Nicked β2-glycoprotein I: a marker of cerebral infarct and a novel role in the negative feedback pathway of extrinsic fibrinolysis

β 2 -Glycoprotein I (β 2 -GPI) is proteolytically cleaved by plasmin in domain V (nicked β 2 -GPI), being unable to bind to phospholipids. This cleavage may occur in vivo and elevated plasma levels of nicked β 2 -GPI were detected in patients with massive plasmin generation and fibrinolysis turnover. In this study, we report higher prevalence of elevated ratio of nicked β 2 -GPI against total β 2 -GPI in patients with ischemic stroke (63%) and healthy subjects with lacunar infarct (27%) when compared to healthy subjects with normal findings on magnetic resonance imaging (8%), suggesting that nicked β 2 -GPI might have a physiologic role beyond that of its parent molecule in patients with thrombosis. Several inhibitors of extrinsic fibrinolysis are known, but a negative feedback regulator has not been yet documented. We demonstrate that nicked β 2 -GPI binds to Glu-plasminogen with K D of 0.37 × 10 –6 M, presumably mediated by the interaction between the fifth domain of nicked β 2 -GPI and the fifth kringle domain of Glu-plasminogen. Nicked β 2 -GPI also suppressed plasmin generation up to 70% in the presence of tissue plasminogen activator, plasminogen, and fibrin. Intact β 2 -GPI lacks these properties. These data suggest that β 2 -GPI/plasmin-nicked β 2 -GPI controls extrinsic fibrinolysis via a negative feedback pathway loop.