In Vitro Evaluation of the Inhibitory Effects of Eculizumab and C1 Esterase Inhibitor on Human and Rhesus Monkey Complement

BACKGROUND: Complement inhibitors are important therapeutic drugs after xenotransplantation. However, it is still unclear whether the commonly used clinical complement inhibitors eculizumab and C1 esterase inhibitor (C1-INH) can effectively inhibit complement activation in nonhuman primates. METHODS: The anti-complement activities of eculizumab and C1-INH were evaluated using an in vitro model of normal human serum (NHS) or normal rhesus monkey serum (NRS)-mediated complement-dependent cytotoxicity (CDC), with an immortalized porcine aortic endothelial cell line (iPEC) as the target. RESULTS: Pretreatment of the NHS with various doses of eculizumab potently inhibited the cell lysis of iPECs. FACS analysis showed that eculizumab potently suppressed the deposition of C5b-9, but not C3c and C4c, on iPECs. However, although eculizumab pretreatment of the NRS had a dose-dependent inhibitory effect on the killing of iPECs, the effect was much weaker than that for similarly treated NHS. Pretreatment of NHS and of NRS with C1-INH produced almost identical dose-dependent inhibitory effects on the killing of iPECs. At the highest concentration (10 IU/mL) of C1-INH used, the CDC was reduced by only about 75%. In addition, C1-INH had moderate inhibitory effects on the deposition of C3c, C4c, and C5b-9 in both humans and rhesus monkeys. When C1-INH was given intravenously to rhesus monkeys at a dose of 17.5 IU/kg, the serum-mediated cell lysis detected in vitro was only slightly reduced, and the maximum inhibition was about 20%. CONCLUSION: The inhibitory effect of eculizumab on human complement was significantly stronger than it was on rhesus monkey complement, showing obvious species specificity. The inhibitory effect of C1-INH on complement in humans and rhesus monkeys was similar, but a higher concentration of C1-INH was required in monkeys to achieve a significant inhibitory effect. These results provide a reference basis for the selection of complement inhibitors after clinical or preclinical xenotransplantation.