Alarmins and their pivotal role in the pathogenesis of spontaneous abortion: insights for therapeutic intervention.

Prior to the implantation of the embryo into the uterine lining, maternal physiological preparations for the upcoming new life commence. Angiogenesis-related factors enhance endometrial epithelial cell adhesion and receptivity, significantly augmenting vascular proliferation within the endometrium [ 128 ]. The establishment of an adequate vascular network is critical for the fetus to obtain essential nutrients throughout gestation [ 129 ]. Additionally, angiogenesis-related factors are pivotal in fostering fetal immune tolerance within the maternal environment. During embryo implantation, these factors activate monocyte-mediated immunity [ 130 ], facilitate the recruitment and activation of macrophages [ 131 – 133 ], and regulate the differentiation and maturation of dendritic cells [ 134 ]. Notably, these crucial molecular entities are also modulated by alarmins (Fig.  1 ). Fig. 1 Alarmins influence pregnancy outcomes through angiogenic factors. Solid lines indicate promotion; dashed lines indicate suppression Alarmins influence pregnancy outcomes through angiogenic factors. Solid lines indicate promotion; dashed lines indicate suppression Low expression of HMGB1 can downregulate hypoxia-inducing factor 1α (HIF1α), subsequently inducing the expression of placental growth factor (PIGF) in placental tissue [ 135 ]. IL-33 increases nitric oxide (NO) synthesis by endothelial cells via activation of the ST2/TRAF6–Akt–eNOS signaling pathway, thereby promoting angiogenesis and vascular leakage [ 136 ]. IL-1β promotes angiogenesis by inducing the MAPK cascade and HIF1-α-mediated upregulation of VEGF [ 137 ]. Additionally, IL-1β regulates other growth factors that promote angiogenesis. For instance, IL-1β stimulates corneal endothelial cells (CEC), leading to the induction of NF-κB-dependent fibroblast growth factor-2 (FGF-2), which promotes endothelium–mesenchymal transformation (EndoMT) [ 138 ]. IL-1β can also directly alter vascular permeability by regulating the expression of intercellular junction components and indirectly increase vascular permeability by inducing the permeability-promoting factor R-spondin3 (RSPO3) [ 139 ]. S100A13 and FGF-1 may have dual roles in endometriosis development, mainly involving new angiogenesis and vascular stability regulation. These effects significantly impact the development and pathophysiological processes of endometriosis [ 140 ]. HSP90 promotes tumor angiogenesis and growth through PRKD2 integration into the NF-κB/VEGF signaling pathway [ 141 ]. HSP90 binds to eNOS and induces NO production through the activation of the PI3K–Akt pathway to promote angiogenesis of coronary endothelial cells [ 142 ]. It is worth noting that cffDNA differs from other alarmins in that its elevated levels exhibit anti-angiogenic and pro-inflammatory effects [ 143 ].

HMGB1 is a high-mobility group protein that plays a crucial regulatory role in the nucleus. Beyond its nuclear functions, HMGB1 also acts as a DAMP outside cells, stimulating inflammation and immune responses [ 23 , 24 ]. HMGB1 has been implicated in various diseases such as cancer [ 25 ], cardiovascular disease [ 26 ], and neurological disorders [ 27 ]. In pregnancy-related pathological conditions, including premature birth, placental dysfunction, intrauterine growth restriction, and preeclampsia, HMGB1 can contribute to these complications through the activation of inflammatory mediators [ 28 ]. Recent studies suggest a potential association between HMGB1 and SA. Several studies have demonstrated significantly higher serum levels of HMGB1 in patients with SA compared to normal pregnant women [ 29 – 31 ]. This indicates that HMGB1 may affect pregnancy stability by promoting an inflammatory response. Additionally, abnormal expression of HMGB1 has been observed in placental tissue, which may be associated with dysregulation of the maternal immune system [ 31 , 32 ]. Such dysregulation could potentially lead to adverse pregnancy outcomes following embryo implantation. Current proposed mechanisms underlying HMGB1 activity and its role in SA are as follows: First, Zou et al. discovered that the activation of HMGB1-RAGE/TLR2/TLR4-NF-κ B pathway [ 31 ] is not the only factor at the maternal–fetal interface in patients with URSA. We also observed high expression of inflammatory factors such as NLRP-3, Caspase-1, and GSDMD [ 33 ]. The activation of these inflammatory signals contributes to the development of URSA. Second, studies have shown that HMGB1 in the villi and decidua of URSA patients is not only actively secreted by immune cells [ 33 ], but also passively released by decidual cells [ 34 ]. This provides new insights into the pathological mechanism of URSA. Wang et al. reported a shift in the subcellular localization of HMGB1 from the nucleus to the cytoplasm and extracellular regions, indicating that HMGB1 has multiple biological functions in different locations and environments, closely related to its role in URSA development. Additionally, Fu and his colleagues found a positive correlation between HMGB1 levels in the peripheral blood of URSA patients and Th1 cytokine levels, while a negative correlation observed with Th2 cytokine levels. This suggests that URSA may be associated with a Th1/Th2 imbalance at the maternal–fetal interface, potentially regulated by HMGB1 [ 29 ]. In a lipopolysaccharide (LPS)-induced HTR8/SVneo cell model, knocking out the HMGB1 gene inhibited the expression of Beclin1 and LC3 in HTR-8/SVneo cells induced by endotoxin. This restoration of cell growth and migration suggests that HMGB1 may contribute to the inflammatory response at the uterine–placental interface in abortion through the induction of autophagy [ 32 ]. Additionally, the rs2249825C/G polymorphism of the HMGB1 gene is associated with an increased risk of recurrent pregnancy loss (RPL) and can lead to elevated gene expression in chorionic villi [ 35 ]. IL-33, a member of the IL-1 cytokine family, is expressed in various cell types, including endothelial, epithelial, and stromal cells [ 36 , 37 ]. As a nuclear factor, IL-33 plays a dual role in maintaining body homeostasis and regulating inflammatory responses. It activates multiple signal transduction pathways, such as MAPK, NF-kB, and JNK, by binding to membrane-bound ST2 receptors, thereby promoting immune activation and inflammatory responses [ 38 , 39 ]. Additionally, soluble ST2 (sST2) acts as a decoy receptor for IL-33 [ 40 ], modulating its biological activity and influencing cellular responses [ 38 , 41 ]. IL-33 is crucial in regulating Th2-type immune responses [ 39 , 42 ] and has been shown to extend cardiac allograft survival in transplantation immunology [ 43 ]. As an alarmin, IL-33 is implicated in various inflammatory and immune diseases [ 42 , 44 , 45 ], including asthma [ 46 ], pulmonary fibrosis [ 47 ], and rheumatoid arthritis [ 48 ]. In SA research, the role of IL-33 has garnered attention in recent years. Evidence suggests that IL-33 plays a role in immune regulation during pregnancy and may contribute to SA. However, the relationship between IL-33 and RSA remains inconsistent. Some studies have reported an association between increased serum levels of IL-33 and RSA [ 49 – 51 ], while others have indicated decreased serum level of IL-33 levels in relation to RSA [ 52 , 53 ]. Recent conference findings demonstrated the presence of IL-33 and sST2 in the cervical mucus of RSA patients during early pregnancy, suggesting that an imbalance in the IL-33/sST2 signaling pathway might be involved in RSA [ 54 ]. Another study observed a significant decrease in sST2 content in the endometrium of RSA patients [ 55 ]. Furthermore, this study found a positive correlation between IL-33 levels and age and the number of miscarriages, and a negative correlation with the number of deliveries [ 49 ]. The activation of IL-33/ST2L/sST2 axis in human endometrial stromal cells is crucial for successful embryo implantation and development [ 56 ]. Failure to activate this axis may result in pregnancy complications, such as preeclampsia [ 57 ] and miscarriage [ 58 ]. Additionally, an imbalance in the IL-33/ST2–axl–efferocytosis axis can induce metabolic reprogramming of decidual macrophages, causing miscarriage [ 50 ]. There are also variations in IL-33 gene polymorphisms among different ethnicities and geographical locations [ 52 , 53 , 59 , 60 ]. However, further research is needed to fully understand the specific effects and regulatory mechanisms of IL-33. Additional studies are urgently required to determine the precise role and therapeutic potential of IL-33 in SA. IL-1β, a key inflammatory mediator, is produced by trophoblast cells in the placenta during early pregnancy. It plays a crucial role in the invasion, adhesion, and tissue repair of trophoblast cells [ 61 , 62 ]. Additionally, IL-1β is essential for maintaining endometrial immune tolerance, which is necessary for the successful implantation of a semi-allogeneic blastocyst into the uterus [ 63 ]. Furthermore, IL-1β is critical for placental maturation, fetal organ development, and the regulation of Th1/Th2 cell immune responses [ 64 – 66 ]. It also promotes cervical relaxation [ 67 , 68 ], preparing for eventual delivery [ 69 ]. However, IL-1β is not always beneficial during pregnancy. Excessive secretion or imbalance can adversely affect pregnancy outcomes [ 70 , 71 ]. Research suggests that polymorphisms in IL-1β may alter its production levels, thereby affecting an individual’s susceptibility to certain infections, inflammation, and malignant diseases [ 72 , 73 ]. A meta-analysis of 12 studies showed an association between IL-1β polymorphisms and RPL in a recessive genetic model [ 74 ]. However, this correlation varied across ethnic groups and geographic regions. For example, multiple studies have shown that IL-1β polymorphisms may be a significant risk factor for RPL in Chinese women [ 75 , 76 ]. In contrast, studies on women from UK, North India, or Iran and Azerbaijan did not find a significant correlation between IL-1β gene polymorphisms and RPL [ 77 – 79 ]. It was found that IL-B*2 gene carrying functional gene polymorphism is not an independent factor of RPL, meaning IL-B*2 alone is not associated with RPL [ 80 ]. Equils and his team suggested that the impact of the IL-1 system on pregnancy outcomes might vary depending on experimental models, race, and disease status [ 65 ]. Loeb’s study found that IL-1β expression significantly increased in the decidua of patients with SA and RSA, indicating that a pro-inflammatory environment could disrupt the Th1/Th2 balance [ 81 ]. Additionally, IL-1β has the potential to promote blood coagulation, which may contribute to miscarriage. Previous studies have also shown that lower maternal serum IL-1β levels may indicate poor implantation conditions, such as preterm birth or preeclampsia in full-term pregnancy [ 82 ]. Interestingly, high levels of IL-1β after in vitro fertilization (IVF) are associated with a reduced risk of miscarriage [ 83 ]. Furthermore, IL-1β levels can predict the risk of RPL in ANA-positive female patients and serve as a prognostic marker after Toxoplasma gondii infection [ 84 , 85 ]. The S100 protein family comprises a group of acidic calcium-binding proteins essential for placental development and pregnancy-related diseases [ 86 ]. This family includes approximately 25 different members, all characterized by a distinctive calcium-binding helical domain, but lacking enzymatic activity [ 87 , 88 ]. S100 proteins influence numerous intracellular processes and biological functions by interacting with various molecular partners, including those involved in cell proliferation, differentiation, migration, energy metabolism, and maintenance of cell structure [ 89 ]. Extracellularly, S100 proteins interact with specific membrane receptors, such as G protein-coupled receptors, RAGEs, and TLRs, to regulate inflammatory responses and immune regulation [ 90 ]. These activities are critical for successful embryo implantation and development, as they require precise regulation of placental and embryonic growth and interaction with the mother. With cells, S100 proteins coordinate intracellular Ca 2+ homeostasis and cell differentiation through interactions with several target enzymes, cytoskeletal components, transcription factors, and nucleic acids [ 91 ]. Therefore, S100 proteins play an important role in maintaining cellular homeostasis and adapting to changing developmental needs. Existing research underscores the significance of S100 proteins as a crucial regulatory factor in pregnancy, particularly during embryo implantation and placenta formation. Anomalies in S100 protein expression have been associated with an increased risk of miscarriage and other complications during pregnancy. For instance, Nair et al. first revealed that overexpression of S100A8 and S100A9 in the endometrial decidua leads to an accumulation of white blood cells at the maternal–fetal interface, reducing utero-placental blood flow, causing thrombosis, and inducing hypoxic conditions in placental tissue [ 92 ]. These mechanisms link S100A8 to embryonic miscarriage. S100A8 also affects embryo implantation and increases the risk of miscarriage, associating it with endometrial inflammation in early pregnancy loss patients [ 93 ]. Additionally, Rull’s study found significant differences in S100A8 gene expression in placenta tissue between recurrent miscarriage patients and those with normal pregnancies [ 94 ]. A recent study discovered that S100A8 helps maintain immune homeostasis, and estrogen can compensate for the decrease in S100A8 expression by increasing IL-10 levels [ 95 ]. S100A11 has been extensively studied for its involvement in various human diseases, including wound healing, stress response, rheumatoid arthritis, Alzheimer’s disease, and cancer [ 96 , 97 ]. In 2006, Liu et al. first reported a decrease in S100A11 in trophoblast differentiation dysfunction in early abortion placenta [ 98 ]. Subsequent research in 2012 demonstrated that knocking out S100A11 in mouse models during pregnancy impacts embryo implantation, negatively affecting endometrial receptivity and the expression of immune response-related factors [ 99 ]. Recent findings suggest that Circular RNA forkhead box P1 (circFOXP1) can enhance the expression of S100A11 by targeting miR-143-3p, thereby regulating the viability, apoptosis, migration, invasion, and epithelial–mesenchymal transition process of trophoblast cells [ 100 ]. The expression of S100P in the placenta of patients with SA is significantly lower compared to pregnant women with normal pregnancies [ 101 ]. Choi and colleagues suggested that decreased S100P gene expression is associated with poor embryo implantation [ 102 ]. Previous research has demonstrated the crucial role of S100P in embryo implantation [ 103 ]. Moreover, incorrect positioning of S100P within cell substructures may be linked to miscarriage and infertility, making accurate localization of S100P in the nucleus critical for successful embryo implantation [ 104 ]. Furthermore, S100P not only slows down the accumulation of lipid droplets during trophoblast syncytium formation in early pregnancy [ 105 ], but also influences this formation process by regulating Yes-associated protein 1 (YAP1) [ 106 ]. This section highlights several alarmins with substantial potential value based on limited studies. First, heat shock proteins (HSPs) are protective proteins found in organisms that enhance cellular tolerance to various adversities. They are also expressed in human reproductive tissues, particularly the placenta [ 107 ]. Sotiriou’s study found that the expression levels of HSP70 and HSP90 in the chorionic villi of early-stage missed pregnancies were higher than those in full-term placentas [ 108 ]. This suggests that these proteins may play a key role in miscarriage. Compared to ectopic pregnancy, the serum levels of HSP-10, HSP-27, and PSG-11 were significantly higher in women at risk of miscarriage [ 109 ]. Peng et al. also found that the apoptosis rate and mRNA expression of the HSP70 gene were significantly higher in the SA group than in the artificial abortion group, indicating that HSP70 may contribute to SA by promoting apoptosis [ 110 ]. If the ratio of HSP60 to HSP70 reaches or exceeds 6 before 12 weeks of pregnancy, the likelihood of miscarriage increases [ 111 ]. Second, serum amyloid A (SAA) is a protein involved in immune regulation during the acute-phase response [ 112 ]. It is released in response to infection, inflammation, or trauma [ 113 ]. SAA levels are closely related to reproductive system diseases and maternal health [ 114 – 117 ]. Studies indicate that SAA may be a potential risk factor for SA [ 118 ] and can be used to detect danger signs during pregnancy [ 119 ]. SAA1, as a trigger of aseptic inflammation in fetal membranes during delivery, can upregulate a series of chemokines through TLR4 and formyl peptide receptor 2 (FPR2), inducing various chemokine pathways [ 120 ]. Measuring SAA in early pregnancy can serve as a simple, fast, and non-invasive method to help predict abortion and determine whether it is caused by infectious factors [ 121 ]. It is important to note that although elevated SAA indicates inflammation, high levels do not always signify the presence of inflammation or disease. Literature reviews suggest that maternal dietary patterns high in animal protein, cholesterol, and/or low in fiber may lead to elevated plasma SAA levels [ 122 ]. Finally, as early as 1997, the presence of cell-free fetal DNA (cffDNA) in maternal blood was discovered, highlighting its potential in non-invasive prenatal diagnosis [ 123 ]. Increased cffDNA levels often accompany pregnancy complications such as preeclampsia [ 124 ], intrauterine growth restriction [ 125 ], and premature birth [ 126 ]. Studies also show that cffDNA levels tend to increase in miscarriage patients [ 127 ].

Alarmins are pivotal in SA, with their interaction with PRRs being essential for immune response modulation, inflammation regulation, embryogenesis, and miscarriage prevention. Specifically, HMGB1, IL-33, IL-1β, and S100 proteins are involved in embryo implantation and establishment of immune tolerance by regulating maternal angiogenic factors. Moreover, alarmins, as mediators of sterile inflammation, negatively impact pregnancy via their pro-inflammatory properties. Investigating the pro-inflammatory mechanisms not only elucidates the embryo’s recognition and response pathways during early gestation, but also may identify novel therapeutic targets for improving pregnancy outcomes (see Table  1 ). Future studies should further explore the effects of age, BMI, and AMH levels on the role of alarmins in SA to develop more individualized treatment strategies. In addition, given the potential role of alarmins in other pregnancy complications, further studies of their mechanisms in conditions such as placental abruption, fetal growth restriction, and preeclampsia are needed to fully understand this area. Through multi-angle and multi-level research, we are expected to gain a deeper understanding of the mechanism of action of alarmins and their targeted therapeutic strategies, ultimately enhancing clinical outcomes related to pregnancy. Table 1 The mechanism and targeting drugs of alarmins induced SA Alarmins Mechanism Drugs HMGB1 Promote the production of inflammatory cytokines through RAGE/TLR2,4/NF-κB pathway [ 31 ] Regulation of Th1/Th2 balance at maternal/fetal interface [ 29 ] HMGB1 induces an inflammatory response at the utero-placental interface by inducing autophagy [ 32 ] Inhibition of placental growth factor (PIGF) expression [ 135 ] Aspirin, LMWH, rTM, GL [ 162 ] IL-33 Failure of IL-33/ST2L/sST2 axis to activate normally causes miscarriage [ 54 ] IL-33/ST2–axl–efferocytosis axis imbalance induces metabolic reprogramming of decidual macrophages, leading to miscarriage [ 50 ] Promotes angiogenesis and vascular leakage through the ST2/TRAF6–Akt–eNOS signaling pathway [ 136 ] LMWH [ 163 ]; tacrolimus [ 164 ] IL-1β High expression of IL-1β can disrupt Th1/Th2 balance, leading to miscarriage [ 81 ] IL-1β promotes angiogenesis and changes vascular permeability [ 136 , 139 ] Anakinra [ 165 ]; canakinumab [ 166 ] S100s The overexpression of S100A8 and S100A9 will lead to the accumulation of white blood cells at the maternal–fetal interface, reduce uterine and placental blood flow, cause thrombus, and lead to hypoxia of placental tissue [ 92 ] S100A8 affects embryo implantation and increases the risk of miscarriage [ 93 ] S100A13 affects neovascularization and regulation of vascular stability [ 140 ] Paquinimod, tasquinimod [ 167 ] HSP The expression levels of HSP70 and HSP90 were high in the chorionic membrane of early embryo closure [ 108 ] Serum levels of HSP-10 and HSP-27 increased in women with miscarriage [ 109 ] Geldanamycin [ 168 ] SAA SAA1 induces multiple chemokine pathways through TLR4/FPR2 during delivery [ 120 ] Atorvastatin [ 169 ] cffDNA cffDNA levels tend to increase in patients with miscarriage [ 127 ] HCQ [ 170 ] The mechanism and targeting drugs of alarmins induced SA Promote the production of inflammatory cytokines through RAGE/TLR2,4/NF-κB pathway [ 31 ] Regulation of Th1/Th2 balance at maternal/fetal interface [ 29 ] HMGB1 induces an inflammatory response at the utero-placental interface by inducing autophagy [ 32 ] Inhibition of placental growth factor (PIGF) expression [ 135 ] Failure of IL-33/ST2L/sST2 axis to activate normally causes miscarriage [ 54 ] IL-33/ST2–axl–efferocytosis axis imbalance induces metabolic reprogramming of decidual macrophages, leading to miscarriage [ 50 ] Promotes angiogenesis and vascular leakage through the ST2/TRAF6–Akt–eNOS signaling pathway [ 136 ] High expression of IL-1β can disrupt Th1/Th2 balance, leading to miscarriage [ 81 ] IL-1β promotes angiogenesis and changes vascular permeability [ 136 , 139 ] The overexpression of S100A8 and S100A9 will lead to the accumulation of white blood cells at the maternal–fetal interface, reduce uterine and placental blood flow, cause thrombus, and lead to hypoxia of placental tissue [ 92 ] S100A8 affects embryo implantation and increases the risk of miscarriage [ 93 ] S100A13 affects neovascularization and regulation of vascular stability [ 140 ] The expression levels of HSP70 and HSP90 were high in the chorionic membrane of early embryo closure [ 108 ] Serum levels of HSP-10 and HSP-27 increased in women with miscarriage [ 109 ]

The term alarmins, coined by J. Oppenheim in 2005, defines endogenous, constitutively expressed, chemotactic and immune-activated protein/peptides that are secreted or released due to degranulation, cell injury or death, or immune-induced response [ 1 ]. By interacting with chemotaxis and pattern recognition receptors (PRRs) such as toll-like receptors (TLRs), receptor for advanced glycation end products (RAGEs), alarmins stimulate immune cells in host defense and play the role of intercellular signal defense. In cases of severe injury and maximum stimulation, excessive release of alarmins can cause significant damage to the body [ 2 ]. Therefore, alarmins are also referred to as damage-related molecular patterns (DAMPs) [ 3 ], and these terms are used interchangeably. Abortion is the termination of a pregnancy before the fetus is viable. According to the World Health Organization (WHO), abortion is categorized into spontaneous abortion (SA) and induced abortion. SA refers to the death and expulsion of the fetus before 20 weeks of gestation due to natural causes such as chromosomal abnormalities, maternal health issues, or environmental factors. Induced abortion is the artificial termination of pregnancy through medical or surgical means. This review specifically examines SA and its pathogenesis, focusing on the significant role of alarmins in this process to offer insights into potential therapeutic interventions. The risk of experiencing a miscarriage in a pregnancy is estimated to be 15.3% of all confirmed pregnancies, with a 95% confidence interval ranging from 12.5% to 18.7%. The overall prevalence rate of a single miscarriage in women was found to be 10.8% (10.3–11.4%), while the prevalence rate of two miscarriages was 1.9% (1.8–2.1%) [ 4 ]. When a woman experiences two or more miscarriages, it is commonly referred to as recurrent spontaneous abortion (RSA) [ 5 ]. It is worth noting that approximately 50–75% of RSA patients endure both mental and physical distress as a result of these miscarriages [ 6 ]. The association between SA and alarmins was investigated to gain new insights and enhance understanding of pregnancy failure. Existing research on SA primarily examines traditional immune cells and cytokines [ 7 – 9 ]. However, considering that alarmins act as early mediators of the immune response, they may have a significant impact on establishing maternal immune tolerance to the embryo. Studying alarm hormones can provide insights into how the mother identifies and reacts to the embryo in early pregnancy, as well as how the immune system can be adjusted during this time to optimize pregnancy outcomes. Additionally, inflammation and stress responses play a crucial role in embryonic development, with alarmins being key regulators of inflammatory responses and cellular stress [ 10 , 11 ]. Investigating these molecules and their impact on pregnancy may uncover new therapeutic targets for reducing the risk of miscarriage. Furthermore, alarmins not only activate the immune system to combat infection and maintain tissue balance, but can also trigger excessive immune responses that result in damage to the embryo [ 2 , 12 ]. The study of this dual action can contribute to a better understanding of the intricate immune balance during pregnancy and how immune responses can be adjusted without affecting normal pregnancy. In addition to immune factors, individual characteristics such as age, body mass index (BMI) and anti-Mullerian hormone (AMH) levels are also important factors affecting the risk of miscarriage. Age is an important independent risk factor for miscarriage, and studies have shown that the risk of miscarriage rises significantly as a woman ages [ 13 ]. The relationship between BMI and pregnancy outcome is complex, and both high and low BMI may increase the risk of miscarriage [ 14 ]. AMH, an important marker of ovarian reserve, is also strongly associated with the risk of miscarriage, and low levels of AMH generally predict a poor pregnancy outcome [ 15 ]. The interaction of these traditional risk factors with the immune system may work together to influence pregnancy outcomes. Besides SA, alarmins may also have a connection with other pregnancy complications, including placental abruption [ 16 , 17 ], fetal growth restriction [ 18 ], and preeclampsia [ 19 , 20 ]. Investigating the role of these molecules in different complications may uncover shared pathological mechanisms. The reason why alarmins are important as upstream molecules in signaling pathways is that they are the first line of stress response molecules and can quickly respond to tissue damage or infection and initiate an immune response [ 2 , 21 ]. Combined with various PRRs, such as TLRs and NLRs [ 2 , 22 ], they can trigger multiple signaling pathways, leading to the activation of various immune cells, the production of cytokines and chemical factors, and other immune regulatory processes [ 1 , 12 ]. By exerting local and systemic effects, alarmins have the ability to modulate the maternal immune environment, consequently impacting pregnancy outcomes. As the relationship between SA and alarmins remains largely unexplored, this field of research holds significant potential for further development and advancement.

Alarmins are integral components of sterile inflammation, referring to endogenous molecules released due to tissue and cellular damage in the absence of pathogenic infection [ 144 ]. These molecules can activate inflammatory responses and play a crucial role in immune modulation under non-infectious conditions, significantly impacting embryo implantation and the maintenance of pregnancy. The correlation between alarmins and sterile inflammation is paramount for elucidating the mechanisms underlying SA, optimizing therapeutic strategies, and enhancing pregnancy outcomes, as illustrated in Fig.  2 . Fig. 2 Alarmin-mediated molecular signaling pathways and targeted therapeutic drugs related to spontaneous abortion. Different types of alarmins mediate the same signaling pathways through different PRRs, leading to inflammation and miscarriage. The solid line indicates that it has been confirmed in SA, while the dashed line has not yet been confirmed in SA Alarmin-mediated molecular signaling pathways and targeted therapeutic drugs related to spontaneous abortion. Different types of alarmins mediate the same signaling pathways through different PRRs, leading to inflammation and miscarriage. The solid line indicates that it has been confirmed in SA, while the dashed line has not yet been confirmed in SA Firstly, research has demonstrated that HMGB1 binding to the RAGE receptor [ 145 , 146 ] facilitates the secretion of inflammation-associated cytokines and mediators via the MAPK pathway. This revelation paves the way for novel therapeutic strategies involving MAPK inhibitors to attenuate HMGB1-induced cytokine release, presenting new therapeutic potential for SA patients. Secondly, IL-33 exhibits dual roles during pregnancy. It can perturb macrophage equilibrium, resulting in miscarriage, while simultaneously fostering the proliferation and invasion of decidual stromal cells through the NF-κB and MAPK pathways, thus sustaining normal pregnancy [ 147 ]. Moreover, it can incite inflammatory responses in immune cells in patients with hepatic disorders [ 148 ]. Thirdly, it is significant that IL-1β in the bloodstream originates not only from immune cell secretion, but also from tissue cell expression. IL-1β from tissue cells exerts an effect akin to TNF-α in promoting inflammation [ 149 ]. Unmodified autocrine IL-1β and GSDMD both activate Caspase-1 cleavage via the NLRP3 pathway [ 150 ]. IL-1β primarily binds to IL-1R1, activating the NF-κB pathway and initiating an inflammatory response [ 151 , 152 ]. Additionally, it has been identified that S100A8/A9 contributes to cytokine production in osteoarthritis patients, enhancing the pro-inflammatory cytokine response by activating the NF-κB and MAPK pathways. It is noteworthy that NF-κB activation is not impeded by MAPK inhibitors [ 153 ]. S100A12 and S100B can bind to the RAGE receptor and relay signals to the NF-κB pathway [ 154 ]. Consequently, Sunahori et al. have proposed that RAGE may act as a universal receptor for the S100 protein family [ 153 ]. It has been discovered that HSPB8 (HSP22) serves as a ligand for TLR4 and is highly expressed in the synovial tissues of osteoarthritis patients [ 155 ]. Additionally, SAA not only binds to RAGE and TLR2/4, but also to FPR, instigating inflammation [ 156 – 158 ]. It is crucial to further investigate whether these receptors exhibit cell-specific expression. Moreover, cffDNA can activate the NF-κB pathway by binding to TLR9, leading to adverse pregnancy outcomes. However, this effect is mitigated by oral administration of the TLR9 inhibitor chloroquine [ 159 ]. Current understanding suggests that cffDNA-triggered TLR9 activation is associated with maternal inflammation, vascular dysfunction, and hypertension [ 160 ]. Previous studies have shown elevated TLR9 expression in the placenta of preeclamptic women [ 161 ]. Currently, drugs that target HMGB1 include aspirin, low molecular weight heparin (LMWH), recombinant thrombomodulin (rTM), and glycyrrhizin (GL) [ 162 ]. Drugs that target IL-33 such as LMWH [ 163 ] and tacrolimus [ 164 ] (see in Fig.  2 ). However, several other alarmin-targeting drugs have shown efficacy in other diseases, but the evidence for the prevention of SA is insufficient. As an IL-1 receptor antagonist, anakinra inhibits inflammation by blocking IL-1β from binding to its receptor [ 165 ]. Canakinumab is a monoclonal antibody that specifically targets and inhibits the biological activity of IL-1β [ 166 ]. Paquinimod can inhibit the activity of S100A9 and reduce inflammation. Tasquinimod, which also inhibits S100A9, is primarily used to treat prostate cancer and has anti-inflammatory properties [ 167 ]. Geldanamycin is an HSP90 inhibitor that exhibits anti-tumor and anti-inflammatory properties by interfering with HSP90 function [ 168 ]. Statins, such as atorvastatin, reduce SAA levels and have anti-inflammatory properties [ 169 ]. Hydroxychloroquine (HCQ) may play a role in reducing cffDNA-associated inflammation through immune regulation [ 170 ]. The study of alarmins and SA remains in the exploratory phase, with a limited number of available therapeutic options. Metformin, a widely utilized hypoglycemic agent for the treatment type 2 diabetes and has been shown to reduce miscarriage rates in patients with polycystic ovary syndrome (PCOS). Studies have shown that metformin may help prevent SA by improving insulin sensitivity and reducing inflammatory markers [ 171 ]. Polyphenols are known for their antioxidant and anti-inflammatory properties and are widely found in fruits, vegetables and tea. Studies have found that polyphenols are able to protect endometrial cells and embryos from damage by reducing oxidative stress and inflammatory responses [ 172 ], and therefore have the potential to be candidates for preventing SA. This presents new opportunities for improving outcomes for patients experiencing SA and serves as a focal point for future research.