Liposome delivery carrier
Characteristics of liposome carriers. Liposomes are
vesicles formed by self-assembly of phospholipid molecules, which can
wrap water-soluble antigens in the center of the vesicles (proteins,
peptides, nucleic acids, etc.) (Fig. 1). While lipophilic substances can
be embedded into the lipid bilayer by adsorption or stable chemical
covalence. Liposomes are potential candidates for vaccine delivery
because of their versatility, biocompatibility, and biodegradability
[9]. As the length and saturation of the phospholipid molecule of
liposomes is different, the membrane
fluidity of the liposomes also changes [10]. The phase change
temperature (Tc) of the phospholipid determines the morphology of
phospholipids [11]. When the ambient temperature is lower than the
liposome, the liposome is in a rigid state, and the cargo can be stored
in the cavity. When the ambient temperature increases to liposome Tc,
the liposome exhibits maximum permeability, and all goods can be
released. The Tc of a single phospholipid molecule is determined, but
the formulation of mixed different phospholipid molecules is usually
used when preparing the liposome, and the Tc can be adjusted to the most
suitable temperature. Further, the addition of cholesterol helps to
maintain the conformational order of the lipid bilayer and increase the
mechanical stiffness of the liposome, thus achieving the purpose of
fluidity of the protective film [12]. Therefore, the addition of
cholesterol to the construction of liposomes can significantly reduce
the permeability of liposome membrane, prevent liposome leakage at
higher ambient temperature, and enhance the ability of liposome vesicles
to resist changes in external conditions [13]. Previous liposome
studies had shown that polyethylene glycol (PEG) modification could
effectively prolong the circulation time of drugs in vivo [14,
15]. Surface PEG modification has a spatially stable effect on the
structure of liposomes, preventing the agglutination and fusion of
liposomes and preventing the binding of plasma proteins to liposomes,
thus producing a spatially stable effect on liposomes [16]. Surface
charge is not only one of the key factors affecting liposomes stability,
but also one of the key factors affecting liposomes immunostimulant.
Depending on the surface charge, the liposomes can be divided into
cationic liposomes, anionic liposomes, and neutral liposomes. The
entrapment efficiency and stability of liposomes are closely related to
the surface charge. First, the entrapment efficiency of liposomes
largely depends on the electrostatic interaction between liposomes and
goods, so the surface charge determines the interaction between
liposomes, biological components, and immune cells, affecting the
loading and release of liposome antigens. The change of physical size
will cause different transport rates of liposomes to antigen-presenting
cells (APC), resulting in different immune effects. It is believed that
the smaller the particle size of liposomes, the higher the stability,
and the liposomes with the particle size of about 100 nm have better
enhanced permeability and retention effect (EPR) in the tumor site
[17]. When the particle size is smaller than 150 nm, it mainly
promotes the development of Th2 cells; when the particle size larger
than 200 nm, the typical Th1 immune response is developed [18, 19].