Schematic representation of the role of sialic acid recognition sites on factor H binding and function on the endothelial cell surface. (A) Under normal conditions, small amounts of C3b bind to host cells. CCP1-4 of factor H binds to C3b, and CCP19-20 binds to C3b and to polyanions (mostly sialic acid) on the cell surface, thereby forming a high-affinity ternary complex that facilitates factor I–mediated inactivation of C3b to iC3b and accelerated decay of any C3bBb that may form (not shown). CCP6-8 also binds to the cell surface via heparan sulfate. (B) Addition of exogenous wild-type CCP19-20 (wt 19-20) interferes with binding of factor H to the cell surface and to C3b, thereby allowing amplification of complement to proceed, with generation of the C3 convertase (C3bBb), liberation of anaphylatoxins C3a and C5a, and assembly of the lytic membrane attack complex, C5b-9. (C) Recombinant forms of CCP19-20 with aHUS-associated mutations at sialic acid recognition sites (Sialo-mut), fail to interfere with factor H binding to the polyanionic cell surface and to C3b, thereby allowing factor H to exhibit protective effects, as in panel A. CCP19-20 forms with mutations outside of the sialic acid recognition site behaved as in panel B. Addition of excess recombinant CCP6-8, known to bind to anionic heparan sulfate on the cell surface, also does not interfere with factor H binding or function, indicating the greater importance of sialic acid recognition by factor H.