Budding, vesiculation and permeabilization of phospholipid membranes—evidence for a feasible physiologic role of β2-glycoprotein I and pathogenic actions of anti-β2-glycoprotein I antibodies

The significance of autoantibodies against β2-glycoprotein I

The antiphospholipid syndrome (APS) is defined by the persistent presence of antiphospholipid antibodies in patients with a history of thrombosis and/or pregnancy morbidity, including fetal loss. APS is an autoimmune disease with a confusing name because the pathologic auto-antibodies are shown to be directed against the plasma protein β(2)-glycoprotein I and not against phospholipids. In fact, auto-antibodies that recognize phospholipids themselves are not associated with thrombosis but with infectious diseases. One of the intriguing questions is why autoantibodies against β(2)-glycoprotein I are so commonly found in both patients and the healthy. Several potential mechanisms have been suggested to explain the increased thrombotic risk in patients with these autoantibodies. In this overview, we will summarize our knowledge on the etiology of the autoantibodies, and we will discuss the evidence that identify autoantibodies against β(2)-glycoprotein I as the culprit of APS.

Budding of giant unilamellar vesicles induced by an amphitropic protein β2-glycoprotein I

β(2)-glycoprotein I (β(2)GPI) is a plasma protein capable of binding reversibly to membranes, and is classified among the amphitropic proteins. Part of the protein intercalates into the outer membrane leaflet, altering the difference between the preferred areas of the membrane leaflets, which results in membrane shape transformations. Budding, as a specific example of such shape transformations, was studied using giant unilamellar vesicles. Our aim was to identify the vesicle parameters that influence the degree of membrane budding by studying this process qualitatively and quantitatively. A simple theoretical model has been developed and assessed against the experimental observations. The results show that β(2)GPI binds in a concentration dependent manner, causing transitions between vesicle shapes with increasing numbers of buds. Higher numbers of buds are characteristic of larger and/or more flaccid vesicles. When the vesicle membrane is strained, a higher β(2)GPI concentration is needed to produce the same effects as on the unstrained vesicle. Vesicles were found to be highly individual in their behaviour, so each was treated individually. Specific vesicle behaviour was found to be the consequence of the neck between the main vesicle body and the buds, which could be either open, closed for the exchange of solution, or closed for the exchange of both solution and membrane.

Excessive exposure to anionic surfaces maintains autoantibody response to beta(2)-glycoprotein I in patients with antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune prothrombotic disorder associated with autoantibodies to phospholipid (PL)-binding proteins, such as beta(2)-glycoprotein I (beta(2)GPI). We have recently reported that binding of beta(2)GPI to anionic PL facilitates processing and presentation of the cryptic beta(2)GPI epitope that activates pathogenic autoreactive T cells. To clarify mechanisms that induce sustained presentation of the dominant antigenic beta(2)GPI determinant in patients with APS, T-cell proliferation induced by beta(2)GPI-treated phosphatidylserine liposome (beta(2)GPI/PS) was evaluated in bulk peripheral blood mononuclear cell cultures. T cells from patients with APS responded to beta(2)GPI/PS in the presence of immunoglobulin G (IgG) anti-beta(2)GPI antibodies derived from APS plasma, and this response was completely inhibited either by the depletion of monocytes or by the addition of anti-FcgammaRI antibody. These findings indicate that efficient presentation of the cryptic determinants can be achieved by monocytes undergoing FcgammaRI-mediated uptake of beta(2)GPI-bound anionic surfaces in the presence of IgG anti-beta(2)GPI antibodies. Finally, beta(2)GPI-bound oxidized LDL or activated platelets also induced the specific T-cell response. Continuous exposure to these anionic surfaces may play a critical role in maintaining the pathogenic anti-beta(2)GPI antibody response in patients with APS.

Coalescence of phospholipid membranes as a possible origin of anticoagulant effect of serum proteins

Interactions between phospholipid membranes (made of palmitoyloleoylphosphatidylcholine, cardiolipin and cholesterol) after addition of beta2 glycoprotein I (beta2GPI) or anti-beta2GPI antibodies or a mixture of both were studied by observing giant phospholipid vesicles under the phase contrast microscope. Both, negatively charged and neutral vesicles coalesced into complexes and adhered to the bottom of the observation chamber in the presence of beta2GPI in solution. Anti-beta2GPIs alone or previously mixed with beta2GPI caused coalescence of charged but not neutral vesicles, i.e. for neutral membranes the effect of beta2GPI was abolished by the presence of anti-beta2GPIs. Since the presence of the above adhesion mediators can prevent fragmentation of the membrane we propose a (new) possible anticoagulant mechanism for some serum proteins by preventing the release of prothrombogenic microexovesicles into circulation.

V Meeting of the European Forum on Antiphospholipid Antibodies

The "V Meeting of the European Forum on Antiphospholipid Antibodies" was organized in Barcelona, Catalonia, Spain, on December 2-3, 2005. This biannual meeting becomes a real forum for the interchange of information on current research on the field antiphospholipid antibodies and the antiphospholipid syndrome, as well as a starting point for many new research projects. About 170 physicians and researchers from a great variety of medical specialities (internists, rheumatologists, hematologists, obstetricians, pediatricians, neurologists, cardiologists, immunologists, and biologists, among others) coming from 20 European countries attended the sessions. This report summarizes the main studies and new research projects presented during this Forum Meeting.

Interaction of giant phospholipid vesicles containing cardiolipin and cholesterol with beta2-glycoprotein-I and anti-beta2-glycoprotein-I antibodies

Antiphospholipid syndrome is characterized with thrombotic events and/or pregnancy morbidity and antiphospholipid antibodies (aPL). The most common antigen for aPL is beta2-glycoprotein-I (beta(2)GPI), a plasma protein binding to negatively charged phospholipids. The influence of aPL on coagulation is not well understood. Giant phospholipid vesicles (GPVs) are a convenient in vitro system for studying interactions between phospholipid membranes and proteins resulting in the change of the vesicles' configuration. We aimed to set up an in vitro model and to study changes in the morphology of GPVs with high content of cardiolipin upon addition of beta(2)GPI and/or IgG fraction of a patient with antiphospholipid syndrome (APS). Addition of the IgG fraction of the APS patient caused lateral segregation of the membrane inclusions and adhesion of GPVs. Addition of beta(2)GPI caused adhesion of GPVs. Addition of both, the patient IgG fraction and beta(2)GPI caused adhesion of vesicles to the glass slides and to each other, formation of pores and burst of vesicles. Our results indicate that adhesion of the cardiolipin-containing vesicles does not seem specific for added proteins, rather, it indicates electrostatic and curvature-mediated interactions between the membrane constituents.

Cell Membrane Microparticles in Blood and Blood Products: Potentially Pathogenic Agents and Diagnostic Markers

Cell membrane microparticles (MPs) circulate in the blood of healthy donors, and their elevated counts have been documented in various pathologies. Microparticles are phospholipid microvesicles of 0.05 to 1.5 microm in size, containing certain membrane proteins of their parental cells. Thus, different phenotypes of MPs derived from platelets, blood cells, endothelial cells, and some other cell types have been identified in plasma. Microparticles are released by various stimuli including shear stress, complement attack, or proapoptotic stimulation. Microparticle release is a highly controlled process and likely independent from metabolic energy. Elevated MPs in various diseases indicate their diagnostic importance, particularly in vascular pathologies. Moreover, MPs in blood possess a broad spectrum of biologic activities. Microparticles may facilitate cell-to-cell interactions, induce cell signaling, or even transfer receptors between different cell types. The physiological roles of MPs in various tissue defense processes have been suggested and the pathophysiologic implications of MPs in thrombosis, inflammation, cancer metastasis, or response to pathogens have been proposed. This is important for transfusion medicine because MPs are present in both plasma and cellular blood products. Thus, the investigation of potentially pathogenic effects of MPs in blood products and of MP release associated with blood product processing and storage have yet to come.