1. Introduction

Antiphospholipid syndrome (APS) is a complex clinical syndrome of an autoimmune nature, characterized by a pathological increase in antiphospholipid antibodies (APA) at the intersection of several medical disciplines. Its main clinical manifestations are recurrent venous and arterial thromboses, thrombocytopenia as some somatic diseases, and obstetric complications characteristic of pregnancy are habitual miscarriage, antenatal death, and premature birth. APS is a pathology related not only to obstetrics and gynecology, but also to rheumatology, neurology, and cardiology, which is associated with thrombotic conditions, microangiopathies leading to disability, reproductive losses, and hypercoagulation associated with an increased need for lifelong anticoagulants [7,10].

According to world statistics, AFA is detected in approximately 1-5% of the healthy population, while in this category of patients, pronounced clinical symptoms manifest in 30-40% of cases. APS is most often observed in sexually active women aged 20-40 years, primary APS occurs not only in women, but also in men, its ratio is approximately 5:1. Among them, CAPS is a rare, rapidly progressive, multi-organ generalized thrombotic microangiopathy, which can lead to multiple organ failure. CAPS occurs in less than 1% of all APS cases, with a mortality rate of up to 50%. Surgical interventions, hormonal therapy, and discontinuation of anticoagulant therapy are significant triggering factors in its development [13,17].

About 25% of typical miscarriages of unknown etiology are associated with APS, which once again confirms the importance of the pathology for the reproductive sphere. In addition to miscarriage, APS can be complicated by premature birth, fetal death in the uterus, placental dysfunction, and congenital malformations. In APS, recurrent thrombosis can significantly reduce the patient's quality of life and lead to the need for long-term use of anticoagulants, as well as disability of patients due to increased risk of ischemic stroke and myocardial infarction against the background of arterial thrombosis. Official statistical data on the prevalence of APS in Uzbekistan are limited, but it has been confirmed that the available data are close to global indicators [8,14,16].

APS occurs on the basis of complex mechanisms of pathogenesis. In this case, it can also occur due to the formation of antiphospholipid antibodies (APA), disruption of anticoagulant mechanisms, and stimulation of genetic predisposition as a result of certain trigger factors.

1. Formation of antiphospholipid antibodies. APA - antibodies to cardiolipin, β2-glycoprotein I, and lupus anticoagulant, which are at the center of a number of immune pathologies. They bind to cell membrane phospholipids, increase the activity of endothelial cells, activate platelets and monocytes, and enhance the external and internal mechanisms of blood clotting. APA increases the expression of adhesive molecules such as E-selectin, VCAM-1, and ICAM-1, enhancing platelet and leukocyte adhesion and aggregation [9,16].

2. Disruption of anticoagulant mechanisms. APA reduces the activity of natural anticoagulants protein C, protein C, and annexin B. This increases the risk of thrombosis.

3. Genetic predisposition. The etiology of APS has not been fully established, but the significance of genetic factors has been proven. In particular: the alleles HLA-DR4, HLA-DR7, HLA-DRw53 increase the predisposition to APS and occur in 10-15% of cases of habitual miscarriage and antenatal mortality.

4. Trigger factors, viral and bacterial infections, certain medications (chlorpromazine, hydralazine), surgical interventions, severe injuries can trigger the development of APS by activating autoimmune reactions.

The significance of APS during pregnancy is associated with its increased hemokoagulation. During pregnancy, APA causes placental dysfunction through the following mechanisms:

1. Disruption of trophoblast invasion. APA acts on trophoblastic cells: suppresses invasion, disrupts the physiological remodeling of the spiral arteries, and leads to placental ischemia.

2. Effect of β2-glycoprotein I. It limits the development of syncytiotrophoblasts and leads to the following complications: preeclampsia, habitual miscarriage, antenatal fetal death.

3. Activation of the complement system. APA activates the C5b-9 complement complex, causing local inflammation, microthrombosis, and hypoperfusion in the placenta.

4. Atherogenesis and the risk of arterial thrombosis. APA acts on low-density lipoproteins, accelerating the development of atherosclerosis and increasing the predisposition to arterial thrombosis.

5. Autoimmune hemolysis. Under the influence of APA, platelets and erythrocytes can be destroyed, leading to hemolytic anemia and thrombocytopenia.

Non-criterial manifestations: reticular livedo, migraine, transient ischemic attacks. It was shown that the types of APA are associated with thrombotic and obstetric complications in some patients even when the classical laboratory criteria were negative. This leads to the formation of the concept of "seronegative APS," which justifies the need for the use of an extended immunological panel in diagnosis. The main clinical manifestations of seronegative APS are practically indistinguishable from classical APS. Patients experience recurrent venous and arterial thromboses, microcirculatory disorders, thrombocytopenia, as well as obstetric complications such as habitual miscarriage, antenatal fetal death, severe and early-onset preeclampsia, and placental insufficiency. This circumstance is the basis for assessing seronegative APS as a clinically dangerous and often late-diagnosed pathology. Changes in the titer of antiphospholipid antibodies over time, limited sensitivity of laboratory tests, and a transient decrease in antibodies during analysis are also important in the development of seronegative APS. In some cases, antibodies may be temporarily undetectable against the background of infections, pregnancy, or immunomodulatory therapy. Thus, seronegative antiphospholipid syndrome is a clinically significant, but less studied form of antiphospholipid syndrome, requiring early diagnosis and individualized preventive approaches in obstetric practice. In-depth study of this issue is of great importance in reducing reproductive losses and preventing severe thrombotic complications.

CAPS manifests as acute thrombotic microangiopathy with damage to the brain, heart, lungs, kidneys, and other organs. The clinical presentation may be similar to the clinical picture of "acute abdomen" syndrome or disseminated intravascular coagulation (DIC) [11].

The diagnosis of APS is based on a combination of clinical and laboratory criteria defined by international recommendations (Saporo criteria updated in 2006 and ACR/EULAR criteria, 2020). At least one clinical and one laboratory criterion is required for diagnosis.

Laboratory criteria

1. Volchan anticoagulant (VA): determined using coagulation tests that show an increase in the time of irreversible blood clotting when mixed with donor plasma [126; 97-118].

2. Cardiolipin antibodies (CLA): IgG/IgM, detected twice at intervals of 12 weeks in medium/high titers using enzyme immunoassay (ELISA).

3. Antibodies to β2-glycoprotein I: IgG/IgM, as well as ELISA with the same interval.

In the studies of Miyakis et al. (2016), the revision of the Saporo criteria serves to standardize the diagnosis of APS. In this study, clinical and laboratory criteria were determined, which became an important basis for the correct diagnosis of APS. According to this author's proposal, triple positivity (VA+AKL+anti-β2-GP1) is associated with a high risk of thrombosis and obstetric complications. It has been proven that new methods, such as immunochemiluminescent analysis (ELISA) and linear immunological analysis (LIA), increase diagnostic sensitivity [5].

Pierangeli S.S., in a study conducted in 2023, studied the morphological and morphometric features of APS. This includes instrumental methods, namely vascular Doppler ultrasonography, MRI/CT for detecting thrombosis, and neuroimaging in brain manifestations. Pathomorphological examination of biopsies (skin, kidneys) confirmed the presence of thrombotic microangiopathy without significant inflammation of the vascular wall. APS is distinguished by systemic vasculitis, hereditary thrombophilia, DIC syndrome, and infectious diseases (tuberculosis, hepatitis). Strokes or repeated miscarriages are detected in 10-20% of cases of APS in the early reproductive period [4].

Prevention of APS is aimed at reducing the risk of thrombotic and obstetric complications. The main approaches include:

1. Primary prevention:

In patients with AFK without clinical manifestations, with a high risk of thrombosis (triple positive test), low-dose aspirin (75-100 mg/day) is recommended.

- Control of modifiable risk factors: hypertension, dyslipidemia, smoking.

2. Secondary prevention:

Patients with venous thrombosis are recommended lifelong treatment with targeted vitamin K antagonists (varfarin) - IHM 2.0-3.0.

In obstetric APS: a combination of low molecular weight heparin (PMH) and aspirin in low doses is used.

In CAPS: emergency therapy with glucocorticoids, plasmapheresis, and intravenous immunoglobulin is recommended. Recent studies have shown that complement inhibitors (ekulisumab) are promising for the prevention of thrombosis in CAPS.

- Direct oral anticoagulants (rivaroksaban) are being studied, but their effectiveness in APS is limited due to the high risk of recurrence.

3. Risk monitoring and differentiation are carried out:

- Regular determination of AFL titers and monitoring of the Ministry of Public Education.

- Classification of patients into high and low risk groups based on the AFL profile.

In studies conducted by Galli et al. (2203), it was shown that antibodies against lupus anticoagulant and β2-glycoprotein I correlate with the risk of thrombosis to a high degree. This study was an important step in the laboratory diagnosis of APS [6].

Optimization of prevention requires an interdisciplinary approach, including rheumatologists, hematologists, and obstetrician-gynecologists, as well as personalized strategies, taking into account the clinical and laboratory profile of the patient.

Impact of APS on implantation and pregnancy complications antiphospholipid syndrome and miscarriage

Antiphospholipid syndrome (APS) affects a number of biological mechanisms that are crucial for maintaining pregnancy, leading to disruption of the implantation process and the development of various obstetric complications. The main pathogenetic point observed in APS is a disruption of cytotrophoblast invasion, which disrupts the sequence of physiological processes necessary for the normal development of the placenta, placental perfusion, and fetal viability. For perfect implantation and adequate placental formation, deep invasion of the trophoblast into the deciduated endometrium and physiological transformation of the spiral arteries are necessary. In APS, antiphospholipid antibodies (aFL) disrupt these mechanisms in several ways.

1. Direct cytotoxic effect on the trophoblast. Antiphospholipid antibodies bind to phospholipid complexes in the membranes of trophoblast cells, activating apoptosis processes, suppressing the proliferative activity of the trophoblast, and disrupting syncytialization. These changes reduce the invasive potential of the trophoblast, make implantation clinically unfavorable, and contribute to the termination of pregnancy in the early stages.

Disorders of the mechanisms of adhesion and invasion. Adhesive molecules and proteolytic enzymes are important for the penetration and migration of the trophoblast into the decidual layer. The following changes are observed in APS. Decreased expression of adhesion molecules: the expression of integrins, cadgerins decreases, and the ability of the trophoblast to strengthen in the endometrium is weakened.

A decrease in the activity of proteolytic enzymes leads to a decrease in the expression of matrix metalloproteinases (MMR-2 and MMR-9), which leads to the loss of the trophoblast's ability to destroy and deeply invade tissue structures. An increase in the expression of TIMP-1 and TIMP-2, an increase in the activity of protease inhibitors causes an imbalance between MMR/TIMP and leads to a further decrease in invasion.

Disorders of physiological transformation of the spiral arteries. Insufficient invasion of the trophoblast leads to the preservation of the muscular layer of the spiral arteries. This condition is characterized by narrowing of the arteries and high resistance. As a result, chronic placental ischemia, fetal growth retardation, preeclampsia, and early gestational losses can develop.

2. Pathological activation of hemostasis and inflammatory systems. Autoantibodies in APS interact with trophoblasts and endothelium and participate in the formation of a prothrombotic environment through the following mechanisms. This is accompanied by activation of the complement system (especially the C5a component), increased expression of tissue factor (TF), an increase in adhesive molecules such as VCAM-1, ICAM-1, E-selectin, platelet activation, and increased thromboxane A2 synthesis. This leads to the formation of microthrombi in the lumens of the vessels of the basal plate and placental vessels, which in turn leads to further deterioration of placental perfusion.

Endothelial dysfunction. aFL causes damage to endothelial cells, exacerbating the thrombotic process through vasoconstriction, microcirculation disorders, and the release of inflammatory mediators. This creates the basis for a more severe course of perinatal complications and periodic relapses characteristic of APS.

In a study conducted by Rand et al. (2020), the interaction between aFL and annexin B was studied in depth. Anneksin B forms a natural anticoagulant layer on the placental surface. As a result of AFL disrupting the stabilization of this layer, anticoagulant protection decreases, a prothrombotic environment is created on the surface of the trophoblast, and microthromboses develop. These circumstances confirm that thrombotic complications in APS are the main pathogenetic mechanism.

The role of endothelial dysfunction in the formation of fetal loss in AFS

The pathophysiology of obstetric complications associated with antiphospholipid syndrome (AFS) involves a complex of multifactorial, interrelated, and mutually reinforcing processes. Among them, endothelial dysfunction and disturbances in the hemostasis system are of particular importance as a central pathogenetic link. These processes form a "pathological circle" that leads to thrombotic microangiopathy, placental insufficiency, impaired placental perfusion and, as a result, termination of pregnancy. Endothelial layer of uterine and placental vessels is the main target for pathogenic effect of aFL. Whereas the endothelium normally has anticoagulant, vasodilatory, and anti-inflammatory properties, in AFS it switches to a procoagulant, vasoconstrictor, and pro-inflammatory phenotype. A major pathology in AFS is associated with endothelial dysfunction. It develops based on the following mechanism. Direct connection of AFL with endothelial cells αFL–β₂-glycoprotein I (β₂-GPI) complexes bind to special receptors on the surface of endotheliocytes — annexin A2, β₂-GPI V-domain receptors, as well as tool-like receptors such as TLR4 and TLR6. This interaction activates key intracellular signaling pathways—NF-κB and p38/MAPK cascades. These cascades lead to the activation of endotheliocytes, as a result of which their physiological anticoagulant functions are weakened, and a prothrombotic environment is formed and blood thickens. It causes the activation of inflammatory cytokines and the induction of a procoagulant state. It is accompanied by increased expression of adhesion molecules (VCAM-1, ICAM-1, E-selectin) in the endothelium, increased leukocyte adhesion and transmigration, increased synthesis of pro-inflammatory cytokines (TNF-a, IL-1b, IL-6), increased expression of endothelial and monocyte tissue factor. These conditions lead to inflammation and increased thrombogenic environment. On the other hand, it has a negative effect on the weakening of the physiological anticoagulant system. In AFS, several joints of the anticoagulant system are derailed. In this, the expression of thrombomodulin and endothelial protein-S receptor (EPCR) decreases, resistance to active protein S (APC-R) occurs under the influence of aFL. The decrease in the activity of protein C and C limits the inactivation of factors Va and VIIIa, as a result of which the activity of antithrombin III, which has an anticoagulant effect, decreases. These changes dramatically activate the extrinsic pathway of blood coagulation and accelerate the formation of fibrin clots by increasing thrombin generation. In addition, activation of platelets and formation of procoagulant microvesicles αFL–β₂-GPI complexes bind to receptors such as ApoER2′, GPIb-IX-V on the surface of platelets, which leads to changes such as activation of platelets, increased aggregation, release of procoagulant substances from granules, formation of procoagulant microvesicles. These microvesicles provide a phospholipid platform for assembly of coagulation complexes, resulting in attenuation of fibrinolysis.

An increase in the level of PAI-1 (plasminogen activator inhibitor) reduces fibrinolytic activity, makes it difficult for clots to break up, and the prothrombotic environment is further strengthened. On top of that, as a result of a violation of the synthesis of vasoactive substances - a decrease in the synthesis of nitric oxide (NO) and prostacyclin (PGI₂), the vasodilation and antiaggregant effect decreases. The synthesis of endothelin-1 (ET-1) increases and vasospasm increases. These changes increase local ischemia and further reduce placental perfusion.

Immunological mechanisms of pathogenesis of antiphospholipid syndrome

The pathogenesis of antiphospholipid syndrome is characterized by a complex interaction between innate and adaptive immunity, which ultimately leads to the loss of protective functions and the predominance of the pathological effects of antiphospholipid antibodies. Although the mechanisms that trigger the production of AFL are not fully understood, dysregulation of T- and V-cell junctions of immunity plays a key role. The main event is the activation of autoreactive V-cells that produce pathogenic aFL. The main antigen targets for AFL are not phospholipids, but plasma proteins that bind to them, the main one being β2-glycoprotein I. This molecule normally exists in an inactive form in plasma, but upon contact with activated cell membranes, changes its shape and forms an antigenic determinant recognized by autoreactive V-cells. This process leads to their proliferation, differentiation into plasma cells and mass production of autoantibodies of the IgG class with a high affinity for β2-GPI. Without cooperation with T-helpers (CD4+), activation of B-cells and production of high-affinity IgG is impossible. Dendritic cells present modified β2-GPI peptides in complex with MHC II T-lymphocyte molecules, which leads to clonal expansion of T-helpers, resulting in the formation of a pathological autoimmune response. The pathogenicity of AFL is determined by its ability to recognize β2-GPI molecules. αFL-β2-GPI complexes bind to receptors on the surface of endothelial cells and activate intracellular signaling cascades. This induces a procoagulant phenotype of the endothelium, increases the expression of adhesion molecules and tissue factor, secretes pro-inflammatory cytokines and chemokines, and decreases the activity of endogenous anticoagulant systems. Under the influence of AFA, receptors are activated, platelets and other blood clotting factors are activated in the endothelium, blood thickens, and anti-inflammatory interleukins are activated. In this case, inflammatory factors damage the trophoblast, microthrombosis develops in the inter-villi space and blood vessels in the area of the basal plate, blood circulation is restricted, placental hypoxia develops, oxidative stress develops, and as a result, a miscarriage is observed. These pathogenetic mechanisms cause obstetric complications such as recurrent miscarriage, severe preeclampsia, fetal growth retardation, and premature placental abruption. Endothelial dysfunction is the main pathogenetic link of AFS, causing disorders in the blood coagulation system, thrombotic microangiopathy, and insufficient placental perfusion, which is the morphological basis of miscarriage, preeclampsia, and other severe obstetric complications. The understanding of these mechanisms justifies the use of not only anticoagulant, but also endothelioprotective therapy.

Immunohistochemical characteristics of the endometrium in spontaneous miscarriage against the background of the pathology of antiphospholipid syndrome

Although the hormonal, immunological, metabolic causes associated with antiphospholipid syndrome have been studied to date, there are very few sources in the world literature about its histological and especially immunohistochemical analysis. Therefore, in this thesis, we decided to investigate the causes of AFS at the tissue and cellular level. It is known that AFS is directly related not only to defects in the implantation process, but also to defects in early placentation and embryogenesis. This factor alone creates the need for deep morphological research, structural and functional study of AFS. Immunohistochemical analysis allows assessment of subtle molecular changes at the cellular level that cannot be detected by conventional histological analysis. The most important immunohistochemical changes are mainly related to the process of trophoblast fusion. It evaluates changes in cell adhesion-related molecule expression. In a 2024 study conducted by the German Association of Reproductive Medicine, endometrial biopsies from women with AFS miscarriage showed decreased expression of the integrins αvβ3 and α4β1 responsible for the formation of the receptive field, resulting in delayed pinopodia formation, resulting in impaired trophoblast nidation. In addition, a decrease in the expression of L-selectins and their ligands is detected, which prevents trophoblast adhesion to the endometrium.

Hughes G.R. (2019) found that pathology associated with angiogenesis disorders in the endometrium and placenta is the pathomorphological basis of AFS miscarriage. In the immunohistochemical analysis carried out, the hypothesis that miscarriage may occur due to the imbalance between proangiogenic and antiangiogenic factors was confirmed. It was confirmed that the expression of VEGF, VEGF-1 and CD31 in the abortive material and in the samples taken from the endometrial layer and the placenta is significantly decreased, which leads to a decrease in the processes of vascular remodeling, as a result of which hypoxia in the fetal-maternal-placental system, together with an increase in the expression of sFLT-1 and endoglin (CD105), leads to the formation of local ischemia and a violation of decidual tissue perfusion, as a result of which the chorion and early it creates an unfavorable microenvironment for the development of the placenta, and at a later stage, it was found to cause the death or miscarriage of the fetus [3].

In AFS, the expression of CD56dim NK-cells, an inflammatory cytokine with cytotoxic activity against the endometrium, increases, the number of CD68-positive macrophages reflecting local inflammation increases, the expression of TNF-α and IFN-γ increases, the level of IL-10 decreases, and a shift of the cytokine profile towards the dominance of Th1-reactions is observed. The predominance of Th1-cytokines leads to trophoblast damage, increased apoptosis and disruption of decidual tissue formation, which seriously affects the maintenance of early pregnancy. In parallel with this process, there are changes in the processes of proliferation and apoptosis, as a result of which the expression level of Ki-67, ER, PR, BCL-2 decreases, and the proapoptotic BAX marker increases several times compared to the norm, which disrupts endometrial regeneration and functional maturity and reduces receptivity. In addition, due to the decrease in the amount of L-selectin, blood coagulation accelerates, fibrin expression increases, due to the increase in the amount of VCAM-1 and ICAM-1, microthrombosis and hypercoagulation occur, as a result of which this leads to the obliteration of blood vessels that supply the fetus with nutrients and blood, and the formation of microinfarcts in the placenta. Underlying these processes is endothelial dysfunction, which leads to the initiation of a cascade of other processes due to local hypoxia. Identified immunohistochemical changes allow us to consider normal miscarriage in AFS as a multi-component process associated with joint disorders of endothelial function, angiogenesis, local immune response and cell adhesion, and these markers are used for the selection of individual treatment tactics, including anticoagulant, immunomodulatory and anticytokine therapy, and early detection of endometrial failure in predicting the risk of reproductive losses.

Erkan et al. (2024) and co-authors' large-scale study in Turkey focused on an in-depth analysis of the role of the complement system in the pathogenesis of catastrophic antiphospholipid syndrome (KAFS) and is one of the first fundamental works in this direction. In the study, it was shown that the course of KAFS with rapidly developing multithrombotic processes is closely related to the excessive activation of the complement cascade. In particular, it was noted that the activation of complement fragments at the level of C5 acts as a trigger for extensive damage to the endothelium, the appearance of microthrombosis in tissues, and a sharp disruption of organ function. On this basis, the researchers evaluated the drug eculizumab, a C5 inhibitor, as a potential therapeutic strategy in KAFS. Clinical observations and late phase analysis showed that pathological activation of the complement cascade was inhibited when eculizumab was used, a sharp decrease in the formation of microangiopathic thromboses, stabilization of hemodynamic indicators and a significant reduction in episodes of organ dysfunction were observed. Importantly, a 21.0–29.0% reduction in the incidence of KAFS was noted in patients receiving eculizumab therapy. This data confirms the importance of this drug as rescue therapy and suggests that the use of complement inhibitors in the clinical management of catastrophic AFS may be one of the effective directions in the future [2].

Meroni W.W. (2018) investigated the relationship between inflammation and thrombosis in AFS. It was shown that aFL activates endothelial cells and induces inflammatory processes through the production of cytokines. Violation of placentation is observed due to endothelial dysfunction, and local inflammation prevents adequate remodeling of spiral arteries by cytotrophoblast. The arteries become narrow and unable to supply the blood needed by the growing fetus. Ischemia and oxidative stress occur due to decreased perfusion in chronic hypoxia of the placental tissue, which causes oxidative stress, further damages the trophoblast and endothelium, and enhances the pathological process. Thus, endothelial dysfunction and systemic disorders of hemostasis are integral and mutually aggravating pathogenetic links of AFS during pregnancy. Primary damage to the AFL endothelium triggers a cascade of reactions in the hemostasis system toward hypercoagulation. The resulting placental thrombotic microangiopathy and placental disruption is a direct morphological substrate for the development of obstetric complications such as recurrent miscarriage, severe preeclampsia, fetal growth retardation, and premature migration of a normally located placenta. This understanding justifies the use of not only anticoagulant but also potentially endothelioprotective therapy in patients with AFS [1].

2. Conclusions

The analyzed literature sources describe antiphospholipid syndrome (AFS) as one of the leading immunopathological causes of normal miscarriage in women of reproductive age. AFS is a multifactorial autoimmune syndrome that causes obstetric complications due to the disruption of blood coagulation system, endothelium and trophoblast functions in connection with the pathological production of antiphospholipid antibodies (anti-cardiolipin, anti-β₂-glycoprotein I, volchanochnyy anticoagulant).