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Regulation of the alternative pathway of human complement by C1q
Z Fishelson1, H J Müller-Eberhard
1Department of Chemical Immunology, Weizmann Institute of Science, Rehovot, Israel.
Insights
C1q protein directly binds to C3b, inhibiting the alternative pathway of complement (APC). This interaction down-regulates complement activity on surfaces activating both classical and alternative pathways.
Area of Science:
- Immunology
- Biochemistry
Background:
- The alternative pathway of complement (APC) plays a crucial role in innate immunity.
- Understanding the regulation of APC is vital for controlling inflammatory and autoimmune responses.
Purpose of the Study:
- To investigate the interaction between C1q and C3b.
- To elucidate the effect of this interaction on C3b activities within the APC.
Main Methods:
- Purified C1q was used to study its effects on C3b deposition and lysis mediated by APC.
- Hemagglutination assays were employed to demonstrate direct binding of C1q to C3b.
- Inhibition studies using EDTA, MgEGTA, and various complement factors were performed.
Main Results:
- C1q significantly inhibited C3b deposition and lysis of erythrocytes by APC.
- C1q blocked the formation of the C3 convertase (C3bBb) and the binding of Factors B and H to C3b-coated erythrocytes.
- Direct, dose-dependent binding of C1q to C3b was confirmed, with optimal activity observed after heating C1q.
Conclusions:
- C1q binds to C3b via its globular heads in a calcium-dependent manner, adjacent to Factor B and H binding sites.
- This C1q-C3b interaction serves as a regulatory mechanism for the APC.
- The findings suggest C1q down-regulates APC activity on surfaces that activate both classical and alternative complement pathways.
Abstract:
The interaction of C1q with C3b and its effect on C3b activities in the alternative pathway of complement (APC) have been studied. Purified C1q markedly inhibited C3b deposition on and lysis of rabbit erythrocytes by the isolated cytolytic APC. It also blocked formation of the C3 convertase, C3b, Bb as well as binding of Factors B and H to sheep erythrocytes (E) bearing C3b. The direct and specific binding of C1q to C3b was clearly demonstrated using the hemagglutination technique at low ionic strength (0.1 M NaCl). C1q concns of 2 micrograms/ml and higher agglutinated, in a dose-dependent fashion, EC3b but not E, EC3bi or EC3d. Addition of C1r and C1s to C1q and formation of C1 did not affect its capacity to agglutinate EC3b. The C1q-mediated agglutination of EC3b was inhibited by EDTA, MgEGTA, C3b and Factor B but not by native C3 or collagen. Heating C1q (56 degrees C) markedly potentiated its agglutinating activity whereas collagenase-treated C1q lost most of its activity. Taken together, these results suggest that C1q binds through its "heads" and in the presence of calcium ions to a site on C3b that is adjacent to the Factor B and Factor H binding sites. This interaction may down-regulate the activity of the alternative pathway of complement on surfaces which activate both the classical and alternative pathways of complement.