Hydroxychloroquine protects the annexin A5 anticoagulant shield from disruption by antiphospholipid antibodies: evidence for a novel effect for an old antimalarial drug.

Annexin A5 (AnxA5) is a potent anticoagulant protein that crystallizes over phospholipid bilayers (PLBs), blocking their availability for coagulation reactions. Antiphospholipid antibodies disrupt AnxA5 binding, thereby accelerating coagulation reactions. This disruption may contribute to thrombosis and miscarriages in the antiphospholipid syndrome (APS). We investigated whether the antimalarial drug, hydroxychloroquine (HCQ), might affect this prothrombotic mechanism. Binding of AnxA5 to PLBs was measured with labeled AnxA5 and also imaged with atomic force microscopy. Immunoglobulin G levels, AnxA5, and plasma coagulation times were measured on cultured human umbilical vein endothelial cells and a syncytialized trophoblast cell line. AnxA5 anticoagulant activities of APS patient plasmas were also determined. HCQ reversed the effect of antiphospholipid antibodies on AnxA5 and restored AnxA5 binding to PLBs, an effect corroborated by atomic force microscopy. Similar reversals of antiphospholipid-induced abnormalities were measured on the surfaces of human umbilical vein endothelial cells and syncytialized trophoblast cell lines, wherein HCQ reduced the binding of antiphospholipid antibodies, increased cell-surface AnxA5 concentrations, and prolonged plasma coagulation to control levels. In addition, HCQ increased the AnxA5 anticoagulant activities of APS patient plasmas. In conclusion, HCQ reversed antiphospholipid-mediated disruptions of AnxA5 on PLBs and cultured cells, and in APS patient plasmas. These results support the concept of novel therapeutic approaches that address specific APS disease mechanisms.

A functional RANKL polymorphism associated with younger age at onset of rheumatoid arthritis.

OBJECTIVE: We previously observed the association of the co-occurrence of the HLA-DRB1 shared epitope (SE) and RANKL single-nucleotide polymorphisms (SNPs) with younger age at the onset of rheumatoid arthritis (RA) in 182 rheumatoid factor (RF)-positive European American patients with early-onset RA. The aim of this study was to fine-map the 48-kb RANKL region in the extended cohort of 210 European American RF-positive patients with early RA, to seek replication of RA-associated SNPs in additional RA cohorts of 501 European Americans and 298 African Americans, and to explore the functional consequences of RA-associated SNPs. METHODS: SNP genotyping was conducted using pyrosequencing or TaqMan polymerase chain reaction (PCR) assays. Associations of rs7984870 with RANKL expression in plasma, peripheral blood mononuclear cells, and isolated T cells were quantified using enzyme-linked immunosorbent assay and reverse transcription-PCR. Site-directed mutagenesis of rs7984870 within the 2-kb RANKL promoter was performed to drive the luciferase reporter gene in osteoblast and stromal cell lines. Interaction of DNA and protein was determined by electrophoretic mobility shift assay. RESULTS: A single promoter SNP, rs7984870, was consistently significantly associated with earlier age at the onset of RA in 3 independent seropositive (RF or anti-cyclic citrullinated peptide antibody) RA cohorts but not in seronegative RA patients. The C risk allele of rs7984870 conferred 2-fold higher plasma RANKL levels in RF-positive patients with RA, significantly elevated RANKL messenger RNA expression in activated normal T cells, and increased promoter activity after stimulation in vitro via differential binding to the transcription factor SOX5. CONCLUSION: The RANKL promoter allele that increased transcription levels upon stimulation might promote interaction between activated T cells and dendritic cells, predisposing to a younger age at the onset of RA in seropositive European American and African American patients.

Novel autoantibodies against the activated coagulation factor IX (FIXa) in the antiphospholipid syndrome that interpose the FIXa regulation by antithrombin.

We previously reported that some human antiphospholipid Abs (aPL) in patients with the antiphospholipid syndrome (APS) bind to the homologous enzymatic domains of thrombin and the activated coagulation factor X (FXa). Moreover, some of the reactive Abs are prothrombotic and interfere with inactivation of thrombin and FXa by antithrombin (AT). Considering the enzymatic domain of activated coagulation factor IX (FIXa) is homologous to those of thrombin and FXa, we hypothesized that some aPLs in APS bind to FIXa and hinder AT inactivation of FIXa. To test this hypothesis, we searched for IgG anti-FIXa Abs in APS patients. Once the concerned Abs were found, we studied the effects of the Ab on FIXa inactivation by AT. We found that 10 of 12 patient-derived monoclonal IgG aPLs bound to FIXa and that IgG anti-FIXa Abs in APS patients were significantly higher than those in normal controls (p < 0.0001). Using the mean + 3 SD of 30 normal controls as the cutoff, the IgG anti-FIXa Abs were present in 11 of 38 (28.9%) APS patients. Importantly, 4 of 10 FIXa-reactive monoclonal aPLs (including the B2 mAb generated against beta(2)-glycoprotein I significantly hindered AT inactivation of FIXa. More importantly, IgG from two positive plasma samples were found to interfere with AT inactivation of FIXa. In conclusion, IgG anti-FIXa Ab occurred in approximately 30% of APS patients and could interfere with AT inactivation of FIXa. Because FIXa is an upstream procoagulant factor, impaired AT regulation of FIXa might contribute more toward thrombosis than the dysregulation of the downstream FXa and thrombin.

Identification of anti-prothrombin antibodies in the anti-phospholipid syndrome that display the prothrombinase activity.

OBJECTIVE: Prothrombin (PT) is one of the most important antigenic targets for aPL antibodies; however, the prothrombotic mechanism of anti-PT (aPT) antibodies in APS is not fully clarified. Considering that some autoantibodies possess the enzymatic activity, the aim of this study was to test the hypothesis that some aPT antibodies in APS may display prothrombinase activity. METHODS: Six APS patient-derived PT-reactive monoclonal antibodies (mAbs) were analysed for prothrombinase activity on PT. One mAb with prothrombinase activity was examined for its proteolytic activity on PT. In addition, IgG was purified from plasma samples positive with IgG aPT antibodies, and their prothrombinase activity analysed. RESULTS: Initial analysis of six mAbs revealed that, upon incubation with PT, IS6 mAb displayed prothrombinase activity and catalysed the proteolysis of PT to fragments. Analysis of plasma samples revealed that 9/21 (42.8%) APS patients had IgG antibodies against PT, based on a cut-off value equal to mean + 3 S.D. of the level in 21 normal controls. Importantly, of those samples positive for IgG aPT antibodies, two polyclonal IgG (P1 and P2) also displayed prothrombinase activity. CONCLUSIONS: In this study, we showed that some aPT antibodies displayed prothrombinase activity. Such catalytic aPT antibodies may contribute to thrombosis in APS.

Hydroxychloroquine directly reduces the binding of antiphospholipid antibody-beta2-glycoprotein I complexes to phospholipid bilayers.

Treatment with the antimalarial drug hydroxychloroquine (HCQ) has been associated with reduced risk of thrombosis in the antiphospholipid (aPL) syndrome (APS) and, in an animal model of APS, with reduction of experimentally induced thrombosis. Recognition of beta2-glycoprotein I (beta2GPI) by aPL antibodies appears to play a major role in the disease process. We therefore used the techniques of ellipsometry and atomic force microscopy (AFM) to investigate whether HCQ directly affects the formation of aPL IgG-beta2GPI complexes on phospholipid bilayers. HCQ, at concentrations of 1 mug/mL and greater, significantly reduced the binding of aPL-beta2GPI complexes to phospholipid surfaces and THP-1 (human acute monocytic leukemia cell line) monocytes. The drug also reduced the binding of the individual proteins to bilayers. This HCQ-mediated reduction of binding was completely reversed when the HCQ-protein solutions were dialyzed against buffer. HCQ also caused modest, but statistically significant, reductions of clinical antiphospholipid assays. In conclusion, HCQ reduces the formation of aPL-beta2GPI complexes to phospholipid bilayers and cells. This effect appears to be due to reversible interactions between HCQ and the proteins and may contribute to the observed reduction of thrombosis in human and experimental APS. These results support the possibility that HCQ, or analogous molecules, may offer novel nonanticoagulant therapeutic strategies for treating APS.

Antibodies to serine proteases in the antiphospholipid syndrome.

It is generally accepted that the major autoantigen for antiphospholipid antibodies (aPL) in the antiphospholipid syndrome (APS) is beta(2)-glycoprotein I (beta(2)GPI). However, a recent study has revealed that some aPL bind to certain conformational epitope(s) on beta(2)GPI shared by the homologous enzymatic domains of several serine proteases involved in hemostasis and fibrinolysis. Importantly, some serine protease-reactive aPL correspondingly hinder anticoagulant regulation and resolution of clots. These results extend several early findings of aPL binding to other coagulation factors and provide a new perspective about some aPL in terms of binding specificities and related functional properties in promoting thrombosis. Moreover, a recent immunological and pathological study of a panel of human IgG monoclonal aPL showed that aPL with strong binding to thrombin promote in vivo venous thrombosis and leukocyte adherence, suggesting that aPL reactivity with thrombin may be a good predictor for pathogenic potentials of aPL.

Clinical characteristics and laboratory findings of 252 Chinese patients with anti-phospholipid syndrome: comparison with Euro-Phospholipid cohort.

This study aims to characterize the Chinese Han patients with anti-phospholipid syndrome (APS) and compare the data with those of the Euro-Phospholipid cohort. We conducted a single center study consisting of 252 patients with definite APS from 2000 to 2015. We analyzed the clinical and laboratory characteristics of our cohort and compared the data with those of the Euro-Phospholipid cohort. Our cohort consisted of 216 females and 36 males, with a mean age at entry into this study of 41 years (range 11-74 years). Of these patients, 69 (27.4%) patients had primary APS, and 183 (72.6%) had secondary APS (SAPS), including 163 (64.7%) patients had systemic lupus erythematosus (SLE). Thrombotic events occurred in 190 (75.4%) patients, and the most common ones were deep vein thrombosis (40.1%) and stroke (23.8%), which were similar to the reports of the Euro-Phospholipid cohort. In contrast, our cohort had less pulmonary embolism (6.7%). Among 93 females with 299 pregnancy episodes, the rates of early (<10 weeks) and late fetal loss (≥10 weeks) were, respectively, 37.8% and 24.4%. The latter was significantly higher than that of the Euro-Phospholipid cohort. Moreover, 7 APS nephropathy patients (characterized histopathologically by thrombotic microangiopathy) and 8 catastrophic APS patients were found in our cohort. Anti-cardiolipin antibodies (aCL) were detected in 169 (67.1%) patients, lupus anti-coagulant (LA) was detected in 83 (32.9%), and anti-ß2 glycoprotein I antibodies (anti-ß2GPI) in 148 (58.7%) patients. These results show that some clinical manifestations of APS may vary among different racial groups.

Relative importance of different human aPL derived heavy and light chains in the binding of aPL to cardiolipin.

INTRODUCTION: Previous studies have suggested the importance of somatic mutations and arginine, asparagine and lysine residues in the complementarity determining regions (CDRs) of antiphospholipid antibodies (aPL) implicated in the pathogenesis of the antiphospholipid antibody syndrome. The relative contributions of the heavy and light chains of aPL in binding to cardiolipin (CL) were assessed by pairing the heavy and light chains of two IgG, beta(2)GPI dependent aPL (IS4 and CL24) with different partner chains from other IgG, beta(2)GPI independent aPL (UK4) and anti-DNA antibodies (B3 and 33H11). METHODS: Four heavy (V(H)) and five light (V(L)) chain variable sequences from three aPL and two anti-DNA antibodies were cloned into expression vectors containing appropriate gamma(1), lambda or kappa constant region cDNA. Paired combinations of heavy and light chain expression plasmids were transfected into COS-7 cells allowing transient expression of whole IgG molecules, which were harvested and tested for the ability to bind CL and DNA by enzyme-linked immunosorbant assay (ELISA). RESULTS: Whole IgG was produced from 19 heavy/light chain combinations. IS4V(H) was dominant in conferring the ability to bind CL with four of the five V(L) tested. The identity of the V(L) region paired with IS4V(H) was important in determining the strength of binding to CL. IS4V(H) contains multiple arginine residues in CDR3, which may have accumulated due to antigen driven selection. It is likely that these arginine residues may interact with CL. The combination B3V(H)/B3V(L) also bound CL, but none of the other 14 combinations showed any binding in this assay. CONCLUSION: Whole IgG molecules capable of binding CL were produced by in vitro expression in COS-7 cells. Arginine residues play important roles in binding to CL and double-stranded DNA. However, different patterns of mutation to arginine are associated with binding to each of these antigens.

Molecular and genetic characterizations of five pathogenic and two non-pathogenic monoclonal antiphospholipid antibodies.

Antiphospholipid syndrome (APS) is an autoimmune disease that is characterized by thrombosis, recurrent fetal loss and thrombocytopenia. Antiphospholipid antibodies, detected by enzyme-linked immunoabsorbent assays (aCL) and/or in vitro blood clotting assays (LAC) are strongly associated with APS. Both the molecular structures used by pathogenic antiphospholipid antibodies and the genetic mechanisms leading to their production are unknown. We describe here the variable region genes of seven IgG antiphospholipid antibodies derived from two APS patients. Of these, five are pathogenic as defined in a mouse model of thrombosis and two are not. Analyses of the expressed variable region genes show no preferential V gene usage. However, similar to anti-DNA antibodies, pathogenic antiphospholipid antibodies contain an increased number of arginine residues in the third complimentarity-determining region (CDR3) of their H chains. The increased accumulation of arginine residues in the V(H) CDR3 may act to enhance antigen binding, promote disease and point to the importance of the H chain in the pathogenic potential of certain antiphospholipid antibodies.

Anti-phospholipid human monoclonal antibodies inhibit CCR5-tropic HIV-1 and induce beta-chemokines.

Traditional antibody-mediated neutralization of HIV-1 infection is thought to result from the binding of antibodies to virions, thus preventing virus entry. However, antibodies that broadly neutralize HIV-1 are rare and are not induced by current vaccines. We report that four human anti-phospholipid monoclonal antibodies (mAbs) (PGN632, P1, IS4, and CL1) inhibit HIV-1 CCR5-tropic (R5) primary isolate infection of peripheral blood mononuclear cells (PBMCs) with 80% inhibitory concentrations of <0.02 to approximately 10 microg/ml. Anti-phospholipid mAbs inhibited PBMC HIV-1 infection in vitro by mechanisms involving binding to monocytes and triggering the release of MIP-1alpha and MIP-1beta. The release of these beta-chemokines explains both the specificity for R5 HIV-1 and the activity of these mAbs in PBMC cultures containing both primary lymphocytes and monocytes.

Plasmin immunization preferentially induces potentially prothrombotic IgG anticardiolipin antibodies in MRL/MpJ mice.

OBJECTIVE: To test the hypothesis, utilizing 2 experimental mouse models, that plasmin is an important autoantigen that drives the production of certain IgG anticardiolipin (aCL) antibodies in patients with the antiphospholipid syndrome. METHODS: BALB/cJ and MRL/MpJ mice were immunized with Freund's complete adjuvant in the presence or absence of human plasmin. The mouse sera were analyzed for production of IgG antiplasmin, IgG aCL, and IgG anti-beta(2)-glycoprotein I (anti-beta(2)GPI) antibodies. IgG monoclonal antibodies (mAb) were generated from the plasmin-immunized MRL/MpJ mice with high titers of aCL, and these 10 mAb were studied for their binding properties and functional activity in vitro. RESULTS: Plasmin-immunized BALB/cJ mice produced high titers of IgG antiplasmin only, while plasmin-immunized MRL/MpJ mice produced high titers of IgG antiplasmin, IgG aCL, and IgG anti-beta(2)GPI. Both strains of mice immunized with the adjuvant alone did not develop IgG antiplasmin or IgG aCL. All 10 of the IgG mAb bound to human plasmin and cardiolipin, while 4 of 10 bound to beta(2)GPI, 3 of 10 bound to thrombin, and 4 of 10 bound to the activated coagulation factor X (FXa). Functionally, 4 of the 10 IgG mAb inhibited plasmin activity, 1 of 10 hindered inactivation of thrombin by antithrombin III, and 2 of 10 inhibited inactivation of FXa by antithrombin III. CONCLUSION: Plasmin immunization leads to production of IgG antiplasmin, aCL, and anti-beta(2)GPI in MRL/MpJ mice, but leads to production of only IgG antiplasmin in BALB/cJ mice. IgG mAb generated from plasmin-immunized MRL/MpJ mice bind to various antigens and exhibit procoagulant activity in vitro. These results suggest that plasmin may drive potentially prothrombotic aCL in genetically susceptible individuals.

Antiphospholipid antibodies and the antiphospholipid syndrome: pathogenic mechanisms.

Antiphospholipid antibodies (Abs) are associated with thrombosis and are a risk factor for recurrent pregnancy loss and obstetric complications in patients with the antiphospholipid syndrome. It is generally accepted that the major autoantigen for aPL Abs is beta (2) glycoprotein I, which mediates the binding of aPL Abs to target cells (i.e., endothelial cells, monocytes, platelets, trophoblasts, etc.) leading to thrombosis and fetal loss. This article addresses molecular events triggered by aPL Abs on endothelial cells, platelets, and monocytes and complement activation, as well as a review of the current knowledge with regard to the putative receptor(s) recognized by aPL Abs on target cells as well as novel mechanisms that involve fibrinolytic processes. A section is devoted to the description of thrombotic and inflammatory processes that lead to obstetric complications mediated by aPL Abs. Based on experimental evidence using in vitro and in vivo models, new targeted therapies for treatment and/or prevention of thrombosis and pregnancy loss in antiphospholipid syndrome are proposed.

Some antiphospholipid antibodies recognize conformational epitopes shared by beta2-glycoprotein I and the homologous catalytic domains of several serine proteases.

OBJECTIVE: To test the hypothesis that some antiphospholipid antibodies (aPL) in patients with the antiphospholipid syndrome (APS) recognize a conformational epitope shared by beta2-glycoprotein I (beta2GPI; the major autoantigen for the antiphospholipid antibodies) and the homologous catalytic domains of several serine proteases (such as thrombin, activated protein C [APC], and plasmin) involved in hemostasis. METHODS: We generated 4 new IgG monoclonal aPL (2 screened against beta2GPI, 1 against thrombin, and 1 against protein C) from 2 APS patients. The monoclonal antibodies (mAb) were analyzed for binding to beta2GPI, thrombin, APC, and plasmin, as well as for anticardiolipin antibody (aCL) activity. To demonstrate a shared epitope between beta2GPI and a serine protease, 1 mAb was studied by cross-inhibition analysis. RESULTS: Both of the IgG anti-beta2GPI mAb bound to thrombin, APC, and plasmin. On the other hand, the 1 anti-thrombin mAb and the 1 anti-protein C mAb also bound to beta2GPI. Moreover, the binding of 1 cross-reactive mAb to beta2GPI was inhibited by alpha-thrombin (which contains only the catalytic domain of thrombin). All 4 mAb displayed aCL activity. CONCLUSION: Taken together with the findings that some aCL bind to several serine proteases that participate in hemostasis and share homologous catalytic domains, these data demonstrate that some aCL in APS patients recognize one or more conformational epitopes shared by beta2GPI and the catalytic domains of disease-relevant serine proteases.

Some plasmin-induced antibodies bind to cardiolipin, display lupus anticoagulant activity and induce fetal loss in mice.

The combined presence of anti-phospholipid Ab (aPL), thrombosis, and/or fetal loss is recognized as the antiphospholipid syndrome (APS). aPL include anti-cardiolipin Ab (aCL) and/or lupus anticoagulants (LAC, detected as Ig that prolong certain in vitro phospholipid (PL)-restricted blood clotting tests); both aCL and LAC are the diagnostic Ab for APS. Studies show that aPL represent a heterogeneous group of Ab, which recognize various PL, PL-binding plasma proteins, and/or PL-protein complexes. Recently, we found that five of seven patient-derived IgG monoclonal aCL react with thrombin, activated protein C, and plasmin. All three proteins are trypsin-like serine proteases (SP), and are highly homologous in their catalytic domains. Importantly, among these SP autoantigens, the reactive aCL bind to plasmin with the highest affinity, suggesting that plasmin may serve as a major driving autoantigen for some aCL in approximately 30% of APS patients who are positive for IgG anti-plasmin Ab. To test this hypothesis, we immunized BALB/c mice with human plasmin and analyzed immune sera for aCL activity and reactivity with relevant SP. We found that some immune sera displayed aCL activity and/or bound to test SP. Subsequently, eight mAb were obtained and studied. The results revealed that one mAb displayed the aCL and the LAC activities and induced fetal loss when injected into pregnant mice. Immunohistological analyses of placentas revealed extensive deposits of activated C3 components. Combined, these data demonstrate that plasmin may serve as a driving Ag for some pathogenic aPL.

Antiphospholipid antibodies and the antiphospholipid syndrome: an update on treatment and pathogenic mechanisms.

PURPOSE OF REVIEW: The antiphospholipid syndrome is a disorder of recurrent thrombosis, pregnancy loss and thrombocytopenia associated with the presence of antiphospholipid antibodies and persistently positive anticardiolipin or lupus anticoagulant positive tests. Since its recognition in the 1980s, growing interest in the field, not only with respect to diagnosis and treatment, but also regarding the pathogenesis of antiphospholipid antibodies, has emerged. RECENT FINDINGS: First, this review addresses the recently updated classification criteria for diagnosis and treatment of the antiphospholipid syndrome. A discussion on the newly described potential beneficial roles of hydroxychloroquine and the statins for the treatment of antiphospholipid syndrome-associated clinical manifestations is included. Importantly, this article analyzes recent data that examine the molecular and intracellular events that antiphospholipid antibodies trigger in target cells, as well as new findings in the identification of the receptors for these antibodies on the membrane of those cells. A separate section discusses novel pathogenic mechanisms of antiphospholipid antibodies, including the activation of complement and their interaction with homologous catalytic domains of several serine proteases of the coagulation system. SUMMARY: Understanding the molecular interactions and the intracellular signaling that antiphospholipid antibodies trigger, new therapeutic and targeted strategies to ameliorate clinical manifestations in patients with antiphospholipid syndrome may be established.

Antibodies against the activated coagulation factor X (FXa) in the antiphospholipid syndrome that interfere with the FXa inactivation by antithrombin.

Antiphospholipid Ab have been shown to promote thrombosis and fetal loss in the antiphospholipid syndrome (APS). Previously, we found IgG anti-thrombin Ab in some APS patients that could interfere with inactivation of thrombin by antithrombin (AT). Considering that activated coagulation factor X (FXa) is homologous to thrombin in the catalytic domains and is also regulated primarily by AT, we hypothesized that some thrombin-reactive Ab may bind to FXa and interfere with AT inactivation of FXa. To test these hypotheses, we studied reactivity of eight patient-derived monoclonal IgG antiphospholipid Ab with FXa and the presence of IgG anti-FXa Ab in APS patients and investigated the effects of FXa-reactive mAb on AT inactivation of FXa. The results revealed that six of six thrombin-reactive IgG mAb bound to FXa and that the levels of plasma IgG anti-FXa Ab in 38 APS patients were significantly higher than those in 30 normal controls (p < 0.001). When the mean plus 3 SDs of the 30 normal controls was used as the cutoff, 5 of 38 APS patients (13.2%) had IgG anti-FXa Ab. Importantly, three of six FXa-reactive mAb significantly inhibited AT inactivation of FXa. Combined, these results indicate that anti-FXa Ab may contribute to thrombosis by interfering with the anticoagulant function of AT on FXa in some APS patients.

Identification of polyclonal and monoclonal antibodies against tissue plasminogen activator in the antiphospholipid syndrome.

OBJECTIVE: To test the hypotheses that some plasmin-reactive anticardiolipin antibodies (aCL) may bind to tissue plasminogen activator (tPA) and that some of the tPA-reactive aCL may inhibit tPA activity. METHODS: We studied the reactivity of 8 patient-derived monoclonal aCL with tPA and examined the presence of IgG anti-tPA antibodies in patients with the antiphospholipid syndrome (APS). The effects of the reactive monoclonal aCL on the activity of tPA were also examined. RESULTS: Six patient-derived plasmin-reactive monoclonal aCL bound to tPA. Analysis of plasma samples revealed that 10 of 80 APS patients (12.5%) and 1 of 81 systemic lupus erythematosus patients (1.2%) had antibodies against fibrin-associated tPA, based on a cutoff value equal to the mean + 2SD of the level in 28 normal subjects. Of the 6 tPA-reactive monoclonal aCL, 2 of them (CL1 and CL15) inhibited tPA activity. CONCLUSION: Some of the plasmin-reactive aCL in APS patients may bind to tPA. Of the tPA-reactive aCL, some (such as CL1 and CL15) may inhibit tPA activity and, thus, may be prothrombotic in the host.

A thrombin-cross-reactive anticardiolipin antibody binds to and inhibits the anticoagulant function of activated protein C.

OBJECTIVE: To test the hypotheses that some thrombin-reactive anticardiolipin antibodies (aCL) may bind to protein C (PC) and/or activated PC (APC), and that some of the PC- and APC-reactive aCL may inhibit PC activation and/or the function of APC. METHODS: We studied the reactivity of patient-derived monoclonal aCL with PC and APC. We examined the effects of the reactive antibodies on PC activation and on the activity of APC in plasma coagulation. RESULTS: Five of 5 patient-derived, thrombin-reactive monoclonal aCL bound to PC and APC. In addition, 1 patient-derived monoclonal antiprothrombin antibody (APT) that displayed aCL activity and reacted with thrombin also bound to PC and APC. Of these 6 PC- and APC-reactive aCL/APT, all failed to inhibit PC activation, but 1 (CL15) shortened the plasma coagulation time in the presence of exogenous APC and thus inhibited the anticoagulant function of APC. CONCLUSION: Most of the thrombin-reactive aCL in patients with antiphospholipid syndrome may bind to PC and APC. Of the APC-reactive aCL, some (like CL15) may inhibit the anticoagulant function of APC and are thus likely to be prothrombotic in the host.

Identification of anti-plasmin antibodies in the antiphospholipid syndrome that inhibit degradation of fibrin.

The combined presence of anti-phospholipid Ab (aPL) and thrombosis is recognized as the antiphospholipid syndrome (APS). The aPL represent a heterogeneous group of Ab that recognize various phospholipids (PL), PL-binding plasma proteins, and/or PL-protein complexes. Recently, we found the presence of antithrombin Ab in some APS patients and that some of these anti-thrombin Ab could inhibit thrombin inactivation by antithrombin. Considering that thrombin is homologous to plasmin, which dissolves fibrin, we hypothesize that some APS patients may have Ab that react with plasmin, and that some anti-plasmin Ab may interfere with the plasmin-mediated lysis of fibrin clots. To test this hypothesis, we searched for anti-plasmin Ab in APS patients and then studied those found for their effects on the fibrinolytic pathway. The results revealed that seven of 25 (28%) APS patients have IgG anti-plasmin Ab (using the mean OD plus 3 SD of 20 normal controls as the cutoff) and that six of six patient-derived IgG anti-thrombin mAb bind to plasmin with relative K(d) values ranging from 5.6 x 10(-8) to 1 x 10(-6) M. These K(d) values probably represent affinities in the higher ranges known for human IgG autoantibodies against protein autoantigens. Of these mAb, one could reduce the plasmin-mediated lysis of fibrin clots. These findings suggest that plasmin may be an important driving Ag for some aPL B cells in APS patients, and that the induced anti-plasmin Ab may act either directly, by binding to plasmin and inhibiting its fibrinolytic activity, or indirectly, by cross-reacting with other homologous proteins in the coagulation cascade to promote thrombosis.

Human monoclonal antiphospholipid antibodies disrupt the annexin A5 anticoagulant crystal shield on phospholipid bilayers: evidence from atomic force microscopy and functional assay.

The antiphospholipid (aPL) syndrome is an autoimmune condition that is marked by recurrent pregnancy losses and/or systemic vascular thrombosis in patients who have antibodies against phospholipid/co-factor complexes. The mechanism(s) for pregnancy losses and thrombosis in this condition is (are) not known. Annexin A5 is a potent anticoagulant protein, expressed by placental trophoblasts and endothelial cells, that crystallizes over anionic phospholipids, shielding them from availability for coagulation reactions. We previously presented data supporting the hypothesis that aPL antibody-mediated disruption of the anticoagulant annexin A5 shield could be a thrombogenic mechanism in the aPL syndrome. However, this has remained a subject of controversy. We therefore used atomic force microscopy, a method previously used to study the crystallization of annexin A5, to image the effects of monoclonal human aPL antibodies on the crystal structure of the protein over phospholipid bilayers. In the presence of the aPL monoclonal antibodies (mAbs) and beta(2)-GPI, the major aPL co-factor, structures presumed to be aPL mAb-antigen complexes were associated with varying degrees of disruption to the annexin A5 crystallization pattern over the bilayer. In addition, measurements of prothrombinase activity on the phospholipid bilayers showed that the aPL mAbs reduced the anti-coagulant effect of annexin A5 and promoted thrombin generation. These data provide morphological evidence that support the hypothesis that aPL antibodies can disrupt annexin A5 binding to phospholipid membranes and permit increased generation of thrombin. The aPL antibody-mediated disruption of the annexin A5 anticoagulant shield may be an important prothrombotic mechanism in the aPL syndrome.