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The molecular basis for the difference in immune hemolysis activity of the Chido and Rodgers isotypes of human
Insights
Human complement C4 (complement component 4) has two isotypes, C4A and C4B, with C4B showing four times greater hemolytic activity. This difference stems from C4B’s superior deposition efficiency onto target cells, not altered cleavage rates.
Area of Science:
- Immunogenetics
- Complement System Biology
- Protein Polymorphism
Background:
- Human complement C4 (C4) exhibits structural polymorphism, with two genetic loci producing C4A and C4B isotypes.
- These isotypes differ in charge, apparent molecular weight, and blood group antigen association.
- Previous studies suggested C4B has higher immune hemolysis activity than C4A in whole plasma.
Purpose of the Study:
- To investigate the molecular basis for functional differences in hemolytic activity between purified human C4A and C4B isotypes.
- To elucidate the mechanisms underlying the observed activity disparities.
- To clarify the role of deposition efficiency and nucleophile preference in C4 isotype function.
Main Methods:
- Purification of C4A and C4B from individual donors lacking one isotype.
- Comparative analysis of hemolytic activity using immune hemolysis assays.
- Examination of C1s cleavage rates, C3 convertase assembly and decay kinetics, and Factor I-mediated cleavage.
- Assessment of C4b deposition efficiency onto sheep erythrocytes.
- Analysis of nascent C4b thioester bond reactivity with different nucleophiles.
Main Results:
- The C4B:C4A hemolytic activity ratio was approximately 4:1.
- No significant differences were found in C1s cleavage rates, C3 convertase kinetics, or Factor I-mediated cleavage.
- A fourfold greater deposition efficiency of C4B onto C1-bearing erythrocytes quantitatively explained the functional difference.
- Nascent C4b of C4A preferentially acylates amino groups, while C4B prefers hydroxyl groups.
Conclusions:
- The higher hemolytic activity of C4B is primarily due to its enhanced deposition efficiency onto target cells.
- The differential reactivity of the C4 isotypes' thioester bonds with surface nucleophiles (hydroxyl vs. amino groups) dictates their functional outcome.
- The observed functional difference does not imply C4A impairment but reflects the target cell's surface chemistry.
Abstract:
Human C4 displays a structural polymorphism which is consistent with there being two closely linked genetic loci coding for this protein. These give rise to two C4 isotypes, designated C4A and C4B, which can be distinguished by charge and apparent m.w. differences in their respective alpha-chains and by the presence or absence of the Chido/Rodgers blood group antigens. Previous qualitative studies of C4 immune hemolysis activity in whole plasma had suggested that the C4B isotype was functionally more active. By using purified C4A and C4B isolated from individual donors known serologically to possess only one of the C4 isotypes, we examined the molecular basis for the differences in their respective hemolytic activities. It was found that the C4B:C4A hemolytic activity ratio was approximately 4:1. This fourfold difference could not be accounted for by a commensurate difference in the cleavage rate of the two isotypes by C1s by differences in the kinetics of assembly or intrinsic decay of the respective C3 convertase enzymes, or by differences in the rate of isotypic C4b cleavage by factor I in the presence of C4bp . However, the fourfold greater deposition efficiency of nascent C4b of the C4B isotype onto the surface of C1-bearing sheep erythrocytes quantitatively accounted for the observed difference in immune hemolysis function. It was further found that the thioester bond of nascent C4b of the C4A isotype preferentially transacylates onto amino group nucleophiles, whereas in the C4B isotype, acylation of hydroxyl groups is strongly preferred. Thus, the difference in immune hemolysis activity between the two C4 isotypes does not necessarily indicate an impairment of function in C4A; it may merely be a reflection of the relative abundance at the surface of a C1-bearing target of hydroxyl and amino groups capable of being acyl acceptors for nascent C4b. Finally, we also present evidence showing that the apparent m.w. difference between the alpha-chains of the C4A and C4B isotypes is not due to differences in protein glycosylation.