The liposome particle size is an important parameter because it strongly affects content release from liposomes as a result of different bilayer curvatures and lipid packing. Earlier, we developed pH-responsive polysaccharide-derivative-modified liposomes that induced content release from the liposomes under weakly acidic conditions. However, the liposome used in previous studies size was adjusted to 100-200 nm. The liposome size effects on their pH-responsive properties were unclear. For this study, we controlled the polysaccharide-derivative-modified liposome size by extrusion through polycarbonate membranes having different pore sizes. The obtained liposomes exhibited different average diameters, in which the diameters mostly corresponded to the pore sizes of polycarbonate membranes used for extrusion. The amounts of polysaccharide derivatives per lipid were identical irrespective of the liposome size. Introduction of cholesterol within the liposomal lipid components suppressed the size increase in these liposomes for at least three weeks. These liposomes were stable at neutral pH, whereas the content release from liposomes was induced at weakly acidic pH. Smaller liposomes exhibited highly acidic pH-responsive content release compared with those from large liposomes. However, liposomes with 50 mol% cholesterol were not able to induce content release even under acidic conditions. These results suggest that control of the liposome size and cholesterol content is important for preparing stable liposomes at physiological conditions and for preparing highly pH-responsive liposomes for drug delivery applications.
pH-Sensitive branched β-glucan-modified liposomes promote antigen uptake and activation of dendritic cells in vitro , inducing anti-tumor immune responses in vivo .
Specific delivery to antigen presenting cells (APC) and precise control of the intracellular fate of antigens are crucial to induce cellular immunity that directly and specifically attacks cancer cells. We previously achieved cytoplasmic delivery of antigen and activation of APC using carboxylated curdlan-modified liposomes, which led to the induction of cellular immunity in vivo. APCs express mannose receptors on their surface to recognize pathogen specifically and promote cross-presentation of antigen. In this study, mannose-residue was additionally introduced to carboxylated curdlan as a targeting moiety to APC for further improvement of polysaccharide-based antigen carriers. Mannose-modified curdlan derivatives were synthesized by the condensation between amino group-introduced mannose and carboxy group in pH-sensitive curdlan. Mannose residue-introduced carboxylated curdlan-modified liposomes showed higher pH-sensitivity than that of liposomes modified with conventional carboxylated curdlan. The introduction of mannose-residue to the liposomes induced aggregation in the presence of Concanavalin A, indicating that mannose residues were presented onto liposome surface. Mannose residue-introduced carboxylated curdlan-modified liposomes exhibited high and selective cellular association to APC. Furthermore, mannose residue-introduced carboxylated curdlan-modified liposomes promoted cross-presentation of antigen and induced strong antitumor effects on tumor-bearing mice. Therefore, these liposomes are promising as APC-specific antigen delivery systems for the induction of antigen-specific cellular immunity.
Size is one of the important factors for antigen carriers because it influences cellular interaction, pharmacokinetics, immune cell activation and the types of immune responses. We developed pH-sensitive polysaccharide derivative-modified liposomes as antigen carriers for induction of antigen-specific immune responses. For an earlier study, the size of these liposomes was controlled to within 100–200 nm considering the suitable size for endocytosis. However, since most antigen-presenting cells can engulf larger particles, the proper liposome size and its effect on the induction of cellular versus humoral immune responses remains unclear. Here, we examined the effects of polysaccharide derivative-modified liposome size on immune responses. Liposome size was controlled by using an extrusion method by passing liposomes through a polycarbonate membrane of different pore sizes. Small liposomes encapsulating model antigenic protein suppressed antigen-expressing tumour growth effectively and increased the IgG2a/IgG1 ratio early after liposome administration, suggesting that Th1 response was mainly induced. Large liposomes increased antigen-specific IgG1 and IgG2a production for 56 days compared with small liposomes. These results suggest that small liposomes modified with pH-responsive polysaccharide derivatives could be effective for cancer immunotherapy whereas large liposomes could be suitable for induction of antibody production towards vaccination.
