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].