Autoantibodies to kininogen-phosphatidylethanolamine complexes augment thrombin-induced platelet aggregation

Anti-phospholipid autoantibodies in human diseases

Anti-phospholipid autoantibodies are a group of antibodies that can specifically bind to anionic phospholipids and phospholipid protein complexes. Recent studies have reported elevated serum anti-phospholipid autoantibody levels in patients with antiphospholipid syndrome, systemic lupus erythematosus, rheumatoid arthritis, metabolic disorders, malaria, SARS-CoV-2 infection, obstetric diseases and cardiovascular diseases. However, the underlying mechanisms of anti-phospholipid autoantibodies in disease pathogenesis remain largely unclear. Emerging evidence indicate that anti-phospholipid autoantibodies modulate NETs formation, monocyte activation, blockade of apoptotic cell phagocytosis in macrophages, complement activation, dendritic cell activation and vascular endothelial cell activation. Herein, we provide an update on recent advances in elucidating the effector mechanisms of anti-phospholipid autoantibodies in the pathogenesis of various diseases, which may facilitate the development of potential therapeutic targets for the treatment of anti-phospholipid autoantibody-related disorders.

Pathophysiology of the Antiphospholipid Antibody Syndrome

The antiphospholipid syndrome is characterized by antibodies directed against phospholipid-binding proteins and phospholipids attached to cell membrane receptors, mitochondria, oxidized lipoproteins, and activated complement components. When antibodies bind to these complex antigens, cells are activated and the coagulation and complement cascades are triggered, culminating in thrombotic events and pregnancy morbidity that further define the syndrome. The phospholipid-binding proteins most often involved are annexins II and V, β2-glycoprotein I, prothrombin, and cardiolipin. A distinguishing feature of the antiphospholipid syndrome is the "lupus anticoagulant". This is not a single entity but rather a family of antibodies directed against complex antigens consisting of β2-glycoprotein I and/or prothrombin bound to an anionic phospholipid. Although these antibodies prolong in vitro clotting times by competing with clotting factors for phospholipid binding sites, they are not associated with clinical bleeding. Rather, they are thrombogenic because they augment thrombin production in vivo by concentrating prothrombin on phospholipid surfaces. Other antiphospholipid antibodies decrease the clot-inhibitory properties of the endothelium and enhance platelet adherence and aggregation. Some are atherogenic because they increase lipid peroxidation by reducing paraoxonase activity, and others impair fetal nutrition by diminishing placental antithrombotic and fibrinolytic activity. This plethora of destructive autoantibodies is currently managed with immunomodulatory agents, but new approaches to treatment might include vaccines against specific autoantigens, blocking the antibodies generated by exposure to cytoplasmic DNA, and selective targeting of aberrant B-cells to reduce or eliminate autoantibody production.

[Seronegative antiphospholipid syndrome: Myth or reality?]

The antiphospholipid syndrome (APS) is a systemic autoimmune disease characterized by thrombosis and/or obstetrical manifestations and the persistent presence, at least 12 weeks apart, of antiphospholipid antibodies (aPL) such as lupus anticoagulant (LA) and/or anticardiolipin antibodies (ACL) and/or anti-β2 glycoprotein I antibodies (aβ2GPI). The finding of patients with clinical profile highly suggestive of APS but who are negative for conventional biological criteria has led to the concept of seronegative APS. In the last few years, new antigen targets and methodological approaches have been employed to more clearly identify this syndrome in patients with thrombosis or obstetrical complications without conventional aPL. Although seronegative APS is still controversial, there is increasing recognition of the existence of this subgroup. However, clinical relevance of non conventional aPL need to be confirmed by efforts toward standardizing new biological tools and longitudinal studies involving large cohort of patients.

Can cryoglobulins interfere with the measurement of IgM antiphosphatidylethanolamine antibodies by ELISA?

Clinical manifestations of the antiphospholipid antibody syndrome (APS) have been recently related to the presence of phosphatidylethanolamine antibodies (aPE). However, it is well known that some molecules such as cryoglobulins, immunoglobulins that undergo a reversible precipitation at low temperatures, may interfere with biological assays. With this in view, we report the case of a patient with APS who was positive for both IgM aPE and type III cryoglobulinemia. Moreover, we show for this patient a potential implication of aPE in the cryoprecipitate formation. To further analyze the potential association between cryoglobulins and aPE, and also the possible consequences for aPE assay, we selected 55 patients according to positivity for both IgM aPE and cryoglobulinemia. Determination of IgM aPE levels was made before and after removal of cryoprecipitate from the serum. Of the 55 selected patients, 52 (95%) presented no significant difference for IgM aPE levels before and after cryoprecipitation. These results were ascertained whatever the aPE levels and clinical status of the patient. Taken together, our results indicate that cryoprecipitation does not interfere in most cases (95%) with the dosage of IgM aPE. Thus, IgM aPE do not appear to be involved in the formation of the cryoprecipitate.

Antiphosphatidylethanolamine antibodies in recurrent early pregnancy loss and mid-to-late pregnancy loss*

AIM

Associations have been reported between antiphospholipid antibodies (aPL), mainly anticardiolipin antibodies (aCL) and/or the lupus anticoagulant, and recurrent pregnancy losses (RPL). However, relatively few studies describing antiphosphatidylethanolamine antibodies (aPE) have been reported. We describe the prevalence of aPL to both cardiolipin and phosphatidylethanolamine in patients with RPL.

METHODS

Patients with recurrent early pregnancy losses (n = 145) and mid-to-late pregnancy loss(es) (n = 26) were screened for aPE and aCL.

RESULTS

In patients with recurrent early pregnancy losses, prevalence of immunoglobulin G (IgG) aPE (17.9%, P = 0.001) and immunoglobulin M (IgM) aPE (12.4%, P = 0.01) was significantly higher than in the control group. In patients with mid-to-late pregnancy loss(es), prevalence of IgM aPE (19.2%, P = 0.008) and IgG aCL (23.1%, P = 0.02) was significantly higher than in the control group.

CONCLUSION

Our data suggest that aPE may be a risk factor in patients with mid-to-late pregnancy loss(es) as well as recurrent early pregnancy losses.

Antiphospholipid antibodies: discovery, definitions, detection and disease

Antiphospholipid antibodies (aPL) are immunoglobulins of IgG, IgM and IgA isotypes that target phospholipid (PL) and/or PL-binding plasma proteins. Detection of aPL in the laboratory is done currently by both immunoassays and functional coagulation tests. Convention defines aPL specificity in immunoassays according to the particular PL substrate present, for example aPS represents antiphosphatidylserine antibodies. This may be technically incorrect inasmuch as a particular PL may be responsible for binding and highly concentrating a specific plasma protein, the latter then becomes the target for the aPL. The binding of beta(2)GP-I (apolipoprotein H) to the negatively charged PL, cardiolipin (CL) provides a good example of this circumstance. In contrast, aPL which specifically prolong coagulation times in in vitro are called lupus anticoagulants (LA). The precise PL target(s) of the aPL responsible for LA activities are unknown and often debated. The persistent finding of aPL in patients in association with abnormal blood clotting and a myriad of neurological, obstetrical and rheumatic disorders often compounded by autoimmune diseases has led to an established clinical diagnosis termed antiphospholipid syndrome (APS). The common denominator for these APS patients is the presence of circulating aPL on two or more occasions and the observation of events attributable to abnormal or accelerated blood clotting somewhere in vivo. The purpose of this review is to collect, collate, and consolidate information concerning aPL.

Kininogen domain 3 contains regions recognized by antiphosphatidylethanolamine antibodies

Antiphosphatidylethanolamine antibodies (APE) have been described in patients with thrombotic diseases and recurrent pregnancy loss (RPL). It has been reported that certain APE are not specific for phosphatidylethanolamine (PE) per se, but are directed to PE-binding plasma proteins, called kininogens. Our recent in vitro data suggest that APE may recognize the domain 3 (D3) region of kininogens. In this study, we have used synthetic peptides that span the D3 of kininogens in inhibition and direct binding studies to identify epitopes that are sites for binding APE. Our present data demonstrate that among 24 RPL patients who were positive for kininogen-dependent immunoglobulin (IgG) APE, 17 patients (70.8%) recognized the LDC27 peptide. We mapped the APE-binding region on D3 using plasma from a RPL patient (X) who had a high titer of IgG APE that recognized LDC27. APE of patient X recognized a 13-residue segment in LDC27, named CNA13. Leu331-Met357 (LDC27) and Cys333-Lys345 (CNA13) are located on the carboxyl-terminal portion of kininogen D3, which is known as the major kininogen heavy chain cell attachment site where it overlaps its cysteine protease inhibitory region. Because APE interferes with the balance of hemostasis in vitro, APE may therefore induce a similar condition in patients thereby causing thrombosis and RPL.

Antiphospholipid antibodies and kininogens in pathologic pregnancies: a review

PROBLEM

Recently, evidence has accumulated for the presence of the kallikrein-kinin system or plasma contact system in the fetoplacental unit. The Kallikrein-kinin system or plasma contact system consists of three essential plasma proteins. These are coagulation factor XII, prekallikrein and high molecular weight kininogen. Deficiencies of these proteins and antiphospholipid antibodies are frequent hemostasis-related abnormalities found in unexplained recurrent aborters.

METHOD OF STUDY

Review of existing data.

RESULTS

Reports of antiphosphatidylethanolamine antibodies (aPE) with similar or identical pathogenic associations as those described for anticardiolipin antibodies (aCL) are found in the literature. We showed a strong association between recurrent pregnancy losses and aPE, the latter of which recognizes kininogens, and kininogen-binding proteins, factor XI and prekallikrein. The reports of aPE are reviewed, the function of the kininogens are summarized, and their role in pregnancy is discussed.

CONCLUSIONS

Because kallikrein-kinin system may play an important role in pregnancy especially in fetoplacental unit, disruption of this system may be a risk factor for early gestational losses.

Certain autoantibodies to phosphatidylethanolamine (aPE) recognize factor XI and prekallikrein independently or in addition to the kininogens

Recent evidence shows that many antiphospholipid antibodies (aPL) to negatively-charged phospholipid (PL) do not target anionic PL per se, but are specific for anionic PL-binding plasma proteins, for example, beta(2)-glycoprotein I (beta(2)-GPI) and prothrombin. We also reported that certain antiphosphatidylethanolamine antibodies (aPE) are not specific for phosphatidylethanolamine (PE) per se, but are directed to PE-binding plasma proteins, high molecular weight kininogen (HK), and low molecular weight kininogen (LK). Additional studies have shown that certain aPE failed to recognize purified kininogens but continued to produce aPE ELISA reactivity in the presence of semipurified HK preparations containing the HK binding proteins, factor XI (FXI) and prekallikrein (PK). We therefore investigated if certain of these aPE recognized FXI and/or PK. In this study we observed that aPE can recognize contact proteins FXI and PK independently or in combination with HK. Since contact proteins such as HK, PK and factor XII (FXII) have anti-coagulant and profibrinolytic functions, the pathophysiological role of aPE has yet to be elucidated. We propose that aPE of different specificities may initiate or promote characteristics pathological conditions in patients with thrombosis or recurrent pregnancy losses.

Anti-phosphatidylethanolamine (aPE) antibodies: a survey

Reports of anti-phosphatidylethanolamine antibodies (aPE) with similar or identical pathogenic associations as those described for anticardiolipin (aCL) and anti-phosphatidylserine (aPS) are found in the literature. In some instances, aPE is the sole antiphospholipid antibody (aPL) observed. Lupus anticoagulant antibodies (LA) appear to represent a subset of aPE as hexagonal phase PE can specifically inhibit the LA ability to prolong clotting times. As documented for aPL to the negatively charged phospholipids (PL), plasma proteins have been implicated for a positive aPE signal in the ELISA. Other aPE appear independent of known PE-binding plasma proteins. Among the described PE-binding proteins are high and low molecular weight kininogens (HMWK and LMWK) and the HMWK-binding proteins, factor XI and prekallikrein. Recently prothrombin has been added to this list. The reports of aPE published since 1986 are reviewed and discussed in this presentation.

Plasma contact system, kallikrein-kinin system and antiphospholipid-protein antibodies in thrombosis and pregnancy

Coagulation factor XII, prekallikrein and high molecular weight kininogen are known as plasma contact proteins in the intrinsic pathway of blood coagulation. Deficiencies of these proteins are not associated with clinical bleeding despite marked prolongation of in vitro surface-activated coagulation time. Paradoxically, studies suggest that these proteins have anticoagulant and profibrinolytic activities. In fact, association between deficiencies of these proteins as well as recurrent thrombosis has been reported. Also deficiencies of these proteins and antiphospholipid antibodies are frequent haemostasis-related abnormalities found in unexplained recurrent aborters. Recently, evidence has accumulated for the presence of the kallikrein-kinin system or plasma contact system in the fetoplacental unit. This suggests that the plasma contact system may also have an important role in pregnancy. Several studies have reported the presence of autoantibodies to the contact proteins in patients with SLE, thrombosis and recurrent pregnancy loss. These autoantibodies are often in association with antiphospholipid antibodies and lupus anticoagulants. Contact proteins may be added to the list of proteins to which autoantibodies are produced in patients assigned to antiphospholipid antibody syndrome.

Prevalence and heterogeneity of antiphosphatidylethanolamine antibodies in patients with recurrent early pregnancy losses

OBJECTIVE

To describe the prevalence of antiphospholipid antibodies to both anionic and zwitterionic phospholipids in women with early recurrent pregnancy losses (RPLs).

DESIGN

Retrospective data analysis.

SETTING

Tokai University Hospital, Kanagawa, Japan.

PATIENT(S)

One hundred thirty-nine patients with unexplained early RPLs.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Enzyme-linked immunosorbent assays were used to measure autoantibodies to phosphatidylethanolamine, cardiolipin, and phosphatidylserine.

RESULT(S)

Twenty-eight (20.1%), 17 (12.2%), and 2 (1.4%) patients of the 139 total patients were positive for immunoglobulin (Ig) G, IgM, and IgA antiphosphatidylethanolamine antibodies, respectively. Because 3 patients had two isotypes, 44 (31.7%) of the patients were positive for antiphosphatidylethanolamine antibodies. Six patients (4.3%) and 1 patient (0.7%) were positive for IgG and IgM antiphosphatidylserine antibodies, respectively. Seven patients (5%) were positive for beta2-glycoprotein I-independent anticardiolipin IgG, and 1 patient was positive for beta2-glycoprotein I-dependent anticardiolipin IgG. Two patients (1.4%) had lupus anticoagulant.

CONCLUSION(S)

Our data show a statistically stronger association between RPLs and antiphosphatidylethanolamine antibodies than between RPLs and antibodies to anionic phospholipids for early gestational losses. Our data suggest that antiphosphatidylethanolamine antibodies may be a risk factor in patients with early RPLs.

Current insights into the "antiphospholipid" syndrome: clinical, immunological, and molecular aspects

Advances in defining the target antigen(s) for the autoantibodies in the APS highlight the inadequacies of the current classification of these autoantibodies into anticardiolipin and LA antibodies. The discovery that beta 2GPI is the target antigen for the autoantibodies detected in solid-phase immunoassays has opened a number of areas of research linking these autoantibodies to atherogenesis and thrombus formation. Although the role of beta 2GPI in the regulation of blood coagulation in unclear, current evidence suggests that anti-beta 2GPI antibodies interfere with its "normal" role and appear to promote a procoagulant tendency. The expansion of research in this area and the diversity of the clinical manifestations of patients with APS have resulted in the inclusion of molecular biologists and pharmaceutical companies joining immunologists, hematologists, rheumatologists, obstetricians, neurologists, vascular surgeons, and protein and lipid biochemists in attempting to understand the pathophysiology of this condition. Although the published literature may result in conflicting results and introduce new controversies, developing standardized laboratory methods and extrapolation of in vitro experimental results to the vivo situation will advance our understanding of the regulation of the immune system and its interaction with normal hemostatic mechanisms. Since the authors' last review in 1991, the study and understanding of the pathophysiology of APS have evolved from lipid biochemistry to molecular techniques that may eventually provide specific therapies for the clinical manifestations of this condition. Although current treatment has improved the morbidity associated with this condition, especially in improving pregnancy outcomes, future therapies, as outlined in this review, may specifically address the biological abnormalities and have fewer side effects. Better diagnostic tools, such as magnetic resonance imaging with perfusion studies, will allow the study of the true incidence and prevalence of vascular flow changes/tissue ischemia and infarction associated with aPL antibodies and help determine treatment and prophylaxis for APS patients. APS is still the only hypercoagulable condition where both arterial and venous beds can be affected independently or in the same individual.

Phospholipid binding plasma proteins required for antiphospholipid antibody detection--an overview

PROBLEM

Antibodies to phospholipid antigens (aPA) are associated with thrombosis thrombocytopenia and recurrent pregnancy loss. Contemporary data show many aPA target phospholipid-binding plasma proteins and not phospholipids. The purpose of this overview is to describe several phospholipid-binding proteins and provide data to demonstrate how the interaction between phospholipids and phospholipid binding proteins results in expression of neo-autoantigenic epitopes.

METHOD

Review of existing data.

RESULTS

Illustrations of how certain plasma proteins beta 2 glycoprotein I, prothrombin, high and low molecular weight kininogens interact with the anionic phospholipids cardiolipin and phosphatidylserine and the zwitterionic phospholipid, phosphatidylethanolamine are shown and discussed. A model of aPA mediated thrombosis is presented.

CONCLUSIONS

Some aPA recognize phospholipids directly, however, the majority and many which correlate with pathology target phospholipid binding proteins. Published data indicate that aPA represent a constellation of antibodies with multiple specificities. Insight into mechanisms responsible for aPA-associated thrombosis should provide a basis for treatment.