Some Anticardiolipin Antibodies Recognize a Combination of Phospholipids With Thrombin-Modified Antithrombin, Complement C4b-Binding Protein, and Lipopolysaccharide Binding Protein

Autoantibodies against complement proteins in patients with antiphospholipid syndrome: Prevalence and clinical associations

Seventy-seven patients with antiphospholipid syndrome were tested for autoantibodies against C1q, C3, FB, FH, and C4bp. Fifty-seven patients had at least one anti-complement antibody. IgM anti-FH positivity was associated with thrombosis when anti-C3 and anti-FB were, negatively or positively, associated with various noncriteria manifestations of antiphospholipid syndrome.

The Complex Relationship between C4b-Binding Protein, Warfarin, and Antiphospholipid Antibodies

BACKGROUND

 Low levels of total C4b-binding protein (C4BPt), a circulating inhibitor of the classical/lectin complement pathways, were observed in patients with antiphospholipid antibodies (aPLs) and during warfarin treatment.

OBJECTIVES

 To investigate the associations between aPL and C4BPt in patients with persistently positive (++) aPL, with/without clinical manifestations and systemic lupus erythematosus (SLE), and in controls. Furthermore, we explored the impact of anticoagulation on C4BPt and in relation to complement activation.

METHODS

 In a cross-sectional design we investigated defined subgroups: primary (p) antiphospholipid syndrome (APS, N = 67), aPL++ individuals without clinical manifestations (aPL carriers, N = 15), SLE-aPL++ (N = 118, among them, secondary [s] APS, N = 56), aPL negative (-) SLE (SLE-aPL-, N = 291), and 322 controls. Clinical characteristics, including treatment, were tabulated. C4BPt was determined with a magnetic bead method. Complement proteins (C1q, C2, C3, C4, C3a, C3dg, sC5b-9, factor I [FI]) were measured. A mediation analysis was performed to decompose the total effect of aPL++ on C4BPt into the direct and indirect effects of aPL++ through warfarin.

RESULTS

 Overall, C4BPt is 20% decreased in aPL++ patients, regardless of SLE, APS, clinical manifestations, and aPL profile. C4BPt levels associate positively with complement proteins C1q, C2, C3, and C4, and negatively with complement activation product C3dg. In the SLE group, warfarin treatment contributes to approximately half of the C4BPt reduction (9%) CONCLUSION:  Both aPLs and warfarin are associated with C4BPt reduction. Complement activation in aPL++ patients may partly be explained by impaired inhibition through depressed C4BPt levels. Further studies are needed to understand the clinical implications.

Antiphospholipid syndrome: critical analysis of the diagnostic path

Antiphospholipid syndrome (APS) is diagnosed in the presence of vascular thrombosis or pregnancy morbidity occurring in patients with circulating antiphospholipid antibodies (lupus anticoagulant [LA] and/or IgG/IgM anticardiolipin [aCL] and/or IgG/IgM anti-beta2glycoprotein I [abeta2GPI] antibodies). Each test may identify different autoantibodies; a single test makes the diagnosis possible when positive on two or more occasions at least 12 weeks apart. However, single test positivity may be unrelated to pathogenic antibodies, which are now considered to be a subclass of abeta2GPI antibodies directed against the domain I of this protein. Conversely, all three positive tests identify a single class of abeta2GPI antibodies, thus identifying high-risk patients with APS.

The role of antiphospholipid antibodies toward the protein C/protein S system in venous thromboembolic disease

The association between venous thromboembolism (VTE) and antibodies anti-Protein C (PC)/Protein S (PS) is still uncertain. We performed a case-control study to determine the risk of VTE related to the presence of these auto-antibodies considered independently of the presence of lupus anticoagulant (LAC) or anti-cardiolipin antibodies (ACA). One hundred thirty-five patients with idiopathic VTE and 164 healthy subjects were enrolled. Anti-PC and anti-PS antibodies (both IgG and IgM) were assessed using commercially available ELISA kits. Among cases there was a higher prevalence of elevated anti-PC IgM antibodies than in controls (OR 2.44, 95%CI 1.00-5.94). The presence of anti-PC IgG and anti-PS IgG and IgM antibodies was also higher in cases than in controls, but the difference was not statistically significant. Only five patients had both anti-PC or anti-PS antibodies and LAC or ACA. We performed a stepwise multivariate logistic regression analysis showing that anti-PC IgM>958 percentile was a significant predictor of VTE after adjustment for LAC or ACA (OR 2.52, 95%CI 1.01-6.24)). Larger prospective studies are needed to confirm this finding.

Pathogenic antiphospholipid antibody: an antigen-selected needle in a haystack

Antiphospholipid antibodies represent a heterogeneous group of autoantibodies directed against anionic phospholipids (PLs) usually linked to protein cofactors. Their presence during the antiphospholipid syndrome is associated with risks of thrombosis and fetal losses. Among 5 randomly selected monoclonal antiphospholipid antibodies, all originating from a single patient suffering from this autoimmune disease, only 1 induced fetal losses when passively injected into pregnant mice. Its antiphospholipid activity was dependent on annexin A5, and its variable regions contained mainly 3 replacement mutations. To clarify the role of these mutations in the pathogenicity of the antibody, they were in vitro reverted to the germ line configuration. The resulting “germ line” antibody reacted with multiple self-antigens and only partially lost its reactivity against PLs, but it was no more dependent on annexin A5 and, more importantly, was no more pathogenic. This study illustrates that the in vivo antigen-driven maturation process of natural autoreactive B cells can be responsible for pathogenicity. (Blood. 2004;104:1711-1715)

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.

A versatile high-throughput screen for inhibitors of lipid kinase activity: development of an immobilized phospholipid plate assay for phosphoinositide 3-kinase gamma

The family of phosphoinositide 3-kinases (PI3K) regulates fundamental cellular responses such as proliferation, apoptosis, motility, and adhesion. In particular, the PI3K gamma isoform plays a critical role in the control of cell migration. Despite the attractiveness of PI3-kinases as drug targets, drug discovery efforts have been hampered by the lack of appropriate lipid kinase assay formats suitable for high-throughput screening. The authors report the development of a simple and robust 384-well plate assay that is based on(33) P-phosphate transfer from radiolabeled [gamma(33) P]ATP to phosphatidylinositol immobilized on Maxisorp plates. The established assay format for PI3K gamma was easily adapted to the automated screening platform and was successfully employed for high-throughput screening. Enzymatic and inhibition characteristics of recombinant human PI3K gamma determined with the plate assay are in very good agreement with previously reported values determined in other assay formats. Maximal catalytic activity of PI3K gamma was observed at pH 7.0. The apparent K(m) value for ATP using a 1:1 mixture of phosphatidylinositol and phosphatidylserine was determined to be 7.3 microM (6.0-8.6 microM, 95% confidence interval [CI]). IC(50) values for known PI3-kinase inhibitors were determined to be 1.45 nM (1.17-1.80 nM, 95% CI) for wortmannin and estimated from partial inhibition data to be 1400, 2830, and 21,400 nM for quercetin, LY294002, and staurosporine, respectively. This novel assay approach allows for screening of inhibitors of lipid kinases in high-throughput mode and thereby may facilitate the identification of novel inhibitory structures for drug development.

Identification and characterization of a peptide mimetic that may detect a species of disease-associated anticardiolipin antibodies in patients with the antiphospholipid syndrome

OBJECTIVE

To test the feasibility of applying a mimetic (specific for a patient-derived prothrombotic anticardiolipin antibody [aCL]) to study the homologous, disease-associated aCL in patients with antiphospholipid syndrome (APS).

METHODS

We used the CL15 monoclonal aCL to screen 17 phage-display peptide libraries. Peptides (corresponding to recurrent peptide sequences) and their derivatives were synthesized and analyzed for binding to CL15 and for their abilities to inhibit CL15 from binding to cardiolipin. A peptide was chosen and used to study CL15-like IgG aCL in plasma samples from patients with APS, patients with systemic lupus erythematosus (SLE) but without APS, and normal healthy donors.

RESULTS

Library screening with CL15 yielded 4 recurrent peptide sequences. Analyses of peptides showed that peptide CL154C reacted with antibody CL15 and inhibited binding of CL15 to cardiolipin, indicating that peptide CL154C may be a peptide mimetic for the CL15 aCL. Initial studies with plasma samples revealed that CL154C-reactive IgG was present (positivity defined as the mean + 3 SD optical density of the 25 normal controls) in 15 of 21 APS patients and 1 of 12 SLE patients.

CONCLUSION

These findings suggest that it is feasible to develop a specific enzyme-linked immunosorbent assay for each immunologically and functionally distinct disease-associated aCL. Additional testing of CL154C with a larger number of APS patients and SLE patients, as well as identification of peptide mimetics for each distinct aCL, will reveal the diagnostic potential of CL154C and other mimetics in identifying patients with aCL who are at risk of developing life-threatening thrombosis.

A perturbation of antithrombin-III and protein C coupling associates with an increase of anti-beta2-glycoprotein I antibody in non-antiphospholipid antibody syndrome cerebral ischemia

Anti-beta -glycoprotein I antibody (abetaGPI) has been recognized in raising the risk of cerebral ischemia in patients with antiphospholipid antibody syndrome (APS), especially by protein C (PC) axis perturbation. Although a high potential is also seen in non-APS patients, the mechanism is substantially unknown. In the present study, we examined the effect of abetaGPI on PC and antithrombin-III (AT-III) activity in non-APS patients with non-cardiac cerebral ischemia (NCCI). A total of 111 NCCI patients and 30 healthy controls were enrolled. They were free of APS manifestation, and their anticardiolipin antibody and lupus anticoagulant tests were within normal range. There were 14.4% patients found to have an abnormal increase of blood abetaGPI. The PC, AT-III, albumin, aminotransferases, creatinine, prothrombin time and activated partial thromboplastin time did not differ between our patients and controls, or patients with or without increased abetaGPI. However, a marked decrease of the PC/AT-III ratio was found in patients with increased abetaGPI. The correlation between PC and AT-III activity was highly significant in patients with an increase of abetaGPI (P = 0.001), only marginal in controls (P = 0.042), and was insignificant in patients with a normal abetaGPI (P = 0.277). The abetaGPI did not correlate to PC or AT-III activity in either patients or controls. These findings suggest that high PC/AT-III coupling may relate to NCCI in non-APS patients associated with an increase of abetaGPI. This coupling effect seems not to be caused by abetaGPI directly.

Identification of anti-thrombin antibodies in the antiphospholipid syndrome that interfere with the inactivation of thrombin by antithrombin

The combined presence of anti-phospholipid (PL) Ab, including lupus anticoagulants (LAC) and/or anticardiolipin Ab (aCL), and thrombosis is recognized as the antiphospholipid syndrome (APS). LAC are detected as an inhibitory effect on PL-restricted in vitro blood coagulation tests, and are comprised mainly of Ab against beta(2) glycoprotein I and prothrombin (PT). Recently, anti-PT Ab (aPT) were found to be associated with thrombosis by some investigators, although this is not confirmed by others. Considering that aPT are heterogeneous in patients and that PT is converted into thrombin, we hypothesize that certain aPT in patients may bind to thrombin, and that some of such anti-thrombin Ab may interfere with thrombin-antithrombin (AT) interaction and thus reduce the AT inactivation of thrombin. To test this hypothesis, we searched for anti-thrombin Ab in APS patients and then studied those found for their effects on the AT inactivation of thrombin. The results revealed that most, but not all, aPT-positive patient plasma samples contained anti-thrombin Ab. To study the functional significance of these Ab, we identified six patient-derived mAb that bound to both PT and thrombin. Of these mAb, three could reduce the AT inactivation of thrombin, whereas others had minimal effect. These findings indicate that some aPT in patients react with thrombin, and that some of such anti-thrombin Ab could inhibit feedback regulation of thrombin. Because the latter anti-thrombin Ab are likely to promote clotting, it will be important to develop specific assays for such Ab and study their roles in thrombosis in APS patients.

The clonal analysis of anticardiolipin antibodies in a single patient with primary antiphospholipid syndrome reveals an extreme antibody heterogeneity

The mechanism underlying the prothrombotic state that characterizes the primary antiphospholipid syndrome proves to be difficult to define mainly because of the variety of the phospholipid and protein targets of antiphospholipid antibodies that have been described. Much of the debate is related to the use of polyclonal antibodies during the different antiphospholipid assays. To better describe the antiphospholipid antibodies, a strategy was designed to analyze the reactivity of each one antibody making up the polyclonal anticardiolipin activity, breaking down this reactivity at the clonal level. This was performed in a single patient with primary antiphospholipid syndrome by combining (1) the antigen-specific selection of single cells sorted by flow cytometry using structurally bilayered labeled anionic phospholipids and (2) the cloning of immunoglobulin (Ig) variable (V) region genes originating from individual IgG anticardiolipin-specific B cells by a single-cell polymerase chain reaction technique. The corresponding V regions were cloned in order to express human recombinant antibodies in insect cells by a baculovirus expression system. The molecular analysis, the fine specificity, and the protein cofactor dependency of the first 5 monoclonal IgG anticardiolipins are reported here. This clonal analysis reveals the extreme heterogeneity of these antibodies, which could account for the difficulties in the previous attempts to define the pathogenic antiphospholipid response. This approach should help to unravel the complex antiphospholipid immune response and the mechanism of the prothrombotic state associated with these antibodies, but it could also shed some light on their possible origins. (Blood. 2001;97:3820-3828)

Monovalent binding of autoantibodies to beta2-glycoprotein I, detected using surface plasmon resonance at low antigen density

The precise mechanism of interaction between autoantibodies and beta2-glycoprotein I (beta2GPI) and the experimental conditions to be used for their detection are still under debate. Until now, these interactions have been studied under static conditions. We have investigated the interactions of purified IgG from 25 lupus anticoagulant-positive patients with immobilized beta2GPI under flow conditions by real-time analysis based on surface plasmon resonance technology. Sensor chips were coated with purified human beta2GPI coupled to dextran via amino groups at low densities (1.4, 1.8 or 2. 4 ng beta2GPI/mm2). Four patients' IgG displayed efficient binding and had the highest so-called antiphospholipid IgG levels by enzyme-linked immunosorbent assay (ELISA) and the highest absorbance values in an anti- beta2GPI ELISA at a beta2GPI density reported to be around 12 ng/mm2. Binding of antibodies to the beta2GPI sensor chips proved to be dependent upon the IgG concentration and beta2GPI density and was inhibited by a rabbit antibody against beta2GPI. Similar association and dissociation profiles were observed for the four efficient binders. The fast rate of dissociation limited the binding of autoantibodies to beta2GPI and was highly suggestive of a monovalent association, confirmed by binding of Fab fragments under similar experimental conditions. In conclusion, monovalent binding of low-affinity antibodies to beta2GPI immobilized at a density as low as 1.8 ng/mm2 could be detected using surface plasmon resonance.