Amide-linked brartemicin glycolipids exhibit Mincle-mediated agonist activity in vitro

Recent Advances in the Development of Mincle-Targeting Vaccine Adjuvants

The Macrophage-inducible C-type lectin (Mincle) is a pattern-recognition receptor (PRR), which has shown much promise as a molecular target for the development of TH1/TH17-skewing vaccine adjuvants. In 2009, the first non-proteinaceous Mincle ligands, trehalose dimycolate (TDM) and trehalose dibehenate (TDB), were identified. This prompted a search for other Mincle agonists and the exploration of Mincle agonists as vaccine adjuvants for both preventative and therapeutic (anti-cancer) vaccines. In this review, we discuss those classes of Mincle agonists that have been explored for their adjuvant potential. These Mincle agonists have been used as stand-alone adjuvants or in combination with other pathogen-associated molecular patterns (PAMPs) or immunomodulatory agents. We will also highlight recently identified Mincle ligands with hitherto unknown adjuvanticity. Conjugate vaccines that contain covalently linked adjuvants and/or adjuvant-antigen combinations are also presented, as well as the different formulations (e.g., oil-in-water emulsions, liposomes, and particulate delivery systems) that have been used for the codelivery of antigens and adjuvants. Insofar the reader is presented with a thorough review of the potential of Mincle-mediated vaccine adjuvants, including historical context, present-day research and clinical trials, and outstanding research questions, such as the role of ligand presentation and Mincle clustering, which, if better understood, will aid in the development of the much-needed TH1/TH17-skewing vaccine adjuvants.

Myeloid C-type lectin receptors in host-pathogen interactions and glycan-based targeting

Lectin-glycan interactions play a crucial role in the immune system. An important class of lectins in the innate immune system is myeloid C-type lectin receptors (CLRs). Myeloid CLRs act as pattern recognition receptors and are predominantly expressed by myeloid cells, such as macrophages, dendritic cells, and neutrophils. In innate immunity, CLRs contribute to self/non-self discrimination. While the recognition of pathogen-associated molecular patterns (PAMPs) by CLRs may contribute to a protective immune response, CLR engagement can also be exploited by pathogens for immune evasion. Since various CLRs act as endocytic receptors and trigger distinct signaling pathways in myeloid cells, CLR targeting has proven useful for drug/antigen delivery into antigen-presenting cells and the modulation of immune responses. This review covers recent discoveries of pathogen/CLR interactions and novel approaches for CLR targeting within the period of the past two years.

Systematic Evaluation of Regiochemistry and Lipidation of Aryl Trehalose Mincle Agonists

The Macrophage-Inducible C-type Lectin receptor (Mincle) plays a critical role in innate immune recognition and pathology, and therefore represents a promising target for vaccine adjuvants. Innovative trehalose-based Mincle agonists with improved pharmacology and potency may prove useful in the development of Th17-mediated adaptive immune responses. Herein, we report on in vitro and in silico investigations of specific Mincle ligand-receptor interactions required for the effective receptor engagement and activation of Th17-polarizing cytokines. Specifically, we employed a library of trehalose benzoate scaffolds, varying the degree of aryl lipidation and regiochemistry that produce inflammatory cytokines in a Mincle-dependent fashion. In vitro interleukin-6 (IL-6) cytokine production by human peripheral blood mononuclear cells (hPBMCs) indicated that the lipid regiochemistry is key to potency and maximum cytokine output, with the tri-substituted compounds inducing higher levels of IL-6 in hPBMCs than the di-substituted derivatives. Additionally, IL-6 production trended higher after stimulation with compounds that contained lipids ranging from five to eight carbons long, compared to shorter (below five) or longer (above eight) carbon chains, across all the substitution patterns. An analysis of the additional cytokines produced by hPBMCs revealed that compound 4d, tri-substituted and five carbons long, induced significantly greater levels of interleukin-1β (IL-1β), tumor necrosis factor- α (TNF-α), interleukin-23 (IL-23), and interferon- γ (IFN-γ) than the other compounds tested in this study. An in silico assessment of 4d highlighted the capability of this analogue to bind to the human Mincle carbohydrate recognition domain (CRD) efficiently. Together, these data highlight important structure-activity findings regarding Mincle-specific cytokine induction, generating a lead adjuvant candidate for future formulations and immunological evaluations.

Probing Isosteric Replacement for Immunoadjuvant Design: Bis-Aryl Triazole Trehalolipids are Mincle Agonists

Herein, we report the modular synthesis and immunological activity of seven bis-aryl triazole trehalolipids (1a-1g) as Brartemicin analogs. The compounds comprised one or two octyloxy (C8) alkyl chains and were synthesized using the venerable CuAAc reaction between the respective aryl acetylenes and a trehalose diazide. A Mincle reporter cell assay revealed that all lipidated analogs activated Mincle. Two compounds, 1c and 1d, produced strong Mincle-dependent immune responses in vitro. The activity was dependent on the degree of alkylation and regiochemistry, with 1c and 1d showing significantly increased IL-1β production in vitro compared to monoalkylated compounds and dialkylated compounds lacking ortho substitution. Molecular docking of 1c positioned the triazole in proximity to Arg-183, which may offer additional interactions that could explain the binding affinity for this class of ligand. These findings demonstrate the capability of triazole-linked Brartemicin analogs as Mincle-mediated Th1/Th17 vaccine adjuvants.

Development of a drug discovery approach from microbes with a special focus on isolation sources and taxonomy

After the successful discoveries of numerous antibiotics from microorganisms, frequent reisolation of known compounds becomes an obstacle in further development of new drugs from natural products. Exploration of biological sources that can provide novel scaffolds is thus an urgent matter in drug lead screening. As an alternative source to the conventionally used soil microorganisms, we selected endophytic actinomycetes, marine actinomycetes, and actinomycetes in tropical areas for investigation and found an array of new bioactive compounds. Furthermore, based on the analysis of the distribution pattern of biosynthetic gene clusters in bacteria together with available genomic data, we speculated that biosynthetic gene clusters for secondary metabolites are specific to each genus. Based on this assumption, we investigated actinomycetal and marine bacterial genera from which no compounds have been reported, which led to the discovery of a variety of skeletally novel bioactive compounds. These findings suggest that consideration of environmental factor and taxonomic position is critically effective in the selection of potential strains producing structurally unique compounds.

Aryl-functionalised α,α'-Trehalose 6,6'-Glycolipid Induces Mincle-independent Pyroptotic Cell Death

α,α'-Trehalose 6,6'-glycolipids have long been known for their immunostimulatory properties. The adjuvanticity of α,α'-trehalose 6,6'-glycolipids is mediated by signalling through the macrophage inducible C-type lectin (Mincle) and the induction of an inflammatory response. Herein, we present an aryl-functionalised trehalose glycolipid, AF-2, that leads to the release of cytokines and chemokines, including IL-6, MIP-2 and TNF-α, in a Mincle-dependent manner. Furthermore, plate-coated AF-2 also leads to the Mincle-independent production of IL-1β, which is unprecedented for this class of glycolipid. Upon investigation into the mode of action of plate-coated AF-2, it was observed that the treatment of WT and Mincle^-/- bone marrow derived macrophages (BMDM), murine RAW264.7 cells, and human monocytes with AF-2 led to lytic cell death, as evidenced using Sytox Green and lactate dehydrogenase assays, and confocal and scanning electron microscopy. The requirement for functional Gasdermin D and Caspase-1 for IL-1β production and cell death by AF-2 confirmed pyroptosis as the mode of action of AF-2. The inhibition of NLRP3 and K⁺ efflux reduced AF-2 mediated IL-1β production and cell death, and allowed us to conclude that AF-2 leads to Capase-1 dependent NLRP3 inflammasome-mediated cell death. The unique mode of action of plate-coated AF-2 was surprising and highlights how the physical presentation of Mincle ligands can lead to dramatically different immunological outcomes.

6-C-Linked trehalose glycolipids signal through Mincle and exhibit potent adjuvant activity

Many studies have investigated the Mincle-mediated agonist activity of α,α'-trehalose-6,6́-glycolipids, however, none have considered how the position, or absence, of the ester moiety influences Mincle-mediated agonist activity. We prepared a variety of 6-C-linked α,α'-trehalose glycolipids containing inverted esters, ketone, carboxy or no carbonyl moieties. Modelling studies indicated that these derivatives bind to the CRD of Mincle in a manner similar to that of the prototypical Mincle agonist, trehalose dibehenate (TDB), with NFAT-GFP reporter cell assays confirming that all compounds, apart from derivatives with short alkyl chains, led to robust Mincle signalling. It was also observed that a carbonyl moiety was needed for good Mincle-mediated signalling. The ability of the compounds to induce mIL-1 β and mIL-6 production by bone marrow-derived macrophages (BMDMs) further demonstrated the agonist activity of the compounds, with the presence of a carbonyl moiety and longer lipid chains augmenting cytokine production. Notably, a C20 inverted ester led to levels of mIL-1β that were significantly greater than those induced by TDB. The same C20 inverted ester also led to a significant increase in hIL-1β and hIL-6 by human monocytes, and exhibited no toxicity, as demonstrated using BMDMs in an in vitro Sytox Green assay. The ability of the inverted ester to enhance antigen-mediated immune responses was then determined. In these studies, the inverted ester was found to augment the OVA-specific Th1/Th7 immune response in vitro, and exhibit adjuvanticity that was better than that of TDB in vivo, as evidenced by a significant increase in IgG antibodies for the inverted ester but not TDB when using OVA as a model antigen.

The trehalose glycolipid C18Brar promotes antibody and T-cell immune responses to Mannheimia haemolytica and Mycoplasma ovipneumoniae whole cell antigens in sheep

Bronchopneumonia is a common respiratory disease in livestock. Mannheimia haemolytica is considered the main causative pathogen leading to lung damage in sheep, with Mycoplasma ovipneumoniae and ParaInfluenza virus type 3, combined with adverse physical and physiological stress, being predisposing factors. A balance of humoral and cellular immunity is thought to be important for protection against developing respiratory disease. In the current study, we compared the ability of the trehalose glycolipid adjuvant C18Brar (C18-alkylated brartemicin analogue) and three commercially available adjuvant systems i.e., Quil-A, Emulsigen-D, and a combination of Quil-A and aluminium hydroxide gel, to stimulate antibody and cellular immune responses to antigens from inactivated whole cells of M. haemolytica and M. ovipneumoniae in sheep. C18Brar and Emulsigen-D induced the strongest antigen-specific antibody responses to both M. haemolytica and M. ovipneumoniae, while C18Brar and Quil-A promoted the strongest antigen-specific IL-17A responses. The expression of genes with known immune functions was determined in antigen-stimulated blood cultures using Nanostring nCounter technology. The expression levels of CD40, IL22, TGFB1, and IL2RA were upregulated in antigen-stimulated blood cultures from animals vaccinated with C18Brar, which is consistent with T-cell activation. Collectively, the results demonstrate that C18Brar can promote both antibody and cellular responses, notably Th17 immune responses in a ruminant species.

Water-soluble trehalose glycolipids show superior Mincle binding and signaling but impaired phagocytosis and IL-1β production

The tremendous potential of trehalose glycolipids as vaccine adjuvants has incentivized the study of how the structures of these ligands relate to their Mincle-mediated agonist activities. Despite this, structure-activity work in the field has been largely empirical, and less is known about how Mincle-independent pathways might be affected by different trehalose glycolipids, and whether Mincle binding by itself can serve as a proxy for adjuvanticity. There is also much demand for more water-soluble Mincle ligands. To address this need, we prepared polyethylene glycol modified trehalose glycolipids (PEG-TGLs) with enhanced water solubility and strong murine Mincle (mMincle) binding and signaling. However, only modest cytokine and chemokine responses were observed upon the treatment of GM-CSF treated bone-marrow cells with the PEG-TGLs. Notability, no IL-1β was observed. Using RNA-Seq analysis and a representative PEG-TGL, we determined that the more water-soluble adducts were less able to activate phagocytic pathways, and hence, failed to induce IL-1β production. Taken together, our data suggests that in addition to strong Mincle binding, which is a pre-requisite for Mincle-mediated cellular responses, the physical presentation of trehalose glycolipids in colloidal form is required for inflammasome activation, and hence, a strong inflammatory immune response.

Monocytes Elicit a Neutrophil-Independent Th1/Th17 Response Upon Immunization With a Mincle-Dependent Glycolipid Adjuvant

Successful subunit vaccination with recombinant proteins requires adjuvants. The glycolipid trehalose-dibehenate (TDB), a synthetic analog of the mycobacterial cord factor, potently induces Th1 and Th17 immune responses and is a candidate adjuvant for human immunization. TDB binds to the C-type lectin receptor Mincle and triggers Syk-Card9-dependent APC activation. In addition, interleukin (IL)-1 receptor/MyD88-dependent signaling is required for TDB adjuvanticity. The role of different innate immune cell types in adjuvant-stimulated Th1/Th17 responses is not well characterized. We investigated cell recruitment to the site of injection (SOI) and to the draining lymph nodes (dLNs) after immunization with the TDB containing adjuvant CAF01 in a protein-based vaccine. Recruitment of monocytes and neutrophils to the SOI and the dramatic increase in lymph node cellularity was partially dependent on both Mincle and MyD88. Despite their large numbers at the SOI, neutrophils were dispensable for the induction of Th1/Th17 responses. In contrast, CCR2-dependent monocyte recruitment was essential for the induction of Th1/Th17 cells. Transport of adjuvant to the dLN did not require Mincle, MyD88, or CCR2. Together, adjuvanticity conferred by monocytes can be separated at the cellular level from potential tissue damage by neutrophils.