Uptake of Archaeobacterial Liposomes and Conventional Liposomes by Phagocytic Cells

Archaeal ether lipids improve internalization and transfection with mRNA lipid nanoparticles

Neutral and positively charged archaeal ether lipids (AEL) have been studied for their utilization as novel delivery systems for pDNA, showing efficient immune response with a strong memory effect while lacking noticeable toxicity. Recent technological advances placed mRNA lipid nanoparticles (LNPs) at the forefront of next-generation delivery systems; however, no study has examined AELs in mRNA delivery yet. In this study, we investigated either a crude lipid extract or the purified tetraether lipid caldarchaeol from Sulfolobus acidocaldarius as potential novel excipients for mRNA LNPs. Depending on their molar share in the respective LNP, particle uptake, and mRNA expression levels could be increased by up to 10-fold in in vitro transfection experiments using both primary cell sources (HSMM) and established cell lines (Caco-2, C2C12) compared to a well-known reference formulation. This increased efficiency might be linked to a substantial effect on endosomal escape, indicating fusogenic and lyotropic features of AELs. This study shows the high value of archaeal ether lipids for mRNA delivery and provides a solid foundation for future in vivo experiments and further research.

Archaeal lipids

The major archaeal membrane glycerolipids are distinguished from those of bacteria and eukaryotes by the contrasting stereochemistry of their glycerol backbones, and by the use of ether-linked isoprenoid-based alkyl chains rather than ester-linked fatty acyl chains for their hydrophobic moieties. These fascinating compounds play important roles in the extremophile lifestyles of many species, but are also present in the growing numbers of recently discovered mesophilic archaea. The past decade has witnessed significant advances in our understanding of archaea in general and their lipids in particular. Much of the new information has come from the ability to screen large microbial populations via environmental metagenomics, which has revolutionised our understanding of the extent of archaeal biodiversity that is coupled with a strict conservation of their membrane lipid compositions. Significant additional progress has come from new culturing and analytical techniques that are gradually enabling archaeal physiology and biochemistry to be studied in real time. These studies are beginning to shed light on the much-discussed and still-controversial process of eukaryogenesis, which probably involved both bacterial and archaeal progenitors. Puzzlingly, although eukaryotes retain many attributes of their putative archaeal ancestors, their lipid compositions only reflect their bacterial progenitors. Finally, elucidation of archaeal lipids and their metabolic pathways have revealed potentially interesting applications that have opened up new frontiers for biotechnological exploitation of these organisms. This review is concerned with the analysis, structure, function, evolution and biotechnology of archaeal lipids and their associated metabolic pathways.

Archaeosomes: New Generation of Liposomes Based on Archaeal Lipids for Drug Delivery and Biomedical Applications

Archaeosomes are a new generation of stable liposomes composed of natural ether lipids extracted from archaea, or synthetic archaeal lipids. Archaea constitute a domain of single-celled microorganisms that are structurally similar to but evolutionarily distinct from bacteria. They synthesize unique membrane lipids with isoprenoid hydrocarbon side chains attached via an ether linkage to the glycerol-phosphate backbone. Compared to the ester linkages found in the lipids of Eukarya and bacteria, the ether linkages in archaeal lipids are more stable in various environmental conditions such as high/low temperatures, acidic or alkaline pH, bile salts, and enzymatic hydrolysis. This feature has intrigued scientists to use archaeal lipids to prepare archaeosomes with superior physicochemical stability and utilize them as effective carriers to deliver various cargos of biomedical importance such as drugs, proteins, peptides, genes, and antioxidants to the target site. Archaeosomes carrying antigens and/or adjuvants are also proven to be better candidates for stimulating antigen-specific, humoral, and cell-mediated immune responses, which broadens their scope in vaccine delivery. These properties associated with excellent biocompatibility and a safety profile provide numerous advantages to the archaeosomes to function as a versatile delivery system. This mini-review will provide an overview of the unique features of archaeal lipids, preparation and characterization of archaeosomes, and emphasize the prospects related to drug delivery and other biomedical applications.

Vesicular and Planar Membranes of Archaea Lipids: Unusual Physical Properties and Biomedical Applications

Liposomes and planar membranes made of archaea or archaea-like lipids exhibit many unusual physical properties compared to model membranes composed of conventional diester lipids. Here, we review several recent findings in this research area, which include (1) thermosensitive archaeosomes with the capability to drastically change the membrane surface charge, (2) MthK channel's capability to insert into tightly packed tetraether black lipid membranes and exhibit channel activity with surprisingly high calcium sensitivity, and (3) the intercalation of apolar squalane into the midplane space of diether bilayers to impede proton permeation. We also review the usage of tetraether archaeosomes as nanocarriers of therapeutics and vaccine adjuvants, as well as the biomedical applications of planar archaea lipid membranes. The discussion on archaeosomal therapeutics is focused on partially purified tetraether lipid fractions such as the polar lipid fraction E (PLFE) and glyceryl caldityl tetraether (GCTE), which are the main components of PLFE with the sugar and phosphate removed.

Mechanistic insight into the induction of cellular immune responses by encapsulated and admixed archaeosome-based vaccine formulations

Archaeosomes are liposomes formulated using total polar lipids (TPLs) or semi-synthetic glycolipids derived from archaea. Conventional archaeosomes with entrapped antigen exhibit robust adjuvant activity as demonstrated by increased antigen-specific humoral and cell-mediated responses and enhanced protective immunity in various murine infection and cancer models. However, antigen entrapment efficiency can vary greatly resulting in antigen loss during formulation and variable antigen:lipid ratios. In order to circumvent this, we recently developed an admixed archaeosome formulation composed of a single semi-synthetic archaeal lipid (SLA, sulfated lactosylarchaeol) which can induce similarly robust adjuvant activity as an encapsulated formulation. Herein, we evaluate and compare the mechanisms involved in the induction of early innate and antigen-specific responses by both admixed (Adm) and encapsulated (Enc) SLA archaeosomes. We demonstrate that both archaeosome formulations result in increased immune cell infiltration, enhanced antigen retention at injection site and increased antigen uptake by antigen-presenting cells and other immune cell types, including neutrophils and monocytes following intramuscular injection to mice using ovalbumin as a model antigen. In vitro studies demonstrate SLA in either formulation is preferentially taken up by macrophages. Although the encapsulated formulation was better able to induce antigen-specific CD8⁺ T cell activation by dendritic cells in vitro, both encapsulated and admixed formulations gave equivalently enhanced protection from tumor challenge when tested in vivo using a B16-OVA melanoma model. Despite some differences in the immunostimulatory profile relative to the SLA (Enc) formulation, SLA (Adm) induces strong in vivo immunogenicity and efficacy, while offering an ease of formulation.

Better Adjuvants for Better Vaccines: Progress in Adjuvant Delivery Systems, Modifications, and Adjuvant-Antigen Codelivery

Traditional aluminum adjuvants can trigger strong humoral immunity but weak cellular immunity, limiting their application in some vaccines. Currently, various immunomodulators and delivery carriers are used as adjuvants, and the mechanisms of action of some of these adjuvants are clear. However, customizing targets of adjuvant action (cellular or humoral immunity) and action intensity (enhancement or inhibition) according to different antigens selected is time-consuming. Here, we review the adjuvant effects of some delivery systems and immune stimulants. In addition, to improve the safety, effectiveness, and accessibility of adjuvants, new trends in adjuvant development and their modification strategies are discussed.

Biotechnology of extremely thermophilic archaea

Although the extremely thermophilic archaea (Topt ≥ 70°C) may be the most primitive extant forms of life, they have been studied to a limited extent relative to mesophilic microorganisms. Many of these organisms have unique biochemical and physiological characteristics with important biotechnological implications. These include methanogens that generate methane, fermentative anaerobes that produce hydrogen gas with high efficiency, and acidophiles that can mobilize base, precious and strategic metals from mineral ores. Extremely thermophilic archaea have also been a valuable source of thermoactive, thermostable biocatalysts, but their use as cellular systems has been limited because of the general lack of facile genetics tools. This situation has changed recently, however, thereby providing an important avenue for understanding their metabolic and physiological details and also opening up opportunities for metabolic engineering efforts. Along these lines, extremely thermophilic archaea have recently been engineered to produce a variety of alcohols and industrial chemicals, in some cases incorporating CO2 into the final product. There are barriers and challenges to these organisms reaching their full potential as industrial microorganisms but, if these can be overcome, a new dimension for biotechnology will be forthcoming that strategically exploits biology at high temperatures.

A membrane-spanning macrocyclic bolaamphiphile lipid mimic of archaeal lipids

The synthesis of a 72-membered macrocyclic tetraester bolaamphiphile is accomplished in six chemical steps from commercially available starting materials using copper-accelerated azide–alkyne coupling to close the macrocycle in high yield. Related diester amphiphiles and an acyclic tetraester bolaamphiphile were also prepared. The set of lipids bearing nitrophenyl phosphate head groups were incorporated into phospholipid vesicles but failed to undergo phosphate hydrolysis in basic conditions, undergoing efficient elimination in competition. The same lipid cores bearing phosphate-linked nitrobenzoxadiazole (NBD) head groups also incorporated into phospholipid vesicles and the NBD fluorescence was quenched with cobalt ions. The proportion of membrane-spanning bolaamphiphiles was determined from the ratio of cobalt quenching in the presence and in the absence of a detergent. The macrocyclic bolaamphiphile is incorporated into phospholipid vesicles such that 48 ± 4% of the NBD head groups are in the outer leaflet, consistent with a membrane-spanning orientation. The acyclic bolaamphiphile is incorporated with 75 ± 3% of the NBD head groups accessible to quencher in the absence of a detergent suggesting U-shaped incorporation in the outer leaflet of the bilayer membrane. In ring size and spanning ability, the macrocyclic bolaamphiphile mimics naturally occurring macrocyclic archaeal lipids.

Microvesicles: ubiquitous contributors to infection and immunity

MVs, which can be subgrouped into exosomes, SVs, and OMVs, are secreted by eukaryotic and prokaryotic cells. Many previously inexplicable phenomena can be explained by the existence of these vesicles, as they appear to be important in a wide range of biologic processes, such as intercellular communication and transfer of functional genetic information. In this review, we discuss the immunologic roles of MVs during sterile insult and infectious disease. MVs contribute to clotting initiation, cell recruitment, and neovascularization during wound healing. In the context of pathogen infection, both the host and the pathogen use MVs for communication and defense. MVs are exploited by various viruses to evade the host immune response and contribute to viral spread. Bacteria produce MVs that contain virulence factors that contribute to disease pathology and antibiotic resistance. This review summarizes the role of MVs in the pathology and resolution of disease.

Cytotoxicity and uptake of archaeosomes prepared from Aeropyrum pernix lipids

Archaeon Aeropyrum pernix K1 is an obligate aerobic hyperthermophilic organism with C25,25-archeol membrane lipids with head groups containing inositol. Interactions of archaeosomes, liposomes prepared from lipids of A. pernix, with mammalian cells in vitro were studied. In vitro cytotoxicity was tested on five different cell lines: rodent mouse melanoma cells (B16-F1) and Chinese hamster ovary (CHO) cells, and three human cell lines—epithelial colorectal adenocarcinoma cells (CACO-2), liver hepatocellular carcinoma cell line (Hep G2) and endothelial umbilical vein cell line (EA.hy926). Archaeosomes were nontoxic to human Hep G2, CACO-2 and mildly toxic to rodent CHO and B16-F1 cells but showed strong cytotoxic effect on EA.hy926 cells. Confocal microscopy revealed that archaeosomes are taken up by endocytosis. The uptake of archaeosomes and the release of loaded calcein are more prominent in EA.hy926 cells, which is in line with high toxicity toward these cells. The mechanisms of uptake, release and action in these cells as well as in vivo functioning have to be further studied for possible targeted drug delivery.

Synthetic archaeosome vaccines containing triglycosylarchaeols can provide additive and long-lasting immune responses that are enhanced by archaetidylserine

The relation between archaeal lipid structures and their activity as adjuvants may be defined and explored by synthesizing novel head groups covalently linked to archaeol (2,3-diphytanyl-sn-glycerol). Saturated archaeol, that is suitably stable as a precursor for chemical synthesis, was obtained in high yield from Halobacterium salinarum. Archaeosomes consisting of the various combinations of synthesized lipids, with antigen entrapped, were used to immunize mice and subsequently determine CD8⁺ and CD4⁺-T cell immune responses. Addition of 45 mol% of the glycolipids gentiotriosylarchaeol, mannotriosylarchaeol or maltotriosylarchaeol to an archaetidylglycerophosphate-O-methyl archaeosome, significantly enhanced the CD8⁺ T cell response to antigen, but diminished the antibody titres in peripheral blood. Archaeosomes consisting of all three triglycosyl archaeols combined with archaetidylglycerophosphate-O-methyl (15/15/15/55 mol%) resulted in approximately additive CD8⁺ T cell responses and also an antibody response not significantly different from the archaetidylglycerophosphate-O-methyl alone. Synthetic archaetidylserine played a role to further enhance the CD8⁺ T cell response where the optimum content was 20–30 mol%. Vaccines giving best protection against solid tumor growth corresponded to the archaeosome adjuvant composition that gave highest immune activity in immunized mice.

Immunogenic properties of archaeal species found in bioaerosols

The etiology of bioaerosol-related pulmonary diseases remains poorly understood. Recently, archaea emerged as prominent airborne components of agricultural environments, but the consequences of airway exposure to archaea remain unknown. Since subcomponents of archaea can be immunogenic, we used a murine model to study the pulmonary immune responses to two archaeal species found in agricultural facilities: Methanobrevibacter smithii (MBS) and Methanosphaera stadtmanae (MSS). Mice were administered intranasally with 6.25, 25 or 100 µg of MBS or MSS, once daily, 3 days a week, for 3 weeks. MSS induced more severe histopathological alterations than MBS with perivascular accumulation of granulocytes, pronounced thickening of the alveolar septa, alveolar macrophages accumulation and increased perivascular mononucleated cell accumulation. Analyses of bronchoalveolar lavage fluids revealed up to 3 times greater leukocyte accumulation with MSS compared to MBS. Instillation of 100 µg of MBS or MSS caused predominant accumulation of monocyte/macrophages (4.5×10(5) and 4.8×10(5) cells/ml respectively) followed by CD4(+) T cells (1.38×10(5) and 1.94×10(5) cells/ml respectively), B cells (0.73×10(5) and 1.28×10(5) cells/ml respectively), and CD8(+) T cells (0.20×10(5) and 0.31×10(5) cells/ml respectively) in the airways. Both archaeal species induced similar titers of antigen-specific IgGs in plasma. MSS but not MBS caused an accumulation of eosinophils and neutrophils in the lungs, which surprisingly, correlated inversely with the size of the inoculum. Stronger immunogenicity of MSS was confirmed by a 3 fold higher accumulation of myeloid dendritic cells in the airways, compared to MBS. Thus, the dose and species of archaea determine the magnitude and nature of the pulmonary immune response. This is the first report of an immunomodulatory role of archaeal species found in bioaerosols.

Extremophiles and their application to veterinary medicine

Extremophiles can be defined as organisms that can survive in extreme environments that cannot support mammalian life. They include microorganisms that can tolerate temperature extremes, extremes of pH, salinity, hydrostatic pressure and ionizing radiation, as well as low oxygen tension, desiccation and the presence of heavy metals. Psychrophilic organisms also include fish in polar waters and animals that withstand freezing. Rare examples of thermophilic pathogens exist, and the main category of extremophilic animal pathogens comprises psychrophilic and psychrotrophic microorganisms that cause fish diseases, e.g. Flavobacterium psychrophilum, Moritella viscosa, Aliivibrio wodanis and Aliivibrio salmonicida. The most widely known application of an extremophile product in veterinary medicine is DNA polymerase from thermophiles, which is a mainstay of PCR-based diagnostics for an extensive range of animal pathogens. DNA polymerases and other extremophile enzymes are also used in many molecular biology applications and animal genomics. Other extremophile products may find application in veterinary medicine in the future. These include enzymes in biosensors, compatible solutes in skin care products, drug excipients, treatments for respiratory disease, radioprotectants, peptide antibiotics, archaeal lipids for drug delivery and anti-cancer therapeutics.

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.

Archaeosomes made of Halorubrum tebenquichense total polar lipids: a new source of adjuvancy

Background

Archaeosomes (ARC), vesicles prepared from total polar lipids (TPL) extracted from selected genera and species from the Archaea domain, elicit both antibody and cell-mediated immunity to the entrapped antigen, as well as efficient cross priming of exogenous antigens, evoking a profound memory response. Screening for unexplored Archaea genus as new sources of adjuvancy, here we report the presence of two new Halorubrum tebenquichense strains isolated from grey crystals (GC) and black mood (BM) strata from a littoral Argentinean Patagonia salt flat. Cytotoxicity, intracellular transit and immune response induced by two subcutaneous (sc) administrations (days 0 and 21) with BSA entrapped in ARC made of TPL either form BM (ARC-BM) and from GC (ARC-GC) at 2% w/w (BSA/lipids), to C3H/HeN mice (25 μg BSA, 1.3 mg of archaeal lipids per mouse) and boosted on day 180 with 25 μg of bare BSA, were determined.

Results

DNA G+C content (59.5 and 61.7% mol BM and GC, respectively), 16S rDNA sequentiation, DNA-DNA hybridization, arbitrarily primed fingerprint assay and biochemical data confirmed that BM and GC isolates were two non-previously described strains of H. tebenquichense. Both multilamellar ARC mean size were 564 ± 22 nm, with -50 mV zeta-potential, and were not cytotoxic on Vero cells up to 1 mg/ml and up to 0.1 mg/ml of lipids on J-774 macrophages (XTT method). ARC inner aqueous content remained inside the phago-lysosomal system of J-774 cells beyond the first incubation hour at 37°C, as revealed by pyranine loaded in ARC. Upon subcutaneous immunization of C3H/HeN mice, BSA entrapped in ARC-BM or ARC-GC elicited a strong and sustained primary antibody response, as well as improved specific humoral immunity after boosting with the bare antigen. Both IgG1 and IgG2a enhanced antibody titers could be demonstrated in long-term (200 days) recall suggesting induction of a mixed Th1/Th2 response.

Conclusion

We herein report the finding of new H. tebenquichense non alkaliphilic strains in Argentinean Patagonia together with the adjuvant properties of ARC after sc administration in mice. Our results indicate that archaeosomes prepared with TPL from these two strains could be successfully used as vaccine 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.

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.

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.

Archaeosomes induce long-term CD8+ cytotoxic T cell response to entrapped soluble protein by the exogenous cytosolic pathway, in the absence of CD4+ T cell help

The unique ether glycerolipids of Archaea can be formulated into vesicles (archaeosomes) with strong adjuvant activity for MHC class II presentation. Herein, we assess the ability of archaeosomes to facilitate MHC class I presentation of entrapped protein Ag. Immunization of mice with OVA entrapped in archaeosomes resulted in a potent Ag-specific CD8(+) T cell response, as measured by IFN-gamma production and cytolytic activity toward the immunodominant CTL epitope OVA(257-264). In contrast, administration of OVA with aluminum hydroxide or entrapped in conventional ester-phospholipid liposomes failed to evoke significant CTL response. The archaeosome-mediated CD8(+) T cell response was primarily perforin dependent because CTL activity was undetectable in perforin-deficient mice. Interestingly, a long-term CTL response was generated with a low Ag dose even in CD4(+) T cell deficient mice, indicating that the archaeosomes could mediate a potent T helper cell-independent CD8(+) T cell response. Macrophages incubated in vitro with OVA archaeosomes strongly stimulated cytokine production by OVA-specific CD8(+) T cells, indicating that archaeosomes efficiently delivered entrapped protein for MHC class I presentation. This processing of Ag was Brefeldin A sensitive, suggesting that the peptides were transported through the endoplasmic reticulum and presented by the cytosolic MHC class I pathway. Finally, archaeosomes induced a potent memory CTL response to OVA even 154 days after immunization. This correlated to strong Ag-specific up-regulation of CD44 on splenic CD8(+) T cells. Thus, delivery of proteins in self-adjuvanting archaeosomes represents a novel strategy for targeting exogenous Ags to the MHC class I pathway for induction of CTL response.

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.

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.

A structural comparison of the total polar lipids from the human archaea Methanobrevibacter smithii and Methanosphaera stadtmanae and its relevance to the adjuvant activities of their liposomes

Mice were immunized with bovine serum albumin (BSA) entrapped within archaeosomes (i.e. liposomes) composed of the total polar lipids (TPL) from the two methanogenic archaea common to the human digestive tract. Methanobrevibacter smithii archaeosomes boosted serum anti-BSA antibody to titers comparable to those achieved with Freund's adjuvant, whereas Methanosphaera stadtmanae archaeosomes were relatively poor adjuvants. An explanation for this difference was sought by analysis of the polar lipid composition of each archaeobacterium. Fast atom bombardment mass spectrometry and NMR analyses of the purified lipids revealed a remarkable similarity in the ether lipid structures present in each TPL extract. However, the relative amounts of each lipid species varied dramatically. The phospholipid fraction in M. stadtmanae TPL was dominated by archaetidylinositol (50 mol% of TPL) and the glycolipid fraction by beta-Glcp-(1,6)-beta-Glcp-(1,1)-archaeol (36 mol%), whereas in M. smithii extracts, both caldarchaeol and archaeol lipids containing a phosphoserine head group were relatively abundant. Liposomes prepared from purified archaetidylinositol and from M. stadtmanae TPL supplemented with increasing amounts of phosphatidylserine elicited poor humoral responses to encapsulated BSA. A dramatic loss in the adjuvanticity of M. smithii archaeosomes was seen upon incorporation of 36 mol% of the uncharged lipid diglucosyl archaeol and, to a lesser extent, of 50 mol% of archaetidylinositol. Interestingly, the relative rates of uptake of M. smithii and M. stadtmanae archaeosomes by phagocytic cultures in vitro were similar. Thus, the lipid composition may influence archaeosome adjuvanticity, particularly a high diglucosyl archaeol and/or archaetidyl inositol content, resulting in a low adjuvant activity.

Synthetic Approaches to Novel Archaeal Tetraether Glycolipid Analogues

Symmetrical and unsymmetrical archaeal tetraether glycolipid analogues have been prepared. The syntheses are based upon the elaboration of lipid cores from versatile chiral starting materials followed by simultaneous or sequential introduction of polar headgroups. Three pathways (A-C) were elaborated for the synthesis of stereochemically defined lipids 14-16 characterized by a straight bridging spacer and two dihydrocitronellyl chains attached to glycerol units at the sn-3 and sn-2 positions, respectively. Pathway C appeared to be particularly advantageous for the synthesis of tetraether 9, which possesses a cyclopentane unit as found in thermoacidophilic lipids. Diglycosylated lipids 4-6 were produced in 49-53% yields by reaction of diols 14-16 with beta-D-galactofuranosyl donor 31, whereas unsymmetrical lipids possessing either two different carbohydrate units 7 or a saccharidic moiety and a phosphate group 8 were efficiently prepared from monoprotected diol 35. These compounds represent the first examples of tetraether-type analogues containing a phosphate unit and/or glycosyl moieties.

Coenzyme Q10 in vesicles composed of archaeal ether lipids or conventional lipids enhances the immuno-adjuvanticity to encapsulated protein

Cellular accumulation, tissue distribution, and immuno-adjuvanticity were evaluated for liposomal CoQ10 prepared from either distearoylphosphatidylcholine:dicetylphosphate:cholesterol (4:1:5, mol. ratio) (conventional liposomes) or from the total polar lipids of the archaeon Methanosarcina mazei (archaeosomes). Liposomal CoQ10 vesicles of approximately 100 nm diameter, containing up to 179 mumol of CoQ10 per mg of lipid have been evaluated using J774A.1 macrophages and Balb/c mice. Archaeosomes uptake by J774A.1 macrophages was better than with the conventional liposome, and the incorporation of CoQ10 enhanced the uptake of both lipid vesicle types. All vesicle types were detected in the liver and spleen of mice (4-27% of injected dose) within 3 h of intraperitoneal injection. Moreover, incorporation of CoQ10 into lipid vesicles enhanced the immuno-adjuvanticity of both conventional liposomes and archaeosomes, to achieve approximately a doubling in the titres of BSA-specific antibody in sera to 169 and 430 micrograms ml-1, respectively. Increases in IgG1 and IgG2a/2b accounted for most of the CoQ10-induced increases in anti-BSA titres. These results are rationalized on the basis of surface hydrophobicity and opsonization changes induced by the presence of CoQ10 in vesicles. We suggest that liposomal CoQ10 has potential as a new generation of vaccine delivery system to enhance the immune response. Its use as a novel delivery system may be particularly effective under pathological conditions where the occurrence of an oxidative stress condition significantly impairs the immune system functions.

Archaeosomes as novel antigen delivery systems

The humoral immune response mounted in BALB/c mice against bovine serum albumin or cholera toxin B subunit was compared when the antigens were associated with liposomes composed of either archaeal ether lipids or conventional lipids. Antibody titres in sera from mice immunised intraperitoneally were elevated to an extent comparable to those achieved with Freund's adjuvant by encapsulating bovine serum albumin in archaeal lipid vesicles (archaeosomes) of about 200 nm diameter. Comparison among six archaeosome and three conventional liposome compositions established that archaeosomes were generally much superior in potentiating an immune response. Further, only two immunisations, at the most, were needed to achieve close to the maximum antibody titre, as shown with archaeosomes composed of the polar lipids from Methanobrevibacter smithii, an inhabitant of the human colon. A similar positive response to presenting the more immunogenic cholera B subunit protein to the immune system of mice was shown for M. smithii archaeosomes. Encapsulation of the antigen in the archaeosome was necessary to achieve the full humoral response. This represents the first study to our knowledge where archaeosomes have been evaluated in vivo as possible antigen carriers.

Identification of beta-L-gulose as the sugar moiety of the main polar lipid Thermoplasma acidophilum

The main polar lipid (MPL) of Thermoplasma acidophilum has been purified and its structure determined. NMR, mass spectrometry, and capillary gas chromatography-mass spectrometry experiments have shown that the previously unidentified sugar moiety of MPL is the rare sugar L-gulose. MPL is thus a tetraether lipid with cyclopentane rings and head groups of phosphoglycerol, as previously reported, and beta-L-gulopyranose. Further, MPL is also the dominant lipid found in lipid extracts from another species of the Thermoplasma genus, T. volcanium, suggesting that L-gulose may represent a dominant sugar moiety of the polar lipids biosynthesized by this archaeobacterial genus. Minor phospholipids were tentatively identified as diether and hydroxydiether analogs of phosphatidylglycerol, and phosphatidylinositol.