Complement receptor 1/CD35 is a receptor for mannan-binding lectin

Mannan-binding lectin (MBL), a member of the collectin family, is known to have opsonic function, although identification of its cellular receptor has been elusive. Complement C1q, which is homologous to MEL, binds to complem ent receptor 1 (CR1/CD35), and thus we investigated whether CR1 also functi ons as the MBL receptor. Radioiodinated MBL bound to recombinant soluble CR 1 (sCR1) that had been immobilized on plastic with an apparent equilibrium dissociation constant of 5 nM. N-acetyl-D-glucosamine did not inhibit sCR1- MBL binding, indicating that the carbohydrate binding site of MBL is not in volved in binding CR1. C1q inhibited MBL binding to immobilized sCR1, sugge sting that MBL and C1q might bind to the same or adjacent sites on CR1. MBL binding to polymorphonuclear leukocytes (PMNs) was associated positively w ith changes in CR1 expression induced by phorbol myristate acetate. Finally , CR1 mediated the adhesion of human erythrocytes to immobilized MBL and fu nctioned as a phagocytic receptor on PMNs for MBL-immunoglobulin G opsonize d bacteria. Thus, MBL binds to both recombinant sCR1 and cellular CR1, whic h supports the role of CR1 as a cellular receptor for the collectin MBL.