Stabilization of homologous and heterologous cell-bound amplification convertases, C3bBb, by C3 nephritic factor

Abstract

C3 nephritic factor (C3NeF) may be found in the sera of patients with membranoproliferative glomerulonephritis or partial lipodystrophy. It is capable of activating the alternative pathway in normal human serum; purified C3NeF has been shown to bind to the amplification convertase of complement, C3bBb. The binding of C3NeF to C3bBb results in stabilization of the otherwise labile C3 convertase. Decay of the convertase is accompanied by release of Bi and C3NeF from C3b. To determine whether stabilization of C3bBb occurs by the binding of C3NeF to C3bBb itself, to C3b or Bb alone, homologous and heterologous cell-bound convertases were prepared with C3^hu, B^hu, C3^rat and B^rat and exposed to C3NeF or properdin. It was found that properdin induced stabilization of C3b^huBb^hu, C3b^ratBb^hu, C3b^huBb^rat and C3b^ratBb^rat in a dose-dependent manner. On the other hand, nine out of ten C3NeF preparations were only capable of stabilizing C3b^huBb^hu and C3b^ratBb^hu and not C3b^huBb^rat and C3b^ratBb^rat. To determine whether binding of [¹²⁵I]-C3NeF to the various convertases occurred, cell-bound convertase were prepared in the presence of excess B^hu and B^rat, washed and further incubated with [¹²⁵I]-C3NeF; the cells were then washed and the amount of cell-bound [¹²⁵I]-C3NeF was measured. As in the stabilization experiments C3NeF bound only to C3^huBb^hu and C3b^ratBb^hu. The binding of C3NeF was always directly related to the presence of Bb^hu in the convertase. The results obtained with nine out of ten C3NeF preparations suggest that C3NeF is an autoantibody directed against antigenic determinants on Bb^hu, which are exposed after interaction of Bb^hu with C3b^hu or C3b^rat. One out of ten C3NeF preparations showed reactivity with both cell-bound C3b alone and cell-bound C3bBb. These reactivities could be separated by absorption with cell-bound C3b^hu.