Breakdown of C3 covalently bound to F(ab')2 immune complexes after complement activation by the alternative pathway

The activation and subsequent degradation of C3 covalently bound to immune complexes (IC) has been studied by using immune aggregates antiovalbumin-125I-F(ab')2-ovalbumin or 125I-C3 in the presence of serum. Kinetic experiments were performed in order to establish the physiological sequence of C3 degradation as a function of time. The results indicated: The interaction C3-IC, as analyzed in SDS-PAGE, results in bands of high molecular weight corresponding to C3 alpha-65-Fd and C3 alpha-41-Fd covalent complexes. In the first 7 min only C3 alpha-65-Fd complexes were detected. From 7 to 15 min a progressive increase in the C3 alpha-41-Fd complexes occurs. After this time the ratio C3 alpha-65-Fd/C3 alpha-41-Fd was kept constant for at least 45 min. Hence, C3b covalently bound to F(ab')2 IC is degraded in serum much faster than when it is bound to IgG IC. The spatial distribution of the Fab arms in the IC appears to be a critical feature in providing a protective environment for C3b. The orientation of the Fab arms was dependent on the presence of the Fc regions.