Requirements for the solubilization of immune aggregates by complement: assembly of a factor B-dependent C3-convertase on the immune complexes.

Abstract

During the solubilization of immune precipitates (BSA[bovine serum albumin]-rabbit antibodies to BSA) by human complement, at least 3 stages can be distinguished. Generation of alternative pathway C3[3rd complement component]-convertase sites associated with the immune complexes is the 1st. During the 1st minutes of interaction between the immune aggregates and serum, before any solubilization takes place, properdin (P), factor B and C3 moieties are incorporated into the lattice. The washed precipitates have C3-convertase activity, which can be completely inhibited by antibodies to factor B, but not to C2. The assembly of the convertase is temperature-dependent and does not take place in the absence of Mg2+. The immune complex-associated C3-convertase activity decays rapidly at 37.degree. C, but it can be restored by addition of purified factor B and properdin. Amplification is the 2nd. When the aggregates bearing C3-convertase are incubated with purified C3, solubilization takes place. Solubilization may be caused by the accumulation of a large number of C3 fragments on the Ag-Ab [antigen-antibody] lattice. In solubilized complexes, the molar ratios of Ab/C3 are close to 1. Spontaneous release is the 3rd. The final step in the solubilization process is a secondary reaction, during which some rearrangement of the lattice takes place. It occurs in medium devoid of serum and does not require divalent cations.