IDENTIFICATION OF A C3bi-SPECIFIC MEMBRANE COMPLEMENT RECEPTOR THAT IS EXPRESSED ON LYMPHOCYTES, MONOCYTES, NEUTROPHILS, AND ERYTHROCYTES*

Membrane complement (C)1 receptors pecific for different parts of the C3 molecule and for /~IH, C5a, and Clq have been described on a variety of different cell types (1). CR1, specific for C3b and C4b, and CRz for C3d have been isolated and shown to be glycoproteins of 205,000 Mr and 72,000 Mr, respectively (2-4). Recently, the f l lH receptor also was isolated successfully using an anti- /~lH idiotypic ant ibody (5). 2 Specific antibodies to isolated C receptors indicated that a common structure for CR1 is shared with erythrocytes, lymphocytes, monocytes, and neutrophils (2, 3) and that CR ~ is restricted to B lymphocytes (4). Because earlier studies had shown that monocytes (6, 7) and neutrophils (8) bound EAC1-3d, it was thought that these phagocytic ell types expressed a C3d-specific receptor that was similar to lymphocyte CR2. However, it now appears likely that the EAC 1-3d reagents used in these previous studies contained bound C3bi and little or no bound C3d. Previously, EAC 1-3d were prepared by treatment of EAC 1-3b with purif ied C3b inactivator (C3bINA) because it was believed that C3bINA removed C3c from the complexes, leaving only bound C3d. Subsequently, it was demonstrated that cleavage of f luid-phase C3b with purif ied C3bINA (9) resulted in formation of the C3bi fragment, and that EAC1-3b or EC3b treated with purif ied C3bINA contained only C3bi (EAC1-3bi or EC3bi) and no C3d (10). Further cleavage of C3bi into C3c and C3d required trypsin (9) o