A simple solid phase radioimmunoassay specific for human C3b to detect C3b receptors on human lymphoblastoid cell surfaces.

Our aim was to detect C3b receptors on human lymphoblastoid cells using a solid phase radioimmunoassay (RIA) specific for human C3b. RIA was performed by coupling rabbit antihuman C3b to acrylamide beads to make immunobeads. The specificity and sensitivity of binding of 125I-C3b to immunobeads allowed the detection of as little as 6 X 10(-10) M unlabeled human C3b. The cells were incubated with a C3b concentration (10(-9) M) giving 25% inhibition in the RIA. The concentration of unbound C3b was then measured in the cell supernatants using RIA. Results showed that: (a) loss of C3b antigen in the cell supernatant was not due to degradation of C3b molecules, (b) C3b binding could be detected at 37 degrees C on the four B cell lines, but not on the two T cell lines or on the two non T-non B cell lines tested, (c) C3b bound on the B lymphoblastoid cells was not cleaved, neither into iC3b nor C3c and C3d fragments, supporting the presence of C3b receptors on the cells tested. This method allows screening of a variety of C3b receptor-positive cells.