Synthesis of thiol-reactive lipopeptide adjuvants. Incorporation into liposomes and study of their mitogenic effect on mouse splenocytes

Synthetic analogues of triacylated and diacylated lipopeptides derived from the N-terminal domain of respectively bacterial and mycoplasmal lipoproteins are highly potent immunoadjuvants when administered either in combination with protein antigens or covalently linked to small peptide epitopes. Because of their amphipathic properties, lipopeptides, such as S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-cysteinyl-alanyl-glycine (Pam(3)CAG), can be conveniently incorporated into liposomes and serve as anchors for antigens that are linked to them. To design vaccination constructs based on synthetic peptides and liposomes as vectors. we have accordingly synthesized a series of lipopeptides that differ by the number (Pam(3)C vs Pam(2)C) and nature of the acyl chains (palmitoyl vs oleoyl) and by the presence at their C-terminus of thiol-reactive functions, such as maleimide or bromoacetyl. When incorporated into liposomes, these latter functionalized lipopeptides allow, in aqueous media, a well controlled chemoselective conjugation of HS-peptides to the surface of the vesicles. Using a BALB/c mice splenocyte proliferation assay ([(3)H]thymidine incorporation), we have measured the lymphocyte activation potency of the different lipopeptides. We found that, compared to their free (emulsified) forms, the liposomal lipopeptides were endowed with enhanced mitogenic activities; i.e., up to 2 orders of magnitude for Pam(3)CAG which was more potent than Pam(2)CAG. The impact of functionalization on the cellular activity of Pam(3)CAG was dependent on the thiol-reactive group introduced: whereas the bromoacetyl derivative retained its full activity, the presence of a maleimide group virtually abolished the lymphocyte activation of the lipopeptide. Finally, the substitution of saturated palmitoyl chains by unsaturated oleoyl chains was inhibitory. Thus, thiol-reactive Ol(3)CAG derivatives were the least active mitogens in our assay. Taken together, our findings are of importance for the further optimization of antigen-specific liposomal-based synthetic vaccines; the bromoacetyl derivative of Pam(3)CAG should be a promising lipopeptide derivative serving as an anchor for peptide epitopes while retaining its lymphocyte activation activity.     

Rapid isolation and characterization of bacterial lipopeptides using liquid chromatography and mass spectrometry analysis

The structural analysis of post‐translational modifications (PTMs) of lipoproteins is difficult due to the hydrophobic properties of their fatty acid moieties. At the present time, the relative positions of fatty acid components on the N‐acyl‐S‐diacylglycerylcysteine core structure has not been specifically identified in any natural or bacterial expressed recombinant lipoproteins. In this study, we describe a rapid solid‐phase extraction using acetonitrile and isopropanol method that can be performed manually to isolate large amounts of relatively pure lipopeptides generated by the limited tryptic‐digestion of recombinant lipoproteins. Using these lipopeptides and LC/MS mass spectra analysis, two groups of N‐terminal lipidated (diacyl or triacyl) molecules that differ by one fatty acid unit were successfully identified. This LC/MS method also provided the separation of lipopeptides differing by 14 Da for the on‐line MS identification. Multiple‐stage fragmentation analyses of the di‐ and triacyl lipopeptides using both the positive and negative ion modes enabled to identify the putative structure of the N‐acyl‐S‐diacylglycerylcysteine containing an amide bond to palmitic acid at the N‐terminal cysteine, a palmitic acid at sn1 position, and an unsaturated fatty acid of either hexadecenoic acid, cyclopropaneoctanoic acid, oleic acid and nonadecenoic acid at sn2 position of diacylglycerol residue through ester bonding. For diacyl lipoprotein, the saturated palmitoyl fatty acid group is absent at sn1 position of glycerol‐derived lipid residue of lipopeptide.     

Liposome-based Systems for Anti-tumor Vaccination: Influence of Lipopeptide Adjuvants

We have developed liposome-based synthetic constructs incorporating peptide epitope(s) (ErbB2 p63-67 CTL which is overexpressed in many tumors and/or HA 307-319 T-helper) and lipopeptide adjuvants (Pam3CysSerSer, Pam3CysAlaGly) in order to elicit an anti-tumor immune response. The epitopes, derivatized with a linker containing a cysteine residue, were conjugated on preformed vesicles (dia. ～ 100 nm) containing lipopeptides functionalized with thiol reactive groups (maleimide or bromoacetyl). The therapeutic efficacy of these constructs was evaluated on a Balb/c mice tumor model inoculated with syngenic murine renal carcinoma (Renca) cells expressing human ErbB2 (Her2/neu) receptor. A successful therapeutic vaccination was obtained which was antigen specific. Furthermore, it appeared that the nature of the polar head group of the lipopeptide adjuvant and also its type of functionalization influence the efficacy of the construct. In our study, the best results were obtained with formulations containing a Pam₃CSS anchor in association with the CTL and Th epitopes. Considering these promising results studies are in progress with a new generation of liposomes that incorporate a neutral lipid – lacking adjuvant properties – that serves as anchor of the peptide epitopes and new adjuvants synthesized in our laboratory, which are screened for their antitumour activity in a therapeutic setting.     

Immunostimulants and Toll-like receptor ligands obtained by screening combinatorial lipopeptide collections.

Synthetic lipopeptides carrying the head group of bacterial lipoproteins are specific ligands of Toll-like receptors (TLR). The three fatty acids containing lipopeptides with the tripalmitoyl-S-glyceryl-cysteinyl N-terminus (Pam(3)Cys) are agonists of TLR2. The structurally related lipopeptides with a head group lacking the fatty acyl residue at the amino-terminus (Pam(2)Cys) stimulate TLR2 and 6. To investigate the influence of the peptide chain of lipohexapeptides with a free N-terminus with regard to their ability to enhance B-cell proliferation, a randomized S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-(R)-cysteinyl-pentapeptide amide collection Pam(2)CysXXXXX and 5 x 19 subcollections (Pam(2)CysOXXXX, Pam(2)CysXOXXX, Pam(2)CysXXOXX, Pam(2)CysXXXOX, Pam(2)CysXXXXO, O: all protein amino acids except Cys) were prepared by parallel solid-phase synthesis. The collection represents synthetic lipopeptide analogues of the numerous bacterial lipoproteins and of mycoplasma lipoprotein. Each of the 95 subcollections is characterized by one defined and four degenerated amino acid positions thus comprising 19(4) individual lipopeptides with free N-terminal amino groups. High-performance liquid chromatography electrospray mass spectrometry (HPLC-ESI-MS) was applied for the analytical characterization of the lipohexapeptide amide subcollections and for the individual lipohexapeptide amides. The subcollections were tested for polyclonal activation of murine spleen cells, deconvolution led to highly active single S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-(R)-cysteinyl-pentapeptide amides.     

Structural requirement for the agonist activity of the TLR2 ligand Pam2Cys

Synthetic lipopeptides have demonstrated great potential as a vaccine strategy for eliciting cellular and humoral immunity. One of the most potent lipid moieties used is S-[2,3-bis(palmitoyloxy)propyl]cysteine (Pam2Cys). Pam2Cys binds to and activates dendritic cells by engagement of Toll like receptor 2 (TLR 2). In this study, we have investigated the structural requirement of the agonist activity of Pam2Cys by varying the three structural elements of the core structure S-(2,3-dihydroxypropyl)-cysteine namely (1) the α-amino group of the cysteine residue (2) the sulphur atom of the cysteine residue and (3) the 2,3-dihydroxypropyl moiety. Four novel analogues of Pam2Cys were made and each of these analogues were incorporated into vaccine constructs and examined for immunogenicity. Our results demonstrate that (1) the potency of the peptide vaccine is least affected by removal of the amino group (2) substitution of the sulphur atom with an amide bond leads to significant reduction of biological activity (3) removal of the amino group and at the same time substitution of the sulphur with an amide bond significantly decreases the biological activity (4) in the two analogues in which the sulphur atom is replaced with an amide bond the analogue containing the 1,3-dihydroxypropyl moiety demonstrates higher activity than the one which contains 2,3-dihydroxypropyl. In conclusion, the results demonstrate strict structural requirements for agonist activity of the TLR2 ligand Pam2Cys.     

Liposome-based systems for anti-tumor vaccination: influence of lipopeptide adjuvants

We have developed liposome-based synthetic constructs incorporating peptide epitope(s) (ErbB2 p63-67 CTL which is overexpressed in many tumors and/or HA 307-319 T-helper) and lipopeptide adjuvants (Pam3CysSerSer, Pam3CysAlaGly) in order to elicit an anti-tumor immune response. The epitopes, derivatized with a linker containing a cysteine residue, were conjugated on preformed vesicles (dia. approximately 100 nm) containing lipopeptides functionalized with thiol reactive groups (maleimide or bromoacetyl). The therapeutic efficacy of these constructs was evaluated on a Balb/c mice tumor model inoculated with syngenic murine renal carcinoma (Renca) cells expressing human ErbB2 (Her2/neu) receptor. A successful therapeutic vaccination was obtained which was antigen specific. Furthermore, it appeared that the nature of the polar head group of the lipopeptide adjuvant and also its type of functionalization influence the efficacy of the construct. In our study, the best results were obtained with formulations containing a Pam3CSS anchor in association with the CTL and Th epitopes. Considering these promising results studies are in progress with a new generation of liposomes that incorporate a neutral lipid--lacking adjuvant properties--that serves as anchor of the peptide epitopes and new adjuvants synthesized in our laboratory, which are screened for their antitumour activity in a therapeutic setting.     

TLR1- and TLR6-independent recognition of bacterial lipopeptides

Bacterial cell walls contain lipoproteins/peptides, which are strong modulators of the innate immune system. Triacylated lipopeptides are assumed to be recognized by TLR2/TLR1-, whereas diacylated lipopeptides use TLR2/TLR6 heteromers for signaling. Following our initial discovery of TLR6-independent diacylated lipopeptides, we could now characterize di- and triacylated lipopeptides (e.g. Pam(2)C-SK(4), Pam(3)C-GNNDESNISFKEK), which have stimulatory activity in TLR1- and in TLR6-deficient mice. Furthermore, for the first time, we present triacylated lipopeptides with short length ester-bound fatty acids (like PamOct(2)C-SSNASK(4)), which induce no response in TLR1-deficient cells. No differences in the phosphorylation of MAP kinases by lipopeptide analogs having different TLR2-coreceptor usage were observed. Blocking experiments indicated that different TLR2 heteromers recognize their specific lipopeptide ligands independently from each other. In summary, a triacylation pattern is necessary but not sufficient to render a lipopeptide TLR1-dependent, and a diacylation pattern is necessary but not sufficient to render a lipopeptide TLR6-dependent. Contrary to the current model, distinct lipopeptides are recognized by TLR2 in a TLR1- and TLR6-independent manner.     

TLR2-dependent induction of IL-10 and Foxp3+ CD25+ CD4+ regulatory T cells prevents effective anti-tumor immunity induced by Pam2 lipopeptides in vivo

16 S-[2,3-bis(palmitoyl)propyl]cysteine (Pam2) lipopeptides act as toll-like receptor (TLR)2/6 ligands and activate natural killer (NK) cells and dendritic cells (DCs) to produce inflammatory cytokines and cytotoxic NK activity in vitro. However, in this study, we found that systemic injection of Pam2 lipopeptides was not effective for the suppression of NK-sensitive B16 melanomas in vivo. When we investigated the immune suppressive mechanisms, systemic injection of Pam2 lipopeptides induced IL-10 in a TLR2-dependent manner. The Pam2 lipopeptides increased the frequencies of Foxp3(+)CD4(+) regulatory T (T reg) cells in a TLR2- and IL-10- dependent manner. The T reg cells from Pam2-lipopeptide injected mice maintained suppressor activity. Pam2 lipopeptides, plus the depletion of T reg with an anti-CD25 monoclonal antibody, improved tumor growth compared with Pam2 lipopeptides alone. In conclusion, our data suggested that systemic treatment of Pam2 lipopeptides promoted IL-10 production and T reg function, which suppressed the effective induction of anti-tumor immunity in vivo. It is necessary to develop an adjuvant that does not promote IL-10 and T reg function in vivo for the future establishment of an anti-cancer vaccine.     

Solid-phase synthesis and application of double-fluorescent-labeled lipopeptides, containing a CTL-epitope from the measles fusion protein.

The mechanism which enables lipopeptides to induce cytotoxicity is not known. By preparing fluorescent-labeled lipopeptides one might unravel the mechanism of their entry into the cell and their intracellular pathway. A method of preparing double-fluorescent-labeled peptides by solid-phase chemistry is described. As model peptides we have chosen analogs of the sequence RRYPDAVYL, which occurs in the measles fusion protein (F438-446) and is an epitope for cytotoxic T lymphocytes. The peptides Pal-K(TMR)KKKRRYPDAVK(FL)L (7) and Pal-K(FL)KKKRRYPDAVK(TMR)L (8), in which Pal is palmitoyl and K(TMR) and K(FL) are Nepsilon-carboxytetramethylrhodamine- and Nepsilon-carboxyfluorescein-labeled lysyl residues, respectively, were prepared and obtained in approximately 30% yield after purification by high-performance liquid chromatography. The fluorescence of fluorescein and tetramethylrhodamine in lipopeptide Pal-K(TMR)KKKRRYPDAVK(FL)L (7) was quenched to 98-99% due to intramolecular interaction of the labels. On incubation with trypsin (i.e. cleavage at the KKKRR-site) the fluorescence of both labels was restored. The intracellular routing of lipopeptide Pal-K(TMR)KKKRRYPDAVK(FL)L was studied with human melanoma cell line, Mel/J, which was transfected with human leukocyte antigen B*2705. It appeared that the double-fluorescent-labeled lipopeptide was able to induce antigen-specific cytotoxicity. Furthermore, preliminary confocal microscopical studies indicated that this lipopeptide is observed intracellularly.     

Design and study of lipopeptide inhibitors on preventing aggregation of human islet amyloid polypeptide residues 11‐20

Type 2 diabetes mellitus, a kind of conformational disease, has become an epidemic disease, which seriously endangers the quality of life and health of human beings. The deposition of human islet amyloid polypeptide (hIAPP) has been considered as one of the major pathological features of type 2 diabetes mellitus. As lipopeptides have some hydrophobic groups, which are similar to the reported aggregation inhibitors, and some lipopeptides could prevent cells from depositing of amyloid fibrils, several potential lipopeptide inhibitors have been engineered and synthesized, which have been assessed for their inhibitory effect in preventing amyloid fibrils formation of hIAPP_11‐20 by using the conventional thioflavin‐T fluorescence assay and new technique microscale thermophoresis (MST). The final amyloid fibrils of hIAPP_11‐20 were characterized by transmission electron microscopy. Results suggested that with the increasing length of alkyl chain, the antiaggregation efficiency of lipopeptide inhibitors towards hIAPP_11‐20 increased gradually. Meanwhile, the amount of arginines, which represent the head groups of lipopeptides, may also have some influence. The binding events also showed that the inhibitory efficiency of these lipopeptide inhibitors was enhanced with the increase of affinities between lipopeptides and hIAPP_11‐20, which were obtained from MST. This study demonstrated the efficiency of lipopeptides in inhibiting the aggregation of hIAPP_11‐20 and proved that MST could be regarded as an appropriate and rapid method to screen potential inhibitors of hIAPP_11‐20 or other amyloid proteins. This study also broadens the types of inhibitors on inhibiting amyloid formation of hIAPP.     

Characterization of Surfactin-like Cyclic Depsipeptides Synthesized by Bacillus pumilus from Ascidian Halocynthia aurantium

A marine bacterium (KMM 1364), identified as Bacillus pumilus, was isolated from the surface of ascidian Halocynthia aurantium. Structural analysis revealed that the strain KMM 1364 produced a mixture of lipopeptide surfactin analogs with major components with molecular masses of 1035, 1049, 1063, and 1077. The variation in molecular weight represents changes in the number of methylene groups in the lipid and/or peptide portions of the compounds. Structurally, these lipopeptides differ from surfactin in the substitution of the valine residue in position 4 by leucine, and have been isolated as two carboxy-terminal variants, with valine or isoleucine in position 7. As constituents of the lipophilic part of the peptides, only β-hydroxy-C₁₅-, β-hydroxy-C₁₆-, and a high amount of β-hydroxy-C₁₇ fatty acid were determined.     

Lipopeptide derivatives of bacterial lipoprotein constitute potent immune adjuvants combined with or covalently coupled to antigen or hapten

Lipopeptide analogues of the N-terminus of bacterial lipoprotein consisting of N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-(R)-cysteine (Pam3Cys) attached to one to five further amino acids [Pam3Cys-Ser-Ser-Asn-Ala, Pam3Cys-Ser-(Lys)4, Pam3Cys-Ala-Gly, and Pam3Cys-Ser] were investigated for biological activity. In vitro, the compounds proved to be potent activators for Balb/c splenocytes as determined by proliferation assays. When given in vivo in combination with SRBC, Pam3Cys-Ser and Pam3Cys-Ala-Gly acted as immunoadjuvants enhancing the antigen specific IgM response after 7, and the IgG response after 14 days. In combination with dinitrophenylated bovine serum albumin (BSA(Dnp)), especially the amphiphilic and water-soluble lipohexapeptide Pam3Cys-Ser-(Lys)4 constituted a potent immune adjuvant. The lipopeptide was able to fully replace Freund's complete adjuvant (FCS) enhancing both anti-Dnp IgM and IgG in Balb/c mice. The hapten Dnp was also coupled directly--or via the spacer molecule 1,6-diaminohexane (HMD)--to the synthetic lipopeptides. The chemically defined low-molecular-mass conjugates obtained were capable of inducing anti-hapten-specific IgM and IgG without further adjuvants or carriers. The anti-hapten responses induced by these chemically uniform lipopeptide-hapten conjugates were, however, less pronounced than the response to the conventional heterogeneous hapten-protein conjugate BSA(Dnp), and only a weak boost effect was observed. Our results show that defined lipopeptides are novel immunoadjuvants either combined with or covalently linked to antigens or haptens.     

A novel lipopeptide produced by a Pacific Ocean deep-sea bacterium, Rhodococcus sp. TW53

Aims: Our goal was to find a novel, biosurfactant‐producing bacterium from Pacific Ocean deep‐sea sediments. Methods and Results: An oil‐degrading biosurfactant‐producing bacterium TW53 was obtained from deep‐sea sediment, and was identified through 16S rDNA analysis as belonging to the genus Rhodococcus. It lowered the surface tension of its culture to 34·4 mN m^?1. Thin layer chromatography (TLC) showed that the crude biosurfactants of TW53 were composed of lipopeptides and free fatty acids (FA). The lipopeptides were purified with column chromatography and then hydrolysed with 6 mol l^?1 HCl. Gas chromatography‐mass spectrometry analysis showed that the hydrolyte in the hydrophobic fraction contained five kinds of FA with chain lengths of C₁₄–C₁₉, and C₁₆H₃₂O₂ was a major component making up 59·18% of the total. However, 3‐hydroxyl FA was not found, although it is usually found in lipopeptides. Silica gel TLC revealed that the hydrolyte in the hydrophilic fraction was composed of five kinds of amino acids; consistently, ESI‐Q‐TOF‐MS analysis confirmed the composition results and provided their sequence tentatively as Ala‐Ile‐Asp‐Met‐Pro. Furthermore, the yield and CMC (critical micelle concentrations) of purified lipopeptides were examined. The purified product reduced the surface tension of water to 30·7 mN m^?1 with a CMC value of 23·7 mg l^?1. These results suggest that Rhodococcus sp. TW53 produces a novel lipopeptide that we have named rhodofactin. Conclusion: The deep‐sea isolate Rhodococcus sp. TW53 was the first reported lipopeptide‐producing bacterium of this genus. The lipopeptides had novel chemical compositions. Significance and Impact of the Study: Rhodococcus sp. TW53 has potential in the exploration of new biosurfactants and could be used in bioremediation of marine oil pollution.     

Crystal structure of the TLR1-TLR2 heterodimer induced by binding of a tri-acylated lipopeptide

TLR2 in association with TLR1 or TLR6 plays an important role in the innate immune response by recognizing microbial lipoproteins and lipopeptides. Here we present the crystal structures of the human TLR1-TLR2-lipopeptide complex and of the mouse TLR2-lipopeptide complex. Binding of the tri-acylated lipopeptide, Pam(3)CSK(4), induced the formation of an "m" shaped heterodimer of the TLR1 and TLR2 ectodomains whereas binding of the di-acylated lipopeptide, Pam(2)CSK(4), did not. The three lipid chains of Pam(3)CSK(4) mediate the heterodimerization of the receptor; the two ester-bound lipid chains are inserted into a pocket in TLR2, while the amide-bound lipid chain is inserted into a hydrophobic channel in TLR1. An extensive hydrogen-bonding network, as well as hydrophobic interactions, between TLR1 and TLR2 further stabilize the heterodimer. We propose that formation of the TLR1-TLR2 heterodimer brings the intracellular TIR domains close to each other to promote dimerization and initiate signaling.     

Soluble proteins induce strong CD8+ T cell and antibody responses through electrostatic association with simple cationic or anionic lipopeptides that target TLR2

The low immunogenicity exhibited by most soluble proteins is generally due to the absence of molecular signatures that are recognized by the immune system as dangerous. In this study, we show that electrostatic binding of synthetic branched cationic or anionic lipopeptides that contain the TLR-2 agonist Pam(2)Cys markedly enhance a protein's immunogenicity. Binding of a charged lipopeptide to oppositely charged protein Ags resulted in the formation of stable complexes and occurs at physiologic pH and salt concentrations. The induction of cell-mediated responses is dependent on the electrostatic binding of lipopeptide to the protein, with no CD8(+) T cells being elicited when protein and lipopeptide possessed the same electrical charge. The CD8(+) T cells elicited after vaccination with lipopeptide-protein Ag complexes produced proinflammatory cytokines, exhibited in vivo lytic activity, and protected mice from challenge with an infectious chimeric influenza virus containing a single OVA epitope as part of the influenza neuraminidase protein. Induction of a CD8(+) T cell response correlated with the ability of lipopeptide to facilitate Ag uptake by DCs followed by trafficking of Ag-bearing cells into draining lymph nodes. Oppositely charged but not similarly charged lipopeptides were more effective in DC uptake and trafficking. Very high protein-specific Ab titers were also achieved by vaccination with complexes composed of oppositely charged lipopeptide and protein, whereas vaccination with similarly charged constituents resulted in significant but lower Ab titers. Regardless of whether similarly or oppositely charged lipopeptides were used in the induction of Ab, vaccination generated dominant IgG1 isotype Abs rather than IgG2a.     

Effect of Pam3Cys induced protection on the therapeutic efficacy of miltefosine against experimental visceral leishmaniasis

Prophylactic potential of synthetic bacterial lipopeptide and a TLR2 agonist, Pam3Cys was first evaluated against experimental visceral leishmaniasis in rodent model. After establishing the potential its effect on therapeutic efficacy of miltefosine was also studied. Pam3Cys showed 74.64% inhibition in parasitic establishment when administered by ip route at a dose of 100 μg/animal spaced at two weeks, i.e. on day −7 and +7 of challenge with Leishmania donovani amastigotes. However, when aforesaid dose of Pam3Cys was given with sub-curative dose of miltefosine (2.5 mg/kg for 5 days) its efficacy enhanced from 49.80% to 92.25%. These findings revealed that this lipopeptide has potential protective efficacy which significantly enhanced the therapeutic efficacy of miltefosine used at low dose against Leishmania infection and warrants detailed investigations on its possible immunopotentiatory actions.     

Characterization of lipopeptides from <Emphasis Type="Italic">Paenibacillus</Emphasis> sp. (IIRAC30) suppressing <Emphasis Type="Italic">Rhizoctonia solani</Emphasis>

The main aim was to identify the active compound against Rhizoctonia solani produced by the cassava endophyte Paenibacillus sp. IIRAC-30. The compounds produced were extracted with ethyl acetate and purified by Sephadex column prior to analysis by Q-TOF mass spectrometry. A C₁₅-lipopeptide with an estimated molecular weight of 1036 Da and homologues were identified. The lipopeptide had a cyclic structure, which was deduced by interpreting the ESI–MS/MS spectra of main protonated homologues containing 15:0 FA, and the amino acid composition was Glu-Leu-Leu-Val-Asp-Leu-Leu. Therefore, the lipopeptides produced by isolate IIRAC-30 was characterized as a surfactin series. Thus, the main mechanism used by Paenibacillus sp. IIRAC-30 to suppress R. solani was elucidated. Furthermore, because lipopeptides active against phytopathogens generally show low toxicity to humans and the environment, the positive findings presented here suggest that the isolate IIRAC-30 could be a possible candidate for biocontrol of R. solani.     

Effect of a highly concentrated lipopeptide extract of <Emphasis Type="Italic">Bacillus subtilis</Emphasis> on fungal and bacterial cells

Lipopeptides produced by Bacillus subtilis are known for their high antifungal activity. The aim of this paper is to show that at high concentration they can damage the surface ultra-structure of bacterial cells. A lipopeptide extract containing iturin and surfactin (5 mg mL⁻¹) was prepared after isolation from B. subtilis (strain OG) by solid phase extraction. Analysis by atomic force microscope (AFM) showed that upon evaporation, lipopeptides form large aggregates (0.1–0.2 μm²) on the substrates silicon and mica. When the same solution is incubated with fungi and bacteria and the system is allowed to evaporate, dramatic changes are observed on the cells. AFM micrographs show disintegration of the hyphae of Phomopsis phaseoli and the cell walls of Xanthomonas campestris and X. axonopodis. Collapses to fungal and bacterial cells may be a result of formation of pores triggered by micelles and lamellar structures, which are formed above the critical micelar concentration of lipopeptides. As observed for P. phaseoli, the process involves binding, solubilization, and formation of novel structures in which cell wall components are solubilized within lipopeptide vesicles. This is the first report presenting evidences that vesicles of uncharged and negatively charged lipopeptides can alter the morphology of gram-negative bacteria.     

The Peptide Sequence of Diacyl Lipopeptides Determines Dendritic Cell TLR2-Mediated NK Activation

Natural killer (NK) cells are lymphocyte effectors that are activated to control certain microbial infections and tumors. Many NK-activating and regulating receptors are involved in regulating NK cell function. In addition, activation of naïve NK cells is fundamentally triggered by cytokines or myeloid dendritic cells (mDC) in various modes. In this study, we synthesized 16 S-[2,3-bis(palmitoyl)propyl]cysteine (Pam2Cys) lipopeptides with sequences designed from lipoproteins of Staphylococcus aureus, and assessed their functional properties using mouse (C57BL/6) bone marrow-derived DC (BMDC) and NK cells. NK cell activation was evaluated by three criteria: IFN-γ production, up-regulation of NK activation markers and cytokines, and NK target (B16D8 cell) cytotoxicity. The diacylated lipopeptides acted as TLR2 ligands, inducing up-regulation of CD25/CD69/CD86, IL-6, and IL-12p40, which represent maturation of BMDC. Strikingly, the Pam2Cys lipopeptides induced mouse NK cell activation based on these criteria. Cell-cell contact by Pam2Cys peptide-stimulated BMDC and NK cells rather than soluble mediators released by stimulated BMDC induced activation of NK cells. For most lipopeptides, the BMDC TLR2/MyD88 pathway was responsible for driving NK activation, while some slightly induced direct activation of NK cells via the TLR2/MyD88 pathway in NK cells. The potential for NK activation was critically regulated by the peptide primary sequence. Hydrophobic or proline-containing sequences proximal to the N-terminal lipid moiety interfered with the ability of lipopeptides to induce BMDC-mediated NK activation. This mode of NK activation is distinctly different from that induced by polyI:C, which is closely associated with type I IFN-inducing pathways of BMDC. These results imply that the MyD88 pathway of BMDC governs an alternative NK-activating pathway in which the peptide sequence of TLR2-agonistic lipopeptides critically affects the potential for NK activation.