Ligand specificities of mouse complement receptor types 1 (CR1) and 2 (CR2) purified from spleen cells

Murine complement receptor type 2 (MCR2) is homologous to human CR2, whereas murine CR1 (MCR1) is structurally and evolutionary different from human CR1. Since ligand specificities of MCR1 and MCR2 are not completely clarified, we analyzed their functional characteristics to correlate them with structural information obtained in molecular biological studies. MCR1 and MCR2 purified from spleen cells were incubated with thiol-Sepharose bearing murine C3b, iC3b, or C3d in the presence or absence of various anti-MCR1 or -MCR1/MCR2 mAbs. Bound and free MCR1 and MCR2 were quantitated by Western blotting or two-site immunoradiometric assay. MCR2 bound to C3d and iC3b similarly efficiently, and 5-fold less efficiently to C3b. These bindings were completely inhibited by MCR1/MCR2-crossreactive antibodies 7G6 and 4E3. MCR1 bound to C3b efficiently and this was partially inhibited by MCR1-monospecific antibody 8C12, but not by 7G6 and 4E3. Combinations of 8C12 and 7G6 or 4E3 completely inhibited MCR1 binding to C3b. Therefore, two binding sites, one unique to MCR1 and the other shared with MCR2, are involved in MCR1 binding to C3b. MCR1 bound also to C3d and this was completely inhibited by 7G6 and 4E3. The binding of this solubilized MCR1 to C3d was as efficient as that of MCR2 to C3d. It seems, therefore, that the site shared by MCR1 and MCR2 that is recognized by both 7G6 and 4E3 binds to C3d and less efficiently to C3b. These results clarify the ligand specificities of MCR1 and MCR2.