LIPOSOME-INDUCED ACTIVATION OF THE CLASSICAL COMPLEMENT PATHWAY DOES NOT REQUIRE IMMUNOGLOBULIN

We have investigated the contribution of immunoglobulin to the liposom e-induced activation of complement in human serum. Liposomes containin g the negatively charged phospholipids cardiolipin, phosphatidylglycer ol or phosphatidylinositol, in addition to phosphatidylcholine and cho lesterol, were used to activate complement in a whole serum system. Th e contribution of immunoglobulin was studied by comparing normal human serum (NHS) to serum depleted of IgG and IgM (DDS). Using hemolytic a ssays of complement function, greater concentrations of phospholipids were required to activate complement in the absence of immunoglobulins . Activation of the classical pathway was confirmed using a Clq ELISA which showed that activation was dependent on the presence of Clq and confirmed that greater concentrations of phospholipids were required t o activate complement in the absence of immunoglobulins. Complement ac tivation was also assessed using crossed immunoelectrophoresis of C3 a ctivation fragments. Using immunoblot analysis, iC3b was detected on t he surface of liposomes exposed to NHS or DDS. These studies demonstra te that when liposomes, containing anionic phospholipids at an equival ent charge to cardiolipin 20 mol%, are exposed to immunoglobulin deple ted serum they become opsonized by complement proteins.