An immunoradiometric assay for erythrocyte complement (C3b) receptor activity applied to a pediatric population with connective tissue disease.

The number of receptors for complement component C3b per erythrocyte reportedly is decreased in over half of adults with systemic lupus erythematosus. We have devised an immunoradiometric assay for C3b receptor (CR1) on erythrocytes, with which one can assess CR1 saturation due to in vivo binding of immune complexes or activated complement fragments (C3b). Using this assay, we examined binding by CR1 in normal adults and newborns, in lupus and juvenile rheumatoid arthritis patients, and in a population of patients with various general medical problems, including other connective tissue diseases. Binding by CR1 was decreased in eight of 15 SLE patients, four of 25 juvenile rheumatoid arthritis patients, and one of 14 patients with other diseases. We found no significant correlation between CR1 binding and either C1q binding, antinuclear antibody titer, results for complement C3 and C4, or the presence of renal disease. Using this assay, we were also able to show that the observed reduction in CR1 binding was not ascribable to prior saturation of CR1 or to blocking antibody against CR1. The assay is precise and easy enough for routine application.