An evaluation of the raji cell assay for immune complexes

The purpose of this investigation was to evaluate the Raji cell assay as a method which detects both immune complexes and antibody directed against Raji cell antigenic determinants. This evaluation was made because antibody to Raji cells could be detected in many samples of sera from patients with leukemia that were being analyzed for immune complexes. The antibody was detected by an antibody-dependent, complement-mediated cytotoxicity assay. Since this antibody, presumably IgG, could bind second antibody (labelled antihuman IgG) used in the Raji cell assay and be interpreted as immune complexes, two model systems were devised to test the relative influence of specific anti-Raji cell antibody on the quantitation of the nonspecific immune complexes. The model systems were: (1) the preparation and test of immune complexes of a uniform size (10S-15S) consisting of rabbit anti-dinitrophenyl-bovine serum albumin and dinitrophenyl bovine serum albumin in antigen excess, and (2) the preparation and test of 7S antibody from rabbits made to Raji cells which showed complement-mediated cytolysis.

The findings suggest that antibody to Raji cells does bind second antibody and can be interpreted as immune complex-equivalents. However, the immune complex model does not induce cytolysis of Raji cells but binds second antibody at a different rate than does antibody to Raji cells. The mixture of the two model preparations bound second antibody additively, indicating that at least two binding sites were involved. This is consistent with immune complexes binding to complement receptors and antibody binding to specific surface determinants. The findings, however, indicated that the amount of antibody in terms of standardized immune complex-equivalents cannot be readily subtracted from the quantity of true immune complexes.

The results show that caution must be exercised in defining the binding of second antibody to Raji cells as indicating only immune complexes. A second assay to detect the presence of antibody to Raji cells should be strongly recommended. The significance of the presence of immune complexes and/or antibody to Raji cells in sera of patients is unknown.