Differing reactivities of human and guinea-pig complement on haptenized liposomes via the alternative pathway

Several haptens coupled to dipalmitoylphosphatidylethanolamine (DPPE) were inserted into the liposome membrane with a base composition of an equimolecular mixture of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol). Haptens used were trinitrophenyl (TNP)-DPPE, TNP-aminocaproyl (TNP-Cap)-DPPE, dinitrophenyl (DNP)-DPPE, DNP-aminocaproyl (DNP-Cap)-DPPE, fluoresceinthiocarbamyl (Fl)-DPPE, azobenzenarsonate-tyrosyl (ABA-Tyr)-DPPE, dansyl (DNS)-DPPE, dabsyl (DABS)-DPPE, dithiopyridyl (DTP)-DPPE and maleimidobenzoyl (MB)-DPPE. Reactivity of those haptenized liposomes with complement via the alternative pathway was assessed by release of trapped fluorescent marker from the liposomes following incubation with dilutions of guinea-pig and human sera in a diluent containing MgCl2 and ethyleneglycol-bis(beta-aminoethyl ether)N,N'-tetraacetate (EGTA). In the diluent (Mg-EGTA-GVB), complement activation via the alternative pathway proceeds while that via the classical pathway is inhibited. Fl-liposomes were found to be extremely sensitive to guinea-pig complement, being lysed by guinea pig serum dilutions of up to 1:76 in Mg-EGTA-GVB. Guinea-pig serum could lyse TNP-Cap-liposomes, DNP-Cap-liposomes, TNP-liposomes, DTP-liposomes, MB-liposomes, DNP-liposomes and ABA-Tyr-liposomes, with the reactivity of the liposomes decreasing in this order. However, the only haptenized liposomes sensitive to human serum in Mg-EGTA-GVB were DTP- and MB-liposomes; the other liposomes including Fl-liposomes being unreactive via the alternative pathway in reaction with human complement.