Antibody-independent C1 activation by E. coli

Area of Science:

• Immunology
• Microbiology
• Biochemistry

Background:

• The complement system, particularly the C1 complex, plays a crucial role in innate immunity.
• Understanding how microbial components interact with the complement system is vital for host defense mechanisms.
• Escherichia coli (E. coli) is a common bacterium with diverse strains that can interact with host immune components.

Purpose of the Study:

• To investigate the direct, antibody-independent interactions between the C1 complex and various strains of E. coli.
• To characterize the binding kinetics, activation rates, and regulatory aspects of C1 interaction with E. coli.
• To identify bacterial surface components involved in C1 binding and activation.

Main Methods:

• Purified C1 complex was incubated with different E. coli strains (J-5, 0111:B4, P, A) in the absence of C1 inhibitor.
• Binding assays were performed to quantify C1 and its subcomponents (C1s2) associated with bacterial surfaces.
• C1 activation and subsequent dissociation of activated C1s were monitored.
• The effect of C1 inhibitor on C1 activation and enzymatic activity was assessed.
• Proteolytic cleavage of C1s and fragment analysis were conducted using techniques like SDS-PAGE.

Main Results:

• Several E. coli strains, including J-5, 0111:B4, P, and A, directly activated C1 in an antibody-independent manner.
• E. coli J-5 exhibited significantly higher C1 activation rates and binding affinity compared to other strains.
• E. coli J-5 uniquely bound C1s2 in the presence of C1q and calcium, indicating specific binding sites.
• Activated C1s dissociated from most strains but remained associated with E. coli J-5.
• C1 inhibitor regulated C1 activation by weaker activators but not by E. coli J-5, though it inhibited the enzymatic activity of bound C1s.
• Analysis revealed that the heavy chain of C1s binds to E. coli J-5, localizing the binding site on the bacterial surface.

Conclusions:

• E. coli strains are direct activators of the complement C1 complex.
• E. coli J-5 possesses distinct surface binding sites for C1q and dimeric C1s, contributing to its potent activation.
• Bacterial surface properties critically influence C1 activation, its regulation by C1 inhibitor, and the fate of activated C1.