Molecular mechanism by which CG001 inhibits complement activation. (A) CG001 inhibited the C3 convertase in AP. In the presence of FB and FD, the C3α chain of C3 was cleaved to C3α′ and released C3a, which was inhibited by CG001 in a dose-dependent manner. (B) CG001 failed to inhibit the C3 convertase in CP. In the presence of C2, C4b, and C1s, C3α was cleaved to C3α′ and C3a was released; however, CG001 failed to block this process even at high concentrations. (C) CG001 promoted C3b degradation. The C3α′ chain of C3b was proteolyzed to C3α′68 and C3α′43 by FI in the presence of FH or CG001, in which CG001 displayed a similar potency to FH. (D) CG001 bound to the specific cell surface with C3b/iC3b deposition. CP and AP were activated in Raji cells, and the deposition of C3b/iC3b and CG001 was determined by a flow cytometry assay. (E) CG001 impaired C5 binding to the cell surface upon CP activation. After CP activation by rituximab and FB-depleted human serum in Raji cells, C5 was recruited to the membrane-bound C5 convertase C4bC2aC3b, and the resultant proteolytic C5b fragment was inserted into the cell surface, all of which could be detected by anti-C5/C5b antibody with a flow cytometry assay. CG001 reduced C5/C5b staining by preventing C5 recruitment and C5b production by binding to the C3b subunit of C5 convertase. Values represent mean ± SD, and experiments were performed in triplicate. ∗P < .05 and ∗∗P < .01.