Production of nephritic factor of the alternative complement pathway by Epstein Barr virus-transformed B cell lines derived from a patient with membranoproliferative glomerulonephritis.

Nephritic factor of the alternative complement pathway (C3NeF) is an IgG autoantibody which binds to and stabilizes the C3 convertase (C3bBb) enzyme, and which has been detected mainly in sera from patients with membranoproliferative glomerulonephritis (MPGN) and partial lipodystrophy. To study the production of C3NeF, mononuclear cells isolated from the peripheral blood of patients with MPGN and C3NeF activity in their sera were infected with Epstein Barr virus (EBV) to establish active B lymphocyte cell lines. By using a modified C3NeF screening assay, we detected C3NeF activity in the supernatant of a B cell line derived from a patient with MPGN Type II, but in none of the supernatants of B cell lines derived from normal individuals. C3NeF-positive supernatants were investigated for their ability to conserve classical or alternative pathway C3 convertase activity by using EAC3bBb and EAC4b2a stabilization assays. C3NeF-positive supernatants stabilized the C3bBb convertase activity, but not the C4b2a convertase activity. Studies of the supernatants, using anti-human IgG affinity columns, showed that the C3NeF activity was in the IgG fraction; furthermore, C3NeF antibody agglutinated sheep erythrocytes coated with C3bBb, but not with C3b alone. On gel electrophoresis, both heavy and light chains of the C3NeF were comparable in size to that of normal human IgG molecules. We conclude that C3NeF, produced in vitro by EBV-transformed B cell lines derived from a patient with MPGN Type II, is functionally identical to the conventional C3NeF in serum. In vitro preparation of homogeneous NeF(s) should greatly facilitate the studies of the role of these autoantibodies in complement dysmetabolism.