Erythrocyte CR1 determination using monoclonal antibody in a microtiter plate ELISA; receptors are not masked by immune complexes.

The microtiter plate ELISA using monoclonal antibody is a specific, sensitive and quantitative technique for measuring CR1 on human erythrocytes. The present investigations established that receptor occupancy by immune complexes did not affect the measurements. The monoclonal anti-CR1 antibody To5 bound unimpeded to receptors that had reacted with an excess of complement-opsonized tetanus toxoid anti-tetanus toxoid complexes prepared at antigen:antibody ratios between 32:1 and 1:8. The CR1 levels on erythrocytes from 11 patients with systemic lupus erythematosus (SLE) were not increased (P greater than 0.30) after release of CR1-bound immune complexes by incubation with factor I. Neither did the serum from these patients contain blocking anti-CR1 activity (P greater than 0.10). Additionally, the number of antigenic CR1 sites in 10 normals and in the 11 patients with SLE was well correlated with the number of functional receptor sites as assessed by binding of soluble complexes (P less than 0.001). These data establish that the true CR1 levels are determined using the microtiter plate ELISA for quantitation of CR1 in patients with diseases involving immune complexes and/or autoantibodies.