Strain-promoted alkyne-azide cycloadditions (SPAAC) reveal new features of glycoconjugate biosynthesis

We have shown that 4-dibenzocyclooctynol (DIBO), which can easily be obtained by a streamlined synthesis approach, reacts exceptionally fast in the absence of a Cu(I) catalyst with azido-containing compounds to give stable triazoles. Chemical modifications of DIBO, such as oxidation of the alcohol to a ketone, increased the rate of strain promoted azide-alkyne cycloadditions (SPAAC). Installment of a ketone or oxime in the cyclooctyne ring resulted in fluorescent active compounds whereas this property was absent in the corresponding cycloaddition adducts; this provides the first example of a metal-free alkyne-azide fluoro-switch click reaction. The alcohol or ketone functions of the cyclooctynes offer a chemical handle to install a variety of different tags, and thereby facilitate biological studies. It was found that DIBO modified with biotin combined with metabolic labeling with an azido-containing monosaccharide can determine relative quantities of sialic acid of living cells that have defects in glycosylation (Lec CHO cells). A combined use of metabolic labeling/SPAAC and lectin staining of cells that have defects in the conserved oligomeric Golgi (COG) complex revealed that such defects have a greater impact on O-glycan sialylation than galactosylation, whereas sialylation and galactosylation of N-glycans was similarly impacted. These results highlight the fact that the fidelity of Golgi trafficking is a critical parameter for the types of oligosaccharides being biosynthesized by a cell. Furthermore, by modulating the quantity of biosynthesized sugar nucleotide, cells might have a means to selectively alter specific glycan structures of glycoproteins.

Multifunctional surface modification of gold-stabilized nanoparticles by bioorthogonal reactions

Nanocarriers that combine multiple properties in an all-in-one system hold great promise for drug delivery. The absence of technology to assemble highly functionalized devices has, however, hindered progress in nanomedicine. To address this deficiency, we have chemically synthesized poly(ethylene oxide)-β-poly(ε-caprolactone) (PEO-b-PCL) block polymers modified at the apolar PCL terminus with thioctic acid and at the polar PEO terminus with an acylhydrazide, amine, or azide moiety. The resulting block polymers were employed to prepare nanoparticles that have a gold core, an apolar polyester layer for drug loading, a polar PEO corona to provide biocompatibility, and three different types of surface reactive groups for surface functionalization. The acylhydrazide, amine, or azide moieties of the resulting nanoparticles could be reacted with high efficiencies with modules having a ketone, isocyanate, or active ester and alkyne function, respectively. To demonstrate proof of principle of the potential of multisurface functionalization, we prepared nanoparticles that have various combinations of an oligo-arginine peptide to facilitate cellular uptake, a histidine-rich peptide to escape from lysosomes, and an Alexa Fluor 488 tag for imaging purposes. It has been shown that uptake and subcellular localization of the nanoparticles can be controlled by multisurface modification. It is to be expected that the modular synthetic methodology provides unique opportunities to establish optimal configurations of nanocarriers for disease-specific drug delivery.

Surface modification of polymeric micelles by strain-promoted alkyne-azide cycloadditions

Organomicelles modified by surface dibenzylcyclooctyne moieties can conveniently be functionalized by strain-promoted alkyne-azide cycloadditions. The ligation approach is highly efficient, does not require toxic reagents and is compatible with a wide variety of functional modules. Interactions of proteins with surface ligands of the micelles have been studied by AFM, which revealed that it leads to disassembly of the particles thereby providing a mechanism for triggered drug release.

Surface functionalization using catalyst-free azide-alkyne cycloaddition

The utility of catalyst-free azide-alkyne [3 + 2] cycloaddition for the immobilization of a variety of molecules onto a solid surface and microbeads was demonstrated. In this process, the surfaces are derivatized with aza-dibenzocyclooctyne (ADIBO) for the immobilization of azide-tagged substrates via a copper-free click reaction. Alternatively, ADIBO-conjugated molecules are anchored to the azide-derivatized surface. Both immobilization techniques work well in aqueous solutions and show excellent kinetics under ambient conditions. We report an efficient synthesis of aza-dibenzocyclooctyne (ADIBO), thus far the most reactive cyclooctyne in cycloaddition to azides. We also describe convenient methods for the conjugation of ADIBO with a variety of molecules directly or via a PEG linker.

Differential induction of innate immune responses by synthetic lipid a derivatives

Recent studies have indicated that lipopolysaccharides (LPS) isolated from particular bacterial strains can bias innate immune responses toward different signal transduction pathways thereby eliciting unique patterns of cytokines. Heterogeneity in the structure of lipid A (the active component of LPS) and possible contaminations with other inflammatory components have made it difficult to confirm these observations and dissect molecular motifs that may be responsible for modulatory properties. To address these issues, we have examined, for the first time, the ability of a range of well defined synthetic lipid As and isolated LPS and lipid A preparations to induce the production of a wide range of cytokines in three different mouse cell types. It was found that, for a given compound, the potencies of production of the various cytokines differed significantly. An additive model, in which a chemical change in the structure of a compound effects the potencies of all cytokines in the same manner, could describe the potencies of the cytokines for all compounds. Thus, no evidence was found that the structure of lipid A can modulate the pattern of cytokine production. In addition, the statistical analysis showed that the relative ordering of the potencies of the compounds was identical in the different cell types and that structural features such as the presence of a 3-deoxy-D-manno-octulosonic acid moiety, anomeric phosphate, lipid length, and acylation pattern were important for pro-inflammatory activity. Finally, it was found that transcriptional and post-transcription control mechanisms determine potencies and efficacies of cytokine production in cell-specific manners.

Glycopeptide-specific monoclonal antibodies suggest new roles for O-GlcNAc

Studies of post-translational modification by beta-N-acetyl-D-glucosamine (O-GlcNAc) are hampered by a lack of efficient tools such as O-GlcNAc-specific antibodies that can be used for detection, isolation and site localization. We have obtained a large panel of O-GlcNAc-specific IgG monoclonal antibodies having a broad spectrum of binding partners by combining three-component immunogen methodology with hybridoma technology. Immunoprecipitation followed by large-scale shotgun proteomics led to the identification of more than 200 mammalian O-GlcNAc-modified proteins, including a large number of new glycoproteins. A substantial number of the glycoproteins were enriched by only one of the antibodies. This observation, combined with the results of inhibition ELISAs, suggests that the antibodies, in addition to their O-GlcNAc dependence, also appear to have different but overlapping local peptide determinants. The monoclonal antibodies made it possible to delineate differentially modified proteins of liver in response to trauma-hemorrhage and resuscitation in a rat model.

Selective labeling of living cells by a photo-triggered click reaction

Phototriggering of the metal-free azide to acetylene cycloaddition reaction was achieved by masking the triple bond of dibenzocyclooctynes as cyclopropenone. Such masked cyclooctynes do not react with azides in the dark. Irradiation of cyclopropenones results in the efficient (Phi(355) = 0.33) and clean regeneration of the corresponding dibenzocyclooctynes, which then undergo facile catalyst-free cycloadditions with azides to give corresponding triazoles under ambient conditions. In situ light activation of a cyclopropenone linked to biotin made it possible to label living cells expressing glycoproteins containing N-azidoacetyl-sialic acid. The cyclopropenone-based phototriggered click chemistry offers exciting opportunities to label living organisms in a temporally and spatially controlled manner and may facilitate the preparation of microarrays.

Chemical synthesis and proinflammatory responses of monophosphoryl lipid A adjuvant candidates

Lipopolysaccharides (LPS), which are structural components of the outer surface membrane of Gram-negative bacteria, trigger innate immune responses through activation of Toll-like receptor 4 (TLR4). Such responses may be exploited for the development of adjuvants and in particular monophosphoryl lipid A (MPLA) obtained by controlled hydrolysis of LPS of Salmonella minnesota, exhibits low toxicity yet possesses beneficial immuno-stimulatory properties. We have developed an efficient synthetic approach for the preparation of a major component of MPLA (1), which has as a key feature the use of allyloxycarbonates (Alloc) as permanent protecting groups for the C-3 and C-4 hydroxyls of the proximal glucosamine unit. The latter protecting groups greatly facilitated deprotection of the fully assembled compound. Furthermore, the amino functions were protected as N-2,2,2-trichloroethoxycarbamates (Troc), which performed efficient neighboring group participation to give selectively 1,2-trans-glycosides and could easily be removed under mild conditions without affecting the permanent Alloc carbonates and anomeric dimethylthexylsilyl (TDS) ether. The synthetic methodology was also employed for the preparation of a monophosphoryl lipid A (2) derivative that has the anomeric center of the proximal sugar modified as a methyl glycoside. Compound 1 was not able to induce cytokine production in mouse macrophages whereas methyl glycoside 2 displayed activity, however it has a lower potency and efficacy than lipid A obtained by controlled hydrolysis S. minnesota. This indicates compound 2 is an attractive candidate for adjuvant development and that 1 is not the active substance of MPLA obtained by controlled hydrolysis of LPS.

Innate immune responses of primary murine macrophage-lineage cells and RAW 264.7 cells to ligands of Toll-like receptors 2, 3, and 4

Although studies have been performed to characterize responses of macrophages from individual anatomical sites (e.g., alveolar macrophages) or of murine-derived macrophage cell lines to microbial ligands, few studies compare these cell types in terms of phenotype and function. We directly compared the expression of cell surface markers and functional responses of primary cultures of three commonly used cells of monocyte-macrophage lineage (splenic macrophages, bone marrow-derived macrophages, and bone marrow-derived dendritic cells) with those of the murine-leukemic monocyte-macrophage cell line, RAW 264.7. We hypothesized that RAW 264.7 cells and primary bone marrow-derived macrophages would be similar in phenotype and would respond similarly to microbial ligands that bind to either Toll-like receptors 2, 3, and 4. Results indicate that RAW 264.7 cells most closely mimic bone marrow-derived macrophages in terms of cell surface receptors and response to microbial ligands that initiate cellular activation via Toll-like receptors 3 and 4. However, caution must be applied when extrapolating findings obtained with RAW 264.7 cells to those of other primary macrophage-lineage cells, primarily because phenotype and function of the former cells may change with continuous culture.

Increasing the antigenicity of synthetic tumor-associated carbohydrate antigens by targeting Toll-like receptors

SYNTHETIC CANCER VACCINES: A number of fully synthetic vaccine candidates have been designed, chemically synthesized, and immunologically evaluated to establish a strategy to overcome the poor immunogenicity of tumor-associated carbohydrates and glycopeptides and to determine the importance of Toll-like receptor (TLR) engagement for antigenic responses against these compounds.Epithelial cancer cells often overexpress mucins that are aberrantly glycosylated. Although it has been realized that these compounds offer exciting opportunities for the development of immunotherapy for cancer, their use is hampered by the low antigenicity of classical immunogens composed of a glycopeptide derived from a mucin conjugated to a foreign carrier protein. We have designed, chemically synthesized, and immunologically evaluated a number of fully synthetic vaccine candidates to establish a strategy to overcome the poor immunogenicity of tumor-associated carbohydrates and glycopeptides. The compounds were also designed to allow study of the importance of Toll-like receptor (TLR) engagement for these antigenic responses in detail. We have found that covalent attachment of a TLR2 agonist, a promiscuous peptide T-helper epitope, and a tumor-associated glycopeptide gives a compound (1) that elicits in mice exceptionally high titers of IgG antibodies that recognize MCF7 cancer cells expressing the tumor-associated carbohydrate. Immunizations with glycolipopeptide 2, which contains lipidated amino acids instead of a TLR2 ligand, gave significantly lower titers of IgG antibodies; this demonstrates that TLR engagement is critical for optimum antigenic responses. Although mixtures of compound 2 with Pam(3)CysSK(4) (3) or monophosphoryl lipid A (4) elicited titers of IgG antibodies similar to those seen with 1, the resulting antisera had impaired ability to recognize cancer cells. It was also found that covalent linkage of the helper T-epitope to the B-epitope is essential, probably because internalization of the helper T-epitope by B-cells requires assistance of the B-epitope. The results presented here show that synthetic vaccine development is amenable to structure-activity relationship studies for successful optimization of carbohydrate-based cancer vaccines.

Binding and Cellular Activation Studies Reveal That Toll-like Receptor 2 Can Differentially Recognize Peptidoglycan from Gram-positive and Gram-negative Bacteria

Although much progress has been made toward the identification of innate immune receptors, far less is known about how these receptors recognize specific microbial products. Such studies have been hampered by the need to purify compounds from microbial sources and a reliance on biological assays rather than direct binding to monitor recognition. We have employed surface plasmon resonance (SPR) binding studies using a wide range of well defined synthetic muropeptides derived from Gram-positive (lysine-containing) and Gram-negative (diaminopimelic acid (DAP)-containing) bacteria to demonstrate that Toll-like receptor 2 can recognize peptidoglycan (PGN). In the case of lysine-containing muropeptides, a limited number of compounds, which were derived from PGN remodeled by bacterial autolysins, was recognized. However, a wider range of DAP-containing muropeptides was bound with high affinity, and these compounds were derived from nascent and remodeled PGN. The difference in recognition of the two classes of muropeptides is proposed to be a strategy by the host to respond appropriately to Gram-negative and -positive bacteria, which produce vastly different quantities of PGN. It was also found that certain modifications of the carboxylic acids of isoglutamine and DAP can dramatically reduce binding, and thus, bacterial strains may employ such modifications to evade innate immune detection. Cellular activation studies employing highly purified PGN from Bacillus licheniformis, Bacillus subtilis, Escherichia coli, Lactobacillus plantarum, Micrococcus luteus, and Staphylococcus aureus support the structure binding relationship. The data firmly establish Toll-like receptor 2 as an innate immune sensor for PGN and provides an understanding of host-pathogen interactions at the molecular level.

Synthetic tetra-acylated derivatives of lipid A from Porphyromonas gingivalis are antagonists of human TLR4

Tetra-acylated lipid As derived from Porphyromonas gingivalis LPS have been synthesized using a key disaccharide intermediate functionalized with levulinate (Lev), allyloxycarbonate (Alloc) and anomeric dimethylthexylsilyl (TDS) as orthogonal protecting groups and 9-fluorenylmethoxycarbamate (Fmoc) and azido as amino protecting groups. Furthermore, an efficient cross-metathesis has been employed for the preparation of the unusual branched R-(3)-hydroxy-13-methyltetradecanic acid and (R)-3-hexadecanoyloxy-15-methylhexadecanoic acid of P. gingivalis lipid A. Biological studies have shown that the synthetic lipid As cannot activate human and mouse TLR2 and TLR4 to produce cytokines. However, it has been found that the compounds are potent antagonist of cytokine secretion by human monocytic cells induced by enteric LPS.

Chemical synthesis and immunological properties of oligosaccharides derived from the vegetative cell wall of Bacillus anthracis

Bacillus anthracis vaccine candidate : Sera of rabbits exposed to live and irradiated-killed spores of B. anthracis Sterne 34F2 or immunized with B. anthracis polysaccharide conjugated to KLH elicited antibodies that recognize isolated polysaccharide and two synthetic trisaccharides providing a proof-of-concept step in the development of vegetative and spore-specific reagents for detection and targeting of non-protein structures of B. anthracis .

Innate immune responses of synthetic lipid A derivatives of Neisseria meningitidis

Differences in the pattern and chemical nature of fatty acids of lipid A of Neisseria meningitides lipooligosaccharides (LOS) and Escherichia coli lipopolysaccharides (LPS) may account for differences in inflammatory properties. Furthermore, there are indications that dimeric 3-deoxy-D-manno-oct-2-ulosonic acid (KDO) moieties of LOS and LPS enhance biological activities. Heterogeneity in the structure of lipid A and possible contaminations with other inflammatory components have made it difficult to confirm these observations. To address these problems, a highly convergent approach for the synthesis of a lipid A derivative containing KDO has been developed, which relies on the ability to selectively remove or unmask in a sequential manner an isopropylidene acetal, 9-fluorenylmethoxycarbonyl (Fmoc), allyloxycarbonate (Alloc), azide, and thexyldimethylsilyl (TDS) ether. The strategy was employed for the synthesis of N. meningitidis lipid A containing KDO (3). Mouse macrophages were exposed to the synthetic compound and its parent LOS, E. coli lipid A (2), and a hybrid derivative (4) that has the asymmetrical acylation pattern of E. coli lipid A, but the shorter lipids of meningococcal lipid A. The resulting supernatants were examined for tumor necrosis factor alpha (TNF-alpha) and interferon beta (IFN-beta) production. The lipid A derivative containing KDO was much more active than lipid A alone and just slightly less active than its parent LOS, indicating that one KDO moiety is sufficient for full activity of TNF-alpha and IFN-beta induction. The lipid A of N. meningitidis was a significantly more potent inducer of TNF-alpha and IFN-beta than E. coli lipid A, which is due to a number of shorter fatty acids. The compounds did not demonstrate a bias towards a MyD88- or TRIF-dependent response.

Robust immune responses elicited by a fully synthetic three-component vaccine

The overexpression of saccharides such as Globo-H, Lewis(Y) and Tn antigen is a common feature of oncogenic transformed cells. Endeavors to exploit this aberrant glycosylation for cancer vaccine development have been complicated by difficulties in eliciting high titers of IgG antibodies against classical conjugates of tumor-associated carbohydrates to carrier proteins. We have designed, chemically synthesized and immunologically evaluated a number of fully synthetic vaccine candidates to establish strategies to overcome the poor immunogenicity of tumor-associated carbohydrates and glycopeptides. We have found that a three-component vaccine composed of a TLR2 agonist, a promiscuous peptide T-helper epitope and a tumor-associated glycopeptide can elicit in mice exceptionally high titers of IgG antibodies that can recognize cancer cells expressing the tumor-associated carbohydrate. The superior properties of the vaccine candidate are attributed to the local production of cytokines, upregulation of co-stimulatory proteins, enhanced uptake by macrophages and dendritic cells and avoidance of epitope suppression.

The influence of the long chain fatty acid on the antagonistic activities of Rhizobium sin-1 lipid A

The lipid A from nitrogen-fixing bacterial species Rhizobium sin-1 is structurally unusual due to lack of phosphates and the presence of a 2-aminogluconolactone and a very long chain fatty acid, 27-hydroxyoctacosanoic acid (27OHC28:0), moiety. This structurally unusual lipid A can antagonize TNF-alpha production by human monocytes induced by Escherichia coli LPS. To establish the relevance of the unusual long chain 27-hydroxyoctacosanoic acid for antagonistic properties, a highly convergent strategy for the synthesis of several derivatives of the lipid A of R. sin-1 has been developed. Compound 1 is a natural R. sin-1 lipid A having a 27-hydroxyoctacosanoic acid at C-2', compound 2 contains an octacosanoic acid moiety at this position, and compound 3 is modified by a short chain tetradecanoic acid. Cellular activation studies with a human monocytic cell line have shown that the octacosanoic acid is important for optimal antagonistic properties. The hydroxyl of the natural 27-hydroxyoctacosanoic moiety does, however, not account for inhibitory activity. The resulting structure-activity relationships are important for the design of compounds for the treatment of septic shock.

Agonistic and antagonistic properties of a Rhizobium sin-1 lipid A modified by an ether-linked lipid

LPS from Rhizobium sin-1 (R. sin-1) can antagonize the production of tumor necrosis factor alpha (TNF-alpha) by E. coli LPS in human monocytic cells. Therefore these compounds provide interesting leads for the development of therapeutics for the prevention or treatment of septic shock. Detailed structure activity relationship studies have, however, been hampered by the propensity of these compounds to undergo beta-elimination to give biological inactive enone derivatives. To address this problem, we have chemically synthesized in a convergent manner a R. sin-1 lipid A derivative in which the beta-hydroxy ester at C-3 of the proximal sugar unit has been replaced by an ether linked moiety. As expected, this derivative exhibited a much-improved chemical stability. Furthermore, its ability to antagonize TNF-alpha production induced by enteric LPS was only slightly smaller than that of the parent ester modified derivative demonstrating that the ether-linked lipids affect biological activities only marginally. Furthermore, it has been shown for the first time that R. sin-1 LPS and the ether modified lipid A are also able to antagonize the production of the cytokine interferon-inducible protein 10, which arises from the TRIF-dependent pathway. The latter pathway was somewhat more potently inhibited than the MyD88-dependent pathway. Furthermore, it was observed that the natural LPS possesses much greater activity than the synthetic and isolated lipid As, which indicates that di-KDO moiety is important for optimal biological activity. It has also been found that isolated R. sin-1 LPS and lipid A agonize a mouse macrophage cell line to induce the production of TNF-alpha and interferon beta in a Toll-like receptor 4-dependent manner demonstrating species specific properties.

Modulation of innate immune responses with synthetic lipid A derivatives

The lipid A moiety of lipopolysaccharides (LPS) initiates innate immune responses by interacting with Toll-like receptor 4 (TLR4), which results in the production of a wide range of cytokines. Derivatives of lipid A show potential for use as immuno-modulators for the treatment of a wide range of diseases and as adjuvants for vaccinations. Development to these ends requires a detailed knowledge of patterns of cytokines induced by a wide range of derivatives. This information is difficult to obtain by using isolated compounds due to structural heterogeneity and possible contaminations with other inflammatory components. To address this problem, we have developed a synthetic approach that provides easy access to a wide range of lipid A's by employing a common disaccharide building block functionalized with a versatile set of protecting groups. The strategy was employed for the preparation of lipid A's derived from E. coli and S. typhimurium. Mouse macrophages were exposed to the synthetic compounds and E. coli 055:B5 LPS, and the resulting supernatants were examined for tumor necrosis factor alpha (TNF-alpha), interferon beta (IFN-beta), interleukin 6 (IL-6), interferon-inducible protein 10 (IP-10), RANTES, and IL-1beta. It was found that for each compound, the potencies (EC50 values) for the various cytokines differed by as much as 100-fold. These differences did not follow a bias toward a MyD88- or TRIF-dependent response. Instead, it was established that the observed differences in potencies of secreted TNF-alpha and IL-1beta were due to differences in the processing of respective pro-proteins. Examination of the efficacies (maximum responses) of the various cytokines showed that each synthetic compound and E. coli 055:B5 LPS induced similar efficacies for the production of IFN-beta and IP-10. However, lipid A's 1-4 gave lower efficacies for the production of RANTES and IL-6 as compared to LPS. Collectively, the presented results demonstrate that cytokine secretion induced by LPS and lipid A is complex, which can be exploited for the development of immuno-modulating therapies.

Modification of the structure of peptidoglycan is a strategy to avoid detection by nucleotide-binding oligomerization domain protein 1

Nucleotide-binding oligomerization domain (NOD) protein 1 (NOD1) and NOD2 are pathogen recognition receptors that sense breakdown products of peptidoglycan (PGN) (muropeptides). It is shown that a number of these muropeptides can induce tumor necrosis factor alpha (TNF-alpha) gene expression without significant TNF-alpha translation. This translation block is lifted when the muropeptides are coincubated with lipopolysaccharide (LPS), thereby accounting for an apparently synergistic effect of the muropeptides with LPS on TNF-alpha protein production. The compounds that induced synergistic effects were also able to activate NF-kappaB in a NOD1- or NOD2-dependent manner, implicating these proteins in synergistic TNF-alpha secretion. It was found that a diaminopimelic acid (DAP)-containing muramyl tetrapeptide could activate NF-kappaB in a NOD1-dependent manner, demonstrating that an exposed DAP is not essential for NOD1 sensing. The activity was lost when the alpha-carboxylic acid of iso-glutamic acid was modified as an amide. However, agonists of NOD2, such as muramyl dipeptide and lysine-containing muramyl tripeptides, were not affected by amidation of the alpha-carboxylic acid of iso-glutamic acid. Many pathogens modify the alpha-carboxylic acid of iso-glutamic acid of PGN, and thus it appears this is a strategy to avoid recognition by the host innate immune system. This type of immune evasion is in particular relevant for NOD1.

Synthesis and biological evaluation of a lipid A derivative that contains an aminogluconate moiety

A highly convergent strategy for the synthesis of several derivatives of the lipid A of Rhizobium sin-1 has been developed. The synthetic derivatives are 2-aminogluconate 3 and 2-aminogluconolactone 4, both of which lack C-3 acylation. These derivatives were obtained by the preparation of disaccharides in which the two amino groups and the C-3' hydroxy group could be modified individually with acyl or beta-hydroxy fatty acyl groups. Detailed NMR spectroscopy and MS analysis of 3 and 4 revealed that, even under neutral conditions, the two compounds equilibrate. The synthetic compounds lack the proinflammatory effects of Escherichia coli lipopolysaccharide (LPS), as indicated by an absence of tumor necrosis factor production. Although 3 and 4 were able to antagonize E. coli LPS, they were significantly less potent than the synthetic compound 2, which is acylated at C-3, and R. sin-1 LPS; these results indicate that the beta-hydroxy fatty acyl group at C-3 contributes to the antagonistic properties of R. sin-1 LPS. Based on a comparison of the biological responses of the synthetic lipid A derivatives with those of the R. sin-1 LPS and lipid A, the 3-deoxy-D-manno-octulosonic moieties appear to be important for the optimal antagonization of enteric LPS-induced cytokine production.

The 2-aminogluconate isomer of rhizobium sin-1 lipid A can antagonize TNF-alpha production induced by enteric LPS

The naturally occurring lipopolysaccharide (LPS) from Rhizobium sin-1, a nitrogen-fixing bacterial species, can prevent the induction of the tumor necrosis factor TNF-alpha induced by enteric LPS. The proximal saccharide moiety of R. sin-1 lipid A can exist in two forms, namely as a 2-aminogluconolactone or 2-aminogluconate. As it is unknown which of these forms is responsible for the antagonistic properties of R. sin-1 lipid A, compound 4 was prepared, and its inflammatory properties were studied. This compound contains a methyl ether at the C-5 hydroxyl, which prevents lactonization and therefore is ideally suited to determine whether the 2-aminogluconate possesses antagonistic properties. Compound 4 was synthesized by a highly convergent approach with a key disaccharide building block functionalized with a set of orthogonal protecting groups. The novel synthetic compound lacks proinflammatory properties, as indicated by an absence of TNF-alpha protein production. This compound was, however, able to antagonize the production of TNF-alpha induced by enteric LPS; this indicates that the 2-aminogluconate form of R. sin-1 lipid A is responsible for its biological properties.

Synthesis and proinflammatory properties of muramyl tripeptides containing lysine and diaminopimelic acid moieties

The unusual amino acid diaminopimelic acid (DAP) was prepared by cross metathesis of appropriately protected vinyl glycine and allyl glycine derivatives. Catalytic hydrogenation of the cross-coupling product resulted in reduction of the double bond and the removal of protecting groups. The resulting compounds were appropriately protected for the polymer-supported and solution-phase synthesis of muramyl tripeptides 2 and 3, which differ in the amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties. Muramyl dipeptide (1, MDP), the DAP-containing muramyl tripeptide 3, and the lysine-containing muramyl tripeptides 4 and 5 induced TNF-alpha gene expression without TNF-alpha protein production in a human monocytic cell line. The observed block in translation could be removed by co-incubation with LPS, resulting in an apparent synergistic effect. Compound 2 did not induce TNF-alpha gene expression, neither did it exhibit a synergistic effect with LPS; this indicates that amidation of the alpha-carboxylic acids of the isoglutamine and DAP moieties results in a loss of biological activity. It is proposed that amidation of alpha-carboxylic acids is a strategy that may be used by pathogens to avoid detection by the innate immune system. Furthermore, the pattern recognition receptors Nod1 and Nod2 have been implicated in the possible induction of a synergistic effect of muropeptides with LPS.