The influence of C3b inactivator (KAF) concentration on the ability of serum to support complement activation.

Minor elevation of the concentration of the C3b inactivator (KAF) in whole serum (by 15-25%) markedly inhibits the capacity of the serum to support complement activation by inulin, aggregated IgG and by low concentrations of CVF. However, there was no effect when large concentrations of CVF were used nor was the spontaneous ageing of C3 slowed by increased KAF concentrations. These findings show that the activity of the C3b feedback cycle can be reduced by raising the KAF concentration above physiological levels. This finding may provide a mechanism for damping down complement activation locally in vivo. It seems that the spontaneous ageing of C3 in vitro does not involve a KAF-inhibitable step and therefore cannot be involved to explain the 'C3 tickover' in vitro.