Immunologic and hematologic properties of antibodies to prothrombin and plasminogen in a mouse model

Prothrombotic mechanisms based on the impairment of fibrinolysis in the antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a clinical autoimmune disorder characterised by thrombosis, venous or arterial, and recurrent pregnancy morbidity in association with the persistence of antiphospholipid antibodies (aPL). The clinical variety of aPL ranges from asymptomatic individuals to those with multiple organ thromboses and failure developing over a short period, also known as catastrophic APS. An increasing number of phospholipid-binding proteins with crucial functions in the regulation of blood coagulation and fibrinolysis are targeted by APS-related autoantibodies. Disruption of fibrinolysis is one of the proposed pathophysiological mechanisms for the APS. There are some experimental data for an association between impaired overall fibrinolytic activity and autoimmune aPL; however, evidence is still inconclusive and more studies are needed in this area. In this article, we review the evidence by which aPL may disturb fibrinolysis.

Antibodies with dual reactivity to plasminogen and complementary PR3 in PR3-ANCA vasculitis

Patients with inflammatory vascular disease caused by anti-neutrophil cytoplasmic autoantibodies (ANCA) can harbor antibodies not only to the autoantigen proteinase 3 (PR3) but also to complementary PR3 (cPR3(105-201)), a recombinant protein translated from the antisense strand of PR3 cDNA. The purpose of this study was to identify potential endogenous targets of anti-cPR3(105-201) antibodies. Patients' plasmapheresis material was tested for the presence of antigens reactive with affinity-purified rabbit and chicken anti-cPR3(105-201) polyclonal antibodies. Antigen-containing fractions were tested with patients' anti-cPR3(105-201) affinity-purified IgG, and putative protein targets were sequenced by mass spectrometry. Unexpectedly, plasminogen was identified as a target of anti-cPR3(105-201). Reactivity of affinity-purified antibodies from two patients was lost when plasminogen was converted to plasmin, indicating restricted specificity. Antiplasminogen antibodies from five patients bound plasminogen at a surface-exposed loop structure within the protease domain. This loop contains an amino acid motif that is also found in a portion of recombinant cPR3(105-201); site-directed mutagenesis of this sequence decreased antibody reactivity by 30%. Functionally, antiplasminogen antibodies delayed the conversion of plasminogen to plasmin and increased the dissolution time of fibrin clots. Serologically, antiplasminogen antibody levels were higher in PR3-ANCA patients (n = 72) than healthy control subjects (n = 63), myeloperoxidase-ANCA patients (n = 34), and patients with idiopathic thrombosis (n = 57; P = 0.001). Of the patients with PR3-ANCA, nine had documented deep venous thrombosis events, five of whom were positive for antiplasminogen antibodies. In summary, capitalizing on interactions with complementary proteins, specifically complementary PR3, this study identified plasminogen as a previously undescribed autoantigen in PR3-ANCA vasculitis.

IgG antibodies to plasminogen and their relationship to IgG anti-beta(2)-glycoprotein 1 antibodies and thrombosis

Reduced fibrinolytic activity has been described in primary antiphospholipid syndrome (PAPS) and may be responsible for thrombotic events. Some evidence supports a relationship between anti-plasminogen (PLG) antibodies, anti-beta(2)-glycoprotein 1 (beta(2)GP1) antibodies, and fibrinolysis, but their relationship is still unclear. The aim of study is to evaluate the association between IgG anti-beta(2)GP1 and IgG anti-PLG antibodies and thrombosis. Two groups of consecutive patients with PAPS and systemic lupus erythematosus (SLE): 32 patients with lupus anticoagulant (LAC), 32 patients without LAC, and 40 healthy controls were included. IgG against beta(2)GP1 and PLG antibodies were measured by enzyme-linked immunosorbent assay, and a value above the 99th percentile of the normal healthy control was considered as positive, and their interrelationship with thrombosis was evaluated by Pearson Chi-squared test. Cross-reactive antibodies binding to PLG and beta(2)GP1 were determined in a competitive and cross-inhibition assay. Levels of fibrinolytic activity in the presence of IgG fractions from patients and healthy controls were examined using a plasmin fluorogenic substrate assay. A high frequency of IgG anti-PLG antibodies (35.9%) was found in 64 patients, and its presence was associated with thrombosis (p = 0.001), which may be due to its ability to inhibit exogenous fibrinolysis. Coexistence of IgG anti-PLG and IgG anti-beta(2)GP1 antibodies was found in 11 of 64 patients and was related with thrombosis (p = 0.001). Cross-reactive antibody binding to PLG and beta(2)GP1 was found in IgG fractions from three patients and a monoclonal anti-beta(2)GP1 antibody BD4, and one of these three patients had thrombotic history. However, no significant association was found between IgG anti-PLG and IgG anti-beta(2)GP1 antibodies in patients. In conclusion, the prevalence of IgG anti-PLG was high in patients with PAPS and SLE and might relate with thrombosis. Cross-reactivity of IgG anti-beta(2)GP1 antibodies with PLG may occur in the sera of patients.

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.

A study on associations between antiprothrombin antibodies, antiplasminogen antibodies and thrombosis

Anti-prothrombin antibodies are a frequent cause of lupus anticoagulant (LAC), a thrombotic risk factor. Prothrombin shares structural homology with plasminogen, a kringle protein with an important role in fibrinolysis. Cross-reactivity between antiprothrombin antibodies and plasminogen has been described. To study associations between LAC, IgG and IgM class antiprothrombin and antiplasminogen antibodies, plasminogen activity levels and thrombosis in selected patients with systemic autoimmune diseases. Patients included forty-six consecutive LAC-positive patients (29 with systemic lupus erythematosus (SLE); 33 with a thrombotic history), 38 patients without LAC (36 with SLE; seven with a history of thrombosis) and 40 healthy controls. In the total group of 84 patient samples, the prevalence of antiprothrombin and antiplasminogen antibodies was 30 and 38%, respectively. There was no significant relationship between the presence of these antibodies. In contrast to presence of antiplasminogen antibodies, presence of antiprothrombin antibodies was statistically significant related to thrombosis. Thirteen samples had antiprothrombin and antiplasminogen antibodies of similar isotype (IgG, n= 4; IgM, n= 9). Of these, all but one had LAC and 11/13 came from patients with a history of thrombosis. Simultaneous presence of IgM-class antiprothrombin and antiplasminogen antibodies had a significant association with thrombosis. Levels of plasminogen activity were similar in samples from healthy controls and patients (with or without antiplasminogen antibodies or thrombosis). Anti-prothrombin antibodies and antiplasminogen antibodies occur frequently in patients with systemic autoimmune disease. Anti-prothrombin antibodies, but not antiplasminogen antibodies are a risk factor for thrombosis. Anti-plasminogen are in most cases unrelated to antiprothrombin antibodies.