Purification and characterization of soluble CD21 from human plasma by affinity chromatography and density gradient centrifugation.

Complement receptor II (CD21) is the receptor for C3d fragments on immune complexes. It also serves as a receptor for Epstein-Barr virus (EBV) and on B-lymphocytes CD21 amplifies signalling through the B cell receptor. CD21 is shed from the surface of the cell and is found circulating in plasma. There, soluble CD21 (sCD21) binds to CD23 and complement fragments, thereby modulating the immune response. sCD21 activates monocytes through binding to membrane CD23. The clinical significance of sCD21 is shown by the increased levels found in the sera of patients with B lymphomas, EBV infections and other lymphoblastoid tumors. In this paper, we report the isolation of soluble CD21 from human plasma using affinity chromatography and density gradient centrifugation. sCD21 was found to be a single 126 kDa molecular species. By determining the sedimentation coefficient, we have calculated the partial specific volume, diffusion coefficient and frictional coefficient of the protein. These values show that the sCD21 isolated from human plasma is an elongated rod-shaped molecule.