Liposomes in allergy and immunology

Archaeobacterial ether lipid liposomes (archaeosomes) as novel vaccine and drug delivery systems

Liposomes are artificial, spherical, closed vesicles consisting of one or more lipid bilayer(s). Liposomes made from ester phospholipids have been studied extensively over the last 3 decades as artificial membrane models. Considerable interest has been generated for applications of liposomes in medicine, including their use as diagnostic reagents, as carrier vehicles in vaccine formulations, or as delivery systems for drugs, genes, or cancer imaging agents. The objective of this article is to review the properties and potential applications of novel liposomes made from the membrane lipids of Archaeobacteria (Archaea). These lipids are unique and distinct from those encountered in Eukarya and Bacteria. Polar glycerolipids make up the bulk of the membrane lipids, with the remaining neutral lipids being primarily squalenes and other hydrocarbons. The polar lipids consist of regularly branched, and usually fully saturated, phytanyl chains of 20, 25, or 40 carbon length, with the 20 and 40 being most common. The phytanyl chains are attached via ether bonds to the sn-2,3 carbons of the glycerol backbone(s). It has been shown only recently that total polar lipids of archaeobacteria, and purified lipid fractions therefrom, can form liposomes. We refer to liposomes made with any lipid composition that includes ether lipids characteristic of Archaeobacteria as archaeosomes to distinguish them from vesicles made from the conventional lipids obtained from eukaryotic or eubacterial sources or their synthetic analogs. In general, archaeosomes demonstrate relatively higher stabilities to oxidative stress, high temperature, alkaline pH, action of phospholipases, bile salts, and serum proteins. Some archaeosome formulations can be sterilized by autoclaving, without problems such as fusion or aggregation of the vesicles. The uptake of archaeosomes by phagocytic cells can be up to 50-fold greater than that of conventional liposome formulations. Studies in mice have indicated that systemic administration of several test antigens entrapped within certain archaeosome compositions give humoral immune responses that are comparable to those obtained with the potent but toxic Freund's adjuvant. Archaeosome compositions can be selected to give a prolonged, sustained immune response, and the generation of a memory response. Tissue distribution studies of archaeosomes administered via various systemic and peroral routes indicate potential for targeting to specific organs. All in vitro and in vivo studies performed to date indicate that archaeosomes are safe and do not invoke any noticeable toxicity in mice. The stability, tissue distribution profiles, and adjuvant activity of archaeosome formulations indicate that they may offer a superior alternative to the use of conventional liposomes, at least for some biotechnology applications.

Effects of adjuvants on the immune response to allergens in a murine model of allergen inhalation: cholera toxin induces a Th1-like response to Bet v 1, the major birch pollen allergen

Based on the fact that type I allergies are frequently elicited by inhalant allergens, we have established a model of aerosol inhalation leading to allergic sensitization in BALB/c mice. Using this model we studied the effects of aluminium hydroxide (Al(OH)3), known to enhance IgE antibody responses, compared with cholera toxin (CT), a potent mucosal adjuvant, on the immune response to birch pollen (BP) and its major allergen Bet v 1. Two groups of BALB/c mice were either systemically immunized with recombinant Bet v 1 in Al(OH)3 and subsequently aerosol exposed to BP allergen, or aerosolized with BP and CT. IgE-mediated skin reactions were only elicited in the mice which had received Bet v 1/Al(OH)3. Allergen-specific serum IgE and IgG1 antibodies dominated in the Al(OH)3 group, IgG2a antibody levels to BP and rBet v 1 were markedly higher in the sera of mice exposed to CT with the allergen. IgA antibodies were only detected in the bronchial lavage of the CT-treated group. Moreover, the latter group displayed consistently higher T cell proliferative responses to BP and interferon-gamma production in vitro. Thus, the systemic immunization with rBet v 1 in Al(OH)3 before inhalation of the BP extract promoted a Th2-like immune response, while CT mixed with the aerosolized BP extract rather induced a Th1-like immune response. In an attempt to reverse these ongoing immune responses we could achieve a shift towards a Th0 response. Immunization with BP extract without adjuvant treatment led to undetectable antibody or cellular immune responses. We conclude from the present study that the induction of an immune response to BP allergen after aerosol inhalation can be directed towards a Th1- or a Th2-like response. Once established, the immune response can be modulated.

Biodistribution of cyclosporin encapsulated in liposomes modified with bioadhesive polymer

The purpose of this investigation was to study the possibility of renewing the immunosuppressive activity of cyclosporin by formulating the compound in liposomes modified with bioadhesive polymers. The liposomes prepared were evaluated both pharmacokinetically and pharmacodynamically. Tissue distribution and plasma pharmacokinetics of cyclosporin and model dye, sudan black, which is as hydrophobic as cyclosporin, were studied in rats after intravenous infusion (10 mg kg-1). The immunosuppressive efficacy of liposomal cyclosporin preparations was studied in the allogenic rat-heart-transplantation model, where cyclosporin therapy (10 mg kg-1) continued for one week. The entrapment of sudan black in liposomes modified with bioadhesive polymers resulted in higher sudan black delivery to the spleen and the liver than with standard sudan-black-loaded liposomes. Among the modified liposomes, those modified with carbopol 941 showed the most remarkable enhancing effect on the delivery of sudan black to these organs and total plasma clearance of sudan black decreased to 38.6 +/- 7.8 mL h-1 kg-1 (standard liposomes, 58.9 +/- 6.4 mL h-1 kg-1). Delivery of cyclosporin to the spleen and the liver was increased approximately twofold by modifying the liposomes with carbopol 941. In the preliminary study on the allogenic rat-heart-transplantation model, the mean survival days of the graft were 18.8 +/- 2.9 days for the group receiving cyclosporin liposomes modified with carbopol 941, 14.2 +/- 4.4 days for the group receiving standard cyclosporin liposomes and 7.6 +/- 0.5 days for the group receiving cyclosporin solution. The encapsulation of cyclosporin in liposomes modified with bioadhesive polymer enhanced the residence time of cyclosporin in the systemic circulation, resulting in approximately twofold greater delivery of cyclosporin to the spleen and liver. However, in the allogenic rat-heart-transplantation model no significant difference was detected between the immunosuppressive efficacy of cyclosporin encapsulated in bioadhesive polymer-modified liposomes and that encapsulated in standard liposomes.

Liposomal presentation of antigens for human vaccines

Liposomes are considered prime candidates to improve the immunogenicity of both antigens with hydrophobic anchor sequences and soluble, nonmembrane proteins or synthetic peptides. During the 20 years since liposomes were first demonstrated to have adjuvant potential, studies have shown that variation in liposomal size, lipid composition, surface charge, membrane fluidity, lipid-protein composition, anchor molecules, and fusogenicity can significantly influence results. In addition, antigen location (e.g., whether it is adsorbed or covalently coupled to the liposome surface or encapsulated in liposomal aqueous compartments) may also be important. Analysis of these variables as well as a comparison of the various techniques used to ensure the efficacy, stability, homogeneity, and safety of liposomal vaccine have been discussed.