Interaction of human C1q with IgG and IgM: revisited.

The first step of activation of the classical complement pathway involves the binding of the globular C1q dornain (gC1q) to the antigen-bound IgG or IgM. To improve our understanding of the mechanism of interaction of gC1q with IgG and IgM, we compared the immunoglobulin binding properties of single-residue mutants of individual globular modules of A and C chains. We found that LyS(A200) and Lys(C170) are significant for binding with both immunoglobulins. In addition, two Clq-specific scFv antibodies known as potent inhibitors of C I q-IgG and -IgM interactions were used in the epitope mapping analysis. A set of important residues, which participate in the Clq epitopes for scFv, were identified: LysC170 for the scFv3(V) epitope and Arg(B108) and Arg(B109) for the scFv10(V) epitope. The ability of scFv3(V) and scFv10(V) to bind preformed Clq-IgG or Clq-IgM complexes differed: scFv3(V) retained its ability to bind Clq, while scFv10(V) lost it. Given the different locations of the epitopes and the varying abilities of both antibodies to bind Clq-IgG and Clq-IgM complexes, we found that residues from the apical surface of Clq [where the scFv3(V) epitope was located] were involved in the initial recognition of IgG and IgM, while Arg(B108) and Arg(B109) are able to interact during the initial recognition as well as during the final binding of immunoglobulins. The reported results provide the first experimental evidence supporting the notion that apical and equatorial surfaces of gClq have consecutive involvement following the gClq reorientation during the interaction with specific Clq ligands.