Incorporation of the major outer membrane protein of Neisseria gonorrhoeae in saponin-lipid complexes (iscoms): chemical analysis, some structural features, and comparison of their immunogenicity with three other antigen delivery systems.

We incorporated the major outer membrane protein (PI) of Neisseria gonorrhoeae into immunostimulating complexes (iscoms) and examined some analytical, physicochemical, and immunological properties of these structures. The immunogenicity was compared with that of three other PI-containing structures, i.e., liposomes, outer membrane complexes produced by the bacterium, and protein-detergent-adjuvant complexes. AIPO4 and dioctadecyldimethylammonium bromide were used as adjuvants. Our results show that iscoms are much more immunogenic than liposomes and protein-detergent complexes but are also much more toxic. The localization of PI in iscoms was investigated. Therefore, the chymotrypsin susceptibility of PI in iscoms was tested, and the incorporation of fragments of PI was determined. Amphiphilic fragments of PI were incorporated in iscoms, but hydrophilic and hydrophobic fragments were not. Chymotrypsin degradation of PI in iscoms indicated that the protein is exposed to the environment in a similar manner as PI in outer membrane complexes, i.e., with both termini anchored in the iscom.