Cationic
liposomes are known as potent adjuvants for
subunit vaccines. The purpose of this work was to study whether the content and the physicochemical properties of the positively charged compound affect the adjuvanticity of cationic
liposomes. Cationic
liposomes containing a cationic compound (
DDA, DPTAP,
DC-Chol, or eDPPC) and a neutral
phospholipid (DPPC) were prepared by the film hydration-extrusion method and loaded with
influenza hemagglutinin (HA) by adsorption. The
liposomes were characterized (hydrodynamic diameter, zeta potential, membrane fluidity, HA loading) and their adjuvanticity was tested in mice. The formulations were administered twice subcutaneously and mouse sera were analyzed for HA-specific
antibodies by ELISA and for HA-
neutralizing antibodies by hemagglutination inhibition (HI) assay. First, the influence of cationic
lipid concentration in the
DC-Chol/DPPC
liposomes (10 vs. 50 mol%) was investigated. The
DC-Chol/DPPC (50:50)
liposomes showed a higher zeta potential and HA loading, resulting in stronger immunogenicity of the HA/
DC-Chol/DPPC (50:50)
liposomes compared to the corresponding (10:90)
liposomes. Next, we used
liposomes composed of 50 mol% cationic
lipids to investigate the influence of the nature of the cationic compound on the adjuvant effect.
Liposomes made of the four cationic compounds showed similar hydrodynamic diameters (between 100 and 170 nm), zeta potentials (between +40 and +50 mV), HA loading (between 55% and 76%) and melting temperatures (between 40 and 55 °C), except for the
DC-Chol liposomes, which did not show any phase transition. HA adjuvanted with the
DC-Chol/DPPC (50:50)
liposomes elicited significantly higher total
IgG1 and
IgG2a titers compared to the other liposomal HA formulations and non-adjuvanted HA. A similar trend was observed for the HI titers. These results show that the adjuvanticity of cationic
liposomes depends on both the content and the physicochemical properties of the charged compound.