Toxicity and Biodistribution of Liposomes of the Main Phospholipid from the ArchaebacteriumThermoplasma Acidophilumin Mice

Polar Lipid Fraction E from Sulfolobus acidocaldarius and Dipalmitoylphosphatidylcholine Can Form Stable yet Thermo-Sensitive Tetraether/Diester Hybrid Archaeosomes with Controlled Release Capability

Archaeosomes have drawn increasing attention in recent years as novel nano-carriers for therapeutics. The main obstacle of using archaeosomes for therapeutics delivery has been the lack of an efficient method to trigger the release of entrapped content from the otherwise extremely stable structure. Our present study tackles this long-standing problem. We made hybrid archaeosomes composed of tetraether lipids, called the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius, and the synthetic diester lipid dipalmitoylphosphatidylcholine (DPPC). Differential polarized phase-modulation and steady-state fluorometry, confocal fluorescence microscopy, zeta potential (ZP) measurements, and biochemical assays were employed to characterize the physical properties and drug behaviors in PLFE/DPPC hybrid archaeosomes in the presence and absence of live cells. We found that PLFE lipids have an ordering effect on fluid DPPC liposomal membranes, which can slow down the release of entrapped drugs, while PLFE provides high negative charges on the outer surface of liposomes, which can increase vesicle stability against coalescence among liposomes or with cells. Furthermore, we found that the zeta potential in hybrid archaeosomes with 30 mol% PLFE and 70 mol% DPPC (designated as PLFE/DPPC(3:7) archaeosomes) undergoes an abrupt increase from −48 mV at 37 °C to −16 mV at 44 °C (termed the ZP transition), which we hypothesize results from DPPC domain melting and PLFE lipid ‘flip-flop’. The anticancer drug doxorubicin (DXO) can be readily incorporated into PLFE/DPPC(3:7) archaeosomes. The rate constant of DXO release from PLFE/DPPC(3:7) archaeosomes into Tris buffer exhibited a sharp increase (~2.5 times), when the temperature was raised from 37 to 42 °C, which is believed to result from the liposomal structural changes associated with the ZP transition. This thermo-induced sharp increase in drug release was not affected by serum proteins as a similar temperature dependence of drug release kinetics was observed in human blood serum. A 15-min pre-incubation of PLFE/DPPC(3:7) archaeosomal DXO with MCF-7 breast cancer cells at 42 °C caused a significant increase in the amount of DXO entering into the nuclei and a considerable increase in the cell’s cytotoxicity under the 37 °C growth temperature. Taken together, our data suggests that PLFE/DPPC(3:7) archaeosomes are stable yet potentially useful thermo-sensitive liposomes wherein the temperature range (from 37 to 42–44 °C) clinically used for mild hyperthermia treatment of tumors can be used to trigger drug release for medical interventions.

Stable archaeal tetraether lipid liposomes for photodynamic application: transfer of carboxyfluorescein to cultured T84 tumor cells

Background: Archaeal membranes have phytanyl ether lipids instead of common fatty acid-glycerol esters in bacterial and eukaryotic cells. Sulfolobus and Thermoplasma species have unique membrane-spanning tetraether lipids (TEL), which form stable liposomes. Recently, we cultured Thermoplasma species from the Indonesian volcano Tangkuban Perahu and isolated TEL. The purpose of this in vitro study is to investigate the transfer of fluorescent dye from stable TEL liposomes to cultured colon carcinoma cells.Methods: TEL was extracted from cultured cells with chloroform-methanol (1:1), then it was fractionated and purified via diethylaminoethyl-cellulose-acetate columns and activated charcoal for the formation of stable liposomes. For the fluorescence exchange assay, TEL liposomes were loaded with water-soluble carboxyfluorescein (CF). Staining experiments were conducted with various cell cultures, and T84 colon carcinoma cells were chosen for the main experiments. Liposome stability was tested by light scattering and electron microscopic size determinations as well as by unspecific CF release at low pH (6.0–7.4) and increased temperature  (4–50°C/70°C).Results: TEL liposomes exhibit high stability and extremely low proton permeability at low pH. CF staining of cultured T84 colon carcinoma cells appeares more intensive from TEL liposomes than from dipalmitoylphosphatidylcholine liposomes.Conclusion: The results of this in vitro study demonstrate CF staining of colon carcinoma cells and high stability of TEL liposomes at low pH, matching the condition in the gastro-intestinal (GI) route and in the urogentital (UG) tract. For this reason, in vivo studies on liposomal fluorescent photosensitizers for topical application of photodynamic cancer therapy in the GI and UG tracts should be carried out.

Design, fabrication, and characterization of archaeal tetraether free-standing planar membranes in a PDMS- and PCB-based fluidic platform

The polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius contains exclusively bipolar tetraether lipids, which are able to form extraordinarily stable vesicular membranes against a number of chemical, physical, and mechanical stressors. PLFE liposomes have thus been considered appealing biomaterials holding great promise for biotechnology applications such as drug delivery and biosensing. Here we demonstrated that PLFE can also form free-standing "planar" membranes on micropores (∼100 μm) of polydimethylsiloxane (PDMS) thin films embedded in printed circuit board (PCB)-based fluidics. To build this device, two novel approaches were employed: (i) an S1813 sacrificial layer was used to facilitate the fabrication of the PDMS thin film, and (ii) oxygen plasma treatment was utilized to conveniently bond the PDMS thin film to the PCB board and the PDMS fluidic chamber. Using electrochemical impedance spectroscopy, we found that the dielectric properties of PLFE planar membranes suspended on the PDMS films are distinctly different from those obtained from diester lipid and triblock copolymer membranes. In addition to resistance (R) and capacitance (C) that were commonly seen in all the membranes examined, PLFE planar membranes showed an inductance (L) component. Furthermore, PLFE planar membranes displayed a relatively large membrane resistance, suggesting that, among the membranes examined, PLFE planar membrane would be a better matrix for studying channel proteins and transmembrane events. PLFE planar membranes also exhibited a sharp decrease in phase angle with the frequency of the input AC signal at ∼1 MHz, which could be utilized to develop sensors for monitoring PLFE membrane integrity in fluidics. Since the stability of free-standing planar lipid membranes increases with increasing membrane packing tightness and PLFE lipid membranes are more tightly packed than those made of diester lipids, PLFE free-standing planar membranes are expected to be considerably stable. All these salient features make PLFE planar membranes particularly attractive for model studies of channel proteins and transmembrane events and for high-throughput drug screening and artificial photosynthesis. This work can be extended to nanopores of PDMS thin films in microfluidics and eventually aid in membrane-based new lab-on-a-chip applications.

Archaebacterial bipolar tetraether lipids: Physico-chemical and membrane properties

Bipolar tetraether lipids (BTL) are abundant in archaea and can be chemically synthesized. The structures of BTL are distinctly different from the lipids found in bacteria and eukaryotes. In aqueous solution, BTL can form extraordinarily stable liposomes with different sizes, lamellarities and membrane packing densities. BTL liposomes can serve as membrane models for understanding the structure-function relationship of the plasma membrane in thermoacidophiles and can be used for technological applications. This article reviews the separation, characterization and structures of BTL as well as the physical properties and technological applications of BTL liposomes. One of the structural features of BTL is the presence of cyclopentane rings in the lipid hydrocarbon core. Archaea use the cyclopentane ring as an adaptation strategy to cope with high growth temperature. Special attention of this article is focused on how the number of cyclopentane rings varies with environmental factors and affects membrane properties.

Archaeal tetraether bipolar lipids: Structures, functions and applications

Archaea have developed specific tools permitting life under harsh conditions and archaeal lipids are one of these tools. This microreview describes the particular features of tetraether-type archaeal lipids and their potential applications in biotechnology. Natural and synthetic tetraether lipid structures as well as their applications in drug/gene delivery, vaccines and proteoliposomes or as lipid films are reviewed.

Bipolar tetraether archaeosomes exhibit unusual stability against autoclaving as studied by dynamic light scattering and electron microscopy

The stability of liposomes made of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius against autoclaving has been studied by using dynamic light scattering and transmission electron microscopy. PLFE lipids have structures distinctly different from those derived from eukaryotes and prokaryotes. PLFE lipids are bipolar tetraether molecules and may contain up to four cyclopentane rings in each of the two dibiphytanyl chains. In the pH range 4-10, PLFE-based archaeosomes, with and without polyethyleneglycol- and maleimide-lipids, are able to retain vesicle size, size distribution, and morphology through at least six autoclaving cycles. The cell growth temperature (65 degrees C vs. 78 degrees C), hence the number of cyclopentane rings in the hydrocarbon chains, does not affect this general conclusion. By contrast, at the same pH range, most conventional liposomes made of monopolar diester lipids and cholesterol or pegylated lipids cannot withhold vesicle size and size distribution against just one cycle of autoclaving. At pH<4, the particle size and polydispersity of PLFE-based archaeosomes increase with autoclaving cycles, suggesting that aggregation or membrane disruption may have occurred at extreme acidic conditions during heat sterilization. Under high salt conditions, dye leakage from PLFE archaeosomes due to autoclaving is significantly less than that from pegylated liposomes composed of conventional lipids. The ability to maintain vesicle integrity after multiple autoclaving cycles indicates the potential usefulness of utilizing PLFE-based archaeosomes as autoclavable and durable drug (including genes, peptides, vaccines, siRNA) delivery vehicles.

Archaeosome adjuvants: immunological capabilities and mechanism(s) of action

Archaeosomes (liposomes comprised of glycerolipids of Archaea) constitute potent adjuvants for the induction of Th1, Th2 and CD8(+) T cell responses to the entrapped soluble antigen. Archaeal lipids are uniquely constituted of ether-linked isoprenoid phytanyl cores conferring stability to the membranes. Additionally, varied head groups displayed on the glycerol-lipid cores facilitate unique immunostimulating interactions with mammalian antigen-presenting cells (APCs). The polar lipid from the archaeon, Methanobrevibacter smithii has been well characterized for its adjuvant potential, and is abundant in archaetidyl serine, promoting interaction with a phosphatidylserine receptor on APCs. These archaeosomes mediate MHC class I cross-priming via the phagosome-to-cytosol TAP-dependent classical processing pathway, and also upregulate costimulation by APCs without overt inflammatory cytokine production. Furthermore, they facilitate potent CD8(+) T cell memory to co-delivered antigen, comparable in magnitude and quality to live bacterial vaccine vectors. Archaeosome vaccines provide profound protection in murine models of infection and cancer. This technology is being developed for clinical application and offers a novel prospect for rational design and development of safe and potent subunit vaccines capable of eliciting T cell immunity against intracellular infections and cancers.

Pressure perturbation and differential scanning calorimetric studies of bipolar tetraether liposomes derived from the thermoacidophilic archaeon Sulfolobus acidocaldarius

Differential scanning calorimetry (DSC) and pressure perturbation calorimetry (PPC) were used to characterize thermal phase transitions, membrane packing, and volumetric properties in multilamellar vesicles (MLVs) composed of the polar lipid fraction E (PLFE) isolated from the thermoacidophilic archaeon Sulfolobus acidocaldarius grown at different temperatures. For PLFE MLVs derived from cells grown at 78 degrees C, the first DSC heating scan exhibits an endothermic transition at 46.7 degrees C, a small hump near 60 degrees C, and a broad exothermic transition at 78.5 degrees C, whereas the PPC scan reveals two transitions at approximately 45 degrees C and 60 degrees C. The endothermic peak at 46.7 degrees C is attributed to a lamellar-to-lamellar phase transition and has an unusually low DeltaH (3.5 kJ/mol) and DeltaV/V (0.1%) value, as compared to those for the main phase transitions of saturated diacyl monopolar diester lipids. This result may arise from the restricted trans-gauche conformational changes in the dibiphytanyl chain due to the presence of cyclopentane rings and branched methyl groups and due to the spanning of the lipid molecules over the whole membrane. The exothermic peak at 78.5 degrees C probably corresponds to a lamellar-to-cubic phase transition and exhibits a large and negative DeltaH value (-23.2 kJ/mol), which is uncommon for normal lamellar-to-cubic phospholipid phase transformations. This exothermic transition disappears in the subsequent heating scans and thus may involve a metastable phase, which is irreversible at the scan rate used. Further, there is no distinct peak in the plot of the thermal expansion coefficient alpha versus temperature near 78.5 degrees C, indicating that this lamellar-to-cubic phase transition is not accompanied by any significant volume change. For PLFE MLVs derived from cells grown at 65 degrees C, similar DSC and PPC profiles and thermal history responses were obtained. However, the lower growth temperature yields a higher DeltaV/V ( approximately 0.25%) and DeltaH (14 kJ/mol) value for the lamellar-to-lamellar phase transition measured at the same pH (2.1). A lower growth temperature also generates a less negative temperature dependence of alpha. The changes in DeltaV/V, DeltaH, and the temperature dependence of alpha can be attributed to the decrease in the number of cyclopentane rings in PLFE at the lower growth temperature. The relatively low DeltaV/V and small DeltaH involved in the phase transitions help to explain why PLFE liposomes are remarkably thermally stable and also echo the proposal that PLFE liposomes are generally rigid and tightly packed. These results help us to understand why, despite the occurrence of thermal-induced phase transitions, PLFE liposomes exhibit a remarkably low temperature sensitivity of proton permeation and dye leakage.

Short-term repeated-dose toxicity profile of archaeosomes administered to mice via intravenous and oral routes

Archaeosomes, liposomes made from polar ether lipids of archaea, show promise for vaccine and drug delivery applications. The potential toxicity of intravenously (14, 70, or 140 mg/kg/day for 5 consecutive days) and orally (gavaged at 55, 275, or 550 mg/kg/day for 10 consecutive days) administered unilamellar archaeosomes, prepared from the total polar lipids (TPLs) extracted from several species of archaea, was assessed in female BALB/c mice. Liposomes prepared from an ester phospholipid composition were included for comparative purposes. Control groups of mice were administered 0.1 ml phosphate-buffered saline (PBS) by either route. Animals were monitored at least once daily for temperature, body weight, and clinical signs of adverse reactions. One day after the last dose, the mice were sacrificed. Blood was collected for selected biochemical/enzyme analyses, and the major organs (heart, lungs, liver, spleen, kidneys) were weighed and examined macroscopically. In addition, the spleens were examined histologically. At the two lower dosages of intravenously administered vesicles, there were no significant indications of toxicity, as compared with the PBS-administered control group. At the highest intravenous dose of 140 mg/kg/day, archaeosomes prepared from the TPL of the extreme halophiles, Halobacterium salinarum and Natronobacterium magadii, indicated potential toxicity, as evidenced by clinical signs (hyperactivity and/or piloerection), drop in body temperature, and loss in body weight. Spleens from mice administered some archaeosomes types, primarily at the highest intravenous dose tested, were enlarged, had increased organ weight, and microscopic examination revealed mild to moderate expansion of the red pulp with increased numbers of hematopoietic cells, but no changes in the white pulp. There were similar clinical signs at one or more of the higher oral doses of the ester liposomes and some of the archaeosome types; however, no other apparent toxicity was observed. Based on this limited mouse study, archaeosomes were generally well tolerated after intravenous or oral delivery at the dosages so indicated in this study.

Safety of archaeosome adjuvants evaluated in a mouse model

Archaeosomes, liposomes prepared from the polar ether lipids extracted from Archaea, demonstrate great potential as immunomodulating carriers of soluble antigens, promoting humoral and cell mediated immunity in the vaccinated host. The safety of unilamellar archaeosomes prepared from the total polar lipids (TPL) of Halobacterium salinarum, Methanobrevibacter smithii or Thermoplasma acidophilum was evaluated in female BALB/c mice using ovalbumin (OVA) as the model antigen. Groups of 6-8 mice were injected (0.1 mL final volume) subcutaneously at 0 and 21 days, with phosphate buffered saline (PBS), 11 microg OVA in PBS, 1.25 mg of antigen-free archaeosomes in PBS (ca 70 mg/kg body wt), or PBS containing 11-20 microg OVA encapsulated in 1.25mg archaeosomes. Animals were monitored daily for injection site reactions, body weight,temperature and clinical signs of adverse reactions. Sera were collected on days 1, 2, 22, and 39 for analyses of creatine phosphokinase. Mice were sacrificed on 39 d, sera were collected for biochemical analyses, and major organs (liver, spleen, kidneys, heart, lungs) were weighed and examined macroscopically. There were no indications of adverse reactions or toxicity associated with any of the archaeosome adjuvants. None of the antigen-free archaeosomes elicited significant anti lipid antibodies when subcutaneously injected (1 mg each at 0, 1, 2, and 4 weeks) in mice, although anti H. salinarum lipid antibodies were detected. These antilipid antibodies cross-reacted with the TPL of T. acidophilum archaeosomes but not with the TPL of M. smithii archaeosomes nor with lipids of ester liposomes made from L-alpha-dimyristoylphosphatidylcholine (DMPC), L-alpha-dimyristoylphosphatidylglycerol (DMPG), and cholesterol (CHOL). In vitro hemolysis assay on mouse erythrocytes indicated no lysis with M. smithii or T. acidophilum archaeosomes at up to 2.5 mg/mL concentration. At this concentration, H. salinarum archaeosomes and DMPC/DMPG/CHOL ester liposomes caused about 2% and 4% hemolysis, respectively. Based on this mouse model evaluation, archaeosomes are well-tolerated and appear relatively safe for potential vaccine applications.

Two-photon fluorescence microscopy studies of bipolar tetraether giant liposomes from thermoacidophilic archaebacteria Sulfolobus acidocaldarius

The effects of temperature and pH on Laurdan (6-lauroyl-2-(dimethylamino)naphthalene) fluorescence intensity images of giant unilamellar vesicles (GUVs) ( approximately 20-150 microm in diameter) composed of the polar lipid fraction E (PLFE) from the thermoacidophilic archaebacteria Sulfolobus acidocaldarius have been studied using two-photon excitation. PLFE GUVs made by the electroformation method were stable and well suited for microscopy studies. The generalized polarization (GP) of Laurdan fluorescence in the center cross section of the vesicles has been determined as a function of temperature at pH 7.23 and pH 2.68. At all of the temperatures and pHs examined, the GP values are low (below or close to 0), and the GP histograms show a broad distribution width (> 0.3). When excited with light polarized in the y direction, Laurdan fluorescence in the center cross section of the PLFE GUVs exhibits a photoselection effect showing much higher intensities in the x direction of the vesicles, a result opposite that previously obtained on monopolar diester phospholipids. This result indicates that the chromophore of Laurdan in PLFE GUVs is aligned parallel to the membrane surface. The x direction photoselection effect and the low GP values lead us to further propose that the Laurdan chromophore resides in the polar headgroup region of the PLFE liposomes, while the lauroyl tail inserts into the hydrocarbon core of the membrane. This unusual L-shaped disposition is presumably caused by the unique lipid structures and by the rigid and tight membrane packing in PLFE liposomes. The GP exhibited, at both pH values, a small but abrupt decrease near 50 degrees C, suggesting a conformational change in the polar headgroups of PLFE. This transition temperature fully agrees with the d-spacing data recently measured by small-angle x-ray diffraction and with the pyrene-labeled phosphatidylcholine and perylene fluorescence data previously obtained from PLFE multilamellar vesicles. Interestingly, the two-photon Laurdan fluorescence images showed snowflake-like lipid domains in PLFE GUVs at pH 7.23 and low temperatures (<20 degrees C in the cooling scan and <24 degrees C in the heating scan). These domains, attributable to lipid lateral separation, were stable and laterally immobile at low temperatures (<23 degrees C), again suggesting tight membrane packing in the PLFE GUVs.

Influence of coenzyme Q10 on tissue distribution of archaeosomes, and pegylated archaeosomes, administered to mice by oral and intravenous routes

The biodistribution of orally and intravenously administered archaeosomes in mice was compared to that of archaeosomes containing either coenzyme Q10 (archaeosome-CoQ10), polyethylene glycol (archaeosome-PEG), or PEG plus CoQ10 (archaeosome-PEG-CoQ10). The archaeosome formulations were prepared by a reverse-phase evaporation method using the total polar lipids from the archaeobacterium Methanosarcina mazei. In the case of oral gavage, the most striking observation was that a significantly (p < 0.05) higher concentration (42.28+/-4.17%) of administered dose was found in the stomach content 3 h after administration of unmodified archaeosomes, as compared to that of archaeosome-CoQ10 (16.98+/-2.48%) and archaeosome-PEG-CoQ10 (5.8 +/-4.05"/ vesicles. This correlated with an increased uptake, notably of the archaeosome-PEG-CoQ 0 vesicles.,into liver and spleen; however, no more than 7% of the administered dose was found in liver, spleen and blood at any time point studied. In the case of intravenous administration, a significantly higher percentage of injected dose of unmodified archaeosomes was found in the liver (66.4 +/-.92%) and spleen (11.445+/-.68%) at 48 h, compared to archaeosome-CoQ10, archaeosome-PEG, and archaeosome-PEG-CoQ10 vesicles. The combination of PEG and CoQ10 significantly prolonged the circulation of archaeosomes in the blood, but after 48 h the amount of the vesicle marker in blood had declined to only about 0.5% of administered dose. These data indicate that the biodistribution of archaeosome formulations given orally or intravenously can be altered significantly by incorporating PEG or CoQ 10, alone or in combination, and these vesicles have the potential to act as a carrier for therapeutics and vaccines.

Molecular modeling of archaebacterial bipolar tetraether lipid membranes

Membranes composed of glycerol dialkylnonitol tetraether (GDNT) lipids from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius have been studied by molecular modeling. GDNT membranes containing eight cyclopentane rings in the molecule are packed much tighter than those without rings. When containing eight cyclopentane rings, the beta-D-galactosyl-D-glucose head-group of GDNT runs almost parallel to the membrane surface. However, when containing no rings, the head-group is oriented perpendicular to the membrane surface. Using molecular dynamics calculations, we have also conducted comparative studies of membrane packing between GDNT and various non-archaebacterial membranes. Compared to gel state dipalmitoylphosphatidylcholine (DPPC) and gel state distearoylphosphatidylcholine (DSPC) bilayers, the GDNT membrane with eight cyclopentane rings has a more negative interaction energy, thus a tighter membrane packing, while the GDNT without rings is less tightly packed than gel state DSPC. Based on the calculated interaction energies, the GDNT membranes (with and without rings) are much more tightly packed than DPhPC (an ester-linked diphytanyl PC) and DPhyPC (an ether-linked diphytanyl PC) bilayers. This suggests that the branched methyl group in the phytanyl chain is not the major contributor of the tight packing of GDNT membranes. The biological implication of this study is that the cyclopentane ring could increase GDNT membrane thermal stability. This explains why the number of cyclopentane rings in archaebacterial lipid increases with increasing growth temperature. Perhaps, through the ring-temperature compensation mechanism the plasma membrane of thermoacidophilic archaebacteria is able to maintain a tight and rigid structure, consequently, a constant proton gradient between the extracellular (pH 2.5) and intracellular compartment (pH 6.5), over a wide range of growth temperatures.

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.

Studies of archaebacterial bipolar tetraether liposomes by perylene fluorescence

Membrane packing and dynamics of bipolar tetraether liposomes composed of the polar lipid fraction E (PLFE) from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius have been studied by perylene fluorescence. At a probe-to-PLFE lipid ratio of 1:400, we have detected an unusual fluorescence intensity increase with increasing temperature, while the fluorescence lifetime changed little. As the ratio was decreased, the intensity anomaly was diminished. At 1:3200 and 1:6400, the anomaly disappeared. A remarkable perylene intensity anomaly was also observed in bilayers composed of saturated monopolar diester phosphatidylcholines at their main phase transition temperatures. These results suggest that the intensity anomaly may be due to probe aggregation caused by tight membrane packing. At the same probe-to-lipid ratio (1:400), however, 1, 2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) and 1, 2-diphytanoyl-sn-glycero-3-phosphoglycerol (DPhPG) liposomes did not exhibit any intensity anomaly with increasing temperature. This suggests that DPhPC and DPhPG liposomes are more loosely packed than PLFE liposomes; thus the branched methyl groups are not the contributing factor of the tight membrane packing found in PLFE liposomes. Using a multiexcitation method, we have also determined the average (R), in-plane (R(ip)), and out-of-plane (R(op)) rotational rates of perylene in PLFE liposomes at various temperatures (20-65 degrees C). R and R(ip), determined at two different probe-to-lipid ratios (1:400 and 1:3200), both undergo an abrupt increase when the temperature is elevated to approximately 48 degrees C. These data suggest that PLFE liposomes are rigid and tightly packed at low temperatures, but they begin to possess appreciable "membrane fluidity" at temperatures close to the minimum growth temperature ( approximately 50 degrees C) of thermoacidophilic archaebacteria.

In vitro assessment of archaeosome stability for developing oral delivery systems

The in vitro stability of archaeosomes made from the total polar lipids of Methanosarcina mazei, Methanobacterium espanolae or Thermoplasma acidophilum, was evaluated under conditions encountered in the human gastrointestinal tract. At acidic pH, multilamellar vesicles (MLV) prepared from T. acidophilum lipids were the most stable, releasing approximately 80, 20, 10 and 5% of encapsulated 14C-sucrose at pH 1.5, 2.0, 2.5 and 6.2, respectively, after 90 min at 37 degrees C. Archaeosomes from M. mazei lipids were the least stable. For each type of total polar lipid, unilamellar vesicles (ULV) were less stable than the corresponding MLV vesicles. Pancreatic lipase had relatively minor effect on the stability of archaeosomes made from either of the three types of total polar lipids, causing the release of 12-27% of the encapsulated 5(6)-carboxyfluorescein (CF) from ULV and MLV after 90 min at 37 degrees C. In simulated human bile at pH 6.2, MLV from M. mazei total polar lipids lost 100% of the encapsulated CF after 90 min at 37 degrees C, whereas those from the polar lipids of M. espanolae or T. acidophilum lost approximately 85% of the marker. Pancreatic lipase and simulated human bile had no synergistic effect on the release of carboxyfluorescein from ULV or MLV prepared from any of the total polar lipids. After 90 min in the combined presence of these two stressors at pH 6.2, the leakage of fluorescein conjugated bovine serum albumin from MLV prepared from T. acidophilum lipids was similar to that of CF, and 13% of the initially present vesicles appeared to be intact. These results indicate that archaeosomes show stability properties indicative of potential advantages in developing applications as an oral delivery system.

Archaeosome vaccine adjuvants induce strong humoral, cell-mediated, and memory responses: comparison to conventional liposomes and alum

Ether glycerolipids extracted from various archaeobacteria were formulated into liposomes (archaeosomes) possessing strong adjuvant properties. Mice of varying genetic backgrounds, immunized by different parenteral routes with bovine serum albumin (BSA) entrapped in archaeosomes ( approximately 200-nm vesicles), demonstrated markedly enhanced serum anti-BSA antibody titers. These titers were often comparable to those achieved with Freund's adjuvant and considerably more than those with alum or conventional liposomes (phosphatidylcholine-phosphatidylglycerol-cholesterol, 1. 8:0.2:1.5 molar ratio). Furthermore, antigen-specific immunoglobulin G1 (IgG1), IgG2a, and IgG2b isotype antibodies were all induced. Association of BSA with the lipid vesicles was required for induction of a strong response, and >80% of the protein was internalized within most archaeosome types, suggesting efficient release of antigen in vivo. Encapsulation of ovalbumin and hen egg lysozyme within archaeosomes showed similar immune responses. Antigen-archaeosome immunizations also induced a strong cell-mediated immune response: antigen-dependent proliferation and substantial production of cytokines gamma interferon (Th1) and interleukin-4 (IL-4) (Th2) by spleen cells in vitro. In contrast, conventional liposomes induced little cell-mediated immunity, whereas alum stimulated only an IL-4 response. In contrast to alum and Freund's adjuvant, archaeosomes composed of Thermoplasma acidophilum lipids evoked a dramatic memory antibody response to the encapsulated protein (at approximately 300 days) after only two initial immunizations (days 0 and 14). This correlated with increased antigen-specific cell cycling of CD4(+) T cells: increase in synthetic (S) and mitotic (G(2)/M) and decrease in resting (G(1)) phases. Thus, archaeosomes may be potent vaccine carriers capable of facilitating strong primary and memory humoral, and cell-mediated immune responses to the entrapped antigen.

Lateral diffusion of the total polar lipids from Thermoplasma acidophilum in multilamellar liposomes

31P NMR lineshapes of multilamellar liposomes composed mostly of a bilayer-spanning tetraether lipid are consistent with rapid axially symmetric motion about the bilayer normal. The residual chemical shift anisotropy of 36 ppm is comparable to that seen for diacylphosphatidylglycerol systems and suggests comparable headgroup motion. The lateral diffusion rates for Thermoplasma acidophilum total polar lipids in mutilamellar liposomes was measured by two dimensional exchange NMR as a function of temperature. At 55 degrees C, near the growth temperature, the rate of lateral diffusion, DL, is comparable to that of diester phospholipids in the Lalpha liquid crystalline phase, having a value of 2 x 10(-8) cm2/s. DL decreases with temperature reaching a value of 8-6 x 10(-9) cm2/s at 30 degrees C. The activation energy Ea for lateral diffusion is estimated to be 10 kcal/mol (approximately 42 kJ/mol). The lateral diffusion rates indicate that the tetraether liposomes have a membrane viscosity at 30 degrees C which is considerably higher than that of diester phospholipids in the liquid crystalline phase.