Identification of the membrane receptor for the complement fragment C3d by means of a monoclonal antibody

The B2 antigen characterized by means of a monoclonal antibody (14) is a 140,000 Mr protein expressed only in certain stages of the differentiation of lymphocytes of the B lineage. Here we examine the relationship between B2 and the membrane complement receptor type 2 (CR2) for the complement fragment C3d (11, 12), which is also associated only with B cells. Both phenotypic markers are distributed in a similar manner among B cell malignancies and, as shown here, among established cell lines. A polypeptide with binding affinity for C3d was isolated from the membrane of B2-positive cells, i.e., tonsil lymphocytes and Raji cells. We found that this C3d-binding protein not only had the same Mr and isoelectric point (pI) as the B2 antigen, but that it was recognized by the monoclonal antibody to B2. However, anti-B2 does not mask the ligand-binding site of CR2 since it does not prevent the interaction of the purified 140,000 Mr polypeptide with immobilized C3d. Rosette formation between tonsil lymphocytes and erythrocyte intermediates bearing C3d was specifically inhibited by anti-B2. In the case of Raji cells, rosette formation was strongly inhibited only when the lymphocytes were sequentially treated with anti-B2 and with a polyclonal antibody against mouse Ig. In short, B2 and CR2 have a similar distribution among normal and malignant cells, have the same Mr and pI under denaturing conditions, and react with a single monoclonal antibody. We conclude that B2 is identical to CR2.