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The kinetics and distribution of C9 and SC5b-9 in vivo: effects of complement activation
J D Greenstein1, P W Peake, J A Charlesworth
1Department of Nephrology, Prince Henry Hospital, Sydney, Australia.
Insights
Complement activation significantly alters the behavior of complement component 9 (C9). During activation, C9 rapidly leaves the plasma and accumulates in organs like the spleen and liver, forming larger complexes.
Area of Science:
- Immunology
- Biochemistry
Background:
- Complement activation is implicated in diseases involving terminal complement complex (TCC) deposition.
- The ninth component of complement (C9) is crucial for TCC's cytolytic and cell-regulating functions.
Purpose of the Study:
- To investigate the in vivo behavior of radiolabeled human C9 and its soluble complex SC5b-9.
- To determine how complement activation affects C9 turnover, distribution, and molecular size.
Main Methods:
- Metabolic studies of radiolabeled C9 and SC5b-9 in rabbits.
- Analysis of C9 distribution and molecular size using techniques like FPLC gel filtration.
- Induction of complement activation using cobra venom factor (CVF).
Main Results:
- In normal rabbits, C9 exhibited a plasma half-life of approximately 26 hours and distributed similarly to serum albumin.
- CVF-induced complement activation led to rapid C9 plasma clearance and accumulation in the spleen and liver.
- CVF administration resulted in the formation of a high molecular weight SC5b-9 complex, while injected SC5b-9 was cleared rapidly.
Conclusions:
- Complement activation profoundly impacts C9 metabolism, causing its rapid removal from circulation and redistribution.
- The formation and rapid clearance of SC5b-9 complexes are key features during complement activation.
- Understanding C9 behavior during complement activation is vital for comprehending its role in disease pathogenesis.
Abstract:
Many diseases associated with complement activation are characterized by tissue deposition of components of the terminal complement complex (TCC). The ninth component of complement (C9) plays an important role in the cytolytic effects, and may contribute to the non-lethal cell-regulating functions of the TCC. In this study we examined the behaviour of radiolabelled human C9 and its soluble complexed form SC5b-9 in vivo in order to determine the effects of complement activation on its turnover, distribution and molecular size. In normal rabbits the metabolic parameters of 125I-C9 (median and range) were: plasma half-life (t1/2) 25.9 (20.6-29.5) h, fractional catabolic rate (FCR) 5.7 (5.3-7.0)%/h, and extravascular/intravascular ratio (EV/IV) 0.7 (0.6-1.1). The distribution of radiolabelled C9 amongst body tissues was similar to that observed for rabbit serum albumin (RSA). Activation of the complement cascade with i.v. injection of cobra venom factor (CVF) resulted in rapid disappearance of C9 from the plasma and accumulation of protein-bound radiolabeled in the spleen (exceeding the plasma concentration) and the liver. RSA metabolism and distribution were unaffected by CVF. Fine performance liquid chromatography (FPLC) gel filtration of plasma samples suggested that monomeric C9 was the only major radiolabelled protein present during normal turnovers, whereas CVF administration was accompanied by the prompt appearance of a high mol. wt species consistent in size with SC5b-9. When injected directly, 125I-SC5b-9 disappeared rapidly from the plasma, falling by 50% in 0.7 (0.6-0.8) h, and less than 15% remaining after 4 h with accumulation of protein-bound label in the spleen and liver. These results demonstrate the complexity of C9 metabolism during complement activation.