Comparative susceptibility of a peptidoglycan monomer from Brevibacterium divaricatum and its anhydromuramyl analogue to hydrolysis with N-acetylmuramyl-L-alanine amidase. Isolation and characterization of anhydromuramyl-peptidoglycan monomer

Comparative study of structurally related peptidoglycan monomer and muramyl dipeptide on humoral IgG immune response to ovalbumin in mouse

Structurally related peptidoglycan monomer (PGM) and muramyl dipeptide (MDP) differ in several aspects of biological activity but have in common immunostimulating properties. Comparative study of the effects of these adjuvants on humoral IgG immune response specific for protein antigen ovalbumin (OVA) was carried out in two inbred mouse strains, CBA and NIH/OlaHsd, and their ability to modulate the bias of immune response towards Th1/Th2 was evaluated. MDP had better adjuvant activity at some points than PGM, whereas both adjuvants stimulated Th2-biased immune response specific for OVA. In comparison to Complete Freund's adjuvant (CFA), as a golden standard of adjuvant action, both PGM and MDP exhibited considerably lower activity. Addition of PGM to Incomplete Freund's adjuvant (IFA) on humoral immune response was studied also, and the effect of such adjuvant formulation was compared to the effect of CFA. While CFA induced the switch towards Th1-biased immune response, the addition of PGM into IFA did have no impact on modulating the immune response towards more pronounced Th2-type of immune response, defined by IFA itself.

Effect of Liposomal Formulations and Immunostimulating Peptidoglycan Monomer (PGM) on the Immune Reaction to Ovalbumin in Mice

The adjuvant activity of liposomes and immunostimulating peptidoglycan monomer (PGM) in different formulations has been studied in mice model using ovalbumin (OVA) as an antigen. PGM is a natural compound of bacterial origin with well-defined chemical structure: GlcNAc-MurNAc-L-Ala-D-isoGln-mesoDpm(epsilonNH2)-D-Ala-D-Ala. It is a non-toxic, non-pyrogenic, and water-soluble immunostimulator. The aim of this study was to investigate the influence of different liposomal formulations of OVA, with or without PGM, on the production of total IgG, as well as of IgG1 and IgG2a subclasses of OVA-specific antibodies (as indicators of Th2 and Th1 type of immune response, respectively). CBA mice were immunized s.c. with OVA mixed with liposomes, OVA with PGM mixed with liposomes, OVA encapsulated into liposomes and OVA with PGM encapsulated into liposomes. Control groups were OVA in saline, OVA with PGM in saline, and OVA in CFA/IFA adjuvant formulation. The entrapment efficacy of OVA was monitored by HPLC method. The adjuvant activity of the mixture of OVA and empty liposomes, the mixture of OVA, PGM, and liposomes and PGM encapsulated with OVA into liposomes on production of total anti-OVA IgG was demonstrated. The mixture of PGM and liposomes exhibited additive immunostimulating effect on the production of antigen-specific IgGs. The analysis of IgG subclasses revealed that encapsulation of OVA into liposomes favors the stimulation of IgG2a antibodies, indicating the switch toward the Th1 type of immune response. When encapsulated into liposomes or mixed with liposomes, PGM induced a switch from Th1 to Th2 type of immune response. It could be concluded that appropriate formulations of antigen, PGM, and liposomes differently affect the humoral immune response and direct the switch in the type of immune response (Th1/Th2).

Effectiveness of novel PGM-containing incomplete Seppic adjuvants in rabbits

Peptidoglycan monomer (PGM) is adjuvant active molecule in experimental mice, although its adjuvanticity is much lower in comparison to potent adjuvants. The novel adjuvant formulations were developed by incorporation of PGM into Montanide ISA 206 and Montanide ISA 720 adjuvants, with the aim to enhance its adjuvanticity by protecting it from the fast degradation and metabolic clearance. Adjuvanticity of the novel adjuvant formulations was tested in rabbits for induction of protein-specific antibodies. Both novel adjuvants ISA206(PGM) and ISA720(PGM) were significantly stronger than Montanide adjuvants themselves, and also significantly more potent than Complete Freund Adjuvant. Montanide ISA 720 was shown as much better carrier of PGM, since the novel ISA720(PGM) adjuvant was significantly stronger adjuvant than the ISA206(PGM).

Purification and Characterization ofL,(L/D)‐Aminopeptidase from Guinea Pig Serum

Mammalian sera contain enzymes that catalyze the hydrolytic degradation of peptidoglycans and molecules of related structure and are relevant for the metabolism of peptidoglycans. We now report on a novel L,(L/D)-aminopeptidase found in human and mammalian sera. The enzyme hydrolyses the pentapeptide L-Ala-D-iso-Gln-meso-DAP(omegaNH(2))-D-Ala-D-Ala yielding the free L-alanine and the respective tetrapeptide (K(M) 18 mM). L,(L/D)-aminopeptidase from guinea pig serum was highly purified in four chromatographic steps, up to 700-fold. Molecular weight of the enzyme was estimated by HPLC to be approximately 175,000. The configuration of alanine obtained by hydrolysis of the pentapeptide was determined by oxidation with L-amino acid oxidase. The amino acids sequence in the respective tetrapeptide was deduced from the results of mass spectrometry. The novel L,(L/D)-aminopeptidase also hydrolyzed alanine-4-nitroanilide (K(M)=0.6 mM) and several peptides comprising L-amino acids. Peptides containing D-amino acid at the amino end and L-Asp-L-Asp were not the substrates for this enzyme. The purified enzyme also exhibited enkephalin degrading activity, hydrolyzing enkephalins comprising L,L- and L,D-peptide bonds. The enzyme was inhibited strongly by metal chelating agents, bestatin and amastatin.

Immunogenicity of peptides of measles virus origin and influence of adjuvants

Epitope-based peptide antigens have been under development for protection against measles virus. The immunogenicity of five peptides composed of the same B cell epitope (BCE) (H236-250 of the measles virus hemagglutinin), and different T cell epitopes of measles virus fusion protein (F421-435, F256-270, F288-302) and nucleoprotein (NP335-345) was studied in mice (subcutaneous immunisation). The adjuvant effects of peptidoglycan monomer (PGM), Montanide ISA 720 and 206 were also investigated. Results showed basic differences in peptide immunogenicity that were consistent with already described structural differences. PGM elevated peptide-specific IgG when applied together with four of five tested peptides. A strong synergistic effect was observed after co-immunisation of mice with a mixture containing all five chimeric peptides in small and equal amounts. Results revealed for the first time that immunisation with several peptides having the common BCE generated significantly higher levels of both anti-peptide and anti-BCE IgG in comparison to those obtained after immunisation with a single peptide in much higher quantity. Further improvement of immune response was obtained after incorporation of such a peptide mixture into oil-based adjuvants.

Entrapment of peptidoglycans and adamantyltripeptides into liposomes: an HPLC assay for determination of encapsulation efficiency

The encapsulation of different immunomodulating peptides, the peptidoglycan monomer, its semisynthetic derivatives (Adamant-1-yl)-acetyl-peptidoglycan monomer and Boc-Tyr-peptidoglycan monomer, respectively, and of two diastereoisomers of adamantyltripeptides into the large negatively charged multilamellar liposomes was investigated. The reproducible quantitative method using HPLC was established for the determination of the entrapped compounds. It was shown that the tested compounds could be efficiently incorporated into liposomes using either the film or modified film method. The results confirmed that the peptidoglycans with lipophilic substituents and particularly the adamantyltripeptides were incorporated into liposomes with higher efficiency than the peptidoglycan monomer using either of the described methods. Liposome preparations were stable at 4 degrees C up to seven days as shown by minimal leaking of the entrapped material.

Modified glycopeptides related to cell wall peptidoglycan: conformational studies by NMR and molecular modelling

Polymeric peptidoglycans of bacterial cell walls, and smaller glycopeptides derived from them, exhibit versatile biological activities including immunomodulating properties. Peptidoglycan monomer (PGM) was isolated from Brevibacterium divaricatum and novel lipophilic derivatives of PGM bearing either (adamantyl-1-yl)-acetyl or Boc-Tyr substituents (Ad-PGM and BocTyr-PGM respectively) have recently been synthesized. We have obtained full assignments of the 1H and 13C spectra, using 2D NMR techniques, for all three compounds in DMSO solutions. NOESY/ROESY experiments have provided interproton distance restraints that were used in distance geometry modelling calculations to derive conformational preferences for each of these molecules. These data were supplemented with information available from chemical shifts, temperature dependence of amide proton shifts and proton-proton scalar couplings. Analysis of the results suggest that the lipophilic substituents attached to the Dap(3)- epsilon amino group of the parent PGM molecule introduce changes to the conformational preferences of the peptide moiety. In PGM electrostatic interactions between charged end groups apparently promote folded conformations with participation of the long Dap side chain. Derivatives wherein such interactions are suppressed by acylation of the Dap(3)- epsilon amino group are characterized by more extended conformations of the peptide chain. The new synthetic derivatives exhibit biological properties similar to those of the parent PGM. This may indicate that peripheral parts of the peptide chain such as the C-terminal and end groups of the long Dap side chain do not significantly contribute to the binding to receptors or enzymes participating in the biochemical interactions referred to above.

A spin labelling study of immunomodulating peptidoglycan monomer and adamantyltripeptides entrapped into liposomes

The interaction of immunostimulating compounds, the peptidoglycan monomer (PGM) and structurally related adamantyltripeptides (AdTP1 and AdTP2), respectively, with phospholipids in liposomal bilayers were investigated by electron paramagnetic resonance spectroscopy. (1). The fatty acids bearing the nitroxide spin label at different positions along the acyl chain were used to investigate the interaction of tested compounds with negatively charged multilamellar liposomes. Electron spin resonance (ESR) spectra were studied at 290 and 310 K. The entrapment of the adamantyltripeptides affected the motional properties of all spin labelled lipids, while the entrapment of PGM had no effect. (2). Spin labelled PGM was prepared and the novel compound bearing the spin label attached via the amino group of diaminopimelic acid was chromatographically purified and chemically characterized. The rotational correlation time of the spin labelled molecule dissolved in buffer at pH 7.4 was studied as a function of temperature. The conformational change was observed above 300 K. The same effect was observed with the spin labelled PGM incorporated into liposomes. Such effect was not observed when the spin labelled PGM was studied at alkaline pH, probably due to the hydrolysis of PGM molecule. The study of possible interaction with liposomal membrane is relevant to the use of tested compounds incorporated into liposomes, as adjuvants in vivo.

Adjuvant activity of peptidoglycan monomer and its metabolic products

Peptidoglycan monomer (PGM) is a natural compound of bacterial origin. It is a non-toxic, non-pyrogenic, water-soluble immunostimulator potentiating humoral immune response to ovalbumin (OVA) in mice. It is fast degraded and its metabolic products-the pentapeptide (PP) and the disaccharide (DS)-are excreted from the mammalian organism upon parenteral administration. The present study investigates: (a). whether PGM could influence the long-living memory generation; (b). whether metabolic products retain adjuvant properties of the parent compound and contribute to its adjuvanticity. We report now that mice immunised twice with OVA+PGM had significantly higher anti-OVA IgG levels upon challenge with antigen alone 6 months later in comparison to control group immunised with OVA only. PP and DS were prepared enzymatically in vitro as apyrogenic and chemically pure compounds. When mice were immunised with OVA plus PP and DS, respectively, the level of anti-OVA IgGs in sera was not higher than in mice immunised with OVA alone, while PGM raised the level of specific antibodies. Results implicate that the adjuvant active molecule, capable of enhancing long-living memory generation, is PGM itself, and none of its metabolic products.

Influence of adjuvant-active peptidoglycan monomer on specific T cell responses in mice

Peptidoglycan monomer (PGM) originating from Brevibacterium divaricatum is a non-toxic, non-pyrogenic, water-soluble immunostimulator. It potentiates humoral immune response to ovalbumin (OVA) in mice upregulating both immunoglobulin (IgG) 1 and IgG2a antibody subclasses. This study concerns the influence of PGM on T cell activation and cytokine networks in response to OVA. OVA-specific proliferative response as well as interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) secretion in lymph node cell cultures of immunised mice were studied. Due to pharmacokinetic properties of PGM, namely its fast metabolism and excretion, special emphasis was on choosing the appropriate time for lymph node removal and duration of cell cultivation for each cytokine. PGM treatment in addition to OVA resulted in an increase of lymph node cellularity, stimulation of OVA-specific IFN-gamma and IL-4 production as well as of OVA-specific proliferative response. Results demonstrate that PGM stimulated both Th1 and Th2 subpopulations.

Reversed-phase high-performance liquid chromatographic method for the determination of peptidoglycan monomers and structurally related peptides and adamantyltripeptides

The reversed-phase HPLC method using UV detection was developed for the determination of (a) immunostimulating peptidoglycan monomers represented by the basic structure GlcNAc-MurNAc-L-Ala-D-isoGln-meso-DAP(omegaNH(2))-D-Ala-D-Ala (PGM) and two more lipophilic derivatives, Boc-Tyr-PGM and (Ada-1-yl)-CH(2)-CO-PGM, (b) two diastereomeric immunostimulating adamantyltripeptides L- and D-(adamant-2-yl)-Gly-L-Ala-D-isoGln and (c) peptides obtained by the enzyme hydrolyses of peptidoglycans and related peptides. The enzymes used, N-acetylmuramyl-L-alanine amidase and an L,D-aminopeptidase are present in mammalian sera and are involved in the metabolism of peptidoglycans and related peptides. Appropriate solvent systems were chosen with regard to structure and lipophilicity of each compound. As well, different gradient systems within the same solvent system had to be applied in order to achieve satisfactory separation and retention time. HPLC separation was developed with the aim to use this method for the study of the stability of the tested compounds, the purity during preparation and isolation and for following the enzyme hydrolyses.

Synthesis of novel adamantylacetyl derivative of peptidoglycan monomer--biological evaluation of immunomodulatory peptidoglycan monomer and respective derivatives with lipophilic substituents on amino group

Novel synthetic analogue of immunomodulatory peptidoglycan monomer 1 (PGM), (adamant-1-yl)-CH2CO-PGM (2), was prepared by acylation of epsilon-amino group of diaminopimelic acid with symmetrical (adamant-1-yl)-acetic acid anhydride in the presence of triethylamine. The product was isolated by gel filtration on Sephadex G-25, followed by ion exchange chromatography on SP-Sephadex C-25. The susceptibility of (adamant-1-yl)-CH2CO-PGM to hydrolysis with N-acetylmuramyl-L-alanine amidase was demonstrated, and the product of hydrolysis, (adamant-1-yl) CH2CO-pentapeptide 3, was characterized. Both 2 and 3 are water soluble and non-pyrogenic compounds. Immunomodulatory activity of PGM (adamant-1-yl)-CH2CO-PGM and structurally related derivative Boc-Tyr-PGM was compared in experiments in vivo, in mice, using ovalbumin (OVA) as an antigen. All three tested compounds exhibited comparable immunostimulating effects with respect to the induction of anti-ovalbumin immunoglobulin G. The results of evaluation of biological activity show that the substitution of free amino group in the parent peptidoglycan molecule with bulky lipophilic substituents did not affect the susceptibility to hydrolysis with N-acetylmuramyl-L-alanine amidase and did not alter markedly the immunostimulating activity. The results also indicate that the free amino group in the peptide chain is not a necessary requirement in the mechanism of immunostimulation of tested immunomodulators.

Comparative study of the effects of peptidoglycan monomer and structurally related adamantyltripeptides on humoral immune response to ovalbumin in the mouse

Peptidoglycan monomer, GlcNAc-MurNAc-L-Ala-D-isoGln-mesoDAP(omega NH2)-D-Ala-D-Ala (PGM) originating from Brevibacterium divaricatum and synthetic adamantyltripeptides, diastereoisomers of D,L-(adamant-2-yl)-Gly-L-Ala-D-isoGln (AdTP1 and AdTP2) exhibit immunomodulating activity. An experimental model in the mouse has been established with suboptimal amounts of ovalbumin (OVA) as the immunogen, and parallel testing of adjuvant activity of these three immunomodulators was carried out in Balb/c, C57B16 or CBA mice. Tested compounds (100 or 200 micrograms/mouse) mixed with OVA in saline (50 micrograms/mouse) were administered s.c. Anti-OVA was assayed by ELISA in the sera of mice taken 7 days after the boosters (given on days 14 and 28). The treatment with PGM and one of the diastereoisomers, AdTP2, resulted in significantly higher increase in anti-OVA IgG levels (stimulation index up to 46) with respect to controls and groups treated with AdTP1. The effect of AdTP2 treatment was comparable to that of PGM in most experiments after the first booster, but after the second booster PGM exhibited markedly better effect. PGM and AdTP2 also induced markedly higher levels of IgG1 and IgG2 anti-OVA subclasses than detected in controls and AdTP1 treated mice, indicating that these two immunomodulators might upregulate both Th1-like and Th2-like immune responses.

Preparation of novel conjugates involving immunomodulating peptidoglycan monomer

Peptidoglycan monomer, the disaccharide pentapeptide beta-D-Glcp-N-Ac-(1-->4)-D-Murp-N-Ac-L-Ala-D-mesoA2pm- (epsilonN H2)-D-Ala-D-Ala (PGM) is an immunomodulator. PGM and/or its derivative N-tert-butyloxycarbonyl-L-tyrosyl peptidoglycan monomer (Boc-Tyr-PGM) were coupled to two polysaccharides: the glucuronoxylomannan (GXM) from Cryptococcus neoformans, type B, solubilized by ultrasonic irradiation (MW 12-400 kDa) and to the dextran FP 70 (MW 70 kDa). Both polysaccharides were activated by CNBr. Initially, unprotected PGM was coupled via its amino group to GXM. The reactions yielded 42%-52% of the conjugate, containing only 0.18%-0.31% of PGM. In another approach Boc-Tyr-PGM (having its amino group blocked) was reacted via its free carboxyl group. Both CNBr-activated polysaccharides were first coupled to adipic acid dihydrazide (ADH) and then subsequently coupled to Boc-Tyr-PGM. The dextran conjugate (approximately 80% yield ) contained 6.3% of Boc-Tyr-PGM. The isolation of GXM conjugate required several modifications and it was obtained in lower yield (approximately 30%) but contained 13.7% of Boc-Tyr-PGM. Both conjugates were water soluble and apyrogenic and suitable for further testing of biological activity.

Structural characterization of the cyanelle peptidoglycan of Cyanophora paradoxa by 252Cf plasma desorption mass spectrometry and fast atom bombardment/tandem mass spectrometry

A strategy for the structural characterization of the four major NaBH4-reduced peptidoglycan monomers derived from muramidase-digested peptidoglycan from the cyanelles of the flagellate Cyanophora paradoxa Korschikoff is described. Initial molecular weight determination of these glycopeptides was performed by positive and negative ion plasma desorption mass spectrometry. Due to the presence of two pairs of disaccharide tripeptide and disaccharide tetrapeptide monomers differing in mass by 112 units, respectively, an as yet unknown peptidoglycan modification either at the carbohydrate or at the peptide moiety was assumed. beta-Elimination of the disaccharide unit from the unreduced peptidoglycan monomers yielded the corresponding (modified) N1-lactyltripeptides and -tetrapeptides, respectively. These peptides, N-terminally blocked with lactic acid, unambiguously showed the modification to be located on the peptide moiety. By positive ion fast atom bombardment/hybrid tandem mass spectrometry of the reduced peptidoglycan monomers as well as of the corresponding deglycosylated monomers (= N1-lactylpeptides) the modification was determined to be linked to the glutamic acid moiety. Based on combined data from plasma desorption mass spectrometry, tandem mass spectrometry, accurate mass measurement and amino acid analysis of the acid hydrolysate after derivatization with o-phthaldialdehyde by high-performance liquid chromatography we could establish the structure of the modification as N-acetylputrescine. Finally, the confirmation of the linkage of the glutamic acid to diaminopimelic acid via the gamma-COOH was based on the presence of a-type peptide backbone fragment ions in the positive ion plasma desorption mass spectra of the modified N1-lactylpeptides.

Synthesis and conformational analysis of muramic acid delta-lactam structures and their 4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl) derivatives, characteristic of bacterial spore peptidoglycan

1,6-Anhydro-4-O-benzyl-beta-muramic acid 1',2-lactam (2) was prepared by reduction of 1,6-anhydro-2-azido-4-O-benzyl-2-deoxy-3-O-[(R)-1- methoxycarbonylethyl]-beta-D-glucopyranose (1) followed by cyclisation. Debenzylation of 2 (-->3) and glycosylation of HO-4 with 3,4,6-tri-O-acetyl-2- deoxy-2-phthalimido-beta-D-glucopyranosyl chloride afforded 75% of a beta-(1-->4)-linked disaccharide derivative (7). Removal of the Phth group from 7, then acetylation, and O-deacetylation yielded 4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-amino-1,6-anhydro-3-O- [(R)- 1-carboxyethyl]-2-deoxy-beta-D-glucopyranose 1',2-lactam (10) Acetolysis of the 1,6-anhydro ring in the 4-acetate (4) of 3 and the 3',4',6'-triacetate (9) of 10, with saponification of the products 5 and 11, afforded 2-amino-3-O- [(R)-1-carboxyethyl]-2-deoxy-D-glucopyranose 1',2-lactam (6) and 4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-amino-3- O-[(R)-1-carboxyethyl]-2-deoxy-beta-D-glucopyranose 1',2-lactam (12), respectively. The structure of 12 corresponds to that of the disaccharide unit characteristic of the glycan chains of bacterial spore peptidoglycan. 1H NMR spectroscopy indicated that the beta-D-glucopyranose ring in the 1,6-anhydro 1',2-lactam derivatives adopts the BO,3 conformation. On cleavage of the 1,6-anhydro ring by acetolysis, the D-glucopyranose ring adopts the 4C1 conformation. X-ray analysis of 2, 4, and 5 confirmed the proposed structures. Molecular mechanics and molecular dynamics simulations were used to follow the transformation of the BO,3 conformation of the D-glucopyranose ring via transition states to the 4C1 form.

A competitive radioimmunoassay for peptidoglycan monomer

The preparation and characterization of the essential components to be used in the radioimmunoassay of peptidoglycan monomer (PGM) is described. In order to raise the anti-peptidoglycan monomer antibodies 14C-labelled peptidoglycan monomer-bovine serum albumin conjugate was prepared by the coupling of 14C-peptidoglycan monomer to bovine serum albumin in the presence of glutaraldehyde in 0.1 M NaHCO3 at pH 8.3. The prepared conjugate elicited anti-PGM response in rabbits. A synthetic analog of peptidoglycan monomer, Boc-L-tyrosyl-peptidoglycan monomer was prepared by condensation of unprotected peptidoglycan monomer and N-hydroxysuccinimidester of Boc-L-tyrosine in the presence of triethylamine and the obtained disaccharide-hexapeptide was labelled with Na125I. This compound exhibited the ability of binding to anti-peptidoglycan monomer antibodies. The prepared compounds, namely anti-PGM antibodies and 125I-labelled Boc-L-tyrosyl-peptidoglycan monomer, were used as essential components in competitive radioimmunoassay for peptidoglycan monomer determination in mammalian and human sera and plasma, respectively.