Generation and characterization of monoclonal antibodies against the N-terminus of alpha-2-antiplasmin

Around 70% of circulating alpha-2-antiplasmin (α2AP), the main natural plasmin inhibitor, is N-terminally cleaved between residues Pro12 and Asn13 by antiplasmin-cleaving enzyme. This converts native Met-α2AP into the more potent fibrinolysis inhibitor Asn-α2AP. The Arg6Trp (R6W) polymorphism affects the N-terminal cleavage rate of Met-α2AP in a purified system, with ~8-fold faster conversion of Met(R6)-α2AP than Met(W6)-α2AP. To date, assays to determine N-terminally intact Met-α2AP in plasma have been limited to an ELISA that only measures Met(R6)-α2AP. The aim of this study was to generate and characterize monoclonal antibodies (mAbs) against Met(R6)-α2AP, Met(W6)-α2AP and all α2AP forms (total-α2AP) in order to develop specific Met(R6)-α2AP and Met(W6)-α2AP ELISAs. Recombinant Met(R6)-α2AP, Met(W6)-α2AP and Asn-α2AP were expressed in Drosophila S2 cells. Using hybridoma technology, a panel of 25 mAbs was generated against a mixture of recombinant Met(R6)-α2AP and Met(W6)-α2AP. All mAbs were evaluated for their specific reactivity using the three recombinant α2APs in one-site non-competitive ELISAs. Three mAbs were selected to develop sandwich-type ELISAs. MA-AP37E2 and MA-AP34C4 were selected for their specific reactivity against Met(R6)-α2AP and Met(W6)-α2AP, respectively, and used for coating. MA-AP15D7 was selected for its reactivity against total-α2AP and used for detection. With the novel ELISAs we determined Met(R6)-α2AP and Met(W6)-α2AP levels in plasma samples and we showed that Met(R6)-α2AP was converted faster into Asn-α2AP than Met(W6)-α2AP in a plasma milieu. In conclusion, we developed two specific ELISAs for Met(R6)-α2AP and Met(W6)-α2AP, respectively, in plasma. This will enable us to determine N-terminal heterogeneity of α2AP in plasma samples.