Insertion of Ia and H-2 alloantigens into model membranes.

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Abstract
The study of immune phenomena dependent on the major histocompatibility complex (MHC) would be greatly simplified by the use of MHC antigen [Ag]-containing liposomes in various functional systems. Unilamellar phosphatidylcholine liposomes were constructed containing H-2 and Ia Ag. These molecules were not simply trapped within the aqueous compartment of the liposome as assessed by their accessibility to papain digestion. They were integrally inserted in the liposome bilayer because they could not be dissociated from the liposome with high salt and EDTA concentrations, but could be solubilized by detergent. A sensitive radioimmunoassay showed that the Ia molecules were antigenically active in the liposome environment. Ia and H-2 Ag could be immunoprecipitated from detergent-solubilized liposomes. Liposome-associated Ia activity was compared in the presence and absence of detergent and by showing accessibility of the Ia Ag to papain. The majority of Ia Ag were exposed on the external surface of the liposome. The orientation of MHC Ag in liposomes may closely parallel their natural orientation in the cell membrane.