COMPLEMENT-DEPENDENT DAMAGE TO LIPOSOMES PREPARED FROM PURE LIPIDS AND FORSSMAN HAPTEN

Previous studies have shown that liposomes prepared from the chloroform-soluble fraction (fraction IIa) of sheep erythrocyte membranes release trapped glucose marker when incubated with both rabbit antisheep erythrocyte serum (as a source of antibodies) and guinea pig serum (as a source of complement). In the present investigation, liposomes were prepared from the corresponding IIa fraction obtained from beef erythrocyte membranes (which lack Forssman antigen), and from lipid mixtures containing sphingomyelin (or lecithin), cholesterol, and dicetyl phosphate (or stearylamine). These liposomes do not release glucose when incubated with the rabbit antiserum and guinea pig serum. However, the liposomes were rendered immunologically responsive when pure Forssman hapten, isolated from sheep erythrocytes, was added to beef fraction IIa or the lipid mixtures. The extent, and rate, of marker loss from Forssmansensitized liposomes was dependent upon the amount of hapten incorporated in the lipid mixture used for generation of the liposomes, and upon the concentration of antiserum and complement present during assay. The extent of glucose release was essentially the same from liposomes prepared with either sphingomyelin or lecithin, and was not affected by the charge on the liposomal membrane. Antiserum absorption experiments show that sheep IIa liposomes can bind all of the antibodies required for glucose release from sphingomyelin-cholesterol liposomes which have been sensitized with the Forssman hapten, whereas the latter liposomes can bind only some of the antibodies which induce marker loss from sheep IIa liposomes. In this regard, the Forssman hapten resembles the antigenic material in the methanol-watersoluble fraction (fraction IIb) isolated from sheep erythrocytes which, as reported previously, also has the capacity to confer immune sensitivity to liposomes prepared with artificial lipid mixtures. This and related earlier investigations raise several possibilities regarding the mechanism of immune cytolysis in view of the marked parallelism between the response of the liposomal membrane and natural membranes to antibody and complement. The available data suggest that lipids alone (perhaps in bilayer configuration) may serve as “substrate” for complement, and that cell membranes may not contain any unique and specific endogenous receptor sites (protein and/or carbohydrate) for components of the complement sequence. © 1969, American Chemical Society. All rights reserved.