Identification of Anti-Plasmin Antibodies in the Antiphospholipid Syndrome That Inhibit Degradation of Fibrin

Abstract

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 Kd values ranging from 5.6 × 10−8 to 1 × 10−6 M. These Kd 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.

Anti-Plasminogen Antibodies Compromise Fibrinolysis and Associate with Renal Histology in ANCA-Associated Vasculitis

“…9,12-14 Notably, high affinity monoclonal anti-plasmin antibodies isolated from patients with APS bind plasminogen, thrombin, factor X, and activated protein C with considerably lower affinity. 12,14 Typically, these autoantibodies seem to cross-react with epitopes within the catalytic domains of serine proteases and conformational epitopes in ␤2 glycoprotein 1.…”

“…14 Briefly, 12 M plasmin was incubated with 6 mM TLCK in PBS (pH 7.4) for 100 minutes at 25°C. 14 Using this method, the proteolytic activity of plasmin is reportedly inhibited in excess of 99%.…”

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 Abs derived from APS patients bind to the homologous enzymatic domains of several serine proteases (SP) involved in coagulation, such as thrombin, activated PC, plasmin, tissue plasminogen activator, and FXa (10,12,14,15,17). Moreover, some of these autoantibodies are prothrombotic and interfere with different anticoagulation pathways and/or fibrinolytic process (10,12,14,15,17,32).…”

“…Moreover, some of these autoantibodies are prothrombotic and interfere with different anticoagulation pathways and/or fibrinolytic process (10,12,14,15,17,32). FIXa also belongs to the SP family and its enzymatic domain is homologous to those of thrombin and FXa (33).…”

“…The hallmark aPLs recognize a variety of Ags including various phospholipids (PL), PL-binding proteins, and protein-PL complexes, as well as other factors related to hemostasis (4 -7). The involved proteins include ␤ 2 -glycoprotein I (␤ 2 GPI), prothrombin (PT), protein C (PC), protein S, annexin V, thrombomodulin, thrombin, tissue factor pathway inhibitor, activated PC, endothelial PC/activated PC receptor, plasmin, tissue plasminogen activator, annexin 2, and the activated coagulation factor X (FXa) (5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Functional studies of aPLs have shown that different aPLs may promote thrombosis through different pathways (4,18,19).…”