Cooperation between CR1 (CD35) and CR3 (CD11b/CD18) in the binding of complement‐opsonized particles

We analyzed the binding of sheep eryth- rocytes bearing C3b (EC3b) to cells transfected with human complement receptors. EC3b bound avidly to cells expressing CR! but failed to bind to cells expressing CR3. In the presence of factor I, the binding of EC3b to CR! was transient. Primary monocytes and cotransfected cells expressing both CR! and CR3 mediated a stable rosetting of EC3b, even in the prolonged presence of factor I. This stable adhesion was dependent on the presence of CR3, because blocking CR3 with mAb resulted in the factor I-dependent release of erythrocytes from these cells. A model is proposed in which these two complement receptors cooperate in a unique man- ner. These results suggest that the stable adhesion of complement-opsonized particles to cells expressing CR! and CR3 is actually a dynamic molecular proc- ess in which an important function of leukocyte CR! is to generate the ligands for CR3. J. Leukoc. Biol. 59: 883-890; !996.