ANALYSIS OF THE RECOGNITION MECHANISM OF THE ALTERNATIVE PATHWAY OF COMPLEMENT BY MONOCLONAL ANTI-FACTOR H-ANTIBODIES - EVIDENCE FOR MULTIPLE INTERACTIONS BETWEEN H-BOUND AND SURFACE-BOUND C3B

The ability of the alternative pathway of complement to discriminate t argets as either activators or non-activators is mediated by different binding properties of factor H to surface-associated C3b molecules, I n the present study we have probed the interaction between H and C3b u sing five anti-H mAb, The binding sites of the mAb were mapped by West ern blotting using both recombinant and trypsin-generated H fragments, Two mAb bound to CCP1 (90X, 196X), two to CCPS (MRC OX24, 86X) and on e to CCP8-15a (131X), At a molar ratio 2:1 of I-125-H:mAb all tested m Ab enhanced binding of H to both activator- and non-activator-bound C3 b, At higher concentrations two mAb had an inhibitory effect on H bind ing to surface-associated C3b (OX24, 131X), Thus the mAb 131X inhibits H binding to surface-bound C3b but unlike OX24 it does not bind to th e previously described C3b binding site within or near CCP4-5. These r esults indicate that there is an additional interaction site on factor H for surface-bound C3b.