The complement fragment C3d facilitates phagocytosis by monocytes.

Two receptors for fragments of C3 are described for human monocytes: CR1 and CR3, which bind C3b and iC3b, respectively. Recently a leucocyte receptor that binds C3dg has also been described, designated CR4. We previously reported that IgM-sensitized sheep erythrocytes that are heavily coated with C3d (EAC3d) can bind to human monocytes that have been cultured in fetal calf serum (FCS). Here we determine whether such binding of C3d-coated targets can lead to phagocytosis, and identify the specific monocyte receptor involved in C3d binding. We confirm that EAC3d bearing greater than 10,000 C3d/cell bind to FCS-cultured monocytes. Furthermore, using non-cultured monocytes, we demonstrate that C3d enhances rosette formation of IgG-coated E and, like C3b and iC3b, C3d augments IgG Fc receptor-mediated phagocytosis. Less than 100 C3d/cell are capable of enhancing phagocytosis, whereas 10,000 or more C3d/cell are required for rosette formation with cultured cells. These results indicate that the C3d-binding receptor is present on peripheral blood monocytes but has poor affinity for target particles coated only with C3d. Anti-CR2 monoclonal antibodies, which recognize the C3d receptor of lymphocytes, do not block EAC3d rosette formation with monocytes. In contrast anti-Mol, a monoclonal antibody against CR3, inhibits EAC3d rosettes by approximately 42%. Anti-CR1 increases this effect, but complete inhibition is not achieved. Ethylenediamine tetraacetate also markedly reduces EAC3d rosetting, reducing the numbers to less than 5%. Thus, the C3d-binding receptor on monocytes, unlike CR4, is metal dependent. Together these data indicate that CR3 is predominantly responsible for C3d binding to monocytes.