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The relationship between the binding ability and the rate of activation of the complement component C1
Insights
Complement C1 activation is not solely dependent on C1q binding strength. New findings suggest other interactions are crucial for activating complement C1, even with C1 inhibitor present.
Area of Science:
- Immunology
- Biochemistry
- Complement System
Background:
- The complement system is a crucial part of innate immunity.
- Complement C1 activation is the initial step in the classical complement pathway.
- The interaction between C1q and C1 binders is thought to initiate C1 activation.
Purpose of the Study:
- To investigate the relationship between C1q binding strength and C1 activation rate.
- To determine the factors influencing the activation of complement C1.
- To understand the role of C1 inhibitor in C1 activation.
Main Methods:
- Purified, radiolabeled (125I) complement C1 was used.
- Hydrolysis of C1r and C1s subcomponents was monitored to measure C1 activation rate.
- C1q binding strength was assessed for various C1 binders.
- Activation rates were compared between immune complexes and glutaraldehyde-aggregated IgG.
Main Results:
- C1 activation rate did not correlate with C1q binding strength.
- Immune complexes activated C1 rapidly, while aggregated IgG showed minimal activation.
- Binding strength was similar for both immune complexes and aggregated IgG.
- C1 inhibitor prevented C1r hydrolysis in C1 bound to immune complexes.
Conclusions:
- Complement C1 activation requires an interaction beyond simple C1q binding.
- The classical pathway initiation involves complex molecular interactions.
- C1 inhibitor effectively blocks C1 activation by preventing C1r hydrolysis.
Abstract:
The strength of the bond between C1 and C1 binders (as measured by C1q binding) has been correlated with the ability of the binders to activate C1. The rate of activation of C1 has been studied by following the extent of hydrolysis of the C1r and C1s subcomponents, using a purified preparation of C1 labelled with 125I. The rate of activation of C1 was not correlated with the binding strength between C1q and the C1 binders. Immune complexes were found to activate C1 rapidly, whereas glutaraldehyde-aggregated IgG failed to activate faster than the spontaneous activation seen on incubation of C1 alone; the strength of the bond between C1q and the binders was similar in the two cases. It is suggested that an interaction other than the binding between C1q and C1 binders is necessary for activation of C1. C1 bound to immune complexes was not activated in the presence of C1 inhibitor, indicating that the inhibitor can prevent the hydrolysis of C1r under the test conditions.