and neural regeneration.

Adoptive transfer of regulatory T cells

Following brain injury, Tregs can suppress adaptive immune responses and mitigate glial reactions that lead to neuronal damage, while also directly facilitating various repair and regeneration processes (Liston et al., 2023). Depletion of Tregs results in uncontrolled inflammation, tissue damage, and cell death, and these features can be rescued to some extent by the supplementation of exogenous Tregs. Shi et al. (2021) demonstrated that in the setting of experimental stroke, selective depletion of Tregs leads to reduced oligodendrocyte generation, impairs white matter repair, and hinders functional recovery. Conversely, increasing Treg numbers improves white matter integrity and promotes long-term neurological recovery, highlighting Tregs as critical targets for neural repair in stroke rehabilitation. Wang et al. (2015) injected Tregs into the left lateral ventricle of mice following transient MCAO and found that the proliferation of Mash1⁺ neural stem cells in the subventricular zone was significantly increased. This effect was abrogated by the application of anti-IL-10 antibodies. In another study, CD4⁺Foxp3⁺ Tregs were isolated from Foxp3-GFP mice and administered via tail vein injection to CD3e^–/– mice on day 5 post-stroke (Ito et al., 2019). The adoptively transferred Tregs facilitated neural recovery by interfering with IL-6/STAT3 signaling in astrocytes and microglia via AREG/EFGR signaling, promoting the production of IL-6, which facilitated neural recovery.

Dombrowski et al. (2017) evaluated the role of Tregs in myelin regeneration using FoxP3-DTR knock-in mice, which express the human diphtheria toxin (DT) receptor under the control of the Foxp3 promoter. In FoxP3-DTR mice with DT-induced Treg depletion, intraperitoneal injection of DT-insensitive Tregs (purified from wild-type mice) 24 hours prior to lysophosphatidylcholine-induced spinal cord white matter demyelination effectively restored myelin regeneration. Recently, another study showed that treatment with Tregs derived from young mice reversed myelin damage associated with aging and alleviated negative effects linked to the functional decline of Tregs in older mice (de la Fuente et al., 2024). Overall, these findings indicate that the adoptive transfer of Tregs holds significant therapeutic potential in demyelinating disorders.

Substantial evidence supports the neuroprotective role of Tregs in neurodegenerative disorders (Machhi et al., 2020). The innate and adaptive immune responses of the host influence the early and long-term survival of midbrain dopaminergic neuron grafts, a plausible approach to PD treatment. Park et al. (2023) showed that co-transplanting stem cells with Tregs can modulate immune responses, significantly protect midbrain dopaminergic neurons, and enhance the efficacy and safety of cell replacement therapies for neurodegenerative diseases such as PD (Park et al., 2023). In the early stages following sciatic nerve injury, adoptively transferred lymphocytes migrate to the injury site, releasing pro-inflammatory cytokines that promote Wallerian degeneration, thereby accelerating debris clearance and axonal regeneration (Bombeiro et al., 2020). Hu et al. (2021a) investigated the role of Tregs in alleviating mechanical hypersensitivity and dynamic allodynia resulting from nerve injury. They found that low-dose IL-2 treatment or the adoptive transfer of Tregs significantly alleviated punctate and dynamic allodynia, suggesting that these cells may serve as a potential therapeutic target for pain related to nerve injury.

Effective clinical translation of the obvious potential of Tregs in managing neurological autoimmune diseases and preventing graft rejection requires further optimization of therapeutic strategies. Key challenges include producing these cells in sufficient quantities while ensuring consistent genotypic and phenotypic stability for therapeutic applications. Santos Roballo et al. (2019) developed an innovative approach for localized immunosuppression that involves a single application of Tregs encapsulated in biodegradable PEG-based hydrogels. This procedure was shown to prevent systemic immunosuppression and promote nerve regeneration in recipient rats. Zhang et al. (2023b) designed a perforated microneedle with a core-shell structure that exhibits excellent mechanical performance and a spacious encapsulation cavity to support Treg viability. Additionally, the adjustable channels in the microneedle facilitated cell migration. This strategy successfully achieved percutaneous delivery of Tregs, providing a more efficient way to deliver local cell-based therapies (Table 2).

Table 2.

Comparative analysis of studies using Treg cell adoptive transfer in neurological diseases

Animal model	Formulation type	Injection site	Route	Dosage and frequency	Reference
MCAO	Treg cells	Tail vein	Injection	2 × 106 Treg cells 6 h after MCAO surgery	Shi et al., 2021
MCAO	Treg cells	Tail vein	Injection	1 × 106 Treg cells on d 5	Liston et al., 2023
MCAO	Treg cells	Lateral ventricle	Injection	1 × 105 Treg cells on d 1 after MCAO surgery	Wang et al., 2015
Demyelination model	Treg cells	Intraperitoneal	Injection	1 × 106 Treg cells	Dombrowski et al., 2017
Parkinson’s disease	C4-mDAPs+Treg cells	Striatum of the brain	Injection	1 × 105 mDAPs + 1 × 104–105 Treg cells (mDAPs: differentiated from human iPSCs)	Park et al., 2023
SNI unilateral nerve injury model	Treg cells	Tail vein	Injection	1 × 106 Treg cell administration 3 wk after SNI surgery	Hu et al., 2021a

iPSCs: Induced pluripotent stem cells; MACO: middle cerebral artery occlusion; mDAPs: midbrain dopamine precursor cells; SNI: spared nerve injury; Treg cell: regulatory T cell.

Advancements in clinical research on Tregs

In recent years, a growing number of clinical studies investigated the potential of Tregs in disease treatment, particularly in the realms of autoimmune disease and cancer therapy. In comparison, research on Treg-based therapies for neurological disorders has developed relatively slowly, but is quickly emerging as a focal research point. Several studies indicated that Tregs possess unique and significant therapeutic potential in neurological disorders by modulating neuroinflammation, neurodegeneration, and various pathophysiological processes associated with autoimmunity (Machhi et al., 2020). Clinical trials targeting neurological diseases such as AD, MS, ALS, and PD have gradually been initiated. These studies have explored the safety and efficacy of treatments that involve activating endogenous Tregs through low-dose IL-2 injections or the direct transfer of ex vivo expanded autologous Tregs. In these trials, Tregs have shown multifaceted effects, including the regulation of neuroinflammation, the alleviation of immune-mediated neuronal damage, and the promotion of tissue repair. Table 3 presents a summary of current clinical studies on Treg cell therapy for neurological disorders, illustrating various disease models, therapeutic strategies, and metrics for evaluating efficacy.

Table 3.

Ongoing clinical trials for neurological diseases involving Treg cells (https://clinicaltrials.gov/)

ID	Sponsor	Disease	Enrollment (n)	Strategy	Phase/Status
NCT05468073	Centre Hospitalier St Ann	Alzheimer’s disease	45	Low-dose IL-2 injection	Phase 2
NCT06096090	The Methodist Hospital Research Institute	Alzheimer’s disease	40	Low-dose IL-2 injection	Phase 2
NCT05016427	VTBIO Co. LTD	Alzheimer’s disease	12	Low- and high-dose Treg cells (VT301) injection	Phase 1
NCT03865017	VTBIO Co. LTD	Alzheimer’s disease	20	Autologous Treg cells (GB301) injection	Phase 1 Phase 2
NCT06671236	Novabio Therapeutics	Neurodegenerative diseases	12	Autologous human polyclonal Treg cells (NP001) injection	Phase 1
NCT05245344	Neuromed IRCC	Relapsing-remitting multiple sclerosis, neuromyelitis optica spectrum disorder	154	Evaluate the effect of ozanimod on the proliferation, metabolism, and function of Treg cells	Unknown
NCT04220190	Rapa Therapeutics LLC	Amyotrophic lateral sclerosis	41	Autologous Treg/Th2 cells (RAPA-501) injection	Phase2 Phase 3
NCT04055623	The Methodist Hospital Research Institute	Amyotrophic lateral sclerosis	12	Autologous Treg cells infusion + IL-2 injection	Phase 2
NCT05695521	Cellenkos, Inc.	Amyotrophic Lateral Sclerosis	66	Neurotropic, allogeneic, umbilical cord blood derived Treg (CK0803) injection	Phase 1
NCT03039673	Centre Hospitalier Universitaire de Nimes	Amyotrophic Lateral Sclerosis	304	Low-dose IL-2 injection	Phase 2
NCT06169176	Rapa Therapeutics LLC	Amyotrophic Lateral Sclerosis	Unknown	Autologous hybrid Treg/Th2 cell (RAPA-501) injection	Unknown

IL-2: Interleukin-2; Treg cells: regulatory T cells.

Engineered Tregs for Disease Treatment and Tissue Regeneration

Engineering T cells is a cutting-edge biotechnological approach aimed at enhancing their therapeutic potential through genetic editing, molecular modifications, and chemical alterations (Chen et al., 2022b; Lim, 2022; Wang et al., 2023c). Wang et al. (2022) used genetic editing techniques, such as the CRISPR/Cas9 system, to precisely modify the T-cell genome, thereby enhancing specific immune functions. Additionally, molecular modifications, including the addition of functional molecules (e.g., chimeric antigen receptors; CARs) to the cell surface, provide T cells with improved targeting specificity and efficacy in treating tumors and autoimmune diseases (Guo et al., 2022; Zhu et al., 2022; Foss et al., 2023). Chemical modifications enhance the stability and activity of Treg cell membranes through the use of chemical crosslinkers or small-molecule drugs, thereby improving cell survival, prolonging therapeutic effects, and enhancing efficacy (Jin et al., 2023; Wang et al., 2023c). This section focuses on four key aspects: the genetic engineering of Tregs, acellular engineering strategies for Tregs, in situ recruitment and activation of endogenous Tregs, and the delivery of immunomodulators to convert effector T cells into Tregs (Figure 3).

Figure 3.

Advances in engineered Tregs for neurological diseases and neural regeneration.

The figure summarizes four key aspects of Treg engineering: genetically modified Tregs, cell-free engineering strategies for Tregs, in situ recruitment and activation of endogenous Tregs, and the delivery of immunomodulators to transform effector T cells into Tregs. Created with BioRender.com. AOA: (Aminooxy)-acetic acid; AOA-nCs: (aminooxy)-acetic acid-nanocapsules; BMSC: bone marrow stromal cells; HUVEC: human umbilical vein endothelial cells; LNP: lipid nanoparticle; Th17: T-helper cell 17; Treg: regulatory T cell; TregD-Exos: Treg-derived exosomes.

Genetic editing is commonly applied to enhance Tregs’ immunosuppressive functions, while physical or chemical modifications achieve non-genetic functional enhancements. Specific regulatory factors or biomaterials can induce the recruitment and activation of endogenous Tregs in vivo, and the design of precision delivery systems can facilitate the conversion of effector T cells into Tregs, stabilizing and enhancing their functionality (Tajima et al., 2015; Ishihara et al., 2018). These multidimensional engineering strategies significantly improve the effector functions of Tregs and their adaptability to complex inflammatory microenvironments. They also enhance long-term survival in vivo and provide novel approaches for clinical applications in autoimmune diseases, cancer immunotherapy, and tissue engineering (Li et al., 2018; Loffredo et al., 2024). Notably, the integration of Tregs with biomaterials further drives technological innovation and expands their applications, paving the way for breakthroughs in tissue engineering and regenerative medicine development (Ho et al., 2024; Wang et al., 2024b).

Genetically engineered regulatory T cells

Genetic engineering of T cells has recently emerged as a major topic in the fields of bioengineering and immunotherapy (Shrestha et al., 2020; Zhao et al., 2024). By endowing T cells with new functions or enhancing specific characteristics, this approach enables precise treatment for a variety of diseases (Foss et al., 2023). In the context of tissue regeneration, engineered T cells hold great potential for modulating the immune microenvironment following tissue damage (Tang et al., 2018). For instance, after neural injury, the local microenvironment often exhibits excessive inflammation and a lack of sufficient signals for tissue repair. Engineered T cells can promote nerve regeneration and functional recovery by secreting specific neurotrophic factors or modulating immune responses (Yang et al., 2023a; Gao et al., 2024). Currently, enhancing or regulating the key transcription factor Foxp3 through genetic engineering is a primary approach for precisely modifying Treg functions and treating autoimmune diseases (Georgiev et al., 2019; Korn, 2023). Honaker et al. (2020) used homologous directed repair technology to insert a potent enhancer/promoter combination near the first exon of the Foxp3 gene in Tregs. This approach overcomes the limitations of epigenetic silencing, enabling stable and sustained expression of Foxp3. The engineered Tregs exhibited typical inhibitory markers and significantly enhanced immunoregulatory functions, demonstrating great potential in treating diseases such as type 1 diabetes and rheumatoid arthritis.

In another study, Thatte et al. (2023) developed an ionizable lipid nanoparticle platform for the efficient delivery of Foxp3 mRNA into CD4⁺ T cells, converting them into Foxp3⁺ T cells with a temporary immunosuppressive phenotype (Thatte et al., 2023). This non-permanent genomic modification effectively suppressed the excessive proliferation of effector T cells within a short period, achieving flexible immune regulation suitable for treating acute graft-versus-host disease or inflammatory disorders. These two Foxp3-engineering strategies offer long-term and short-term immune regulation options, demonstrating the immense potential of genetic engineering in immunoregulation and laying a solid foundation for the development of personalized immunotherapy.

Gene-editing tools such as CRISPR-Cas9 have significantly enhanced the precision of T-cell engineering. By knocking out genes associated with inflammatory responses or overexpressing genes that support neural regeneration, dual-function engineered T cells can be customized to effectively suppress inflammation while promoting tissue repair. Wang et al. (2022) used CRISPR/dCas9 nanoparticle technology to program Tregs in situ, successfully preventing excessive inflammatory responses following the transplantation of small-diameter tissue-engineered vascular grafts and promoting neural regeneration. In rat experiments, the edited Tregs upregulated the TET2 gene, reduced the proportion of CCR2⁺ macrophages, and modulated IL-6 levels, maintaining them within a range favorable for neural regeneration. This strategy significantly improved vascular integrity and long-term patency, achieving near-normal vascular regeneration and immune homeostasis after 3 months. These results demonstrate the great potential of Treg genetic reprogramming for tissue regeneration and immune regulation. Another promising immunoregulatory strategy is the design of CAR-Tregs. By expressing CARs that target specific antigens on T cells, these engineered cells can precisely regulate immune responses. D. W. Scott’s research team developed a CAR analog, the B-cell targeting antibody receptor (BAR), which targets the protein antigen ovalbumin and uses the CD28-CD3ζ signaling domain to regulate the activity of B cells and mast cells. In an anaphylaxis mouse model, OVA-BAR Tregs effectively suppressed allergic reactions, providing basic evidence of possible clinical protection against severe allergic reactions in IgE-sensitized individuals (Abdeladhim et al., 2019). Building on this foundation, CAR-Treg applications might be further extended to the field of neural regeneration by targeting neural tissue-specific antigens to localize at sites of nerve injury. These CAR-Tregs could secrete anti-inflammatory cytokines (such as IL-10 and TGF-β), suppress overactivated immune responses, and reduce damage caused by inflammatory mediators to neurons and myelin. Additionally, these cells could release neurotrophic factors (such as brain-derived neurotrophic factor) or interact with glial cells to promote axonal regeneration and myelin repair. Through their diverse applications in immune regulation and tissue repair, CAR-Tregs hold immense potential for precise therapies for a variety of complex diseases, offering innovative directions for clinical development.

A cutting-edge approach in the field of neural regeneration involves the use of Treg-biomaterial composite systems. These systems encapsulate genetically engineered Tregs within biodegradable biomaterials, providing novel therapeutic strategies for repairing neural tissues (Bollyky et al., 2007). The biomaterials not only function as scaffolds to support neural tissue repair but also enhance the local microenvironment by controlling the release of bioactive factors (Santos Roballo et al., 2019). The engineered Tregs express specific chemokines or surface adhesion molecules, facilitating interaction with regenerating nerve fibers and surrounding matrices, which accelerates the reconstruction of neural networks (Hu et al., 2018; Jie et al., 2022). In summary, the genetic engineering of Tregs, when combined with biomaterials, nanotechnology, and high-throughput gene editing techniques, holds significant promise for developing more precise and efficient Treg-based therapeutic strategies. This multidisciplinary collaboration offers innovative solutions for addressing complex diseases, including central and peripheral nerve injuries.

Cell-free Treg engineering strategies

EVs, particularly exosomes, are emerging as critical tools in tissue repair, immune regulation, and regenerative medicine, representing key components of cell-free engineering strategies (Ullah et al., 2021; Fan et al., 2024). These small vesicles, typically ranging from 30 to 150 nm in diameter, are released into the extracellular matrix when multivesicular bodies fuse with the cell membrane (Li et al., 2021d). Rich in biologically active molecules—including proteins, lipids, and nucleic acids such as mRNA and miRNA—exosomes not only reflect the characteristics of their parent cells but can also be delivered to specific cells or tissues, thereby modulating the functions of recipient cells (Kalluri, 2024). Exosomes derived from Tregs are particularly enriched with various anti-inflammatory molecules and factors, enabling them to suppress the activity of effector T cells and promote tissue-specific immune tolerance (Zhang et al., 2023a; Li et al., 2024). For example, Treg-derived exosomes containing miR-142-3p, when delivered to bone marrow MSCs and human umbilical vein endothelial cells, significantly enhance osteogenesis and angiogenesis. Specifically, miR-142-3p promotes osteogenic differentiation of bone marrow MSCs via the TGFBR1/SMAD2 signaling pathway while simultaneously enhancing the proliferation, migration, and angiogenesis of human umbilical vein endothelial cells, thus accelerating bone repair in vivo (Chen et al., 2022a). These naturally secreted nanoscale particles play a vital role in intercellular communication, and their unique properties position them as ideal carriers and effectors for cell-free therapies (Krishnan et al., 2023; Wang et al., 2023a; Zhang et al., 2024).

The advantages of exosome applications stem from their natural biocompatibility and low immunogenicity (Wallen et al., 2022). Moreover, through genetic engineering or chemical modifications, it is possible to construct artificially designed bioactive molecules, synthetic materials, or delivery systems that can regulate cellular behavior and immune responses in vivo, without the need for live cells. The core concept of this strategy is to precisely regulate biological and physical signals within the microenvironment, stimulating the repair potential of innate cells while avoiding the complexities and risks associated with cell-based therapies (Veerman et al., 2019; Zhou et al., 2020; Fan et al., 2024). For instance, Tian et al. (2021) developed a nanomedicine that combines Treg-derived exosomes with an anti-VEGF drug. This system exploits the activity of matrix metalloproteinases in inflammatory lesions to enable the precise release of Treg-derived exosomes and an antiangiogenic drug, thereby suppressing inflammatory responses and VEGF activity. This innovative approach suggested that by carefully controlling drug release and combining the immunoregulatory effects of Treg cell-derived exosomes, it is possible to effectively intervene in pathological processes, such as ocular neovascular diseases, to optimize therapeutic outcomes. Cheng et al. (2022) used a biomaterial engineering strategy to design carriers or scaffolds for delivering secreted protein acidic and cysteine rich (Sparc)-overexpressing Treg-derived EVs, creating a composite hydrogel system. This system responds to specific biomarkers in infarcted areas, such as CXCR2, pH, H₂O₂, and MMP9, enabling targeted EV release. As a result, it reduces myocardial damage, promotes collagen synthesis, and suppresses pro-inflammatory factors such as IL-1, IL-6, and TNF-α, significantly improving cardiac function following acute myocardial infarction. Building on the immunosuppressive functions of Tregs, Li et al. (2021b) devised an innovative biomimetic immunoregulatory strategy by coating poly(lactic-co-glycolic acid) nanoparticles with Treg cell membranes. This approach addresses the impairment of Treg cell function in inflammatory environments such as chronic periodontitis. By preserving the natural immunoregulatory properties of Treg cell membranes, this biomimetic membrane technology effectively targets inflammation sites and modulates the local immune microenvironment, facilitating efficient inflammation control and tissue repair. In summary, cell-free engineering strategies based on Treg-derived exosomes, Treg membranes, and biomimetic nanoparticles have broadened the potential applications of Tregs in inflammation regulation, tissue regeneration, and immune balance, demonstrating significant advantages for treating complex diseases (Zhang et al., 2021b; Zeng et al., 2023). Compared to cell-based therapies, these strategies bypass the technical challenges of cell sourcing, cultivation, and storage, while offering more flexible solutions through modular design (Yang et al., 2023b). With the ongoing integration of synthetic biology, materials science, and nanotechnology, cell-free engineering is poised to become a cornerstone of precision therapy and regenerative medicine, providing groundbreaking opportunities for treating neurological diseases and advancing tissue regeneration (Rau et al., 2021; Liu et al., 2022).

In situ recruitment and activation of endogenous regulatory T cells

In situ recruitment and activation of endogenous Tregs is an innovative strategy to modulate the local immune microenvironment (Zhang et al., 2022). A notable characteristic of Tregs is the persistent expression of CD25, the alpha chain of the trimeric high-affinity IL-2 receptor. Through selective binding to IL-2, Tregs can be preferentially activated to exert immune regulation, a critical goal in the treatment of autoimmune diseases (Attias et al., 2019; Ho et al., 2024). Building on this property, Shi et al. (2021) demonstrated that the administration of IL-2 antibody complexes significantly increased the number of Tregs and improved long-term recovery after stroke. However, the narrow therapeutic window and short half-life of IL-2 limit its broader clinical application. To address this issue, Zhang et al. (2021a) introduced azido amino acids at specific positions in the IL-2 molecule and orthogonally conjugated them with PEG groups via copper-free click chemistry, significantly enhancing the pharmacokinetics and half-life of IL-2. Following subcutaneous injection, PEGylated IL-2 exhibited a high degree of selectivity in binding to the trimeric IL-2R across a broad dosage range, persistently inducing Treg activation and expansion. In mouse models of inflammatory diseases and graft-versus-host disease, PEGylated IL-2 significantly improved therapeutic outcomes while preserving the host’s immune defense against viral infections.

Although systemic cytokine injection can effectively activate the immune system, it may also unwantedly activate immune cells in non-target tissues, leading to toxicity and limiting its clinical application (Quijano-Rubio et al., 2022). To address this issue, Zamecnik et al. (2020) proposed an innovative strategy involving the design of a high-aspect-ratio polycaprolactone nanowire system. These nanowires can form complexes with cytokine-binding antibodies and self-assemble into a porous matrix after subcutaneous injection. This system can remain in vivo for several weeks, eliciting minimal foreign body reactions and effectively resisting clearance. By combining them with the anti-IL-2 antibody JES6-1, these nanowires can capture endogenous IL-2 and selectively activate tissue-resident Tregs. This approach successfully rebalanced the local immune environment in autoimmune disease models involving wild-type and transgenic mice, shifting it toward a Treg-mediated suppressive phenotype (Zamecnik et al., 2020; Zhong et al., 2022). This strategy not only optimizes the pharmacokinetics of the drug but also enhances Treg functions locally, thereby avoiding the side effects associated with systemic immune responses.

Unlike traditional systemic immunoregulatory approaches, in situ recruitment strategies achieve localized immune balance by selectively activating endogenous Tregs that reside in or migrate to the target site. The core of this strategy lies in leveraging biomaterials or drug delivery systems to optimize the local microenvironment, thereby inducing the recruitment, activation, and functional enhancement of Tregs (Ishihara et al., 2017; Gaharwar et al., 2020). However, while current strategies have demonstrated significant efficacy in Treg recruitment and activation, they do not specifically target Tregs in a particular tissue but instead act systemically or across all tissues (Becker et al., 2024). This limitation underscores the challenges for future clinical translation, particularly in achieving precise regulation of Tregs within specific tissues or immune environments. Future research focusing on developing strategies that can specifically target particular tissues or immune microenvironments is necessary to enhance therapeutic specificity and safety while minimizing unintended immune responses.

Delivery of immunomodulators for regulatory T cell conversion

Drug delivery systems offer great potential for improving therapeutic efficiency and reducing side effects, particularly in the treatment of complex health conditions such as autoimmune diseases (Foss et al., 2023; Mahmoudi et al., 2024). In recent years, research on Treg conversion and functional regulation has highlighted the significant potential of immunomodulators, conversion inducers, and innovative immune-homeostatic particles as drug delivery strategies.

Aminooxy-acetic acid (AOA) is a promising transdifferentiation inducer that promotes the expression of Foxp3 by inhibiting pyridoxal 5′-phosphate-dependent transaminases and reducing the methylation of the Foxp3 locus, thereby inducing the conversion of Th17 cells into Tregs. However, the inability of AOA to cross the BBB limits its application in treating neurodegenerative conditions such as MS. To address this challenge, Shi et al. (2023a) developed an innovative “Trojan horse” drug delivery strategy using Th17 cells. By employing nanocapsule technology, AOA was conjugated to Th17 cells and intravenously administered into MS model mice. These nanocapsules successfully crossed the BBB and released AOA in a pro-oxidant environment, inducing the conversion of Th17 cells into Tregs to significantly reduce inflammation and exert immune regulation in MS mice.

In another study focusing on Treg-mediated immunoregulation, efficient targeted delivery of the immunomodulatory drug fingolimod to draining lymph nodes was achieved through the design of CaCO3/CaP/heparin hybrid nanoparticles modified with a CCL21-specific aptamer. This strategy significantly enhanced the local accumulation of the drug and effectively reduced the incidence of long-term complications in a heart transplantation mouse model by promoting the generation of endogenous Tregs and decreasing the proportion of effector T cells (Che et al., 2022). This approach improved therapeutic specificity through precise delivery, avoiding the side effects of traditional systemic immunosuppression. Chen et al. (2021) developed an immune-homeostatic microparticle system by combining mesoporous silica nanoparticles with polymer microspheres, providing a multifunctional delivery platform. This system utilized monocyte chemoattractant protein-1 (MCP-1) to induce T-cell migration to inflammation sites, while Fas ligand immobilized on the surface of the microparticles triggered apoptosis of activated T cells. Following the phagocytosis of these cells by macrophages, large amounts of TGF-β were produced, further promoting Treg differentiation (Chen et al., 2021).

The aforementioned studies demonstrate the significant potential of innovative drug delivery strategies that combine nanotechnology and bioengineering to target and regulate Treg differentiation and function. By integrating drug release and delivery with specific immune microenvironments, these technologies not only enhance therapeutic efficacy but also reduce the side effects associated with systemic immunosuppression. However, these strategies still face numerous challenges in terms of clinical translation, such as the controllability of delivery systems, targeting specificity, and drug stability (Rong et al., 2020; Shan and Wu, 2024). Future research should focus on developing more precise and efficient delivery technologies tailored to the characteristics of immune microenvironments in diseased or injured tissues. This approach will help accelerate the translation of these promising technologies from the laboratory to the clinic.

Limitations

This review comprehensively covers relevant research on the multifaceted roles of Tregs in diseases and tissue injury, with a particular focus on their immunological and non-immunological mechanisms in neurological disorders and tissue repair. However, we acknowledge some limitations in this review. First, research in this field remains relatively scarce, limiting the breadth of analysis and highlighting the necessity for further exploration. Second, distinguishing between the immunological and non-immunological pathways of Tregs in experimental models poses a major challenge, as these mechanisms often overlap and interact in complex ways. Third, the technical complexity associated with studying neurological models, particularly those focusing on the brain and other CNS structures, remains a substantial hurdle. In the context of spinal cord research, exploration in this area remains limited, resulting in a significant gap in our understanding of the mechanisms underlying Treg-mediated spinal repair. Fourth, despite advancements in the field, many mechanisms, particularly those bridging the immunological and non-immunological functions of Tregs, remain unclear. Further targeted and comprehensive research to address these gaps will help establish a robust foundation for biomedical engineering applications in regenerative medicine.

Discussion and Future Perspectives

In recent years, the role of the immune system in tissue repair and regeneration has garnered significant attention in the field of regenerative medicine. However, our understanding of the mechanisms by which the immune system—particularly specific immune cell subsets—regulates the tissue healing process remains limited (Adusei et al., 2021; Wang et al., 2023c, 2024a). Tregs present a promising avenue for research focusing on the treatment of neurological conditions, immune-related diseases, and tissue regeneration (Arpaia et al., 2015; Liston et al., 2023; Loffredo et al., 2024).

In this review, we first introduce the multifaceted roles of Tregs in diseases and tissue injury, encompassing traditional immunological functions and emerging non-immunological mechanisms. As part of their well-characterized immunological functions, Tregs suppress the activation and proliferation of effector T cells by secreting anti-inflammatory cytokines, thereby reducing excessive immune responses against pathogens or self-antigens. Furthermore, Tregs play a critical role in establishing central and peripheral tolerance by regulating the development of self-reactive T cells in the thymus and promoting immune tolerance to non-pathogenic antigens in the periphery, which helps prevent autoimmune diseases (Savage et al., 2020; Dikiy and Rudensky, 2023).

In addition to regulating T cells, Tregs influence B-cell antibody production and memory formation through cytokine secretion and cell-to-cell contact mechanisms. They also modulate the functions of innate immune cells, such as macrophages, neutrophils, and natural killer cells, further balancing immune responses (Sakaguchi et al., 2020; Ho et al., 2024). Through these multilayered immunoregulatory mechanisms, Tregs play a central role in suppressing inflammation, maintaining tissue homeostasis, and promoting repair under pathological conditions.

This review summarizes the non-immunological pathways of Tregs, emphasizing their critical roles in interacting with stem cells during immune regulation and neural tissue repair. Tregs and stem cells work together to maintain tissue homeostasis and promote regeneration through complex intercellular signaling networks. In the stem cell microenvironment, Tregs secrete anti-inflammatory cytokines that suppress local inflammation, reduce immune-mediated damage to stem cells, and protect their survival and function (Loffredo et al., 2024). Additionally, Tregs release pro-regenerative signals that enhance the proliferation, differentiation, and functionality of stem cells, thereby supporting tissue regeneration (Qi et al., 2018). At the same time, exosomes or factors derived from stem cells can regulate Treg metabolism and signaling pathways, improving their migration and function in injured tissues (Chen et al., 2022a). These synergistic interactions between Tregs and stem cells offer novel mechanistic insights into immune regulation and tissue repair, providing a solid foundation for developing regenerative medicine and cell-based therapies that leverage the potential of both cell types. However, several key questions regarding their mechanisms of action remain unresolved. For instance, it is unclear whether Treg-mediated immunosuppression and tissue repair require the cooperation of innate immune cells. It is uncertain also whether these processes depend on specific antigens, whether there are functional differences and interrelationships between tissue-resident Treg cells in the nervous system and peripheral Tregs, and what the optimal therapeutic window is for Treg-mediated repair. As research into the repair mechanisms of Tregs advances, addressing these challenges will enhance our understanding and establish a robust theoretical foundation for Treg-based therapies.

To achieve success in future personalized cell therapies for neurological diseases and tissue regeneration, therapeutic strategies must be scalable, demonstrate appropriate efficacy, and provide long-lasting effects without causing significant adverse events. Current adoptive cell transfer strategies have shown promise in treating neurological disorders and promoting regeneration across a variety of neurodegenerative diseases, autoimmune conditions, and age-related regenerative impairments. However, several barriers continue to limit the clinical therapeutic potential of Tregs (Li et al., 2021a, c; Wu et al., 2022; Liu et al., 2023). Under good manufacturing practice conditions, Tregs can be expanded ex vivo to generate sufficient quantities for clinical applications. This approach addresses challenges such as their survival and persistence in vivo, the stability of their functional phenotype, and the selective engagement or inhibition of antigen-specific responses in specific disease contexts (Hickey et al., 2019; Foss et al., 2023; Eckman et al., 2024). Despite these challenges, we remain cautiously optimistic about the prospects of utilizing Treg cells to slow or even reverse the progression of neurological diseases, especially given the notable successes of passive cell therapies in various other conditions.

The integration of biomaterials into immunoregulatory therapies has the potential to revolutionize tissue engineering and regenerative medicine (Gaharwar et al., 2020). Current strategies for modulating immune cell responses primarily focus on targeting pro-inflammatory or anti-inflammatory activities within the tissue microenvironment, particularly by suppressing the inflammatory activity of immune cells to promote healing and tissue regeneration. By incorporating biomaterials into these therapeutic designs, it is possible to achieve localized delivery of cytokines and other molecules that target inflammatory processes while precisely controlling their release kinetics (Gaharwar et al., 2020; Wu et al., 2022; Wang et al., 2023c). In regenerative therapies and biomaterial implants, greater attention must be paid to the role of Treg cells. Progress has already been made in modulating Treg cell activity, providing a robust theoretical foundation for designing biomaterials that support immune responses. Beyond existing biological approaches, such as genetic engineering and cell-free therapies, modifications to material properties through physical and chemical techniques (e.g., size, shape, surface characteristics) can also directly influence the therapeutic efficacy of Treg cells (Chaudhuri et al., 2020; Yuan et al., 2021; Shi et al., 2023b). Future immunoregulatory therapies may further leverage these biomaterial properties, particularly in controlling the delivery of cytokines, chemokines, and other regulatory factors, thereby demonstrating greater potential in fine-tuning immune responses (Eppler and Jewell, 2019; Madhusudanan et al., 2020; Li et al., 2021c; Lou and Mooney, 2022). In the field of tissue repair and immune engineering, a major future challenge will be to strike a balance between encouraging beneficial immune responses and ensuring that immunoregulatory technologies harmoniously integrate with the native tissue microenvironment. This integration is essential for modifying immune responses and supporting tissue regeneration. Overall, the role of Treg cells in the nervous system and the development of related therapeutic strategies open up an exciting new avenue of research in Treg cell medicine. This research will deepen our understanding of repair mechanisms and advance the development of cell therapies, ultimately leading to precise cellular drug therapies.

Funding Statement

Funding: This work was supported by the National Natural Science Foundation of China, Nos. 32271389, 31900987 (both to PY); the Natural Science Foundation of Jiangsu Province, No. BK20230608 (to JJ).

Data availability statement:

Not applicable.