Differential Complement Activation Pathways Promote C3b Deposition on Native and Acetylated LDL thereby Inducing Lipoprotein Binding to the Complement Receptor 1

Abstract

Lipoproteins can induce complement activation resulting in opsonization and binding of these complexes to complement receptors. We investigated the binding of opsonized native LDL and acetylated LDL (acLDL) to the complement receptor 1 (CR1). Binding of complement factors C3b, IgM, C1q, mannose-binding lectin (MBL), and properdin to LDL and acLDL were investigated by ELISA. Subsequent binding of opsonized LDL and acLDL to CR1 on CR1-transfected Chinese Hamster Ovarian cells (CHO-CR1) was tested by flow cytometry. Both native LDL and acLDL induced complement activation with subsequent C3b opsonization upon incubation with normal human serum. Opsonized LDL and acLDL bound to CR1. Binding to CHO-CR1 was reduced by EDTA, whereas MgEGTA only reduced the binding of opsonized LDL, but not of acLDL suggesting involvement of the alternative pathway in the binding of acLDL to CR1. In vitro incubations showed that LDL bound C1q, whereas acLDL bound to C1q, IgM, and properdin. MBL did neither bind to LDL nor to acLDL. The relevance of these findings was demonstrated by the fact that ex vivo up-regulation of CR1 on leukocytes was accompanied by a concomitant increased binding of apolipoprotein B-containing lipoproteins to leukocytes without changes in LDL-receptor expression. In conclusion, CR1 is able to bind opsonized native LDL and acLDL. Binding of LDL to CR1 is mediated via the classical pathway, whereas binding of acLDL is mediated via both the classical and alternative pathways. Binding of lipoproteins to CR1 may be of clinical relevance due to the ubiquitous cellular distribution of CR1.