Functional Impairment of Activated Regulatory T Cells as a Central Immunological Defect in Endometriosis

To dissect the causal contribution of Tregs to endometriosis pathophysiology, a Foxp3 DTR mouse model enabling transient, selective depletion of Foxp3 + Tregs upon diphtheria toxin (DT) administration was employed [ 38 ]. This approach offers a powerful in vivo system for examining how the absence of Treg‐mediated immune suppression influences the establishment and progression of endometriosis‐like lesions [ 39 ]. Following Treg depletion, Foxp3 DTR/DT mice developed significantly increased numbers and total weights of endometriosis‐like lesions compared with control mice ( p  < 0.05) [ 24 ]. These findings indicate that Treg deficiency directly facilitates the adhesion, survival, and expansion of ectopic endometrial tissue. The lesions exhibited markedly higher Ki‐67 immunoreactivity, confirming enhanced cellular proliferation, and displayed positive staining for cytokeratin, vimentin, and estrogen receptor‐α (ERα), consistent with estrogen‐dependent growth mechanisms characteristic of human endometriotic lesions [ 24 ]. The mechanistic basis of lesion exacerbation in the absence of Tregs is rooted in the amplification of inflammatory and angiogenic pathways. Quantitative PCR analysis revealed elevated mRNA expression of multiple inflammatory mediators, including IL‐6, CCL2, MIP‐1, and MIP‐2, as well as the pro‐angiogenic factor VEGF, within the lesions of Treg‐depleted mice (Figure  1 ) [ 24 ]. IL‐6 elevation was pronounced not only locally within the lesions but also systemically and in the PF, indicating widespread inflammatory activation ( p  < 0.01) [ 40 , 41 , 42 ]. Because IL‐6 is a pleiotropic cytokine with established roles in promoting endometriotic cell survival, proliferation, and resistance to apoptosis [ 43 , 44 ], its upregulation provides a direct mechanistic link between Treg deficiency and inflammatory disease progression [ 24 ]. Among the upregulated chemokines, CCL2 (C–C motif chemokine ligand 2) plays a particularly critical role. Elevated CCL2 expression in the lesions of Treg‐deficient mice aligns with clinical observations in OE, deep infiltrating endometriosis (DIE), and peritoneal lesions, where high CCL2 levels in the lesion tissue and PF have been consistently reported [ 45 , 46 , 47 , 48 ]. CCL2 promotes recruitment and activation of monocytes/macrophages and has been shown to stimulate endometrial stromal cell survival and invasiveness through Akt and MAPK/Erk1/2 signaling [ 49 ]. Moreover, CCL2 contributes to the recruitment of immunosuppressive cell types such as Tregs and myeloid‐derived suppressor cells (MDSCs) under physiological conditions [ 50 ]. Therefore, the increased CCL2 expression following Treg loss likely represents a dysregulated compensatory mechanism that instead fuels local inflammation and lesion expansion, highlighting a critical feedback loop between Treg deficiency and chemokine‐driven immunopathology [ 24 ]. Endometriosis is well recognized as an angiogenesis‐dependent disease [ 51 , 52 ], and Treg depletion further accentuates this hallmark feature. In patients with endometriosis, peritoneal macrophages exhibit enhanced production of VEGF, which contributes substantially to neovascularization within the peritoneal cavity [ 13 ]. PF VEGF levels are significantly elevated and correlate with disease severity and inflammatory activity [ 13 , 53 ]. In the Treg‐depleted mouse model, VEGF expression was markedly increased within lesions, although systemic VEGF levels in PB and PF remained unchanged [ 24 ]. This pattern suggests that angiogenesis is preferentially driven at local lesion sites, potentially through macrophage‐derived VEGF induced by inflammatory activation. Indeed, Treg depletion resulted in a marked increase in activated macrophages (F4/80 + CD11b + ) within both the lesions and the spleen [ 24 ], underscoring the central role of macrophage activation in amplifying inflammation and promoting neovascularization. Collectively, these findings illustrate a coherent mechanistic framework in which loss of aTreg‐mediated immune suppression triggers a cascade of inflammatory and angiogenic responses driven by IL‐6, CCL2, VEGF, and activated macrophages, thereby directly accelerating the establishment and growth of endometriotic lesions (Figure  1 ). This experimental evidence not only validates the essential role of aTregs in maintaining immune homeostasis within the peritoneal environment but also provides a strong conceptual foundation for therapeutic approaches aimed at restoring or enhancing Treg function. Such Treg‐centered immunomodulatory strategies represent promising nonhormonal treatment options capable of suppressing inflammation and lesion progression while preserving fertility, addressing a critical unmet need in current endometriosis management.

The failure to establish effector‐phase immune tolerance at ectopic sites is expected to have direct consequences for inflammatory regulation within the peritoneal environment. Consistent with this concept, the causal involvement of a local reduction in aTregs in shaping a pro‐inflammatory milieu that exacerbates endometriosis has been functionally demonstrated using a Treg‐depletion mouse model [ 24 ]. This experimental system provided direct evidence that impaired Treg‐mediated immunoregulation is not merely associated with disease progression but is mechanistically responsible for driving pathological inflammation. These findings strongly support the notion that therapeutic strategies aimed at restoring Treg function may effectively suppress disease progression [ 24 , 25 ].

This work was supported in part by Grants‐in‐aid for Scientific Research (Grant nos. 18K16808, 20K18196, and 23K08804 from Yukiko Tanaka and 20K22983 from Eiko Maeda) from the Japan Society for the Promotion of Science and under the affiliation of the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

Several mechanisms may contribute to the reduced proportion of aTregs observed in endometriotic lesions. Chronic inflammatory environments are known to influence the stability and functional differentiation of Treg populations [ 26 , 54 ]. In particular, pro‐inflammatory cytokines such as IL‐6 can destabilize Foxp3 expression and promote the conversion of Tregs into Th17‐like cells, thereby reducing the pool of functionally suppressive Tregs [ 54 , 55 ]. Because elevated IL‐6 levels are a consistent feature of the PF and lesions of patients with endometriosis, this cytokine‐driven Treg plasticity may contribute to the local deficiency of aTregs [ 40 ]. Microbial and innate immune signals may further exacerbate this process. Bacterial components such as lipopolysaccharide (LPS), which may enter the peritoneal cavity through retrograde menstruation, activate Toll‐like receptor 4 (TLR4) signaling and promote NF‐κB–mediated inflammatory responses [ 16 , 56 ]. Persistent activation of these pathways can create a microenvironment that impairs Treg stability and favors effector T‐cell differentiation [ 26 ]. In addition, microbial factors such as Fusobacterium , which have recently been detected in endometriotic tissues [ 57 ], may further amplify inflammatory signaling and contribute to the disruption of local immune tolerance. Interestingly, alterations in aTreg populations have also been reported in other immune‐mediated conditions, including autoimmune diseases, chronic inflammatory disorders, and cancer [ 26 , 58 , 59 ]. In these contexts, changes in the frequency or functional integrity of specific Treg subsets have been linked to disease activity and clinical outcomes. These parallels suggest that qualitative alterations in Treg subsets, rather than simple changes in total Treg numbers, may represent an important immunological mechanism underlying chronic inflammatory diseases such as endometriosis.

Advancing Treg‐based immunotherapies toward clinical application requires careful consideration of several technical and safety challenges. A primary issue is the maintenance of Treg cell stability within inflammatory environments. Under such conditions, Treg cells may lose Foxp3 expression and acquire effector phenotypes, including Th17‐like characteristics, resulting in diminished suppressive capacity and potential exacerbation of inflammatory pathology [ 17 , 58 , 67 , 68 ]. Therefore, ensuring stable Foxp3 expression following adoptive transfer is essential for therapeutic efficacy. Several pharmacological approaches have demonstrated the ability to enhance Treg stability, including low‐dose IL‐2, which selectively expands and stabilizes Tregs [ 69 ], histone and protein deacetylase inhibitors that reinforce Foxp3‐associated transcriptional programs [ 70 ], and rapamycin, which supports Treg survival by inhibiting mTOR‐dependent effector differentiation [ 71 ]. IL‐7 also contributes to the long‐term maintenance of resting Treg populations, and elevated IL‐7 levels reported in plasma and ectopic lesions of endometriosis patients suggest that dysregulated IL‐7 signaling may influence the imbalance between resting and aTreg subsets within the disease microenvironment (Figure  3 ) [ 72 , 73 ]. Suppression of lesion progression by adoptive Treg transfer and future therapies. Schematic representation of Treg dysfunction in endometriosis and emerging Treg‐based therapeutic approaches. Loss of Tregs promotes inflammatory cytokine production and lesion progression, whereas adoptive transfer of CD4 + CD25 + Tregs restores immune balance and suppresses lesion growth. Future strategies include enhanced Treg stability through pharmacological modulation, antigen‐specific Tregs for targeted immunosuppression, and activation‐inducible Tregs, in which suppressive activity is restricted to inflammatory microenvironments via inducible regulatory modules. Another major challenge in Treg‐based therapy is achieving sufficient therapeutic specificity while minimizing systemic immunosuppression. Polyclonal Treg infusion carries a risk of broad immune suppression, raising concerns regarding susceptibility to infection and impaired tumor surveillance [ 74 , 75 ]. To address these limitations, antigen‐specific Treg strategies have emerged as a promising alternative. By identifying antigens associated with endometriotic lesions and selectively expanding or engineering Tregs that recognize these targets, it may be possible to achieve localized and potent immunomodulation with reduced off‐target effects. Antigen‐specific Tregs have demonstrated superior suppressive potency compared with polyclonal Tregs in multiple experimental models [ 76 , 77 ]. Furthermore, advances in synthetic biology have enabled the development of engineered Treg systems equipped with activation‐inducible modules, such as cytokine‐responsive biosensors, that restrict suppressive function to inflammatory microenvironments characterized by signals such as TNF‐α (Figure  3 ) [ 76 , 77 ]. Together, these approaches provide a framework for improving the precision, safety, and efficacy of Treg‐based immunotherapy. Collectively, these innovations suggest that refinement of Treg stability, specificity, and environmental responsiveness—rather than the identification of novel Treg subsets—will be critical for translating Treg‐based therapies into effective clinical interventions for endometriosis.

Analyses of functional Treg subsets have clarified that the pathophysiology of endometriosis is characterized by a failure to establish adequate local immunosuppression, a defect that is particularly relevant to infertility. To investigate this, we previously analyzed Treg subsets in PB, PF, normal endometrium (NE), ectopic endometrium (EE), and OE from 27 patients with endometriosis and 28 non‐endometriotic women of reproductive age (20–45 years), using flow cytometry with combined Foxp3 and CD45RA staining. This analysis demonstrated a marked reduction in aTregs, the subset with bona fide suppressive function, in EE and OE compared with NE from non‐endometriotic controls. The proportion of aTregs averaged 8.9% in NE, whereas EE and OE exhibited markedly reduced levels of 1.5% and 2.6%, respectively. In contrast, the overall proportion of Foxp3 + cells did not differ substantially among NE, EE, and OE [ 24 ]. These findings indicate that immune dysregulation in endometriosis arises not from a reduction in total Treg numbers but from a qualitative deficiency in functionally suppressive Treg subsets. In NE from non‐endometriotic women, both total Tregs and aTregs were present at significantly higher proportions than in PB and PF ( p  < 0.01), suggesting that the NE actively establishes a localized immunosuppressive environment [ 24 ]. In contrast, in EE and OE from patients with endometriosis, aTreg proportions were comparable to or not significantly different from those observed in PB or PF [ 24 ], indicating a failure of ectopic tissues to acquire site‐specific immune tolerance. These observations help reconcile inconsistencies reported in earlier studies that relied primarily on quantification of Foxp3 + cells [ 22 ]. Although reductions in Foxp3 + cells in PB have been consistently demonstrated, only a limited number of studies have addressed qualitative or functional alterations within Treg populations [ 26 ]. Indeed, early investigations presented seemingly contradictory findings, including persistence of Foxp3 + cells in eutopic endometrium during the secretory phase with endometriosis [ 19 ] and slightly increased Foxp3 + cell numbers in moderate to severe disease compared with minimal to mild (Stages I/II) disease [ 27 ]. However, such numerical changes alone were insufficient to determine whether immune suppression was functionally intact. A clearer understanding of these discrepancies was provided by our studies applying phenotypic classification of Treg subsets based on FoxP3 and CD45RA expression. Tanaka et al. demonstrated that, despite the apparent abundance of FoxP3 + cells within endometriotic lesions, the proportion of aTregs, the subset with genuine suppressive capacity, is markedly reduced at the local sites [ 24 ]. This finding explains the paradox observed in earlier reports: numerical increases in FoxP3 + cells do not necessarily translate into effective immunoregulation. Rather, it is the qualitative deficit in functionally competent Treg populations, rather than changes in total cell numbers, that underlie the impaired establishment of local immune tolerance in endometriosis. The local deficiency of aTregs is likely to play a central role in the pathophysiology of infertility, a condition frequently associated with endometriosis [ 28 , 29 ]. Successful pregnancy requires the establishment of immune tolerance toward the semi‐allogeneic fetus, and a failure to generate an appropriately immunosuppressive endometrial microenvironment can directly hinder embryo implantation [ 30 , 31 ]. Consistent with this concept, analysis of menstrual cycle‐dependent Treg dynamics revealed that the proportion of aTreg cells in EE from patients with endometriosis (2.2% ± 1.6%) was significantly lower than that in NE from non‐endometriotic controls ( p  < 0.01 and p  < 0.05) [ 32 ]. Whereas the NE actively builds a localized immunosuppressive state in preparation for implantation, ectopic lesions fail to achieve this essential transition. Notably, aTreg proportion in EE did not exceed that observed in endometriosis‐associated PF (E‐PF) at any point during the menstrual cycle, highlighting a persistent inability of ectopic tissues to develop the heightened immune tolerance characteristic of a receptive endometrium. Additional observations showed that, during the secretory phase, aTreg proportion in OE (3.8% ± 3.2%) was modestly but significantly higher than that in PF (2.3% ± 2.7%; p  = 0.038) [ 32 ]. These cycle‐dependent differences suggest that the balance between rTregs and aTregs is disrupted in a phase‐specific manner in endometriosis. This interpretation is supported by reports from other groups describing attenuated cyclical immune variation and reduced Treg proportions in more advanced stages of disease [ 33 , 34 ]. Collectively, these findings indicate that abnormalities in the cyclical regulation of Treg subsets impair the establishment of reproductive immune tolerance, thereby contributing to implantation failure and endometriosis‐associated infertility. Insight into these local immune abnormalities is further strengthened by comparison with systemic Treg dynamics. Analysis of peripheral Treg subsets in healthy women revealed coordinated, cycle‐dependent fluctuations, with increases in total Tregs and aTregs during the periovulatory phase and expansion of rTregs during the luteal phase and early pregnancy [ 35 ]. These observations suggest a functional division of labor among Treg subsets, whereby rTreg act as a preparatory population during the implantation window, while aTreg functions as effector cells mediating immune tolerance upon antigen encounter after conception [ 35 , 36 ]. Within this framework, the reduced proportion of aTregs observed locally in EE can be interpreted as a failure to enter the effector phase of reproductive immune tolerance. Such failure provides a mechanistic explanation for the strong association between endometriosis and infertility and establishes a foundation for understanding how impaired immune regulation predisposes endometriotic lesions to persistent inflammation [ 24 , 32 , 37 ].

Dysfunction of Treg cells not only compromises immune surveillance but also disrupts the broader immunological balance, particularly the axis involving pro‐inflammatory Th17 cells. Accumulating evidence implicates Th17 cells in the pathophysiology of endometriosis, with multiple studies reporting increased frequencies of Th17 cells and elevated concentrations of IL‐17A in the PF and lesion microenvironment of affected women [ 15 , 60 , 61 ]. This Th17‐skewed immune milieu is thought to promote lesion establishment and persistence. Because Treg and Th17 cells arise from reciprocally regulated differentiation pathways, a decline in functional Treg activity creates conditions that favor Th17 expansion and activation [ 54 , 55 ]. Activated Th17 cells secrete IL‐17A and related cytokines that enhance the inflammatory phenotype of endometriotic stromal cells by stimulating IL‐8 production, upregulating COX‐2 expression, and promoting cellular proliferation [ 61 ]. Through these mechanisms, excessive Th17 activity amplifies local inflammation and contributes to disease progression [ 62 ]. Loss of Treg‐mediated regulation further exacerbates this imbalance by promoting inflammatory cytokine production. Treg depletion is associated with elevated levels of cytokines such as IL‐6, which has been shown to destabilize Foxp3 expression and facilitate the conversion of Treg cells into Th17‐like cells under inflammatory conditions [ 54 , 63 ]. This shift contributes to a breakdown of the Th17/Treg balance, thereby accelerating the progression of endometriosis [ 64 ]. Consistent with this model, increased proportions of Th17 cells in the PF of women with endometriosis correlate with disease severity [ 60 ]. Chronic inflammation in endometriosis is further amplified by dysregulation of the innate immune response and by microbial signals. The bacterial contamination hypothesis proposes that bacterial endotoxin, particularly LPS, may enter the peritoneal cavity through retrograde menstruation and contribute to sustained inflammatory activation [ 56 , 65 ]. LPS activates TLR4 on antigen‐presenting cells such as macrophages, triggering MyD88‐dependent signaling and persistent activation of NF‐κB. Because NF‐κB governs the expression of pro‐inflammatory cytokines and regulates pathways controlling cell survival and proliferation, constitutive activation of this signaling axis is considered a major driver of lesion maintenance and expansion (Figure  2 ) [ 15 , 16 ]. Integrated immune network amplified by activated Treg deficiency in endometriosis. Loss of activated regulatory T cells (aTregs) disrupts immune homeostasis and initiates a self‐amplifying inflammatory network within the peritoneal and lesion microenvironment. Reduced aTreg‐mediated suppression promotes Th17 expansion and IL‐17A production, while elevated IL‐6 further destabilizes Treg identity and enhances Th17/Treg plasticity. Concurrently, macrophage activation increases CCL2 and VEGF production, driving inflammation and angiogenesis. Innate immune pathways, including TLR4/NF‐kB signaling triggered by microbial components such as lipopolysaccharide (LPS) and Fusobacterium , further amplify inflammatory responses. Together, these interconnected adaptive and innate immune pathways sustain chronic inflammation and lesion progression in endometriosis. This figure illustrates a simplified conceptual model highlighting key immune interactions amplified by activated Treg deficiency, rather than an exhaustive representation of all molecular pathways involved in endometriosis. Created in Biorender.com . The involvement of microbial factors is further supported by recent evidence demonstrating Fusobacterium infection in approximately 64% of ectopic and eutopic endometrial tissues from patients with endometriosis [ 57 ]. Fusobacterium binds host cell adhesion molecules via its adhesin FadA and promotes fibroblast‐to‐myofibroblast transition (FMT) in endometrial stromal fibroblasts [ 57 ]. Because FMT contributes to fibrosis, tissue remodeling, and chronic inflammation, these findings suggest a direct role for microbial factors in disease pathogenesis. Moreover, the ability of Fusobacterium to activate TLR4/NF‐κB signaling supports a model in which impaired immune surveillance due to Treg dysfunction synergizes with bacteria‐driven inflammatory amplification to accelerate lesion progression (Figure  2 ). Innate lymphoid cells (ILCs), another key component of innate immunity, have also been implicated in reproductive immunology and in the immunopathology of endometriosis. In OE, all ILC subsets, ILC1, ILC2, and ILC3, are increased, reflecting the highly inflammatory microenvironment characteristic of this lesion type [ 66 ]. In addition, the expression of IL‐1β and IL‐23, two key cytokines driving ILC3 differentiation and activation, is significantly elevated in OE compared with NE and EE ( p  < 0.01) (Figure  2 ) [ 66 ]. These findings indicate that, similar to Treg cells, ILC dynamics are strongly shaped by the local microenvironment and vary according to lesion subtype. Collectively, these observations underscore that endometriosis is not solely a hormone‐dependent disorder but a multifactorial inflammatory disease driven by complex interactions among adaptive immunity, innate immunity, microbial signals, and fibrotic processes. In particular, the convergence of TLR4/NF‐κB–mediated inflammation and microbial factors such as Fusobacterium highlights potential therapeutic avenues, including antimicrobial strategies, TLR4 inhibition, and modulation of ILC activity.

A series of studies conducted by our group has established that endometriosis is driven, at least in part, by a localized deficiency in bona fide immunosuppressive Treg (aTregs), revealing a clear immunological defect underlying disease progression. This body of work provides a strong conceptual basis for the development of immunotherapies aimed at replenishing or enhancing Treg cell function, positioning such approaches as promising nonhormonal therapeutic alternatives or complements to existing hormone‐based treatments. Importantly, Treg‐centered immunotherapy holds particular potential for patients who wish to preserve fertility, as it offers a strategy that avoids the reproductive suppression associated with conventional hormonal therapies. Looking forward, future research should aim to develop personalized immune cell therapies tailored to the heterogeneous immunological profiles observed across different subtypes of endometriosis. Such precision immunotherapy has the potential not only to improve therapeutic efficacy but also to reshape the clinical management of endometriosis by addressing the disease at its immunological roots.

The identification of Treg dysfunction in endometriosis by our group provided the conceptual basis for proposing that restoration of Treg‐mediated immune regulation could attenuate disease progression. Building upon this mechanistic insight, our group further evaluated the therapeutic potential of adoptive Treg transfer using a Treg‐cell–depleted Foxp3 DTR mouse model. In this system, intravenous administration of wild‐type CD4 + CD25 + Treg cells markedly suppressed the development of endometriosis‐like lesions. The number of lesions ( p  < 0.0001), their total weight ( p  = 0.0021), and total volume ( p  = 0.0010) were all significantly reduced, demonstrating a clear inhibitory effect on disease progression. These outcomes provide compelling evidence that the loss or reduction of Treg cells contributes directly to lesion expansion and that replenishment of the Treg compartment can restore immune homeostasis in the ectopic tissue environment. At the mechanistic level, adoptive Treg transfer exerted a broad anti‐inflammatory influence within lesions. Expression of cytokines associated with Th1 (Ifng, p  = 0.0101), Th2 (Il4, p  = 0.0051), and Th17 (Il17, p  = 0.0177) Th‐cell lineages was significantly diminished, indicating that Treg supplementation restrains multiple effector pathways simultaneously. In addition, both mRNA expression ( p  = 0.0317) and circulating levels ( p  = 0.0002) of the pro‐inflammatory cytokine IL‐6 were markedly reduced, further underscoring the capacity of adoptively transferred Treg cells to mitigate local and systemic inflammation. Taken together, these findings reinforce the concept that Treg insufficiency is a driving factor in the immunopathology of endometriosis and that therapeutic strategies aimed at restoring Treg function may offer significant clinical benefit. The demonstration that exogenous Treg cells can counteract inflammatory cytokine networks and suppress lesion progression highlights adoptive Treg transfer as a promising immunomodulatory approach, particularly for disease states characterized by impaired Treg activity.

Endometriosis affects 5%–10% of women of reproductive age and has a broad range of manifestations, including chronic pelvic pain, dysmenorrhea, dyspareunia, and infertility, all of which significantly impair patients' quality of life (QOL) [ 1 , 2 ]. Its prevalence is particularly high among women with infertility, reaching 30%–50% [ 3 ]. The most widely accepted explanation for its pathogenesis is the theory of retrograde menstruation, which proposes that endometrial cells present in menstrual blood flow backward through the fallopian tubes into the peritoneal cavity, where they adhere to the peritoneum and proliferate, forming lesions [ 4 , 5 ]. However, a fundamental paradox exists: although almost all women experience retrograde menstruation, only a small subset develops endometriosis. This observation strongly suggests that dysfunction of the host immune system, specifically a failure of immune surveillance that would normally eliminate ectopic endometrial tissue, is essential for the establishment and progression of the disease [ 6 ]. The microenvironment of endometriotic lesions is characterized by chronic inflammation exacerbated by endocrine factors such as excessive estrogen production and progesterone resistance [ 7 , 8 , 9 , 10 , 11 ]. Estrogen promotes cellular proliferation, while progesterone resistance reduces the endometrium's ability to suppress inflammatory responses [ 12 ]. Additionally, angiogenesis mediated by vascular endothelial growth factor (VEGF) contributes to the development and persistence of lesions [ 13 ]. Abnormal activities of various immune cells, including macrophages, natural killer (NK) cells, dendritic cells (DCs), and helper T (Th) cells, have also been reported [ 14 , 15 ]. For example, in the peritoneal fluid (PF) of patients with endometriosis, increases in macrophages and Th cells have been observed, whereas NK CD16 + cells are decreased and cytotoxic NK CD57 + cells are increased [ 14 ], suggesting alterations in the function of immune defense mechanisms [ 16 ]. Among the various immune abnormalities, regulatory T cells (Tregs), which play a central role in immunological self‐tolerance and the maintenance of immune homeostasis, represent a particularly important population for understanding the dysregulated immune responses associated with endometriosis. Tregs are commonly characterized by the expression of transcription factor Foxp3 (forkhead box P3), which functions as a master regulator of Treg differentiation and suppressive activity [ 17 ]. Tregs suppress excessive immune activation and prevent autoreactive T cell responses, thereby maintaining immune balance. Nevertheless, previous studies examining Treg involvement in endometriosis produced inconsistent and sometimes contradictory findings, with some reports describing increased Foxp3 + cells in ectopic endometrial tissues [ 18 , 19 ], while others observed no significant differences [ 20 ]. For instance, the proportion of CD4 + CD25 high Foxp3 + Tregs has been reported to be higher in the PF of patients with OE than in their peripheral blood (PB) [ 21 ]. Conversely, other studies have demonstrated a pattern in which Foxp3 + cells are increased locally within lesions but decreased in PB [ 22 ]. To reconcile these discrepancies and enable more accurate assessment of Treg function, increasing attention has been directed toward the functional heterogeneity of the Treg population. Miyara et al. demonstrated that human Treg cells can be subdivided into three distinct subsets based on Foxp3 and CD45RA: resting Tregs (rTregs; Foxp3 low CD45RA + ), activated Tregs (aTregs; Foxp3 high CD45RA − ), which possess potent immunosuppressive properties, and non‐suppressive Tregs (non‐Tregs; Foxp3 low CD45RA − ) (Figure  1 ) [ 23 ]. Building on this functional classification, our laboratory has focused on aTreg and proposed a revised pathological concept in which immune dysfunction in endometriosis arises not from alterations in total Tregs numbers, but from a qualitative impairment of the suppressive Treg compartment characterized by a local deficiency of aTregs [ 24 ]. This insight suggests that restoration of Treg‐mediated immune regulation may provide a foundation for developing effective nonhormonal therapeutic strategies for endometriosis [ 25 ]. Functional deficiency of activated regulatory T cells (aTregs) as a central mechanism in endometriosis. In the normal endometrium, activated Tregs (aTregs) are locally enriched and maintain reproductive immune tolerance by suppressing excessive inflammatory responses, supporting immune homeostasis, and promoting endometrial receptivity. In endometriosis, despite preserved or increased numbers of total Foxp3 + regulatory T cells, the proportion of functionally suppressive aTregs is selectively reduced within ectopic endometrial lesions. This qualitative defect in Treg‐mediated immune regulation results in enhanced activation of inflammatory pathways, including increased IL‐6, CCL2, and VEGF production, macrophage activation, and angiogenesis. Collectively, these changes create a pro‐inflammatory microenvironment that promotes lesion persistence, progression, and impaired reproductive immune tolerance, thereby linking endometriosis to infertility.

The authors declare no conflicts of interest.