Loss of autoantibody activity by alteration in autoantigen.

C3 nephritic factor (C3NeF) is an autoantibody produced by patients with membranoproliferative glomerulonephritis (MPGN); it is thought to be responsible for the continued breakdown of C3 in these patients. We have studied three patients with MPGN for periods of 7 to 17 years who were noted to develop a normal C3 and factor B level in the face of persistent circulating C3NeF. In addition, C3NeF obtained from other sera were inactive when added to these patients' sera. When C3NeF was isolated from these patients' sera, however, it was completely active when added to normal human serum as a source of C3 and factor B. Removal of the C3 and factor B from these patients' sera by passage over anti-C3 and anti-factor B columns followed by reconstitution with C3 and factor B isolated from normal serum allowed C3NeF to be active in these sera. Replacement of C3 alone did not restore activity to these sera but replacement with factor B alone restored full C3NeF activity. These data suggest some alteration in autoantigen (factor B) as a mechanism for reduction in autoantibody (C3NeF) activity.