The activation of the C3b feedback cycle with human complement components. II. Using components of the alternative pathway.

The C3 convertase of the C3b feedback cycle was generated in the fluid phase from C3b and factors B and D. It was shown to be an effective decomplementing agent when reacted with human serum. Many of the effects observed were dependent on the operation of the feedback during the ongoing reaction. Oxidization of factor B with iodine did not significantly increase the potency or the stability of the enzyme. Treatment of C3b with antrypol inhibited the formation of the enzyme. In preliminary experiments in the rat, the convertase was successfully formed in vivo, accompanied by cleavage of C3, consumption of C5, C6 and C7, and a biphasic change in the circulating neutrophils.