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. Unfractioned, heat-aggregated human gamma globulin (ΔIgG) could be detected at 20 ng/ml when diluted in buffer only. The detection threshold changed to 40–80 ng ΔIgG/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 ?3 S.D. above the mean of controls). Comparison with platelet aggregation test results revealed a highly significant correlation between the two methods (P < 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.