Three monoclonal antibodies against the serpin protease nexin-1 prevent protease translocation

Protease nexin-1 (PN-1) belongs to the serpin. family and is an in, hibitor of thrombin, plasmin, urokinase-type plasminogen activator, and matriptase. Recent studies have suggested PN-1 to play important roles in vascular-, neuro-, and tumour-biology. The serpin inhibitory mechanism consists of the serpin presenting its so-called reactive centre loop as a substrate to its target protease, resulting in a covalent complex with the inactivated enzyme. Previously, three mechanisms, have been proposed for the inactivation of serpins by monoclonal antibodies: steric blockage of protease recognition, conversion to an inactive conformation or induction of serpin substrate behaviour. Until now, no inhibitory antibodies against PN-1 have been thoroughly characterised. Here we report the development of three monoclonal antibodies binding specifically and with high affinity to human PN-1. The antibodies all abolish the protease inhibitory activity of PN-1. In the presence of the antibodies, PM-1 does not form a complex with its target proteases, but is recovered in a reactive centre cleaved form., Using site-directed mutagenesis, we mapped the three overlapping epitopes to an area spanning the gap between the loop connecting alpha-helix F with beta-strand 3A and the loop connecting alpha-helix A with beta-strand 1B. We conclude that antibody binding causes a direct blockage of the final critical step of protease translocation, resulting in abortive inhibition and premature release of reactive centre cleaved, PM-1. These new antibodies will provide a powerful tool to study the in vivo role of PN-1s protease inhibitory activity.