Synthetic β-1,2-Mannosyloxymannitol Glycolipid from the Fungus Malassezia pachydermatis Signals through Human Mincle

Card9 and MyD88 differentially regulate Th17 immunity to the commensal yeast Malassezia in the murine skin

The fungal community of the skin microbiome is dominated by a single genus, Malassezia. Besides its symbiotic lifestyle at the host interface, this commensal yeast has also been associated with diverse inflammatory skin diseases in humans and pet animals. Stable colonization is maintained by antifungal type 17 immunity. The mechanisms driving Th17 responses to Malassezia remain, however, unclear. Here, we show that the C-type lectin receptors Mincle, Dectin-1, and Dectin-2 recognize conserved patterns in the cell wall of Malassezia and induce dendritic cell activation in vitro, while only Dectin-2 is required for Th17 activation during experimental skin colonization in vivo. In contrast, Toll-like receptor recognition was redundant in this context. Instead, inflammatory IL-1 family cytokines signaling via MyD88 were also implicated in Th17 activation in a T cell-intrinsic manner. Taken together, we characterized the pathways contributing to protective immunity against the most abundant member of the skin mycobiome. This knowledge contributes to the understanding of barrier immunity and its regulation by commensals and is relevant considering how aberrant immune responses are associated with severe skin pathologies.

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.

Card9 and MyD88 differentially regulate Th17 immunity to the commensal yeast Malassezia in the murine skin

The fungal community of the skin microbiome is dominated by a single genus, Malassezia. Besides its symbiotic lifestyle at the host interface, this commensal yeast has also been associated with diverse inflammatory skin diseases in humans and pet animals. Stable colonization is maintained by antifungal type 17 immunity. The mechanisms driving Th17 responses to Malassezia remain, however, unclear. Here, we show that the C-type lectin receptors Mincle, Dectin-1, and Dectin-2 recognize conserved patterns in the cell wall of Malassezia and induce dendritic cell activation in vitro, while only Dectin-2 is required for Th17 activation during experimental skin colonization in vivo. In contrast, Toll-like receptor recognition was redundant in this context. Instead, inflammatory IL-1 family cytokines signaling via MyD88 were also implicated in Th17 activation in a T cell-intrinsic manner. Taken together, we characterized the pathways contributing to protective immunity against the most abundant member of the skin mycobiome. This knowledge contributes to the understanding of barrier immunity and its regulation by commensals and is relevant considering how aberrant immune responses are associated with severe skin pathologies.

Archaeal Glycerolipids Are Recognized by C-Type Lectin Receptor Mincle

Recently, various metabolites derived from host microbes have been reported to modulate the immune system, with potential involvement in health or diseases. Archaea, prokaryotic organisms, are present in the human body, but their connection with the host is largely unknown when compared to other microorganisms such as bacteria. This study focused on unique glycerolipids from symbiotic methanogenic archaea and evaluated their activities toward an innate immune receptor. The results revealed that archaeal lipids were recognized by the C-type lectin receptor Mincle and induced immune responses. A concurrent structure-activity relationship study identified the key structural features of archaeal lipids required for recognition by Mincle. Subsequent gene expression profiling suggested qualitative differences between the symbiotic archaeal lipid and the pathogenic bacteria-derived lipid. These findings have broad implications for understanding the function of symbiotic archaea in host health and diseases.

Characterisation of the bovine C-type lectin receptor Mincle and potential evidence for an endogenous ligand

Innate immune receptors that form complexes with secondary receptors, activating multiple signalling pathways, modulate cellular activation and play essential roles in regulating homeostasis and immunity. We have previously identified a variety of bovine C-type lectin-like receptors that possess similar functionality than their human orthologues. Mincle (CLEC4E), a heavily glycosylated monomer, is involved in the recognition of the mycobacterial component Cord factor (trehalose 6,6'-dimycolate). Here we characterise the bovine homologue of Mincle (boMincle), and demonstrate that the receptor is structurally and functionally similar to the human orthologue (huMincle), although there are some notable differences. In the absence of cross-reacting antibodies, boMincle-specific antibodies were created and used to demonstrate that, like the human receptor, boMincle is predominantly expressed by myeloid cells. BoMincle surface expression increases during the maturation of monocytes to macrophages. However, boMincle mRNA transcripts were also detected in granulocytes, B cells, and T cells. Finally, we show that boMincle binds to isolated bovine CD4⁺ T cells in a specific manner, indicating the potential to recognise endogenous ligands. This suggests that the receptor might also play a role in homeostasis in cattle.

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.

From the banal to the bizarre: unravelling immune recognition and response to microbial lipids

Microbes produce a rich array of lipidic species that through their location in the cell wall and ability to mingle with host lipids represent a privileged class of immune-active molecules. Lipid-sensing immunity recognizes microbial lipids from pathogens and commensals causing immune responses. Yet microbial lipids are often heterogeneous, in limited supply and in some cases their structures are incompletely defined. Total synthesis can assist in structural determination, overcome supply issues, and provide access to high-purity, homogeneous samples and analogues. This account highlights synthetic approaches to lipidic species from pathogenic and commensal bacteria and fungi that have supported immunological studies involving lipid sensing through the pattern recognition receptor Mincle and cell-mediated immunity through the CD1-T cell axis.

Candida albicans steryl 6-O-acyl-α-D-mannosides agonize signalling through Mincle

The C-type lectin receptor Mincle binds Candida albicans and has been implicated in its pathobiology, but the molecular effectors responsible have not been identified. We report the synthesis of cholesteryl and ergosteryl 6-O-acyl-α-d-mannosides, produced by C. albicans mycelium, and demonstrate their ability to signal through human and mouse Mincle.

Chemical Strategies to Boost Cancer Vaccines

Personalized cancer vaccines (PCVs) are reinvigorating vaccine strategies in cancer immunotherapy. In contrast to adoptive T-cell therapy and checkpoint blockade, the PCV strategy modulates the innate and adaptive immune systems with broader activation to redeploy antitumor immunity with individualized tumor-specific antigens (neoantigens). Following a sequential scheme of tumor biopsy, mutation analysis, and epitope prediction, the administration of neoantigens with synthetic long peptide (SLP) or mRNA formulations dramatically improves the population and activity of antigen-specific CD4+ and CD8+ T cells. Despite the promising prospect of PCVs, there is still great potential for optimizing prevaccination procedures and vaccine potency. In particular, the arduous development of tumor-associated antigen (TAA)-based vaccines provides valuable experience and rational principles for augmenting vaccine potency which is expected to advance PCV through the design of adjuvants, delivery systems, and immunosuppressive tumor microenvironment (TME) reversion since current personalized vaccination simply admixes antigens with adjuvants. Considering the broader application of TAA-based vaccine design, these two strategies complement each other and can lead to both personalized and universal therapeutic methods. Chemical strategies provide vast opportunities for (1) exploring novel adjuvants, including synthetic molecules and materials with optimizable activity, (2) constructing efficient and precise delivery systems to avoid systemic diffusion, improve biosafety, target secondary lymphoid organs, and enhance antigen presentation, and (3) combining bioengineering methods to innovate improvements in conventional vaccination, "smartly" re-educate the TME, and modulate antitumor immunity. As chemical strategies have proven versatility, reliability, and universality in the design of T cell- and B cell-based antitumor vaccines, the union of such numerous chemical methods in vaccine construction is expected to provide new vigor and vitality in cancer treatment.

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.

Malassezia Yeasts in Veterinary Dermatology: An Updated Overview

Lipophilic yeasts of the genus Malassezia are important skin commensals and opportunistic skin pathogens in a variety of animals. The species M. pachydermatis was first isolated from the skin of a captive Indian rhinoceros with an exfoliative dermatitis in 1925, recognized as an important otic pathogen of dogs in the 1950's, and finally accepted, after several years of controversy, as a common cause of canine dermatitis in the 1990's. Since then, there has been considerable research into the biology of Malassezia yeasts and their interaction with their animal hosts. In dogs and cats, M. pachydermatis is associated with ceruminous otitis externa and a "seborrhoeic" dermatitis, wherein pruritic, erythematous skin lesions, often with brown/black greasy, malodourous material matting hairs, preferentially develop in intertriginous areas. Skin disease is favored by folds, underlying hypersensitivity disorders, endocrinopathies, defects of cornification, and in cats, various visceral paraneoplastic syndromes. Diagnosis is based on detecting the yeast in compatible skin lesions, usually by cytology, and observing a clinical and mycological response to therapy. Treatment normally comprises topical or systemic azole therapy, often with miconazole-chlorhexidine shampoos or oral itraconazole or ketoconazole. Management of concurrent diseases is important to minimize relapses. Historically, wild-type Malassezia isolates from dogs and cats were typically susceptible to azoles, with the exception of fluconazole, but emerging azole resistance in field strains has recently been associated with either mutations or quadruplication of the ERG11 gene. These observations have prompted increased interest in alternative topical antifungal drugs, such as chlorhexidine, and various essential oils. Further clinical trials are awaited with interest.