Patients with CHI exhibit significant pathological changes in placental function, and multiple studies have confirmed its association with fetal growth restriction (FGR). Research indicates that the incidence of fetal growth restriction among CHI patients ranges from 51.6% to 73%. Clinical observations of 111 CHI patients revealed that the rate of FGR was significantly higher than that of the control group (70.4% vs. 0.9%, p < 0.001) (13). An analysis of 69 CHI cases from 1977 to 2009 revealed an FGR incidence of 66.7%, with a live birth rate of less than 54%. Similarly, a Dutch cohort study found that among 38 CHI patients, the incidence of FGR was 51.6%. These cases were also associated with microstructural abnormalities, including placental vascular lesions and fibrosis in the chorionic mesenchyme.

CHI is correlated with fetal miscarriage. Statistics indicate that the rate of spontaneous miscarriage among CHI patients ranges from 9.5% to 33%, with early miscarriages occurring more frequently than late ones. A pathological analysis of 178 placentas in France revealed that 73% of CHI cases were associated with fetal growth restriction, with 9.5% resulting in miscarriage (5). A study involving 69 pregnancies with CHI found that the rate of early spontaneous miscarriage was 30.4%, while the rate for late miscarriage was 13.0% (14). An analysis of 151 CHI cases in France from 2000 to 2020 indicated that early miscarriages constituted 20.0%, while late miscarriages accounted for 4.8% (5). Additionally, the occurrence of CHI is often associated with recurrent miscarriages. In a study conducted in Leiden, Netherlands, the miscarriage rate among 38 women with CHI reached as high as 42% (15).

The syncytiotrophoblast (STB) develops from the fusion of trophoblast cells in the blastocyst’s outer layer, serving as the core functional unit of the maternal-fetal interface. It is central to nutrient and material exchange in the placenta, secreting hormones like human chorionic gonadotropin (hCG) and progesterone to maintain placental development and function. Importantly, the STB also plays a crucial role in immune tolerance and regulation at the maternal-fetal interface, In the placentas of CHI patients, STB was observed in the inflammatory lesion region (31). Previous studies found that normal syncytiotrophoblasts do not express MHC class II molecules. However, recent research has shown that MHC class II molecules could be detected on syncytiotrophoblasts layer in a pathological condition, which may underlie immune recognition and inflammatory responses in CHI (32).

CD200 is a type I membrane glycoprotein widely expressed in fetal-derived placental trophoblast cells (33). At the maternal-fetal interface, the high expression of CD200 on fetal membrane glycoproteins binds to CD200R on the surface of maternal myeloid immune cells, inhibiting the NF-κB/MAPK pathway through phosphatase cascade. Thereby, the signals break the maternal immune system (34). It inhibits dendritic cell maturation and antigen presentation, drives macrophages toward anti-inflammatory M2 polarization (35). simultaneously downregulates Th1-type pro-inflammatory factors (IFN-γ, TNF-α), and upregulates Th2-type anti-inflammatory factors IL-10, thereby inducing Treg expansion and Th2 shift (33). The CD200/CD200R axis also synergizes with molecules such as indoleamine 2,3-dioxygenase (IDO) and FasL to form a fetal “invisibility cloak,” preventing maternal rejection (36). The expression level of the anti-inflammatory molecule CD200R in CHI placental villous trophoblast cells is significantly reduced, which may be associated with abnormal Treg proliferation and immune homeostasis imbalance at the maternal-fetal interface (37).

Besides, the exonuclease CD39 is an immune signaling molecule highly expressed on the surface of trophoblast cells. CD39 hydrolyzes extracellular pro-inflammatory ATP/ADP to generate AMP, which is further converted by CD73 into the immunosuppressive adenosine, thereby establishing a “high adenosine-low ATP” microenvironment at the maternal-fetal interface (38). Adenosine is the core molecule in the formation of an inhibitory immune homeostasis. Adenosine binds to the A2A receptor on maternal immune cells, inhibiting NK cell cytotoxicity, T cell activation, and DC maturation, while promoting Treg expansion and M2 macrophage polarization (39, 40). It also synergizes with tolerance molecules such as PD-L1 and HLA-G to prevent maternal rejection of the semi-allogeneic fetus (41). The reduced expression of the immunosuppressive enzyme CD39 in the trophoblast cells of the placenta disrupts the immune tolerance at the maternal-fetal interface (42).

Additionally, the elevated expression of the adhesion molecule ICAM-1 in the trophoblast cells enhances the adhesion capacity of macrophages, facilitating their accumulation in the intervillous space (43). Furthermore, the increased expression of the inflammatory receptor TLR1 in the trophoblast layer promotes the formation of inflammation in the intervillous space (44).

In CHI placental lesions, trophoblast cells induce large numbers of CD68⁺ monocytes in the villous spaces, which is also the primary pathological feature of CHI.

Macrophages are one of the primary immune cells at the maternal-fetal interface. Research indicates that in the CHI placenta, macrophages primarily originate from maternal circulation rather than from fetal Hofbauer cells. The infiltration of macrophages predominates in this lesion, accounting for 80% of inflammatory cells (45). Macrophages play crucial roles in antigen presentation, immune modulation, and the repair of inflammatory damage. In the CHI lesion, macrophages display a mixed M1/M2 polarization phenotype. The M1 subset typically expresses high levels of pro-inflammatory factors such as Tumor Necrosis Factor-alpha (TNF-α) and Interleukin-6 (IL-6), while the M2 subset suppresses local T cell activation by secreting Interleukin-10 (IL-10) and Transforming Growth Factor-beta (TGF-β) (46). Furthermore, studies on CHI have found that macrophages synthesize and release Interleukin-1 beta (IL-1β) through the NLR family pyrin domain containing 3 (NLRP3) inflammasome pathway, leading to its maturation and release, which in turn induces placental inflammation (47). It is important to note that macrophages mediate the remodeling of the extracellular matrix and tissue repair during placental inflammation in preeclampsia (48), and they are also a significant source of proteins for collagen deposition and fibrosis fetal interface in parturition and preterm birth (49).

Macrophages are the primary cells involved in immune regulation and antigen presentation. In classical immune responses, macrophages present antigens bound to MHC class II molecules to CD4⁺ T lymphocytes, which constitutes a critical step in the immune response.

As the most numerous immune cells in the body, T lymphocytes play a central role in immune recognition and response at the maternal-fetal interface (50). CD4⁺ T cells and CD8⁺ T cells are the effector cells responsible for self-limiting immune recognition of antigens, which trigger immune rejection responses. In CHI, there is a significant presence of CD4⁺ T cells and CD8⁺ T cells in the placental villous space, with their ratio being nearly 1:1.

A plethora of research has indicated that the expression of cytotoxic T lymphocytes (CTL), a population of immune cells differentiated from CD8⁺ T cells, is elevated in patients diagnosed with Chronic Hypertension in Pregnancy in comparison to levels observed during a normal pregnancy (51). During the immune induction phase of inflammation in CHI, CD8⁺ T cells may recognize paternal antigen-MHC I complexes presented by macrophages, which is similar to the classical immune recognition pathway. This recognition leads to their activation, proliferation, and differentiation into CTL cells, exerting cytotoxic effects. CTL cells could induce target cell apoptosis by expressing perforin and granzyme B through the Fas/FasL pathway.

Besides, CD4⁺ T cells play the core role in immune transplant rejection. CD4⁺ T cells are activated by recognizing MHC-II molecule-antigen complex and differentiates into Th1 lymphocytes, which assist CD8⁺ T cells in exerting cytotoxic functions. The expression of MHC II-antigen complex by trophoblasts in CHI is a potential activation binding site for CD4⁺ T cells. CD4⁺ T cells may play a role in CHI similar to that in liver transplant rejection, where IFN-γ and TNF-α secreted by Th1 cells exacerbate the inflammatory response, disrupt immune tolerance, and trigger a cascade amplification of the inflammatory response (52).

Additionally, Treg cells are vital for the development and maintenance of placental function. The appropriate number and functionality of Tregs are critical for achieving maternal-fetal immune tolerance. Insufficient Treg numbers and activity can lead to complications such as recurrent miscarriage and preeclampsia (53). Tregs bind to CD80 and CD86 on antigen-presenting cells (APCs) through cytotoxic T lymphocyte-associated protein 4 (CTLA-4), competitively inhibiting costimulatory signals for CD4⁺T cells (54). A significant infiltration of Tregs is observed in the CHI chorionic space, accompanied by reduced Foxp3 expression. This situation reflects a compensatory mechanism for the imbalance at the maternal-fetal interface and immune tolerance in CHI (55). Additionally, Treg cells secrete immunomodulatory factors like TGF-β to suppress the activity of CD4⁺ T cells, preventing maternal immune rejection of the fetus (56).

CHI placental pathology is multi-layered and multi-faceted, involving not only cellular changes but also significant molecular alterations.

IL-1β plays a crucial role in placental inflammation and is implicated in various pregnancy complications, particularly in placental immune regulation. It is likely that IL-1β is secreted by infiltrating macrophages and trophoblast cells via the NLRP3 inflammasome pathway. IL-1β promotes trophoblast invasion and angiogenesis by activating the PI3K/Akt-VEGF pathway. It also enhances maternal-fetal immune tolerance by inducing NK cells in the decidua to secrete IL-8 and GM-CSF (57). In addition, low concentrations of IL-1β upregulate HLA-G expression in trophoblast cells while simultaneously suppressing the excessive activation of maternal T cells (58).

In the placenta of CHI, the expression level of the IL-1β gene increases by 3.9 times, according to reference (59). Under pathological conditions (such as preeclampsia and chronic villitis), macrophages trigger the secretion and synthesis of sFLT-1 through a pro-inflammatory cascade involving NLRP3 inflammasomes and Gasdermin D (GSDMD) (60). These proteins antagonize the binding of VEGF-A and PlGF to the vascular endothelial cell receptor VEGFR-1/2, leading to increased NO synthesis, elevated ROS production, and heightened vascular permeability, thereby inducing vascular spasm (61). Excess placental soluble fms-like tyrosine kinase 1 (sFlt1) may contribute to endothelial dysfunction, hypertension, and proteinuria in preeclampsia. Additionally, IL-1β recruits CD8⁺ T cells and neutrophils, causing damage to the villi (57). This recruitment also stimulates prostaglandin synthesis, which can lead to preterm labor (62).

Once IL-1β presses the “start button” for inflammation, TNF-α immediately takes the stage, acting as an “accelerator” that amplifies and sustains this inflammatory storm.

Tumor necrosis factor-alpha (TNF-α) is a multifunctional pro-inflammatory cytokine that plays a role in immune regulation, apoptosis, and vascular function during both physiological and pathological processes in the placenta (63). In patients with chronic hypoxia-induced (including CHI) conditions, the TNF-α levels in placental tissue show a significant positive correlation with the risk of fetal growth restriction and preterm birth (64). TNF-α is primarily secreted by infiltrating macrophages and CD8⁺ T cells, with a smaller secreted by trophoblasts under stress conditions (65). In the placenta of cases with idiopathic FGR, TNF-α expression is significantly increased in trophoblastic giant cells and vascular endothelial cells. This increase may lead to fetal developmental restriction by inhibiting placental angiogenesis or directly damaging trophoblast function (66). In preeclampsia and gestational diabetes, elevated placental TNF-α levels is associated with insulin resistance, abnormal expression of advanced glycation end products and their receptor for advanced glycation end products. This pro-inflammatory microenvironment can induce localized oxidative stress and endothelial dysfunction in the placenta (67), which may lead to maternal hypertension and proteinuria (68).

TNF-α expression is essential for the formation of immune tolerance in early pregnancy. In the normal placenta development, TNF-α induces trophoblasts to express HLA-G, which inhibits the cytotoxicity of NK cells, thereby maintaining maternal-fetal immune tolerance (69). The soluble tumor necrosis factor receptor 1 (sTNFR1) secreted by placental tissue specifically neutralizes the pro-inflammatory effects of TNF-α, and this local immune regulation mechanism effectively suppresses autoimmune responses (70).

TNF-α-driven placental inflammatory signals can activate and amplify the complement system cascade reaction, which is a key marker of immune-mediated tissue damage.

C4d is a breakdown product activated by the classical complement pathway and is commonly associated with antibody-mediated immune responses (71). The deposition of the complement breakdown product C4d in placental inflammation is a significant pathological marker for pregnancy complications related to antiphospholipid antibodies (aPL) (72). Recent studies have shown that C4d plays a crucial role in organ transplant rejection and maternal-fetal immune tolerance (73). In the placenta of CHI, the deposition of C4d is distributed either diffusely or focally, and the amount of deposition is positively correlated with the severity of the disease (74). The abnormal expression of HLA class II molecules (such as HLA-DR and HLA-DQ) in placental trophoblast cells is significantly positively correlated with C4d deposition. This finding supports the idea that maternal anti-HLA antibodies may drive inflammatory responses and macrophage infiltration through the classical complement pathway (32).

The deposition of complement fragment C4d in CHI may be involved in the formation of pathological deposits in intercellular spaces due to the activation of the complement cascade (75). C4d is produced during the complement cascade and induces the formation of the membrane attack complex (MAC), which directly damages vascular endothelial and stromal cells while releasing anaphylatoxins C3a and C5a (76). Chemokines recruit monocytes, exacerbating local inflammatory responses. This leads to chronic inflammatory infiltration of the villous stroma and disruption of the epithelial barrier (77). Furthermore, C4d may enhance the pro-fibrotic microenvironment in conjunction with TGF-β. The deposition of C4d can directly promote the conversion of local fibrinogen to fibrin, exacerbating the formation of fibrotic networks in the villous space and the deposition of extracellular matrix, ultimately hindering maternal-fetal blood exchange (78).

In addition, although some studies have used C4d expression levels as an auxiliary diagnostic criterion, given lack of studies specifically investigating C4d as a diagnostic marker (27). the discriminatory value of C4d in CHI remains controversial (75). Along with C4d-mediated complement attack and inflammatory damage, changes in TGF-β signaling are also commonly observed in placental tissue. Changes in TGFβR1 expression and activity may reflect the body’s attempt to curb excessive inflammation and promote tissue repair.

In the placenta, transforming growth factor-β receptor 1 (TGF-βR1) acts as a receptor for transforming growth factor-β (TGF-β), playing a multifaceted role in regulating trophoblast differentiation, maintaining immune balance, and supporting angiogenesis to uphold pregnancy stability (79). TGF-βR1 and type II receptor (TβRII) are primarily expressed in the syncytiotrophoblast of the placenta, as well as in extravillous trophoblasts and the chorionic plate (80).

TGFβR1 is significantly upregulated in the placenta of CHI, and it may influence the pathological process of CHI by inhibiting trophoblast invasion, regulating maternal-fetal immune tolerance, and modulating inflammatory repair. It is a key cytokine that regulates the pathological changes in chronic placental inflammation (81). In preeclampsia, TGFβR1 is found to function in the remodeling of uterine spiral arteries by inhibiting trophoblast invasion (82). Besides, TGFβR1 plays an important role in immune tolerance and inflammatory repair at the maternal-fetal interface (83). In patients with recurrent miscarriage, the expression and activity of TGF-β are reduced, which can inhibit Treg proliferation and differentiation, leading to immune dysfunction at the maternal-fetal interface (84). The decreased ability of monocytes/macrophages to synthesize TGF-β1 fails to effectively suppress excessive inflammatory responses, leading to chronic inflammation (85). Furthermore, in the repair of inflammatory damage associated with chronic liver disease and pulmonary fibrosis, pro-inflammatory factors in the tissue microenvironment, such as TNF-α and IL-6, collaborate with TGF-β to induce fibroblast differentiation and extracellular matrix deposition (86).

Matrix metalloproteinases (MMPs) are a superfamily of proteases that depend on metal ions, such as zinc and calcium, as cofactors. They are capable of degrading critical components of the extracellular matrix (ECM), including collagen and elastin, leading to the disruption and remodeling of tissue structure, playing roles in placental development, immune regulation, and tissue remodeling (87). Tissue inhibitor of metalloproteinases (TIMPs) maintains tissue microenvironment homeostasis by inhibiting MMP-driven matrix degradation and excessive inflammatory responses, thereby preventing inflammation spread and tissue damage.

During tissue repair, MMP-2 participates in the clearance of damaged ECM, while TIMP-1 helps control the extent of degradation and promotes the deposition of new matrix (87, 88). Increased TIMP-1 activity is considered a key factor in promoting ECM accumulation and fibrosis formation (89). Abnormal MMP expression is the mechanism underlying the formation of massive perivillous fibrin deposition in CHI pathology (90–92). Additionally, TIMP-1 activation of the CD63/β1 integrin receptor complex in oligodendrocytes promotes the conversion of macrophages to an anti-inflammatory phenotype (M2 type), increasing IL-10 and TGF-β secretion while inhibiting the release of pro-inflammatory factors such as TNF-α and IL-6 (93).

In addition to MMPs, fibronectin is another important active molecule in the process of massive deposition around villi.

Fibronectin deposition is a biomarker for diagnosing placental diseases and predicting recurrences (94). Fibronectin is a core molecule of the coagulation system, formed from fibrinogen after activation by thrombin. Its dynamic balance is crucial for hemostasis, inflammation, and tissue repair (95). In the placenta, fibronectin moderately deposits in the intervillous space, covering approximately 5%-10% of the area. This forms a temporary scaffold that supports the branching of the chorionic tree and the structural integrity of the maternal-fetal interface (96). Fibronectin promotes the fusion of trophoblast cells into a multinucleated syncytiotrophoblast through integrin (αvβ3) signaling, enhancing hormone secretion (such as hCG) and barrier function (97).

A significant feature of CHI pathology is the massive perivillous fibrin deposition (MPFD) around the chorionic villi. In this context, fibrin serves both as a product of inflammation-coagulation cross-reaction and as a key mediator driving placental damage (98). The maternal interface immune rejection activates mononuclear-macrophages, which release pro-inflammatory factors such as IL-6 and IL-1β. This process leads to endothelial damage in the intervillous space and thrombin generation, subsequently promoting the conversion and accumulation of fibrinogen into fibrin (99). Additionally, the necrosis of placental trophoblasts releases cellular debris and mitochondrial DNA. This activates Toll-like receptor 9 and the complement system, such as C5a, enhancing the local coagulation cascade and exacerbating fibrin deposition (100). Furthermore, under chronic hypoxic conditions, HIF-1α promotes the synthesis of plasminogen activator inhibitor-1 and inhibits plasmin activity. This ultimately hinders the clearance of fibrin (101). Together, these mechanisms contribute to increased fibrin deposition around the chorionic villi, negatively affecting placental function.

The impact of fibrin deposition covering the placental intervillous space on villous function manifests in two ways: physical obstruction and disruption of cellular signaling. The physical barrier created by fibrin mechanically hinders the exchange of oxygen and nutrients between mother and fetus. It also obstructs the migration of trophoblasts and immune cells, thereby suppressing placental development and villous vascularization (102). In cases of chronic hypoxia-induced (CHI), the area of fibrin deposition is negatively correlated with the birth weight percentile of newborns, indicating that fibrin accumulation affects placental exchange efficiency (103). Pathological observations reveal that in severe cases of CHI, the area affected by fibrin can reach 40%-60% of the placental volume, leading to extensive placental infarction and functional impairment (104). Furthermore, fibrin recruit macrophages and stimulates the activation of complement components C3d and C4d in the intervillous space, exacerbating immune rejection and inflammatory responses at the maternal-fetal interface (1).

CHI pathological diagnosis is based on placental histological examination, with routine examination content including observation of placental tissue morphology, CD68+ histiocyte positivity screening, and examination of fibrin deposition around the villi. Quantitative immunohistochemical analysis of placental pathology in CHI showed that the density of CD68+ macrophages in the CHI group was 88 ± 23 per unit area (HPF 40×), while in the control group it was 8 ± 5 per unit area (P < 0.001) (45). CHI pathology is classified into three grades—Grade 1 (5%-10%), Grade 2 (10%-50%), and Grade 3 (>50%)—which exhibit a significant dose-response relationship with perinatal outcomes (P<0.0001). The neonatal survival rate for Grade 3 patients was only 16.1%, significantly lower than that for Grade 2 (59%) and Grade 1 (86.5%) (P = 0.0002) (45). The risk of disease recurrence for Grade 2 and Grade 3 patients was 3.8 times higher than that for Grade 1 patients. Pathological grading is of great significance for predicting pregnancy outcomes and recurrence (105).

Placental alkaline phosphatase (PLAP) is an enzyme specifically expressed in placental cells. It regulates active transport across cell membranes and calcium-phosphate metabolism, providing nutrition to the fetus. There is academic debate regarding the clinical significance of alkaline phosphatase (ALP) levels in CHI patients from different regions. A retrospective cohort study from Canada involving 33 patients found that 31.6% (10/33) of CHI cases exhibited elevated serum ALP levels (>125 U/L), suggesting that ALP could serve as a reference indicator for inflammatory activity (106). However, a prospective cohort study from Japan in 2017 (n=58) conducted a multivariable regression analysis and found no significant correlation between fluctuations in ALP levels related to CHI (elevated group vs. normal group) and adverse pregnancy outcomes such as fetal growth restriction (OR = 1.32, 95% CI 0.75-2.34) or preterm birth (OR = 1.15, 95% CI 0.82-1.61) (p>0.05). This study also emphasized that the clinical value of PLAP as a specific diagnostic marker for CHI needs to be validated through large-scale multicenter studies (107).

The role of HLA antibodies as biochemical markers for CHI remains contentious within the international academic community. Some studies report that anti-paternal HLA-I/II antibodies may be detected in the placental tissue of CHI patients, with positivity rates reaching up to 75% in certain case reports (40). HLA antibodies may be associated with fluctuations in the expression levels of HLA-B and HLA-DRB1 alleles in peripheral blood. Additionally, some cases exhibit abnormal deposition of complement C4d in placental tissue (107). However, a recent cohort study from the UK found no statistically significant difference in the overall positivity rates of anti-HLA antibodies between the CHI group and healthy controls, indicating that the diagnostic specificity of a single HLA antibody marker has not yet met clinical requirements.