Assembly and regulation of the complement amplification loop in blood: the role of C3b-C3b-IgG complexes

Amplification of complement activation in blood and serum starts on multi-protein complexes that act as precursors of an alternative C3 convertase. Among these covalently linked C4b-, C3b-, and IgG-containing complexes C3b-C3b-IgG complexes represent the major species containing C3b and IgG. Recent work on their purification and characterization is discussed. Special emphasis is placed on the arrangement of ester bonds in these complexes and their dual type of partial protection from inactivation. Partial protection from inactivation is mediated by properdin which binds to these complexes in the complete absence of any other complement protein. High dose IgG, known to stimulate inactivation of these complexes, appears to lower properdin binding in a process that also involves factor H. Properdin stimulates factor B binding to these complexes and renders them far better precursors of a C3 convertase than C3b. The available information allows a suggestion for a new scheme on how the amplification loop is assembled and regulated in blood and serum.