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Introduction

Antiphospholipid syndrome (APS) is an autoimmune disorder whose clinical manifestations are characterized by arterial and/or venous thrombosis, pathological pregnancy and persistent positive antiphospholipid antibodies (aPLs). A patient can be diagnosed with APS when at least one clinical criterion and at least one laboratory criterion are present (1). Clinical criteria include pregnancy pathology or vascular thrombosis (2). Laboratory criteria include the presence of titers or high titers of IgG and/or IgM in anti-cardiolipin (aCL) and/or lupus anticoagulants on at least two occasions, at least 6 weeks apart. The Sapporo criteria (also known as the Sydney Criteria), revised in 2006, added IgG/IgM anti-β2glycoprotein 1 (anti-β2GPI) antibody testing to the laboratory criteria, and the positive detection interval was extended from 6 weeks to 12 weeks(2). 'Non-standard' manifestations include thrombocytopenia, APS-associated nephropathy reticulum and cognitive impairment (3). In addition, 'non-standard' autoantibodies such as IgA isotypes (IgA aCL and IgA anti-β2GPI) have also been reported, but their clinical relevance is still controversial (3,4).

To date, the pathogenesis of APS has remained elusive. Although the role of T helper (Th) cells in the production of aPL antibodies has long been recognized (5), there are less data on the role of T cells in the development of APS. Studies have shown that the activation of T lymphocytes and the production of corresponding cytokines can be induced in patients with APS through different ways, which can cause an imbalance among immune cells, immune molecules and the immune system, and damage immune homeostasis in vivo (6), which is related to thrombosis or pathological pregnancy. It has been reported that vitamin D (VD) has a pleiotropic effect, such as anti-oxidation, preventing inflammatory response, reducing immune-mediated damage and anticancer (7-10). Low serum VD levels are associated with autoimmune diseases (11). In vitro studies have shown that high VD levels directly inhibit T-cell proliferation and cell-cycle progression, inhibit the release of pro-inflammatory factors and transform the Th to the Th2 phenotype (12). This suggests that VD is essential for regulating T cells. In patients with APS, decreased serum VD levels are significantly correlated with thrombosis or obstetric complications (13-17), but their relationship with T cells remains to be further studied. Therefore, it is required to pay attention to the role of the relationship between VD and T cells in APS, which provides a relevant basis for the prevention and treatment of APS-related pathological pregnancy and thrombosis. In the present article, the relevant literature was reviewed to understand the potential mechanistic role of VD in APS protection. At the same time, this review summarizes the progress of VD in the treatment of APS and its possible adverse reactions, providing new insights for the selection of adjuvant therapy for APS.

Epidemiology of APS and VD insufficiency or deficiency

The estimated annual incidence of APS is 21 cases per 100,000 individuals, with a prevalence of 50 cases per 100,000 individuals (18). The male-to-female ratio of primary APS is ~1:3.5, and secondary APS is mainly systemic lupus erythematosus (SLE)-related, with a ratio of 1:7 (18). Catastrophic APS accounts for <1% of all APS cases (19). In the normal adult population, the optimal concentration of serum 25-hydroxyVD [25(OH)D] is 30-50 ng/ml (75-125 nmol/l), while concentrations indicating insufficient VD are 20-30 ng/ml (50-75 nmol/l). VD deficiency is defined as levels <20 ng/ml (<50 nmol/l) (1). The reported prevalence of 25(OH)D levels <50 nmol/l (or 20 ng/ml) is estimated to be 24% in the US population and 37% in Canada, with an even higher prevalence in Europe, reaching ~40% (20-23). In a survey of the normal adult population, 67.4% of women had VD deficiency and ~87% of women were affected by VD insufficiency, whereas ~41% of men had VD deficiency, with prevalence rates varying across studies (24). In patients with APS, the total prevalence of VD deficiency and insufficiency was reported to be 32.2 and 61.5%, respectively. The incidence of thrombotic events in VD-deficient patients with APS was 62.9% and the incidence of adverse pregnancy outcomes was 49.5% (13,25).

VD and its receptor

VD is a lipophilic steroid hormone, mainly in the forms of VD2 and VD3 (26). VD2 is synthesized by the ultraviolet irradiation of yeast ergosterol, a compound known as ergocalciferol (27). While it cannot be synthesized in the human body, it can be obtained through food sources, such as edible fungi and supplements (28). VD3 is synthesized in the human body by ultraviolet irradiation of 7-dehydrocholesterol in the skin into cholecalciferol. It can also be obtained from fatty fish, eggs and dairy products. VD is absorbed, transported to the liver and hydroxylated, mainly by 25-hydroxylase, and converted to 25(OH)D, which is subsequently bound to VD-binding proteins and transported to the kidney to form 1,25(OH)2D via the action of 1-α25 hydroxylase. Researchers have emphasized that 1,25(OH)2D can also be synthesized through autocrine and paracrine processes in other body tissues (25); 1,25(OH)2D is the active form of VD, which functions through diffusion or endocytosis at target organ sites (29,30). In addition, circulating 25(OH)D is moderately lipophilic, tightly binds to VD-binding proteins, is prevalent at low concentrations, and is present in two structurally similar forms: 25(OH)D3 and 25(OH)D2, but these two molecules are not as well detected as 25(OH)D because of their low levels in the blood (31).

The biological effects of VD are mainly mediated by the nuclear VD receptor (VDR), a member of the nuclear hormone receptor family and a high-affinity ligand-activated transcription factor (32). As a signaling molecule, VDR binds to 1,25(OH)2D and the heterodimer partner, the retinoid X receptor. This complex interacts with specific DNA sequences in the promoter regions of target genes, known as VD response elements, to regulate their expression (33). VDR is distributed in various cells and tissues throughout the body, such as small intestinal epithelial cells, endothelial cells, bone osteoblasts, the large intestine, distal renal tubules and parathyroid glands (34-36). VDR is expressed in almost all immune cells, including monocytes, macrophages, T and B lymphocytes and DCs, where it regulates the differentiation and proliferation of immune cells. However, its expression levels vary, which indicates that VD is involved in the regulation of inflammation and immune responses (37). T cells are one of the target cells of VD, and VD deficiency can lead to immune disorders, increasing the expression of procoagulant factors, proinflammatory cytokines, adhesion molecules and tissue factors (TF) (36). Simultaneously, all these intermediates are associated with thrombus formation and pathological pregnancy through mechanisms such as increased platelet reactivity, endothelial dysfunction and activation of the coagulation cascade and complement system (36). Therefore, VD plays an important role in the regulation of the immune balance and may have therapeutic potential in the pathophysiology of APS-related thrombosis and obstetric complications.

VD plays a role in maintaining immune balance in T cells and other immune cells

VD is known to differentially regulate innate immune cell subsets; influence cell maturation, metabolism and antigen presentation; and regulate cytokine production (38-40). Immature DCs usually express more VDR, whereas mature DCs show reduced VDR expression (38). This may indicate that mature DCs produce VD locally upon activation and that VD then acts on immature DCs to regulate the immune response (38,41) (Fig. 1); 1,25(OH)2D can inhibit DC maturation markers, such as CD80/CD86 and CD83 (41,42), increase the production of IL-10 and reduce proinflammatory cytokines (42,43). Thus, 1,25(OH)2D promotes an immature, tolerant DC phenotype (44,45), thereby reducing antigen presentation to T cells; 1,25(OH)2D primarily regulates the proliferation of T and B cells, but studies have found that it plays a more important role in regulating T-cell phenotypes (46,47). After binding to autoantigens, autoantibodies can be presented to CD4+ Th cells together with major histocompatibility complex class II molecules to stimulate T-cell activation and the adaptive immune response; 1,25(OH)2D reduces the cytotoxic activity of T cells by downregulating the expression of Fas ligands (48). VD also has various effects on Th cells, such as promoting CD4+ Th-cell differentiation, resulting in a reduction in Th1 and Th17 cells (49). Th1 and Th17 subgroups play key roles in different autoimmune diseases by releasing cytokines that drive inflammatory responses. In addition, 1,25(OH)2D reduces the production of Th1-related cytokines [IL-2, tumor necrosis factor (TNF)-α and interferon (IFN)-γ] and Th17-related cytokines (IL-17, IL-21, IL-22) by inhibiting Th1 and Th17 cells (50) (Fig. 1). Simultaneously, CD4+ T cells are polarized toward the Th2 phenotype by upregulating cytokines, such as IL-4 and IL-5 (50). By enhancing the expression of forkhead box P3, the differentiation of Treg and CD4+ T cells involved in maintaining immune tolerance is induced, while levels of IL-10 and transforming growth factor β1 (TGF-β1) are also increased (50,51), thereby reducing the inflammatory response (Fig. 1). Furthermore, Th cells reduce the formation of autoantibodies by inhibiting the differentiation and proliferation of B cells (Fig. 1). The aforementioned findings indicate that VD can prevent the development of autoimmune diseases by regulating the balance between T-cell subpopulations (Th1/Th2/Th17/Treg) (Fig. 1).

Figure 1 Immunomodulatory balance of T-cell subsets with sufficient VD. After VD and 1,25D bind to the related receptors of DCs, under the action of different cytokines, Th0 cells can promote the differentiation of Th1, Th2, Th17 and Treg cells, but Th2 and Treg cells are the most numerous. At the same time, various cytokines are secreted and the expression of Th2 cytokines and Treg cell cytokines is upregulated. The expression of Th1 and Th17 cytokines was downregulated. At the same time, Th2 cells can inhibit the overactivation of B cells and reduce the formation of autoantibodies. The red T-shaped symbol represents inhibition. The upward red arrow represents upregulation. The downward red arrow represents downregulation. Treg, T-regulatory; Th1, type I T-helper; DC, dendritic cell; VDR, VD receptor; VD, vitamin D; TCR, T-cell receptor; Fox, forkhead box; 1,25D, 1,25(OH)VD.

Potential role of T-cell imbalance caused by insufficient or absent VD in APS pathologic pregnancy

Moderate VD levels are associated not only with pathogen clearance but also with immune regulation in normal pregnancy T-cell subsets. The Th1/Th2 immune response is generally characterized by immunoinflammatory changes that occur during embryo implantation. Th1 cells migrate into the decidua, which is critical for regulating trophoblast invasion and contributes to tissue remodeling and angiogenesis, thereby supporting pregnancy (52). Th1 cells secrete various cytokines, such as IL-2, TNF-α and IFN-γ, which, in addition to providing immune surveillance, prevent excessive invasion of trophoblast cells (53). Under normal conditions, moderate levels of TNF-α protect the placental unit and modulate the adhesion of trophoblast cells to laminin, thereby regulating trophoblast cell invasion (53). In addition to downregulating protease activity and extravillous trophoblastic (EVT) cell apoptosis to prevent excessive EVT invasion, IFN-γ can induce vascular remodeling during the implantation period to better nourish implanted cells (54,55). Therefore, IFN-γ plays a key role in early placental and trophoblast invasion. Toward the end of the implantation period, Th1 immune predominance in the decidua gradually shifts to Th2 predominance. Th2 dominance has been demonstrated during normal pregnancy (56). After Th2 cells infiltrate the decidua basalis, they induce local Th2 dominance through the release of Th2 cytokines (57), such as IL-4, IL-10 and IL-13, which inhibit the development of Th1 and Th17 immunity, promoting allograft tolerance, which favors pregnancy (58). Th17 cells are relatively rare (0.64-1.4%) in the peripheral blood of healthy individuals (59). In the decidua, IL-17+ T-cell counts parallel neutrophil counts, indicating that neutrophil infiltration is closely related to IL-17+ T cells (60), inducing protective immunity against extracellular microorganisms in the uterus, which is associated with immune protection and pregnancy. Following implantation, sufficient VD levels in the decidua promote the differentiation of T-cell subsets into Tregs, which maintain maternal tolerance by inhibiting cytotoxic T and Th1 cells. Tregs can suppress destructive immune responses and prevent autoimmune diseases during pregnancy (61). VD enhances the immunosuppressive function of Tregs; therefore, it can inhibit proinflammatory Th17 cells and related cytokines (such as IL-17), maintaining a Th17/Treg balance (62). This balance facilitates the secretion of growth factors and extracellular matrix remodeling, which supports trophoblastic infiltration, cytotrophoblast development and decidual vessel remodeling (63), thus supporting placental development.

In a healthy human body, sufficient VD is important for embryo implantation, mediating immune tolerance and promoting embryo growth. However, T-cell dysregulation caused by VD insufficiency or deficiency is closely linked to adverse pregnancy outcomes in patients with APS. Insufficient or deficient VD promotes the production of aPLs by B cells and disrupts immune balance, leading to the overexpression of Th1 cytokines (IFN-γ and TNF-α) and the failure of immune tolerance after implantation (64) (Fig. 2). TNF-α increases plasminogen activator inhibitor-1 levels derived from trophoblast cells, reduces trophoblast cell invasion, activates endothelial cells, induces matrix metalloproteinase-9 expression in the decidua and degrades the decidual extracellular matrix, which interferes with normal EVT invasion (65-67) (Fig. 2). IFN-γ has a powerful proinflammatory effect, stimulating chemokine secretion, activating macrophages and increasing phagocytosis, leading to maternal inflammation and subsequent fetal loss (Fig. 2). In addition, these cytokines mediate cytotoxic activity against target cells, which may damage placental vascular endothelial cells (64,68), leading to placental microthrombus formation and subsequent pathological pregnancies (Fig. 2). The high expression of Th1 cytokines in the placentas of patients with VD insufficiency or deficiency, particularly in preeclampsia, suggests that VD exerts a protective effect at the fetomaternal interface (61). After the implantation phase, if the Th1-cell immune advantage is not transferred to a Th2-cell advantage, pathological pregnancy may ensue. However, excessive activation of Th2 cells and the subsequent release of cytokines are not beneficial. The Th2 cytokine IL-4 can induce the activation of autoreactive B cells and the exacerbation of Th2 immunity during pregnancy may worsen autoimmune diseases (69) (Fig. 2). VD deficiency can also lead to abnormal function and reduced numbers of Treg cells in the decidua. This condition is related to insufficient infiltration of extravillous trophoblastic cells and poor remodeling of spiral arteries, which, in turn, results in unstable placental development and a 'shallow' placenta (63). Simultaneously, an overly robust Th17-cell response can induce the activation of decidual natural killer cells and impair the vascular reactivity of the uterine artery, leading to embryo absorption (70). A study showed that, compared with the normal pregnancy group, the proportion of Th17 cells was significantly increased in aborted mice, whereas the proportion of Treg cells and the levels of IL-10 and TGF-β1 were significantly decreased. Following the transplantation of Treg cells from normal pregnant mice into abortion-prone mice, the levels of TGF-β and IL-10 significantly increased, exerting a protective effect on the fetus (63). In addition, transplantation of CD4+ CD25+ Treg-deficient lymphocytes into pregnant T cell-deficient mice resulted in abortion (71). This finding suggests that the absence or insufficiency of Tregs can lead to abortion and that VD insufficiency or deficiency can cause an imbalance in Th1/Th2/Treg/Th17 cells and related cytokines in patients with APS, leading to adverse pregnancy outcomes (Fig. 2).

Figure 2 Potential mechanisms of pathological pregnancy caused by low VD levels in patients with antiphospholipid syndrome. When the VD level is low, it can lead to immune disorders, excessive Th1 and Th17 cells, and reduced Th2 and Treg cells, thus overactivating B cells and forming too many aPLs. At the same time, pro-inflammatory factors are too high and anti-inflammatory factors too low, and there is too much PAI-1 and MMP-9, thus leading to placental inflammation and fetal injury. This can eventually lead to loss of the fetus. The upward red arrow represents upregulation and the downward red arrow represents downregulation. PAI-1, plasminogen activator inhibitor-1. MMP-9, matrix metalloproteinase-9; VD, vitamin D; Treg, T-regulatory; Th, T-helper; Fox, forkhead box; aPLs, antiphospholipid antibodies; EVT, extravillous trophoblastic; IL, interleukin; IFN, interferon; TNF, tumor necrosis factor; TGF, transforming growth factor.

Potential role of T-cell imbalance in thrombosis in APS caused by VD insufficiency or deficiency

In patients with APS, VD deficiency may be closely associated with thrombosis. Normal levels of VD can inhibit endothelial cell angiogenic factors, modulate inflammatory activity, reduce endothelial damage and suppress tissue factor expression, thereby exerting antithrombotic effects. However, when VD deficiency occurs in patients with APS, B cells may become overactivated and differentiate into plasma cells, resulting in the production of more aPL (Fig. 3); aPL recognizes β2GPI and induces plaque-derived T-cell proliferation and increased IFN-γ expression, indicating Th1 cell activation in the artery walls of patients with APS. Local production of IFN-γ and TNF-α following Th1-cell activation may further stimulate these cells to present β2GPI to T cells, driving their differentiation into Th1 effector cells (72), thereby causing endothelial cell damage (Fig. 3). IFN-γ can also enhance calcium flux in neutrophils and the production of reactive oxygen species, leading to the formation of neutrophil extracellular traps and, ultimately, thrombosis (73). In a study examining inflammatory atherosclerotic infiltrates in patients with SLE-APS, researchers identified β2GPI-specific T cells. Among these, 42% were polarized Th1 cells, 38% were Th17/Th1 cells, 15% were polarized Th17 cells and 5% were Th0 cells. Notably, no T cells were polarized into Th2 cells (72). Th2-cell deficiency is an important risk factor for atherosclerosis. Furthermore, most Th cells specifically producing IL-17 in patients with APS were polarized Th17 cells. In the absence of VD, Treg cells lose their inhibitory ability and cannot control the proliferation of IL-17+ T cells, leading to excessive IL-17 production. IL-17 can induce endothelial cells to secrete proinflammatory cytokines (such as IL-6) and chemokines (including IL-8) and increase adhesion molecule levels, particularly in combination with TNF-α. This promotes leukocyte recruitment and endothelial-cell invasion (73,74). In addition, IL-17A promotes thrombosis by activating tissue factors and reducing anticoagulant mediators, such as thrombomodulin and CD55 (73) (Fig. 3). Deficiency in complement inhibitor CD55 disrupts the balance of the complement system, leading to its activation. When VD levels are <40 ng/ml, CD55 expression is downregulated, whereas complement factor B expression is significantly upregulated, indicating complement system activation and readiness for the membrane attack complex formation stage (75) (Fig. 3). Complement factor B, a component of the complement alternative pathway, is responsible for C3/C5 convertase activity. Its upregulation can lead to the excessive production of complement fragments C3a and C5a. In patients with APS, C5a mediates tissue factor expression via C5a receptors, promoting procoagulant activity (76). Simultaneously, the expression of complement component C9 is upregulated in response to reduced VD levels, and its sustained upregulation may increase the synthesis of C5b-9 (77). The release of C5a and the formation of C5b-9 result in endothelial cell activation, tissue factor expression, enhanced neutrophil-endothelial cell interactions, neutrophil extracellular trap formation and a lowered threshold for thrombosis. Severe cases may present with extensive microthrombosis, possibly associated with catastrophic APS (78,79) (Fig. 3).

Figure 3 Potential mechanisms of thrombosis caused by low VD levels in patients with antiphospholipid syndrome. aPLs, antiphospholipid antibodies; VDR, VD receptor; VD, vitamin D; IL, interleukin; IFN, interferon; TNF, tumor necrosis factor; TGF, transforming growth factor; C, complement; 1,25D, 1,25(OH)2D; Th, T-helper; Ne, neutrophil; TF, tissue factor; NETs, neutrophil extracellular traps; CFB, complement factor B; C5b-9, complement membrane attack complex.

The antithrombotic properties of VD have been studied previously; aPL induces endothelial cell activation through Toll-like receptor 4 (TLR-4)-mediated signal transduction and NF-κB activation, which is associated with APS thrombosis (80). Normal VD levels not only inhibit type I IFN synthesis by blocking TLR-4 signal transduction in inflammatory cells and MYD88 innate immune signal transduction adaptor (MyD88) signaling but also downregulate NF-κB and TF expression, thereby inhibiting thrombosis in APS (81,82). This evidence indicates that insufficient or deficient VD is related to APS thrombosis. In APS, aPLs can also upregulate angiogenic factors and adhesion molecules, such as intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1, also known as ELAM-1). VD can inhibit the expression of these molecules and reduce endothelial cell damage (83,84). To further confirm the role of VD, several studies have performed VDR knockouts in mice. After VDR knockout, platelet aggregation increased significantly, the expression of anticoagulant genes (including anticoagulant glucagon protein, antithrombin and thrombomodulin) was downregulated and the expression of the procoagulant TF gene was upregulated, increasing thrombus formation (85). In APS, the absence of VD weakens the peroxisome proliferator-activated receptor (PPAR)γ pathway, increases superoxide production and impairs endothelial progenitor cell repair capacity (86). Simultaneously, the weakened PPARγ pathway enhances angiotensin II-induced reactive oxygen species production and activates NF-κB. This activation induces cytokines, such as TNF-α, IL-6, ICAM-1, VCAM-1 and E-selectin, which promote vascular injury and thrombosis (36).

VD insufficiency or deficiency in autoimmune diseases

VD has immunomodulatory properties and sufficient VD generally exerts immunosuppressive effects. Autoimmune diseases are often triggered by external factors (including infections), leading to immune-cell dysregulation, particularly in T cells. This dysregulation results in an imbalance in T-cell subsets, with Th17 and Th1 cells and their associated proinflammatory cytokines becoming significantly elevated, causing excessive immune responses. This strong immune response can attack multiple organs or tissues simultaneously, causing systemic damage. When VD levels are low, autoimmune reactions may become further aggravated, leading to disease progression and exacerbation of clinical symptoms. In recent years, increasing attention has been paid to the impact of low VD levels on autoimmune diseases. Studies have demonstrated the importance of VD in diseases, such as SLE, type 1 diabetes mellitus, multiple sclerosis, autoimmune thyroid disease, rheumatoid arthritis and psoriasis (87-118). In addition to APS, the role of VD in these autoimmune diseases is summarized in Table I. It is worth noting that the type and dose of VD used in the study of different autoimmune diseases may be different (91,92,95,96,99,106,111,112,117).

Table I Role of VD in different rheumatic diseases.

Table I

Role of VD in different rheumatic diseases.

Disease	T cells in the pathogenesis of disease	Therapeutic mechanism of VD	Dose of VD	Efficacy
T1DM	When VD levels are low, Treg cells inhibit defects in the activity and proliferation of autoreactive CD4+ and CD8+ T cells (87). Promotion of the production of Th1 and Th17 cells, reduction of the ratio of Th1/Th2 and stimulation of the release of proinflammatory cytokines such as IFN-γ, IL-1β and TNF-α by immune cells. The differentiation of CD4+ T cells into Th2 and Treg cells was reduced and anti-inflammatory cytokines such as IL-4, IL-10 and TGF-β were weakened (88,89). This results in autoimmune destruction of β-cells in the islets.	Promotes CD4+ T cells to differentiate into Th2 and Treg cells, reduces the production of Th1 and Th17 cells and decreases the proportion of Th1/Th2. Stimulates immune cells to release anti-inflammatory cytokines while weakening the production of proinflammatory cytokines (89,90).	Oral cholecalciferol 4,000 and 10,000 IU/d or 70 IU/kg/d (91,92)	Significant improvement in glycemic control and preservation of pancreatic β-cell function, increase in regulatory T cells. Prevention of its related micro- and macrovascular complications (91-93).
AITD	The imbalance between Th1/Th2 cells and Th17/Treg cells mediates the production of Th1 cytokines (IL-2, IFN-γ and TNF-α), promotes the proliferation and differentiation of B cells into plasma cells and induces the expression of autoantibodies (50,94).	VD inhibits Th1-cell proliferation as well as the Th1-mediated cytokine production (IL-2, IFN-γ and TNF-α) and modulates Th2 cell and cytokines production (IL-4, IL-5 and IL-10) inducing the Th2 phenotype (50). Reduces the proliferation and differentiation of B cells into plasma cells (50).	Oral cholecalciferol(4,000 IU/d) or 50,000 IU VD (95,96).	Interferes with late activation of monocytes and T cells, significantly reducing TgAb and TSH levels (95,96).
PsO	The immune complex activates resident DCs and then releases IL-23 to activate T lymphocytes, promoting differentiation into Th17. IL-16 promotes differentiation into Th1. These cells produce three major cytokines, IL-17, IL-22 and IFN, which promote the proliferation of keratinocytes (97-99).	Inhibits the production of IL-17 and directly inhibits the proliferation of keratinocytes (100).	Oral 1α(OH)D (1.0 μg/d) or VD 5,000 IU/d (99).	Promotes the expression of anti-inflammatory factor (IL-10), decreases the PASI score and plasma homocysteine level and the expression of proinflammatory factors (TNF-α, IL-1β and IL-6) (100). It can also significantly improve psoriatic arthritis (101,102).
RA	T cells secrete cytokines to activate B cells and autoantibody production increases. The Th1- and Th17-cell response increases with the increase of the pro-inflammatory cytokine IL-17. In addition, TNF-α, IL-6 and IL-1 production increase, stimulating synovial cells (103).	VD can inhibit Th17-mediated inflammation and inhibit IL-6 and TNF-β levels, while Th17-inducing cytokines (IL-1β, IL-6 and IL-23) synergistically enhance the pro-regulatory effect of 1,25-(OH)2D3 on the T-cell phenotype (104,105). Decreased Th17-induced osteoclast activity and RA-related bone resorption (104).	8,000-50,000 IUs/wk of oral VD or equivalent or oral 1,25(OH) 2D (106).	Significant improvement in the DAS28, ESR and tender joint count but not in the pain VAS (106,107).
SLE	SLE is characterized by hyperfunction of Th1 and Th2 cells. Cytokines secreted by Th1 cells (TNF-α, IL-2, IFN-γ) participate in the activation of CD8+ T cells. Th2 cytokines (such as IL-4, IL-10, etc.) can cause excessive activation of B cells, produce autoantibodies and cause tissue damage (108,109). Th17/Treg cell imbalance allows Th17 cells to enter inflammatory tissues, such as the kidney, and promotes inflammation by increasing the production of cytokine (IL-17) (108,110).	Improved naïve CD4+ T cells, regulatory T cells; reduced Th1 and Th17 cells, memory B cells, anti-DNA antibodies (111).	Oral Cholecalciferol, 100,000 IU/wk or ergocalciferol, 50,000 IU/wk, calcium/VD 200 IU/twice daily. (111,112).	Decreased SLE disease activity and fatigue symptoms (113,114). Reduced SELENA-SLEDAI, decreased urine protein-to-creatinine ratio (111).
MS	When the VD level is insufficient, T-cell subsets are unbalanced, Treg-cell activity is inhibited, and Th1/Th17-cell activity is enhanced, which increases the release of pro-inflammatory factors and decreases the anti-inflammatory factors. These immune cells and inflammatory factors can damage the nervous system (115).	VD upregulates Treg-cell activity, downregulates Th1/Th17-cell activity, decreases the release of pro-inflammatory factors (IL-2, IL-17, IFN-γ and TNF-α) and increases the release of anti-inflammatory factors (IL-4, IL-10 and TGF-β) (115,116).	Oral 20,000 IU/d (117).	Slight trends of reduced annual recurrence, better MRI results and lower EDSS progression (117,118).

[i] AIH, autoimmune hepatitis; AITD, autoimmune thyroid disorders; DC, dendritic cell; RA, rheumatoid arthritis; PsO, psoriasis; MS, multiple sclerosis; T1DM, type 1 diabetes; VD, vitamin D; TgAb, antithyroglobulin; IL, interleukin; Th, T-helper; Treg. T-regulatory; TSH, thyroid-stimulating hormone; PASI, psoriasis area and severity index; TNF-α, tumor necrosis factor α; IL, interleukin; IFN, interferon; DAS28; Disease Activity Score 28; ESR, erythrocyte sedimentation rate; VAS, visual analogue scale; SELENA-SLEDAI, safety of estrogens in lupus erythematosus national assessment-systemic lupus erythematosus disease activity index; MRI, magnetic resonance imaging; EDSS, expanded disability status scale; wk, week; d, day.

Benefits of VD supplementation in APS

Since the discovery of the VDR in cells across various systems, the pleiotropic effects of VD have garnered much attention. VD supplementation is important for regulating the immune system in APS. In a study, patients were divided into two groups: One received an intensive cholecalciferol regimen (300,000 IU at baseline, maintained at 50,000 IU/month and 850,000 IU/year), and the other received a standard regimen (25,000 IU/month and 300,000 IU/year). The intensive regimen improved the balance of B- and T-cell homeostasis, with an increase in Tregs and Th2 cells and a decrease in Th17, Th1 and memory B cells (111,119). VD supplementation has been shown to correct immune dysregulation in patients with autoimmune diseases. In a VDR knockout mouse model, platelet aggregation was significantly increased, the expression of anticoagulant genes (including glucagon protein, antithrombin and thrombomodulin) was downregulated and the TF gene was upregulated (120). After the injection of exogenous lipopolysaccharide, VDR-knockout mice developed multi-organ thrombosis compared to wild-type controls, suggesting VD's potential as an antithrombotic agent (120); aPL antibodies, such as anti-β2GPI-Abs, can trigger the clotting cascade through multiple mechanisms, including inducing TF expression. VD may act as an immunomodulator with antithrombotic effects. One study evaluated 179 patients with APS and found that 49.5% of patients with APS had VD deficiency (serum levels ≤15 ng/ml), which was significantly associated with thrombosis (58 vs. 42%; P<0.05). Neurological and ophthalmic manifestations, pulmonary hypertension, reticular skin changes and skin ulcers were also significantly associated (121). In the same study, anti-β2GPI antibodies from four patients with APS were purified and co-cultured with VD [1,25(OH)2D, 10 nm]. VD inhibited anti-β2GPI antibody-induced TF expression. Furthermore, VD reduced the overexpression of adhesion molecules, such as VCAM-1 and ICAM-1, in cultures incubated with aPL IgG antibodies (121). However, the effects of VD supplementation in thrombotic APS have remained to be studied prospectively. Clinical trials investigating VD supplementation for thrombosis prevention have yielded conflicting results. For instance, in one trial involving patients with metastatic prostate cancer, those receiving high doses of calcitriol had significantly fewer thrombotic events compared to placebo-treated patients, even after adjusting for prior thrombotic history and anticoagulation therapy (122). Conversely, in the general population, large trials have not demonstrated any significant benefits of VD supplementation in preventing venous thromboembolism (123,124).

Currently, APS-related thrombosis therapy, intensive therapy with warfarin and other vitamin K antagonists (VKA), are standard treatments. For the first thrombotic event in APS, switching from a direct oral anticoagulant to warfarin or another VKA is recommended if aPL is triple-positive and direct oral anticoagulants are already in use. The first-line treatment for arterial and small-vessel thrombosis should be VKA (125). Low-dose aspirin and moderate-intensity anticoagulant therapy are recommended (126). For patients with recurrent thrombosis, heparin (including enoxaparin) at a dose of 1.5 mg/kg administered via subcutaneous injection is recommended (127), followed by anticoagulant therapy with warfarin, with a target international normalized ratio of 2.0 to 3.0 (128). Heparin not only inhibits the activation of T cells and neutrophils but also suppresses complement activation. VD has effects similar to those of an anticoagulant (including heparin), such as immune modulation, inhibition of T-cell overactivation and suppression of complement activation. In addition, supplementing VD as an adjunctive therapy for APS-related thrombosis has additional effects (125). The central role of inflammation in aPL-mediated thrombosis supports VD's potential as a treatment for APS (129). Among its many roles, VD has an immunomodulatory effect on inflammatory activity, inhibits the overactivation of monocytes, thereby reducing endothelial damage, and suppresses the expression of angiogenic factors and TF in endothelial cells (14,125). VD can also inhibit TLR-4, block the signaling pathway of the adaptor protein MyD88 and its downstream NF-κB in inflammatory cells, suppress inflammation and immune responses, and upregulate thrombomodulin (14,125), thus playing an antithrombotic role.

Regarding adverse pregnancies, the total prevalence of aPL was significantly higher in women with recurrent pregnancy loss (RPL) in the VD insufficiency or deficiency groups than in the normal VD group. VD deficiency is more common in patients with RPL having APS, and these patients are at greater risk of autoimmune abnormalities, including APS (130). For patients with insufficient or deficient VD combined with RPL, studies have found no significant difference in the proportion of Th1/Th2 cytokine-expressing CD3+/CD4+ Th cells when VD supplementation is administered compared with patients with normal VD levels combined with RPL (129). In another study, 35 patients with RPL and insufficient VD levels were treated with 0.5 μg/day of 1,25(OH)2D for 2 months. The proportion of TNF-α-producing Th cells decreased significantly after treatment compared with that before treatment (131); 1,25(OH)2D can reduce IFN-γ production and the number of IFN-γ+ CD4+ T cells (132). In addition, after treatment with 3.3 million IU of intramuscular VD injections in patients with RPL, the number of Th17 cells in the treatment group was significantly lower than that in the control group and the ratio of Th17/Treg cells decreased significantly more than that in the control group (133). In another study, patients with RPL having VD deficiency were administered 2,000 IU of oral VD capsules per day. After 2 months of treatment, the Treg/Th17 ratio was significantly higher than that in women who did not receive VD supplementation (134). These findings suggest that VD supplementation can provide immunomodulatory benefits in RPL and may serve as an alternative therapy for APS-related thrombosis and adverse pregnancies.

At present, there are certain limitations to the efficacy of VD supplementation in patients with ASP. First, of the existing articles, most of them focus on pre-clinical studies (in vitro studies and animal experiments), and there are also certain retrospective studies, but the changes in serum 25(OH)D levels after VD supplementation are mainly mentioned (135,136). There may be differences between preclinical studies and human studies, such as the impact on aPL (121,135), and retrospective studies often lack a lot of information, e.g. T-cell subsets were not reviewed to analyze which T-cell subtypes were affected by VD. Therefore, it is esteemed that there will be more large-scale prospective studies in the future to investigate the efficacy of VD in the treatment of patients with APS, particularly with regard to the effects of VD on T cells (both in thrombosis and obstetric complications). In addition, a meta-analysis was conducted on the basis of many large-scale prospective studies, so as to better evaluate the optimal dose range, safety and efficacy of VD in patients with APS, particularly the effects of correcting the T-cell imbalance after VD supplementation on thrombosis and obstetric complications, further supporting the present conclusions and making them more general.

VD-related adverse reactions

Due to the increasing awareness of VD's benefits, supplements are being used more widely. Currently, there is no evidence that increasing the daily dose of VD to 50 μg (2,000 IU) causes serious adverse effects in the general population. Considering that the minimum beneficial dose for bone health is 20 μg (800 IU), it is reasonable to recommend a daily dose of 20 to 50 μg (800-2,000 IU). These recommendations are supported by scholars, with evidence levels ranging from 2 to 4 (137). In 2011, the Institute of Medicine reported the upper limits of VD intake, highlighting the possible side effects of short-term high-dose VD use and long-term VD supplementation. However, toxicity may occur when serum 25(OH)D levels in patients are ≥150 ng/ml (375 nmol/l). Acute VD toxicity is usually caused by a serum 25(OH)D concentration of >150 ng/ml (375 nmol/l) following doses >10,000 IU/day. Long-term use of doses >4,000 IU/day may lead to chronic VD toxicity, resulting in 25(OH)D concentrations ranging from 50 to 150 ng/ml (138). The manifestations of VD poisoning are typically multisystemic and are primarily due to hypercalcemia. Signs and symptoms of poisoning are presented in Table II (118,139-141). Generally speaking, the more common side effects are on the gastrointestinal tract, such as vomiting, nausea, thirst, anorexia and constipation (118,139-141).

Table II Possible manifestations of vitamin D poisoning.

Table II

Possible manifestations of vitamin D poisoning.

Manifestations in different systems or organs	Symptoms or signs
Neuropsychiatric	Cognitive disturbances, drowsiness, confusion, apathy, psychosis, depression, stupor and coma
Gastrointestinal	Recurrent vomiting, abdominal pain, polydipsia, anorexia, constipation and peptic ulcers to pancreatitis
Cardiovascular	Hypertension, shortened QT interval, ST segment elevation, bradyarrhythmias, first degree heart block
Renal system	Polyuria, polydipsia, dehydration, hypercalciuria, nephrolithiasis, nephrocalcinosis and renal failure
Eyes	Keratopathy
Ears	Hearing impairment or loss
Joints	Periarticular calcinosis, pain, limited movement
Vascular system	Widespread vascular calcification

In a study from Brazil, 21 patients who took cholecalciferol supplementation for an average of 1 year (average dose, 87,000 IU/day) were enrolled. Of note, 17 patients experienced relapses, new MRI lesions or increased disabilities. The authors also observed direct side effects of VD supplementation, including gastrointestinal symptoms, seizures, severe hypercalcemia, kidney failure, kidney stones and renal calcification (142). In another case, a patient who received high-dose cholecalciferol for 20 months (78 million IU cumulatively, 130,000 IU/day on average) developed nausea, vomiting, muscle weakness, reversible hypercalcemia and acute kidney injury (143). In addition, high VD doses and specific treatment regimens may increase adverse effects. For instance, a study of 2,256 women >70 years of age administered 500,000 IU of VD per year (~1,400 IU/day) reported a 26% increase in fracture risk within the first 3 months after dosing (144). Similarly, annual intramuscular injections of 300,000 IU of VD (~820 IU/day) were associated with a 49% increase in hip fracture risk (145). A case study from Brazil reported a 19-year-old male hospitalized for anorexia, nausea and vomiting after consuming 300 ml of an enteral formulation containing vitamin A, D and E (totaling 5,000,000 IU of VD). Laboratory tests revealed a serum 25(OH)D level of 150 ng/ml, total calcium of 14.8 mg/dl (normal reference range: 8.4-11.0 mg/dl) and serum creatinine of 2.88 mg/dl (normal reference range: 0.9-1.3 mg/dl). Hypercalcemia and acute kidney injury were treated successfully with fluid rehydration, diuretics and zoledronic acid (146). In a randomized clinical trial, participants were divided into three groups: A low-dose control group receiving 24,000 IU/month of VD3, a group receiving 60,000 IU/month and a group receiving 24,000 IU/month plus 300 μg calcifediol. Over 12 months, the group receiving 60,000 IU/month had a higher risk of falls compared with that of the control group (147). Although VD is generally safe, adverse events can occur; therefore, clinicians should carefully monitor patients receiving supplementation.

In addition, there are of course other causes of VD poisoning events, e.g., an Italian study reported 3 cases of severe hypercalcemia and renal insufficiency associated with VD poisoning. The patients had been treated with VD preparations; the prescribed dose was 600 IU, but the actual content was 52,800 IU, which was related to formulation errors and long-term use. The study highlights the need to prepare supplements such as VD according to precise rules in order to make the amount of the potentially toxic ingredient the correct and safe dose (148). The toxicity of VD has also been linked to high doses of over-the-counter supplements that are taken frequently and incorrectly prescribed by doctors as well as incorrectly instructed by pharmacists, e.g., by providing 50,00 IU prescriptions daily instead of once a week. For instance, a patient with MS who initiated a self-prescribing change increased his VD3 intake from 8,000 to 88,000 IU/day over four years, resulting in an upward trend in hypercalcaemia and creatinine rates (149).

Conclusion

APS is an autoimmune disease that causes pathological pregnancy and thrombosis, significantly affecting patients' quality of life and increasing their economic burden. T-cell immune disorders caused by VD deficiency are critical factors in the pathogenesis of APS thrombosis and adverse pregnancy outcomes. VD has potent immunomodulatory effects, helping to regulate T-cell subsets, exerting antithrombotic effects and preventing adverse pregnancy outcomes in APS. VD supplementation is a promising and beneficial treatment for APS. However, studies on the correction of T-cell imbalances through VD supplementation in patients with APS are still lacking, and the specific mechanisms at the molecular and signaling pathway levels remain elusive. Large-scale prospective studies are needed to confirm VD's efficacy and mechanisms in APS treatment. Furthermore, the optimal dosage and safety of VD supplementation should be further explored to develop new therapeutic strategies for APS.

Availability of data and materials

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Authors' contributions

RH, YY and CW wrote the manuscript. XH, DM, RH, YY and YH collected the references and created the tables and figures. JL and XH conceptualised and designed the study. All authors read and approved the final manuscript. Data authentication is not applicable.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

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Funding

No funding was received.

References