Quantitation of C3 nephritic factor of alternative complement pathway by an enzyme-linked immunosorbent assay

We have developed an enzyme-linked immunosorbent assay (ELISA) for the quantitation of C3 nephritic factor of the alternative pathway of complement (NeFA). Incubation of the NeFA-positive serum (patient KS serum) with normal human serum (NHS) in Mg-EGTA resulted in the formation of C3-B-IgG complex. No complex was formed in EDTA. At first this was detected as three types of complexes: C3-IgG, B-IgG and B-C3, by the combination of antibodies. The reaction mixture in Mg-EGTA was filtered through an ACA 22 column, from which the complexes were eluted in the same part as the first protein peak. When IgG purified from KS serum was incubated with NHS in Mg-EGTA, B-C3 complex increased in proportion to the dose of IgG. These results indicated that only one kind of complex consisting of IgG, C3 and B (IgG-C3-B) was generated by the addition of NeFA to NHS. Serum NeFA could be quantified as the titer of B-C3 complex formed after its incubation with NHS in Mg-EGTA. Using the ELISA method, NeFA was positive in five out of six patients with membranoproliferative glomerulonephritis (MPGN) type II and in only one of 17 with MPGN type I. Titers obtained by the new method were in good accordance with those by C3 conversion and C3bBb stabilization assays for NeFA, and the new method was more exact and simple than the conventional methods.