An Improved Method to Study Complement Receptor-Mediated Function of the Fixed Macrophage System in vivo

Abstract. A method to coat unsensitized erythrocytes with fragments of C3 and C4 using autologous serum, in order to study complement receptor-dependent function of the fixed macrophage system, is presented. After incubation with serum under optimal conditions, at least 90% of the cells had C3b/iC3b deposited on the surface, with an average of 20 × 10³ molecules per cell. Elimination of the coated cells by the fixed macrophage system was studied in 12 normal subjects. With a dose of 4.5 × 10⁸ red cells injected, 75% of the cells were eliminated with a half-life of approximately 2.4 ± 0.3 min (n = 7). In subjects receiving ten times more cells, there was a rapid decrease in the amount of C3-coated cells, reaching a nadir with 85% remaining for 4–6 min, after which there was a gradual release of cells for another 20 min (n = 5). In absolute numbers, 3 × 10⁸ of labeled cells were eliminated regardless of the dose injected. The coating procedure presented here is simple, does not introduce heterologous blood components and makes it possible to control the amount and the degree of fragmentation of the C3 and C4 deposited on the erythrocyte surface.