Effect of antigen site and complement receptor status on the rate of cleavage of C3c antigen from red cell bound C3b
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M S Currie1, P K Rustagi, R Wojcieszak
1Department of Medicine, Duke University, Durham, NC.
Insights
The speed of complement C3b breakdown on red blood cells depends on antigen location and complement receptor 1 (CR1) levels. This impacts how C3-coated red cells are cleared from circulation.
Area of Science:
- Immunology
- Hematology
- Complement System Biology
Background:
- The complement system is crucial for innate and adaptive immunity, involving a cascade of protein activations.
- Complement component 3 (C3) plays a central role, with its breakdown products opsonizing pathogens and cells.
- Red blood cells (RBCs) can bind complement fragments, influencing their clearance and immune responses.
Purpose of the Study:
- To investigate the factors influencing the rate of C3b cleavage to C3dg on human red blood cells.
- To determine the impact of antigen characteristics and complement receptor 1 (CR1) density on C3b/iC3b lifespan.
Main Methods:
- Activation of serum complement on human red cells using antibodies against various red cell antigens.
- Quantification of C3b cleavage to C3dg by measuring the loss of bound C3c antigens via radio-labeled monoclonal anti-C3c.
- Correlation analysis between cleavage rates, antigen proximity to the lipid bilayer, and red cell CR1 expression levels.
Main Results:
- Cleavage of C3b to C3dg was significantly faster when C3b was bound to antigens on branched-chain glycoproteins compared to antigens near the lipid bilayer.
- The rate of C3b cleavage directly correlated with the number of complement receptors 1 (CR1) per red cell.
- CR1 acts as a cofactor in the Factor I-mediated cleavage of iC3b, influencing the overall C3b/iC3b lifespan.
Conclusions:
- The lifespan of C3b/iC3b on human red cells is influenced by the location of the bound antigen.
- Red cell CR1 status is a critical determinant of C3b/iC3b cleavage rates, highlighting its cofactor role.
- These findings are important for understanding the clearance mechanisms of C3-coated red blood cells.
Abstract:
C3b was bound to human red cells when serum complement was activated by addition of antibodies directed against different red cell antigens, and the rate of cleavage to C3dg was determined by assay for loss of bound C3c antigens using radiolabeled monoclonal anti-C3c. When C3b was bound by antibodies to antigens on branched-chain glycoproteins, cleavage to C3dg occurred more rapidly than when C3b was bound by antibodies to antigens closer to the red cell lipid bilayer. The rate of cleavage to C3dg also correlated directly with the number of complement receptors (CR1) per red cell, reflecting their role as cofactors in the cleavage of iC3b by factor I. Thus, the life span of C3b/iC3b on human red cells, which may be important for determining the rate and mechanism of clearance of C3-coated red cells, appears to depend on the CR1 status of the red cells and the characteristics of the antigen sites around which complement is bound.