Toll-like receptor activation of human cells by synthetic triacylated lipid A-like molecules

MPL Adjuvant Contains Competitive Antagonists of Human TLR4

Monophosphoryl lipid A (MPL^®) is the first non-alum vaccine adjuvant to achieve widespread clinical and market acceptance, a remarkable achievement given that it is manufactured from a Salmonella enterica endotoxin. To understand how MPL^® successfully balanced the dual mandate of vaccine design—low reactogenicity with high efficacy—clinical- and research-grade MPL was evaluated in human and mouse cell systems. Stimulatory dose response curves revealed that most preparations of MPL are much more active in mouse than in human cell systems, and that the limited efficacy observed in human cells correlated with TLR4 inhibitory activity that resulted in a partial agonist profile. Further analysis of the major components of MPL^® adjuvant prepared synthetically identified two structural variants that functioned as competitive antagonists of human TLR4. A partial agonist profile could be recapitulated and manipulated by spiking synthetic agonists with synthetic antagonists to achieve a broad dose range over which TLR4 stimulation could be constrained below a desired threshold. This report thus identifies mixed agonist–antagonist activity as an additional mechanism by which MPL^® adjuvant is detoxified, relative to its parental LPS, to render it safe for use in prophylactic vaccines.

Dose matters: Direct killing or immunoregulatory effects of natural polysaccharides in cancer treatment

Polysaccharides from natural resources possess anti-tumor activities for decades, but the efficacy of polysaccharides as the adjuvant drugs for cancer treatment at prescribed doses remains open for debate. In this review, molecular mechanisms involved in direct killing effects of polysaccharides, including apoptosis, cell cycle arrest and mitochondria/DNA damage were described. However, the concentrations/doses used to reach the direct killing effects are too high to be applicable. Polysaccharides can also exert anti-tumor effects through immunoregulation at lower doses, and the effects of polysaccharides on natural killer cells, dendritic cells and other lymphocytes for tumor destruction, along with the receptor recognition and downstream signaling pathways, were delineated. Unfortunately, the prescribed doses of polysaccharides are too low to stimulate immunoresponse, resulting in the failure of some clinical trials. Therefore, understanding the sophisticated mechanisms of the immunoregulatory function of natural polysaccharides with refined doses for clinical use will help the standardization of traditional medicine.

Recent progress in the discovery of myeloid differentiation 2 (MD2) modulators for inflammatory diseases

Myeloid differentiation protein 2 (MD2), together with Toll-like receptor 4 (TLR4), binds lipopolysaccharide (LPS) with high affinity, inducing the formation of the activated homodimer LPS-MD2-TLR4. MD2 directly recognizes the Lipid A domain of LPS, leading to the activation of downstream signaling of cytokine and chemokine production, and initiation of inflammatory and immune responses. However, excessive activation and potent host responses generate severe inflammatory syndromes such as acute sepsis and septic shock. MD2 is increasingly being considered as an attractive pharmacological target for the development of potent anti-inflammatory agents. In this Keynote review, we provide a comprehensive overview of the recent advances in the structure and biology of MD2, and present MD2 modulators as promising agents for anti-inflammatory intervention.

Synthesis and immunological evaluation of a low molecular weight saccharide with TLR-4 agonist activity

The paucity of FDA approved adjuvants renders the synthesis, characterization, and use of new compounds as vaccine adjuvants, a necessity. For this purpose, a novel saccharide analog has been synthesized from glucosamine, pyruvylated galactose and 1,4-cyclohexanediol and its biological efficacy was determined in innate immune cells. More specifically, we assessed the production of pro-inflammatory cytokines from the murine monocyte cell line, Raw 264.7 and from C57 BL/6 mouse peritoneal macrophages following exposure to the saccharide analog. Our data conclude that the novel saccharide has immunostimulatory activity on mouse macrophages as indicated by the elevated levels of IL-6 and TNF-α in culture supernatants. This effect was TLR-4-dependent but TLR-2-independent. Our data, suggest TLR-4 agonism; a key feature of vaccine adjuvants.

Small-Molecule Carbohydrate-Based Immunostimulants

In this review, we discuss small-molecule, carbohydrate-based immunostimulants that target Toll-like receptor 4 (TLR-4) and cluster of differentiation 1D (CD1d) receptors. The design and use of these molecules in immunotherapy as well as results from their use in clinical trials are described. How these molecules work and their utilization as vaccine adjuvants are also discussed. Future applications and extensions for the use of these analogues as therapeutic agents will be outlined.

Synthesis of anti-inflammatory α-and β-linked acetamidopyranosides as inhibitors of toll-like receptor 4 (TLR4)

The low-molecular weight isopropyl 2-acetamido-α-glucoside 16 (C34) inhibits toll-like receptor 4 (TLR4) in enterocytes and macrophages in vitro, and reduces systemic inflammation in mouse models of endotoxemia and necrotizing enterocolitis. We used a copper(II)-mediated solvolysis of anomeric oxazolines and an acid-mediated conversion of β-glucosamine and β-galactosamine pentaacetates to generate analogs of 16 at the anomeric carbon and at C-4 of the pyranose ring. These compounds were evaluated for their influence on TLR4-mediated inflammatory signaling in cultured enterocytes and monocytes. Their efficacy was confirmed using a NF-kB-luciferase reporter mouse, thus establishing the first structure-activity relationship (SAR) study in this series and identifying the more efficacious isopropyl 2-acetamido-α-galactoside 17.

TLR activation of tumor-associated macrophages from ovarian cancer patients triggers cytolytic activity of NK cells

We analyzed the functional outcome of the interaction between tumor-associated macrophages (TAMs) and natural killer (NK) cells. TAMs from ascites of ovarian cancer patients displayed an alternatively activated functional phenotype (M2) characterized by a remarkably high frequency and surface density of membrane-bound IL-18. Upon TLR engagement, TAMs acquired a classically activated functional phenotype (M1), released immunostimulatory cytokines (IL-12, soluble IL-18), and efficiently triggered the cytolytic activity of NK cells. TAMs also induced the release of IFN-γ from NK cells, which however was significantly lower compared with that induced by in vitro-polarized M2 cells. Most tumor-associated NK cells displayed a CD56(bright) , CD16(neg) or CD56(bright) , CD16(dim) phenotype, and very poor cytolytic activities, despite an increased expression of the activation marker CD69. They also showed downregulation of DNAM-1, 2B4, and NTB-A activating receptors, and an altered chemokine receptor repertoire. Importantly however, when appropriately stimulated, NK cells from the patients, including those cells isolated from ascites, efficiently killed autologous TAMs that expressed low, "nonprotective" levels of HLA class I molecules. Overall, our data show the existence of a complex tumor microenvironment in which poorly cytolytic/immature NK cells deal with immunosuppressive tumor-educated macrophages.

IL-1β and TLR4 signaling are involved in the aggravated murine acute graft-versus-host disease caused by delayed bortezomib administration

It was shown that the proteasome inhibitor, bortezomib, administered immediately following allogeneic bone marrow transplantation resulted in marked inhibition of acute graft-versus-host disease (aGVHD), with retention of graft-versus-tumor effects. However, continuous bortezomib administration resulted in significant acceleration of graft-versus-host disease-dependent morbidity. We carried out studies to dissect the mechanisms of aggravated aGVHD caused by delayed bortezomib administration. First, we demonstrated that IL-1β was critically involved, and the subsequent aGVHD could be alleviated by IL-1β blockade. Bortezomib treatment after dendritic cell (DC) activation resulted in drastically elevated IL-1β production, whereas bortezomib treatment before DC activation inhibited IL-1β production, suggesting that the timing of bortezomib administration significantly affected IL-1β production by DCs. We further demonstrated that delayed administration of bortezomib accelerated aGVHD through TLR4 signaling. Because the LPS levels were much lower with reduced-intensity conditioning compared with high-dose irradiation, the accelerated graft-versus-host disease-dependent morbidity with delayed bortezomib administration could be rescued by reduced-intensity conditioning. Our studies suggested that TLR4 pathway activation and delayed bortezomib administration amplified the production of IL-1β and other inflammatory cytokines, which resulted in accelerated aGVHD-dependent morbidity. These results indicated that decreased toxicity of continuous bortezomib administration could be achieved by reduced-intensity conditioning or by inhibiting IL-1β.

Toll-like receptor 4 (TLR4) modulation by synthetic and natural compounds: an update

Toll-like receptor 4 (TLR4), together with MD-2, binds bacterial endotoxins (E) with high affinity, triggering formation of the activated homodimer (E.MD-2.TLR4)2. Activated TLR4 induces intracellular signaling leading to activation of transcription factors that result in cytokine and chemokine production and initiation of inflammatory and immune responses. TLR4 also responds to endogenous ligands called danger associated molecular patterns (DAMPs). Increased sensitivity to infection and a variety of immune pathologies have been associated with either too little or too much TLR4 activation. We review here the molecular mechanisms of TLR4 activation (agonism) or inhibition (antagonism) by small organic molecules of both natural and synthetic origin. The role of co-receptors MD-2 and CD14 in the TLR4 modulation process is also discussed. Recent achievements in the field of chemical TLR4 modulation are reviewed, with special focus on nonclassical TLR4 ligands with a chemical structure different from that of lipid A.

Immunostimulation by synthetic lipopeptide-based vaccine candidates: structure-activity relationships

Peptide-based vaccines offer several advantages over conventional whole organism or protein approaches by offering improved purity and specificity in inducing immune response. However, peptides alone are generally non-immunogenic. Concerns remain about the toxicity of adjuvants which are critical for immunogenicity of synthetic peptides. The use of lipopeptides in peptide vaccines is currently under intensive investigation because potent immune responses can be generated without the use of adjuvant (thus are self-adjuvanting). Several lipopeptides derived from microbial origin, and their synthetic versions or simpler fatty acid moieties impart this self-adjuvanting activity by signaling via Toll-like receptor 2 (TLR2). Engagement of this innate immune receptor on antigen-presenting cell leads to the initiation and development of potent immune responses. Therefore optimization of lipopeptides to enhance TLR2-mediated activation is a promising strategy for vaccine development. Considerable structure-activity relationships that determine TLR2 binding and consequent stimulation of innate immune responses have been investigated for a range of lipopeptides. In this mini review we address the development of lipopeptide vaccines, mechanism of TLR2 recognition, and immune activation. An overview is provided of the best studied lipopeptide vaccine systems.

Species-Dependent Blood-Brain Barrier Disruption of Lipopolysaccharide: Amelioration by Colistin In Vitro and In Vivo

The aim of this study was to use in vitro and in vivo models to assess the impact of lipopolysaccharide (LPS) from two different bacterial species on blood-brain barrier (BBB) integrity and brain uptake of colistin. Following repeated administration of LPS from Pseudomonas aeruginosa, the brain-to-plasma ratio of [¹⁴C]sucrose in Swiss outbred mice was not significantly increased. Furthermore, while the brain uptake of colistin in mice increased 3-fold following administration of LPS from Salmonella enterica, LPS from P. aeruginosa had no significant effect on colistin brain uptake. This apparent species-dependent effect did not appear to correlate with differences in plasma cytokine levels, as the concentrations of tumor necrosis factor alpha and interleukin-6 following administration of each LPS were not different (P > 0.05). To clarify whether this species-specific effect of LPS was due to direct effects on the BBB, human brain capillary endothelial (hCMEC/D3) cells were treated with LPS from P. aeruginosa or S. enterica and claudin-5 expression was measured by Western blotting. S. enterica LPS significantly (P < 0.05) reduced claudin-5 expression at a concentration of 7.5 μg/ml. In contrast, P. aeruginosa LPS decreased (P < 0.05) claudin-5 expression only at the highest concentration tested (i.e., 30 μg/ml). Coadministration of therapeutic concentrations of colistin ameliorated the S. enterica LPS-induced reduction in claudin-5 expression in hCMEC/D3 cells and the perturbation in BBB function in mice. This study demonstrates that BBB disruption induced by LPS is species dependent, at least between P. aeruginosa and S. enterica, and can be ameliorated by colistin.