Triacyl-lipopentapeptide adjuvants: TLR2-dependent activation of macrophages and modulation of receptor-mediated cell activation by altering acyl-moieties

A purified recombinant lipopeptide as adjuvant for cancer immunotherapy

Synthetic lipopeptides have been widely used as vaccine adjuvants to enhance immune responses. The present study demonstrated that the tryptic N-terminal fragment of the lipoprotein rlipo-D1E3 (lipo-Nter) induces superior antitumor effects compared to a synthetic lipopeptide. The lipo-Nter was purified and formulated with protein or peptide vaccines to determine if lipo-Nter could be used as a novel adjuvant and could induce antitumor immunity in a cervical cancer model. Purified lipo-Nter activated the maturation of bone marrow-derived dendritic cells (BM-DCs), leading to the secretion of TNF-α through TLR2/6 but not TLR1/2. A recombinant mutant HPV16 E7 (rE7m) protein was mixed with lipo-Nter to immunize the mice; the anti-E7 antibody titers were increased, and the T helper cells were skewed toward the Th1 fate (increased IL-2 and decreased IL-5 secretion). Single-dose injection of rE7m and lipo-Nter inhibited tumor growth, but the injection of rE7m alone did not. Accordingly, lipo-Nter also enhanced the antitumor immunity of the E7-derived peptide but not the synthetic lipopeptide (Pam3CSK4). We demonstrated that the lipo-Nter of a bacterial-derived recombinant lipoprotein is a novel adjuvant that could be used for the development of a new generation of vaccines.

Single nucleotide polymorphisms in toll-like receptor 6 are associated with altered lipopeptide- and mycobacteria-induced interleukin-6 secretion

Toll-like receptors (TLRs) are critical mediators of the immune response to pathogens. The influence of human TLR6 polymorphisms on susceptibility to infection is only partially understood. Most microbes contain lipopeptides recognized by TLR2/1 or TLR2/6 heterodimers. Our aim was to determine whether single nucleotide polymorphisms (SNPs) in TLR6 are associated with altered immune responses to lipopeptides and whole mycobacteria.

We sequenced the TLR6 coding region in 100 healthy South African adults to assess genetic variation and determined associations between polymorphisms and lipopeptide- and mycobacteria-induced IL-6 production in whole blood. We found 2 polymorphisms, C745T and G1083C that were associated with altered IL-6 secretion. G1083C was associated with altered IL-6 levels in response to lipopeptides, Mycobacterium tuberculosis lysate (Mtb, P = 0.018) and BCG (P = 0.039). The 745T allele was also associated with lower NF-κB signaling in response to di-acylated lipopeptide, PAM2 (P = 0.019) or Mtb (P = 0.026) in a HEK293 cell line reconstitution assay, compared with the 745C allele.

We conclude that TLR6 polymorphisms may be associated with altered lipopeptide-induced cytokine responses and recognition of Mtb. These studies provide new insight into the role of TLR6 variation and the innate immune response to human infection.

Toll-like receptor signaling links dietary fatty acids to the metabolic syndrome

PURPOSE OF REVIEW

Dietary saturated fatty acids (SFAs) have been implicated in promoting the metabolic syndrome and atherosclerotic cardiovascular disease. Recent evidence suggests that SFAs promote the metabolic syndrome by activating Toll-like receptor 4 (TLR4). Here we examine emerging molecular evidence that SFAs directly engage pathways of innate immunity, thereby promoting inflammatory aspects of the metabolic syndrome.

RECENT FINDINGS

Accumulation of SFA in the body is tightly regulated by stearoyl-CoA desaturase 1, an enzyme that converts endogenous SFA to monounsaturated fatty acids. Recent studies have demonstrated that the accumulation of SFA seen with genetic deletion or inhibition of stearoyl-CoA desaturase 1 promotes inflammation, TLR4 hypersensitivity, and accelerated atherosclerosis. Therefore, stearoyl-CoA desaturase 1 may play an unexpected role in suppressing inflammation by preventing excessive accumulation of endogenous SFA-derived TLR4 agonists. In parallel, several independent laboratories have demonstrated that TLR4 is necessary for dietary SFAs to induce obesity, insulin resistance, and vascular inflammation in rodent models.

SUMMARY

The metabolic syndrome and atherosclerotic cardiovascular disease have long been linked to dietary SFA intake and inflammation. Recent mechanistic insights into how SFAs and downstream metabolites can potentiate inflammation-driven metabolic disease are discussed here.

Proteomic analysis of and immune responses to Ehrlichia chaffeensis lipoproteins

Ehrlichia chaffeensis is an obligately intracellular gram-negative bacterium and is the etiologic agent of human monocytic ehrlichiosis (HME). Although E. chaffeensis induces the generation of several cytokines and chemokines by leukocytes, E. chaffeensis lacks lipopolysaccharide and peptidoglycan. Bioinfomatic analysis of the E. chaffeensis genome, however, predicted genes encoding 15 lipoproteins and 3 posttranslational lipoprotein-processing enzymes. The present study showed that by use of multidimensional liquid chromatography followed by tandem mass spectrometry, all predicted lipoproteins as well as lipoprotein-processing enzymes were expressed by E. chaffeensis cultured in the human promyelocytic leukemia cell line HL-60. Consistent with this observation, a signal peptidase II inhibitor, globomycin, was found to inhibit E. chaffeensis infection and lipoprotein processing in HL-60 cell culture. To study in vivo E. chaffeensis lipoprotein expression and host immune responses to E. chaffeensis lipoproteins, 13 E. chaffeensis lipoprotein genes were cloned into a mammalian expression vector. When the DNA constructs were inoculated into naïve dogs, or when dogs were infected with E. chaffeensis, the animals developed delayed-type hypersensitivity reactions at cutaneous sites of the DNA construct deposition and serum antibodies to these lipoproteins. This is the first demonstration of lipoprotein expression and elicitation of immune responses by a member of the order Rickettsiales. Multiple lipoproteins expressed by E. chaffeensis in vitro and in vivo may play key roles in pathogenesis and immune responses in HME.

Mycobacterium tuberculosis, macrophages, and the innate immune response: does common variation matter?

Despite the discovery of the tuberculosis (TB) bacillus over 100 years ago and the availability of effective drugs for over 50 years, there remain a number of formidable challenges for controlling Mycobacterium tuberculosis (MTb). Understanding the genetic and immunologic factors that influence human susceptibility could lead to novel insights for vaccine development as well as diagnostic advances to target treatment to those who are at risk for developing active disease. Although a series of studies over the past 50 years suggests that host genetics influences resistance to TB, a comprehensive understanding of which genes and variants are associated with susceptibility is only partially understood. In this article, we review recent advances in our understanding of human variation of the immune system and its effects on macrophage function and influence on MTb susceptibility. We emphasize recent discoveries in human genetic studies and correlate these findings with efforts to understand how these variants alter the molecular and cellular functions that regulate the macrophage response to MTb.

Vaccine adjuvants revisited

The development of new adjuvants for human vaccines has become an expanding field of research in the last thirty years, for generating stronger vaccines capable of inducing protective and long-lasting immunity in humans. Instead of such efforts, with several adjuvant strategies approaching to requirements for their clinical application, limitations like adjuvant toxicity remain to be fully surpassed. Here we summarize the current status of adjuvant development, including regulatory recommendations, adjuvant requirements, and adjuvant categories like mineral salts, tensoactive compounds, microorganism-derived adjuvants, emulsions, cytokines, polysaccharides, nucleic acid-based adjuvants, and a section dedicated to particulate antigen delivery systems. The mechanisms of adjuvanticity are also discussed in the light of recent findings on Toll-like receptors' biology and their involvement on immune activation.

Biosynthesis of emulsan biopolymers from agro-based feedstocks

AIMS

The need for biocompatible, biodegradable, and versatile biopolymers permeates many fields including environmental and food technology. The goal of the study presented here is to establish the utility of agricultural oils as an inexpensive carbon source to produce materials useful for biomedical materials and offer positive attributes in terms of green chemistry.

METHODS AND RESULTS

Structural variants of the complex acylated polysaccharide, emulsan, secreted from Acinetobacter venetianus RAG-1, were biosynthesized in cultures supplemented with agricultural feedstocks to examine the feasibility of conversion of these substrates into value-added biopolymers. Acinetobacter venetianus produced chemically and biologically distinct emulsan variants in culture on soy molasses and tallow oil. These variants possess significant biological function, including macrophage activation and adjuvant activity, in similar range to that observed for the standard emulsan formed on ethanol-fed A. venetianus.

CONCLUSIONS

The results indicate that this novel family of biopolymers can be produced in significant quantities from the readily available renewable agricultural feedstocks and the resulting structures and functions can be correlated to the chemistry of these feedstocks.

SIGNIFICANCE AND IMPACT OF THE STUDY

The significant quantities of agricultural oils produced annually represent an untapped source for bioconversion to valuable products. The results of this study confirm that the important polymer emulsan can be synthesized from this inexpensive carbon source.

Immunostimulatory combinations: designing the next generation of vaccine adjuvants

Agents that activate dendritic cells are essential components for vaccines and can be conceptualized as molecular adjuvants. Other molecular adjuvants affect downstream factors that shape the resulting immune response. This review provides a compendium of recently studied molecular adjuvants, focusing on CD8+ T cell responses, which have important roles in HIV vaccines. Reference is also made to CD8+ T cell antitumor responses, where parallel studies of molecular adjuvants are being pursued. Molecular adjuvants can be considered in the following groups: TNF superfamily molecules such as CD40 ligand; agonists for TLRs; agonists for NAIP, CIITA, HET-E, TP-1-leucine-rich repeat pathway receptors, such as nucleotide-binding and oligomerization domain (NOD)1, NOD2, and cryopyrin; chemokines; ILs; CSFs; IFNs; alarmins; and purinergic P2X7 receptor agonists. Complementing these positively acting agents are strategies to reduce the immunosuppressive effects of CD4+CD25+ regulatory T cells and negatively acting factors such as TGF-beta, IL-10, suppressor of cytokine signaling 1, and programmed cell death-1 using neutralizing antibodies, antisense, and small interfering RNA. Especially effective are combinations of molecular adjuvants, which can elicit a massive expansion of antigen-specific CD8+ T cells and show unprecedented efficacy in vaccine and tumor models. Taken together, these new approaches provide significant incremental progress in the development of vaccines to elicit cell-mediated immunity against HIV and other pathogens.

Conserved nontypeable Haemophilus influenzae-derived TLR2-binding lipopeptides synergize with IFN-beta to increase cytokine production by resident murine and human alveolar macrophages

Nontypeable Haemophilus influenzae (NTHi) is strongly associated with exacerbations of chronic obstructive pulmonary disease, which often coincide with viral respiratory infections. TLR2 contributes importantly to innate immunity to NTHi, but whether this pathway is affected by simultaneous antiviral responses is unknown. To analyze potential interactions, resident murine and human alveolar macrophages (AMphi) were exposed, in the presence or absence of the appropriate rIFN-beta, to synthetic lipopeptides corresponding to the triacylated N-terminal fragments of three outer membrane proteins (OMP) (PCP, P4, and P6) that are highly conserved among different NTHi strains. Synthetic OMP elicited strong release of IL-6, the principal inducer of airway mucin genes, and induced CCL5 and CXCL10 from murine AMphi only when IFN-beta was also present. Surprisingly, combined stimulation by OMPs and IFN-beta also markedly enhanced TNF-alpha release by murine AMphi. Stimulation with PCP plus IFN-beta induced IFN-regulatory factor 1 expression and sustained STAT1 activation, but did not alter the activation of MAPKs or NF-kappaB. AMphi derived from STAT1-deficient mice did not demonstrate increased production of TNF-alpha in response to PCP plus IFN-beta. Analysis of wild-type and STAT1-deficient AMphi using real-time PCR showed that increased TNF-alpha production depended on transcriptional up-regulation, but not on mRNA stabilization. The synergistic effect of synthetic OMP and IFN-beta was conserved between murine AMphi and human AMphi for IL-6, but not for TNF-alpha. Thus, IFN-beta, which is produced by virally infected respiratory epithelial cells, converts normally innocuous NTHi OMP into potent inflammatory stimulants, but does so via different mechanisms in mice and humans.

Toll-like receptor-2 is essential for the development of palmitate-induced insulin resistance in myotubes

Fatty acids can activate proinflammatory pathways leading to the development of insulin resistance, but the mechanism is undiscovered. Toll like receptor 2 (TLR2) recognizes lipids, activates proinflammatory pathways, and is genetically associated with inflammatory diseases. This study aimed to examine the role of TLR2 in palmitate-induced insulin resistance in C2C12 myotubes. Treatment with palmitate rapidly induced the association of myeloid differentiation factor 88 (MyD88) with the TLR2 receptor, activated the stress-linked kinases p38, JNK, and protein kinase C, induced degradation of IkappaBalpha, and increased NF-kappaB DNA binding. The activation of these pathways by palmitate was sensitive and temporally regulated and occurred within the upper physiologic range of saturated fatty acid concentrations in vivo, suggesting a receptor-mediated event and not simple lipotoxicity. When compared with an equimolar concentration of palmitate, fibroblast-stimulating lipopeptide-1, a known TLR2 ligand, was a slightly more potent activator of signal transduction and interleukin (IL)-6 production. Palmitate inhibited insulin signal transduction in C2C12 cells beginning 1-2 h after exposure and reached a maximum at 12-16 h. An antagonist TLR2 antibody, mAb 2.5, led to a 50-60% decrease in palmitate-induced IL-6 production and partially restored insulin signal transduction, whereas an isotype-matched control antibody had no effect. RNA interference-mediated inhibition of TLR2 and MyD88 expression in C2C12 muscle cells resulted in a near complete inhibition of palmitate-induced insulin resistance and IL-6 production. This study provides strong evidence that TLR2 mediates the initial events of fatty acid-induced insulin resistance in muscle.

Mycobacterial trehalose-containing glycolipid with immunomodulatory activity on human CD4+ and CD8+ T-cells

Protection against Mycobacterium tuberculosis is based on cell-mediated immunity, most importantly involving CD4+ and CD8+ T-cell subsets. One of the key features of the tubercle bacillus is its cell envelope, characterized by extremely abundant and specific lipids. The cell-surface glycolipid 2,3-di-O-acyl-trehalose (DAT) has been consistently found in M. tuberculosis strains. In this study, analysis of proliferation, activation markers and cytokine release was performed in human peripheral blood mononuclear cells (PBMC) activated in the presence and absence of DAT. We present evidence that mycobacterial DAT is able to reduce antigen-induced proliferation of human CD4+ and CD8+ T-cell subsets. We show that the effect is associated with a decrease of cells expressing the T-cell surface activation markers CD25 and CD69, and down-modulation of IL-2, IL-12, TNF-alpha and IL-10 cytokines. Data indicating that fine acyl chain structural variations in the trehalose-containing lipid may be involved in the degree of immune modulation are also presented.

Lipopeptide structure determines TLR2 dependent cell activation level

Bacterial lipoproteins/peptides are composed of di-O-acylated-S-(2,3-dihydroxypropyl)-cysteinyl residues N-terminally coupled to distinct polypeptides, which can be N-acylated with a third fatty acid. Using a synthetic lipopeptide library we characterized the contribution of the lipid portion to the TLR2 dependent pattern recognition. We found that the two ester bound fatty acid length threshold is beyond eight C atoms because almost no response was elicited by cellular challenge with analogues carrying shorter acyl chains in HEK293 cells expressing recombinant human TLR2. In contrast, the amide bound fatty acid is of lesser importance. While two ester-bound palmitic acids mediate a high stimulatory activity of the respective analogue, a lipopeptide carrying one amide-bound and another ester-bound palmitic acid molecule was inactive. In addition, species specific LP recognition through murine and human TLR2 depended on the length of the two ester bound fatty acid chains. In conclusion, our results indicate the responsibility of both ester bound acyl chains but not of the amide bound fatty acid molecule for the TLR dependent cellular recognition of canonical triacylated LP, as well as a requirement for a minimal acyl chain length. Thus they might support the explanation of specific immuno-stimulatory potentials of different microorganisms and provide a basis for rational design of TLR2 specific adjuvants mediating immune activation to distinct levels.

MyD88-dependent signaling contributes to protection following Bacillus anthracis spore challenge of mice: implications for Toll-like receptor signaling

Bacillus anthracis is a spore-forming, gram-positive organism that is the causative agent of the disease anthrax. Recognition of Bacillus anthracis by the host innate immune system likely plays a key protective role following infection. In the present study, we examined the role of TLR2, TLR4, and MyD88 in the response to B. anthracis. Heat-killed Bacillus anthracis stimulated TLR2, but not TLR4, signaling in HEK293 cells and stimulated tumor necrosis factor alpha (TNF-alpha) production in C3H/HeN, C3H/HeJ, and C57BL/6J bone marrow-derived macrophages. The ability of heat-killed B. anthracis to induce a TNF-alpha response was preserved in TLR2-/- but not in MyD88-/- macrophages. In vivo studies revealed that TLR2-/- mice and TLR4-deficient mice were resistant to challenge with aerosolized Sterne strain spores but MyD88-/- mice were as susceptible as A/J mice. We conclude that, although recognition of B. anthracis occurs via TLR2, additional MyD88-dependent pathways contribute to the host innate immune response to anthrax infection.