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.
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–β2-glycoprotein I (anti-β2GPI) 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.
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-β2GPI. 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 β2GPI, 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 (AT), and 2 of 10 inhibited inactivation of FXa by AT.
Plasmin immunization leads to production of the IgG mAb antiplasmin, aCL, and anti-β2GPI in MRL/MpJ mice, but leads to production of only IgG-antiplasmin in BALB/cJ mice. IgG mAb generated from the plasmin-immunized MRL/MpJ mice bind to various antigens and exhibit procoagulant activity in vitro. These results suggest that plasmin may drive the potentially prothrombotic activities of aCL in genetically susceptible individuals.