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Activation of Human Complement by Liposomes: A Model for Membrane Activation of the Alternative Pathway

From the Department of Immunology, Rush University, Chicago, Illinois 60612, and the Department of Membrane Biochemistry, Walter Reed Army Institute of Research, Washington, D. C. 20012

Abstract

Liposomal model membranes were found to activate the alternative pathway of human complement. Activation was measured by C3 conversion and component consumption in serum that had been incubated with liposomes. C3 conversion did not require C1 or C2 of the classical pathway, since it was observed in serum from a C1r-deficient patient, serum from a C2-deficient patient, and normal serum in buffer containing EGTA and MgCl₂. The incubation of liposomes with C2-deficient serum resulted in consumption of components C3 through C9 with no consumption of C1 or C4 in a profile typical of alternative pathway activation. The reaction was further shown to require alternative pathway factor D, and to be independent of antibody.

Activation of the alternative pathway was dependent on the membrane composition of the liposomes. A positive charge was required for liposomes to produce C3 conversion. Liposomal cholesterol concentration and phospholipid fatty acyl chain length and unsaturation all influenced activation, suggesting the importance of membrane fluidity. Positively charged liposomes containing dimyristoyl phosphatidylcholine and cholesterol required the presence of certain glycolipids for C3 conversion. The activation of the alternative complement pathway by liposomes of defined membrane composition may provide a suitable model for the study of alternative pathway activation by cellular membranes.

Footnotes

¹ H.G. holds the Thomas J. Coogan, Sr., Chair in Immunology established by Marjorie Lindheimer Everett.