Interaction of C3b(2)--IgG complexes with complement proteins properdin, factor B and factor H: implications for amplification

• Published in: Biochemical journal 2000-07, Vol.349 (Pt 1), p.217-223
• Main Authors: Jelezarova, E, Vogt, A, Lutz, H U
• Format: Article
• Language: English
• Subjects: Complement C3-C5 Convertases - metabolism; Complement C3b - chemistry; Complement C3b - metabolism; Complement Factor B - chemistry; Complement Factor B - metabolism; Complement Factor H - chemistry; Complement Factor H - metabolism; Dimerization; Dose-Response Relationship, Drug; Electrophoresis, Polyacrylamide Gel; Humans; Hydrogen-Ion Concentration; Immunoglobulin G - chemistry; Immunoglobulin G - metabolism; Ions; Kinetics; Properdin - chemistry; Properdin - metabolism; Protein Binding; Radioimmunoassay; Sodium Chloride - pharmacology
• ISSN: 0264-6021; 1470-8728
• DOI: 10.1042/0264-6021:3490217

Nascent C3b can form ester bonds with various target molecules on the cell surface and in the fluid phase. Previously, we showed that C3b(2)--IgG complexes represent the major covalent product of C3 activation in serum [Lutz, Stammler, Jelezarova, Nater and Späth (1996) Blood 88, 184--193]. In the present report, binding of alternative pathway proteins to purified C3b(2)--IgG complexes was studied in the fluid phase by using biotinylated IgG for C3b(2)--IgG generation and avidin-coated plates to capture complexes. Up to seven moles of properdin 'monomer' bound per mole of C3b(2)--IgG at physiological conditions in the absence of any other complement protein. At low properdin/C3b(2)--IgG ratios bivalent binding was preferred. Neither factor H nor factor B affected properdin binding. On the other hand, properdin strongly stimulated factor B binding. Interactions of all three proteins with C3b(2)--IgG exhibited pH optima. An ionic strength optimum was most pronounced for properdin, while factor B binding was largely independent of the salt concentration. C3b(2)--IgG complexes were powerful precursors of the alternative pathway C3 convertase. In the presence of properdin, C3 convertase generated from C3b(2)--IgG cleaved about sevenfold more C3 than the enzyme generated on C3b. C3b(2)--IgG complexes could therefore maintain the amplification loop of complement longer than free C3b.