Thrombin Generation Assay in Antiphospholipid Antibodies Positive Subjects as a Personalized Thrombotic Risk Assessment: State of the Art and Perspectives

PURPOSE OF THE REVIEW

Thrombotic risk assessment in antiphospholipid positive (aPL +) subjects is a major challenge, and the study of in vitro thrombin generation (thrombin generation assays (TGA)) could provide useful information. Activated protein C (APC) sensitivity is involved in thrombotic events in antiphospholipid syndrome patients. We summarized methods used to assess APC sensitivity with TGA and evaluated the prognostic role of APC resistance through literature search.

RECENT FINDINGS

APC resistance induced by aPL is a complex pathway. Several cross-sectional studies assessed APC sensitivity to understand thrombotic event mechanisms in aPL + subjects. Only one prospective cohort had investigated the prognostic impact of APC resistance in aPL + subjects, with a positive and significant correlation between APC sensitivity and the risk of thrombosis during the follow up (hazard ratio, 6.07 [95% CI, 1.69-21.87]). APC resistance assessed with TGA could be associated with thrombotic events in aPL + subjects.

Antiphospholipid antibodies and the risk of thrombosis in myeloproliferative neoplasms

The morbidity and mortality of BCR-ABL-negative myeloproliferative neoplasia (MPN) patients is highly dependent on thrombosis that may be affected by antiphospholipid antibodies (aPLA) and lupus anticoagulant. Our aim was to evaluate the association of the aPLA together with platelet receptor glycoprotein (GP) Ia/IIa c.807C>T CT/TT genotypes and thrombotic complications in patients with MPNs. The study included 108 patients with BCR-ABL-negative MPN with data of previous thrombosis. Two different screening and one confirmatory test for the lupus anticoagulant were performed. Thrombotic complications were present in 59 (54.6%) subjects. aPLA were more frequently found in MPN patients with thrombosis vs no thrombosis (25.4 and 6.1%; p = 0.007). MPN patients with arterial thrombosis were more frequently positive for aPLA vs no arterial thrombosis (38.8 and 11.9%; p = 0.001). aPLA were more frequently found in patients with cerebrovascular events vs other arterial thrombotic complications or no thrombosis, respectively (39.3, 6.1, and 12.9%; p < 0.001). MPN patients with thrombosis were more frequently positive with aPLA and had platelet receptor GP Ia/IIa c.807C>T CT/TT genotypes compared to MPN patients without thrombosis (18.6 and 2.0%; p = 0.006). aPLA alone or with coexistence with platelet receptor GP Ia/IIa c.807C>T CT/TT polymorphism could be associated with thrombotic complications in patients with MPN.

Protein Phosphatase 2A Activation Via ApoER2 in Trophoblasts Drives Preeclampsia in a Mouse Model of the Antiphospholipid Syndrome

[Figure: see text].

Complement Activation and Thrombin Generation by MBL Bound to β2-Glycoprotein I

β2-Glycoprotein I (β2-GPI) is an abundant plasma glycoprotein with unknown physiological function and is currently recognized as the main target of antiphospholipid Abs responsible for complement activation and vascular thrombosis in patients with antiphospholipid syndrome (APS). In this study, we provide evidence that mannose-binding lectin (MBL) binds to β2-GPI in Ca⁺⁺ and a dose-dependent manner and that this interaction activates complement and promotes complement-dependent thrombin generation. Surprisingly, a significant binding was observed between MBL and isolated domains II and IV of β2-GPI, whereas the carbohydrate chains, domain I and domain V, were not involved in the interaction, documenting a noncanonical binding mode between MBL and β2-GPI. Importantly, this interaction may occur on endothelial cells because binding of MBL to β2-GPI was detected on the surface of HUVECs, and colocalization of MBL with β2-GPI was observed on the endothelium of a biopsy specimen of a femoral artery from an APS patient. Because β2-GPI-mediated MBL-dependent thrombin generation was increased after priming the endothelium with TNF-α, our data suggests that this mechanism could play an important yet unrecognized role under physiological conditions and may be upregulated in pathological situations. Moreover, the complement activation and the procoagulant effects of the β2-GPI/MBL complex may contribute to amplify similar activities of anti-β2-GPI Abs in APS and possibly act independently of Abs, raising the issue of developing appropriate therapies to avoid recurrences and disability in patients at risk for these clinical conditions.

Changes of platelet count throughout pregnancy in women with antiphospholipid syndrome

OBJECTIVE

Antiphospholipid antibodies (aPL) activate several cell types, such as endothelial cells, monocytes, neutrophils, fibroblasts, trophoblasts and platelets, thus leading to thrombosis and obstetric complications in patients with antiphospholipid syndrome (APS). The aim of the present study was the longitudinal investigation of platelet count in women with APS. Additionally, platelet count in women with APS who developed preeclampsia during pregnancy were compared to women with APS and uncomplicated pregnancy for potential early detection of preeclampsia.

MATERIAL AND METHODS

This longitudinal study included 65 women with APS, 38 women with preeclampsia and 84 women with normal pregnancies, where platelet count was determined every four weeks, starting in early pregnancy.

RESULTS

Platelet count was significantly lower in women with APS compared to women who developed preeclampsia and normal pregnancies starting at 12 weeks of gestation. The areas under the curve (AUC) for platelet count were 0.765 at 12 weeks of gestation (95% of CI of 0.634-0.896), 0.747 at 20 weeks (95% of CI of 0.600-0.894), 0.719 at 24 weeks (95% of CI of 0.555-0.882), respectively. The cut off points for platelets were 216 at 12-14 weeks of gestation, 226.5 at 20 weeks of gestation, and 163.5 at 24 weeks of gestation, respectively.

DISCUSSION

We demonstrated a significant lower platelet count in women with APS throughout gestation. Additionally, platelet count is significantly decreased in women with APS who developed preeclampsia. According to our results, platelet count seems to have a predictive value for the development of preeclampsia in these women.

Type I interferon gene expression in antiphospholipid syndrome: Pathogenetic, clinical and therapeutic implications

Type I Interferon gene expression has been shown to play an important role in the pathogenesis of several systemic autoimmune disorders, paving the way for its potential use as a surrogate marker or a therapeutic tool. While the concept of type I interferon signature and its correlation with clinical phenotypes and disease activity, along with anti-interferon targeted therapy have been vastly investigated in patients with systemic lupus erythematosus, there is a paucity of data concerning antiphospholipid syndrome patients. In this review, we summarize the current knowledge on the pathogenetic and clinical implications of type I interferon expression in antiphospholipid syndrome and discuss the therapeutic possibility of targeting molecules along the interferon signaling pathway. A number of recent studies have shown a type I interferon gene expression induction in patients with primary antiphospholipid syndrome via the plasmacytoid dendritic cell pathway, toll like receptors (TLRs) such as TLR7 and TLR9, anti-beta2glycoprotein I antibody-mediated neutrophil activation and neutrophil extracellular traps (NETs) release in a TLR4-dependent fashion, and a subsequent B cell and plasmablast activation. An association between type I interferon expression and several demographic, clinical and laboratory characteristics including age, gender, pregnancy complications such as eclampsia, anti-beta2glycoprotein I antibodies, and a negative correlation with hydroxychloroquine and/or statin use, has been shown. Correlation of high interferon scores to worse outcomes in prospective studies could direct the initiation for a prompt treatment in high-risk populations. Potential therapeutic approaches targeting type I interferon production and signaling pathway components might include anti-interferon or interferon receptor monoclonal antibodies, or an interferon based therapeutic vaccine as was indicated from previous systemic lupus erythematosus studies, TLR inhibitors including hydroxychloroquine and anti-TLR antibodies, plasmacytoid dendritic cell inhibition, adenosine-receptor agonists, and plasmablast targeting treatments. Well-designed studies are needed to further assess the immunomodulatory potential of the above targets for therapeutic intervention in patients with primary antiphospholipid syndrome.

Systemic lupus erythematosus: a case-based presentation of renal, neurologic, and hematologic emergencies

INTRODUCTION

Systemic lupus erythematosus (SLE) is a chronic autoimmune disorder with systemic manifestations and multiorgan involvement. Although primarily diagnosed, and managed in the outpatient setting, it can occasionally present with life-threatening complications that require rapid assessment and urgent aggressive therapy. Areas covered: In our review, we explore three organ systems that are often affected in SLE, but have the potential to present as medical emergencies; these are the kidney, the central nervous system, and the hematologic system. We take a case-based approach to each clinical scenario, with information given sequentially in order to reflect "real-life" situations where management decisions need to be made with limited information. We review the acute management, pathophysiology, diagnostic approach, and treatment along with a review of the literature, for lupus nephritis presenting as rapidly progressive glomerulonephritis, acute lupus transverse myelitis, and refractory antiphospholipid syndrome. Expert commentary: At the conclusion of each section, we provide an expert commentary regarding each issue, relating to diagnosis, early management, and current evidence behind treatment recommendations.

Primary antiphospholipid syndrome and antiphospholipid syndrome associated to systemic lupus: Are they different entities?

Primary antiphospholipid syndrome (PAPS) and antiphospholipid syndrome associated to lupus (SAPS) have several overlapping characteristics. As systemic manifestations are also reported in patients with PAPS, and as a subgroup of PAPS patients could evaluate to a SAPS, the differentiation between the two types of APS could be performed based on the clinical experience of the medical teams and is related to a variety of clinical, biological, histological and genetic features. Several data are available in the literature with respect to the identification of distinctive features between these two entities. However, there are some limitation in the interpretation of results issued from studies performed prior to updated Sydney criteria. Based on recent data, a certain number of features more frequent in one type of APS as compared to the other could be distinguished. The major differentiation between these two entities is genetical. New genetic data allowing the identification of specific subgroups of APS are ongoing.

An unusual case of heart attack

We reported a patient with antiphospholipid syndrome (APS) secondary to systemic lupus erythematosus (SLE) who suffered from myocardial infarction and graft occlusion after coronary artery bypass surgery. Our patient illustrates the impact of accelerated atherosclerosis in patients with SLE and APS and the importance of early institution of antithrombotic therapies. In this article, the treatment of APS is summarized. Although the main stay of treatment is anticoagulation, there is emerging evidence to support rituximab as an option for “refractory” cases of APS.

Autologous Hematopoietic Stem Cell Transplantation for Antiphospholipid Syndrome: Case Report and Review of the Literature

Refractory antiphospholipid syndrome represents a challenge for preventing thrombosis that may occur despite adequate anticoagulation and immunomodulation therapy. Here, we report a 35-year-old male patient who presented with variable venous thromboembolic events. Autologous hematopoietic stem cell transplant was performed after conditioning with cyclophosphamide and antithymocyte globulin. Five years after transplant, the treatment continued to show a successful outcome in preventing new thrombotic events. Autologous hematopoietic stem cell transplant represents a chance for cure from antiphospholipid syndrome despite the paucity of reported data so far.

Antiphospholipid antibodies induce thrombosis by PP2A activation via apoER2-Dab2-SHC1 complex formation in endothelium

In the antiphospholipid syndrome (APS), antiphospholipid antibody (aPL) recognition of β2 glycoprotein I promotes thrombosis, and preclinical studies indicate that this is due to endothelial nitric oxide synthase (eNOS) antagonism via apolipoprotein E receptor 2 (apoER2)-dependent processes. How apoER2 molecularly links these events is unknown. Here, we show that, in endothelial cells, the apoER2 cytoplasmic tail serves as a scaffold for aPL-induced assembly and activation of the heterotrimeric protein phosphatase 2A (PP2A). Disabled-2 (Dab2) recruitment to the apoER2 NPXY motif promotes the activating L309 methylation of the PP2A catalytic subunit by leucine methyl transferase-1. Concurrently, Src homology domain-containing transforming protein 1 (SHC1) recruits the PP2A scaffolding subunit to the proline-rich apoER2 C terminus along with 2 distinct regulatory PP2A subunits that mediate inhibitory dephosphorylation of Akt and eNOS. In mice, the coupling of these processes in endothelium is demonstrated to underlie aPL-invoked thrombosis. By elucidating these intricacies in the pathogenesis of APS-related thrombosis, numerous potential new therapeutic targets have been identified.

Arterial stenosis in antiphospholipid syndrome: Update on the unrevealed mechanisms of an endothelial disease

First described in 1983, antiphospholipid syndrome (APS) is an autoimmune condition characterized by the occurrence of recurrent arterial and/or venous thrombosis, and/or pregnancy morbidity, in the setting of persistent presence of antiphospholipid antibodies (aPL). While thrombosis is the most well-known pathogenic mechanism in this disorder, the relevance of some other mechanisms such as arterial stenosis is being increasingly recognized. Arterial stenosis has been first described in the renal arteries in patients with APS, however intracranial and coeliac arteries can also be involved with various and treatable clinical manifestations. The underlying pathophysiology of this stenotic arterial vasculopathy is not fully understood but some recent studies revealed new insights into the molecular mechanism behind this endothelial cell activation in APS. In this review, we discuss these newly discovered mechanisms and highlight the diagnostic and therapeutic modalities of the APS related arterial stenosis.

Further Investigations of the Effects of Anti-β2GP1 Antibodies on Collagen-Induced Platelet Aggregation

Anti-beta-2-glycoprotein 1 (anti-β₂GP1) antibodies are associated with increased thrombotic risk in patients with autoimmune disease. There is conflicting evidence on the effects of anti-β₂GP1 antibodies on platelets, with both enhanced and inhibited aggregation previously reported. However, previous studies did not include isotype antibodies to ensure the observed effects were due to anti-β₂GP1 antibodies. The aims of this study were to (1) investigate the effects of anti-β₂GP1 antibodies on collagen-induced platelet aggregation in parallel with negative control (buffer normal saline) and isotype control antibodies and (2) determine the lupus anticoagulant (LA) activity of anti-β₂GP1 antibodies used. Three animal-derived anti-human-β₂GP1 antibodies (1.25, 2.5, and 5 μg/mL) incubated with healthy platelet-rich plasma were activated by collagen (2.5 μg/mL). Each anti-β₂GP1 antibody demonstrated the inhibition of aggregation compared to negative control, but not to isotype control. No anti-β₂GP1 antibody demonstrated LA activity, suggesting they were probably nonpathological. This study highlights both negative and isotype control markers are important to validate the effects of anti-β₂GP1 antibodies. Assays to measure anti-domain I-β₂GP1 antibodies are recommended to be used in conjunction with functional measures to further investigate the effects of anti-β₂GP1 antibodies.

Antiphospholipid Syndrome: Role of Vascular Endothelial Cells and Implications for Risk Stratification and Targeted Therapeutics

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by venous thromboembolism, arterial thrombosis, and obstetric morbidities in the setting of persistently positive levels of antiphospholipid antibodies measured on 2 different occasions 12 weeks apart. Patients with APS are at increased risk for accelerated atherosclerosis, myocardial infarction, stroke, and valvular heart disease. Vascular endothelial cell dysfunction mediated by antiphospholipid antibodies and subsequent complement system activation play a cardinal role in APS pathogenesis. Improved understanding of their pathogenic function could help in the risk stratification of patients with APS and provide new molecular therapeutic targets.

Antiphospholipid antibodies attenuate endothelial repair and promote neointima formation in mice

Background

Antiphospholipid syndrome patients have antiphospholipid antibodies (aPLs) that promote thrombosis, and they have increased cardiovascular disease risk. Although the basis for the thrombosis has been well delineated, it is not known why antiphospholipid syndrome patients also have an increased prevalence of nonthrombotic vascular occlusion. The aims of this work were to determine if aPLs directly promote medial hypertrophy or neointima formation in mice and to identify the underlying mechanisms.

Methods and Results

Medial hypertrophy and neointima formation invoked by carotid artery endothelial denudation were evaluated in mice administered normal human IgG or aPLs. While aPLs had no effect on medial hypertrophy, they caused exaggerated neointima development. This was related to an aPL‐induced impairment in reendothelialization post denudation, and scratch assays in cell culture revealed that there are direct effects of aPLs on endothelium that retard cell migration. Further experiments showed that aPL antagonism of endothelial migration and repair is mediated by antibody recognition of β2‐glycoprotein I, apolipoprotein E receptor 2, and a decline in bioavailable NO. Consistent with these mechanisms, the adverse impacts of aPLs on reendothelialization and neointima formation were fully prevented by the NO donor molsidomine.

Conclusions

APLs blunt endothelial repair, and there is related aPL‐induced exaggeration in neointima formation after endothelial injury in mice. The initiating process entails NO deficiency mediated by β2‐glycoprotein I recognition by aPLs and apolipoprotein E receptor 2. The modulation of endothelial apolipoprotein E receptor 2 function or NO bioavailability may represent new interventions to prevent the nonthrombotic vascular occlusion and resulting cardiovascular disorders that afflict antiphospholipid syndrome patients.

The relationship between mean platelet volume and thrombosis recurrence in patients diagnosed with antiphospholipid syndrome

Increased mean platelet volume (MPV) is associated with platelet reactivity and is a predictor of cardiovascular risk and unprovoked venous thromboembolism. The aim of our study was to evaluate MPV in patients with confirmed antiphospholipid antibody syndrome (APS) and to identify the correlation between the value of MPV and the recurrence of thrombosis. The studied group consists of 247 patients with a history of thrombosis and/or pregnancy loss (median age 38, range 18–66 years) classified as APS group (n = 70) or APS negative patients (n = 177) according to the updated Sapporo criteria. The control group consisted of 98 healthy subjects. MPV was significantly higher in the group of patients with clinically and laboratory confirmed APS (median 7.85, range 4.73–12.2 fl) in comparison with the controls. It was also higher than in APS negative patients (7.61, range 5.21–12.3 fl). APS patients with triple positivity for antiphospholipid antibodies with respect to Miyakis classification categories had higher MPV values than other APS patients (9.69 ± 1.85 vs. 7.29 ± 1.3 fl, p = 0.001). Recurrent thrombotic episodes were observed in 83 patients, but among the triple positive high-risk patients with APS in 80 % cases (p = 0.0046). In receiver operating characteristic curve analysis, the value of MPV level for thrombosis recurrence prediction in the APS group with sensitivity of 86 % and specificity of 82 % was 7.4 fl. In the multivariate logistic regression model, MPV above 7.4 fl (OR 3.65; 95 % CI 1.38–9.64, p = 0.009) significantly predicts thrombosis recurrence. Our results identify the value of MPV as a prognostic factor of thrombosis recurrence in patients with APS.

Splenectomy is modifying the vascular remodeling of thrombosis

Background

Splenectomy is a clinical risk factor for complicated thrombosis. We hypothesized that the loss of the mechanical filtering function of the spleen may enrich for thrombogenic phospholipids in the circulation, thereby affecting the vascular remodeling of thrombosis.

Methods and Results

We investigated the effects of splenectomy both in chronic thromboembolic pulmonary hypertension (CTEPH), a human model disease for thrombus nonresolution, and in a mouse model of stagnant flow venous thrombosis mimicking deep vein thrombosis. Surgically excised thrombi from rare cases of CTEPH patients who had undergone previous splenectomy were enriched for anionic phospholipids like phosphatidylserine. Similar to human thrombi, phosphatidylserine accumulated in thrombi after splenectomy in the mouse model. A postsplenectomy state was associated with larger and more persistent thrombi. Higher counts of procoagulant platelet microparticles and increased leukocyte–platelet aggregates were observed in mice after splenectomy. Histological inspection revealed a decreased number of thrombus vessels. Phosphatidylserine‐enriched phospholipids specifically inhibited endothelial proliferation and sprouting.

Conclusions

After splenectomy, an increase in circulating microparticles and negatively charged phospholipids is enhanced by experimental thrombus induction. The initial increase in thrombus volume after splenectomy is due to platelet activation, and the subsequent delay of thrombus resolution is due to inhibition of thrombus angiogenesis. The data illustrate a potential mechanism of disease in CTEPH.

Inhibition of nitric oxide and antiphospholipid antibody-mediated thrombosis

The antiphospholipid syndrome (APS) is characterized by recurrent vascular thrombosis, thrombocytopenia, and fetal loss occurring in the presence of antiphospholipid antibodies (aPL). Along with arterial and venous thrombosis and pregnancy complications, patients with APS have an increased risk of myocardial infarction, stroke, and coronary artery disease, resulting from vascular cell dysfunction induced by aPL. Accumulating evidence to date indicates that interactions between circulating aPL and cell surface molecules of target cells, primarily endothelial cells and platelets, underlie the vascular disease phenotypes of APS. However, the molecular basis of APS is poorly understood. Nitric oxide produced by endothelial cells is a key determinant of vascular health that regulates several physiologic processes, including thrombosis, endothelial-leukocyte interaction, vascular cell migration, and the modulation of vascular tone. This review will discuss recent findings that indicate a novel mechanism by which aPL antagonize endothelial cell production of nitric oxide and thereby promote thrombosis.

Anti-β2-glycoprotein I antibodies

Anti-β(2)-glycoprotein I (anti-β(2)GPI) antibodies are the main antiphospholipid antibodies, along with anticardiolipin and lupus anticoagulant, that characterize the autoimmune disease antiphospholipid syndrome (APS). While the exact physiological functions of β(2)GPI are unknown, there is overwhelming evidence that anti-β(2)GPI antibodies are pathogenic, contributing to thrombosis, pregnancy morbidity, and accelerated atherosclerosis in APS and systemic lupus erythematosus patients. The revelation that these antibodies play a central role in the pathogenesis and pathophysiology of APS has driven research to characterize the physiology and structure of β(2)GPI as well as the pathogenic effects of anti-β(2)GPI antibodies. It has also resulted in the development of improved testing methodologies for detecting these antibodies. In this review we discuss the characteristics of β(2)GPI; the generation, pathogenic effects, and standardized testing of anti-β(2)GPI antibodies; and the potential use of therapies that target the β(2)GPI/anti-β(2)GPI interaction in the treatment of APS.

Platelet and endothelial activation in catastrophic and quiescent antiphospholipid syndrome

Antiphospholipid antibodies (aPL) seem to induce a prothrombotic state by activating endothelium and platelets, but no studies have evaluated systematically the effects of aPL from patients with the antiphospholipid syndrome (APS) in quiescent versus catastrophic phase. Our aims were to evaluate the in vitro effects on platelet activation of anti-β2 glycoprotein I (anti-β2GPI) antibodiesisolated from APS patientin either quiescent or catastrophic phase and to investigate ex vivo platelet and endothelial activation in patients with quiescent or catastrophic APS. Anti-β2GPI antibodies were isolated from plasma of a pregnant woman in two different stages of APS (quiescent and catastrophic, respectively). They were co-incubated with washed platelets from healthy controls that were then challenged with TRAP-6 (thrombin receptor activating peptide 6) and the expression of P- selectin (P-sel) on platelets was assessed by flow cytometry. Moreover, plasma samples from six patients with quiescent, four with catastrophic APS and 10 controls were assessed for several markers of platelet and endothelial activation. The results showed that purified anti-β2GPI antibodies co-incubated with platelets enhanced TRAP-6- induced platelet P-sel expression. Notably, anti-β2GPI antibodies isolated during the catastrophic phase enhanced platelet P-sel expression more than antibodies isolated from the same patient in the quiescent stage of disease. Moreover, APS patients had significantly higher plasma levels of soluble (s) Psel, sCD40 ligand, soluble vascular cell adhesion molecule 1 and monocyte chemoattractant protein 1 than control subjects. In addition, sP-sel and von Willebrand factor activity were significantly higher during catastrophic than in quiescent phase.

Elevated plasminogen activator inhibitor type-1 (PAI-1) as contributing factor in pathogenesis of hypercoagulable state in antiphospholipid syndrome

The aim of this study is to explore the role of plasminogen activator inhibitor type 1 (PAI-1) in primary and secondary antiphospholipid syndrome (APS). Thirty patients of APS (24 primary and 6 secondary) were recruited in the study who fulfilled the revised Sapporo criteria. Control groups comprised of age- and sex-matched 10 healthy volunteers and 10 patients each of systemic lupus erythematosus and rheumatoid arthritis without any antecedent thrombotic event and/or APS-related pregnancy morbidity. Serum samples were tested for PAI-1 antigen levels measured by quantitative ELISA. Positivity rate of PAI-1 in patients of primary, secondary as well as total APS patients was significantly higher in relation to age- and sex-matched healthy volunteers (p = 0.010, p = 0.003 and p < 0.001, respectively). Mean ± SEM levels of PAI-1 in primary and secondary as well as total APS patients were significantly higher (p = 0.006, p < 0.001 and p < 0.001) in relation to healthy controls. Correlation of PAI-1 levels (mean ± SEM) with clinical characteristics, that is, thrombosis and pregnancy morbidity, revealed significantly higher levels of PAI-1 (p < 0.001) in patients having thrombosis and APS-related pregnancy morbidity. Elevated PAI-1 level leading to impaired fibrinolysis plays a significant role in producing hypercoagulable state in primary and secondary APS.

Platelets in pulmonary vascular physiology and pathology

Almost a trillion platelets pass through the pulmonary circulation every minute, yet little is known about how they support pulmonary physiology or contribute to the pathogenesis of lung diseases. When considering this conundrum, three questions jump out: Does platelet production in the lungs occur? Why does severe thrombocytopenia—which undercuts the principal physiological role of platelets to effect hemostasis—not lead to pulmonary hemorrhage? Why does atherothrombosis—which platelets initiate, maintain, and trigger is other critically important arterial beds—not develop in the pulmonary artery? The purpose of this review is to explore these and derivative questions by providing data within a conceptual framework that begins to organize a subject that is largely unassembled.

In vivo distribution of β2 glycoprotein I under various pathophysiologic conditions

In vitro studies have documented β2 glycoprotein I (β2GPI) binding to endothelial cells (ECs) and trophoblast using antiphospholipid antibodies. The in vivo binding of β2GPI to these cells and the conditions that favor their interaction have not been investigated. We analyzed the in vivo distribution of cyanine 5.5-labeled β2GPI in mice and evaluated the effect of pregnancy and circulating antibodies on its tissue localization. The signal was detected in the liver by whole body scan and ex vivo analysis. The β2GPI failed to bind to the vascular endothelium and reacted only with the ECs of uterine vessels. In pregnant mice the protein was localized on ECs and trophoblast at the embryo implantation sites. Immunized mice showed a similar β2GPI biodistribution to naive mice but the immunized pregnant animals exhibited a significant increase in fetal loss associated with C3 and C9 deposition at the implantation sites. Treatment of mice with LPS after β2GPI-Cy5.5 injection promoted protein localization on gut and brain ECs associated with IgG, C1q, and C9 deposition in immunized mice. These findings indicate that β2GPI binding to EC requires priming with pro-inflammatory factors which is not needed for uterine and placental localization probably dependent on hormonal changes.

New Insights into the Molecular Basis of the Antiphospholipid Syndrome

Antiphospholipid syndrome is an autoimmune disease characterized by the presence of circulating antiphospholipid antibodies (aPL) that promote thrombosis, pregnancy complications and cardiovascular diseases. Alterations in the function of vascular cells induced by aPL underlie these outcomes. This review will discuss recent findings that indicate a novel mechanism by which aPL antagonize endothelial cell production of nitric oxide and thereby promote thrombosis.

Pathophysiology of the antiphospholipid antibody syndrome

Antiphospholipid antibodies (aPL) are associated with the recurrent pregnancy loss and thrombosis that characterizes the antiphospholipid antibody syndrome (APS). Although the ontogeny of these pathogenic antibodies has not been fully elucidated, there is evidence that indicates the involvement of both genetic and environmental factors. The ability of aPL to induce a procoagulant phenotype in APS patients plays a central role in the development of arterial and venous thrombotic manifestations typical of the disease. Inflammation serves as a necessary link between this procoagulant phenotype and actual thrombus development and is an important mediator of the placental injury seen in APS patients with obstetric complications. Recent evidence has indicated a role for abnormal cellular proliferation and differentiation in the pathophysiology of APS, especially in those patients with pregnancy morbidity and other more atypical manifestations that have no identifiable thrombotic cause. The interplay of genetic and environmental factors responsible for aPL development and the mechanisms by which these antibodies produce disease in APS patients is the focus of this review.

The role of LRP8 (ApoER2') in the pathophysiology of the antiphospholipid syndrome

One of the greatest enigmas in thrombosis research is the observation that one can diagnose a person with a thrombotic risk with a prolongation of the clotting time. Our textbooks have taught us that prolongation of clotting correlates with a tendency to bleed. To confuse our textbook knowledge further, the same patients often have a prolonged bleeding time, a diagnostic test to detect a dysfunction in primary haemostasis. In this paper we critically review the literature that tries to explain the contradiction that exists between in-vitro diagnostic tests and the observed clinical manifestations and discuss our current opinion on how antiphospholipid antibodies can disturb the haemostatic balance.

Antiphospholipid antibodies and the protein C pathway

Haemostasis is a delicate balance between procoagulant and anticoagulant processes. In the human body usually anticoagulant mechanisms prevail over procoagulant mechanisms, thereby preventing a prothrombotic state. The antiphospholipid syndrome is an example in which this balance is shifted to a more prothrombotic state due to the presence of antiphospholipid antibodies. One of the most extensively proposed pathogenic mechanisms within the antiphospholipid syndrome is the inhibition of protein C by antiphospholipid antibodies. Antiphospholipid antibodies have been described to have different actions on the protein C pathway, for example decreasing protein C and/or S plasma levels, inducing increased resistance against activated protein C and lowering thrombin levels (resulting in an impaired protein C activation). This review briefly discusses the actions of protein C in human body but mainly focuses on the effects of antiphospholipid antibodies on the protein C pathway that have been described in literature.

What nephrologists need to know about antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disorder characterized by recurrent arterial or venous thrombosis and/or pregnancy losses, in the presence of persistently elevated levels of anticardiolipin antibodies (aCL) and/or evidence of circulating lupus anticoagulant (LA). The kidney is a major target organ in both primary and secondary APS. With the expanding spectrum of renal diseases associated with APS, and the impact of APS in ESRD care, this subject is of increasing relevance to nephrologists. This review describes the various clinical manifestations and histological features of this syndrome, with reference to the kidney.

Antiphospholipid syndrome review

The antiphospholipid syndrome (APS) is an autoimmune disorder presenting with tissue injury in various organs attributed to large or small vessel thrombosis or, in some instances, possible nonthrombotic inflammatory mechanisms, associated with in vitro evidence of antibodies to certain proteins, or proteinphospholipid complexes. Although the pathophysiology, diagnosis, and management of APS may seem clear and straightforward from a distance, closer inspection reveals a more complex, incomplete, and uncertain image. This article reviews the evolution of APS from the first description of lupus anticoagulant to the current criteria used to guide clinical research, critiques laboratory methods used to identify autoantibodies, comments on prognosis and management, and summarizes insights into the pathophysiology of this elusive disorder.