MONOCLONAL ANTIBODY BINDING TO FACTOR VIII:C

The interactions of monoclonal antibodies with highly purified Factor VIII:c have been studied utilizing the ELISA technique. ELISA plates were coated with Factor VIII:c, protein A purified monoclonal IgG was then added and bound antibody detected with peroxidase labeled antimouse IgG. A Scatchard-Sips plot approach to data analysis was used to calculate binding constants. The binding constants for four antibodies designated BD10, AD7, C7F7 and 39MH8 were as follows: BD10, K_O = 7.1 x 10⁸ M^-1, n = 1.1 (moles antibody/moles ligand); AD7, K_O = 3.1 x 10⁸ M^-1, n = 2.7; C7F7, K_O = 3.6 x 10¹¹M^-1, n = 0.03; 39MH8, K = 6.0 x 10¹¹ M^-1, n = 0.03. The binding constants for C7F7 to the purified carboxy-terminal (residues 1649-2332) 80 kD functional region of the Factor VIII:c molecule were also determined: K_O = 1.0 x 1011 M^-1, n = 0.55. On the basis of these results the following conclusions can be drawn: 1) the antibodies can be divided into two groups: high affinity (suitable for use in immunopurification), C7F7 and 39MH8; low affinity: BD10 and AD7; 2) the antibodies in the low affinity group have valance values two orders of magnitude higher than the high affinity antibodies, C7F7 and 39MH8. The difference might be explained by the high affinity antibody epitopes on the immobilized Factor VIII:c being less exposed to the solution; 3) C7F7 binding to the 80 kD polypeptide, compared to the whole Factor VIII:c molecule, gave virtually identical Kc values, but dramatically different valance values. This suggests that the C7F7 epitope is more accessible on the 80 kD polypeptide.