Archaebacteria thrive in environments characterized by anaeobiosis, saturated salt, and both high and low extremes of temperature and pH. The bulk of their membrane lipids are polar, characterized by the archaeal structural features typified by ether linkage of the glycerol backbone to isoprenoid chains of constant length, often fully saturated, and with sn-2,3 stereochemistry opposite that of glycerolipids of Bacteria and Eukarya. Also unique to these bacteria are macrocyclic archaeol and membrane spanning caldarchaeol lipids that are found in some extreme thermophiles and methanogens. To define the barrier function of archaebacterial membranes and to examine the effects of these unique structural features on permeabilities, we investigated the water, solute (urea and glycerol), proton, and ammonia permeability of liposomes formed by these lipids. Both the macrocyclic archaeol and caldarchaeol lipids reduced the water, ammonia, urea, and glycerol permeability of liposomes significantly (6 -120-fold) compared with diphytanylphosphatidylcholine liposomes. The presence of the ether bond and phytanyl chains did not significantly affect these permeabilities. However, the apparent proton permeability was reduced 3-fold by the presence of an ether bond. The presence of macrocyclic archaeol and caldarchaeol structures further reduced apparent proton permeabilities by 10 -17-fold. These results indicate that the limiting mobility of the midplane hydrocarbon region of the membranes formed by macrocyclic archaeol and caldarchaeol lipids play a significant role in reducing the permeability properties of the lipid membrane. In addition, it appears that substituting ether for ester bonds presents an additional barrier to proton flux.Many archaebacteria thrive in hostile environments, such as hot springs, salt lakes, and acidic or alkaline domains (1, 2). For instance, Methanococcus jannaschii grows optimally at 85°C, pH 6 (3, 4), Thermoplasma acidophilum at 55°C, pH 2.0 (5), and Halobacterium salinarum in near-saturated salt brines (2). The major role of the cell membrane is to provide a selective barrier between the external environment and the inside of the cell. Given the extreme environmental conditions in which these bacteria thrive, it is not surprising that their plasma membranes are composed of lipids that differ markedly in structure and physicochemical properties from the glycerolipids of eubacterial, animal, and plant cell membranes. In these unique bacteria, the membrane lipids are characterized by the presence of ether linkages instead of ester linkages, and they contain regularly branched phytanyl and biphytanyl chains instead of fatty acyl chains (6). The presence of ether rather than ester bonds is thought to contribute to greater chemical stability at extreme pH. Moreover the glycerol ethers contain an sn-2,3 stereochemistry that is opposite that of the naturally occurring sn-1,2 stereochemistry of glycerophospholipids of the other domains (7). The basic lipid core structures of these unique organisms are su...

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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 (ϳ200-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 ϳ300 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.

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