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The functional inhibition of activated C1 inhibitor in normal human serum causes spontaneous consumption of the
Insights
The study reveals that inhibiting C1r blocks spontaneous complement activation, highlighting the role of activated C1 inhibitor (C1INH) in regulating C1 activation in normal human serum (NHS). This research clarifies complement pathway regulation.
Area of Science:
- Immunology
- Biochemistry
Background:
- The complement system is crucial for innate and adaptive immunity.
- Regulation of complement activation is vital to prevent self-tissue damage.
Purpose of the Study:
- To investigate the spontaneous activation of the classical complement pathway in normal human serum (NHS).
- To elucidate the role of activated C1 inhibitor (C1INH) in regulating C1 activation.
Main Methods:
- Functional blockade of complement components (C1r, C1s, C4, C3, factor B) using Fab'-fragments.
- Inhibition experiments in normal human serum (NHS) and treated NHS (EGTA- or EDTA-).
- Analysis of complement component consumption and C5 conversion.
Main Results:
- Inhibition of C1r completely blocked spontaneous complement activation.
- Spontaneous generation of a fluid-phase C3 convertase (C4b2a) was observed upon C1INH inhibition.
- Consumption of C2, C4, C3, and factor B occurred, with minimal C5 conversion.
- Blood cells slightly reduced C3 consumption but not C2 or C4 consumption.
Conclusions:
- The control of C1r hydrolysis by activated C1INH is essential for regulating C1 activation in NHS.
- Spontaneous classical pathway activation can occur independently of non-complement proteases.
- This study provides insights into the intricate regulatory mechanisms of the human complement system.
Abstract:
The human complement components C1r, C1s, C4, C3, factor B, and/or activated C1INH were functionally blocked in normal human serum (NHS) and EGTA- or EDTA-treated NHS by polyclonal monospecific Fab'-fragments to the individual components. The results of inhibition experiments are compatible with the formation of a classical pathway fluid-phase C3 convertase (C4b2a) spontaneously generated by the inhibition of activated C1INH. This process in both NHS and EGTA-NHS was accompanied by the consumption of C2, C4, C3, and factor B but only by poor enhancement of C5 conversion. Blocking subcomponent C1r, completely inhibited spontaneous activation of the complement components, indicating that the control of C1r hydrolysis is the essential role of activated C1INH as a regulator of C1 activation in NHS. Non-complement serum proteases were inactive during the initiation of the activation process. The presence of blood cells during functional inhibition of activated C1INH in NHS slightly decreased the consumption of C3 but not of C2 and C4.