Mechanisms of Self-Assembly of Giant Unilamellar Vesicles in the Army Liposome Formulation (ALF) Family of Vaccine Adjuvants

<b>Background/Objectives:</b> Army Liposome Formulation with QS21 (ALFQ) is a vaccine adjuvant formulation consisting of liposomes that contain saturated zwitterionic and anionic phospholipids, 55 mol% cholesterol, and small molar amounts of monophosphoryl lipid A (MPLA) and QS21 saponin as adjuvants. A unique aspect of ALFQ is that after addition of QS21 to nanoliposomes (<100 nm), the liposomes self-assemble through fusion to form giant (≥1000 nm) unilamellar vesicles (GUVs). The purpose of this study was to introduce and investigate new intermediate structures in the fusion process that we term tethered incomplete microspheres (TIMs), which were discovered by us incidentally as structures that were visible by phase contrast microscopy. <b>Methods:</b> Differential centrifugation; phase contrast microscopy; confocal microscopy of vesicles or TIMs which contain fluorescent chromophores linked to phospholipids or cholesterol; ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) analysis of lipid components of liposomes and TIMs; and dynamic light scattering were all used for the characterization of TIMS. <b>Results and Conclusions:</b> (A) Sizes of TIMs range from overall aggregated structural sizes of ~1 µm to mega sizes of ≥200 µm. (B) Stable TIM structures occur when a fusion process is stopped by depletion of a fusogenic lipid during an evolving fusing of a lipid bilayer membrane. (C) TIMs consist of long-term stable (>2 years), but also metastable, tightly aggregated tear-drop or spherical incomplete GUVs tethered to visible masses of underlying vesicles that are not individually visible. (D) The TIMs and GUVs all contain phospholipid and cholesterol (when present) as bulk lipids. (E) Lyophilized liposomes lacking QS21 saponin, but which still contain MPLA (ALF55lyo), also self-assemble to form GUVs and TIMs. (F) Cholesterol is a required component in nanoliposomes for generation of GUVs and TIMs by addition of QS21. (G) Cholesterol is not required for production of GUVs and TIMs in ALFlyo, but cholesterol greatly reduces and narrows the polydisperse vesicle distribution.