Incomplete inhibition by eculizumab: mechanistic evidence for residual C5 activity during strong complement activation

Markus J. Harder, Nadine Kuhn, Hubert Schrezenmeier, Britta Höchsmann, Inge von Zabern, Christof Weinstock, Thomas Simmet, Daniel Ricklin, John D. Lambris, Arne Skerra, Markus Anliker and Christoph Q. Schmidt

Key points

  • Strong complement activation overrides the terminal pathway inhibition by the anti-C5 antibody eculizumab.

  • The more powerful complement is activated, the less effective is terminal pathway inhibition by diverse anti-C5 agents.


Eculizumab inhibits the terminal, lytic pathway of complement by blocking the activation of the complement protein C5 and shows remarkable clinical benefits in certain complement-mediated diseases. However, several reports suggest that activation of C5 is not always completely suppressed in patients even under excess of eculizumab over C5 indicating that residual C5 activity may derogate the drug's therapeutic benefit under certain conditions. By using eculizumab and the tick-derived C5 inhibitor coversin we determine conditions ex vivo in which C5-inhibition is incomplete. The degree of such residual lytic activity depended on the strength of the complement activator and the resulting surface density of the complement activation product C3b, which auto-amplifies via the alternative pathway (AP) amplification loop. We show that, at high C3b densities required for binding and activation of C5, both inhibitors reduce but do not abolish this interaction. The decrease of C5 binding to C3b clusters in the presence of C5 inhibitors correlated with the levels of residual hemolysis. However, by employing different C5 inhibitors simultaneously residual hemolytic activity could be abolished. The importance of AP-produced C3b clusters for C5 activation in the presence of eculizumab was corroborated by the finding that residual hemolysis after forceful activation of the classical pathway could be reduced by blocking the AP. By providing insights into C5 activation and inhibition, our study delivers the rationale for the clinically observed phenomenon of residual terminal pathway activity under eculizumab treatment with important implications for anti-C5 therapy in general.

  • Submitted August 8, 2016.
  • Accepted December 16, 2016.