Solubilization of an activity regulating C3b function from Raji cell membranes.

A fraction of isolated Raji cell membranes solubilized with 2m KBr which was capable of inhibiting C3-dependent rosettes was examined for its ability to inhibit the alternative pathway of complement. It was shown to decrease alternative pathway-dependent haemolysis of sheep erythrocytes and to accelerate decay of factor B from these cells. It had no effect on C2 decay, the classical pathway analog of factor B. Inhibitory activity solubilized from Raji cells was not removed by immunoadsorption with anti-factor H or anti-factor I, two well-characterized serum C3b-control proteins. It also differed in two functional respects from these proteins. Firstly, it failed to result in cleavage of a peptide bone in C3 which is characteristic of factor I; secondly, its inhibitory activity did not synergize with factor I in inhibiting the alternative pathway, unlike factor H. These results suggest that Raji cells contain a regulatory factor in their membranes for the alternative pathway of complement which is distinct from factors H and I.