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Factor I-dependent inactivation of human complement C4b of the classical pathway by C3b/C4b receptor (CR1, CD35) and
T Masaki1, M Matsumoto, I Nakanishi
1Department of Immunology, Center for Adult Diseases Osaka.
Insights
The study investigated how C3b/C4b receptor (CR1) and membrane cofactor protein (MCP) regulate complement factor C4b inactivation. CR1 and MCP show different optimal conditions and potencies for degrading C4b, impacting the classical complement pathway.
Area of Science:
- Immunology
- Biochemistry
Background:
- Proteolytic inactivation of C4b is essential for regulating the classical complement pathway.
- Plasma protease factor I, along with cofactors like C3b/C4b receptor (CR1) and membrane cofactor protein (MCP), controls cell-bound C4b activity.
- The precise physiological roles and optimal conditions for these cofactors in C4b regulation are not fully understood.
Purpose of the Study:
- To investigate the optimal conditions for the factor I-mediated C4b regulatory system using purified CR1 and MCP cofactors.
- To compare the cofactor activities of CR1 and MCP in the degradation of fluid-phase and liposome-bound C4b.
Main Methods:
- Purified cofactors CR1 and MCP were used with factor I to study C4b degradation under varying conditions.
- Experiments assessed the effects of cofactor/C4b ratio, pH, conductivity, and NP-40 concentration on C4b inactivation.
- Degradation products (C4bi, C4c, C4d) were analyzed to determine cofactor efficiency.
Main Results:
- CR1 efficiently degraded fluid-phase C4b and C4ma into C4bi, with optimal pH at 6.0 and 7.5, enhanced by low conductivity.
- Liposome-bound C4b (LAC4b) degradation by CR1 yielded C4c and C4d, with optimal pH at 6.0.
- MCP degraded C4b and C4ma into C4c and C4d more efficiently than CR1, with optimal pH at 6.0 and requiring NP-40 for full activity, unlike CR1.
Conclusions:
- CR1 and MCP exhibit distinct cofactor activities and optimal conditions for C4b inactivation, influencing classical complement pathway regulation.
- MCP is a more potent cofactor for C4b inactivation than CR1, particularly for membrane-bound C4b.
- Understanding these cofactor dynamics is crucial for comprehending complement system regulation and potential therapeutic interventions.
Abstract:
Proteolytic inactivation of C4b is a crucial step for regulation of the classical complement pathway. A plasma protease factor I and membrane cofactors, C3b/C4b receptor (CR1) and membrane cofactor protein (MCP), participate in the regulation of cell-bound C4b although the physiological potency of these cofactors remains unknown. We have examined the optimal conditions of the factor I-mediated C4b regulatory system using purified cofactors. CR1 being a cofactor at a cofactor/C4b ratio less than 0.1 (w/w), fluid phase C4b, and methylamine-treated C4 (C4ma) were degraded by factor I into C4bi: minimal Cd4 was generated in the fluid phase. Liposome-bound C4b (LAC4b), on the other hand, was degraded into C4c and C4d. CR1 showed two optimal pHs (6.0 and 7.5) for fluid phase C4b, but one (6.0) for LAC4b, and in both cases low conductivity conditions enhanced the C4bi generation. CR1 cofactor activity was barely influenced by the NP-40 concentration. On the other hand, MCP degraded C4b and C4ma, as a factor I-cofactor, more efficiently into C4c and C4d. Though MCP cofactor activity, like that of CR1, was enhanced under low conductivity conditions, it has only one optimal pH, 6.0, in both fluid and solid phases. Furthermore, as in the case of C3b cleavage, a sufficient NP-40 concentration to solubilize membrane was needed for MCP to express full cofactor activity for C4b, in contrast to CR1. MCP was less potent for C4b inactivation than for C3b inactivation, while CR1 acted as a slightly more effective cofactor for C4b cleavage than for C3b cleavage.(ABSTRACT TRUNCATED AT 250 WORDS)