Antibodies against platelet-activating factor in patients with antiphospholipid antibodies

Antiphospholipid antibodies: paradigm in transition

OBJECTIVES

This is a critical review of anti-phospholipid antibodies (aPL). Most prior reviews focus on the aPL syndrome (APS), a thrombotic condition often marked by neurological disturbance. We bring to attention recent evidence that aPL may be equally relevant to non-thrombotic autoimmune conditions, notably, multiple sclerosis and ITP.

ORGANIZATION

After a brief history, the recent proliferation of aPL target antigens is reviewed. The implication is that many more exist. Theories of aPL in thrombosis are then reviewed, concluding that all have merit but that aPL may have more diverse pathological consequences than now recognized. Next, conflicting results are explained by methodological differences. The lupus anticoagulant (LA) is then discussed. LA is the best predictor of thrombosis, but why this is true is not settled. Finally, aPL in non-thrombotic disorders is reviewed.

CONCLUSION

The current paradigm of aPL holds that they are important in thrombosis, but they may have much wider clinical significance, possibly of special interest in neurology.

Cell-derived microparticles and exosomes in neuroinflammatory disorders

All blood cells and the vascular endothelium shed microparticles (MP) from their plasma membranes when suitably stimulated, and assay of MP in patient blood has found increasing application to the monitoring of disease states. In addition, mounting evidence suggests that MP are not mere epiphenomena but play significant roles in the pathophysiology of thromboses, inflammation, and cancers. This chapter endeavors to summarize the limited number of studies thus far done on MP in neurological disorders such as multiple sclerosis (MS), transient ischemic attacks, and the neurological manifestations of antiphospholipid syndrome (APS). In addition, the chapter offers some plausible hypotheses on possible roles of MP in the pathophsyiology of these disorders, chiefly, the hypothesis that MP are indeed important participants in some neuropathologies, especially those which are ischemic in nature, but probably also inflammatory ones. The chapter also goes over the history and general principles of MP studies (e.g., assay methods and pitfalls), comparison with alternative methods (e.g., soluble markers of disease states), subclasses of MP (such as exosomes), and other topics aimed at helping readers to consider MP studies in their own clinical fields. Tables include a listing of bioactive agents known to be carried on MP, many of which were heretofore considered strictly soluble, and some of which can be transferred from cell to cell via MP vectors, for example certain cytokine receptors.

The antiphospholipid syndrome and atherosclerosis: clue to pathogenesis

Regulation of blood vessels is intrinsically tied to inflammatory signaling. Recent research suggests that chronic inflammation is associated with atherosclerosis risk. The antiphospholipid syndrome is a prototypic autoimmune disease. Disturbance of blood vessel homeostasis in this disorder may increase risk for atherosclerosis by mechanisms that are direct (through antibody targeting of blood vessel-regulating proteins) or indirect (via inflammatory mechanisms that have recently been implicated in autoantibody-mediated thrombosis).

No evidence for a putative involvement of platelet-activating factor in systemic lupus erythematosus without active nephritis

BACKGROUND

Platelet-activating factor (PAF) seems to be implicated in systemic lupus erythematosus (SLE) patients with associated renal diseases.

AIMS

In this study, we ensured the role of PAF in SLE patients without renal complications.

METHODS

Blood PAF and acetylhydrolase activity, plasma soluble phospholipase A(2), and the presence of antibodies against PAF were investigated in 17 SLE patients without active nephritis and in 17 healthy controls.

RESULTS

Blood PAF levels were not different (p=0.45) between SLE patients (6.7+/-2.8 pg/ml) and healthy subjects (9.6+/-3.1 pg/ml). Plasma acetylhydrolase activity (the PAF-degrading enzyme) was significantly (p=0.03) elevated in SLE patients (57.8+/-6.4 nmol/min/ml) as compared with controls (37.9+/-2.6 nmol/min/ml). Plasma soluble phospholipase A(2) (the key enzyme for PAF formation) was not different (p=0.6) between SLE patients (59.1+/-5.1 U/ml) and controls (54.7+/-2.4 U/ml). Antibodies against PAF were detected only in 3/17 SLE patients. Flow cytometry analysis did not highlight PAF receptors on circulating leukocytes of SLE patients.

CONCLUSION

This clinical study highlights no evidence for a putative important role of PAF in SLE patients without active nephritis.

Which antiphospholipid antibody tests are most useful?

Despite an active international effort to improve diagnosis and treatment of the antiphospholipid syndrome (Hughes syndrome), there remain problems of lack of standardization and lack of prospective and multivariate epidemiologic analysis which restrict the diagnostic and predictive ability of commercially available tests. Nevertheless, current published series provide some data from which strategic approaches can be used to maximize the efficiency and usefulness of available tests. For further updates on new research and developments of interest to physicians and patients with this syndrome, the following web sites may prove helpful: www.slrapls.org, www.hematology.org, www.acforum.org, www.americanheart.org, www.rarediseases.org, www.aarda.org, and www.lupus.org.

What causes the antiphospholipid syndrome?

The antiphospholipid syndrome (APS) is characterized by unpredictable, sporadic, thrombotic events. The cause of the thrombosis is probably multifactorial and may involve disparate effects of the autoantibodies associated with the syndrome, which are known to interfere with various protein regulators of hemostasis. An integrated theory of pathogenesis that accounts for the diversity of autoantibodies and their effects suggests that cellular inflammation or apoptosis within the vasculature may lead to oxidation or turnover in phospholipid membranes. Thus, normally cryptic, functionally important epitopes of phospholipid-binding proteins are subjected to increased exposure to immune surveillance.

Immunopathogenesis of the antiphospholipid antibody syndrome: an update

The primary antiphospholipid antibody syndrome is characterized clinically by the presence of venous and arterial thrombosis, recurrent fetal loss, and thrombocytopenia. The presence of antiphospholipid antibodies is a central serologic finding in primary antiphospholipid antibody syndrome, and plays a critical role in diagnosis. Contrary to initial reports, it is now widely accepted that these autoantibodies are directed predominantly against two antigens: phospholipid-binding plasma protein beta2-glycoprotein I and prothrombin. The mechanism by which antiphospholipid antibodies cause disease is under vigorous investigation. It is hypothesized that antiphospholipid antibodies induce a procoagulant state by binding to antigens on endothelial cells and trophoblast cell surfaces. Indeed, beta2-glycoprotein I appears to function as a cofactor that facilitates this interaction. The resulting endothelial cell activation is associated with cell-surface expression of adhesion molecules that lead to monocyte adhesion - the first steps in thrombosis. Although the precise mechanism that mediates endothelial cell-platelet interaction have not been fully elucidated, platelet binding to the endothelium appears to be the next phase in thrombosis. Thus, the antiphospholipid antibody may be a triggering or activating factor in placental spiral artery thrombosis and subsequent placental infarction. More recently, a role for annexin V has emerged. Studies suggest that thrombosis in the antiphospholipid syndrome may be due to disruption of the annexin shield by antiphospholipid (and cofactor) antibodies, which results in the increased exposure of trophoblasts and endothelial cells to thrombogenic phospholipids.

Antiphospholipid syndrome and recurrent miscarriages

Sixty percent of recurrent spontaneous abortions are unexplained. Antiphospholipid syndrome is a multisystem disease with the predominant features of venous and arterial thrombosis, recurrent pregnancy loss, foetal death and the presence of antiphospholipid antibodies. Many epidemiological studies focus on antiphospholipid autoantibodies syndrome (APS) as a cause of recurrent spontaneous abortion (RSA). It is found that 7-25% of RSA would have APS as the main risk factor. 'Association not being synonymous with cause', the proportion of abortions due to the APS is difficult to estimate for several reasons: definition of recurrent abortion is variable, the assays for antiphospholipid antibodies are not well standardised, inclusion of patients in the study group according to the antibodies titre is author dependent. Recent studies suggest association of antiphospholipid antibodies syndrome not only with recurrent abortions but also with infertility. New mechanisms are described by which antiphospholipid antibodies could cause placental thrombosis and infarction, acting directly on the surface anticoagulant expressed on trophoblastic cells. Only lupus anticoagulant (LA) and anticardiolipin antibodies (aCL) assays are sufficiently standardised to be usable in routine. Testing for other antiphospholipid antibodies (aPLs) should remain investigational. Several treatments have been proposed: low doses of aspirin, low or immunosuppressive doses of corticosteroids, and preventive or effective dose of heparin, intravenous immunoglobulin.

Clinical importance of antibodies against platelet activating factor in antiphospholipid syndrome manifestations

BACKGROUND

We assessed whether antibodies against platelet activating factor (PAF) are related to the presence of antiphospholipid syndrome (APS) clinical manifestations, in particular thrombosis, in patients with connective tissue diseases.

MATERIALS AND METHODS

Anti-PAF, anticardiolipin (aCL), antibeta2 glycoprotein I (antibeta2GPI) and antiphosphatidylcholine (anti-PC) antibodies were determined in 52 patients with APS, 29 patients with systemic lupus erythematosus (SLE) aCL but without APS, 30 patients with SLE without aCL, and 30 patients with scleroderma. A new enzyme-linked immunosorbent assay (ELISA) was developed for determining anti-PAF antibodies in a bovine serum-free fashion.

RESULTS

The ELISA showed high specificity. Homologous inhibition experiments showed 60-70% inhibition. Anti-PAF antibodies were found in 18/52 APS patients, 10/29 SLE/aCL+ patients, 9/30 SLE/aCL- patients and 3/30 scleroderma patients. Anti-PAF antibodies were significantly associated with anti-PC antibodies (odds ratio [OR] 12. 7, P < 0.01), and there was a modest association with immunoglobulin G (IgG) aCL (OR 3.1, P > 0.10), but not with IgM aCL or antibeta2GPI. Three SLE/aCL+ patients and five SLE/aCL- patients had clinical manifestations characteristic of APS. All these patients had anti-PAF antibodies, while none had high titres of aCL or antibeta2GPI antibodies and only one had anti-PC antibodies. Among the combined APS and SLE groups, the presence of anti-PAF antibodies was significantly associated with clinical manifestations which are characteristic of APS (OR 2.6, P = 0.02). The effect was independent of IgG aCL and antibeta2GPI antibodies.

CONCLUSIONS

Anti-PAF antibodies are common in APS and SLE and comprise an independent factor for the development of thrombosis. Several patients experiencing thromboses have anti-PAF antibodies without other antiphospholipid specificities.

Antibodies to platelet-activating factor are associated with borderline hypertension, early atherosclerosis and the metabolic syndrome

OBJECTIVE

Platelet-activating factor (PAF) is a phospholipid inflammatory mediator which is synthesized by a variety of cells, including monocytes and endothelial cells, and PAF can be retained in activated endothelial cell membranes. Furthermore, PAF-like lipids are produced in other phospholipid membranes as in oxidized LDL. Atherosclerosis is a chronic inflammation in the artery wall, but little is known about the role of immune reactions in the early stages of development of cardiovascular disease. In the present study we investigated if there are antibodies to PAF (aPAF) that may play a role in borderline hypertension and early atherosclerosis.

DESIGN

Seventy-three men with borderline hypertension (BHT) and 73 age-matched normotensive (NT) men (diastolic blood pressure 85-94 and <80 mmHg, respectively) were recruited from a population screening programme. Antibody levels were determined by use of ELISA. Carotid intima-media (IM)-thickness and atherosclerosis was determined by B-mode ultrasonography.

RESULTS

BHT men had 49.3% higher aPAF levels of IgG class than NT controls (P = 0.0007). Antibodies to the biologically inactive lysoPAF did not differ between BHT and NT group. aPAF levels were associated with IM-thickness in the left (P = 0.02) and right (P = 0.009) carotid artery. Furthermore, aPAF levels were enhanced in individuals with the metabolic syndrome (n = 44) as compared to those without (n = 102; P = 0.009), and also significantly associated with insulin levels (P = 0.02) and insulin resistance (P = 0.02).

CONCLUSIONS

aPAF antibodies may reflect early vascular changes and thus serve as novel markers for disease, and they may also be pathogenic, by eliciting an inflammatory reaction in the vascular wall.

Antiphospholipid antibodies and the antiphospholipid antibody syndrome

The antiphospholipid antibody syndrome is a multiple-system disorder characterized by persistently elevated antiphospholipid antibodies and/or arterial or venous thrombosis, thrombocytopenia, or recurrent spontaneous abortion. Anticardiolipin antibodies and the lupus anticoagulant are different classes of antiphospholipid antibodies associated with this disorder. Cutaneous manifestations are common and may be the presenting sign of the underlying disease. This article reviews the clinical manifestations, laboratory assays, histopathologic features, and treatment of the antiphospholipid antibody syndrome.

Pathogenesis of the antiphospholipid antibody syndrome

Endothelial and/or platelet activation likely initiates thrombus formation. Whether antiphospholipid antibody (aPL) is an activator, a toxic response, or a protective response is not clear, nor is it certain whether aPL is germ-line encoded or antigen-driven. The pregnancy model is particularly informative. Alternative hypotheses about thrombogenicity which relegate aPL to the role of bystander have not yet been excluded.

Antiphospholipid antibody: future developments

In September 1991, an NIH workshop on molecular and biological aspects of antiphospholipid antibodies (aPL) identified questions for future studies: are the antibodies defined by the ELISA and by the lupus anticoagulant tests the same? Is aPL directly responsible for disease? What is the antigen? What drives the production of aPL? What is the role of beta 2-glycoprotein I? What accounts for patient heterogeneity? Can a satisfactory animal model be developed? The NIH workshop did not address important clinical questions, including those of pathogenesis and treatment. In 1994 many of these questions have at least partial answers. beta 2-glycoprotein I appears to be an obligatory component of the antigen, abnormal coagulation is the probable central pathogenic event and animal models now exist. There are still critical unknowns that define a future research agenda: the genetics of the aPL syndrome, the relationship of aPL to SLE and mechanisms of pathogenesis (including why clotting is episodic and what is the cellular or anatomical location of the initial injury). Despite a decade of clinical studies, risk prediction for defined patient groups is only now beginning to be studied. There are still almost no randomized, prospective, controlled treatment trials on any aspect of the syndrome nor are there definitive answers regarding which among antiplatelet, anticoagulant or antithrombin therapies is superior, what is the role of immunosuppressive therapy and what experimental therapies might be introduced. The molecular biology of the antigen-antibody interaction will soon be fully understood, then the cellular and the organism biology. Definitive treatment interventions may await this understanding but adequate therapies are available at this time to conduct important and effective prospective clinical trials.