Yimin Shi
Xiufeng Tang- 1Traditional Chinese Medicine Department, Shandong Provincial Maternal and Child Health Care Hospital, Jinan, China
- 2Department of Pharmacy and Shandong Provincial Key Traditional Chinese Medical Discipline of Clinical Chinese Pharmacy, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China
Recurrent pregnancy loss (RPL), particularly its unexplained form (URPL), represents a formidable challenge in reproductive medicine. Although traditionally attributed to local immune imbalances at the maternal-fetal interface, this perspective may not fully account for the condition’s upstream etiological drivers and recurrent nature. This review transcends this limitation by proposing and systematically substantiating an integrative ‘gut-systemic-decidual’ model of immunometabolic dysregulation. We posit that a key pathological cascade in many URPL cases may originate with distal gut dysbiosis, which, through imbalanced metabolite profiles and the leakage of inflammatory molecules such as lipopolysaccharide (LPS), triggers systemic ‘metabolic endotoxemia’ and fundamentally reprograms the metabolic state of circulating immune cells. This systemic ‘first hit’ is compounded when these ‘pre-sensitized’ cells migrate to an equally metabolically stressed and ‘hostile’ decidual microenvironment—a ‘second hit’ characterized by hypoxia and high lactate. This culminates in the functional collapse of the core sentinels of maternal-fetal tolerance, namely regulatory T (Treg) and decidual natural killer (dNK) cells, due to profound metabolic misprogramming. Ultimately, this integrated model elevates the etiological understanding of URPL from a ‘local conflict’ to that of a ‘systemic disease,’ paving the way for the development of dynamic warning systems that integrate multi-omics data and for the design of multi-level precision intervention strategies targeting patient stratification and preventive approaches for the gut, systemic metabolism, and the local microenvironment.
1 Introduction
Recurrent pregnancy loss (RPL) is a major clinical challenge in reproductive medicine, inflicting not only physiological harm but also profound psychological trauma on patients. Despite decades of research, up to 50% of cases remain idiopathic and are classified as unexplained RPL (URPL) (1). It is crucial to recognize that URPL is likely not a single disease entity but rather a common clinical endpoint for a heterogeneous group of underlying pathologies. The immuno-metabolic dysregulation model proposed here may represent a significant, yet not exclusive, pathway contributing to a substantial subset of these cases. Among the many potential etiologies, immune dysregulation is widely considered a key factor strongly associated with URPL. The rationale for this hypothesis lies in the exquisite immunological paradox of a successful pregnancy: the maternal immune system must establish and maintain tolerance to the semi-allogeneic fetus, which carries paternal antigens, while simultaneously preserving its capacity to defend against pathogens (2, 3).
Traditionally, immunological research into RPL has focused on local imbalances in the proportions or functions of immune cells at the maternal-fetal interface, such as the Treg/Th17 cell axis (4). However, these studies often treat the uterus as an isolated local phenomenon, an approach that largely fails to explain the upstream drivers of this local immune dysfunction. Why does the program for establishing immune tolerance repeatedly fail in certain women? What are the root pathophysiological mechanisms?
In recent years, groundbreaking advances in the field of immunometabolism have offered a novel perspective. This field has revealed that the functional state of an immune cell is intricately linked to its intrinsic metabolic programming (5, 6). For instance, tolerogenic regulatory T (Treg) cells rely on oxidative phosphorylation (OXPHOS) to sustain their suppressive function, whereas pro-inflammatory T helper 17 (Th17) cells depend on aerobic glycolysis to support their rapid proliferation and effector functions (7, 8). This discovery implies that the immune cell imbalances observed in RPL may, at their core, represent a profound dysregulation of metabolic programming.
Building on this premise, this review aims to move beyond the confines of traditional local-centric immune studies by proposing an integrative ‘gut-systemic-decidual’ model of immunometabolic dysregulation. We hypothesize that URPL is not a problem confined to the endometrium but rather a systemic disease that originates with a distal homeostatic imbalance (e.g., in the gut), progresses through systemic immunometabolic reprogramming, and ultimately manifests at the maternal-fetal interface. This article will systematically elucidate how gut dysbiosis, as the initial ‘distal disturbance’, triggers a cascade that reshapes the maternal systemic immunometabolic landscape; how these functionally pre-conditioned immune cells, upon migrating into the decidua, encounter a ‘hostile microenvironment’ that is itself metabolically compromised; and how this dual systemic and local pressure ultimately drives the functional failure of key immune cells (notably Treg and dNK cells), thereby dismantling maternal-fetal immune tolerance. By integrating multi-dimensional, recent evidence, we aim to provide a more complete and in-depth explanation for the pathophysiology of URPL and to illuminate new directions for the development of novel diagnostic markers and precision preventive and therapeutic strategies.
2 Gut dysbiosis: the initiating factor of systemic immuno-metabolic dysregulation
A growing body of evidence suggests that the immune dysregulation in RPL is not a localized uterine event but that its pathophysiological roots can be traced to distant organ systems. a concept well-established in the context of the gut-skin and gut-brain axes (9, 10)**. Among these, the gut, as the body’s largest immune organ and microbial reservoir, is emerging as a focal point of investigation. A healthy gut microbiota and its metabolites, such as short-chain fatty acids (SCFAs), are critical regulators for maintaining systemic immune homeostasis (11). SCFAs not only provide energy to colonocytes and maintain intestinal barrier integrity but also directly regulate the differentiation and function of distant immune cells, such as T cells. Classic animal model studies have long established that SCFAs like butyrate, produced by commensal bacteria, are key factors in inducing the differentiation of colonic Treg cells (12, 13), thereby establishing a systemic environment of immune tolerance. A recent landmark study provided direct and compelling evidence for this “gut-immune” axis: in a clinical cohort of patients with unexplained RPL (URPL), gut dysbiosis and the associated reduction in microbial metabolites (such as SCFAs and secondary bile acids) were directly and significantly correlated with a reduction in protective peripheral Treg cells and an increase in pro-inflammatory Th1/Th17 cells (14). Deeper mechanistic investigations further revealed that SCFAs (particularly propionate and butyrate) can directly act on G protein-coupled receptors (GPR43) on the surface of T cells in distant immune organs (such as the spleen), potently promoting their differentiation into immunosuppressive Treg cells and consequently ameliorating a mouse model of fetal loss (15). This provides a clear mechanistic link for the concept that a healthy gut ecosystem actively cultivates a systemic immune tolerance environment favorable for pregnancy through its metabolic output.
However, when the gut microbiota homeostasis is disrupted (gut dysbiosis), this delicate balance collapses. The overgrowth of pathogenic bacteria and the decline of beneficial bacteria lead to a decrease in the production of beneficial metabolites like SCFAs, coupled with an increase in inflammatory molecules such as lipopolysaccharide (LPS) (4). This concept is supported by early evidence: two decades ago, prospective studies began to explore the association between host genetic susceptibility to LPS (such as CD14 gene polymorphisms) and the risk of RPL, laying the early groundwork for an “endotoxin hypothesis” (16). More importantly, dysbiosis is often accompanied by impaired intestinal barrier function, a condition clinically referred to as “leaky gut” (17, 18). This allows microbial products like LPS, which are normally confined to the intestinal lumen, to cross the barrier and enter the bloodstream, triggering a sustained, low-grade systemic inflammatory response (1, 18, 19) that defines “metabolic endotoxemia”. Notably, this pathological cascade may be more complex; for instance, some evidence suggests the existence of a “gut-lung-uterus” axis, where inflammatory signals originating from the gut may first affect the lungs and then extend to the uterus via the circulatory system, forming a multi-organ cascade (20). Furthermore, this gut-decidual axis may not be a one-way street. The profound hormonal shifts during pregnancy, including rising levels of progesterone and estrogen, can themselves modulate the composition and function of the gut microbiota. This suggests a potential bidirectional feedback loop, where pregnancy-induced changes in the gut could either reinforce a healthy state or, in susceptible individuals, exacerbate pre-existing dysbiosis, further amplifying the systemic inflammatory pressure on the maternal-fetal interface.
Circulating LPS, as a potent pathogen-associated molecular pattern (PAMP), exerts widespread “pre-sensitizing” and “metabolic reprogramming” effects on the maternal immune system. The detrimental impact of this systemic inflammatory state is multi-faceted. First, in addition to inducing the production of large quantities of pro-inflammatory cytokines (such as TNF-α and IL-6) from monocytes/macrophages via the classic Toll-like receptor 4 (TLR4) signaling pathway (17), LPS can also activate the complement system. This leads to the generation of the potent inflammatory mediator C5a, which directly mediates placental injury and pregnancy loss through its receptor, C5aR1 (21). This chronic, low-grade inflammatory environment constitutes a “pre-sensitization” process for the maternal immune system, systematically altering the differentiation potential of T cells by inhibiting Treg cell differentiation while promoting the expansion of Th17 cells, thereby disrupting systemic immune balance (22). Second, at a deeper level, LPS fundamentally re-wires the metabolic programs of immune cells. Persistent LPS stimulation induces a shift toward glycolytic metabolism in myeloid cells like monocytes, a pro-inflammatory “war-ready” phenotype that renders them more easily activated to release inflammatory mediators (23). Within the cell, continuous LPS stimulation inhibits the activity of core metabolic sensors like AMPK and SIRT1. This not only drives cells toward a pro-inflammatory phenotype dependent on glycolysis but can also directly trigger a form of metabolism-dependent cell death termed “ferroptosis,” causing direct damage to tissues such as the endometrium (24). Furthermore, LPS can amplify inflammation at a post-transcriptional level through a newly discovered and elegant mechanism. It promotes the sequestration of RC3H1, a protein responsible for degrading TNF-α mRNA, within intracellular stress granules by a protein named MNSFβ. This sequestration prevents the degradation of TNF-α mRNA, leading to its uncontrolled expression and creating a self-amplifying inflammatory loop (25) (Figure 1).
Figure 1. The ‘Gut-Systemic-Decidual’ immunometabolic axis model driving unexplained recurrent pregnancy loss (URPL). This model illustrates the proposed multi-stage pathophysiological cascade in URPL. (1) Gut Dysbiosis: An imbalanced gut microbiota leads to reduced production of beneficial short-chain fatty acids (SCFAs) and increased intestinal permeability (“leaky gut”). (2) Systemic Inflammation: This allows microbial products like lipopolysaccharide (LPS) to enter the circulation, triggering systemic low-grade inflammation and metabolically reprogramming circulating immune cells (e.g., T cells, NK cells) towards a pro-inflammatory state. (3) Decidual Hostility: These pre-sensitized immune cells migrate to the decidua, where a locally hostile metabolic environment (e.g., hypoxia, high lactate) further compromises their function, leading to the collapse of maternal-fetal tolerance and pregnancy failure.
3 The decidual microenvironment: from immune sanctuary to metabolic hostility
If gut dysbiosis is the “distant storm” that initiates the pathological process, then the decidual microenvironment is the local battlefield where this conflict ultimately culminates. Under physiological conditions, the early-pregnancy decidua is a unique immune-privileged microenvironment that actively suppresses immune attacks to protect the embryo through multiple mechanisms. Among these, decidual stromal cells (DSCs) play the role of central coordinators. They not only possess intrinsic immunosuppressive functions but also secrete various cytokines and chemokines to recruit and regulate the phenotype and function of immune cells (especially Treg cells and specialized dNK cells), collectively building a robust niche of immune tolerance (25) (Figure 2). This microenvironment is also metabolically unique: it is a relatively hypoxic yet nutrient-rich area, a distinctive metabolic feature crucial for maintaining immune tolerance (26, 27). Importantly, this privileged status is not a passive “isolation” but a dynamic equilibrium actively constructed and maintained by a sophisticated signaling network between maternal and fetal cells. For example, normal dNK cells secrete pigment epithelium-derived factor (PEDF) to protect DSCs from inflammatory and apoptotic damage, forming a positive protective feedback loop (28).
Figure 2. Schematic comparison of the decidual microenvironment in normal pregnancy and RPL. (A) Normal Pregnancy: A tolerogenic environment is actively maintained, characterized by an abundance of immunomodulatory cells such as decidual NK (dNK) cells, M2-like macrophages, and regulatory T (Treg) cells. Successful spiral artery remodeling ensures adequate oxygen and nutrient supply, supporting metabolic homeostasis. (B) Recurrent Pregnancy Loss (RPL): The microenvironment shifts towards hostility, with increased infiltration of cytotoxic CD8+ T cells and pro-inflammatory M1 macrophages. Defective vascular remodeling results in local hypoxia and accumulation of acidic metabolites like lactate, creating conditions unfavorable for embryonic development.
In patients with RPL, however, this “immune sanctuary” transforms into a metabolically hostile environment (29). This transformation is particularly evident in its spatial dimension. The latest spatial transcriptomics studies have clearly depicted that the implantation site (IZ) of a normal decidua is a protective zone enriched with dNK1 cells and M2-like macrophages. In RPL, however, this protective structure is dismantled; the protective cells are sharply reduced and replaced by an infiltration of cytotoxic CD8+ T cells. In essence, the spatial distribution pattern of cells undergoes a fundamental reversal (30). The root of this spatial collapse lies in the profound dysregulation of molecular and metabolic programs within the microenvironment. Multiple integrative transcriptomic and metabolomic studies have consistently revealed significant metabolic disturbances in the decidual tissue of RPL patients (14, 31, 32). Even at the upstream epigenetic level, RPL-associated villi and decidua exhibit a state of widespread DNA hypermethylation, affecting the gene expression of numerous key signaling pathways (33). This metabolic dysregulation is not merely an intrinsic cellular malfunction but is also driven by aberrant signaling molecules in the microenvironment. For instance, DSCs from RPL patients excessively secrete the extracellular matrix protein Decorin, which acts as a detrimental signaling molecule that directly targets decidual macrophages, inducing their mitochondrial dysfunction (34). Concurrently, systemic lipid imbalance further exacerbates the local pathology. It leads to a sharp increase in downstream inflammatory metabolites of arachidonic acid (AA) in the decidua—oxylipins such as PGE2 and PGF2α—and these inflammatory lipid molecules further shape a pro-inflammatory microenvironment (35).
Amid these chaotic metabolic changes, a hallmark feature is an aberrant upregulation of the glycolytic pathway and an impaired tricarboxylic acid (TCA) cycle, leading to the excessive accumulation of acidic metabolites like lactate (31, 32). It is noteworthy, however, that the dysregulation of glucose metabolism is not a simple “upregulation” or “downregulation” but rather a failure in the control of key metabolic nodes. A pioneering study discovered that the glycolytic intermediate fructose-1,6-bisphosphate (FBP) is a critical signal for inducing tolerogenic interactions between DSCs and macrophages, and its deficiency in RPL may be a significant cause of immune dysregulation (36). Furthermore, autophagy, a core mechanism for maintaining cellular metabolic homeostasis, is severely defective, preventing the timely clearance of damaged organelles and proteins and thus exacerbating cellular stress and dysfunction (37). This dysregulation can even trigger “ferroptosis,” a form of metabolism-dependent cell death, inflicting direct physical damage on decidual and trophoblast cells (23).
This metabolically disordered microenvironment not only directly harms maternal immune cells but also poses a grave threat to the survival and function of trophoblast cells, which are semi-allogeneic. At the same time, the disturbed microenvironment disrupts the intricate metabolic programs of the trophoblasts themselves. For example, the expression of HSD3B1, a key enzyme for progesterone synthesis in invasive extravillous trophoblasts (EVTs), is downregulated, leading to impaired local synthesis of progesterone, an endogenous immunosuppressant (38). The secretion of the critical growth factor GDF15 by trophoblasts may also be reduced, further weakening their invasive capacity (39). Even at a more refined molecular level, trophoblast function is severely disrupted by epigenetic and post-transcriptional dysregulation. For example, the abnormal upregulation of long non-coding RNAs (such as Lnc-HZ05) disrupts cytoskeletal rearrangement, thereby inhibiting the formation of migrasomes, a key structure for trophoblast migration (40).
More importantly, this hostile battlefield is not silent but is filled with erroneous signals and disrupted interactions. This “acidified” and “dysregulated” metabolic microenvironment, shaped by a multitude of factors (41), ultimately exerts a direct and decisive influence on the function of immune cells.
For example, high concentrations of lactate and a hypoxic environment can stabilize the expression of HIF-1α, which plays a dual role in different cells: on one hand, it inhibits the proliferation and function of Treg cells (42); on the other, it promotes the survival and differentiation of pro-inflammatory Th17 cells, thus directly driving the Treg/Th17 imbalance from a metabolic level. Meanwhile, other dysregulated metabolite profiles (such as abnormal tryptophan metabolism) can directly impair the cytotoxic regulation and vascular remodeling capabilities of dNK cells via pathways like the aryl hydrocarbon receptor (AHR) pathway (5). Single-cell sequencing has revealed with unprecedented precision that the decidua in RPL does not suffer from a uniform dysfunction but is instead dominated by pathogenic cell subpopulations. For instance, the massive emergence of “inflammatory” iDSCs and “glycolytic” glyDSCs actively secrete inflammatory factors, remodeling the entire communication network of the microenvironment (43). This breakdown in communication is also reflected between mother and fetus. Normally, fetal villi deliver an inhibitory signal, miR-29a-3p, to maternal dNK cells via exosomes to suppress their aggression. In RPL, however, this critical tolerance signal is significantly weakened, leading to a communication breakdown and the eruption of conflict (44). Therefore, when immune cells that have already been “pre-sensitized” in the systemic circulation arrive at a decidua filled with erroneous cell subpopulations, incorrect signaling molecules, and aberrant metabolic products, not only do they fail to be reprogrammed into a tolerogenic phenotype, but their inherent inflammatory propensity is further amplified and solidified, ultimately leading to an immune attack on the embryo.
4 Functional collapse of key immune cells: the fatal error in metabolic programming
If the deterioration of the decidual microenvironment sets the local stage for the conflict, then the functional collapse of key immune cells within this environment is the direct cause of pregnancy failure. Here, we will focus on two core cell types essential for maintaining maternal-fetal tolerance—regulatory T cells (Tregs) and decidual natural killer (dNK) cells—to dissect how profound errors in their metabolic programming lead to their functional collapse in RPL.
4.1 Insufficiency of regulatory T cells
Treg cells are the central regulators of maternal-fetal immune tolerance (45). In RPL, their functional deficiency is not merely a correlative phenomenon but has a clear causal relationship. Large-scale Mendelian randomization studies have confirmed that a reduction in specific Treg subsets (such as CD39+ resting Tregs) is a root cause of RPL (46). This insufficiency manifests as a dual decline in both quantity and quality, and may ultimately lead to a defect related to immune memory.
In terms of quantity, while the conventional view holds that the overall proportion of Treg cells is decreased in RPL (4), high-resolution mapping with single-cell technologies reveals that the loss is not of generic Tregs, but of specific subsets that exert critical effector functions. One study identified a CCR8+ Treg subset, specifically enriched in the decidua, as the core force maintaining local immunosuppression. This subset is significantly reduced in RPL, and its adoptive transfer in a model system can effectively rescue pregnancy, directly demonstrating its causal role. This loss can be traced to upstream signaling defects, such as reduced expression of the key recruiting chemokine CCL1 (47), or the inability to suppress miR-520a-5p due to the downregulation of circular RNA circDDX21, leading to the abrogation of expression of the master Treg transcription factor FOXP3 at its source (48). In terms of quality, even the Tregs that remain often fall into a state of functional exhaustion. Similar to the tumor microenvironment, Treg cells in the decidua of RPL patients exhibit high expression of a series of inhibitory receptors, including PD-1 and LAG3, signifying a loss of their sustained suppressive capacity (49) (Figure 3).
Figure 3. Function and metabolic programming of Treg cells in the normal and RPL decidual microenvironment. (A) Functional Treg in Normal Pregnancy: Treg cells rely on mitochondrial oxidative phosphorylation (OXPHOS) for sustained FOXP3 expression and immunosuppressive function, a state supported by active AMPK and suppressed mTOR signaling. (B) Dysfunctional Treg in RPL: In the hostile RPL microenvironment (high lactate, hypoxia, LPS), Treg metabolism is rewired towards glycolysis. This shift, driven by suppressed AMPK and activated mTOR signaling, leads to unstable FOXP3 expression, upregulation of exhaustion markers (e.g., PD-1), and impaired suppressive capacity, with a potential to convert into IL-17-producing pathogenic cells.
Underlying this functional collapse is a profound error in metabolic programming. The suppressive function of Tregs is highly dependent on the sustained energy provided by mitochondrial oxidative phosphorylation (OXPHOS) and the metabolic adaptability mediated by FOXO1 (45). However, in the “acidified” and “hypoxic” decidual microenvironment of RPL, the inherent “glycolytic fragility” of Treg cells makes it difficult for them to adapt (50, 51). This metabolic reprogramming is actively driven by dysregulated signaling pathways: the energy sensor AMPK pathway is suppressed, while the mTOR pathway, which promotes glycolysis, is abnormally activated. This imbalance not only inhibits Treg differentiation but also simultaneously promotes the generation of pro-inflammatory Th17 cells, and may even induce the conversion of Tregs into pathogenic, IL-17-secreting cells (52). At the organellar level, endoplasmic reticulum (ER) stress, a core hallmark of metabolic homeostasis imbalance, is also closely associated with Treg functional defects, forming a critical link between metabolic pressure and the collapse of Treg function (53).
However, the reduction in quantity and decline in quality still cannot fully explain the “recurrent” nature of RPL. Recent discoveries regarding Treg immune memory offer a possible explanation. Under normal circumstances, maternal tolerance to the fetus is a memory-like response, mediated by cells known as “ex-Tregs”—cells that were once Tregs but lost FOXP3 expression postpartum. In a subsequent pregnancy, these ex-Tregs can be rapidly reactivated to re-express FOXP3 and re-establish immune tolerance. This reveals that the plasticity of FOXP3 expression is key to successful pregnancy-related immune memory (54). Based on this, a hypothesis can be proposed: the root cause of the repeated failures in RPL may lie in a defect of this “plasticity” and the exhaustion of the “memory cell pool.” Fetal Tregs themselves exist in an unstable, “hypofunctional” state (51). It is conceivable that in the pathological microenvironment of RPL, this instability is amplified, causing Tregs to easily lose FOXP3 expression and undergo premature apoptosis or conversion. More critically, persistent metabolic stress may lead to the epigenetic state of these cells becoming fixed, rendering them unable to be effectively reactivated in subsequent pregnancies, thereby causing a loss of “tolerance memory.” This theory systematically integrates the phenomena of “reduced quantity,” “functional exhaustion,” and “metabolic instability,” providing a new theoretical framework to explain the recurrent nature of RPL.
4.2 Dysfunction of decidual natural killer cells
dNK cells are the most abundant immune cells in the early pregnancy decidua, and their functional prominence in both healthy pregnancy and infertility is well-documented (55). Their core task is to transition from a cytotoxic phenotype in the peripheral blood to a functional phenotype that promotes trophoblast invasion and spiral artery remodeling (6, 55). In RPL, however, this identity remodeling fails, and their function reverses from promoting angiogenesis and immune tolerance to exerting cytotoxic and pro-inflammatory effects.
First, this functional reversal is clearly visible in the macroscopic landscape of cell subsets. Multiple single-cell sequencing studies have consistently shown that in the decidua of RPL patients, the population of subsets with immunomodulatory and pro-angiogenic functions, represented by CD39+ dNK1 cells, is sharply reduced. In contrast, dNK3 and CD18+ dNK subsets, which have pro-inflammatory and cytotoxic potential, undergo abnormal expansion (56, 57). The root of this dramatic shift is a combination of intrinsic cellular factors and the external environment. The intrinsic factor is an imbalance in the transcriptional program of dNK cells, where transcription factors driving tolerance (such as RELB) are suppressed, while those driving inflammation, such as T-bet (TBX21), are activated (58). The external factor is induction by the pathological microenvironment; as previously mentioned, inflammatory stromal cells (iDSCs) directly induce the emergence of pathogenic dNK subsets through spatial interactions (43).
At the heart of this collapse is, again, a profound error in metabolic programming. Normal dNK cells are highly dependent on OXPHOS to maintain their pro-angiogenic and immunomodulatory functions, and their metabolic state is strictly regulated by mTORC1 signaling. In RPL, however, mTORC1 signaling activity is significantly suppressed, leading directly to the collapse of the dNK cell metabolic program (59). The core of this metabolic dysfunction points directly to the mitochondria—the latest evidence has directly linked dNK cell dysfunction to the downregulation of GRIM19, a key protein in the mitochondrial respiratory chain, which constitutes direct evidence of damage to their functional foundation (60).
Ultimately, this comprehensive collapse, spanning from upstream transcription to core metabolism, leads to a twofold deterioration of dNK cell function:
Complete loss of pro-angiogenic and immunomodulatory functions: The loss of the critical CD39+ dNK subset (which supports trophoblasts by secreting M-CSF) has been confirmed to be causal; its functional defect can lead to pregnancy loss in humanized mouse models, which can be rescued by the adoptive transfer of this subset (61). Concurrently, inhibitory signals from trophoblasts, such as exosomal miR-185-5p, can directly inhibit the secretion of vascular endothelial growth factor (VEGF) by dNK cells, thereby blocking spiral artery remodeling (62).
Abnormal activation of cytotoxic and pro-inflammatory functions: The cytotoxicity of dNK cells is activated under the stimulation of an abnormal metabolic microenvironment (e.g., imbalanced tryptophan metabolites) and inflammatory signals (5). One theoretical model suggests that metabolic stress (such as ER stress) can downregulate inhibitory ligands (HLA-C/G) on the surface of trophoblasts, allowing dNK cells to recognize and attack them (63). The elevated levels of Granzyme B in the decidua are direct evidence of this phenomenon (32). At the same time, dNK cells switch to secreting large amounts of pro-inflammatory cytokines such as IFN-γ. This loss of control stems from the failure of multiple inhibitory mechanisms: at the intercellular level, inhibitory exosomal signals from the fetus (such as miR-29a-3p) are interrupted (64); at the intracellular level, the expression of the inhibitory miR-122-5p is downregulated, rendering it unable to suppress the pro-inflammatory transcription factor T-bet (65); and alterations in their surface activating receptor profile (such as NKp46) further exacerbate the pro-inflammatory shift (66).
In summary, the functional collapse of dNK cells is a comprehensive process of dysregulation—from transcription and metabolism to subset structure—driven by both distant and local factors. This functional reversal is a key execution step leading to insufficient embryonic blood supply and direct immune rejection.
5 Diagnostic and therapeutic perspectives: from multidimensional warning to precision intervention
5.1 Building a multidimensional warning system: integration of immuno-metabolic targets
The diagnosis of RPL is gradually evolving from a traditional model reliant on single, static indicators like the Treg/Th17 ratio, whose limited predictive power fails to capture the full picture of the disease (4, 17, 19). As noted in commentaries on the field, the key to building robust predictive models for RPL lies in integrating multi-omics data and deeply exploring the metabolic reprogramming of immune cells (67). Based on this, a new paradigm for a multidimensional warning network that integrates immune status, metabolic fingerprints, and cellular function is emerging.
This paradigm shift has rapidly moved from theory to practice. A predictive model that integrates routine peripheral blood immune and metabolic indicators using machine learning can now predict subsequent pregnancy outcomes with high accuracy (AUC = 0.88), marking a significant advance in macro-level prediction (68). In the realm of non-invasive diagnostics, the use of urine metabolic fingerprints combined with machine learning has enabled rapid, high-throughput screening for RPL, with a diagnostic efficacy (AUC = 0.91) that offers a feasible path for large-scale early warning (69). Beyond mere prediction, this new approach can also achieve deeper mechanistic subtyping. Studies have confirmed a direct link between amino acid metabolism disorders in the serum of RPL patients and Th1/Th2 immune imbalance, and a diagnostic model built on this association has demonstrated extremely high accuracy (AUC = 0.969) (70).
More precise diagnosis requires delving into the key site of the pathological process—the maternal-fetal interface. With unprecedented resolution, spatial transcriptomics has revealed a reduction in protective cell subsets and an infiltration of aggressive cells in the RPL decidua, identifying FOSL2 as a core transcription factor driving this pathological shift and positioning it as a highly promising diagnostic target (30). At the cellular level, the functional defects of key immune cells are being pinpointed to specific molecular events. For instance, the dysfunction of dNK cells has been shown to be closely related to the downregulation of the mitochondrial protein GRIM19, suggesting that GRIM19 could serve as a cellular-level indicator for assessing dNK cell function (60). Similarly, in-depth studies of Treg cell metabolism have revealed the central role of the energy sensor LKB1 and the mitochondrial uncoupling protein UCP3 in maintaining their function, providing a theoretical basis for developing mechanism-based diagnostics based on Treg metabolic status (71, 72). These mechanistic discoveries also offer new tools for evaluating existing therapies; for example, monitoring changes in the metabolic profiles of patients before and after lymphocyte immunotherapy (LIT) may emerge as a novel diagnostic strategy for dynamically assessing therapeutic efficacy and prognosis (73).
A truly comprehensive diagnostic model must expand its perspective to include paternal contributions. The latest research has extended the etiology of RPL to the level of paternal epigenetics, discovering that abnormal m6A modification of sperm mRNA is associated with early embryonic arrest, thus providing a completely new molecular target for diagnosis (74). From a metabolic standpoint, a taurine metabolism defect has also been identified in the sperm of male partners of RPL patients, which is not only a new biomarker but also hints at potential intervention strategies (75).
In summary, the future of RPL diagnosis is evolving from a linear assessment of single indicators to a multi-level, dynamic warning network. This network aims to integrate a portfolio of biomarkers spanning immunology, metabolomics, genetics, and epigenetics, thereby enabling a paradigm shift from “unexplained” to “precision diagnosis.” Several recent reviews have systematically organized this emerging biomarker system, providing a solid theoretical foundation for constructing the next generation of predictive frameworks (76, 77).
5.2 Precision intervention: new therapeutic strategies targeting immuno-metabolism
The new understanding of RPL pathophysiology based on the “gut-systemic-decidual” immuno-metabolic axis is giving rise to a series of novel therapeutic strategies, moving beyond traditional approaches to explore targeted immunomodulation for alloimmune-related RPL (78, 79). These strategies aim for multi-level, precision intervention, from the source and system to the local environment. Reviews have clearly pointed out that targeting microbiota-mediated immune regulation can open new avenues for RPL treatment (80).
5.2.1 Reshaping the source: gut microbiome intervention
The first level of intervention directly targets the initiating step of the pathological process—the gut. The core principle is to restore the production of key metabolites (especially SCFAs) by remodeling the gut microbiota, thereby regulating the systemic immune response. Basic research has confirmed that the maternal gut microbiota and its product, SCFAs, are prerequisites for normal placental development (81). In abortion-prone models, direct supplementation with butyrate not only restores Treg cell numbers but also effectively reduces the rate of pregnancy loss, directly validating the efficacy of “replenishing key metabolites” (82). Clinically, high-quality randomized controlled trials (RCTs) have demonstrated that prebiotics can effectively increase beneficial bacteria and SCFAs in the gut of both mothers and infants (83). While probiotics and fecal microbiota transplantation are promising approaches, their application requires a cautious, strain-specific, and individualized strategy to avoid potential risks (84).
5.2.2 Recalibrating the system: systemic immuno-metabolic therapies
The second level of intervention directly targets the dysregulated systemic immuno-metabolic network, aiming to correct the metabolic programs of immune cells and optimize the uterine microenvironment. The core of this strategy is to target key regulatory nodes of cellular metabolism. Clinical evidence shows that correcting the Th17/Treg imbalance in RIF patients with the mTOR inhibitor rapamycin (sirolimus) can significantly increase the live birth rate, providing direct evidence for “targeting metabolic pathways to correct immune imbalance” (85). Another key metabolic sensor, PPARγ, can be indirectly activated through dietary supplements (such as stigmasterol) to reprogram T cell metabolic patterns and restore the Treg/Th17 balance (86). Furthermore, the use of the classic drug metformin and novel SGLT2 inhibitors can ameliorate metabolic disorders in high-risk RPL populations by regulating pathways such as AMPK. At the same time, the newly discovered biomarker TRAF3, due to its key role in immuno-metabolism, offers new possibilities for assessing RPL risk and the potential efficacy of related therapies (87, 88).
In addition to regulating key nodes, directly supplementing critical metabolic substrates or activating endogenous defense systems is equally important. Supplementation with NAD+ precursors (NR or NMN) has been shown to restore NAD+ levels, improve mitochondrial function, and thereby rescue oocyte quality associated with maternal aging or metabolic diseases (89, 90). In tackling oxidative stress, the strategy has shifted toward activating the body’s own NRF2/HO-1 antioxidant system with micronutrients like N-acetylcysteine (NAC), a strategy proven to mitigate ROS-mediated fetal injury induced by environmental toxins (91, 92). Additionally, some pleiotropic regulators have shown potential. Melatonin has been shown to block the TLR4/MAPK inflammatory pathway in the placenta by remodeling the gut microbiota, reflecting an intervention strategy that targets the “gut-placenta axis” (93). Supplementing with L-arginine can optimize uterine-placental blood perfusion by increasing NO synthesis, creating a favorable physical environment for embryonic development (94).
5.2.3 Reprogramming the endpoint: decidual local immunomodulation
The final level of intervention focuses on the maternal-fetal interface, aiming to directly reprogram dysfunctional immune cells or regulate their key signals. At the molecular level, activating the PD-1/PD-L1 immune checkpoint pathway can inhibit macrophage glycolysis and drive their differentiation toward a protective M2 phenotype, suggesting that PD-1 agonists may become a future therapy to mimic normal pregnancy signals and restore decidual immune homeostasis (95). At the cellular level, classic lymphocyte immunotherapy (LIT) has been shown in multiple clinical trials to effectively increase live birth rates by remodeling the systemic and local immune environment, including reducing the proportion of cytotoxic NK cells and reversing Th1/Th2 and Th17/Treg imbalances (73, 96).
Finally, the significance of all these intervention strategies extends beyond maintaining a single pregnancy. Critical evidence indicates that SCFAs produced by the maternal gut microbiota can cross the placenta and directly participate in the development and metabolic programming of the fetal cardiovascular, nervous, and immune systems. Therefore, targeting the regulation of maternal immuno-metabolic homeostasis during pregnancy is essentially a forward-looking safeguard for the long-term health of the offspring, carrying profound transgenerational significance (97).
6 Conclusion and future perspectives
This review has systematically challenged the traditional view of unexplained recurrent pregnancy loss (URPL) as an isolated uterine event. By integrating the latest evidence from immuno-metabolism, microbiology, and maternal-fetal medicine, we have constructed and substantiated a novel “gut-systemic-decidual” immuno-metabolic dysregulation model. We propose that URPL is not merely a local immune imbalance but a systemic disease initiated by distant gut dysbiosis, propagated through the metabolic reprogramming of systemic immune cells and inflammatory signaling, and ultimately erupting in a metabolically dysregulated decidual microenvironment. From the impaired systemic differentiation of Treg cells due to reduced gut SCFAs, to the “metabolic endotoxemia” triggered by LPS leakage; from the “metabolic hostility” of the decidual microenvironment, to the “functional collapse” of key immune cells (Tregs and dNK cells) due to errors in metabolic programming—this series of interconnected pathological events collectively dismantles maternal-fetal immune tolerance. This integrated model not only provides a deeper, more logical pathophysiological explanation for up to 50% of “unexplained” RPL cases but also opens up entirely new horizons for future research and clinical practice. It is important to acknowledge, however, the limitations of the current evidence. Much of the human data supporting this model is correlational, and future longitudinal studies and intervention trials are essential to firmly establish causality. Moreover, given the heterogeneity of URPL, this model likely explains a significant subset of cases, and the potential for bidirectional interactions within the gut-decidual axis warrants further investigation.
Looking ahead, based on this new paradigm, research in the RPL field will enter a new era of multidimensional, dynamic, and precision-based approaches.
In diagnostics, the future goal is to build a dynamic warning system capable of “full-temporal, full-spatial” risk assessment. This will require moving beyond the current static multi-omics snapshots to develop artificial intelligence (AI) predictive models that can integrate gut microbiota, serum metabolites, the epigenetic status of immune cells, and their real-time functions (such as Treg plasticity and memory status). This approach will be crucial for patient stratification, distinguishing subtypes of URPL based on their primary driver (e.g., ‘gut-driven inflammatory type’ vs. ‘local decidual metabolic type’). We envision that the management of high-risk pregnant women in the future may involve creating their personalized “Digital Twin” models, enabling pre-emptive warnings and guiding targeted preventive approaches through dynamic monitoring and algorithmic prediction before the cascade of immune imbalance is triggered. Furthermore, incorporating paternal factors (such as sperm epigenetic and metabolic defects) as standard parameters into these models will be a critical step toward achieving a truly comprehensive diagnosis.
In therapeutics, intervention strategies will shift from symptomatic treatment to multi-target, mechanism-based precision interventions. Future treatment regimens will be a multidimensional combination, potentially including: (1) “Engineered probiotics”: not just simple strain supplementation, but “Live Biotherapeutic Products (LBPs)” that are engineered to produce specific SCFAs or immunomodulatory molecules; (2) “Treg memory remodeling therapy”: using targeted drugs or cell therapies to repair the potentially depleted “ex-Treg” memory cell pool in RPL patients, restoring their rapid response capability in subsequent pregnancies; and (3) “Decidual targeted delivery systems”: using nanoparticles and other carriers to precisely deliver PD-1 agonists, metabolic regulators, or anti-inflammatory drugs to the maternal-fetal interface, allowing for highly selective intervention in the local microenvironment without affecting systemic immune function. These interventions represent promising preventive approaches that can be tailored to patient subgroups identified through advanced diagnostics.
In basic research, several core scientific questions remain to be answered. We need higher-resolution spatiotemporal multi-omics technologies to track the migration trajectory and metabolic state evolution of individual immune cells from the gut to the decidua in real time. We need more sophisticated humanized mouse models and decidual organoids to validate the causal relationships of key molecules in a setting that more closely resembles the human body. Most importantly, we need to answer an ultimate question: How do we define and maintain an immuno-metabolic homeostasis that is “most favorable for pregnancy”? This is not only for a successful pregnancy but also because the maternal immune-metabolic state during gestation profoundly influences the long-term health of the offspring through “metabolic programming”.
In conclusion, by expanding the etiological understanding of RPL from the local uterine environment to a systemic “gut-systemic-decidual” framework, we not only provide a critical new perspective for understanding this stubborn clinical challenge but also point toward a clearer research direction for safeguarding the long-term health of both mother and child.
Author contributions
YS: Methodology, Conceptualization, Writing – review & editing, Formal analysis, Visualization, Writing – original draft. XT: Writing – original draft, Visualization, Resources, Supervision, Funding acquisition, Writing – review & editing.
Funding
The author(s) declared that financial support was received for this work and/or its publication. This research was funded by the Shandong Provincial Natural Science Foundation, grant number ZR2022QH049, and the Youth Innovation Science and Technology Program of Shandong Provincial Universities, grant number 2023KJ345.
Acknowledgments
The authors would like to thank the reviewers for their valuable comments and suggestions, which have significantly improved the quality of this manuscript.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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References
1. Lu X, Shi Z, Jiang L, and Zhang S. Maternal gut microbiota in the health of mothers and offspring: from the perspective of immunology. Front Immunol. (2024) 15:1362784. doi: 10.3389/fimmu.2024.1362784
2. Aluvihare VR, Kallikourdis M, and Betz AG. Regulatory T cells mediate maternal tolerance to the fetus. Nat Immunol. (2004) 5:266–71. doi: 10.1038/ni1037
3. Samstein RM, Josefowicz SZ, Arvey A, Treuting PM, and R AY. Extrathymic generation of regulatory T cells in placental mammals mitigates maternal-fetal conflict. Cell. (2012) 150:29–38. doi: 10.1016/j.cell.2012.05.031
4. Liu Y, Chen H, Feng L, and Zhang J. Interactions between gut microbiota and metabolites modulate cytokine network imbalances in women with unexplained miscarriage. NPJ Biofilms Microbiom. (2021) 7(1):24. doi: 10.1038/s41522-021-00199-3
5. Moldenhauer LM, Hull ML, Foyle KL, McCormack CD, and Robertson SA. Immune–metabolic interactions and T cell tolerance in pregnancy. J Immunol. (2022) 209:1426–36. doi: 10.4049/jimmunol.2200362
6. Li M-Y, Shen H-H, Cao X-Y, Gao X-X, Xu F-Y, Ha S-Y, et al. Targeting a mTOR/autophagy axis: a double-edged sword of rapamycin in spontaneous miscarriage. Biomed Pharmacother. (2024) 177:116976. doi: 10.1016/j.biopha.2024.116976
7. Michalek RD, Gerriets VA, Jacobs SR, Macintyre AN, MacIver NJ, Mason EF, et al. Cutting edge: distinct glycolytic and lipid oxidative metabolic programs are essential for effector and regulatory CD4+ T cell subsets. J Immunol. (2011) 186:3299–303. doi: 10.4049/jimmunol.1003613
8. Delgoffe GM, Kole TP, Zheng Y, Zarek PE, Matthews KL, Xiao B, et al. The mTOR kinase differentially regulates effector and regulatory T cell lineage commitment. Immunity. (2009) 30:832–44. doi: 10.1016/j.immuni.2009.04.014
9. Salem I, Ramser A, Isham N, and Ghannoum MA. The gut microbiome as a major regulator of the gut-skin axis. Front Microbiol. (2018) 9:1459. doi: 10.3389/fmicb.2018.01459
10. Haidar L, Bănărescu CF, Uţa C, Zimbru E-L, Zimbru R-I, Tîrziu A, et al. Beyond the skin: exploring the gut–skin axis in chronic spontaneous urticaria and other inflammatory skin diseases. Biomedicines. (2025) 13(8):2014. doi: 10.3390/biomedicines13082014
11. Koh A, De Vadder F, Kovatcheva-Datchary P, and Bäckhed F. From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell. (2016) 165:1332–45. doi: 10.1016/j.cell.2016.05.041
12. Arpaia N, Campbell C, Fan X, Dikiy S, van der Veeken J, deRoos P, et al. Metabolites produced by commensal bacteria promote peripheral regulatory T-cell generation. Nature. (2013) 504:451–5. doi: 10.1038/nature12726
13. Furusawa Y, Obata Y, Fukuda S, Endo TA, Nakato G, Takahashi D, et al. Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells. Nature. (2013) 504:446–50. doi: 10.1038/nature12721
14. Li Z, Zheng Y, Zhang M, Wu K, Zhang L, Yao Y, et al. Gut microbiota-derived metabolites associate with circulating immune cell subsets in unexplained recurrent spontaneous abortion. Heliyon. (2024) 10(2):e24571. doi: 10.1016/j.heliyon.2024.e24571
15. Yao Y, Cai X, He D, Zheng Y, Liu M, Zhang M, et al. Short-chain fatty acids regulate T cell heterogeneity to alleviate recurrent spontaneous abortion. Br J Pharmacol. (2025) 182:5762–89. doi: 10.1111/bph.70155
16. Karhukorpi J, Laitinen T, and Karttunen R. Searching for links between endotoxin exposure and pregnancy loss: CD14 polymorphism in idiopathic recurrent miscarriage. Am J Reprod Immunol. (2003) 50:346–50. doi: 10.1034/j.1600-0897.2003.00092.x
17. Cani PD, Amar J, Iglesias MA, Poggi M, Knauf C, Bastelica D, et al. Metabolic endotoxemia initiates obesity and insulin resistance. Diabetes. (2007) 56:1761–72. doi: 10.2337/db06-1491
18. Di Vincenzo F, Del Gaudio A, Petito V, Lopetuso LR, and Scaldaferri F. Gut microbiota, intestinal permeability, and systemic inflammation: a narrative review. Internal Emergency Med. (2023) 19:275–93. doi: 10.1007/s11739-023-03374-w
19. Chen X, Li P, Liu M, Zheng H, He Y, Chen M-X, et al. Gut dysbiosis induces the development of pre-eclampsia through bacterial translocation. Gut. (2020) 69:513–22. doi: 10.1136/gutjnl-2019-319101
20. Huang Y, Fang R, Xiong T, Li W, and Yu N. The relationship between gut microbiota and recurrent spontaneous abortion. Microorganisms. (2025) 13(5):1073. doi: 10.3390/microorganisms13051073
21. Denny KJ, Coulthard LG, Mantovani S, Simmons D, Taylor SM, and Woodruff TM. The role of C5a receptor signaling in endotoxin-induced miscarriage and preterm birth. Am J Reprod Immunol. (2015) 74:148–55. doi: 10.1111/aji.12386
22. Jin J, Gao L, Zou X, Zhang Y, Zheng Z, Zhang X, et al. Gut dysbiosis promotes preeclampsia by regulating macrophages and trophoblasts. Circ Res. (2022) 131:492–506. doi: 10.1161/circresaha.122.320771
23. Wang K, Cai L, Sun J, Liu L, and Gao S. AMPK/SIRT1/GPX4 signaling pathway mediates the protective effect of puerarin against LPS-induced endometritis in mice. J Nutr Biochem. (2025) 146:110072. doi: 10.1016/j.jnutbio.2025.110072
24. Jiang H-y, Gu W-w, Gan J, Yang Q, Shi Y, Lian W-b, et al. MNSFβ promotes LPS-induced TNFα expression by increasing the localization of RC3H1 to stress granules, and the interfering peptide HEPN2 reduces TNFα production by disrupting the MNSFβ–RC3H1 interaction in macrophages. Int Immunopharmacol. (2024) 142(Pt A):113053. doi: 10.1016/j.intimp.2024.113053
25. Nancy P, Tagliani E, Tay C-S, Asp P, Levy DE, and Erlebacher A. Chemokine gene silencing in decidual stromal cells limits T cell access to the maternal-fetal interface. Science. (2012) 336:1317–21. doi: 10.1126/science.1220030
26. Rhee JS, Saben JL, Mayer AL, Schulte MB, Asghar Z, Stephens C, et al. Diet-induced obesity impairs endometrial stromal cell decidualization: a potential role for impaired autophagy. Hum Reproduct. (2016) 31:1315–26. doi: 10.1093/humrep/dew048
27. Yang D, Ding J, Wang Y, Yuan M, Xian S, Zhang L, et al. YY1-PVT1 affects trophoblast invasion and adhesion by regulating mTOR pathway-mediated autophagy. J Cell Physiol. (2020) 235:6637–46. doi: 10.1002/jcp.29560
28. Zheng J, Li Y, Sang Y, Xu L, Jin X, Tao Y, et al. Pigment epithelium-derived factor, a novel decidual natural killer cells-derived factor, protects decidual stromal cells via anti-inflammation and anti-apoptosis in early pregnancy. Hum Reproduct. (2020) 35:1537–52. doi: 10.1093/humrep/deaa118
29. Azizi R, Soltani-Zangbar MS, sheikhansari G, Pourmoghadam Z, Mehdizadeh A, Mahdipour M, et al. Metabolic syndrome mediates inflammatory and oxidative stress responses in patients with recurrent pregnancy loss. J Reprod Immunol. (2019) 133:18–26. doi: 10.1016/j.jri.2019.05.001
30. Sha Q, Yu Q, Chen K, Wang J, Wang F, Jiang C, et al. Spatial transcriptomics of human decidua identifies molecular signatures in recurrent pregnancy loss. Genomics Proteomics Bioinf. (2025) qzaf080. doi: 10.1093/gpbjnl/qzaf080
31. Wen X, Dong P, Liu J, Wang S-J, and Li J. Role of immune inflammation in recurrent spontaneous abortions. J Inflammation Res. (2024) 17:9407–22. doi: 10.2147/jir.S488638
32. Tang T, Fu J, Zhang C, Wang X, Cao H, and Chen L. Exploring the role of endoplasmic reticulum stress in recurrent spontaneous abortion: Identification of diagnostic biomarkers and immune cell interactions. Heliyon. (2024) 10(19):e38964. doi: 10.1016/j.heliyon.2024.e38964
33. Niu Y, Yin L, Zhou Y, Pang X, Li Y, Peng C, et al. Comprehensive analysis of DNA methylation patterns in recurrent miscarriage: imprinted/non-imprinted genes and their regulation across sperm and fetal-maternal tissues. PeerJ. (2025) 13:e20125. doi: 10.7717/peerj.20125
34. Wang L, Wang H, Luo J, Xie T, Mor G, and Liao A. Decorin promotes decidual M1-like macrophage polarization via mitochondrial dysfunction resulting in recurrent pregnancy loss. Theranostics. (2022) 12:7216–36. doi: 10.7150/thno.78467
35. Liu H, Chen H, Han T, Wang X, Dai J, Yang X, et al. Lipid imbalance and inflammatory oxylipin cascade at the maternal-fetal interface in recurrent spontaneous abortion. Heliyon. (2024) 10(24):e40515. doi: 10.1016/j.heliyon.2024.e40515
36. Zhou W-J, Yang H-L, Mei J, Chang K-K, Lu H, Lai Z-Z, et al. Fructose-1,6-bisphosphate prevents pregnancy loss by inducing decidual COX-2 macrophage differentiation. Sci Adv. (2022) 8(8):eabj2488. doi: 10.1126/sciadv.abj2488
37. Sun Y, Li G, Kong M, Li J, Wang S, and Tan Y. Angelica sinensis polysaccharide as potential protectants against recurrent spontaneous abortion: focus on autophagy regulation. Front Med. (2025) 12:1522503. doi: 10.3389/fmed.2025.1522503
38. Vondra S, Kunihs V, Eberhart T, Eigner K, Bauer R, Haslinger P, et al. Metabolism of cholesterol and progesterone is differentially regulated in primary trophoblastic subtypes and might be disturbed in recurrent miscarriages. J Lipid Res. (2019) 60:1922–34. doi: 10.1194/jlr.P093427
39. Yang S-H, Yang L, Shi Y, Xu H-R, Gan J, Shi J-X, et al. GDF15 promotes trophoblast invasion and pregnancy success via the BMPR1A/BMPR2/p-SMAD1 pathway: Implications for recurrent miscarriage. Life Sci. (2025) 371:123586. doi: 10.1016/j.lfs.2025.123586
40. Chen W, Zhang Y, Zhao Z, Yang Y, Guo G, Sun Y, et al. Lnc-HZ05 suppresses trophoblast cell migrasome formation by disrupting TGFβ2 pathway in unexplained recurrent miscarriage. Adv Sci. (2025) 13(2):e17558. doi: 10.1002/advs.202417558
41. Shi LZ, Wang R, Huang G, Vogel P, Neale G, Green DR, et al. HIF1α–dependent glycolytic pathway orchestrates a metabolic checkpoint for the differentiation of TH17 and Treg cells. J Exp Med. (2011) 208:1367–76. doi: 10.1084/jem.20110278
42. Angelin A, Gil-de-Gómez L, Dahiya S, Jiao J, Guo L, Levine MH, et al. Foxp3 reprograms T cell metabolism to function in low-glucose, high-lactate environments. Cell Metab. (2017) 25:1282–93.e7. doi: 10.1016/j.cmet.2016.12.018
43. Bao S, Chen Z, Qin D, Xu H, Deng X, Zhang R, et al. Single-cell profiling reveals mechanisms of uncontrolled inflammation and glycolysis in decidual stromal cell subtypes in recurrent miscarriage. Hum Reproduct. (2023) 38:57–74. doi: 10.1093/humrep/deac240
44. Lin Y, Zhang D, Li Y, Li Y, Li B, and Du M. Decidual NR2F2-expressing CD4+ T cells promote TH2 transcriptional program during early pregnancy. Front Immunol. (2021) 12:670777. doi: 10.3389/fimmu.2021.670777
45. Beier UH, Angelin A, Akimova T, Wang L, Liu Y, Xiao H, et al. Essential role of mitochondrial energy metabolism in Foxp3+ T-regulatory cell function and allograft survival. FASEB J. (2015) 29:2315–26. doi: 10.1096/fj.14-268409
46. Wu J, Cao Q, Liao J, Li Y, Lu G, Gong F, et al. Immunological indicators of recurrent pregnancy loss: A mendelian randomization study. Reprod Sci. (2024) 31:2783–93. doi: 10.1007/s43032-024-01555-2
47. Li Z, Si P, Meng T, Zhao X, Zhu C, Zhang D, et al. CCR8+ decidual regulatory T cells maintain maternal-fetal immune tolerance during early pregnancy. Sci Immunol. (2025) 10:eado2463. doi: 10.1126/sciimmunol.ado2463
48. Duan B, Feng Q, Li L, and Huang J. CircDDX21 alleviates trophoblast dysfunction and Treg differentiation in recurrent spontaneous abortion via miR-520a-5p/FOXP3/PD-L1 axis. J Assist Reprod Genet. (2024) 41:3539–57. doi: 10.1007/s10815-024-03281-9
49. Tsuda S, Shichino S, Tilburgs T, Shima T, Morita K, Yamaki-Ushijima A, et al. CD4+ T cell heterogeneity in gestational age and preeclampsia using single-cell RNA sequencing. Front Immunol. (2024) 15:1401738. doi: 10.3389/fimmu.2024.1401738
50. Leong JY, McGovern N, Mishra A, Wasser M, Kumar P, Tay SH, et al. Epigenetic instability and hypofunctionality of fetal Tregs allow a permissive regulatory environment for T effector memory maturation. Proc Natl Acad Sci. (2025) 122:e2506673122. doi: 10.1073/pnas
51. Shao TY, K J, Harper G, Pham G, Peng Y, Liu J, et al. Reproductive outcomes after pregnancy-induced displacement of preexisting microchimeric cells. Science. (6664) 2023) 381:1324–30. doi: 10.1126/science.adf9325
52. Chen Y, Gan B, Zheng S, Zhao X, Jin L, and Wei J. AMPK-mTOR pathway modulates glycolysis reprogramming in unexplained recurrent spontaneous abortion. BMC Pregnancy Childbirth. (2024) 24(1):840. doi: 10.1186/s12884-024-07054-1
53. Alissafi T, Kalafati L, Lazari M, Filia A, Kloukina I, Manifava M, et al. Mitochondrial oxidative damage underlies regulatory T cell defects in autoimmunity. Cell Metab. (2020) 32:591–604.e7. doi: 10.1016/j.cmet.2020.07.001
54. Galgani M, Insabato L, Calì G, Della Gatta AN, Mirra P, Papaccio F, et al. Regulatory T cells, inflammation, and endoplasmic reticulum stress in women with defective endometrial receptivity. Fertil Steril. (2015) 103:1579–86.e1. doi: 10.1016/j.fertnstert.2015.03.014
55. Shojaei Z, Jafarpour R, Mehdizadeh S, Bayatipoor H, Pashangzadeh S, and Motallebnezhad M. Functional prominence of natural killer cells and natural killer T cells in pregnancy and infertility: A comprehensive review and update. Pathol - Res Pract. (2022) 238:154062. doi: 10.1016/j.prp.2022.154062
56. Wang F, Jia W, Fan M, Shao X, Li Z, Liu Y, et al. Single-cell immune landscape of human recurrent miscarriage. Genom Proteomics Bioinf. (2021) 19:208–22. doi: 10.1016/j.gpb.2020.11.002
57. Chen P, Zhou L, Chen J, Lu Y, Cao C, Lv S, et al. The immune atlas of human deciduas with unexplained recurrent pregnancy loss. Front Immunol. (2021) 12:689019. doi: 10.3389/fimmu.2021.689019
58. Rytkönen KT, Adossa N, Zúñiga Norman S, Lönnberg T, Poutanen M, and Elo LL. Gene regulatory network analysis of decidual stromal cells and natural killer cells. Reprod Sci. (2024) 31:3159–74. doi: 10.1007/s43032-024-01653-1
59. Yan S, Dong J, Qian C, Chen S, Xu Q, Lei H, et al. The mTORC1 signaling support cellular metabolism to dictate decidual NK cells function in early pregnancy. Front Immunol. (2022) 13:771732. doi: 10.3389/fimmu.2022.771732
60. Wang Y, Guo A, Yang L, Han X, Li Q, Liu J, et al. Immune dysregulation of decidual NK cells mediated by GRIM19 downregulation contributes to the occurrence of recurrent pregnancy loss. Mol Cell Biochem. (2024) 480:3117–31. doi: 10.1007/s11010-024-05181-z
61. Jia W, Ma L, Yu X, Wang F, Yang Q, Wang X, et al. Human CD56+CD39+ dNK cells support fetal survival through controlling trophoblastic cell fate: immune mechanisms of recurrent early pregnancy loss. Natl Sci Rev. (2024) 11(6):nwae142. doi: 10.1093/nsr/nwae142
62. Xiong Y, Fang Z, Dong J, Chen S, Mao J, Zhang W, et al. Maternal circulating exosomal miR-185-5p levels as a predictive biomarker in patients with recurrent pregnancy loss. J Assist Reprod Genet. (2023) 40:553–66. doi: 10.1007/s10815-023-02733-y
63. Sen Santara S, Crespo ÂC, Mulik S, Ovies C, Boulenouar S, Strominger JL, et al. Decidual NK cells kill Zika virus–infected trophoblasts. Proc Natl Acad Sci. (2021) 118(47):e2115410118. doi: 10.1073/pnas.2115410118
64. Fang Z, Mao J, Huang J, Sun H, Lu X, Lei H, et al. Increased levels of villus-derived exosomal miR-29a-3p in normal pregnancy than uRPL patients suppresses decidual NK cell production of interferon-γ and exerts a therapeutic effect in abortion-prone mice. Cell Commun Signaling. (2024) 22(1):230. doi: 10.1186/s12964-024-01610-0
65. Liu Y, Li L, Feng T, Zhou W, Liu Y, Mu Y, et al. Abnormal miR-122-5p expression in decidual NK cells and its impact on trophoblast behavior: insights into unexplained recurrent pregnancy loss. Int J Med Sci. (2024) 21:2824–36. doi: 10.7150/ijms.101865
66. Yamamoto M, Fukui A, Mai C, Saeki S, Takayama R, Wakimoto Y, et al. Evaluation of NKp46 expression and cytokine production of decidual NK cells in women with recurrent pregnancy loss. Reprod Med Biol. (2022) 21(1):e12478. doi: 10.1002/rmb2.12478
67. Wei Y, Deng Z, and Yin T. Are we closer to robust predictors of recurrent pregnancy loss by means of integrating different types of omics data? Expert Rev Mol Diagn. (2024) 24:561–3. doi: 10.1080/14737159.2024.2375235
68. Li Y, Wu IXY, Wang X, Song J, Chen Q, and Zhang W. Immunological parameters of maternal peripheral blood as predictors of future pregnancy outcomes in patients with unexplained recurrent pregnancy loss. Acta Obstetr Gynecol Scand. (2024) 103:1444–56. doi: 10.1111/aogs.14832
69. Qu Y, Chen M, Han M, Yu X, Yu X, Fan J, et al. High throughput recurrent pregnancy loss screening: urine metabolic fingerprints via LDI-MS and machine learning. Analyst. (2025) 150:2128–36. doi: 10.1039/d5an00177c
70. Ye X, Ma C, Guo W, Guo Y, Li D-d, Zhou S, et al. Metabolomic analysis reveals potential role of immunometabolism dysregulation in recurrent pregnancy loss. Front Endocrinol. (2024) 15:1476774. doi: 10.3389/fendo.2024.1476774
71. Yang K, Blanco DB, Neale G, Vogel P, Avila J, Clish CB, et al. Homeostatic control of metabolic and functional fitness of Treg cells by LKB1 signalling. Nature. (2017) 548:602–6. doi: 10.1038/nature23665
72. O’Reilly S, O’Connor EB, Muñoz-Wolf N, Leon G, Lavelle EC, Mills KHG, et al. UCP3 reciprocally controls CD4+ Th17 and Treg cell differentiation. PloS One. (2020) 15(11):e0239713. doi: 10.1371/journal.pone.0239713
73. Aslanian-kalkhoran L, Kamrani A, Alipourfard I, Chakari-Khiavi F, Chakari-Khiavi A, Aghebati-Maleki L, et al. The effect of lymphocyte immunotherapy (LIT) in modulating immune responses in patients with recurrent pregnancy loss (RPL). Int Immunopharmacol. (2023) 121:110326. doi: 10.1016/j.intimp.2023.110326
74. Yang T, Liu Y, Lin Z, Chen F, Zhu L, Zhang L, et al. Altered N6-methyladenosine methylation level in spermatozoa messenger RNA of the male partners is related to unexplained recurrent pregnancy loss. Andrology. (2024) 13:531–43. doi: 10.1111/andr.13678
75. Wu H, Zhang X, Yang J, Feng T, Chen Y, Feng R, et al. Taurine and its transporter TAUT positively affect male reproduction and early embryo development. Hum Reproduct. (2022) 37:1229–43. doi: 10.1093/humrep/deac089
76. Ye L and Dimitriadis E. Endometrial receptivity–lessons from “Omics. Biomolecules. (2025) 15(1):106. doi: 10.3390/biom15010106
77. Li L and Baek K-H. Exploring potential biomarkers in recurrent pregnancy loss: A literature review of omics studies to molecular mechanisms. Int J Mol Sci. (2025) 26. doi: 10.3390/ijms26052263
78. Uţa C, Tîrziu A, Zimbru E-L, Zimbru R-I, Georgescu M, Haidar L, et al. Alloimmune causes of recurrent pregnancy loss: cellular mechanisms and overview of therapeutic approaches. Medicina. (2024) 60. doi: 10.3390/medicina60111896
79. Mekinian A, Cohen J, Alijotas-Reig J, Carbillon L, Nicaise-Roland P, Kayem G, et al. Unexplained recurrent miscarriage and recurrent implantation failure: is there a place for immunomodulation? Am J Reprod Immunol. (2016) 76:8–28. doi: 10.1111/aji.12493
80. Yao Y, Ye Y, and Zheng C. The impact of microbiota-mediated immune regulation on recurrent pregnancy loss and intervention strategies. Am J Reprod Immunol. (2025) 94(1):e70121. doi: 10.1111/aji.70121
81. Pronovost GN, Y K, Coley-O’Rourke EJL, Telang SS, Chen AS, Vuong HE, et al. The maternal microbiome promotes placental development in mice. (2023) 9(40):eadk1887. doi: 10.1101/2023.02.15.528712
82. Uchida A, Imai K, Miki R, Hamaguchi T, Nishiwaki H, Ito M, et al. Butyrate-producing bacteria in pregnancy maintenance: mitigating dysbiosis-induced preterm birth. J Trans Med. (2025) 23(1):533. doi: 10.1186/s12967-025-06534-y
83. Jones JM, Reinke SN, Mousavi-Derazmahalleh M, Garssen J, Jenmalm MC, Srinivasjois R, et al. Maternal prebiotic supplementation during pregnancy and lactation modifies the microbiome and short chain fatty acid profile of both mother and infant. Clin Nutr. (2024) 43:969–80. doi: 10.1016/j.clnu.2024.02.030
84. Jendraszak M and Andrusiewicz M. Uncovering hidden connections: the role of the male reproductive system microbiome and gut microorganisms in implantation: a critical review. Crit Rev Clin Lab Sci. (2025) 6:1–31. doi: 10.1080/10408363.2025.2562894
85. Ahmadi M, Abdolmohamadi-vahid S, Ghaebi M, Dolati S, Abbaspour-Aghdam S, Danaii S, et al. Sirolimus as a new drug to treat RIF patients with elevated Th17/Treg ratio: A double-blind, phase II randomized clinical trial. Int Immunopharmacol. (2019) 74:105730. doi: 10.1016/j.intimp.2019.105730
86. Wen S, He L, Zhong Z, Zhao R, Weng S, Mi H, et al. Stigmasterol restores the balance of treg/th17 cells by activating the butyrate-PPARγ Axis in colitis. Front Immunol. (2021) 12:741934. doi: 10.3389/fimmu.2021.741934
87. He Y-b, Li J-y, Chen S-l, Ye R, Fei Y-r, Tong S-y, et al. TRAF3 as a potential diagnostic biomarker for recurrent pregnancy loss: insights from single-cell transcriptomics and machine learning. BMC Pregnancy Childbirth. (2025) 25(1):637. doi: 10.1186/s12884-025-07742-6
88. Elkind-Hirsch KE, Seidemann E, and Harris R. A randomized trial of dapagliflozin and metformin, alone and combined, in overweight women after gestational diabetes mellitus. Am J Obstet Gynecol MFM. (2020) 2(3):100139. doi: 10.1016/j.ajogmf.2020.100139
89. Wei C, Zeng X, Wang K, Wang M, Lei M, Zhu Z, et al. Nicotinamide riboside supplementation protects against maternal diabetes-associated decline in oocyte quality. Reprod (Cambridge England). (2025) 169(5):e240350. doi: 10.1530/rep-24-0350
90. Miao Y, Cui Z, Gao Q, Rui R, and Xiong B. Nicotinamide mononucleotide supplementation reverses the declining quality of maternally aged oocytes. Cell Rep. (2020) 32(5):107987. doi: 10.1016/j.celrep.2020.107987
91. Asghar MA, Wan B, Li L, Zhang J, Tang S, Han H, et al. Micronutrient antioxidant supplementation alleviates valproic acid-induced oxidative stress and male infertility via the NRF2/HO-1 pathway. Redox Biol. (2025) 85:103685. doi: 10.1016/j.redox.2025.103685
92. Yang D, Zhu J, Zhou X, Pan D, Nan S, Yin R, et al. Polystyrene micro- and nano-particle coexposure injures fetal thalamus by inducing ROS-mediated cell apoptosis. Environ Int. (2022) 166:107362. doi: 10.1016/j.envint.2022.107362
93. Wu J-J, Zheng X, Wu C, Ma W, Wang Y, Wang J, et al. Melatonin alleviates high temperature exposure induced fetal growth restriction via the gut-placenta-fetus axis in pregnant mice. J Adv Res. (2025) 68:131–46. doi: 10.1016/j.jare.2024.02.014
94. Wu G, Meininger CJ, McNeal CJ, Bazer FW, and Rhoads JM. Role of L-arginine in nitric oxide synthesis and health in humans. Adv Exp Med Biol. (2021) 1332:167–87. doi: 10.1007/978-3-030-74180-8_10
95. Zhang Y, Ma L, Hu X, Ji J, Mor G, and Liao A. The role of the PD-1/PD-L1 axis in macrophage differentiation and function during pregnancy. Hum Reproduct. (2019) 34:25–36. doi: 10.1093/humrep/dey347
96. Dashti M, Kamrani A, Shahir-Khajeh Z, Heris JA, Aghebati-Maleki L, Danaii S, et al. Impact of lymphocyte immunotherapy (LIT) on fertility rates in recurrent pregnancy loss (RPL) women with antinuclear antibodies: A randomized clinical trial. J Reprod Immunol. (2025) 168:104432. doi: 10.1016/j.jri.2025.104432
Keywords: decidual NK cells, decidual microenvironment, gut dysbiosis, immunometabolism, metabolic reprogramming, recurrent pregnancy loss, regulatory T cells
Citation: Shi Y and Tang X (2026) From gut dysbiosis to decidual hostility: the immuno-metabolic crosstalk driving recurrent pregnancy loss. Front. Immunol. 16:1746620. doi: 10.3389/fimmu.2025.1746620
Received: 14 November 2025; Accepted: 29 December 2025; Revised: 19 December 2025;
Published: 21 January 2026.
Edited by:
Maria Laura Zenclussen, National Scientific and Technical Research Council (CONICET), ArgentinaReviewed by:
Silvia Giugliano, Humanitas University, ItalyCristina Uţa, Victor Babes University of Medicine and Pharmacy, Romania
Copyright © 2026 Shi and Tang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Xiufeng Tang, dGFuZ3hmOTI3QDEyNi5jb20=