Protection of mice from lethal endotoxemia by use of an ornithine-containing lipid or a serine-containing lipid

Ornithine lipid is a partial TLR4 agonist and NLRP3 activator

Gram-negative bacterial lipopolysaccharides (LPSs) trigger inflammatory reactions through Toll-like receptor 4 (TLR4) and prime myeloid cells for inflammasome activation. In phosphate-limited environments, bacteria reduce LPS and other phospholipid production and synthesize phosphorus-free alternatives such as amino-acid-containing lipids like the ornithine lipid (OL). This adaptive strategy conserves phosphate for other essential cellular processes and enhances bacterial survival in host environments. While OL is implicated in bacterial pathogenicity, the mechanism is unclear. Using primary murine macrophages and human mononuclear cells, we elucidate that OL activates TLR4 and induces potassium efflux-dependent nucleotide-binding domain and leucine-rich repeat-containing pyrin protein 3 (NLRP3) activation. OL upregulates the expression of NLRP3 and pro-interleukin (IL)-1β and induces cytokine secretion in primed and unprimed cells. By contrast, in the presence of LPS, OL functions as a partial TLR4 antagonist and reduces LPS-induced cytokine secretion. We thus suggest that in phosphate-depleted environments, OL replaces LPS bacterial immunogenicity, while constitutively present OL may allow bacteria to escape immune surveillance.

Genetic mapping of microbial and host traits reveals production of immunomodulatory lipids by Akkermansia muciniphila in the murine gut

The molecular bases of how host genetic variation impacts the gut microbiome remain largely unknown. Here we used a genetically diverse mouse population and applied systems genetics strategies to identify interactions between host and microbe phenotypes including microbial functions, using faecal metagenomics, small intestinal transcripts and caecal lipids that influence microbe-host dynamics. Quantitative trait locus (QTL) mapping identified murine genomic regions associated with variations in bacterial taxa; bacterial functions including motility, sporulation and lipopolysaccharide production and levels of bacterial- and host-derived lipids. We found overlapping QTL for the abundance of Akkermansia muciniphila and caecal levels of ornithine lipids. Follow-up in vitro and in vivo studies revealed that A. muciniphila is a major source of these lipids in the gut, provided evidence that ornithine lipids have immunomodulatory effects and identified intestinal transcripts co-regulated with these traits including Atf3, which encodes for a transcription factor that plays vital roles in modulating metabolism and immunity. Collectively, these results suggest that ornithine lipids are potentially important for A. muciniphila-host interactions and support the role of host genetics as a determinant of responses to gut microbes.

Lipid-protein interactions regulating the canonical and the non-canonical NLRP3 inflammasome

The inflammatory response is a complex regulated effector mechanism of the innate immune system that is initiated after tissue injury or infection. The NLRP3 inflammasome is an important initiator of inflammation by regulating the activation of caspase-1, the maturation of pro-inflammatory cytokines and the induction of pyroptotic cell death. Numerous studies demonstrate that the NLRP3 inflammasome could be modulated by lipids, existing a relation between lipids and the activation of different inflammatory processes. In this review we will summarize how the mechanism of NLRP3 inflammasome activation is regulated by different lipids and how these lipids control specific cellular localization of NLRP3 during activation. Although being a cytosolic protein, NLRP3 interacts with lipids accessible in neighbor membranes. Also, the modulation of NLRP3 by endogenous lipids has been found causative of different metabolic diseases and bacterial-pathogenic lipids lead to NLRP3 activation during infection. The understanding of the modulation of the NLRP3 inflammasome by lipids has resulted not only in a better knowledge about the mechanism of NLRP3 activation and its implication in disease, but also opens a new avenue for the development of novel therapeutics and vaccines, as NLRP3 could be modulated by synthetic lipids used as adjuvants.

Polar lipids of Burkholderia pseudomallei induce different host immune responses

Melioidosis is a disease in tropical and subtropical regions of the world that is caused by Burkholderia pseudomallei. In endemic regions the disease occurs primarily in humans and goats. In the present study, we used the goat as a model to dissect the polar lipids of B. pseudomallei to identify lipid molecules that could be used for adjuvants/vaccines or as diagnostic tools. We showed that the lipidome of B. pseudomallei and its fractions contain several polar lipids with the capacity to elicit different immune responses in goats, namely rhamnolipids and ornithine lipids which induced IFN-γ, whereas phospholipids and an undefined polar lipid induced strong IL-10 secretion in CD4⁺ T cells. Autologous T cells co-cultured with caprine dendritic cells (cDCs) and polar lipids of B. pseudomallei proliferated and up-regulated the expression of CD25 (IL-2 receptor) molecules. Furthermore, we demonstrated that polar lipids were able to up-regulate CD1w2 antigen expression in cDCs derived from peripheral blood monocytes. Interestingly, the same polar lipids had only little effect on the expression of MHC class II DR antigens in the same caprine dendritic cells. Finally, antibody blocking of the CD1w2 molecules on cDCs resulted in decreased expression for IFN-γ by CD4⁺ T cells. Altogether, these results showed that polar lipids of B. pseudomallei are recognized by the caprine immune system and that their recognition is primarily mediated by the CD1 antigen cluster.

Ornithine lipids and their structural modifications: from A to E and beyond

Ornithine lipids (OLs) are phosphorus-free membrane lipids that are widespread in eubacteria, but absent from archaea and eukaryotes. They contain a 3-hydroxy fatty acyl group attached in amide linkage to the α-amino group of the amino acid ornithine. A second fatty acyl group is ester-linked to the 3-hydroxy position of the first fatty acid. About 25% of the bacterial species whose genomes have been sequenced are predicted to have the capacity to form OLs. Distinct OL hydroxylations have been described in the ester-linked fatty acid, the amide-linked fatty acid, and the ornithine moiety. These modifications often seem to form part of a bacterial stress response to changing environmental conditions, allowing the bacteria to adjust membrane properties by simply modifying already existing membrane lipids without the need to synthesize new lipids.

Brucella abortus ornithine lipids are dispensable outer membrane components devoid of a marked pathogen-associated molecular pattern

The brucellae are α-Proteobacteria facultative intracellular parasites that cause an important zoonosis. These bacteria escape early detection by innate immunity, an ability associated to the absence of marked pathogen-associated molecular patterns in the cell envelope lipopolysaccharide, lipoproteins and flagellin. We show here that, in contrast to the outer membrane ornithine lipids (OL) of other Gram negative bacteria, Brucella abortus OL lack a marked pathogen-associated molecular pattern activity. We identified two OL genes (olsB and olsA) and by generating the corresponding mutants found that olsB deficient B. abortus did not synthesize OL or their lyso-OL precursors. Liposomes constructed with B. abortus OL did not trigger IL-6 or TNF-α release by macrophages whereas those constructed with Bordetella pertussis OL and the olsB mutant lipids as carriers were highly active. The OL deficiency in the olsB mutant did not promote proinflammatory responses or generated attenuation in mice. In addition, OL deficiency did not increase sensitivity to polymyxins, normal serum or complement consumption, or alter the permeability to antibiotics and dyes. Taken together, these observations indicate that OL have become dispensable in the extant brucellae and are consistent within the trend observed in α-Proteobacteria animal pathogens to reduce and eventually eliminate the envelope components susceptible of recognition by innate immunity.

Identification of a gene required for the formation of lyso-ornithine lipid, an intermediate in the biosynthesis of ornithine-containing lipids

Under phosphate-limiting conditions, some bacteria replace their membrane phospholipids by lipids not containing any phosphorus. One of these phosphorus-free lipids is an ornithine-containing lipid (OL) that is widespread among eubacteria. In earlier work, we had identified a gene (olsA) required for OL biosynthesis that probably encodes an O-acyltransferase using acyl-acyl carrier protein (acyl-AcpP) as an acyl donor and that converts lyso-ornithine lipid into OL. We now report on a second gene (olsB) required for OL biosynthesis that is needed for the incorporation of radiolabelled ornithine into OL. Overexpression of OlsB in an olsA-deficient mutant of Sinorhizobium (Rhizobium) meliloti leads to the transient accumulation of lyso-ornithine lipid, the biosynthetic intermediate of OL biosynthesis. Overexpression of OlsB in Escherichia coli is sufficient to cause the in vivo formation of lyso-ornithine lipid in this organism and is the cause for a 3-hydroxyacyl-AcpP-dependent acyltransferase activity forming lyso-ornithine lipid from ornithine. These results demonstrate that OlsB is required for the first step of OL biosynthesis, in which ornithine is N-acylated with a 3-hydroxy-fatty acyl residue in order to obtain lyso-ornithine lipid. OL formation in a wild-type S. meliloti is increased upon growth under phosphate-limiting conditions. Expression of OlsB from a broad host range vector leads to the constitutive formation of relatively high amounts of OL (12-14% of total membrane lipids) independently of whether strains are grown in the presence of low or high concentrations of phosphate, suggesting that in S. meliloti the formation of OlsB is usually limiting for the amount of OL formed in this organism. Open reading frames homologous to OlsA and OlsB were identified in many eubacteria and although in S. meliloti the olsB and olsA gene are 14 kb apart, in numerous other bacteria they form an operon.

Membrane lipids in plant-associated bacteria: their biosyntheses and possible functions

Membrane lipids in most bacteria generally consist of the glycerophospholipids phosphatidylglycerol, cardiolipin, and phosphatidylethanolamine (PE). A subset of bacteria also possesses the methylated derivatives of PE, monomethylphosphatidylethanolamine, dimethylphosphatidylethanolamine, and phosphatidylcholine (PC). In Sinorhizobium meliloti, which can form a nitrogen-fixing root nodule symbiosis with Medicago spp., PC can be formed by two entirely different biosynthetic pathways, either the PE methylation pathway or the recently discovered PC synthase pathway. In the latter pathway, one of the building blocks for PC formation, choline, is obtained from the eukaryotic host. Under phosphorus-limiting conditions of growth, S. meliloti replaces its membrane phospholipids by membrane-forming lipids that do not contain phosphorus; namely, the sulfolipid sulfoquinovosyl diacylglycerol, ornithine-derived lipids, and diacylglyceryl-N,N,N-trimethylhomoserine. Although none of these phosphorus-free lipids is essential for growth in culture media rich in phosphorus or for the symbiotic interaction with the legume host, they are expected to have major roles under free-living conditions in environments poor in accessible phosphorus. In contrast, sinorhizobial mutants deficient in PC show severe growth defects and are completely unable to form nodules on their host plants. Even bradyrhizobial mutants with reduced PC biosynthesis can form only root nodules displaying reduced rates of nitrogen fixation. Therefore, in the cases of these microsymbionts, the ability to form sufficient bacterial PC is crucial for a successful interplay with their host plants.

Identification of a gene required for the biosynthesis of ornithine-derived lipids

Phospholipids are the membrane-forming constituents in all living organisms. In addition to phosphorus-containing lipids, the membranes of numerous bacteria contain significant amounts of phosphorus-free polar lipids, often derived from amino acids. Although lipids derived from the amino acid ornithine are widespread among bacteria, their biosynthesis is unknown. Here, we describe the isolation of mutants of Sinorhizobium meliloti deficient in the biosynthesis of ornithine-derived lipids (OL). Complementation of such mutants with a sinorhi-zobial cosmid gene bank, subcloning of the complementing fragment and sequencing of the subclone led to the identification of a gene (olsA) coding for a presumptive acyltransferase. Amplification of this gene and its expression in OL-deficient mutant backgrounds of S. meliloti demonstrates that it is required for OL biosynthesis. An OL-deficient mutant of S. meliloti disrupted in olsA shows wild type-like growth behaviour and is capable of inducing nitrogen-fixing nodules on legume hosts. A lyso-ornithine lipid-dependent acyltransferase activity forming OL requires acyl-AcpP as the acyl donor and expression of the olsA gene.

Modulation of chemotaxis, O(2)(-) production and myeloperoxidase release from human polymorphonuclear leukocytes by the ornithine-containing lipid and the serineglycine-containing lipid of Flavobacterium

The ornithine-containing lipid (OL) and the serineglycine-containing lipid (SGL) of Flavobacterium activated and modulated the functions of human polymorphonuclear leukocytes (PMNs). The OL and the SGL strongly activated fMet-Leu-Phe- and interleukin-8-induced chemotaxis of PMNs at the concentration of 0.1 microg ml(-1), and a synthetic OL also activated the function of PMNs. Further, the OL strongly activated O(2)(-) production from PMNs. Although the OL and the SGL slightly modulated myeloperoxidase release from PMNs, inhibition effects of their component fatty acid analogues were observed. O(2)(-) production-inducing activity is a common biological activity between the OL and bacterial lipopolysaccharides, but OL and SGL, unlike lipopolysaccharide, are potent activators of PMN chemotaxis.

Ornithine-containing lipids stimulate CD14-dependent TNF-alpha production from murine macrophage-like J774.1 and RAW 264.7 cells

The ornithine-containing lipids (OL)-induced cytokine production pattern in macrophage-like J774.1 and RAW 264.7 cells was different from that in the peritoneal macrophages previously reported. OLs, as well as lipopolysaccharide (LPS) of Escherichia coli, strongly induced tumor necrosis factor (TNF) alpha but not interleukin (IL)-1beta in J774.1 cells. In the RAW cells, IL-1beta, TNF-alpha and prostaglandin E(2) were strongly induced by the OLs and LPS. OL- and serine-glycine-containing lipid (SGL)-induced TNF-alpha production in J774.1 and RAW 264.7 cells required serum. However, in CD14-deficient LR-9 cells, TNF-alpha was not induced by the OLs in the presence or absence of serum. OLs and a SGL almost completely inhibited the binding of (125)I-LPS to J774.1 cells. These results suggested that OLs and SGL activate macrophages via the CD14-dependent pathway.

A typical bacterial ornithine-containing lipid Nalpha-(D)-[3-(hexadecanoyloxy)hexadecanoyl]-ornithine is a strong stimulant for macrophages and a useful adjuvant

Nalpha-[3-(Hexadecanoyloxy)hexadecanoyl]-ornithine is a typical bacterial ornithine-containing lipid (OL). The configuration of the 3-hydroxy fatty acids in the OL was proved to be D by using HPLC with chiral column. For this analysis, Nalpha-(D or L)-[3-(hexadecanoyloxy)hexadecanoyl]-L-ornithine were synthesized and used as standards. The typical bacterial OL, as well as the synthesized one, exhibited strong interleukin-1- and prostaglandin E2-inducing activities, and further, it induced the production of high IgG anti-tetanus toxoid antibodies in mice. The typical OL is expected to be utilized as a nontoxic, potent adjuvant.

Adjuvanticity of an ornithine-containing lipid of Flavobacterium meningosepticum as a candidate vaccine adjuvant

Ornithine-containing lipids (OrnL) extracted from Flavobacterium meningosepticum have been reported to have various biological activities such as B-cell mitogenicity and macrophage activation to generate interleukin-1 and prostaglandin E2. We, using ovalbumin (OVA) as an antigen, evaluated the adjuvant activity of OrnL as an immunological adjuvant in BALB/c mice. OrnL showed the function of forming liposome-like vesicles retaining biological activities when prepared as either small unilamellar or dehydration-rehydration vesicles. Although OrnL was not shown to have enough entrapping efficacy for use as a vaccine adjuvant, phosphatidylglycerol (PG) and cholesterol (CHOL) added to stabilize the vesicle membrane increased the entrapping efficacy to the same extent as that of conventional liposomes. Furthermore, the stabilized OrnL vesicles tolerated centrifugation to remove non-entrapped antigens. Completely antigen-entrapped OrnL vesicles including Pg and CHOL induced a significantly greater enhancement of IgG antibody production than did aluminum hydroxide gel in BALB/c mice from week 6. These results indicate that OrnL can be utilized as an immunological adjuvant for vaccines.

Hypothermic response of mice to ornithine-containing lipids and to endotoxin

The hypothermic response of mice to ornithine-containing lipids (Orn-Ls) of the form alpha-N-(3-acyloxyacyl)-ornithine and to endotoxin (Escherichia coli 0111:B4 lipopolysaccharide [LPS]) was studied. After the administration of Orn-L or LPS to C3H/HeSlc mice, body temperature decreases were determined at 30-min intervals by inserting a thermistor into the rectum of each mouse. When Orn-L (750 microg) or LPS (70 microg) was injected into the mice, body temperature decreases of 0.8 and 2.0 degrees C, respectively, occurred 1.8 to 2.0 h later. These body temperature decreases were completely suppressed by the preadministration of indomethacin. When anti-tumor necrosis factor alpha (TNF-alpha) antibody was administered before the administration of Orn-L or LPS, only the body temperature decrease by LPS was suppressed. The body temperature decrease by Orn-L was suppressed by anti-interleukin-1beta (IL-1beta) antibody preadministration. Next, in order to study IL-1beta and TNF-alpha mRNA expression in macrophages, peritoneal macrophages were collected 40 min after the administration of Orn-L or LPS to mice. The expression of IL-1beta mRNA by stimulation with Orn-L was as strong as that by stimulation with LPS, but the expression of TNF-alpha mRNA by stimulation with Orn-L was very weak. Our previous studies of in vitro macrophage activation by Orn-L proved that strong induction of IL-1 and prostaglandin E2 generation by Orn-L occurred (Y. Kawai and K. Akagawa, Infect. Immun. 57:2086-2091, 1989). From these experiments, the weak body temperature decrease in mice caused by Orn-L was found to be mediated by cytokines different from those which mediate the strong body temperature decrease caused by LPS. Namely, it was caused by prostaglandin E2 being mediated by IL-1 but not by TNF-alpha.

Multistep regulation mechanisms for tolerance induction to lipopolysaccharide lethality in the tumor-necrosis-factor-alpha-mediated pathway. Application of non-toxic monosaccharide lipid A analogues for elucidation of mechanisms

Lipid A is the active principle of lipopolysaccharide (LPS). Synthetic lipid A analogues with monosaccharide backbones, GLA-60, GLA-69 and GLA-58, which exhibit potent, weak and scarce agonistic activities of LPS, respectively, induced tolerance against LPS lethality in galactosamine-(GalN)-sensitized mice while none of them were pyrogenic in rabbits. The tolerance-inducing mechanisms were investigated focusing on the regulation of tumor-necrosis-factor-alpha(TNF-alpha)-mediated lethal pathway of LPS. Induction of serum TNF-alpha in LPS-challenged mice was suppressed by prior administration of these analogues as well as LPS. Prior treatment of murine macrophages with the substances suppressed LPS-stimulated TNF-alpha production in the culture supernatant and TNF-alpha mRNA expression in the cells as well. Lethal toxicity of TNF-alpha in GalN-sensitized mice was effectively suppressed by prior treatment with LPS, GLA-60 and GLA-69 but not by GLA-58. This protective effect was suggested to be mediated by endogenous TNF-alpha, which was induced by prior treatment with the effective substances, because either neutralization of endogenously induced TNF-alpha activity with an antibody or deletion of its induction by using LPS-resistant C3H/HeJ mice reduced the protective effect, and a detectable amount of TNF-alpha was produced by stimulating macrophages with the effective substances but not with GLA-58. These results indicated that multiple regulation steps (one is prior to and the others are following TNF-alpha production) are participating in the tolerance induction by LPS and some lipid A analogues and that GLA-58 is a characteristic compound which induces the tolerance by only blocking the step prior to TNF-alpha production.