Immunologic detection of phosphatidylserine externalization during thrombin-induced platelet activation

The Problem of Apoptotic Processes Reversibility

Apoptosis is the best understood variant of regulated cell death, which has been considered irreversible for a long time. To date, an increasing amount of data has been accumulating indicating that key events of apoptosis, such as the externalization of phosphatidylserine, mitochondrial outer membrane permeabilization, caspase activation, DNA damage, and cytoplasmic blebbing are not irreversible and can be involved in the normal cell functioning not associated with the induction of apoptosis. Anastasis - cell recovery after induction of apoptosis - can occur following elimination of proapoptotic stimuli. This can facilitate survival of damaged or tumor cells. This review describes key processes of apoptosis, which do not necessarily lead to cell death during normal cell activity as well as anastasis. Understanding mechanisms and consequences of apoptotic processes reversibility, on the one hand, could contribute to the improvement of existing therapeutic approaches for various diseases, including malignant neoplasms, and, on the other hand, could open up new possibilities for protecting cellular elements of tissues and organs from death during treatment of degenerative pathologies.

Ligand-based vascular targeting of disease

This review illustrates the basic principles of ligand-based vascular targeting and presents some of the most advanced results obtained in this field, not only in terms of biopharmaceuticals, which are currently being investigated in clinical and preclinical studies, but also in terms of enabling technologies that facilitate target and ligand discovery. Whereas most of the vascular targeting research activities have so far concentrated on tumoral angiogenesis, the development of non-oncological applications has recently gained momentum and is likely to become an important area of modern pharmaceutical research.

Vascular imaging of solid tumors in rats with a radioactive arsenic-labeled antibody that binds exposed phosphatidylserine

PURPOSE

We recently reported that anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of vascular endothelial cells in tumors, probably in response to oxidative stresses present in the tumor microenvironment. In the present study, we tested the hypothesis that a chimeric monoclonal antibody that binds phosphatidylserine could be labeled with radioactive arsenic isotopes and used for molecular imaging of solid tumors in rats.

EXPERIMENTAL DESIGN

Bavituximab was labeled with (74)As (beta(+), T(1/2) 17.8 days) or (77)As (beta(-), T(1/2) 1.6 days) using a novel procedure. The radionuclides of arsenic were selected because their long half-lives are consistent with the long biological half lives of antibodies in vivo and because their chemistry permits stable attachment to antibodies. The radiolabeled antibodies were tested for the ability to image subcutaneous Dunning prostate R3227-AT1 tumors in rats.

RESULTS

Clear images of the tumors were obtained using planar gamma-scintigraphy and positron emission tomography. Biodistribution studies confirmed the specific localization of bavituximab to the tumors. The tumor-to-liver ratio 72 h after injection was 22 for bavituximab compared with 1.5 for an isotype-matched control chimeric antibody of irrelevant specificity. Immunohistochemical studies showed that the bavituximab was labeling the tumor vascular endothelium.

CONCLUSIONS

These results show that radioarsenic-labeled bavituximab has potential as a new tool for imaging the vasculature of solid tumors.

Progesterone-Modulated Phosphatidylserine Externalization in Apoptosis and Activation of Jurkat Cells

PROBLEM

During pregnancy the elevated levels of progesterone (Pg) have immunomodulating effects. It is important to follow-up Pg effects on basic biological processes at cell level as apoptosis and activation which was the aim of this study.

METHODS OF STUDY

Jurkat cells cultured in the presence or absence of Pg were used as a model system. Apoptosis was induced by H(2)O(2) and activation by phorbol myriastate acetate. The induced changes in the phosphatidylserine (PS) externalization and cell surface CD69 expression were followed by fluorescence-activated cell sorter and immunofluorescence.

RESULTS

After the induction of apoptosis PS externalizes in 52.3% of Jurkat cells. Cells cultured with Pg show tendency to a decrease of PS positive cells (42%). The opposite effect is observed in activated cells--PS externalization increase from 33.8% of control cells to 40.1% of Pg-treated cells.

CONCLUSIONS

These findings would suggest that by increasing activation and decreasing apoptosis Pg could regulate local immune system during pregnancy.

A monoclonal antibody that binds anionic phospholipids on tumor blood vessels enhances the antitumor effect of docetaxel on human breast tumors in mice

Anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of viable vascular endothelial cells in tumors, providing an excellent marker for tumor vascular targeting. We recently raised an IgG monoclonal antibody, 3G4, which binds to anionic phospholipids in a beta2-glycoprotein I-dependent manner. It inhibited tumor growth in a variety of rodent tumor models by stimulating antibody-dependent cellular cytotoxicity toward tumor vessels. In the present study, we tested the hypothesis that docetaxel, which is known to have antivascular effects on tumors, might induce exposure of anionic phospholipids on tumor vasculature and, thus, enhance the antitumor activity of 3G4. Treatment of human umbilical vascular endothelial cells with subtoxic concentrations of docetaxel (20 pmol/L) in vitro caused anionic phospholipids to be externalized without inducing apoptosis. Docetaxel treatment of mice increased the percentage of tumor vessels that expose anionic phospholipids from 35% to 60%. No induction of phosphatidylserine was observed on vessels in normal tissues even after systemic treatment with docetaxel. Treatment of mice bearing orthotopic MDA-MB-435 human breast tumors with 3G4 plus docetaxel inhibited tumor growth by 93%. Treatment of mice bearing disseminated MDA-MB-435 tumors with 3G4 plus docetaxel reduced the average number of tumor colonies in the lungs by 93% and half the animals did not develop tumors. In both tumor models, the antitumor effect of the combination was statistically superior (P < 0.01) to that of docetaxel or 3G4 alone. Combination therapy reduced the tumor vessel density and plasma volume in tumors to a greater extent than did the individual drugs. The combination therapy was no more toxic to the mice than was docetaxel alone. These results indicate that, as an adjuvant therapy, 3G4 could enhance the therapeutic efficacy of docetaxel in breast cancer patients.

Dynamics of membrane translocation of phosphatidylserine during apoptosis detected by a monoclonal antibody

Translocation of phosphatidylserine (PS) to the outer leaflet of the cellular membrane seems to be a key step in apoptosis and cell activation. In this paper, the production and characterization of a monoclonal antibody designated as Mab 1H6 is described which does not show cross reactivity with others anionic phospholipids. It is demonstrated that Mab1H6 can recognize externalized PS at early stages after the induction of apoptosis shown by both flow cytometry and immunofluorescence. Our results show that translocation of PS can be detected as early as 5 min by immunofluorescence and 10 min by flow cytometry after the treatment of cells and a specific dynamics is observed concerning the location and distribution of the staining. These data prove that antibody Mab 1H6 can be used as a specific probe for detection of PS translocation.

Autoimmune disease as a cause of reproductive failure

High-risk pregnancy is the most common clinical association with antiphospholipid antibodies; the principal manifestations are pregnancy loss and early preeclampsia. Membership in this family of antibodies is continually growing and includes antibodies against a variety of phospholipids, phospholipid-protein complexes, and phospholipid-binding proteins. The current information in the literature is inadequate to clearly implicate a subgroup of antiphospholipid antibodies or a particular pathophysiologic mechanism as being responsible for poor pregnancy outcomes. It is clear, however, that prevalent diagnostic tests for LA and aCL are extremely useful to identify many of these patients, but are inadequate for diagnosis of all patients with autoimmune pregnancy loss or to elucidate the pathophysiology. Many patients who present clinically with autoimmune-like pregnancy complications currently are negative in tests for LA or aCL, but have antibodies against annexin V, phosphatidylserine, or other relevant antigens. The greatest risk for a complicated pregnancy is conveyed by a subgroup of antibodies that affect the normal function of placental trophoblast. As clinical laboratory tests designed to detect more members of the antiphospholipid antibody family become available, understanding of this complicated disease (APS) will increase.

Phosphatidylserine is a marker of tumor vasculature and a potential target for cancer imaging and therapy

PURPOSE

(1) To determine whether exposure of phosphatidylserine (PS) occurs on vascular endothelium in solid tumors in mice. (2) To determine whether PS exposure can be induced on viable endothelial cells in tissue culture by conditions present in the tumor microenvironment.

METHODS AND MATERIALS

Externalized PS in vivo was detected by injecting mice with a monoclonal anti-PS antibody and examining frozen sections of tumors and normal tissues for anti-PS antibody bound to vascular endothelium. Apoptotic cells were identified by anti-active caspase-3 antibody or by TUNEL assay. PS exposure on cultured endothelial cells was determined by 125I-annexin V binding.

RESULTS

Anti-PS antibody bound specifically to vascular endothelium in six tumor models. The percentage of PS-positive vessels ranged from 4% to 40% in different tumor types. Vascular endothelium in normal organs was unstained. Very few tumor vessels expressed apoptotic markers. Hypoxia/reoxygenation, acidity, inflammatory cytokines, thrombin, or hydrogen peroxide induced PS exposure on cultured endothelial cells without causing loss of viability.

CONCLUSIONS

Vascular endothelial cells in tumors, but not in normal tissues, externalize PS. PS exposure might be induced by tumor-associated oxidative stress and activating cytokines. PS is an abundant and accessible marker of tumor vasculature and could be used for tumor imaging and therapy.

True anti-anionic phospholipid immunoglobulin M antibodies can exert lupus anticoagulant activity

True (cofactor-independent) anticardiolipin antibodies (aCL) are thought to lack lupus anticoagulant (LA) activity and pathogenic potential. A serum monoclonal immunoglobulin Mlambda (mIgMlambda) with aCL and LA activities found in a man with a splenicIgMlambda+ B-cell lymphoplasmacytic lymphoma (LPL) without thrombotic events has been characterized. LPL-derived hybridoma clones (designated HY-FRO) producing the serum mIgMlambda were obtained. mIgMlambda secreted by HY-FRO grown in protein-free culture medium, like that purified from serum, (i) showed binding, in a cofactor-free system, to solid-phase CL and phosphatidylserine (PS) and to the membrane of PS-expressing cells (apoptotic cells and activated platelets); (ii) failed to bind neutral phospholipids (PL), beta2Glycoprotein, histone, ssDNA, dsDNA, human IgG and umbilical vein endothelial cells. Absorption with apoptotic cells abolished its binding to anionic plate-bound CL and PS. IgMlambda-FRO used poorly mutated VH and Vlambda region genes, with a pattern that was inconsistent with an antigen-driven selection. Basic amino acids were present in the IgH complementarity determining region 3 (CDR3), which can be important for binding to anionic PL. These findings demonstrate unequivocally that true anti-anionic PL IgM antibodies can exert LA and indicate this anti-PL type does not involve thrombophilia.

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.

Ultrastructural localization of monoclonal antiphospholipid antibody binding to rat brain

Antiphospholipid antibodies (aPL), in the presence or absence of systemic lupus erythematosus, are associated with a number of neurologic complications. However, the role aPL play in pathology is unclear. A thrombotic etiology seems likely for many associated disorders, but not for others. Here we describe aPL-reactive sites in the central nervous system (CNS). Previously, using light microscopy, we showed direct binding of two monoclonal phosphatidylserine-reactive antibodies (aPS) to ependyma and myelin of fixed cat brain. In this study we determined the ultrastructural localization of their binding sites in rat CNS using immunogold electron microscopy techniques. Both monoclonal antibodies reacted strongly with myelin, preferentially with the major dense line formed by the cytoplasmic apposition of the oligodendrocyte plasma membrane. Both monoclonal antibodies also reacted with an antigen that appears associated with the axoneme in cilia of ependymal and choroid plexus epithelium. One monoclonal aPS also showed some reactivity with brain vascular endothelium and reacted slightly with mitochondria, while the other aPS did not react with these structures. While the etiology of aPL-associated neurologic disorders remains unclear, our data suggest possible target sites within the CNS with which aPL can react.

Regulation of phosphatidylserine exposure at the cell surface by the serine--base exchange enzyme system during CD95-induced apoptosis

Early in the apoptotic process, CD95 induces a translocation of phosphatidylserine (PtdSer) from the inner to the outer leaflet of the cellular plasma membrane. In mammalian cells, PtdSer is only synthesized through a calcium-dependent exchange of the polar head group of pre-existing phospholipids, either phosphatidylcholine or phosphatidylethanolamine, by a serine. Using a pharmacological approach, we examined the influence of PtdSer synthesis on CD95-induced PtdSer exposure at the surface of Jurkat cells. We found that CD3/TCR triggering or thapsigargin treatment of Jurkat cells was accompanied both by a decreased PtdSer synthesis and by a strong reduction of CD95-induced PtdSer at the cell surface, as monitored by fluorescence-activated cell sorting (FACS) analysis of annexin V-fluorescein isothiocyanate (FITC)-labeled cells. PtdSer synthesis through the serine-base exchange enzyme system thus appeared as one of the mechanisms implicated in the recently discovered CD3/TCR-induced down-regulation of CD95-induced apoptosis. Conversely, increasing the activity of the serine-base exchange enzyme system with different drugs, either the K+ channel blocker quinine, the cationic amphiphil stearylamine, or three different calmodulin antagonists, chlorpromazine, trifluoperazine, and N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W7), resulted in an increased appearance of PtdSer at the surface of CD95-treated cells. Both PtdSer synthesis and CD95-induced annexin V-FITC reactivity were abrogated in ATP-depleted cells. Also, modifying the membrane potential with valinomycin (hyperpolarization) or either gramicidin or KCl (depolarization) demonstrated a strong relationship between PtdSer synthesis and annexin V-FITC reactivity in CD95-treated cells. Together, our results indicate that CD95-induced exposure of PtdSer at the cell surface could be regulated by the activity of the serine-base exchange enzyme system.

Antiphospholipid antibodies and reproduction: the antiphospholipid antibody syndrome

In women who have a diagnosis of APS (both clinical and laboratory criteria) the chance for successful pregnancy is reduced. In these cases, treatment appears to be a clear option, particularly in the case of prior thromboembolic events. The current preference of treatment for women with RPL and aPL antibodies is subcutaneous heparin and aspirin. This treatment should begin with a positive pregnancy test and continue postpartum. It is unclear, at this time, what treatment, if any, is required for women who do not meet all the criteria for diagnosis of APS, but who are known to have aPL antibodies. In some cases, these women were tested because of a prior false-positive test for syphilis, with subsequent identification of aPL antibodies. More recently, women undergoing IVF were tested and found to have an increased incidence of aPL antibodies. It was suggested that aPL antibodies are associated with infertility and failure to implant. However, a summary of published reports indicate that positive aPL antibodies in patients undergoing IVF do not influence ongoing pregnancy rates. This subject, however, remains an area of active investigation because aPL antibodies were shown to interact with the syncytiotrophoblast and cytotrophoblast layers and could, theoretically, after implantation.

The role of placental trophoblast in the pathophysiology of the antiphospholipid antibody syndrome

PROBLEM

The antiphospholipid (aPL) antibody syndrome is characterized by severe pregnancy complications, the cause of which remains unknown. We hypothesized that the placental trophoblast is a target for aPLs.

METHOD OF STUDY

The effects of monoclonal aPLs on trophoblast function, including the invasion of JAR into matrigel-coated filters and the effects of annexin V expression on BeWo, were investigated using choriocarcinoma models.

RESULTS

aPLs against phosphatidylserine (PS) significantly (P < 0.001) decreased the migration of JAR across the membrane. In the annexin V studies, undifferentiated BeWo did not express surface annexin V. After differentiation, BeWo expressed surface annexin V, which was removed in the presence of aPLs, resulting in increased binding of prothrombin.

CONCLUSIONS

PS is expressed on the trophoblast surface during differentiation and invasion of extracellular matrix. Our data suggest that aPLs against PS can directly affect trophoblast function by limiting the depth of decidual invasion and by concurrently creating a procoagulant surface on trophoblast exposed to the maternal circulation.

Antiphosphatidylserine antibody removes annexin-V and facilitates the binding of prothrombin at the surface of a choriocarcinoma model of trophoblast differentiation

OBJECTIVE

Trophoblast differentiation is associated with externalization of phosphatidylserine from the inner to the outer surface of the plasma membrane. In this study we tested the hypothesis that concurrent externalization and binding of annexin-V blocks the phosphatidylserine-rich surface from acting as a site for activation of coagulation and that antiphospholipid antibodies lead to a procoagulant state by preventing annexin-V binding.

STUDY DESIGN

A choriocarcinoma model of trophoblast differentiation, forskolin-activated BeWo cells and immunoperoxidase techniques were used to determine surface and cytoplasmic localization of annexin-V related to differentiation. Monoclonal immunoglobulin M antibodies against phosphatidylserine- and cardiolipin-dependent antigens were used to determine the effects of antiphospholipid antibodies on annexin-V localization and on the binding of prothrombin to the BeWo surface.

RESULTS

During differentiation BeWo cells externalized phosphatidylserine and increased the expression of surface annexin-V. Monoclonal antibody against phosphatidylserine removed annexin-V from the BeWo surface and increased binding of prothrombin.

CONCLUSION

Antiphosphatidylserine antibody induces sites for prothrombin binding on the surface of a BeWo model of trophoblast, most likely by removing annexin-V. This mechanism could explain the frequent observation of increased thrombosis at the maternal-fetal interface in miscarriages associated with antiphospholipid antibodies.

Aminophospholipid translocase activity in JEG-3; a choriocarcinoma model of cytotrophoblast differentiation

The plasma membrane is characterized by a non-symmetrical distribution of phospholipids; the outer monolayer of the plasma membrane consists primarily of phosphatidylcholine (PC), and the aminophospholipids, phosphatidylserine (PS) and phosphatidylethanolamine (PE), preferentially reside in the inner monolayer. Asymmetry is maintained by a membrane associated ATP-dependent aminophospholipid translocase that preferentially relocates PS and PE from the outer to the inner monolayer. Although in most cells the translocase minimizes expression of PS on the outer surface, differentiating trophoblasts express increasing levels of surface PS. One possible explanation of prolonged PS externalization is that trophoblasts lack an effective aminophospholipid translocase. To test this hypothesis, fluorescent PC and PS analogues, NBD-PC and NBD-PS, were introduced into the plasma membrane of a choriocarcinoma model of trophoblast, JEG-3 cells. After incubation, the fluorescent lipid remaining on the outer monolayer was removed by incubation with fetal bovine serum. JEG-3 cells selectively translocated 80 per cent of the NBD-PS without significant translocation of NBD-PC. The process was significantly inhibited by N-ethylmaleimide (NEM) and vanadate. It is concluded that this model of trophoblast contains an active aminophospholipid translocase.

Antibodies against phospholipids other than cardiolipin: potential roles for both phospholipid and protein

Autoantibodies to phospholipids other than cardiolipin have received less attention, to date, than anti-cardiolipin antibodies. This review focuses on these antibodies and potential roles for both phospholipid and protein in their reactivity. We review data in the literature indicating that antibodies to phosphatidylethanolamine and some lupus anticoagulant antibodies recognize phospholipid-binding proteins in association with phospholipid. Kininogens appear to be involved in the binding of antibodies to phosphatidylethanolamine, while phosphatidylserine-binding proteins, such as prothrombin and annexin V, have been implicated in lupus anticoagulant antibody recognition. These proteins bind to phospholipids that normally reside in the inner monolayer of the cell membrane, suggesting that exposure of these lipids is necessary for protein binding and antibody recognition to occur. In contrast, other autoantibodies, in particular those reactive with erythrocytes, appear to be directed at phospholipids that normally occur in the outer membrane leaflet, such as phosphatidylcholine. In summary, there is clearly accumulating evidence that antibodies to phospholipids other than cardiolipin recognize epitopes on phospholipid-binding proteins. It is not clear whether recognition of these epitopes is due to an increase in antigen density or a change in the protein or phospholipid structure, but it is likely that both protein and phospholipid structure play an important role in the in vivo interactions of these antibodies.

A model for the antiphospholipid antibody syndrome: monoclonal antiphosphatidylserine antibody induces intrauterine growth restriction in mice

OBJECTIVE

Antiphospholipid antibodies are associated with clinical intrauterine growth restriction. In this study we investigated whether immunoglobulin M monoclonal antibodies against phosphatidylserine or cardiolipin or cross-reactive with both phospholipids would induce intrauterine growth restriction in an experimental model of the antiphospholipid antibody syndrome.

STUDY DESIGN

Balb/c or CD-1 mice were injected intraperitoneally on day 8 of pregnancy with three immunoglobulin M monoclonal antibodies that differentiated between cardiolipin- and phosphatidylserine-dependent antigens or with control immunoglobulin M monoclonal antibodies against irrelevant antigens. The animals were killed on day 15 of pregnancy and placental and fetal weights were measured.

RESULTS

Monoclonal antibody 3SB9b, which reacted in enzyme-linked immunosorbent assays with phosphatidylserine but not cardiolipin, induced a significant reduction in both fetal and placental weights. Monoclonal antibodies BA3B5C4, which was cross-reactive with cardiolipin and phosphatidylserine, and D11A4, which reacted with cardiolipin, did not alter fetoplacental weights.

CONCLUSION

An antiphospholipid antibody that reacts with phosphatidylserine induces significant fetal and placental intrauterine growth restriction in a mouse model for the antiphospholipid antibody syndrome, but those that react with cardiolipin do not.

Reversal of thrombocytopenia following oral anticoagulation in two patients with primary antiphospholipid syndrome

We report on two patients with the primary antiphospholipid syndrome who had significant thrombocytopenia. In both patients platelet number improved soon after initiation of oral anticoagulation. Possible mechanisms for the resolution of thrombocytopenia are discussed.

Antiphospholipid antibody binding to bilayer-coated glass microspheres

Thrombosis, recurrent fetal loss, and thrombocytopenia are clinical manifestations associated with circulating antibodies that recognize cardiolipin (CL)- or phosphatidylserine (PS)-dependent antigens. Enzyme-linked immunosorbent assays (ELISAs) are generally used to determine the presence and specificity of antiphospholipid antibodies (aPLs). However, the presentation of the phospholipid antigen in the ELISA assay is unknown. In this study, we determined the specificity of three mouse monoclonal aPLs for phospholipid bilayer membranes. These monoclonal aPLs had been characterized by ELISA to have different specificities for CL and PS and were designated BA3B5C4 (CL+/PS+), 3SB9b (CL-/PS+), and D11A4 (CL+/PS-). Bilayers composed of 0-100% PS or CL in phosphatidylcholine (PC) were formed on the surface of 1.6 microns diameter glass microspheres to permit analysis by flow cytometry. BA3B5C4 and 3SB9b bound specifically to both PS- and CL-containing bilayers, and binding increased with increasing percentage of anionic phospholipid. The threshold for PS-dependent binding was 20 mol% PS for both BA3B5C4 and 3SB9b. For CL-dependent binding, the threshold was below 25 mol% CL for both of these antibodies. Binding to PS-containing bilayers was tested as a function of ionic strength for BA3B5C4 and 3SB9b. The ionic strength dependence of the binding suggested that the intermolecular attractive forces between anti-PS antibodies and PS-containing bilayers are predominantly multiple weak electrostatic bonds. D11A4 bound only to bilayers composed of 100% PS and 100% PC, and this antibody did not bind to CL-containing bilayers. The binding specificities of these aPLs to bilayer membranes suggest that, in this system, the conformation of the epitope involving CL, and perhaps PS, is different from that expressed in the routine clinical ELISA. Two of the monoclonal antibodies reacted in this model system at the low levels of PS typically externalized in the plasma membranes of activated platelets, apoptopic lymphocytes, and senescent red blood cells: thus, these surfaces are plausible candidates for the site of pathologically relevant antibody interactions.

Monoclonal antiphosphatidylserine antibody reactivity against human first-trimester placental trophoblasts

OBJECTIVE

To investigate the binding of antibodies against negatively charged phospholipids (antiphospholipid antibodies) to human placenta, we tested the reactivity of three mouse monoclonal antiphospholipid antibodies against first-trimester human placenta.

STUDY DESIGN

Formalin-fixed and frozen sections of first-trimester placentas were stained by immunoperoxidase with three mouse monoclonal antibodies. Each monoclonal antibody reacted differently with cardiolipin and phosphatidylserine, 3SB9b reacted with phosphatidylserine, D11A4 reacted with cardiolipin, and BA3B5C4 reacted with both.

RESULTS

3SB9b reacted strongly with the syncytiotrophoblastic layer of both formalin-fixed and frozen placental tissue. Sporadic reactivity was observed against the cytotrophoblastic layer. BA3B5C4 reacted strongly and specifically with cytotrophoblastic cells. D11A4 reacted minimally or, more commonly, not at all.

CONCLUSION

The trophoblastic layer directly in contact with the maternal circulation is most reactive with antiphospholipid antibodies that react with phosphatidylserine rather than cardiolipin, suggesting that the trophoblasts may potentially be directly damaged by antiphospholipid antibodies through mechanisms unrelated to thrombosis. In addition, the differential reactivity of 3SB9b and BA3B5C4 suggests that the antigenic conformation involving phosphatidylserine on the cytotrophoblast is altered concurrent with fusion into the syncytium.

Expression of phosphatidylserine-dependent antigens on the surface of differentiating BeWo human choriocarcinoma cells

PROBLEM

Antiphospholipid antibodies (aPLs) are associated with pregnancy loss, pregnancy-induced hypertension, and intrauterine growth retardation. We have previously reported that phosphatidylserine (PS)-dependent antigens are expressed in formalin-fixed cells concurrent with differentiation in a choriocarcinoma model (BeWo) of cytotrophoblast. That study, however, could not differentiate between cytoplasmic or surface antigen expression.

METHOD

Three monoclonal aPLs that differentiate between PS- and cardiolipin (CL)-dependent antigens were reacted with BeWo, with or without forskolin activation, before fixation, and antibody binding was evaluated by immunoperoxidase techniques.

RESULTS

Activation with forskolin induced a PS-dependent antigenic determinant on the surface on BeWo cells. CL-reactive monoclonal antibodies did not react with the cell surface, whether forskolin treated or not.

CONCLUSION

These observations demonstrate that a PS-dependent antigen is expressed on the surface of a model of differentiating cytotrophoblastic cells and should be accessible in vivo to circulating aPLs.

Monoclonal IgM antiphosphatidylserine antibody reacts against cytoskeleton-like structures in cultured human umbilical cord endothelial cells

PROBLEM

It has been proposed that antibodies against phospholipid-dependent antigens (aPLs), induce recurrent pregnancy loss and thrombosis through modulation of endothelial cell function, yet aPLs have not been conclusively shown to bind with endothelial cells.

METHOD

Using indirect immunofluorescence we investigated the anti-endothelial cell reactivity of three monoclonal antibodies that differentiate between the phospholipids cardiolipin (CL) and phosphatidylserine (PS): BA3B5C4 (CL+/PS+); 3SB9b (CL-/PS+); and D11A4 (CL+/PS-). Cultured umbilical cord endothelial cells were prepared without fixation or with cold acetone fixation.

RESULTS

None of the aPLs reacted with endothelial cells prepared without fixation. 3SB9B reacted strongly with cytoskeletal-like components in acetone-fixed cells, whereas BA3B5C4 and D11A4 were unreactive. The cytoskeletal-like binding of 3SB9b was completely blocked by a monoclonal antibody against vimentin, whereas antibodies against tubulin or actin were not inhibitory. Lipid extraction of the cells destroyed the 3SB9b reactive antigen without affecting the reactivity of anti-vimentin.

CONCLUSION

These results suggest that phospholipid-dependent antigenic determinants are not expressed on the surface of resting endothelial cells but that a PS-dependent antigenic determinant is associated with endothelial cell intermediate filaments.

Monoclonal antibodies directed against the E2 protein (MIC2 gene product) induce exposure of phosphatidylserine at the thymocyte cell surface

Monoclonal antibodies (mAbs) directed against E2, a 32-kDa transmembrane protein encoded by the MIC2 gene located in the pseudoautosomal region, induce a transbilayer movement of phosphatidylserine and, to a lesser extent, phosphatidylethanolamine in human thymocytes and a Jurkat T lymphocytes. The translocation of phosphatidylserine has been evidenced by using either derivatization of anionic phospholipids with trinitrobenzenesulfonate (TNBS) or cytofluorimetry after labeling of cells with antiphosphatidylserine antibodies. The perturbation of membrane phospholipids induced by anti-E2 mAbs was further evidenced by labeling the cells with merocyanine 540. The specificity of anti-E2-induced perturbations of membrane asymmetry was tested by using a number of mAbs able to activate T cells, including CD3 and CD2. The results strongly suggest that anti-E2-induced changes in PtdSer are related to cell aggregation since the same mAbs specifically induce the aggregation of both thymocytes and Jurkat cells and since the E2 molecule has been previously implicated in the adhesive properties of human T cells with erythrocytes.