Pathogenesis of the antiphospholipid syndrome revisited: time to challenge the dogma

Antiphospholipid antibodies and risk of post-COVID-19 vaccination thrombophilia: The straw that breaks the camel's back?

Antiphospholipid antibodies (aPLs), present in 1–5 % of healthy individuals, are associated with the risk of antiphospholipid syndrome (APS), which is the most common form of acquired thrombophilia. APLs may appear following infections or vaccinations and have been reported in patients with COronaVIrus Disease-2019 (COVID-19). However, their association with COVID-19 vaccination is unclear. Notably, a few cases of thrombocytopenia and thrombotic events resembling APS have been reported to develop in recipients of either adenoviral vector- or mRNA-based COVID-19 vaccines.

The aim of this review is therefore to speculate on the plausible role of aPLs in the pathogenesis of these rare adverse events.

Adenoviral vector-based vaccines can bind platelets and induce their destruction in the reticuloendothelial organs. Liposomal mRNA-based vaccines may instead favour activation of coagulation factors and confer a pro-thrombotic phenotype to endothelial cells and platelets. Furthermore, both formulations may trigger a type I interferon response associated with the generation of aPLs. In turn, aPLs may lead to aberrant activation of the immune response with participation of innate immune cells, cytokines and the complement cascade. NETosis, monocyte recruitment and cytokine release may further support endothelial dysfunction and promote platelet aggregation. These considerations suggest that aPLs may represent a risk factor for thrombotic events following COVID-19 vaccination, and deserve further investigations.

The Weight of IgA Anti-β2glycoprotein I in the Antiphospholipid Syndrome Pathogenesis: Closing the Gap of Seronegative Antiphospholipid Syndrome

The specific value of IgA Anti-β2glycoprotein I antibodies (aB2GP1) in the diagnosis and management of antiphospholipid syndrome (APS) is still controversial and a matter of active debate. The relevance of the IgA aB2GP1 isotype in the pathophysiology of APS has been increasingly studied in the last years. There is well know that subjects with multiple positive APS tests are at increased risk of thrombosis and/or miscarriage. However, these antibodies are not included in the 2006 APS classification criteria. Since 2010 the task force of the Galveston International Congress on APS recommends testing IgA aB2GP1 isotype in patients with APS clinical criteria in the absence of criteria antibodies. In this review, we summarize the molecular and clinical “state of the art” of the IgA aB2GP in the context of APS. We also discuss some of the characteristics that may help to evaluate the real value of the IgA aB2GP1 determination in basic research and clinical practice. The scientific community should be aware of the importance of clarifying the role of IgA aB2GP1 in the APS diagnosis.

Induction of tissue factor expression by anti-β2-glycoprotein I is mediated by tumor necrosis factor α

Antiphospholipid antibodies (aPL) are heterogeneous and there is evidence that binding specificity determines which cellular effects they can trigger. We have therefore hypothesised that the induction of tissue factor (TF) in monocytes and endothelial cells by aPL depends on their binding specificity. To further investigate this, we have analyzed the ability of three human monoclonal aPL with distinctly different binding specificities to induce transcription and cell surface expression of TF in monocytes and endothelial cells. Results with human monoclonal aPL were validated with IgG-fractions obtained from patients with APS. We confirmed previous results that a lipid reactive human monoclonal aPL rapidly induced TF transcription and cell surface expression in monocytes and endothelial cells. A monoclonal aPL reactive against β2 glycoprotein I (β2GPI) induced TF with a delayed time course. This was fully dependent on the induction of tumor necrosis factor alpha (TNFα) secretion as capture of TNFα by adalimumab prevented TF induction. This pattern was confirmed with patient IgG fractions. Both lipid reactive and anti-β2GPI induced TF transcription. Unexpectedly, this activity of anti-β2GPI was mediated fully by TNFα which was secreted in response to incubation with anti-β2GPI. The role of TNFα in mediating TF induction by anti-β2GPI may have wider implications for APS pathogenesis.

Discovery and characterization of 2 novel subpopulations of aPS/PT antibodies in patients at high risk of thrombosis

Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies are often detected in patients with antiphospholipid syndrome (APS), but how aPS/PT engage prothrombin at the molecular level remains unknown. Here, the antigenic determinants of immunoglobulin G aPS/PT were investigated in 24 triple-positive APS patients at high risk of thrombosis by using prothrombin mutants biochemically trapped in closed and open conformations, and relevant fragments spanning the entire length of prothrombin. Two novel unexpected findings emerged from these studies. First, we discovered that some aPS/PT are unique among other anti-prothrombin antibodies insofar as they efficiently recognize prothrombin in solution after a conformational change requiring exposure of fragment-1 to the solvent. Second, we identified and characterized 2 previously unknown subpopulations of aPS/PT, namely type I and type II, which engage fragment-1 of prothrombin at different epitopes and with different mechanisms. Type I target a discontinuous density-dependent epitope, whereas type II engage the C-terminal portion of the Gla-domain, which remains available for binding even when prothrombin is bound to the phospholipids. Based on these findings, APS patients positive for aPS/PT were classified into 2 groups, group A and group B, according to their autoantibody profile. Group A contains mostly type I antibodies whereas group B contains both type I and type II antibodies. In conclusion, this study offers a first encouraging step toward unveiling the heterogeneity of anti-prothrombin antibodies in correlation with thrombosis, shedding new light on the mechanisms of antigen-autoantibody recognition in APS.

Pathogenesis of antiphospholipid syndrome: recent insights and emerging concepts

Introduction: Even though our understanding of the antiphospholipid syndrome (APS) has improved tremendously over the last decades, we are still not in a position to replace symptomatic anticoagulation by pathogenesis based causal treatments. Areas covered: Recent years have provided further insights into pathogenetically relevant mechanisms. These include a differentiation of pathogenic subtypes of antiphospholipid antibodies (aPL), novel mechanisms modulating disease activity, for example, extracellular vesicles and microRNA, and novel players in pathogenesis, for example, neutrophils and neutrophil extracellular traps (NETs). Expert commentary: It is evident that aPL induce a proinflammatory and procoagulant state and recent data suggest that different aPL species activate different signaling pathways which sometimes converge into a common cellular response. This implies that presence of more than one aPL species may disproportionally increase the risk for the major manifestations of APS, that is, thrombosis and fetal loss. Further delineation of the pathogenic mechanisms will hopefully provide clues to causal rather than symptomatic treatments of APS.

The anti-thrombotic effects of vitamin D and their possible relationship with antiphospholipid syndrome

The importance of the immunomodulatory effects of vitamin D has recently been associated with autoimmune and chronic inflammatory diseases. Vitamin D deficiency has been linked to the development of autoimmune conditions. Antiphospholipid syndrome is an autoimmune disease characterized by thrombotic events and obstetric complications in patients with antiphospholipid antibodies. Current data show that patients with antiphospholipid syndrome have a high prevalence of vitamin D deficiency even without classic risk factors. Several studies have suggested vitamin D may have anti-thrombotic functions. In antiphospholipid syndrome, low vitamin D serum levels have been associated with thrombotic manifestations, suggesting a possible protective role of vitamin D in antiphospholipid syndrome. This literature review presents current evidence on the haemostatic functions of vitamin D and their possible relationship with the clinical manifestations of antiphospholipid syndrome.

Cellular and Molecular Mechanisms of Anti-Phospholipid Syndrome

The primary anti-phospholipid syndrome (APS) is characterized by the production of antibodies that bind the phospholipid-binding protein β2 glycoprotein I (β2GPI) or that directly recognize negatively charged membrane phospholipids in a manner that may contribute to arterial or venous thrombosis. Clinically, the binding of antibodies to β2GPI could contribute to pathogenesis by formation of immune complexes or modification of coagulation steps that operate along cell surfaces. However, additional events are likely to play a role in pathogenesis, including platelet and endothelial cell activation. Recent studies focus on neutrophil release of chromatin in the form of neutrophil extracellular traps as an important disease contributor. Jointly, the participation of both the innate and adaptive arms of the immune system in aspects of the APS make the complete understanding of crucial steps in pathogenesis extremely difficult. Only coordinated and comprehensive analyses, carried out in different clinical and research settings, are likely to advance the understanding of this complex disease condition.

Pathophysiological insights into the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is characterized by venous and/or arterial thrombosis and severe pregnancy morbidity in presence of antiphospholipid antibodies (aPL). While there is compelling evidence that aPL cause the clinical manifestations of APS, the underlying mechanisms are still a matter of scientific debate. This is mainly related to the broad heterogeneity of aPL. There are three major types of aPL: The first one binds to (anionic) phospholipids, e.g. cardiolipin, in absence of other factors (cofactor independent aPL). The second type binds to phospholipids only in presence of protein cofactors, e.g. ß2-glycoprotein I (ß2GPI) (cofactor dependent aPL). The third type binds to cofactor proteins directly without need for phospholipids. It is widely believed that cofactor independent aPL (type 1) are associated with infections and, more importantly, non-pathogenic, while pathogenic aPL belong to the second and in particular to the third type. This view, in particular with regard to type 1 aPL, has not been undisputed and novel research data have shown that it is in fact untenable. We summarize the available data on the pathogenetic role of aPL and the implications for diagnosis of APS and future research.

Antiphospholipid antibody-induced cellular responses depend on epitope specificity : implications for treatment of antiphospholipid syndrome

Essentials Antiphospholipid antibodies (aPL) are heterogeneous and induce different cellular responses. We analyzed signaling events induced by different monoclonal and patient aPL in monocytes. Two major signaling pathways involving either NADPH-oxidase or LRP8 were identified. Our data suggest that these two pathways mediate the majority of aPL effects on monocytes.

SUMMARY

Background Antiphospholipid antibodies (aPLs) contribute to the pathogenesis of the antiphospholipid syndrome (APS) by induction of an inflammatory and procoagulant state in different cell types, and several signaling pathways have been described. Objectives To investigate whether signaling depends on the epitope specificity of aPLs. Methods Cellular effects of three human monoclonal aPLs with distinctly different epitope specificities were analyzed in vitro. Expression of tumor necrosis factor-α mRNA by mouse and human monocytes was the major readout. Analysis included cells from genetically modified mice, and the use of specific inhibitors in human monocytes. Data were validated with IgG isolated from 20 APS patients. Results Cofactor-independent anticardiolipin aPLs activated monocytes by induction of endosomal NADPH oxidase. Activation could be blocked by hydroxychloroquine (HCQ). Anti-β2 -glycoprotein I aPL activated monocytes by interacting with LDL receptor-related protein 8 (LRP8). This could be blocked by rapamycin. Analysis of 20 APS patients' IgG showed that all IgG fractions activated the same two pathways as the monoclonal aPL, depending on their epitope patterns as determined by ELISA. Monocyte activation by APS IgG could be blocked completely by HCQ and/or rapamycin, suggesting that in most, if not all, APS patients there is no other relevant signaling pathway. Conclusions aPLs activate two major proinflammatory signal transduction pathways, depending on their epitope specificity. HCQ and rapamycin, either alone or in combination, completely suppress signaling by APS IgG. These observations may provide a rationale for specific treatment of APS patients according to their aPL profile.

Risk of developing antiphospholipid antibodies following viral infection: a systematic review and meta-analysis

Objective The objective of this paper is to conduct a systematic review and meta-analysis on the risk of developing elevated antiphospholipid (aPL) antibodies and related thromboembolic and/or pregnancy events following a viral infection. Method We searched Medline, EMBASE, Web of Science, PubMed ePubs, and Cochrane Central Register of Controlled Trials through June 2016. Independent observational studies of elevated aPL antibodies in patients with a viral infection compared with controls or patients with lupus were included. Results We analyzed 73 publications for 60 studies. Human immunodeficiency virus (HIV) and hepatitis C virus (HCV) were most commonly reported. Compared with healthy controls, patients with HIV were more likely to develop elevated anticardiolipin (aCL) antibodies (risk ratio (RR) 10.5, 95% confidence interval (CI) 5.6-19.4), as were those with HCV (RR 6.3, 95% CI 3.9-10.1), hepatitis B virus (HBV) (RR 4.2, 95% CI 1.8-9.5), and Epstein-Barr virus (EBV) (RR 10.9 95% CI 5.4-22.2). The only statistically significant increased risk for anti-β2-glycoprotein I (anti-β2-GPI) antibodies was observed in patients with HCV (RR 4.8 95% CI 1.0-22.3). Compared with patients with lupus, patients with HIV were more likely to develop elevated aCL antibodies (RR 1.8, 95% CI 1.3-2.6), and those with EBV, elevated anti-β2-GPI antibodies (RR 2.2, 95% CI 1.3-3.9). Thromboembolic events were most prevalent in patients with elevated aPL antibodies who had HCV (9.1%, 95% CI 3.0-18.1), and HBV (5.9%, 95% CI 2.0-11.9) infections, and pregnancy events were most prevalent in those with parvovirus B19 (16.3%, 95% CI 0.78-45.7). However, compared to virus-infected patients with negative aPL antibodies, the only statistically significant increased risk was observed in those with HCV and positive aPL. Conclusions Viral infection can increase the risk of developing elevated aPL antibodies and associated thromboembolic events. Results are contingent on the reported information.

Hormonal contraception and the development of autoimmunity: A review of the literature

Estrogens and progestins are known to have profound effects on the immune system and may modulate the susceptibility to autoimmune diseases. A comprehensive literature search was carried out using PubMed for any of 153 autoimmune disease terms and the terms contraception, contraceptive, or their chemical components with limits of Humans + Title or Abstract. Over 1,800 titles were returned and scanned, 352 papers retrieved and reviewed in depth and an additional 70 papers retrieved from the bibliographies. Based on this review, substantial evidence exists linking the use of combined oral contraceptives to a lower incidence of hyperthyroidism, an increase in multiple sclerosis, ulcerative colitis, Crohn's disease, Systemic Lupus Erythematosus, and interstitial cystitis. Progesterone only contraceptives are linked to progesterone dermatitis and in one large developing world concurrent cohort study are associated with increases in arthropathies and related disorders, eczema and contact dermatitis, pruritis and related conditions, alopecia, acne, and urticaria. Hormonal contraceptives modulate the immune system and may influence the susceptibility to autoimmune diseases with significant increases in risk for several autoimmune diseases.

SUMMARY

Hormonal contraceptives (HCs), such as the "pill," Norplant, and vaginal rings, are very potent hormones that have effects on the immune system, which is made up of white blood cells and lymph nodes and normally defends the body against invading bacteria, viruses and parasites. This review looked at the association of HC use to the development of autoimmune diseases, where the immune system turns against the body and causes damage to organs. There is good evidence that HC use is associated with an increased risk of several serious autoimmune diseases such as Crohn's disease (which causes inflammation of the bowels), Lupus (which causes inflammation in many organs), and interstitial cystitis (which causes inflammation in the bladder). Several other rarer autoimmune diseases are also linked to HC use. People contemplating the use of HCs should be informed of these risks.

Pathogenesis and management of antiphospholipid syndrome

Antiphospholipid antibodies are a heterogeneous group of autoantibodies that have clear associations with thrombosis and pregnancy morbidity, and which together constitute the 'antiphospholipid syndrome' (APS). However, the pathophysiology of these complications is not well understood and their heterogeneity suggests that more than one pathogenic process may be involved. Diagnosis remains a combination of laboratory analysis and clinical observation but there have been significant advances in identifying specific pathogenic features, such as domain I-specific anti-β2-glycoprotein-I antibodies. This in turn has pointed to endothelial and complement activation as important factors in the pathogenesis of APS. Consequently, although anticoagulation remains the standard treatment for thrombotic APS and during pregnancy, the realisation that these additional pathways are involved in the pathogenesis of APS has significant implications for treatment: agents acting outside the coagulation system, such as hydroxychloroquine for pregnancy complications and sirolimus as an inhibitor of the mammalian target of rapamycin (mTOR) pathway, are now under evaluation and represent a radical change in thinking for haematologists. Conventional anticoagulation is also under challenge from new, direct acting anticoagulants. This review will provide a comprehensive overview of the evolving understanding of APS pathogenesis and how this and novel therapeutics will alter diagnosis and management.

Cardiovascular risk factors are major determinants of thrombotic risk in patients with the lupus anticoagulant

BACKGROUND

Patients with the lupus anticoagulant (LA) are at an increased risk of thrombotic events, which in turn increase the risk of death. Understanding the determinants of thrombotic risk in patients with LA may pave the way towards targeted thromboprophylaxis. In the Vienna Lupus Anticoagulant and Thrombosis Study (LATS), we systematically evaluate risk factors for thrombotic events in patients with LA.

METHODS

We followed 150 patients (mean age: 41.3 years, female gender: n = 122 (81.3%), history of thrombosis or pregnancy complications: n = 111 (74.0%)), who tested repeatedly positive for LA until development of thrombosis, death, or censoring. The primary endpoint was a composite of arterial or venous thrombotic events (TEs).

RESULTS

During a median follow-up of 9.5 years (range: 12 days-13.6 years) and 1076 person-years, 32 TEs occurred (arterial: n = 16, venous: n = 16; cumulative 10-year TE incidence: 24.3%). A prolonged lupus-sensitive activated partial thromboplastin time (aPTT-LA) (adjusted subdistribution hazard ratio (SHR) = 2.31, 95% CI: 1.07--5.02), diabetes (adjusted SHR = 4.39, 95% CI: 1.42-13.57), and active smoking (adjusted SHR = 2.31, 95% CI: 1.14-5.02) emerged as independent risk factors of both arterial and venous thrombotic risk. A risk model that includes a prolonged lupus-sensitive aPTT, smoking, and diabetes enabled stratification of LA patients into subgroups with a low, intermediate, and high risk of thrombosis (5-year TE risk of 9.7% (n = 77), 30.9% (n = 51), and 56.8% (n = 22).

CONCLUSIONS

Long-term thrombotic risk in patients with LA is clustered within subjects harboring typical cardiovascular risk factors in addition to a prolonged lupus-sensitive aPTT, whereas patients with none of these risk factors represent a large subgroup with a low risk of thrombosis.

The Laboratory Diagnosis of the Antiphospholipid Syndrome

The Antiphospholipid Syndrome (APS) is classified based on the presence of both clinical and laboratory criteria. Both sets of criteria are subject to much review and intense research as it is becoming increasingly clear that no single test is specific for defining this autoimmune disorder. A number of leading international bodies have released guidelines in an attempt to improve the laboratory testing and reporting. The current review is an appraisal of some of the literature pertaining to the laboratory testing.