Specific inhibition of the classical complement pathway by C1q-binding peptides

Undesired activation of the complement system is a major pathogenic factor contributing to various immune complex diseases and conditions such as hype racute xenograft rejection. We aim for prevention of complement-mediated da mage by specific inhibition of the classical complement pathway, thus not a ffecting the antimicrobial functions of the complement system via the alter native pathway and the lectin pathway. Therefore, 42 peptides previously se lected from phage-displayed peptide libraries on basis of C1q binding were synthesized and examined for their ability to inhibit the function of C1q. From seven peptides that showed inhibition of C1q hemolytic activity but no inhibition of the alternative complement pathway, one peptide (2J) was sel ected and further studied. Peptide 2J inhibited the hemolytic activity of C 1q from human, chimpanzee, rhesus monkey, rat, and mouse origin, all with a similar dose-response relationship (IC50 2-6 muM). Binding of C1q to pepti de 2J involved the globular head domain of C1q. In line with this interacti on, peptide 2J dose-dependently inhibited the binding of C1q to IgG and blo cked activation of C4 and C3 and formation of C5b-9 induced via classical p athway activation, as assessed by ELISA. Furthermore, the peptide strongly inhibited the deposition of C4 and C3 on pig cells following their exposure to human xenoreactive Abs and complement. We conclude that peptide 2J is a promising reagent for the development of a therapeutic inhibitor of the ea rliest step of the classical complement pathway, i.e., the binding of C1q t o its target.