Flow cytometric detection and quantitation of immune complexes using human C1q-coated microspheres

A solid phase human C1q-binding fluorescent immunoassay for the measurement of immune complexes in human serum was developed. The solid phase used was 5 micron diameter polystyrene microspheres. Serum immune complexes bound to the C1q-coated microspheres were measured by flow cytometry using fluoresceinated anti-human IgG, and heat-aggregated human IgG as a standard. Patient samples were assayed and results compared to a standard fluoroimmunometric C1q-binding immune complex assay. Greater differences in circulating immune complexes were observed between the healthy control group mean and the mean of the patient values in the microsphere-flow cytometric method than were seen in the standard assay. In the microsphere-flow cytometric assay, the mean patient value was 7.5 times greater than the control mean, whereas in the standard assay the mean patient value was 2.8 times the control mean. Preliminary results suggest greater sensitivity of the microsphere-flow cytometric method over the other method.