CHARACTERIZATION OF COMMON NEOANTIGENIC EPITOPES GENERATED IN PLASMINOGEN-ACTIVATOR INHIBITOR-1 AFTER CLEAVAGE OF THE REACTIVE CENTER LOOP OR AFTER COMPLEX-FORMATION WITH VARIOUS SERINE PROTEINASES

Plasminogen activator inhibitor-1 (PAI-1), an important risk factor fo r thrombotic diseases, is a member of the superfamily of serine protei nase inhibitors. To define structural rearrangements occurring during interaction between PAI-1 and its target proteinases we have raised mo noclonal antibodies against the PAI-1/t-PA complex. Thirteen out of 40 1 monoclonal antibodies reacted preferentially with the PAI-1/t-PA com plex as compared to free PAI-1 or free t-PA. Detailed characterization revealed the presence of two non-overlapping neoantigenic epitopes in the PAI-1/t-PA complex. Both neoantigenic epitopes were also exposed after complex formation between PAI-1 and either urokinase-type plasmi nogen activator, plasmin or thrombin as well as after cleavage of the reactive site loop of non-inhibitory substrate type PAI-1 variants. Th us, we have identified two neoanitigenic epitopes, localized entirely in PAI-1, and commonly exposed after complex formation of active PAI-1 with various proteinases or after cleavage of substrate PAI-1. These monoclonal antibodies should facilitate further studies on the mechani sm of interaction between various PAI-1 forms and its target proteinas es.