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Updated: Apr 25, 2026
Measuring the 50% Haemolytic Complement CH50 Activity of Serum
Published on: March 29, 2010
Complotype affects the extent of down-regulation by Factor I of the C3b feedback cycle in vitro
Insights
Genetic variations in complement proteins C3 and Factor H influence disease risk. Higher Factor I doses are needed to regulate complement pathways in individuals with risk alleles, suggesting Factor I as a potential therapeutic for inflammatory diseases.
Area of Science:
- Immunology
- Genetics
- Complement System Biology
Background:
- Age-related macular degeneration (AMD) and kidney diseases are linked to specific genetic polymorphisms in complement proteins C3 and Factor H.
- These polymorphisms (C3 R102G, Factor H V62I, Y402H) affect an individual's predisposition to these inflammatory conditions.
- The alternative complement pathway plays a crucial role in the pathogenesis of these diseases.
Purpose of the Study:
- To investigate how common C3 and Factor H polymorphisms influence the regulation of the alternative complement pathway.
- To determine if individuals with genetic risk factors for AMD and kidney disease exhibit altered responses to Factor I in complement regulation.
- To assess the potential of exogenous Factor I as a therapeutic agent for inflammatory diseases linked to complement dysregulation.
Main Methods:
- Sera from normal subjects were genotyped for C3 (R102G) and Factor H (V62I, Y402H) polymorphisms.
- Subjects were categorized into three groups based on their genotypes: homozygous for risk alleles, heterozygous for all three, and homozygous for low-risk alleles.
- The alternative complement pathway was activated using zymosan, and the effect of adding exogenous Factor I on the formation and breakdown of iC3b was measured in vitro.
Main Results:
- Individuals with at-risk complement genotypes required higher concentrations of Factor I to achieve similar down-regulation of the alternative complement pathway.
- Both the maximum amount of iC3b formed and the rate of its conversion to C3dg were affected by Factor I dosage, with differences observed between genotype groups.
- The breakdown of iC3b to C3dg, a key step in reducing complement-induced inflammation, was less efficient in individuals with risk alleles without sufficient Factor I.
Conclusions:
- Sera from individuals with different complement genotypes exhibit predictable variations in alternative complement pathway regulation in vitro.
- The findings support the hypothesis that exogenous Factor I can effectively down-regulate the C3b feedback cycle.
- Exogenous Factor I shows promise as a therapeutic strategy for managing age-related macular degeneration and other inflammatory diseases associated with complement hyperactivity.
Abstract:
Sera from a large panel of normal subjects were typed for three common polymorphisms, one in C3 (R102G) and two in Factor H (V62I and Y402H), that influence predisposition to age-related macular degeneration and to some forms of kidney disease. Three groups of sera were tested; those that were homozygous for the three risk alleles; those that were heterozygous for all three; and those homozygous for the low-risk alleles. These groups vary in their response to the addition of exogenous Factor I when the alternative complement pathway is activated by zymosan. Both the reduction in the maximum amount of iC3b formed and the rate at which the iC3b is converted to C3dg are affected. For both reactions the at-risk complotype requires higher doses of Factor I to produce similar down-regulation. Because iC3b reacting with the complement receptor CR3 is a major mechanism by which complement activation gives rise to inflammation, the breakdown of iC3b to C3dg can be seen to have major significance for reducing complement-induced inflammation. These findings demonstrate for the first time that sera from subjects with different complement alleles behave as predicted in an in-vitro assay of the down-regulation of the alternative complement pathway by increasing the concentration of Factor I. These results support the hypothesis that exogenous Factor I may be a valuable therapeutic aid for down-regulating hyperactivity of the C3b feedback cycle, thereby providing a treatment for age-related macular degeneration and other inflammatory diseases of later life.
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