Selection and characterization of a novel factor XI inhibiting peptide by using phage display technology

English: The role of factor XI in hemostasis can be seen as a combination of a procoagulant action (the formation of fibrin) and an antifibrinolytic action (the protection of fibrin). High levels of factor XI lead to a prolonged down regulation of fibrinolysis and therefore a risk of thrombosis (Meijers et ai, 2000). Under disease conditions associated with Disseminated Intravascular Coagulation (DIC), the continuous exposure to excess TF typically exhaust the available tissue factor pathway inhibitor (TFPI), leading to rampant thrombin generation by factor XI feedback and therefore also a risk of thrombosis (0sterud and Bjerlid, 2001). I selected possible .inhibitors of factor XI using phage display technology. I started the phage display selection by biopanning in immuno-tubes and eluted the factor XI binding phages non-specifically from the immuno-tube. I did four selection rounds, to enrich the factor XI binding phages. I found only two strong factor XI binding phage clones from a linear 12-mer phage library. Both phage clones bound dose dependently and with a high affinity to factor XI. Both clones also lengthened the partial thromboplastin time (aPTT) dose dependently. The amino acid sequences of the peptides displayed on these two clones indicate that both peptides contain three amino acid sequences of HMWK and thrombin. One clone also contains a three amino acid sequence of factor XII. None of them contains a three amino acid sequence of factor IX. I synthesize a linear peptide with the corresponding sequence as the peptide displayed on the clone that was prevented from binding to factor XI by both factor IX and thrombin. I characterized the peptide by studying its effect on the aPTT. This peptide lengthens the aPTT dose dependently. The lengthening in aPTT of our peptide however indicates that I have selected an inhibitor of the contact system factors of coagulation. In summary, this study shows that the phage display can be used to select novel factor XI inhibitors from random peptide libraries. With further studies, this peptide may be developed as an antithrombotic.