IDENTIFICATION OF A C3bi-SPECIFIC MEMBRANE COMPLEMENT RECEPTOR THAT IS EXPRESSED ON LYMPHOCYTES, MONOCYTES, NEUTROPHILS, AND ERYTHROCYTES* BY GORDON D. ROSS:I: ANO

M e m b r a n e complement (C)1 receptors specific for different par ts of the C3 molecule and fo r /~ IH , C5a, and Clq have been descr ibed on a var ie ty 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). Recent ly , the f l l H receptor also was isolated successfully using an ant i /~ lH id io typic an t ibody (5). 2 Specific ant ibodies to isolated C receptors ind ica ted that a common s t ructure for CR1 is shared with erythrocytes, lymphocytes , monocytes, and neutrophi ls (2, 3) and that CR~ is restr icted to B lymphocytes (4). Because earl ier studies had shown tha t monocytes (6, 7) and neutrophi ls (8) bound EAC1-3d, it was thought that these phagocyt ic cell types expressed a C3d-specific receptor that was s imilar to lymphocy te CR2. However , it now appears likely that the EAC 1-3d reagents used in these previous studies conta ined bound C3bi and li t t le or no bound C3d. Previously, EAC 1-3d were p repared by t rea tment of EAC 1-3b with purif ied C3b inac t iva tor (C3bINA) because it was bel ieved that C 3 b I N A removed C3c from the complexes, leaving only bound C3d. Subsequent ly , it was demons t ra ted that c leavage of f lu id-phase C3b with purif ied C 3 b I N A (9) resulted in format ion of the C3bi f ragment , and that EAC1-3b or EC3b t rea ted with purif ied C 3 b I N A conta ined only C3bi (EAC1-3bi or EC3bi) and no C3d (10). Fur the r cleavage of C3bi into C3c and C3d required t rypsin (9) or