Activation of dendritic cells by liposomes prepared from phosphatidylinositol mannosides from Mycobacterium bovis bacillus Calmette-Guerin and adjuvant activity in vivo

Biophysical Interaction Landscape of Mycobacterial Mycolic Acids and Phenolic Glycolipids with Host Macrophage Membranes

Lipidic adjuvant formulations consisting of immunomodulatory mycobacterial cell wall lipids interact with host cells following administration. The impact of this cross-talk on the host membrane's structure and function is rarely given enough consideration but is imperative to rule out nonspecific perturbation underlying the adjuvant. In this work, we investigated changes in the plasma membranes of live mammalian cells after exposure to mycobacterial mycolic acid (MA) and phenolic glycolipids, two strong candidates for lipidic adjuvant therapy. We found that phenolic glycolipid 1 softened the plasma membrane, lowering membrane tension and stiffness, but MA did not significantly change the membrane characteristics. Further, phenolic glycolipid 1 had a fluidizing impact on the host plasma membrane, increasing the fluidity and the abundance of fluid-ordered-disordered coexisting lipid domains. Notably, lipid diffusion was not impacted. Overall, MA and, to a lesser extent, phenolic glycolipid 1, due to minor disruption of host cell membranes, may serve as appropriate lipids in adjuvant formulations.

Spotlight on mycobacterial lipid exploitation using nanotechnology for diagnosis, vaccines, and treatments

Tuberculosis (TB), historically the most significant cause of human morbidity and mortality, has returned as the top infectious disease worldwide, under circumstances worsened by the COVID-19 pandemic's devastating effects on public health. Although Mycobacterium tuberculosis, the causal agent, has been known of for more than a century, the development of tools to control it has been largely neglected. With the advancement of nanotechnology, the possibility of engineering tools at the nanoscale creates unique opportunities to exploit any molecular type. However, little attention has been paid to one of the major attributes of the pathogen, represented by the atypical coat and its abundant lipids. In this review, an overview of the lipids encountered in M. tuberculosis and interest in exploiting them for the development of TB control tools are presented. Then, the amalgamation of nanotechnology with mycobacterial lipids from both reported and future works are discussed.

Generation of Liposomes to Study the Effect of Mycobacterium Tuberculosis Lipids on HIV-1 cis- and trans-Infections

Tuberculosis (TB) is the leading cause of death among HIV-1-infected individuals and Mycobacterium tuberculosis (Mtb) co-infection is an early precipitate to AIDS. We aimed to determine whether Mtb strains differentially modulate cellular susceptibility to HIV-1 infection (cis- and trans-infection), via surface receptor interaction by their cell envelope lipids. Total lipids from pathogenic (lineage 4 Mtb H37Rv, CDC1551 and lineage 2 Mtb HN878, EU127) and non-pathogenic (Mycobacterium bovis BCG and Mycobacterium smegmatis) Mycobacterium strains were integrated into liposomes mimicking the lipid distribution and antigen accessibility of the mycobacterial cell wall. The resulting liposomes were tested for modulating in vitro HIV-1 cis- and trans-infection of TZM-bl cells using single-cycle infectious virus particles. Mtb glycolipids did not affect HIV-1 direct infection however, trans-infection of both R5 and X4 tropic HIV-1 strains were impaired in the presence of glycolipids from M. bovis, Mtb H37Rv and Mtb EU127 strains when using Raji-DC-SIGN cells or immature and mature dendritic cells (DCs) to capture virus. SL1, PDIM and TDM lipids were identified to be involved in DC-SIGN recognition and impairment of HIV-1 trans-infection. These findings indicate that variant strains of Mtb have differential effect on HIV-1 trans-infection with the potential to influence HIV-1 disease course in co-infected individuals.

Recognition of Mycobacteria by Dendritic Cell Immunoactivating Receptor

Mycobacteria have unique lipids on their cell walls, and the structures and physiological activities of these lipid components have been the subject of many studies. Although the host receptors for mycobacterial lipid have long been elusive, in recent years C-type lectin receptors (CLRs) have been reported to recognize these components. The dendritic cell immunoactivating receptor (DCAR), a CLR member, is encoded by Clec4b1. DCAR, which was identified in 2003, is reported to be associated with the immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptor protein, the Fc receptor γ chain (FcRγ). However, its physiological ligand and biological function were unknown. We recently identified DCAR as an activating receptor for mycobacteria. DCAR recognizes acylated phosphatidyl-inositol mannosides (PIMs) in mycobacteria to promote Th1 responses during mycobacterial infection. This review summarizes recent discoveries about the ligands and immunological roles of DCAR.

Novel trisaccharide based phospholipids as immunomodulators

A focused library of novel mannosylated glycophospholipids was synthesized employing imidate coupling and H-phosphate phosphorylation methods. All novel glycophospholipids were evaluated for their receptor interactions by molecular docking studies. Docking studies revealed dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) specific interaction of the glycophospholipid ligand P4 acts, which was further confirmed by in vitro DC-SIGN expression on monocyte-derived dendritic cells (MoDCs). Further, in vitro and in vivo immunomodulatory activity among the six compounds (P1-P6) examined, compound P4 displayed good immunopotentiation and adjuvant properties as indicated by the induced cytokine expression and enhanced ovalbumin (OVA) specific antibody (IgG) titers. Phosphatidylinositol mannosides (PIMs) analogues in the present study enforced the immunomodulatory properties, truncating parent PIMs or tailor-made of PIMs may bring the novel efficacious molecules, which will be useful in vaccine preparation against different diseases.

Biocompatible chitosan nanoparticles as an efficient delivery vehicle for Mycobacterium tuberculosis lipids to induce potent cytokines and antibody response through activation of γδ T cells in mice

The activation of cell-mediated and humoral immune responses to Mycobacterium tuberculosis (Mtb) is critical for protection against the pathogen and nanoparticle-mediated delivery of antigens is a more potent way to induce different immune responses. Herein, we show that mice immunized with Mtb lipid-bound chitosan nanoparticles (NPs) induce secretion of prominent type-1 T-helper (Th-1) and type-2 T-helper (Th-2) cytokines in lymph node and spleen cells, and also induces significantly higher levels of IgG, IgG1, IgG2 and IgM in comparison to control mice. Furthermore, significantly enhanced γδ-T-cell activation was observed in lymph node cells isolated from mice immunized with Mtb lipid-coated chitosan NPs as compared to mice immunized with chitosan NPs alone or Mtb lipid liposomes. In comparison to CD8⁺ cells, significantly higher numbers of CD4⁺ cells were present in both the lymph node and spleen cells isolated from mice immunized with Mtb lipid-coated chitosan NPs. In conclusion, this study represents a promising new strategy for the efficient delivery of Mtb lipids using chitosan NPs to trigger an enhanced cell-mediated and antibody response against Mtb lipids.

Conserved Molecular Superlattices in a Series of Homologous Synthetic Mycobacterial Cell-Wall Lipids Forming Interdigitated Bilayers

Synthetic analogues of the cell-wall lipid monomycoloyl glycerol (MMG) are promising as next-generation vaccine adjuvants. In the present study, the thermotropic phase behavior of an array of synthetic MMG analogues was examined by using simultaneous small- and wide-angle X-ray scattering under excess water conditions. The MMG analogues differed in the alkyl chain lengths and in the stereochemistry of the polar glycerol headgroup or of the lipid tails (native-like versus alternative compounds). All MMG analogues formed poorly hydrated lamellar phases at low temperatures and inverse hexagonal (H₂) phases at higher temperatures prior to melting. MMG analogues with a native-like lipid acid configuration self-assembled into noninterdigitated bilayers whereas the analogues displaying an alternative lipid acid configuration formed interdigitated bilayers in a subgel (L_c') state. This is in contrast to previously described interdigitated phases for other lipids, which are usually in a gel (L_β) state. All investigated MMG analogues displayed an abrupt direct temperature-induced phase transition from L_c' to H₂. This transition is ultimately driven by the lipid chain melting and the accompanying change in molecular shape. No intermediate structures were found, but the entire array of MMG analogues displayed phase coexistence during the lamellar to H₂ transition. The structural data also showed that the headgroups of the MMG analogues adopting the alternative lipid acid configuration were ordered and formed a two-dimensional molecular superlattice, which was conserved regardless of the lipid tail length. To our knowledge, the MMG analogues with an alternative lipid acid configuration represent the first example of a lipid system showing both interdigitation and superlattice formation, and as such could serve as an interesting model system for future studies. The MMG analogues are also relevant from a subunit vaccine perspective because they are well-tolerated and display promising immunopotentiating activity. The structural characterization described here will serve as a prerequisite for the rational design of nanoparticulate adjuvants with specific and tailored structural features.

C-Type Lectin Receptor DCAR Recognizes Mycobacterial Phosphatidyl-Inositol Mannosides to Promote a Th1 Response during Infection

Phosphatidyl-inositol mannosides (PIM) are glycolipids unique to mycobacteria and other related bacteria that stimulate host immune responses and are implicated in mycobacteria pathogenicity. Here, we found that the FcRγ-coupled C-type lectin receptor DCAR (dendritic cell immunoactivating receptor; gene symbol Clec4b1) is a direct receptor for PIM. Mycobacteria activated reporter cells expressing DCAR, and delipidation of mycobacteria abolished this activity. Acylated PIMs purified from mycobacteria were identified as ligands for DCAR. DCAR was predominantly expressed in small peritoneal macrophages and monocyte-derived inflammatory cells in lungs and spleen. These cells produced monocyte chemoattractant protein-1 (MCP-1) upon PIM treatment, and absence of DCAR or FcRγ abrogated MCP-1 production. Upon mycobacterial infection, Clec4b1-deficient mice showed reduced numbers of monocyte-derived inflammatory cells at the infection site, impaired IFNγ production by T cells, and an increased bacterial load. Thus, DCAR is a critical receptor for PIM that functions to promote T cell responses against mycobacteria.

Novel adjuvant formulations for delivery of anti-tuberculosis vaccine candidates

There is an urgent need for a new and improved vaccine against tuberculosis for controlling this disease that continues to pose a global health threat. The current research strategy is to replace the present BCG vaccine or boost BCG-immunity with subunit vaccines such as viral vectored- or protein-based vaccines. The use of recombinant proteins holds a number of production advantages including ease of scalability, but requires an adjuvant inducing cell-mediated immune responses. A number of promising novel adjuvant formulations have recently been designed and show evidence of induction of cellular immune responses in humans. A common trait of effective TB adjuvants including those already in current clinical testing is a two-component approach combining a delivery system with an appropriate immunomodulator. This review summarizes the status of current TB adjuvant research with a focus on the division of labor between delivery systems and immunomodulators.

Ac2PIM-responsive miR-150 and miR-143 target receptor-interacting protein kinase 2 and transforming growth factor beta-activated kinase 1 to suppress NOD2-induced immunomodulators

Specific and coordinated regulation of innate immune receptor-driven signaling networks often determines the net outcome of the immune responses. Here, we investigated the cross-regulation of toll-like receptor (TLR)2 and nucleotide-binding oligomerization domain (NOD)2 pathways mediated by Ac2PIM, a tetra-acylated form of mycobacterial cell wall component and muramyl dipeptide (MDP), a peptidoglycan derivative respectively. While Ac2PIM treatment of macrophages compromised their ability to induce NOD2-dependent immunomodulators like cyclooxygenase (COX)-2, suppressor of cytokine signaling (SOCS)-3, and matrix metalloproteinase (MMP)-9, no change in the NOD2-responsive NO, TNF-α, VEGF-A, and IL-12 levels was observed. Further, genome-wide microRNA expression profiling identified Ac2PIM-responsive miR-150 and miR-143 to target NOD2 signaling adaptors, RIP2 and TAK1, respectively. Interestingly, Ac2PIM was found to activate the SRC-FAK-PYK2-CREB cascade via TLR2 to recruit CBP/P300 at the promoters of miR-150 and miR-143 and epigenetically induce their expression. Loss-of-function studies utilizing specific miRNA inhibitors establish that Ac2PIM, via the miRNAs, abrogate NOD2-induced PI3K-PKCδ-MAPK pathway to suppress β-catenin-mediated expression of COX-2, SOCS-3, and MMP-9. Our investigation has thus underscored the negative regulatory role of Ac2PIM-TLR2 signaling on NOD2 pathway which could broaden our understanding on vaccine potential or adjuvant utilities of Ac2PIM and/or MDP.

Targeting myeloid cells using nanoparticles to improve cancer immunotherapy

While nanoparticles have traditionally been used to deliver cytotoxic drugs directly to tumors to induce cancer cell death, emerging data suggest that nanoparticles are likely to generate a larger impact on oncology through the delivery of agents that can stimulate antitumor immunity. Tumor-targeted nanocarriers have generally been used to localize chemotherapeutics to tumors and thus decrease off-target toxicity while enhancing efficacy. Challengingly, tumor heterogeneity and evolution render tumor-intrinsic approaches likely to succumb to relapse. The immune system offers exquisite specificity, cytocidal potency, and long-term activity that leverage an adaptive memory response. For this reason, the ability to manipulate immune cell specificity and function would be desirable, and nanoparticles represent an exciting means by which to perform such manipulation. Dendritic cells and tumor-associated macrophages are cells of the myeloid lineage that function as natural phagocytes, so they naturally take up nanoparticles. Dendritic cells direct the specificity and potency of cellular immune responses that can be targeted for cancer vaccines. Herein, we discuss the specific criteria needed for efficient vaccine design, including but not limited to the route of administration, size, morphology, surface charge, targeting ligands, and nanoparticle composition. In contrast, tumor-associated macrophages are critical mediators of immunosuppression whose trans-migratory abilities can be exploited to localize therapeutics to the tumor core and which can be directly targeted for elimination or for repolarization to a tumor suppressive phenotype. It is likely that a combination of targeting dendritic cells to stimulate antitumor immunity and tumor-associated macrophages to reduce immune suppression will impart significant benefits and result in durable antitumor responses.

Highly purified mycobacterial phosphatidylinositol mannosides drive cell-mediated responses and activate NKT cells in cattle

Mycobacterial lipids play an important role in the modulation of the immune response upon contact with the host. Using novel methods, we have isolated highly purified phosphatidylinositol mannoside (PIM) molecules (phosphatidylinositol dimannoside [PIM2], acylphosphatidylinositol dimannoside [AcPIM2], diacyl-phosphatidylinositol dimannoside [Ac2PIM2], acylphosphatidylinositol hexamannoside [AcPIM6], and diacylphosphatidylinositol hexamannoside [Ac2PIM6]) from virulent Mycobacterium tuberculosis to assess their potential to stimulate peripheral blood mononuclear cell (PBMC) responses in Mycobacterium bovis-infected cattle. Of these molecules, one (AcPIM6) induced significant levels of gamma interferon (IFN-γ) in bovine PBMCs. Three PIM molecules (AcPIM6, Ac2PIM2, and Ac2PIM6) were shown to drive significant proliferation in bovine PBMCs. AcPIM6 was subsequently used to phenotype the proliferating cells by flow cytometry. This analysis demonstrated that AcPIM6 was predominantly recognized by CD3(+) CD335(+) NKT cells. In conclusion, we have identified PIM lipid molecules that interact with bovine lymphocyte populations, and these lipids may be useful as future subunit vaccines or diagnostic reagents. Further, these data demonstrate, for the first time, lipid-specific NKT activation in cattle.

Mycobacterial outer membrane is a lipid bilayer and the inner membrane is unusually rich in diacyl phosphatidylinositol dimannosides

Mycobacterium species, including the human pathogen Mycobacterium tuberculosis, are unique among Gram-positive bacteria in producing a complex cell wall that contains unusual lipids and functions as a permeability barrier. Lipids in the cell wall were hypothesized to form a bilayer or outer membrane that would prevent the entry of chemotherapeutic agents, but this could not be tested because of the difficulty in extracting only the cell-wall lipids. We used reverse micellar extraction to achieve this goal and carried out a quantitative analysis of both the cell wall and the inner membrane lipids of Mycobacterium smegmatis. We found that the outer leaflet of the outer membrane contains a similar number of hydrocarbon chains as the inner leaflet composed of mycolic acids covalently linked to cell-wall arabinogalactan, thus validating the outer membrane model. Furthermore, we found that preliminary extraction with reverse micelles permitted the subsequent complete extraction of inner membrane lipids with chloroform-methanol-water, revealing that one-half of hydrocarbon chains in this membrane are contributed by an unusual lipid, diacyl phosphatidylinositol dimannoside. The inner leaflet of this membrane likely is composed nearly entirely of this lipid. Because it contains four fatty acyl chains within a single molecule, it may produce a bilayer environment of unusually low fluidity and may slow the influx of drugs, contributing to the general drug resistance phenotype of mycobacteria.

Role of B-1 cells in the immune response against an antigen encapsulated into phosphatidylcholine-containing liposomes

B-1 lymphocytes comprise a unique subset of B cells that differ phenotypically, ontogenetically and functionally from conventional B-2 cells. A frequent specificity of the antibody repertoire of peritoneal B-1 cells is phosphatidylcholine. Liposomes containing phosphatidylcholine have been studied as adjuvants and their interaction with dendritic cells and macrophages has been demonstrated. However, the role of B-1 cells in the adjuvanticity of liposomes composed of phosphatidylcholine has not been explored. In the present work, we studied the contribution of B-1 cells to the humoral response against ovalbumin (OVA) encapsulated into dipalmitoylphosphatidylcholine (DPPC) and cholesterol-containing liposomes. BALB/X-linked immunodeficient (xid) mice, which are deficient in B-1 cells, showed quantitative and qualitative differences in the anti-OVA antibody response compared with wild-type animals after immunization with these liposomes. The OVA-specific immune response was significantly increased in the BALB/xid mice when reconstituted with B-1 cells from naive BALB/c mice. Our results indicate the internalization of DPPC-containing liposomes by these cells and their migration from the peritoneal cavity to the spleen. Phosphatidylcholine significantly contributed to the immunogenicity of liposomes, as DPPC-containing liposomes more effectively stimulated the anti-OVA response compared with vesicles composed of dipalmitoylphosphatidylglycerol. In conclusion, we present evidence for a cognate interaction between B-1 cells and phosphatidylcholine liposomes, modulating the immune response to encapsulated antigens. This provides a novel targeting approach to assess the role of B-1 cells in humoral immunity.

Targeting C-type lectin receptors with multivalent carbohydrate ligands

C-type lectin receptors (CLRs) represent a large receptor family including collectins, selectins, lymphocyte lectins, and proteoglycans. CLRs share a structurally homologous carbohydrate-recognition domain (CRD) and often bind carbohydrates in a Ca²⁺-dependent manner. In innate immunity, CLRs serve as pattern recognition receptors (PRRs) and bind to the glycan structures of pathogens and also to self-antigens. In nature, the low affinity of CLR/carbohydrate interactions is overcome by multivalent ligand presentation at the surface of cells or pathogens. Thus, multivalency is a promising strategy for targeting CLR-expressing cells and, indeed, carbohydrate-based targeting approaches have been employed for a number of CLRs, including asialoglycoprotein receptor (ASGPR) in the liver, or DC-SIGN expressed by dendritic cells. Since CLR engagement not only mediates endocytosis but also influences intracellular signaling pathways, CLR targeting may allow for cell-specific drug delivery and also the modulation of cellular functions. Glyconanoparticles, glycodendrimers, and glycoliposomes were successfully used as tools for CLR-specific targeting. This review will discuss different approaches for multivalent CLR ligand presentation and aims to highlight how CLR targeting has been employed for cell specific drug delivery. Major emphasis is directed towards targeting of CLRs expressed by antigen-presenting cells to modulate immune responses.

Physicochemical and biological characterization of synthetic phosphatidylinositol dimannosides and analogues

Native phosphatidylinositol mannosides (PIMs), isolated from the cell wall of Mycobacterium bovis, and synthetic PIM analogues have been reported to offer a variety of immunomodulating properties, including both suppressive and stimulatory activity. While numerous studies have examined the biological activity of these molecules, the aim of this research was to assess the physicochemical properties at a molecular level and correlate these characteristics with biological activity in a mouse model of airway eosinophilia. To accomplish this, we varied the flexibility and lipophilicity of synthetic PIMs by changing the polar headgroup (inositol- vs glycerol-based core) and the length of the acyl chains of the fatty acid residues (C0, C10, C16, and C18). A series of six phosphatidylinositol dimannosides (PIM2s) and phosphatidylglycerol dimannosides (PGM2s) were synthesized and characterized in this study. Langmuir monolayer studies showed that surface pressure-area (π-A) isotherms were greatly influenced by the length of the lipid acyl chains as well as the steric hindrance and volume of the headgroups. In aqueous solution, lipidated PIM2 and PGM2 compounds were observed to self-assemble into circular aggregates, as confirmed by dynamic light scattering and transmission electron microscopic investigations. Removal of the inositol ring but retention of the three-carbon glycerol unit maintained biological activity. We found that the deacylated PGM2, which did not show self-organization, had no effect on the eosinophil numbers but did have an impact on the expansion of OVA-specific CD4(+) Vα2Vβ5 T cells.

Phosphatidylinositol di-mannoside and derivates modulate the immune response to and efficacy of a tuberculosis protein vaccine against Mycobacterium bovis infection

Mycobacterium bovis infects a wide range of hosts, including domestic livestock, wildlife, and humans. Development of an effective vaccine protecting against bovine tuberculosis would provide a cost-effective tuberculosis control strategy. The objective of this study was to investigate the ability of phosphatidylinositol di-mannoside (PIM(2)) and its derivatives to modulate cell-mediated immunity in vivo in a bovine tuberculosis mouse model in response to a relevant antigen, namely a fusion protein of mycobacterial proteins Ag85A and ESAT-6. The addition of synthetic PIM(2) to the vaccine resulted in a significant reduction in lung bacterial counts and a cytokine profile indicating a Th 1 type immune response. The addition of the other PIM(2) derivatives to the vaccine or the fusion protein alone did not result in reduced lung bacterial counts; moreover, the addition of PIM(2)ME appeared to negate the induction of an antigen-specific interferon-γ response and protection against tuberculosis. In conclusion, this study provides further evidence that PIMs can function as potent adjuvants for protein or sub-unit vaccines, but subtle structural differences among PIMs can markedly alter the type of immune response induced.

Synthesis and Toll-like receptor 4 (TLR4) activity of phosphatidylinositol dimannoside analogues

A series of five PIM(2) analogues were synthesized and tested for their ability to activate primary macrophages and modulate LPS signaling. Structural changes included replacement of the fatty acid esters of the phosphatidyl moiety of PIM(2) with the corresponding ether or amide. An AcPIM(2) analogue possessing an ether linkage was also prepared. The synthetic methodology utilized an orthogonally protected chiral myo-inositol starting material that was conveniently prepared from myo-inositol in just two steps. Important steps in the synthetic protocols included the regio- and α-selective glycosylation of inositol O-6 and introduction of the phosphodiester utilizing phosphoramidite chemistry. Replacement of the inositol core with a glycerol moiety gave compounds described as phosphatidylglycerol dimannosides (PGM(2)). Biological testing of these PIM compounds indicated that the agonist activity was TLR4 dependent. An ether linkage increased agonist activity. Removal of the inositol ring enhanced antagonist activity, and the presence of an additional lipid chain enhanced LPS-induced cytokine production in primary macrophages. Furthermore, the interruption of the LPS-induced 2:2 TLR4/MD-2 signaling complex formation by PIM(2) represents a previously unidentified mechanism involved in the bioactivity of PIM molecules.

Chemical synthesis and immunosuppressive activity of dipalmitoyl phosphatidylinositol hexamannoside

Phosphatidylinositol mannosides (PIMs) isolated from mycobacteria have been identified as an important class of phosphoglycolipids with significant immune-modulating properties. We present here the synthesis of dipalmitoyl phosphatidylinositol hexamannoside (PIM(6)) 1 and the first reported functional biology of a synthetic PIM(6). Key steps in the synthetic protocol included the selective glycosylation of an inositol 2,6-diol with a suitably protected mannosyl donor and construction of the glycan core utilizing a [3 + 4] thio-glycosylation strategy. The target 1 was purified by reverse phase chromatography and characterized by standard spectroscopic methods, HPLC, and chemical modification by deacylation to dPIM(6). The (1)H NMR spectrum of synthetic dPIM(6) obtained from 1 matched that of dPIM(6) obtained from nature. PIM(6) (1) exhibited dendritic cell-dependent suppression of CD8(+) T cell expansion in a human mixed lymphocyte reaction consistent with the well established immunosuppressive activity of whole mycobacteria.

Incorporation of a synthetic mycobacterial monomycoloyl glycerol analogue stabilizes dimethyldioctadecylammonium liposomes and potentiates their adjuvant effect in vivo

The combination of delivery systems such as cationic liposomes and immunopotentiating molecules is a promising approach for the rational design of vaccine adjuvants. In this study, a synthetic analogue of the mycobacterial lipid monomycoloyl glycerol (MMG), referred to as MMG-1, was synthesized and combined with the cationic surfactant dimethyldioctadecylammonium (DDA). The purpose of the study was to provide a thorough pharmaceutical characterization of the resulting DDA/MMG-1 binary system and to evaluate how incorporation of MMG-1 affected the adjuvant activity of DDA liposomes. Thermal analyses demonstrated that MMG-1 was incorporated into the DDA lipid bilayers, and cryo-transmission electron microscopy (TEM) confirmed that liposomes were formed. The particles had a polydisperse size distribution and an average diameter of approximately 400 nm. Evaluation of the colloidal stability indicated that at least 18 mol% MMG-1 was required to stabilize the DDA liposomes as the average particle size remained constant during storage for 6 months. The improved colloidal stability is most likely caused by increased hydration of the lipid bilayer. This was demonstrated by studying Langmuir-Blodgett monolayers of DDA and MMG-1 which revealed an increased surface pressure in the presence of high concentrations of MMG-1 when the DDA/MMG-1 monolayers were fully compressed, indicating an increased interaction with water due to enhanced hydration of the lipid head groups. Finally, immunization of mice with the tuberculosis fusion antigen Ag85B-ESAT-6 and DDA/MMG-1 liposomes induced a strong cell-mediated immune response characterized by a mixed Th1/Th17 profile and secretion of IgG1 and IgG2c antibodies. The Th1/Th17-biased immunostimulatory effect was increased in an MMG-1 concentration-dependent manner with maximal observed effect at 31 mol% MMG-1. Thus, incorporation of 31 mol% MMG-1 into DDA liposomes results in an adjuvant system with favorable physical as well as immunological properties.

A synthetic analogue of phosphatidylinositol mannoside is an efficient adjuvant

We recently described the synthesis of an ether linked analogue of phosphatidylinositol dimannoside (PIM(2)ME). In the current study, PIM(2)ME was found to significantly enhance the release of the key Th1 cytokine interleukin-12 (IL-12) by dendritic cells (DCs) of naive mice in vitro, but not interleukin-10 (IL-10). Based on this result, it was hypothesized that PIM(2)ME would be an effective adjuvant for cell-mediated immune responses. Injections of PIM(2)ME alone did not lead to weight loss and did not have toxic side effects, based on biomarkers of toxicity in serum,demonstrating that the compound induced no apparent adverse side effects. Mice were vaccinated with the core antigens of the hepatitis C virus by itself or with three different adjuvants, namely PIM(2)ME, a commercial preparation of monophosphoryl lipid A (MPL) or a preparation of aluminium hydroxide gel (alum). A control group of animals received the antigen only with no adjuvants. Immune responses to the Hepatitis C viral antigens were monitored by measuring antigen-specific production of interferon-gamma (IFN-gamma), the p40 subunit of interleukin-12 (IL-12) and interleukin-10 (IL-10) to assess cell-mediated immune responses. Vaccination of mice with Hepatitis C viral antigens with the adjuvant PIM(2)ME led to a significant increase in cell-mediated immune responses (IFN-gamma and IL-12). Injection of Hepatitis C viral antigens in alum led to no enhancement of the cell-mediated immune response. We conclude that PIM(2)ME is an efficacious adjuvant for enhancing cell-mediated immunity, and induces no observable adverse effects.

Liposome-based vaccines

Here, we report methods of preparation of liposome vaccine formulations for the entrapment of antigenic peptides and antigen encoding plasmid DNAs. Two examples of liposomal vaccine formulations producing highly effective immune responses are given. Firstly, a formulation with encapsulated antigenic peptides derived from the hepatitis C virus NS4 and the core proteins, and secondly, the encapsulation of a plasmid DNA encoding the gp33 glycoprotein of the lymphocytic choriomeningitis virus (LCMV). Vaccination with liposomal HCV peptides in HLA-A2 transgenic mice by subcutaneous injections induced strong cytotoxic T cell responses as shown by lysis of human target cells expressing HCV proteins. The immunogenicity of the liposomal peptide vaccines was further enhanced by incorporation of immunostimulatory CpG oligonucleotide sequences, shown by a strong increase of the frequency of IFN-gamma secreting cells that persisted at high levels for long periods of time. With the LCMV model, we could show that upon intradermal injection, plasmid-DNA liposomes formed LCMV gp33 antigen depots facilitating long-lasting in vivo antigen loading of dendritic cells (DC), followed by a strong immune response. Our data show that liposomal formulations of peptide or plasmid-DNA vaccines are highly effective at direct in vivo antigen loading and activation of DC leading to protective antiviral and anti-tumor immune responses.

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).

Archaeosome adjuvants: immunological capabilities and mechanism(s) of action

Archaeosomes (liposomes comprised of glycerolipids of Archaea) constitute potent adjuvants for the induction of Th1, Th2 and CD8(+) T cell responses to the entrapped soluble antigen. Archaeal lipids are uniquely constituted of ether-linked isoprenoid phytanyl cores conferring stability to the membranes. Additionally, varied head groups displayed on the glycerol-lipid cores facilitate unique immunostimulating interactions with mammalian antigen-presenting cells (APCs). The polar lipid from the archaeon, Methanobrevibacter smithii has been well characterized for its adjuvant potential, and is abundant in archaetidyl serine, promoting interaction with a phosphatidylserine receptor on APCs. These archaeosomes mediate MHC class I cross-priming via the phagosome-to-cytosol TAP-dependent classical processing pathway, and also upregulate costimulation by APCs without overt inflammatory cytokine production. Furthermore, they facilitate potent CD8(+) T cell memory to co-delivered antigen, comparable in magnitude and quality to live bacterial vaccine vectors. Archaeosome vaccines provide profound protection in murine models of infection and cancer. This technology is being developed for clinical application and offers a novel prospect for rational design and development of safe and potent subunit vaccines capable of eliciting T cell immunity against intracellular infections and cancers.

Distribution coefficient of rifabutin in liposome/water system as measured by different methods

Distribution coefficient (D) of rifabutin in liposome/water system was measured by phase separation and fluorescence probe quenching techniques. D values were identical suggesting that rifabutin is fully immersed into lipid bilayer. Structural studies of phospholipid bilayer employing (31)P NMR spectroscopy demonstrated that introduction of rifabutin does not alter the bilayer structure. A scheme of the rifabutin position in lipid bilayer based on the calculated size of rifabutin molecule is proposed.

A reduced antigen load in vivo, rather than weak inflammation, causes a substantial delay in CD8+ T cell priming against Mycobacterium bovis (bacillus Calmette-Guérin)

Regardless of the dose of Ag, Ag presentation occurs rapidly within the first few days which results in rapid expansion of the CD8+ T cell response that peaks at day 7. However, we have previously shown that this rapid priming of CD8+ T cells is absent during infection of mice with Mycobacterium bovis (bacillus Calmette-Guérin (BCG)). In this study, we have evaluated the mechanisms responsible for the delayed CD8+ T cell priming. Because BCG replicates poorly and survives within phagosomes we considered whether 1) generation of reduced amounts of Ag or 2) weaker activation by pathogen-associated molecular patterns (PAMPs) during BCG infection is responsible for the delay in CD8+ T cell priming. Using rOVA-expressing bacteria, our results indicate that infection of mice with BCG-OVA generates greatly reduced levels of OVA, which are 70-fold lower in comparison to the levels generated during infection of mice with Listeria monocytogenes-expressing OVA. Furthermore, increasing the dose of OVA, but not PAMP signaling during BCG-OVA infection resulted in rapid Ag presentation and consequent expansion of the CD8+ T cell response, indicating that the generation of reduced Ag levels, not lack of PAMP-associated inflammation, was responsible for delayed priming of CD8+ T cells. There was a strong correlation between the relative timing of Ag presentation and the increase in the level of OVA in vivo. Taken together, these results reveal that some slowly replicating pathogens, such as mycobacteria, may facilitate their chronicity by generating reduced Ag levels which causes a substantial delay in the development of acquired immune responses.

Phosphatidylinositol Mannoside Ether Analogues: Syntheses and Interleukin-12-Inducing Properties

Phosphatidylinositol mannosides (PIMs) isolated from mycobacteria have been identified as an important class of glycolipids with significant immune modulating properties. We present here the syntheses of phosphatidylinositol dimannoside ether analogues 2 and 3 and evaluate their interleukin-12 (IL-12)-inducing properties along with dipalmitoyl PIM2 (1) in an in vitro bovine dendritic cell assay. Both synthetic PIM analogues and synthetic dipalmitoyl PIM2 (1) were effective at enhancing IL-12 production by immature bovine dendritic cells. Unexpectedly, ether analogue 2 was significantly more active than dipalmitoyl PIM2 (1) which indicates that modified PIM compounds can be strongly immunoactive and may have significant adjuvant activities.

Intraperitoneal immunization with oligomannose-coated liposome-entrapped soluble leishmanial antigen induces antigen-specific T-helper type immune response in BALB/c mice through uptake by peritoneal macrophages

The present study demonstrates that the intraperitoneal administration of soluble leishmanial antigen (SLA) entrapped in liposomes coated with neoglycolipids containing oligomannose residues (mannopentaose or mannotriose) strongly induces an antigen-specific T-helper type 1 (Th1) immune response in BALB/c mice. In response to in vitro stimulation with SLA, spleen cells from mice that had received oligomannose-coated liposomes encasing SLA (SLA-OML) displayed greater interferon (IFN)-gamma and interleukin (IL)-2 production and lower IL-4 and IL-5 production than spleen cells from mice that had received SLA alone, indicating that the SLA-specific Th1 immune response had predominantly been induced in the mice that had received SLA-OML. After subsequent infection with Leishmania major, mice that had received SLA-OML were effectively protected against the disease, with a predominant production of IFN-gamma. OML were preferentially and rapidly incorporated into peritoneal macrophages, and the transplantation of macrophages containing SLA-OML into the peritoneal cavity also induced protection against L. major infection. Thus, SLA-OML were shown to successfully induce a specific Th1 immune response capable of controlling L. major infection in BALB/c mice through the effective uptake of OML by peritoneal macrophages.

New packaging method of mycobacterial cell wall using octaarginine-modified liposomes: enhanced uptake by and immunostimulatory activity of dendritic cells

Despite the potential of mycobacterial cell wall (CW) components to serve as immunotherapeutic agents, this application is hampered by the molecules' unfavorable physicochemical properties, such as its high molecular weight, poor solubility and negatively charged nature. Here we describe a new mycobacterial CW delivery system that uses an efficient and simple packaging method. This is achieved by incorporating mycobacterial CW into liposomes and attaching arginine octamers (R8) to the liposome surface. R8-modified liposomes improve the uptake of mycobacterial CW by dendritic cells (DC) and enhance its immunostimulatory activity. High R8 surface density promoted high levels of mycobacterial CW uptake by DC compared to low density R8-modified liposomes. Maturation markers (CD80, CD86, MHC Class II molecules) showed significantly enhanced expression on DC pulsed with high density R8-modified liposomes containing mycobacterial CW. Moreover, R8-modified liposomes with mycobacterial CW incorporated induced production of IL-12 p40 by DC, at levels similar to those produced by lipopolysaccharide-pulsed DC. We assert that R8-modified liposomes with mycobacterial CW incorporated should have tremendous potential as immune-potentiating agents.

TLR2 but not TLR4 signalling is critically involved in the inhibition of IFN-gamma-induced killing of mycobacteria by murine macrophages

Gamma-interferon (IFN-gamma) plays a determinant role in activating macrophages that are critical to control Mycobacterium tuberculosis infection. However, M. tuberculosis can escape killing by attenuating the response of macrophages to IFN-gamma by blocking the transcription of a subset of IFN-gamma inducible genes. This inhibition occurs after signalling through Toll-like receptor 2 (TLR2). While most studies have investigated the inhibition of IFN-gamma responsive genes after TLR2 signalling, the present study focuses on the functional implications of inhibition of IFN-gamma signalling in macrophages with regard to mycobacteria killing. Here, we provide evidence that exposure of the murine macrophage cell line J774 to the TLR2 ligands; 19-kDa or zymosan, but not the TLR4 ligand LPS, inhibits IFN-gamma-induced killing of Mycobacterium bovis Bacillus Calmette-Guérin (BCG). Moreover, exposure of bone marrow-derived macrophages (BMM) from TLR4-deficient and wild-type (WT), but not from TLR2-deficient mice to 19-kDa lipoprotein (19-kDa) or zymosan, results in an impairment of IFN-gamma-mediated killing. We demonstrate that 19-kDa and zymosan inhibit the ability of IFN-gamma to activate murine macrophages to kill BCG without inhibiting nitric oxide (NO) or tumour necrosis factor (TNF) production. Finally, we demonstrate that the inhibitory effect of 19-kDa on IFN-gamma signalling is overcome with increasing amounts of IFN-gamma indicating that the refractoriness could be reversed at optimal IFN-gamma concentrations. The critical role of TLR2 but not TLR4 signalling in the inhibition of IFN-gamma promoted killing of mycobacteria is discussed.

Phosphatidylinositol mannoside from Mycobacterium tuberculosis binds alpha5beta1 integrin (VLA-5) on CD4+ T cells and induces adhesion to fibronectin

The pathological hallmark of the host response to Mycobacterium tuberculosis is the granuloma where T cells and macrophages interact with the extracellular matrix (ECM) to control the infection. Recruitment and retention of T cells within inflamed tissues depend on adhesion to the ECM. T cells use integrins to adhere to the ECM, and fibronectin (FN) is one of its major components. We have found that the major M. tuberculosis cell wall glycolipid, phosphatidylinositol mannoside (PIM), induces homotypic adhesion of human CD4+ T cells and T cell adhesion to immobilized FN. Treatment with EDTA and cytochalasin D prevented PIM-induced T cell adhesion. PIM-induced T cell adhesion to FN was blocked with mAbs against alpha5 integrin chain and with RGD-containing peptides. Alpha5beta1 (VLA-5) is one of two major FN receptors on T cells. PIM was found to bind directly to purified human VLA-5. Thus, PIM interacts directly with VLA-5 on CD4+ T lymphocytes, inducing activation of the integrin, and promoting adhesion to the ECM glycoprotein, FN. This is the first report of direct binding of a M. tuberculosis molecule to a receptor on human T cells resulting in a change in CD4+ T cell function.

Fine discrimination in the recognition of individual species of phosphatidyl-myo-inositol mannosides from Mycobacterium tuberculosis by C-type lectin pattern recognition receptors

The Mycobacterium tuberculosis (M.tb) envelope is highly mannosylated with phosphatidyl-myo-inositol mannosides (PIMs), lipomannan, and mannose-capped lipoarabinomannan (ManLAM). Little is known regarding the interaction between specific PIM types and host cell C-type lectin pattern recognition receptors. The macrophage mannose receptor (MR) and dendritic cell-specific ICAM-3-grabbing nonintegrin on dendritic cells engage ManLAM mannose caps and regulate several host responses. In this study, we analyzed the association of purified PIM families (f, separated by carbohydrate number) and individual PIM species (further separated by fatty acid number) from M.tb H(37)R(v) with human monocyte-derived macrophages (MDMs) and lectin-expressing cell lines using an established bead model. Higher-order PIMs preferentially associated with the MR as demonstrated by their reduced association with MDMs upon MR blockade and increased binding to COS-1-MR. In contrast, the lower-order PIM(2)f associated poorly with MDMs and did not bind to COS-1-MR. Triacylated PIM species were recognized by MDM lectins better than tetra-acylated species and the degree of acylation influenced higher-order PIM association with the MR. Moreover, only higher-order PIMs that bind the MR showed a significant increase in phagosome-lysosome fusion upon MR blockade. In contrast with the MR, the PIM(2)f and lipomannan were recognized by DC-SIGN comparable to higher-order PIMs and ManLAM, and the association was independent of their degree of acylation. Thus, recognition of M.tb PIMs by host cell C-type lectins is dependent on both the nature of the terminal carbohydrates and degree of acylation. Subtle structural differences among the PIMs impact host cell recognition and response and are predicted to influence the intracellular fate of M.tb.

Determinant role for Toll-like receptor signalling in acute mycobacterial infection in the respiratory tract

Toll-like receptors (TLRs) are a vital component of the innate branch of the immune system in its battle against mycobacterial infections. Extensive in vitro studies have demonstrated a role for both TLR2 and TLR4 in recognition of mycobacterial components, whereas the in vivo situation appears less clear, with results depending on the infection model. In the present work, the importance of TLR signalling in the course of mycobacterial infection was investigated in a human-like infection model using TLR-knockout mice. TLR2(-/-) and TLR4(-/-) mice infected with Mycobacterium tuberculosis by aerosol, or for the first time, intranasally with Mycobacterium bovis bacillus Calmette-Guérin (BCG), displayed increased susceptibility at an early stage of infection in the respiratory tract, while at a later stage of infection, the TLR deficiency appeared to be overcome. The higher susceptibility was correlated to impaired pro-inflammatory responses to BCG components, and reduced induction of anti-bacterial activity by infected macrophages from TLR2(-/-) mice, and to a lesser extent from TLR4(-/-) mice. These findings demonstrate a role for TLR signalling in protection against mycobacterial infection specifically in the respiratory tract at the acute phase, whereas the TLR deficiency can be compensated at a later stage of infection.

Mannose receptor-targeted vaccines

Targeting antigens to endocytic receptors on professional antigen-presenting cells (APCs) represents an attractive strategy to enhance the efficacy of vaccines. Such APC-targeted vaccines have an exceptional ability to guide exogenous protein antigens into vesicles that efficiently process the antigen for major histocompatibility complex class I and class II presentation. Efficient targeting not only requires high specificity for the receptor that is abundantly expressed on the surface of APCs, but also the ability to be rapidly internalised and loaded into compartments that contain elements of the antigen-processing machinery. The mannose receptor (MR) and related C-type lectin receptors are particularly designed to sample antigens (self and non-self), much like pattern recognition receptors, to integrate the innate with adaptive immune responses. In fact, a variety of approaches involving delivery of antigens to the MR have demonstrated effective induction of potent cellular and humoral immune responses. Yet, although several lines of evidence in diverse experimental systems attest to the efficacy of targeted vaccine strategies, it is becoming increasingly clear that additional signals, such as those afforded by adjuvants, may be critical to elicit sustained immunity. Therefore, MR-targeted vaccines are likely to be most efficacious in vivo when combined with agents that elicit complementary activation signals. Certainly, a better understanding of the mechanism associated with the induction of immune responses as a result of targeting antigens to the MR, will be important in exploiting MR-targeted vaccines not only for mounting immune defenses against cancer and infectious disease, but also for specific induction of tolerance in the treatment of autoimmune disease.

Activation of antigen-presenting cells by DNA delivery vectors

Gene-based modulation of immune functions is a promising means of eliciting protective immunity and induction of tolerance. Novel viral and non-viral DNA delivery systems are being investigated to achieve efficient gene transfer into mammalian cells. Antigen-presenting cells (APCs), in particular dendritic cells, are crucial targets in this context due to their capacity to initiate and direct effector functions. The increasing relevance of APCs as targets of DNA vectors calls for an assessment of vector-driven activation of these cells. For viral vectors, a putative pathway of APC activation would be Toll-like receptor signalling for certain RNA genome viruses. On the other hand, non-viral vectors appear to mature APCs by interaction of polymeric particulates or bioactive lipids with cellular mechanisms. The rational design of DNA-based therapies is possible only when the intrinsic effects of the vector and immune modulation originating from the DNA are delineated. This paper will summarise recent reports of adjuvant properties of viral and non-viral delivery systems.