Evaluation of the Fibrin Binding Profile of two Anti-fibrin Monoclonal Antibodies

Summary Two anti-fibrin monoclonal antibodies, MAbs 1H10 and 5F3, raised to human freeze-fractured fibrin and thrombin-treated N-terminal disulphide knot (T-NDSK), respectively, were compared for epitope binding to various domains of the fibrinogen/fibrin moiety. Using plasmin-mediated fibrinogen digests, immunoblots showed that both MAbs crossreacted strongly with fragments X and Y, weakly with fragment-E and not at all with fragment D. Purified fragments D and E used in an ELISA confirmed that MAbs 1H10 and 5F3 cross-reacted in a dose-response fashion with the isolated fragment-E, while there was no reaction with fragment-D. The two MAbs were similarly shown to react with fibrin-derived fragment-E. Surface Plasmon Resonance (SPR) technology, employed to further evaluate the epitopes in fibrin, showed that MAb 1 HI0 had a higher affinity for fragment-E (KD = 8.04 × 10-9 M) than MAb 5F3 (KD = 1.13 × 10-8 M). Individual association and dissociation rate constants of 7.97 × 105 M-1S-1 and 3.97 × 10-3S-1, respectively, for MAb 1H10, and 5.16 × 105 M-1s-1 and 3.62 × 10-3s-1, respectively, for MAb 5F3 were also obtained. A SPR inhibition assay confirmed that MAb 1H10 had a greater affinity for fragment-E than MAb 5F3. However individual isolated polypeptide chains of fibrinogen fragment E (E-Aα, E-Bβ, E-γ) showed no reaction with the two antibodies in ELISA, immunoblot or SPR analysis procedures. Furthermore, SPR pair-wise epitope mapping analysis revealed that MAbs 1H10 and 5F3 have in fact distinct epitopes on fragment-E. These distinct epitopes appeared to be a conformational amalgam of linear sequences in two or three of the polypeptide chains of fragment-E, or distinct conformational epitopes on one of the three subunit chains alone.