Complement receptor binding of C3b-coated cells treated with C3b inactivator, beta 1H globulin and trypsin.

Treatment of 125I-C3b bound to EAC1423b with C3b inactivator (C3bINA) and beta 1H globulin (beta 1H) cleaved the alpha-chain of C3b into 65,000- and 42,000-dalton fragments, both of which remained disulfide-bonded to the intact beta-chain (C3bi). Subsequent treatment with trypsin (0.1 microgram/ml) released 125I into the supernatant and yielded cells coated with a 33,000-dalton fragment of alpha-chain, presumably C3d. These results are in agreement with those obtained by others using fluid phase C3b. C3b-coated cells (EAC1423b) adhered to complement (C) receptors on human erythrocytes, glomeruli, and monocytes. C3bi-coated cells adhered to the receptors on glomeruli and monocytes, but not to those on human erythrocytes. C3d-coated cells adhered only to the monocyte receptors. The findings suggest that the glomerular C receptor recognizes portions of the C3 molecule different from those recognized by either the erythrocyte or monocyte receptors.