Effective induction of anti-phospholipid and anticoagulant antibodies in normal mouse

Pathogenetic potential of antiphospholipid antibodies

Antiphospholipid antibodies are autoantibodies that recognize phospholipid-binding proteins such as beta2 glycoprotein (beta2GP)-I, prothrombin or annexins. These antibodies have been associated with arterial or venous thrombotic events and pregnancy morbidity. The molecular mechanisms responsible for the pathogenetic potential of these antibodies include: resistance to activated protein C, acquired Factor XII deficiency resulting in suppression of intrinsic fibrinolytic activity, activation of endothelial cells through the nuclear factor kappaB pathway leading to tissue factor upregulation, adhesion molecule and cytokine expression and activation of platelets. Opposite effects, such as the potentiation of the inhibitory action of beta2GPI on the activation of Factor XI, make the dynamics of the interaction of these antibodies with the coagulation system rather complex. Many of the above functions can be mediated by signaling through molecules of the tumor necrosis factor receptor family, such as CD40, which is recognized by purified anti-beta2GPI antibodies.

Viral origin of antiphospholipid antibodies: endothelial cell activation and thrombus enhancement by CMV peptide-induced APL antibodies

Our observations and those from others, give further support to our hypothesis that "autoimmune aPL" may be generated by immunization with products from bacteria or viruses after incidental exposure or infection. We also were able to generate APS-like syndrome in a strain of mice susceptible to autoimmunity, indicating that other factors such as genetics are likely to be involved in the development of APS. Furthermore, not all aPL antibodies generated by immunization with bacterial or viral products are pathogenic. Based on the clinical experience and on the numerous reports indicating presence of aPL in a large number of infectious diseases, it may be expected that not all aPL antibodies produced during infection will be pathogenic. We hypothesize that a limited number aPL antibodies induced by certain viral/bacterial products would be pathogenic in certain groups of predisposed individuals. Identification of these bacterial and/ or viral agents may help to find strategies for the prevention of production of aPL "pathogenic" antibodies. Alternatively, free peptides may be used to induce tolerance against aPL production.

Origin of antiphospholipid antibodies

Our observations and those from others give further support to our hypothesis that "autoimmune aPL" may be generated by immunization with products from bacteria or viruses after incidental exposure or infection. We also were able to generate an APS-like syndrome in a strain of mice susceptible to autoimmunity, indicating that other factors such as genetic factors are likely to be involved in development of APS. Furthermore, not all aPL generated by immunization with bacterial or viral products were pathogenic. Based on the clinical experience and on the numerous reports indicating the presence of aPL in large number of infectious diseases, it may be expected that not all aPL produced during infection are pathogenic. We hypothesize that a limited number aPL induced by certain viral or bacterial products would be pathogenic in certain groups of predisposed individuals. Identification of those bacterial or viral agents may help to find strategies for the prevention of production of "pathogenic" aPL. Alternatively, free peptides may be used to induce tolerance against aPL production.

GDKV-Induced Antiphospholipid Antibodies Enhance Thrombosis and Activate Endothelial Cells In Vivo and In Vitro

Antiphospholipid (aPL) Abs are associated with thrombosis, pregnancy loss, and thrombocytopenia in patients with systemic lupus erythematosus or primary antiphospholipid syndrome (APS). β2-Glycoprotein I (β2GPI), a phospholipid-binding serum protein, is involved in aPL binding to phospholipids. aPL can be generated in mice by immunization with β2GPI, and these Abs are thrombogenic and cause pregnancy loss in mice. The objective of this study is to determine whether aPL induced by immunization with the phospholipid-binding site of β2GPI are thrombogenic and whether they activate endothelial cells (EC) in vivo and in vitro. Murine monoclonal aPL were generated from spleen cells of a mouse immunized with GDKV, a synthetic 15-aa peptide spanning Gly274–Cys288 in the fifth domain of human β2GPI, which represents the phospholipid-binding site of β2GPI. The Abs generated had aPL and anti-β2GPI activities. The effect of these Abs on thrombus formation and on EC activation in vivo was determined using a mouse model of thrombosis and microcirculation that enables examination of the adhesion of leukocyte to EC as an indication of EC activation as well as adhesion molecule expression using in vitro ELISA analysis. Mice injected with this monoclonal aPL showed a significant increase in leukocyte sticking and also produced larger thrombi that persisted longer. Exposure to GDKV-induced aPL for 4 h significantly increased surface Ag expression of E-selectin, ICAM-1, and VCAM-1. These data indicate that aPL induced by immunization with the phospholipid binding site of β2GPI are thrombogenic and activate endothelial cells.

Origin of antiphospholipid antibodies: induction of aPL by viral peptides

The clinical associations of antiphospholipid antibodies (aPL) are well recognized but the mechanism(s) causing the production of these antibodies are not yet known. We demonstrated the induction of pathogenic aPL antibodies that caused intrauterine fetal death and transverse myelopathy due to spinal cord infarction in mice by immunization with foreign beta2GPI. We also induced aPL and anti-beta2-GPI in mice by immunization with PL-binding viral peptides and hypothesized that in APS patients, aPL may be induced by beta2GPI-like-PL-binding products of common human bacteria and viruses.

A subgroup of multiple sclerosis patients with anticardiolipin antibodies and unusual clinical manifestations: do they represent a new nosological entity?

The presence of antibodies to cardiolipin (ACL Abs) has been reported in some patients suffering from multiple sclerosis (MS), especially of the "neuromyelitic" type. In addition, bright T2-imaging foci (unidentified bright objects) are occasionally detected on brain magnetic resonance imaging (MRI) scans, in patients with antiphospholipid syndrome. From a cohort of 100 patients with a probable or definite diagnosis of MS according to Poser's criteria, we isolated a subgroup of 20 patients (8 males and 12 females) consistently positive for ACL Abs. These patients were followed up neurologically for 1 to 3 years and brain MRI scanning and a complete autoimmune screening were performed. Nineteen (19 of 20) of our patients had the classic neuroimaging features of MS (multiple white-matter T2 bright foci on the MRI scan). The most common neurological syndrome was chronic, slowly progressing myelopathy (presenting as myelopathy, neuromyelitis optica, or spinocerebellar syndrome; 15 of 20), and optic neuropathy (6 of 20). Headache was a dominant symptom in 8 of 20 patients. Less common symptoms included cognitive and psychiatric disorders and chronic fatigue. The mean levels of ACL Abs were 38.8+/-28.2 GPL (normal values up to 7.5). Oligoclonal bands in the cerebrospinal fluid were detected in only 3 of 20 patients. Patients were treated with acetylsalycilic acid and occasionally with short courses of steroids. The progression of the chronic myelopathic/spinocerebellar syndrome was slower than expected in MS (only 2 patients deteriorated whereas 4 improved during a mean follow-up period of 20.8+/-7.1 months). We conclude that patients with probable or definite diagnosis of MS, and consistently elevated levels of ACL Abs show a slower progression and some atypical (for MS) features, such as persistent headaches and absence of oligoclonal bands in the cerebrospinal fluid. In these patients, other, presumably vascular, mechanisms may be involved in the pathogenesis of the neurological symptoms. Therefore, management should include antiplatelet or even anticoagulant agents.

Monoclonal Anti-Cardiolipin Antibodies from New Zealand Black × New Zealand White F1 Mice React to Thrombomodulin

The reactivity with and affinity for thrombomodulin (TM) of monoclonal anti-cardiolipin Abs (MoaCL), derived from a New Zealand Black × New Zealand White F1 (NZB/W F1) mouse, were studied to investigate the pathogenicity of anti-cardiolipin Abs (aCL). Four of eighteen MoaCL were found to react with rabbit TM when examined using ELISA. These four MoaCL also reacted with synthetic peptide that included the epidermal growth factor-like domain of human TM, a binding site for thrombin. The reaction with TM of these four MoaCL was inhibited by bovine thrombin. When the affinity for TM of the MoaCL was determined, the dissociation constants (Kd) ranged from 4.8 × 10−9 to 4.7 × 10−8 M. By contrast, examination of the affinity for cardiolipin (CL) gave values from 8.3 × 10−6 to 7.4 × 10−5 M. Thus, these MoaCL reacted to TM with a higher affinity than to CL. Moreover, these MoaCL also bound to TM on HUVEC and down-regulated the expression level of TM on the surface of HUVEC due to internalization of TM. The binding of thrombin to TM is known to initiate rapid protein C activation, and complexes of activated protein C and protein S show anticoagulatory activity. Thus, the present studies suggest that certain pathogenic aCL cross-react with TM and induce down-regulation of TM on endothelial cells, followed by induction of thrombosis.

The lupus anticoagulant/antiphospholipid syndrome

Lupus anticoagulants and anticardiolipin antibodies have been strongly associated with the risk of thrombosis, recurrent fetal loss, thrombocytopenia, and a number of other clinical manifestations that together have been referred to as the antiphospholipid syndrome. Despite growing evidence of the significance of this relationship, the pathogenetic mechanisms involved are largely unknown. Recent data suggest strongly that lupus anticoagulants (LACs) and anticardiolipin antibodies (ACAs) are antibodies to protein-phospholipid complexes rather than to phospholipids, as had originally been thought, and that other protein-phospholipid complexes, not recognized by standard assays for LACs or ACAs, may also exist in patients with the antiphospholipid syndrome. Although very recent experimental data may lead to new therapeutic approaches in this syndrome, at present we can only deal with the thrombotic risk by the use of long-term anticoagulation. This chapter reviews current methods of diagnosis, concepts of pathogenesis, and the basis for an approach to anticoagulation in patients at risk for thrombosis or other manifestations of the antiphospholipid syndrome.

Cerebrovascular disease with antiphospholipid antibodies: immune mechanisms, significance, and therapeutic options

There has been a recent, dramatic surge in interest in antiphospholipid antibodies and associated clinical disorders, especially focal ischemic cerebrovascular disease. Antiphospholipid antibodies are a heterogeneous group of antibodies with varying specificities. Coagulation assays will detect lupus anticoagulants while enzyme-linked immunosorbent assays detect anticardiolipin antibodies. There are numerous potential links between antiphospholipid antibodies and coagulation disorders, including interaction of antiphospholipid antibodies and a cofactor, beta 2-glycoprotein I, which itself is involved in coagulation mechanisms. While the specific mechanism of antiphospholipid antibody-related coagulopathy is unknown, it is clear that antiphospholipid antibodies are associated with an immune-mediated prothrombotic state. Patients with the highest titers of IgG antiphospholipid antibodies have a relatively high risk of recurrent thrombotic events, especially stroke, deep venous thrombosis, and spontaneous abortion. Because of limited controlled, prospective data, current therapy remains empiric and directed at coagulation mechanisms, immune mechanisms, or both.

Monoclonal antibody to phosphatidylserine inhibits Na+/K(+)-ATPase activity

A monoclonal IgG, directed to phosphatidylserine (PS1G3), partially (40-50%) inhibited Na+/K(+)-ATPase activity (forward running reaction cycle) without affecting the K0.5 values for Na+,K+ and MgATP. The Hill or interaction coefficients (nH) for Na+ and K+ for this reaction were reduced from 3.0 to 1.6 and from 1.6 to 0.8, respectively. The K(+)-stimulated p-nitrophenylphosphatase activity (p-NPPase), which is a partial reaction sequence of the Na+/K(+)-ATPase system (but in the backward running mode), was inhibited more strongly (about 70%) due to an increase in K+/substrate antagonism. In this system K0.5 and nH values for both p-nitrophenyl phosphate (p-NPP) and K+ were increased by the mAb. At the maximally inhibitory concentration of PS1G3 the Vmax of the p-NPPase was also reduced. Partial reactions, which were inhibited by PS1G3, are: (1) the Na(+)-activated phosphorylation (non-competitive vs. Na+), (2) the Rb+ occlusion (competitive vs. Rb+). Partial reactions not harmed by PS1G3 are: (3) the K(+)-dependent dephosphorylation, (4) the K(+)-dependent E1 + K+<-->E2K transition. We conclude that PtdSer is involved in cation occlusion, possibly by forming part of the access gate.

Induction of phospholipid-binding antibodies in mice and rabbits by immunization with human beta 2 glycoprotein 1 or anticardiolipin antibodies alone

Anticardiolipin (aCL) antibodies are autoantibodies present in high concentrations in patients with the antiphospholipid syndrome (APS), a disorder of recurrent thrombosis and pregnancy loss. What induces aCL antibodies is uncertain, but a recent report suggested that immunization of mice with beta 2 glycoprotein 1 (beta 2 GP1) in Freund's complete adjuvant (FCA) resulted in aCL antibody production in the recipient mice. Since this observation might explain how autoantibodies might be induced by poor immunogens, such as phospholipids, we decided to explore the question further. In our first series of experiments, we found that aCL antibodies were induced in mice by beta 2GP1 mixed with adjuvants that did not contain lipids (Adju-Prime or aluminium hydroxide). This excluded the possibility that antibody induction occurred because beta 2GP1 formed complexes with lipids in FCA. We also found that aCL antibodies always appeared before anti-beta 2GP1 antibodies, excluding the possibility that aCL antibodies were directed to beta 2GP1 or were induced by formation of anti-idiotypic antibodies (to anti-beta 2GP1). In experiments, we found that immunization of mice with human IgG antibodies from patients with the APS (IgG-APS), also induced aCL antibodies. Immunization with pure bovine serum albumin (BSA) did not induce aCL antibodies. We propose that aCL antibodies are induced by proteins with high avidity for phospholipids. These proteins may be bound to phospholipids when introduced, or may bind circulating phospholipids, so transforming phospholipid molecules into immunogens. Similar mechanisms might explain autoantibody induction to other poor immunogens.

Release of 14-kDa group-II phospholipase A2 from activated mast cells and its possible involvement in the regulation of the degranulation process

Group II phospholipase A2 was detected in appreciable amounts in rat peritoneal mast cells. The effect of several inhibitors specific to 14-kDa group-II phospholipase A2, including two proteinaceous inhibitors and a product of microorganisms with a low molecular mass, on mast-cell activation was examined. When rat peritoneal mast cells were sensitized with IgE and then challenged with antigen, the specific phospholipase-A2 inhibitors suppressed histamine release in a concentration-dependent manner. By contrast, these inhibitors showed no effect on prostaglandin generation under the same conditions. Histamine release from rat peritoneal mast cells subjected to non-immunochemical stimuli, such as concanavalin A, the Ca2+ ionophore A23187, compound 48/80 and substance P was also suppressed. When rat peritoneal mast cells were treated with 14-kDa-group-II-phospholipase-A2-specific inhibitors, washed and stimulated, histamine release was not affected appreciably. Similar suppressive effects of the inhibitors on histamine release were observed with mouse cultured bone-marrow-derived mast cells. When bone-marrow-derived mast cells were activated, they secreted both a soluble and an ecto-enzyme form of 14-kDa group-II phospholipase A2, although appearance of the enzyme associated with the external surface of cells was observed transiently. An appreciable amount of membrane phospholipids was degraded during activation of mast cells, which was decreased by treatment with 14-kDa-group-II-phospholipase-A2 inhibitor. These observations suggest that degranulation and eicosanoid generation in mast cells are regulated independently by discrete phospholipases A2 and that the 14-kDa group-II phospholipase A2 released from mast cells during activation may play an essential role in the progression of the degranulation process.

Induction of antiphospholipid autoantibodies by immunization with beta 2 glycoprotein I (apolipoprotein H)

A subset of patients with systemic lupus erythematosus has autoantibodies to acidic phospholipids. Since lipids are poor immunogens, the mechanism responsible for the induction of these antibodies is unclear. Immunization of a normal rabbit and normal mice with purified human beta 2-glycoprotein I (apolipoprotein H) resulted in the production of high levels of two non-cross-reactive antibody populations, anti-apolipoprotein H, and antiphospholipid. The antiphospholipid antibodies had binding specificities indistinguishable from autoantibodies obtained from human and murine lupus. These findings suggest a novel mechanism for the induction of antiphospholipid autoantibodies.

Lupus-like anticoagulant properties of murine monoclonal antibodies to beta 2-glycoprotein I

The lipid-binding inhibitor of coagulation, beta 2-glycoprotein I (beta 2GPI), has been shown to form the antigen to which some autoantibodies against anionic phospholipids (aPL) are directed. Six murine monoclonal antibodies (MAbs) of the IgG1 isotype were raised against human beta 2GPI and could be subdivided into three groups on the basis of mutual competition experiments. MAbs 9G1 and 8C3 (group A) markedly inhibited the binding of immunoglobulins from aPL-positive sera to beta 2GPI-coated wells. Using a lipid-based solid-phase radioimmunoassay, the MAbs interacted with both anionic phospholipids and phosphatidylethanolamine, but not phosphatidylcholine, in a beta 2GPI-dependent manner. A cross-reaction between beta 2GPI from several (including bovine) species was seen with one of the MAbs (9G1). All six MAbs induced dose-dependent prolongation of the DAPTT, DRVVT, KCT and TTI clotting times of human plasma, whereas 9G1 was the sole antibody to be inhibitory with plasma from bovine origin. Synergistic inhibitory effects were observed with MAbs used in pairs provided that they did not compete with each other for beta 2GPI binding. The anticoagulant activity of the MAbs was fully neutralized by the addition of freeze-thawed platelets. The MAbs described here resemble lupus anticoagulants in several respects which makes them valuable to study the involvement of beta 2GPI in the autoimmune thrombotic pathophysiology.