Regulation of the complement-mediated elimination of red blood cells modified with biotin and streptavidin

Abstract

Red blood cells (RBC) modified with biotin and streptavidin (SA) present an interesting potential drug delivery system. Biotinylation and SA attachment, however, alter the biocompatibility of RBC. We have reported that polyvalent SA attachment induces lysis of biotinylated RBC (b-RBC) by homologous complement via the alternative pathway. Lysis occurs due to inactivation of the membrane regulators of complement, DAF and CD59, cross-linked by SA. However, monovalent SA attachment does not induce lysis. On the basis of these findings we hypothesized that reduction of the biotin surface density on b-RBC would allow for monovalent SA attachment to b-RBC and that such SA/b-RBC should then be stable in the circulation. In the present work we injected into rats several different radiolabeled RBC probes: rat RBC biotinylated to varying degrees (bn-RBC, where bn represents the input micromolar concentration of biotinylating agent), as well as SA/bn-RBC. Extensively biotinylated rat RBC (b700-RBC, stable in serum in vitro) were rapidly cleared from the bloodstream. We further found that extensively biotinylated human b1000-RBC bound C3b from serum in vitro without detectable lysis, and that rat b700-RBC bound to isolated macrophages in a complement-dependent fashion. Therefore, nonlytic C3b flxation and uptake of C3b-carrying b700-RBC by macrophages appears to be the mechanism leading to clearance of b700-RBC in vivo. Moderately biotinylated RBC (b70-RBC and b240-RBC) were stable in serum in vitro. SA attachment to b240-RBC led to their rapid lysis in serum in vitro, lysis in the bloodstream, and clearance by the liver and spleen. SA attachment to b70-RBC led to fast elimination of SA/b70-RBC from the bloodstream, while in vitro SA/ b70-RBC were stable in serum. Modestly biotinylated RBC (b22-RBC) demonstrated only marginally decreased 60-min survival in the bloodstream regardless of SA attachment. Our in vitro studies indicate that b23-RBC bound approximately 10(5) SA molecules per cell, and the resulting SA/b23-RBC bound 5 x 10(4) molecules of biotinylated IgG (b-IgG) per cell. About 60% of the injected dose of b-IgG/SA/b23-RBC labeled with 51Cr was detected in the rat blood cells 1 day after iv injection. To assess whether b-IgG/SA/b23-RBC circulate in the bloodstream as a stable complex, we have injected 125I-labeled b-IgG/ SA/51Cr-labeled b23-RBC in rats. Up to 60 min after injection, both radiolabels display similar level in bloodstream. Up to 3 h after injection, about 70% of 125I was detected in the blood cells. In contrast, 100% of 125I was detected in plasma after injection of nonconjugated 125I-labeled b-IgG. Thus, major portion of SA/b23-RBC-attached b-IgG circulates as a complex with RBC. About 30% of RBC-bound b-IgG undergoes detachment from the carrier b-RBC, probably in the pulmonary capillaries, because lung level of 125I was twice as high as that of 51Cr. Therefore, the surface density of biotin on the b-RBC membrane appears to play a key role in regulating complement-mediated clearance of bn-RBC and SA/bn-RBC from the bloodstream. Modest biotinylation generates b-IgG/SA/b23-RBC circulating for several hours as stable immunoerythrocytes without detectable lysis or marked elimination, and it may be possible to use these RBC in a drug delivery system.