Effect of supraphysiologic levels of C1-inhibitor on the classical, lectin and alternative pathways of complement

Mol Immunol. 2007 Mar;44(8):1819-26. doi: 10.1016/j.molimm.2006.10.003. Epub 2006 Nov 13.


C1-inhibitor is increasingly used experimentally and clinically in inflammatory conditions like septicemia and ischemia-reperfusion injury. Several mechanisms may account for the anti-inflammatory effects of C1-inhibitor, including inhibition of complement. The aim of the present study was to investigate and compare the supraphysiologic effect of C1-inhibitor on the three complement pathways. Novel assays for specific evaluation of the classical, lectin and alternative pathways were employed using normal human serum supplemented with increasing concentrations of C1-inhibitor. Solid-phase classical- and lectin pathway activation was dose-dependently and significantly reduced up to 85% in the range of 2-28 times physiologic C1-inhibitor concentration. The lectin pathway was more potently inhibited than the classical at low doses. A functional lectin pathway assay demonstrated a significant reduction of C4 deposition up to 86% even at low concentration of C1-inhibitor and documented the effect to be at the level of MBL/MASPs. In contrast, C1-inhibitor had no effect on solid-phase alternative pathway activation, but significantly reduced cobra venom factor-induced fluid-phase activation up to 88%. The negative controls albumin and IgG had no effect on complement activation. The positive inhibitory controls compstatin (C3 inhibition), EDTA- or MBL-deficient sera reduced complement activation by 82-100%. We conclude that C1-inhibitor in high physiologic doses differentially inhibits all three-complement pathways. The inhibition pattern was strikingly different in the classical and lectin pathway, compared to the alternative. Previous studies interpreting the effects of C1-inhibitor as only due to classical pathway inhibition needs reconsideration. The data has implications for the therapeutic use of C1-inhibitor.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Complement C1 Inhibitor Protein / chemistry
  • Complement C1 Inhibitor Protein / metabolism
  • Complement C1 Inhibitor Protein / pharmacology*
  • Complement C1 Inhibitor Protein / therapeutic use
  • Complement C4 / chemistry
  • Complement C4 / metabolism
  • Complement Pathway, Alternative / drug effects*
  • Complement Pathway, Alternative / physiology
  • Complement Pathway, Classical / drug effects
  • Complement Pathway, Classical / physiology
  • Complement Pathway, Mannose-Binding Lectin / drug effects
  • Dose-Response Relationship, Drug
  • Humans
  • Mannose-Binding Lectin / chemistry*
  • Mannose-Binding Lectin / metabolism
  • Mannose-Binding Protein-Associated Serine Proteases / chemistry*
  • Mannose-Binding Protein-Associated Serine Proteases / metabolism
  • Reperfusion Injury / drug therapy
  • Reperfusion Injury / metabolism
  • Sepsis / drug therapy
  • Sepsis / metabolism


  • Complement C1 Inhibitor Protein
  • Complement C4
  • Mannose-Binding Lectin
  • Mannose-Binding Protein-Associated Serine Proteases