Heterogeneity of nephritic factor and its identification as an immunoglobulin

Complement C3 nephritic factor (NeF) produces alternative pathway-meciated C3 cleavage by binding to and stabilizing the alternative pathway C3 convertase, C3bBb. Some studies have suggested that NeF is an immunoglobulin, while others conclude that it is a distinct serum protein. The heterogeneity of NeF was evaluated by electrophoresis and isoelectric focusing of NeF-containing serum followed by hemolytic demonstration of NeF activity in agar gel. With each method, diffuse cathodal zones or multiple bands of hemolysis developed, which revealed remarkable variations in patterns from patient to patient. NeF activity was absorbed by and eluted from insolubilized antibody to Fc and Fab fragments of IgG. Immunoabsorption of six NeF-containing sera with insolubilized anti-kappa and anti-lambda light chain antisera revealed that NeF had kappa antigenic determinants in three, lambda antigenic determinants in one, and both kappa and lambda antigenic determinants in two. These data indicate that NeF is an oligoclonal immunoglobulin. Because NeF binds to the alternative pathway C3 convertase, C3bBb, we suggest that it is an antibody to a conformational antigen of the C3-factor B complex, and thereby stabilizes this complex.