Archaeobacterial Ether Lipid Liposomes (Archaeosomes) as Novel Vaccine and Drug Delivery Systems

ABSTRACT:?

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 Archaeo-bacteria (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.     

Impact of Mycobacterium Avium Infection on Macrophage-Liposome Interaction

Abstract

Mycobacterium avium (M. avium) can survive within macrophages, and alters various functions of its cellular host. Liposomes and other particulate drug carriers have been investigated as a means for enhancing drug delivery to the intracellular site of infection via the endocytic pathway. We have investigated the impact of M. avium infection on the endocytosis, retention, and intracellular disposition of liposomes, as these are essential macrophage functions that may determine the activity of liposome-encapsulated antibiotics. J774, a macrophage-like cell line, was infected with M. avium at various multiplicities. Infected cells accumulated and retained liposomes in a manner similar to uninfected cells, regardless of the time after infection. Moreover, the intracellular trafficking and disposition of liposomes was unaltered in J774 cells that contained live or heat-killed M. avium, as evidenced by the vesicular pH which liposomes encountered within cells. These results suggest that specific essential cellular functions involved in processing particulate drug carriers remain intact in macrophages infected with M. avium.     

Systemic Activation of Macrophages by Liposomes Containing Muramyltripeptide Phosphatidylethanolamine for therapy of Cancer Metastasis

Abstract

The heterogeneous response of metastases to conventional therapy is a major cause of failure in cancer treatment. Evidence that activated macrophages can recognize and destroy neoplastic cells in vitro without regard to their phenotypic diversity has stimulated efforts to develop effective approaches to the activation of macrophages in situ. Systemic administration of liposomes containing immunomodulators activates macrophages in situ and augments host destruction of spontaneous metastases.

Liposomes are a useful carrier system for the transport of agents to phagocytic cells in vivo. Once in the circulation, liposomes are cleared by phagocytic cells, and this passive localization provides an effective mechanism for targeting liposome-entrapped materials, such as muramyltripeptide phosphatidylethanolamine (MTP-PE), to macrophages.

Macrophage destruction of metastases in vivo is significant, provided that the total tumor burden at start of treatment is minimal. For this reason, we advocate using chemotherapy or radiotherapy first to reduce the tumor burden in patients with metastases. Tumoricidal macrophages that can differentiate neoplastic from bystander nonneoplastic cells are then used to destroy the few tumor cells that escape destruction by conventional therapeutic methods.     

Stealth Liposomes: Five Years On

Abstract

Incorporation of polymers, such as polyethylene glycol (PEG-lipid derivatives, or glycolipids, such as monosialoganglioside GM_1; into liposomes results in sterically stabilized liposomes which have several advantages over liposome formulations traditionally used in the past, including reduced recognition and uptake by macrophages, extended circulation half-lives, dose-independent pharmacokinetics, and increased uptake in vivo by solid tumours. PEG-lipid derivatives such as PEG-distearoylphosphatidylethanolamine (PEG-DSPE) are particularily useful because of their ease of preparation and relative lack of expense. Optimum molecular weight of the PEG headgroup is approximately 2000 daltons and optimum concentration in the bilayer is 5 to 7 mol% of phospholipids. Pegylated liposomes have the additional advantage of allowing.     

Intrahepatic Distribution of Long-Circulating Liposomes Containing Polyethylene Glycol) Distearoyl Phosphatidylethanolamine

Abstract

We investigated the intrahepatic distribution in rats of liposomes of 85 or 130 nm diameter, which were sterically stabilized with a polyethylene glycol) derivative of phosphatidylethanolamine (PEG-PE) so as to increase their circulation time in blood. Various times after intravenous injection of radiolabeled ([³H-]cholesterylether) liposomes, parenchymal and non-parenchymal cells of the liver were isolated and their radioactivity content was determined. Control liposomes of 85 nm without PEG-PE distributed in an approximately 80:20 ratio to hepatocytes (H) and macrophages (M), respectively; the 130-nm control liposomes showed a 50:50 H/M distribution. Incorporation of PEG-PE reduced the rate of total liver uptake about 4-fold for liposomes of either size and shifted the H/M ratio to 60:40 for the smaller vesicles and to 40:60 for the larger ones. For both liposome sizes, PEG-PE apparently causes a shift in intrahepatic distribution in favor of the macrophages. It is concluded that PEG-PE has a stronger inhibitory effect on liposome uptake by hepatocytes than on uptake by macrophages. Attempts to shift liposome uptake more in favor of hepatocytes, by incorporation of lactosylceramide, failed. This compound, although causing an increase in hepatic uptake, particularly for the 130-nm liposomes, shifted the H/M ratio further towards the macrophages. We conclude that the galactose moiety of the glycolipid is sufficiently exposed on the surface of (PEG-PE)-containing liposomes to allow interaction with the galactose-binding lectin at the surface of the liver macrophage and that the extent of exposure is dependent on vesicle size.     

Subcutaneous Administration of Liposomes for Lymphatic Targeting

Abstract

Liposomes have received considerable interest for targeting to regional lymph nodes after s.c. administration. Detailed information on factors influencing lymphatic uptake and lymph node localization of s.c. administered liposomes is, however, not readily available. The present paper provides a short overview of the outcome of recently performed studies on factors potentially affecting lymphatic disposition of liposomes after s.c. injection into rats. An important factor influencing lymphatic disposition was found to be the anatomical site of injection. S.c. injection into the dorsal side of the foot or in the footpad resulted in relatively high uptake (about 40% of the injected dose (%ID)) of small liposomes (mean size about 0.10 μm) from the site of injection compared to uptake from the s.c. injection site at the flank from which uptake was low (< 5 %ID). Liposome size was found to be the most important liposome characteristic influencing lymphatic disposition of s.c. administered liposomes. Small, liposomes (mean size about 0.04 μm) were taken up by the lymphatic system to a relatively high extent (about 74 %ID) compared to large, non-sized liposomes which remained present almost completely at the site of injection. Small liposomes were less efficiently retained by regional lymph nodes than larger liposomes. Liposomal lipid composition did not influence lymphatic disposition significantly with one exception: lymph node localization of liposomes was substantially enhanced by inclusion of phosphatidylserine into the liposomal bilayers. Remarkably, lymphatic uptake and lymph node localization was only slightly affected by distearoylphosphatidylethanolamine-poly(ethyleneglycol) (DSPE-PEG¹) mediated steric stabilization of the liposome surface. Studies designed to elucidate the intranodal fate of liposomes confirmed that liposomes are mainly taken up by lymph node macrophages. Small liposomes may also be taken up by other cells such as endothelial cells. In addition, it was found that PEG-liposomes retained by lymph nodes are also taken up by lymph node macrophages.     

The potent adjuvant activity of archaeosomes correlates to the recruitment and activation of macrophages and dendritic cells in vivo

The unique glycerolipids of ARCHAEA: can be formulated into vesicles (archaeosomes) with potent adjuvant activity. We studied the effect of archaeosomes on APCs to elucidate the mechanism(s) of adjuvant action. Exposure of J774A.1 macrophages to archaeosomes in vitro resulted in up-regulation of B7.1, B7.2, and MHC class II molecules to an extent comparable to that achieved with LPS. Similarly, incubation of bone marrow-derived DCs with archaeosomes resulted in enhanced expression of MHC class II and B7.2 molecules. In contrast, conventional liposomes made from ester phospholipids failed to modulate the expression of these activation markers. APCs treated with archaeosomes exhibited increased TNF production and functional ability to stimulate allogenic T cell proliferation. More interestingly, archaeosomes enhanced APC recruitment and activation in vivo. Intraperitoneal injection of archaeosomes into mice led to recruitment of Mac1alpha(+), F4/80(+) and CD11c(+) cells. The expression of MHC class II on the surface of peritoneal cells was also enhanced. Furthermore, peritoneal cells from archaeosome-injected mice strongly enhanced allo-T cell proliferation and cytokine production. The ability of archaeosome-treated APCs to stimulate T cells was restricted to Mac1alpha(high), B220(-) cells in the peritoneum. These Mac1alpha(high) cells in the presence of GM-CSF gave rise to both F4/80(+) (macrophage) and CD11c(+) (dendritic) populations. Overall, the activation of APCs correlated to the ability of archaeosomes to induce strong humoral, T helper, and CTL responses to entrapped Ag. Thus, the recruitment and activation of professional APCs by archaeosomes constitutes an efficient self-adjuvanting process for induction of Ag-specific responses to encapsulated Ags.     

Stability of Liposomes Prepared from the Total Polar Lipids of Methanosarcina Mazei is Affected by the Specific Salt form of the Lipids

Abstract

The total polar lipids (TPL) extracted from the archaeobacterium Metha-nosarcina mazei were predominantly in the form of sodium and potassium salts. Upon passage of this natural salt form of TPL (ns-TPL) through a silica gel G column (s-TPL), the molar ratio of the monovalent cations, sodium plus potassium, to that of the divalent cations, magnesium plus calcium, decreased from about 11:1 to 2:1. Liposomes (archaeosomes) made from ns-TPL were unable to efficiently retain low molecular weight aqueous markers such as 5(6)-carboxyfluorescein (CF). In phosphate buffered saline (PBS, pH 7.14), between 60-90% and 90-100% of entrapped CF leaked out during 21 h storage at room temperature (20-22°) and 3 h at 50°, respectively. However, archaeosomes made with the s-TPL, as well as from TPL converted to the salt forms of predominantly sodium, potassium, calcium or magnesium, were significantly less leaky. The archaeosomes made with s-TPL were the most stable, showing less than 7% leakage after 21 h at room temperature and only 14% leakage of CF after 3 h at 50°. The stability of s-TPL archaeosomes (21 h, 20-22°) was not affected when Tris-HCI buffer (pH 7.4) was used instead of PBS. However, the inclusion of 1 or 10 mM EGTA in the Tris-HCI buffer increased the amount of CF leaked, to about 25% and 100%, respectively. Except for some differences in phosphatidylserine and phosphati-dylglycerol, the lipid compositions of ns-TPL, s-TPL, and the magnesium form of TPL were similar, as determined by thin layer chromatography of labeled lipids. Archaeosomes prepared from s-TPL and ns-TPL had -“P NMR spectra that were similar to each other, but distinct from those of vesicles prepared from the ester lipid dimyristoylphosphatidylcholine. The types and relative proportions of cations associated with the lipids of M. mazei prior to their hydration and vesicle formation have a major influence, although other factors such as lipid composition may have an effect, on the permeability of the bilayer to low molecular weight compounds.     

Ultradeformable Archaeosomes for Needle Free Nanovaccination with Leishmania braziliensis Antigens

Total antigens from Leishmania braziliensis promastigotes, solubilized with sodium cholate (dsLp), were formulated within ultradeformable nanovesicles (dsLp-ultradeformable archaeosomes, (dsLp-UDA), and dsLp-ultradeformable liposomes (dsLp-UDL)) and topically administered to Balb/c mice. Ultradeformable nanovesicles can penetrate the intact stratum corneum up to the viable epidermis, with no aid of classical permeation enhancers that can damage the barrier function of the skin. Briefly, 100 nm unilamellar dsLp-UDA (soybean phosphatidylcholine: Halorubrum tebenquichense total polar lipids (TPL): sodium cholate, 3:3:1 w:w) of -31.45 mV Z potential, containing 4.84 ± 0.53% w/w protein/lipid dsLp, 235 KPa Young modulus were prepared. In vitro, dsLp-UDA was extensively taken up by J774A1 and bone marrow derive cells, and the only that induced an immediate secretion of IL-6, IL-12p40 and TNF-α, followed by IL-1β, by J774A1 cells. Such extensive uptake is a key feature of UDA ascribed to the highly negatively charged archaeolipids of the TPL, which are recognized by a receptor specialized in uptake and not involved in downstream signaling. Despite dsLp alone was also immunostimulatory on J774A1 cells, applied twice a week on consecutive days along 7 weeks on Balb/c mice, it raised no measurable response unless associated to UDL or UDA. The highest systemic response, IgGa2 mediated, 1 log lower than im dsLp Al₂O₃, was elicited by dsLp-UDA. Such findings suggest that in vivo, UDL and UDA acted as penetration enhancers for dsLp, but only dsLp-UDA, owed to its pronounced uptake by APC, succeeded as topical adjuvants. The actual TPL composition, fully made of sn2,3 ether linked saturated archaeolipids, gives the UDA bilayer resistance against chemical, physical and enzymatic attacks that destroy ordinary phospholipids bilayers. Together, these properties make UDA a promising platform for topical drug targeted delivery and vaccination, that may be of help for countries with a deficient healthcare system.     

DIMENSIONAL ANALYSIS AND SCALE-UP IN THEORY AND INDUSTRIAL APPLICATION*

A comparative study between archaeosomes, lipid lamellar vesicles made from archaea polar lipids, and conventional phospholipids liposomes was carried out, aiming at evaluating the properties and the potential of archaeosomes as novel colloidal carriers for effective drug delivery to the skin. Betamethasone dipropionate (BMD)–loaded archaeosomes and conventional liposomes were prepared by the thin-lipid film and sonication procedures, using, respectively, archaeal lipids extracted from archaea Halobacterium salinarum and enriched soy phosphatidylcholine. Vesicular formulations were characterized by assessing vesicle size, zeta potential, incorporation efficiency, and morphology. In order to investigate the effect of the incorporation in the two different colloidal carrier systems on the (trans)dermal delivery of BMD, in vitro drug permeation studies through full-thickness pig skin were carried out by using Franz diffusion vertical cells by testing both archaeal and liposomal dispersions. Interestingly, archaeosomes appeared to be the most effective carriers for the model drug, achieveing a major drug penetration and accumulation in the skin strata, especially in the epidermis. This can, presumably, be due to the enhanced archaeosomal bilayer fluidity, as indicated by the rheological studies that provided insight into the viscoelastic properties of all the studied systems. The available data suggest that suitably developed archaeosomes may hold great promise as delivery vehicles for topical applications.     

Cationic Liposomes Containing Disulfide Bonds in Delivery of Plasmid DNA

Abstract

Cationic liposomes are non-viral gene transfer vectors for in vitro and in vivo experiments. In the present studies, we investigated whether a disulfide linkage in a cationic lipid was reducible by cell lysate resulting in the release of plasmid DNA and enhanced gene transfection. We also investigated if the differences in transgene production were from differences in total amount of cellular associated plasmid DNA. We systematically compared the gene transfection of disulfide bond containing-cationic lipid, 1', 2'-dioleoyl-sn-glycero-3'-succinyl-2-hydroxyethyl disulfide ornithine conjugate (DOGSDSO), its non-disulfide-containing analog, 1', 2'-dioleyl-sn-glycero-3'-succinyl-1, 6-hexanediol ornithine conjugate (DOGSHDO), 1, 2-dioleoyl-3-trimethylammonium-propane (DOTAP). Two transgene reporter systems (i.e., luciferase and green fluorescent protein (GFP)) were used to address transgene transgene expression and transgene efficiency. Experiments with the luciferase expression plasmid resulted in transgene activity up to 11 times greater transgene production for the disulfide containing lipid in at least two different cell lines, COS 1 and CHO cells. When transgene expression was determined by GFP activity, DOGSDSO liposomes were four times greater than the non-disulfide lipid or positive control (DOTAP) liposomes. By quantifying nucleic acid uptake by flow cytometry it was also demonstrated that increase expression was not solely from an increase in cellular plasmid DNA accumulation. These results demonstrate that cationic lipids containing a disulfide linkage are a promising method for gene transfer.     

Archaeosomes varying in lipid composition differ in receptor-mediated endocytosis and differentially adjuvant immune responses to entrapped antigen

Archaeosomes prepared from total polar lipids extracted from six archaeal species with divergent lipid compositions had the capacity to deliver antigen for presentation via both MHC class I and class II pathways. Lipid extracts from Halobacterium halobium and from Halococcus morrhuae strains 14039 and 16008 contained archaetidylglycerol methylphosphate and sulfated glycolipids rich in mannose residues, and lacked archaetidylserine, whereas the opposite was found in Methanobrevibacter smithii, Methanosarcina mazei and Methanococcus jannaschii. Annexin V labeling revealed a surface orientation of phosphoserine head groups in M. smithii, M. mazei and M. jannaschii archaeosomes. Uptake of rhodamine-labeled M. smithii or M. jannaschii archaeosomes by murine peritoneal macrophages was inhibited by unlabeled liposomes containing phosphatidylserine, by the sulfhydryl inhibitor N-ethylmaleimide, and by ATP depletion using azide plus fluoride, but not by H. halobium archaeosomes. In contrast, N-ethylmaleimide failed to inhibit uptake of the four other rhodamine-labeled archaeosome types, and azide plus fluoride did not inhibit uptake of H. halobium or H. morrhuae archaeosomes. These results suggest endocytosis of archaeosomes rich in surface-exposed phosphoserine head groups via a phosphatidylserine receptor, and energy-independent surface adsorption of certain other archaeosome composition classes. Lipid composition affected not only the endocytic mechanism, but also served to differentially modulate the activation of dendritic cells. The induction of IL-12 secretion from dendritic cells exposed to H. morrhuae 14039 archaeosomes was striking compared with cells exposed to archaeosomes from 16008. Thus, archaeosome types uniquely modulate antigen delivery and dendritic cell activation.     

Liposomal modification with uronate, which endows liposomes with long circulation in vivo, reduces the uptake of liposomes by J774 cells in vitro.

For overcoming rapid removal of liposomes from the bloodstream, we developed reticuloendothelial system (RES)-avoiding liposomes modified with a uronic acid derivative, palmityl-D-glucuronide (PGlcUA). In this current study, we examined the in vitro interaction of PGlcUA-liposomes with J774 cells derived from mouse macrophages. Liposomal association with J774 cells at 37 degrees C did not increase compared with the binding at 4 degrees C when liposomes were modified with PGlcUA. RES-avoiding ability was not specifically endowed by glucuronate but by uronates in general, since palmityl-D-galacturonide showed a similar effect on liposomal clearance in vivo and liposomal uptake in vitro. These facts indicate that modification of the liposomal surface with uronic acid derivatives endows liposomes with a long circulation time in the bloodstream by reducing their uptake by macrophages.     

Liposomes and Skin Tolerance

Abstract

The authors evaluate different situations where there is an increase or a decrease of complexed liposome tolerance on the skin and in particular of the substances they convey. Since liposomes are made of physiological membrane components, namely phospholipids, they are considered as atoxic microspherules, that are non immunogenic and histocompatible when applied to the skin. When produced in the laboratory (synthetic liposomes) the purity and stability of the lipids making up the liposomes must be kept under strict control. Any undesired side effect should not be attributed to the phospholipids as liposomes, but to the substances (both medical and non) that they convey; indeed side effects result from the interaction and activity of these active principles. On the other hand, the drugs or active principles delivered to the skin by the liposomes have a longer and more concentrated effect and they enter into the systemic circulation in very small amounts, and consequently local and general tolerance is very much enhanced.     

Liposome-mediated cytosolic delivery of macromolecules and its possible use in vaccine development.

In the majority of bacterial and viral infections the generation of cytotoxic T cells is of particular interest because such pathogens are able to escape the host defence mechanisms by surviving intracellularly within the phagocytic cells. To generate a CD8+ T lymphocyte response against exogenous antigens, the prerequisite is their delivery into the cytosol followed by processing and presentation along with class I major histocompatibility complex (MHC-I) molecules. In the present study we describe the method of liposome-based delivery of antigens and other macromolecules into the cytosol of target cells. To develop safe and effective methods for generating CD8+ T lymphocytes, we exploited the fusogenic character of lipids derived from lower organisms, that is baker's yeast (Saccharomyces cerevisiae). The degree of fusion with model membrane systems using yeast lipid liposomes varied from 40-70%, as opposed to 1-8% observed with egg PtdCho liposomes, depending on the assay system used. The fusion of yeast lipid liposomes with macrophages resulted in effective delivery of the entrapped solutes into the cytoplasmic compartment. This was further supported by the inhibition of cellular protein synthesis in J774 A1 cells by ricin A, encapsulated in the yeast lipid liposomes. Interestingly, the model antigen ovalbumin, when entrapped in the yeast lipid liposomes, successfully elicited antigen reactive CD8+ T cell responses. It may be concluded that the liposomes made of lipids derived from S. cerevisiae can spontaneously fuse with macrophages, delivering a significant portion of their contents into the cytoplasmic compartment of the cells.     

Liposomes composed of unsaturated lipids for membrane modification of human erythrocytes

Abstract

Previous studies have shown that certain saturated lipids protect red blood cells (RBCs) during hypothermic storage but provide little protection during freezing or freeze-drying, whereas various unsaturated lipids destabilize RBCs during hypothermic storage but protect during freezing and freeze-drying. The protective effect of liposomes has been attributed to membrane modifications. We have previously shown that cholesterol exchange and lipid transfer between liposomes composed of saturated lipids and RBCs critically depends on the length of the lipid acyl chains. In this study the effect of unsaturated lipids with differences in their number of unsaturated bonds (18:0/18:1, 18:1/18:1, 18:2/18:2) on RBC membrane properties has been studied. RBCs were incubated in the presence of liposomes and both the liposomal and RBC fraction were analyzed by Fourier transform infrared spectroscopy (FTIR) after incubation. The liposomes caused an increase in RBC membrane conformational disorder at suprazero temperatures. The fluidizing effect of the liposomes on the RBC membranes, however, was found to be similar for the different lipids irrespective of their unsaturation level. The gel to liquid crystalline phase transition temperature of the liposomes increased after incubation with RBCs. RBC membrane fluidity increased linearly during the first 8 hours of incubation in the presence of liposomes. The increase in RBC membrane fluidity was found to be temperature dependent and displayed Arrhenius behaviour between 20 and 40°C, with an activation energy of 88 kJ mol^-1. Taken together, liposomes composed of unsaturated lipids increase RBC membrane conformational disorder, which could explain their cryoprotective action.     

Photosensitive Liposomes

Abstract

Photosensitive lipids and liposomes may be designed by a variety of strategies. These include the photochemical modification of individual lipids in the bilayer; the photoinduced change in the association of polyelectrolytes with liposomes; and the photoinitiated polymerization of some or all of the lipids in the liposome. The interaction of light with photosensitive liposomes can cause bilayer reorganization with possible applications in imaging, sensing, as well as therapeutics. The latter is the focus of this review.     

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.     

On the Molecular Mechanism of Steric Stabilization of Liposomes in Biological Fluids

Abstract

Liposomes with specific surface modification overcome rapid in vivo uptake by cells of the mononuclear phagocytic system (MPS) resulting in prolonged circulation in the blood. The structure-function relationship of this effect has been examined by measurements both in vitro and in vivo. The results are reviewed and compared with those from liposomes without surface modification. For example, in the best cases with polyethylene glycol-derivatized phosphatidylethanolamine (PEG-PE) up to 35% of the injected dose remains in the blood and less than 10% is taken up by the two major organs of the MPS, liver and spleen, after 24 hr. This compares with less than 1% in the blood and up to 40% uptake for liposomes without PEG-PE. Steric stabilization has been proposed as a theoretical basis for these results, and some initial results testing this basis have been reported. Here, we discuss steric stabilization in terms of the physico-chemical properties of the liposomes.     

Biological Action of Liposomes on Main Pathogenetic Mechanisms of Inflammation

Abstract

Small neutral liposomes from phosphatidylholine and cholesterol (7:5 molar ratio) were used to test their antiexudative and antialterative action on rats with acute foot oedema and chemical necrosis of soft hip tissues. The action of liposomes on erythrocytes and liver cells homogenates depending on the peroxidation level of liposomal lipids was studied in vitro. Bactericidal action of liposomes was tested on main types of conditionally pathogenic microorganisms. The interaction of liposomes with blood serum components was studied in vitro. Showed that liposomes reduced the degree of tissue hyperhydratation and shortened time of healing chemically induced necrosis. Rate of liposomal lipids oxidation had a great influence on erythrocytes resistance and lipids oxidation in liver cells homogenates. Showed that liposomes had an evident bactericidal action and could adsorbe toxic components (middle mass peptides) of blood serum. The results showed that liposomes could be used as independent drug to influence on main pathogenetic mechanisms of local and general inflammatory disease.