In vivo clearance studies of the terminal fluid-phase complement complex in rabbits

The present study was directed at obtaining information on the in vivo elimination rate of SC5b-9, the terminal fluid-phase product of complement activation. A sandwich ELISA based on the use of mono- and polyclonal antibodies was constructed that permitted quantitation of rabbit SC5b-9 in plasma. Rabbit serum was activated with inulin in vitro to generate SC5b-9, and the activated serum was applied intravenously in normal and C6-deficient rabbits. Elimination of SC5b-9 in normal rabbits was rapid, half-life in the range of 30-50 min. No differences were noted between the clearance of homologous rabbit and heterologous human SC5b-9, SC5b-9 concentrations returned to basal levels 2-3 h after application. Plasma of C6-deficient rabbits contained no SC5b-9 and these animals displayed an even more effective clearance capacity for the complex. Quantitative considerations indicated that basal plasma SC5b-9 levels in healthy animals result from a spontaneous turnover rate of approximately 0.2% of C5-C9 components per h. When multiple doses of SC5b-9 were injected in sequence, the same half-life and total elimination time were found as with single-dose experiments. The results demonstrate the existence of an effective clearance mechanism for SC5b-9, consistent with recent findings that SC5b-9 plasma levels are very low not only in healthy adults, but also in the majority of patients with complement-consuming diseases.