Synthesis of Branched Trehalose Glycolipids and Their Mincle Agonist Activity

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.

Synthesis and Biological Evaluation of Trehalose-based Bi-aryl Derivatives as C-type Lectin Ligands

The identification of Mincle as the C-type lectin receptor on innate immune cells responsible for binding TDM and the realization that this receptor could be key to productive vaccines for mycobacterial infection has raised interest in the development of synthetic Mincle ligands as novel adjuvants. We recently reported on the synthesis and evaluation of Brartemicin analog UM-1024 that demonstrated Mincle agonist activity, exhibiting potent Th1/Th17 adjuvant activity that was greater than that of trehalose dibehenate (TDB). Our pursuit to understand Mincle/ligand relationships and improve the pharmacologic properties of the ligands has expanded and continues to reveal new and exciting structure activity relationships. Herein we report the synthesis of novel bi-aryl trehalose derivatives in good to excellent yields. These compounds were evaluated for their ability to engage the human Mincle receptor and tested for the induction of cytokines from human peripheral blood mononuclear cells. A preliminary structure-activity relationship (SAR) of these novel bi-aryl derivatives revealed that bi-aryl trehalose ligand 3D showed relatively high potency in cytokine production in comparison to trehalose glycolipid adjuvant TDB and the natural ligand TDM and induced dose-dependent, Mincle selective stimulation in hMincle HEK reporter cells. Also, through computational studies, we provide an insight into the potential mode of binding of 6,6'-Biaryl trehalose compounds on human Mincle receptor.

Novel trehalose-based excipients for stabilizing nebulized anti-SARS-CoV-2 antibody

COVID-19 is caused by the infection of the lungs by SARS-CoV-2. Monoclonal antibodies, such as sotrovimab, showed great efficiency in neutralizing the virus before its internalization by lung epithelial cells. However, parenteral routes are still the preferred route of administration, even for local infections, which requires injection of high doses of antibody to reach efficacious concentrations in the lungs. Lung administration of antibodies would be more relevant requiring lower doses, thus reducing the costs and the side effects. But aerosolization of therapeutic proteins is very challenging, as the different processes available are harsh and trigger protein aggregation and conformational changes. This decreases the efficiency of the treatment, and can increase its immunogenicity. To address those issues, we developed a series of new excipients composed of a trehalose core, a succinyl side chain and a hydrophobic carbon chain (from 8 to 16 carbons). Succinylation increased the solubility of the excipients, allowing their use at relevant concentrations for protein stabilization. In particular, the excipient with 16 carbons (C₁₆TreSuc) used at 5.6 mM was able to preserve colloidal stability and antigen-binding ability of sotrovimab during the nebulization process. It could also be used as a cryoprotectant, allowing storage of sotrovimab in a lyophilized form during weeks. Finally, we demonstrated that C₁₆TreSuc could be used as an excipient to stabilize antibodies for the treatment against COVID-19, by in vitro and in vivo assays. The presence of C₁₆TreSuc during nebulization preserved the neutralization capacity of sotrovimab against SARS-CoV-2 in vitro; an increase of its efficacy was even observed, compared to the non-nebulized control. The in vivo study also showed the wide distribution of sotrovimab in mice lungs, after nebulization with 5.6 mM of excipient. This work brings a solution to stabilize therapeutic proteins during storage and nebulization, making pulmonary immunotherapy possible in the treatment of COVID-19 and other lung diseases.

Tunable Strategy for the Asymmetric Synthesis of Sulfoglycolipids from Mycobacterium tuberculosis To Elucidate the Structure and Immunomodulatory Property Relationships

We developed a versatile asymmetric strategy to synthesize different classes of sulfoglycolipids (SGLs) from Mycobacterium tuberculosis. The strategy features the use of asymmetrically protected trehaloses, which were acquired from the glycosylation of TMS α-glucosyl acceptors with benzylidene-protected thioglucosyl donors. The positions of the protecting groups at the donors and acceptors can be fine-tuned to obtain different protecting-group patterns, which is crucial for regioselective acylation and sulfation. In addition, a chemoenzymatic strategy was established to prepare the polymethylated fatty acid building blocks. The strategy employs inexpensive lipase as a desymmetrization agent in the preparation of the starting substrate and readily available chiral oxazolidinone as a chirality-controlling agent in the construction of the polymethylated fatty acids. A subsequent investigation on the immunomodulatory properties of each class of SGLs showed how the structures of SGLs impact the host innate immunity response.

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.

Trehalose diesters containing a polar functional group-modified lipid moiety: Synthesis and evaluation of Mincle-mediated signaling activity

Mincle, a C-type lectin receptor (CLR), activates the innate immune system by recognizing certain complex lipid compounds. In this study, we designed and synthesized trehalose disteate (TDS) and dibehenate (TDB), containing a polar-functional group in the middle of fatty acid moieties, based on a model of the Mincle-glycolipids interaction. The modified fatty acids were prepared using hydroxy fatty acids as common intermediates, and conjugated with an appropriate trehalose moiety to synthesize the desired trehalose diesters. TDE derivatives containing the modified fatty acid have different Mincle-mediated signaling activities depending on the position of the functional group and the length of the lipids. The newly developed TDE derivatives exhibit signaling activity comparable or superior to that of TDS or TDB, and the results suggest that Mincle tolerates polar functional groups at a certain position of the lipid chain of TDE. The introduction of the polar functional groups into the lipid moiety of the glycolipids also resulted in improved solubility in polar solvents, which would be advantageous for various analyses and applications.

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.

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

Lipidated derivatives of the natural product brartemicin show much promise as vaccine adjuvants due to their ability to signal through the Macrophage Inducible C-type Lectin (Mincle). We synthesised three lipophilic amide-linked brartemicin derivatives and compared their agonist activity to that of their ester-linked counterparts in vitro. We demonstrate that the brartemicin amide derivatives activate bone-marrow-derived macrophages (BMDMs) in a Mincle-dependent manner, as evidenced by the production of the pro-inflammatory cytokine IL-1β in wildtype but not Mincle^-/- cells. The amide derivatives showed activity that was as good as, if not better than, their ester counterparts. Two of the amide derivatives, but none of the ester-derivatives, also led to the production of IL-1β by human-derived monocytes. As the production of IL-1β is a good indicator of vaccine adjuvanticity potential, these findings suggest that amide-linked brartemicin derivatives show particular promise as vaccine adjuvants.

Emerging glyco-based strategies to steer immune responses

Glycan structures are common posttranslational modifications of proteins, which serve multiple important structural roles (for instance in protein folding), but also are crucial participants in cell-cell communications and in the regulation of immune responses. Through the interaction with glycan-binding receptors, glycans are able to affect the activation status of antigen-presenting cells, leading either to induction of pro-inflammatory responses or to suppression of immunity and instigation of immune tolerance. This unique feature of glycans has attracted the interest and spurred collaborations of glyco-chemists and glyco-immunologists to develop glycan-based tools as potential therapeutic approaches in the fight against diseases such as cancer and autoimmune conditions. In this review, we highlight emerging advances in this field, and in particular, we discuss on how glycan-modified conjugates or glycoengineered cells can be employed as targeting devices to direct tumor antigens to lectin receptors on antigen-presenting cells, like dendritic cells. In addition, we address how glycan-based nanoparticles can act as delivery platforms to enhance immune responses. Finally, we discuss some of the latest developments in glycan-based therapies, including chimeric antigen receptor (CAR)-T cells to achieve targeting of tumor-associated glycan-specific epitopes, as well as the use of glycan moieties to suppress ongoing immune responses, especially in the context of autoimmunity.

Design of Trehalose-Based Amide/Sulfonamide C-type Lectin Receptor Signaling Compounds

Mincle agonists have been shown to induce inflammatory cytokine production, such as tumor necrosis factor-alpha (TNF) and promote the development of a Th1/Th17 immune response that might be crucial to development of effective vaccination against pathogens such as Mycobacterium tuberculosis. As an expansion of our previous work, a library of 6,6'-amide and sulfonamide α,α-d-trehalose compounds with various substituents on the aromatic ring was synthesized efficiently in good to excellent yields. These compounds were evaluated for their ability to activate the human C-type lectin receptor Mincle by the induction of cytokines from human peripheral blood mononuclear cells. A preliminary structure-activity relationship (SAR) of these novel trehalose diamides and sulfonamides revealed that aryl amide-linked trehalose compounds demonstrated improved activity and relatively high potency cytokine production compared to the Mincle ligand trehalose dibehenate adjuvant (TDB) and the natural ligand trehalose dimycolate (TDM) inducing dose-dependent and human-Mincle-specific stimulation in a HEK reporter cell line.

Trehalose diamide glycolipids augment antigen-specific antibody responses in a Mincle-dependent manner

Many studies have investigated how trehalose glycolipid structures can be modified to improve their Macrophage inducible C-type lectin (Mincle)-mediated adjuvanticity. However, in all instances, the ester-linkage of α,ά-trehalose to the lipid of choice remained. We investigated how changing this ester-linkage to an amide influences Mincle signalling and agonist activity and demonstrated that Mincle tolerates this functional group change. In in vivo vaccination studies in murine and ovine model systems, using OVA or Mannheimia haemolytica and Mycoplasma ovipneumoniae as vaccine antigens, respectively, it was demonstrated that a representative trehalose diamide glycolipid was able to enhance antibody-specific immune responses. Notably, IgG titres against M. ovipneumoniae were significantly greater when using trehalose dibehenamide (A-TDB) compared to trehalose dibehenate (TDB). This is particularly important as infection with M. ovipneumoniae predisposes sheep to pneumonia.

6,6'-Aryl trehalose analogs as potential Mincle ligands

6,6'-Aryl trehalose derivatives have been synthesized with a view towards identifying novel Th-17-inducing vaccine adjuvants based on the high affinity Mincle ligand Brartemicin. The initial structure-activity relationships of these novel trehalose-based compounds were investigated. All compounds have been evaluated for their ability to engage the Mincle receptor and induce a potential pro-Th17 cytokine profile from human peripheral blood mononuclear cells based on IL-6 production in human peripheral blood mononuclear cells. The preliminary biological characterization of the designed analogs presented in this paper should aid in the future design and testing of more affine ligands that may foster the discovery of novel adjuvants with improved pharmacological properties.

Design of potent Mincle signalling agonists based on an alkyl β-glucoside template

The innate immune receptor Mincle senses lipid-based molecules derived from pathogens, commensals and altered self. Based on emerging structure-activity relationships we design simple alkyl 6-O-acyl-β-d-glucosides that are effective agonists of Mincle and signal with potency on par with the prototypical ligand trehalose dimycolate.

Stereochemistry, lipid length and branching influences Mincle agonist activity of monoacylglycerides

Herein, we report on the synthesis of a series of enantiomerically pure linear, iso-branched, and α-branched monoacyl glycerides (MAGs) in 63-72% overall yield. The ability of the MAGs to signal through human macrophage inducible C-type lectin (hMincle) using NFAT-GFP reporter cells was explored, as was the ability of the compounds to activate human monocytes. From these studies, MAGs with an acyl chain length ≥C22 were required for Mincle activation and the production of interleukin-8 (IL-8) by human monocytes. Moreover, the iso-branched MAGs led to a more pronounced immune response compared to linear MAGs, while an α-branched MAG containing a C-32 acyl chain activated cells to a higher degree than trehalose dibehenate (TDB), the prototypical Mincle agonist. Across the compound classes, the activity of the sn-1 substituted isomers was greater than the sn-3 counterparts. None of the representative compounds were cytotoxic, thus mitigating cytotoxicity as a potential mediator of cellular activity. Taken together, 6h (sn-1, iC26+1), 8a (sn-1, C32) and 8b (sn-3, C32) exhibited the best immunostimulatory properties and thus, have potential as vaccine adjuvants.

Synthesis of trehalose glycolipids

Chemical synthesis of trehalose glycolipids such as DAT, TDM, SL-1, SL-3, and Ac₂SGL from MTb, emmyguyacins from fungi, succinoyl trehalose from rhodococcus, and maradolipids from worms, as well as mycobacterial oligosaccharides is reviewed.

ortho-Substituted lipidated Brartemicin derivative shows promising Mincle-mediated adjuvant activity

Structure activity relationship studies of lipidated Brartemicin analogues have revealed the potent adjuvant activity of ortho-substituted Brartemicin analogue 5a, which was better than that of p-OC₁₈ (5c) and C18dMeBrar (4).