Porcine C1q and the solid-phase immunoassay of human immune complexes

Experiments were undertaken to determine if porcine C1q could replace human C1q in the solid-phase immunoassay of human immune complexes (ICs). Porcine C1q was obtained by a two-cycle precipitation method involving dialysis against chelating agents in low ionic strength buffer. C1q was adsorbed to polystyrene beads and in vivo- or in vitro-formed ICs binding to the solid-phase C1q were detected with 125I-labeled or horseradish peroxidase-conjugated anti-human gamma antibodies. Unfractionated, heat-aggregated human gamma globulin (delta IgG) could be detected at 20 ng/ml when diluted in buffer only. The detection threshold changed to 40-80 ng delta IgG/ml when the assay was run with buffer containing normal human serum diluted 1: 1000 (the serum dilution used for detecting natural ICs). Analysis of systemic lupus erythematosus sera revealed that 60% contained highly significant levels of ICs (binding greater than or equal to 3 S.D. above the mean of controls). Comparison with platelet aggregation test results revealed a highly significant correlation between the two methods (P less than 0.0001), even though each assay detected ICs in several serum specimens negative in the other test. These results demonstrate that porcine C1q can functionally replace human C1q in the solid-phase immunoassay of human ICs. Since porcine blood is normally a waste product of the meat-processing industry, it is an obvious source of easily isolated C1q for use in such an assay.