Generation of multiple fluid-phase C3b:plasma protein complexes during complement activation: possible implications...

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• The complement system prevents tissue damage through tightly regulated activation.
• C3 cleavage to C3b is a critical step controlled by various mechanisms.
• Uncontrolled complement activation can lead to autoimmune and inflammatory diseases.

Purpose of the Study:

• To investigate the formation and nature of C3b complexes in the fluid-phase during complement activation.
• To determine the role of these complexes in C3 glomerulopathies.
• To explore the potential pathological consequences of dysregulated complex formation.

Main Methods:

• Triggering complement activation in normal human serum and plasma.
• Utilizing serum depleted of specific complement proteins (C3, factors B, D).
• Inactivating the thioester bond of C3.
• Generating complexes in vitro using purified C3, factors B, D, and a target protein with Mg(2+).
• Analyzing plasma samples from patients with dense deposit disease and C3 glomerulonephritis.

Main Results:

• C3b:plasma protein complexes were observed during complement activation, independent of the pathway.
• Complex formation requires C3, factors B, and D, and an intact C3 thioester bond.
• In vitro studies confirmed complex generation with purified components.
• Complexes consist of C3b covalently bound to a plasma protein (1:1 molar ratio), with C3b rapidly degraded by factors H and I.
• Spontaneous C3b:protein complex formation was detected in patients with dense deposit disease and, to a lesser extent, C3 glomerulonephritis, but not in healthy controls.

Conclusions:

• C3b:protein complexes are a normal product of fluid-phase complement activation.
• Their excessive generation or defective clearance may contribute to C3 glomerulopathies.
• These findings support the hypothesis that C3 glomerulopathies result from fluid-phase complement dysregulation.