Autoantibody to complement neoantigens in membranoproliferative glomerulonephritis

With the exception of C3 nephritic factor, autoantibody formation has not been commonly associated with membranoproliferative nephritis (MPGN). We measured autoantibodies (nephritic factors) to the C3 convertases C3bBb (NFa) and C3bBbP (NFt), which result in fast and slow C3 activation, respectively, and to a neoantigen on C1q fixed to a solid phase (spC1q) in sera from 29 patients with MPGN type I, 26 with type II, and 28 with type III. Autoantibody formation was common in all MPGN types. An autoantibody to a C3 convertase neoantigen was identified in more than 75% of the hypocomplementemic MPGN sera tested. Anti-C3bBb (NFa) was present in 81% of patients with MPGN type II but was rarely found in either type I or type III. Anti-C3bBbP (NFt) was common in both MPGN I and III. Anti-spC1q was present in 74% of patients with type I and in 38% and 48% of types II and III MPGN, respectively. Patients with MPGN types I, II, and III had one and two serum autoantibodies detected significantly more frequently than did a group of healthy subjects. The presence of any one autoantibody was not specifically associated with the presence of any other autoantibody. The results indicate that multiple autoantibody formation is common in all MPGN types. MPGN II, and possibly MPGN I, tend to form more specific autoantibodies.