Interactions of the third component of complement (C3) with cross-linked dextran. II. Demonstration of an alternate pathway activation as binding mechanism of C3 to cross-linked dextran.

In previous investigations we could show that incubation of cross-linked dextran (Sephadex) with normal human serum results in the binding of the third component of complement to the Sephadex beads. In this paper, data are presented which demonstrate that not only human but also guinea pig C3 reacts with Sephadex and that this binding is due to an alternate pathway of C3 activation. This conclusion was drawn, since a) C3 is bound also from guinea pig serum with total deficiency of C4, b) the reaction can be completely blocked by EDTA but only diminished by EGTA and c) the reaction turned out to be temperature dependent with an optimum at 37 degrees C and could be abolished by diluting the serum more than 1:16. The ability of cross-linked dextran to activate C3 via the alternate pathway seems to be due to conformational changes, since in our experiments soluble dextran of the same source was found to be ineffective in this respect. Implications from these findings and possible applications are discussed.