The C-type Lectin Receptor CLEC12A Recognizes Plasmodial Hemozoin and Contributes to Cerebral Malaria Development

Synthetic hemozoin as a nanocarrier for cross-presentation

It is known that conventional antigen presentation involves phagocytosis of antigens followed by its internalization in endocytic compartments and presentation of epitopes through MHC class II molecules for CD4 T cells. However, since 1976 a cross-presentation pathway has been studied, in which CD8 T cells are activated via MHC class I with antigens acquired through phagocytosis or endocytosis by dendritic cells (DCs). Among some important molecules involved in the cross-presentation, the C-type lectin receptor of the Dectin-1 cluster (CLECs), particularly the CLEC9A receptor, not only is expressed in dendritic cells but also presents a pivotal role in this context. In special, CLEC12A has been highlighted as a malaria pigment hemozoin (HZ) receptor. During Plasmodium infection, hemozoin crystals defend the parasite against heme toxicity within erythrocytes, as well as the released native HZ elicits pro-inflammatory responses and can induce cross-presentation. Particularly, this crystal can be synthesized from hematin anhydride and mimics the native form, and the gaps generated between the nanocrystal domains during its synthesis allow for substance coupling followed by its coating. Therefore, this study aimed to assess whether synthetic hemozoin (sHz) or hematin anhydride could be a nanocarrier and promote cross-presentation in dendritic cells. Firstly, it was verified that sHz can carry coated and coupled antigens, the compounds can associate to LAMP1-positive vesicles and decrease overall intracellular pH, which can potentially enhance the cross-presentation of ovalbumin and Leishmania infantum antigens. Thus, this study adds important data in the molecular intricacies of antigen presentation by showing not only the sHz immunomodulatory properties but also its potential applications as an antigen carrier.

Myeloid C-type lectin receptors in innate immune recognition

C-type lectin receptors (CLRs) expressed by myeloid cells constitute a versatile family of receptors that play a key role in innate immune recognition. Myeloid CLRs exhibit a remarkable ability to recognize an extensive array of ligands, from carbohydrates and beyond, and encompass pattern-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), and markers of altered self. These receptors, classified into distinct subgroups, play pivotal roles in immune recognition and modulation of immune responses. Their intricate signaling pathways orchestrate a spectrum of cellular responses, influencing processes such as phagocytosis, cytokine production, and antigen presentation. Beyond their contributions to host defense in viral, bacterial, fungal, and parasitic infections, myeloid CLRs have been implicated in non-infectious diseases such as cancer, allergies, and autoimmunity. A nuanced understanding of myeloid CLR interactions with endogenous and microbial triggers is starting to uncover the context-dependent nature of their roles in innate immunity, with implications for therapeutic intervention.

Mechanistic insights into the C-type lectin receptor CLEC12A-mediated immune recognition of monosodium urate crystal

CLEC12A, a member of the C-type lectin receptor family involved in immune homeostasis, recognizes MSU crystals released from dying cells. However, the molecular mechanism underlying the CLEC12A-mediated recognition of MSU crystals remains unclear. Herein, we reported the crystal structure of the human CLEC12A-C-type lectin-like domain (CTLD) and identified a unique "basic patch" site on CLEC12A-CTLD that is necessary for the binding of MSU crystals. Meanwhile, we determined the interaction strength between CLEC12A-CTLD and MSU crystals using single-molecule force spectroscopy. Furthermore, we found that CLEC12A clusters at the cell membrane and seems to serve as an internalizing receptor of MSU crystals. Altogether, these findings provide mechanistic insights for understanding the molecular mechanisms underlying the interplay between CLEC12A and MSU crystals.

Pathogenetic mechanisms and treatment targets in cerebral malaria

Malaria, the most prevalent mosquito-borne infectious disease worldwide, has accompanied humanity for millennia and remains an important public health issue despite advances in its prevention and treatment. Most infections are asymptomatic, but a small percentage of individuals with a heavy parasite burden develop severe malaria, a group of clinical syndromes attributable to organ dysfunction. Cerebral malaria is an infrequent but life-threatening complication of severe malaria that presents as an acute cerebrovascular encephalopathy characterized by unarousable coma. Despite effective antiparasite drug treatment, 20% of patients with cerebral malaria die from this disease, and many survivors of cerebral malaria have neurocognitive impairment. Thus, an important unmet clinical need is to rapidly identify people with malaria who are at risk of developing cerebral malaria and to develop preventive, adjunctive and neuroprotective treatments for cerebral malaria. This Review describes important advances in the understanding of cerebral malaria over the past two decades and discusses how these mechanistic insights could be translated into new therapies.

Secreted NS1 proteins of tick-borne encephalitis virus and West Nile virus block dendritic cell activation and effector functions

The flavivirus non-structural protein 1 (NS1) is secreted from infected cells into the circulation and the serum levels correlate with disease severity. The effect of secreted NS1 (sNS1) on non-infected mammalian immune cells is largely unknown. Here, we expressed recombinant sNS1 proteins of tick-borne encephalitis virus (TBEV) and West Nile virus (WNV) and investigated their effects on dendritic cell (DC) effector functions. Murine bone marrow-derived DCs (BMDCs) showed reduced surface expression of co-stimulatory molecules and decreased release of pro-inflammatory cytokines when treated with sNS1 of TBEV or WNV prior to poly(I:C) stimulation. Transcriptional profiles of BMDCs that were sNS1-exposed prior to poly(I:C) stimulation showed two gene clusters that were downregulated by TBEV or WNV sNS1 and that were associated with innate and adaptive immune responses. Functionally, both sNS1 proteins modulated the capacity for BMDCs to induce specific T-cell responses as indicated by reduced IFN-γ levels in both CD4+ and CD8+ T cells after BMDC co-cultivation. In human monocyte-derived DCs, poly(I:C)-induced upregulation of co-stimulatory molecules and cytokine responses were even more strongly impaired by TBEV sNS1 or WNV sNS1 pretreatment than in the murine system. Our findings indicate that exogenous flaviviral sNS1 proteins interfere with DC-mediated stimulation of T cells, which is crucial for the initiation of cell-mediated adaptive immune responses in human flavivirus infections. Collectively, our data determine soluble flaviviral NS1 as a virulence factor responsible for a dampened immune response to flavivirus infections. IMPORTANCE The effective initiation of protective host immune responses controls the outcome of infection, and dysfunctional T-cell responses have previously been associated with symptomatic human flavivirus infections. We demonstrate that secreted flavivirus NS1 proteins modulate innate immune responses of uninfected bystander cells. In particular, sNS1 markedly reduced the capacity of dendritic cells to stimulate T-cell responses upon activation. Hence, by modulating cellular host responses that are required for effective antigen presentation and initiation of adaptive immunity, sNS1 proteins may contribute to severe outcomes of flavivirus disease.

A C-type lectin-like receptor CD302 in yellow drum (Nibea albiflora) functioning in antibacterial activity and innate immune signaling

Molecular dissection of disease resistance against Vibrio harveyi infection in yellow drum at the genome-wide level uncovered a C-type lectin-like receptor cluster of differentiation CD302 (named as YdCD302) in our previous study. Here, the gene expression pattern of YdCD302 and its function in mediating the defense response to V. harveyi attack were investigated. Gene expression analysis demonstrated that YdCD302 was ubiquitously distributed in various tissues with the highest transcript abundance in liver. The YdCD302 protein exhibited agglutination and antibacterial activity against V. harveyi cells. Binding assay indicated that YdCD302 can physically interact with V. harveyi cells in a Ca2+-independent manner, and the interaction can activate reactive oxygen species (ROS) production in the bacterial cells to induce RecA/LexA-mediated cell death. After infection with V. harveyi, the expression of YdCD302 can be up-regulated significantly in the main immune organs of yellow drum and potentially further trigger the cytokines involved innate immunity. These findings provide insight into the genetic basis of the disease resistance trait in yellow drum and shed light on the functioning of the CD302 C-type lectin-like receptor in host-pathogen interactions. The molecular and functional characterization of YdCD302 is a significant step towards a better understanding of disease resistance mechanisms and the development of new strategies for disease control.

Targeting dendritic cells to advance cross-presentation and vaccination outcomes

Dendritic cells (DCs) are a complex network of specialised antigen-presenting cells that are critical initiators of adaptive immunity. Targeting antigen directly to DCs in situ is a vaccination strategy that selectively delivers antigen to receptors expressed by DC subtypes. This approach exploits specific DC subset functions of antigen uptake and presentation. Here, we review DC-targeted vaccination strategies that are designed to elicit effective cross-presentation for CD8+ T cell immunity. In particular, we focus on approaches that exploit receptors highly expressed by mouse and human cDCs equipped with superior cross-presentation capacity. These receptors include DEC205, Clec9A and XCR1. Targeting DC receptors Clec12A, Clec4A4 and mannose receptor is also reviewed. Outcomes of DC-targeted vaccination in mouse models through to human clinical trials is discussed. This is a promising new vaccination approach capable of directly targeting the cross-presentation pathway for prevention and treatment of tumours and infectious diseases.

Clec12a tempers inflammation while restricting expansion of a colitogenic commensal

Regulation of the microbiota is critical to intestinal health yet the mechanisms employed by innate immunity remain unclear. Here we show that mice deficient in the C-Type-lectin receptor, Clec12a developed severe colitis, which was dependent on the microbiota. Fecal-microbiota-transplantation (FMT) studies into germfree mice revealed a colitogenic microbiota formed within Clec12a -/- mice that was marked by expansion of the gram-positive organism, Faecalibaculum rodentium . Treatment with F. rodentium was sufficient to worsen colitis in wild-type mice. Macrophages within the gut express the highest levels of Clec12a. Cytokine and sequencing analysis in Clec12a -/- macrophages revealed heighten inflammation but marked reduction in genes associated with phagocytosis. Indeed, Clec12a -/- macrophages are impaired in their ability to uptake F. rodentium. Purified Clec12a had higher binding to gram-positive organisms such as F. rodentium . Thus, our data identifies Clec12a as an innate immune surveillance mechanism to control expansion of potentially harmful commensals without overt inflammation.

CLEC12A Binds to Legionella pneumophila but Has No Impact on the Host's Antibacterial Response

Legionella pneumophila is an intracellular pathogen that can cause severe pneumonia after the inhalation of contaminated aerosols and replication in alveolar macrophages. Several pattern recognition receptors (PRRs) have been identified that contribute to the recognition of L. pneumophila by the innate immune system. However, the function of the C-type lectin receptors (CLRs), which are mainly expressed by macrophages and other myeloid cells, remains largely unexplored. Here, we used a library of CLR-Fc fusion proteins to search for CLRs that can bind the bacterium and identified the specific binding of CLEC12A to L. pneumophila. Subsequent infection experiments in human and murine macrophages, however, did not provide evidence for a substantial role of CLEC12A in controlling innate immune responses to the bacterium. Consistently, antibacterial and inflammatory responses to Legionella lung infection were not significantly influenced by CLEC12A deficiency. Collectively, CLEC12A is able to bind to L. pneumophila-derived ligands but does not appear to play a major role in the innate defense against L. pneumophila.

Immune gene expression in the mosquito vector Culex quinquefasciatus during an avian malaria infection

Plasmodium relictum is the most widespread avian malaria parasite in the world. It is listed as one of the 100 most dangerous invasive species, having been responsible for the extinction of several endemic bird species, and the near-demise of several others. Here we present the first transcriptomic study focused on the effect of P. relictum on the immune system of its vector (the mosquito Culex quinquefasciatus) at different times post-infection. We show that over 50% of immune genes identified as being part of the Toll pathway and 30%-40% of the immune genes identified within the Imd pathway are overexpressed during the critical period spanning the parasite's oocyst and sporozoite formation (8-12 days), revealing the crucial role played by both these pathways in this natural mosquito-Plasmodium combination. Comparison of infected mosquitoes with their uninfected counterparts also revealed some unexpected immune RNA expression patterns earlier and later in the infection: significant differences in expression of several immune effectors were observed as early as 30 min after ingestion of the infected blood meal. In addition, in the later stages of the infection (towards the end of the mosquito lifespan), we observed an unexpected increase in immune investment in uninfected, but not in infected, mosquitoes. In conclusion, our work extends the comparative transcriptomic analyses of malaria-infected mosquitoes beyond human and rodent parasites and provides insights into the degree of conservation of immune pathways and into the selective pressures exerted by Plasmodium parasites on their vectors.

The diverse roles of C-type lectin-like receptors in immunity

Our understanding of the C-type lectin-like receptors (CTLRs) and their functions in immunity have continued to expand from their initial roles in pathogen recognition. There are now clear examples of CTLRs acting as scavenger receptors, sensors of cell death and cell transformation, and regulators of immune responses and homeostasis. This range of function reflects an extensive diversity in the expression and signaling activity between individual CTLR members of otherwise highly conserved families. Adding to this diversity is the constant discovery of new receptor binding capabilities and receptor-ligand interactions, distinct cellular expression profiles, and receptor structures and signaling mechanisms which have expanded the defining roles of CTLRs in immunity. The natural killer cell receptors exemplify this functional diversity with growing evidence of their activity in other immune populations and tissues. Here, we broadly review select families of CTLRs encoded in the natural killer cell gene complex (NKC) highlighting key receptors that demonstrate the complex multifunctional capabilities of these proteins. We focus on recent evidence from research on the NKRP1 family of CTLRs and their interaction with the related C-type lectin (CLEC) ligands which together exhibit essential immune functions beyond their defined activity in natural killer (NK) cells. The ever-expanding evidence for the requirement of CTLR in numerous biological processes emphasizes the need to better understand the functional potential of these receptor families in immune defense and pathological conditions.

Mycobacterial mycolic acids trigger inhibitory receptor Clec12A to suppress host immune responses

Mycobacteria often cause chronic infection. To establish persistence in the host, mycobacteria need to evade host immune responses. However, the molecular mechanisms underlying the evasion strategy are not fully understood. Here, we demonstrate that mycobacterial cell wall lipids trigger an inhibitory receptor to suppress host immune responses. Mycolic acids are major cell wall components and are essential for survival of mycobacteria. By screening inhibitory receptors that react with mycobacterial lipids, we found that mycolic acids from various mycobacterial species bind to mouse Clec12A, and more potently to human Clec12A. Clec12A is a conserved inhibitory C-type lectin receptor containing immunoreceptor tyrosine-based inhibitory motif (ITIM). Innate immune responses, such as MCP-1 production, and PPD-specific recall T cell responses were augmented in Clec12A-deficient mice after infection. In contrast, human Clec12A transgenic mice were susceptible to infection with M. tuberculosis. These results suggest that mycobacteria dampen host immune responses by hijacking an inhibitory host receptor through their specific and essential lipids, mycolic acids. The blockade of this interaction might provide a therapeutic option for the treatment or prevention of mycobacterial infection.

Understanding inhibitory receptor function in neutrophils through the lens of CLEC12A

Neutrophils are the first leukocytes recruited from the circulation in response to invading pathogens or injured cells. To eradicate pathogens and contribute to tissue repair, recruited neutrophils generate and release a host of toxic chemicals that can also damage normal cells. To avoid collateral damage leading to tissue injury and organ dysfunction, molecular mechanisms evolved that tightly control neutrophil response threshold to activating signals, the strength and location of the response, and the timing of response termination. One mechanism of response control is interruption of activating intracellular signaling pathways by the 20 inhibitory receptors expressed by neutrophils. The two inhibitory C-type lectin receptors expressed by neutrophils, CLEC12A and DCIR, exhibit both common and distinct molecular and functional mechanisms, and they are associated with different diseases. In this review, we use studies on CLEC12A as a model of inhibitory receptor regulation of neutrophil function and participation in disease. Understanding the molecular mechanisms leading to inhibitory receptor specificity offers the possibility of using physiologic control of neutrophil functions as a pharmacologic tool to control inflammatory diseases.

Pathogenicity and transcriptome analysis of a strain of Vibrio owensii in Fenneropenaeus merguiensis

Vibrio is an important conditional pathogen in shrimp aquaculture. This research reported a dominant bacteria strain E1 isolated from a shrimp tank with the method of biofloc culture, which was further identified as Vibrio owensii. To understand the interaction between V. owensii and the host shrimp, we studied the pathogenicity of the V. owensii and the molecular mechanisms of the Fenneropenaeus merguiensis immunity during the Vibrio invasion. Drug susceptibility tests showed that V. owensii was resistant to antibiotics streptomycin oxacillin, tetracycline, minocycline, and aztreonam, but highly sensitive to cefazolin, cefotaxime, and ciprofloxacin, and moderately sensitive to cefotaxime, ampicillin, and piperacillin. Lethal concentration 50 (LC₅₀) test was performed to evaluate the toxicity of V. owensii to F. merguiensis. The LC₅₀ of V. owensii infected F. merguiensis after 24, 48, 72, 96, 120, 144 and 168 h were 1.21 × 10⁷, 1.68 × 10⁶, 6.36 × 10⁵, 2.15 × 10⁵, 7.58 × 10⁴, 5.55 × 10⁴ and 4.33 × 10⁴ CFU/mL. In order to explore the molecular response mechanism of F. merguiensis infected with V. owensii, the hepatopancreas of F. merguiensis were sequenced at 24 hpi and 48 hpi, and a total 40,181 of unigenes were obtained. Through comparative transcriptomic analysis, 86 differentially expressed genes (DEGs) (including 38 up-regulated DEGs, and 48 down-regulated DEGs) and 305 DEGs (including 150 up-regulated DEGs, and 155 down-regulated DEGs) were identified at 24 hpi and 48 hpi, respectively. Annotation and classification analysis of these 391 DEGs showed that most of the DEGs were annotated to metableolic and immune pathways, which indicated that F. merguiensis responded to the invasion through the regulation of material metableolism and immune system genes during V. owensii infection. In the KEGG enrichment analysis, some pathways related to immune response were significantly influenced by V. owensii infection, including phagosome, MAPK signalling pathway and PI3K-Akt signalling pathway. In addition, some pathways related to the warburg effect were also significantly enriched after V. owensii infection, including pyruvate metableolism, glycolysis/gluconeogenesis, and citrate cycle (TAC cycle). Further analysis showed that C-type lectins and ficolin were also play important roles in the immune response of F. merguiensis against V. owensii infection. The current research preliminarily revealed the immune response of F. merguiensis to V. owensii infection at the molecular level, which provided valuable information to further understand the disease control and the interaction between shrimp and Vibrio.

Self-referential immune recognition through C-type lectin receptors

The term "lectin" is derived from the Latin word lego- (aggregate) (Boyd & Shapleigh, 1954). Indeed, lectins' folds can flexibly alter their pocket structures just like Lego blocks, which enables them to grab a wide-variety of substances. Thus, this useful fold is well-conserved among various organisms. Through evolution, prototypic soluble lectins acquired transmembrane regions and signaling motifs to become C-type lectin receptors (CLRs). While CLRs seem to possess certain intrinsic affinity to self, some CLRs adapted to efficiently recognize glycoconjugates present in pathogens as pathogen-associated molecular patterns (PAMPs) and altered self. CLRs further extended their diversity to recognize non-glycosylated targets including pathogens and self-derived molecules. Thus, CLRs seem to have developed to monitor the internal/external stresses to maintain homeostasis by sensing various "unfamiliar" targets. In this review, we will summarize recent advances in our understanding of CLRs, their ligands and functions and discuss future perspectives.

Malaria parasite heme biosynthesis promotes and griseofulvin protects against cerebral malaria in mice

Heme-biosynthetic pathway of malaria parasite is dispensable for asexual stages, but essential for mosquito and liver stages. Despite having backup mechanisms to acquire hemoglobin-heme, pathway intermediates and/or enzymes from the host, asexual parasites express heme pathway enzymes and synthesize heme. Here we show heme synthesized in asexual stages promotes cerebral pathogenesis by enhancing hemozoin formation. Hemozoin is a parasite molecule associated with inflammation, aberrant host-immune responses, disease severity and cerebral pathogenesis. The heme pathway knockout parasites synthesize less hemozoin, and mice infected with knockout parasites are protected from cerebral malaria and death due to anemia is delayed. Biosynthetic heme regulates food vacuole integrity and the food vacuoles from knockout parasites are compromised in pH, lipid unsaturation and proteins, essential for hemozoin formation. Targeting parasite heme synthesis by griseofulvin-a FDA-approved antifungal drug, prevents cerebral malaria in mice and provides an adjunct therapeutic option for cerebral and severe malaria.

Vector and Host C-Type Lectin Receptor (CLR)-Fc Fusion Proteins as a Cross-Species Comparative Approach to Screen for CLR-Rift Valley Fever Virus Interactions

Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus endemic to Africa and the Arabian Peninsula, which causes diseases in humans and livestock. C-type lectin receptors (CLRs) represent a superfamily of pattern recognition receptors that were reported to interact with diverse viruses and contribute to antiviral immune responses but may also act as attachment factors or entry receptors in diverse species. Human DC-SIGN and L-SIGN are known to interact with RVFV and to facilitate viral host cell entry, but the roles of further host and vector CLRs are still unknown. In this study, we present a CLR–Fc fusion protein library to screen RVFV–CLR interaction in a cross-species approach and identified novel murine, ovine, and Aedes aegypti RVFV candidate receptors. Furthermore, cross-species CLR binding studies enabled observations of the differences and similarities in binding preferences of RVFV between mammalian CLR homologues, as well as more distant vector/host CLRs.

Hemozoin: a Complex Molecule with Complex Activities

Purpose of Review

Malaria is a disease caused by parasites that reside in host red blood cells and use hemoglobin as a nutrient source. Heme released by hemoglobin catabolism is modified by the parasite to produce hemozoin (HZ), which has toxic effects on the host. Experimentation aiming to elucidate how HZ contributes to malaria pathogenesis has utilized different preparations of this molecule, complicating interpretation and comparison of findings. We examine natural synthesis and isolation of HZ and highlight studies that have used multiple preparations, including synthetic forms, in a comparative fashion.

Recent Findings

Recent work utilizing sophisticated imaging and detection techniques reveals important molecular characteristics of HZ synthesis and biochemistry. Other recent studies further refine understanding of contributions of HZ to malaria pathogenesis yet highlight the continuing need to characterize HZ preparations and contextualize experimental conditions in the in vivo infection milieu.

Summary

This review highlights the necessity of collectively determining what is physiologically relevant HZ. Characterization of isolated natural HZ and use of multiple preparations in each study are recommended with application of in vivo studies whenever possible. Adoption of such practices is expected to improve reproducibility of results and elucidate the myriad of ways that HZ participates in malaria pathogenesis.

Additional C-type lectin receptors mediate interactions with Pneumocystis organisms and major surface glycoprotein

Introduction. Pathogen-associated molecular patterns' (PAMPs) are microbial signatures that are recognized by host myeloid C-type lectin receptors (CLRs). These CLRs interact with micro-organisms via their carbohydrate recognition domains (CRDs) and engage signalling pathways within the cell resulting in pro-inflammatory and microbicidal responses.Gap statement. In this article, we extend our laboratory study of additional CLRs that recognize fungal ligands against Pneumocystis murina and Pneumocystis carinii and their purified major surface glycoproteins (Msgs).Aim. To study the potential of newly synthesized hFc-CLR fusions on binding to Pneumocystis and its Msg.Methods. A library of new synthesized hFc-CLR fusions was screened against Pneumocystis murina and Pneumocystis carinii organisms and their purified major surface glycoproteins (Msgs) found on the respective fungi via modified ELISA. Immunofluorescence assay (IFA) was implemented and quantified to verify results. mRNA expression analysis by quantitative PCR (q-PCR) was employed to detect respective CLRs found to bind fungal organisms in the ELISA and determine their expression levels in the mouse immunosuppressed Pneumocystis pneumonia (PCP) model.Results. We detected a number of the CLR hFc-fusions displayed significant binding with P. murina and P. carinii organisms, and similarly to their respective Msgs. Significant organism and Msg binding was observed for CLR members C-type lectin domain family 12 member A (CLEC12A), Langerin, macrophage galactose-type lectin-1 (MGL-1), and specific intracellular adhesion molecule-3 grabbing non-integrin homologue-related 3 (SIGNR3). Immunofluorescence assay (IFA) with the respective CLR hFc-fusions against whole P. murina life forms corroborated these findings. Lastly, we surveyed the mRNA expression profiles of the respective CLRs tested above in the mouse immunosuppressed Pneumocystis pneumonia (PCP) model and determined that macrophage galactose type C-type lectin (Mgl-1), implicated in recognizing terminal N-acetylgalactosamine (GalNAc) found in the glycoproteins of microbial pathogens was significantly up-regulated during infection.Conclusion. The data herein add to the growing list of CLRs recognizing Pneumocystis and provide insights for further study of organism/host immune cell interactions.

Fc-conjugated C-type lectin receptors: Tools for understanding host-pathogen interactions

The use of soluble fusion proteins of pattern recognition receptors (PRRs) used in the detection of exogenous and endogenous ligands has helped resolve the roles of PRRs in the innate immune response to pathogens, how they shape the adaptive immune response, and function in maintaining homeostasis. Using the immunoglobulin (Ig) crystallizable fragment (Fc) domain as a fusion partner, the PRR fusion proteins are soluble, stable, easily purified, have increased affinity due to the Fc homodimerization properties, and consequently have been used in a wide range of applications such as flow cytometry, screening of protein and glycan arrays, and immunofluorescent microscopy. This review will predominantly focus on the recognition of pathogens by the cell membrane-expressed glycan-binding proteins of the C-type lectin receptor (CLR) subgroup of PRRs. PRRs bind to conserved pathogen-associated molecular patterns (PAMPs), such as glycans, usually located within or on the outer surface of the pathogen. Significantly, many glycans structures are identical on both host and pathogen (e.g. the Lewis (Le) X glycan), allowing the use of Fc CLR fusion proteins with known endogenous and/or exogenous ligands as tools to identify pathogen structures that are able to interact with the immune system. Screens of highly purified pathogen-derived cell wall components have enabled identification of many unique PAMP structures recognized by CLRs. This review highlights studies using Fc CLR fusion proteins, with emphasis on the PAMPs found in fungi, bacteria, viruses, and parasites. The structure and unique features of the different CLR families is presented using examples from a broad range of microbes whenever possible.

Hemozoin-mediated inflammasome activation limits long-lived anti-malarial immunity

During acute malaria, most individuals mount robust inflammatory responses that limit parasite burden. However, long-lived sterilizing anti-malarial memory responses are not efficiently induced, even following repeated Plasmodium exposures. Using multiple Plasmodium species, genetically modified parasites, and combinations of host genetic and pharmacologic approaches, we find that the deposition of the malarial pigment hemozoin directly limits the abundance and capacity of conventional type 1 dendritic cells to prime helper T cell responses. Hemozoin-induced dendritic cell dysfunction results in aberrant Plasmodium-specific CD4 T follicular helper cell differentiation, which constrains memory B cell and long-lived plasma cell formation. Mechanistically, we identify that dendritic cell-intrinsic NLRP3 inflammasome activation reduces conventional type 1 dendritic cell abundance, phagocytosis, and T cell priming functions in vivo. These data identify biological consequences of hemozoin deposition during malaria and highlight the capacity of the malarial pigment to program immune evasion during the earliest events following an initial Plasmodium exposure.

Expanding the Known Repertoire of C-Type Lectin Receptors Binding to Toxoplasma gondii Oocysts Using a Modified High-Resolution Immunofluorescence Assay

The environmental stage of the apicomplexan Toxoplasma gondii oocyst is vital to its life cycle but largely understudied. Because oocysts are excreted only by infected felids, their availability for research is limited. We report the adaptation of an agarose-based method to immobilize minute amounts of oocysts to perform immunofluorescence assays. Agarose embedding allows high-resolution confocal microscopy imaging of antibodies binding to the oocyst surface as well as unprecedented imaging of intracellular sporocyst structures with Maclura pomifera agglutinin after on-slide permeabilization of the immobilized oocysts. To identify new possible molecules binding to the oocyst surface, we used this method to screen a library of C-type lectin receptor (CLR)-human IgG constant region fusion proteins from the group of related CLRs called the Dectin-1 cluster against oocysts. In addition to CLEC7A that was previously reported to decorate T. gondii oocysts, we present experimental evidence for specific binding of three additional CLRs to the surface of this stage. We discuss how these CLRs, known to be expressed on neutrophils, dendritic cells, or macrophages, could be involved in the early immune response by the host, such as oocyst antigen uptake in the intestine. In conclusion, we present a modified immunofluorescence assay technique that allows material-saving immunofluorescence microscopy with T. gondii oocysts in a higher resolution than previously published, which allowed us to describe three additional CLRs binding specifically to the oocyst surface.

IMPORTANCE Knowledge of oocyst biology of Toxoplasma gondii is limited, not the least due to its limited availability. We describe a method that permits us to process minute amounts of oocysts for immunofluorescence microscopy without compromising their structural properties. This method allowed us to visualize internal structures of sporocysts by confocal microscopy in unprecedented quality. Moreover, the method can be used as a low- to medium-throughput method to screen for molecules interacting with oocysts, such as antibodies, or compounds causing structural damage to oocysts (i.e., disinfectants). Using this method, we screened a small library of C-type lectin receptors (CLRs) present on certain immune cells and found three CLRs able to decorate the oocyst wall of T. gondii and which were not known before to bind to oocysts. These tools will allow further study into oocyst wall composition and could also provoke experiments regarding immunological recognition of oocysts.

Toxocara canis and Toxocara cati Somatic and Excretory-Secretory Antigens Are Recognised by C-Type Lectin Receptors

Toxocara canis and Toxocara cati, the worldwide occurring intestinal roundworms of canids and felids, represent an important public health threat due to various disease manifestations in humans. Host recognition of pathogens is mediated by pattern recognition receptors (PRRs). Myeloid C-type lectin receptors (CLRs) are PRRs and recognise carbohydrate structures of various pathogens. As Toxocara excretory-secretory products (TES) are predominantly composed of glycoconjugates, they represent suitable targets for CLRs. However, the range of host-derived CLRs recognising Toxocara spp. is still unknown. Using a CLR-hFc fusion protein library, T. canis and T. cati L3 somatic antigens (TSOM) were bound by a variety of CLRs in enzyme-linked immunosorbent assay (ELISA), while their TES products interacted with macrophage galactose-type lectin-1 (MGL-1). Two prominent candidate CLRs, MGL-1 and macrophage C-type lectin (MCL), were selected for further binding studies. Immunofluorescence microscopy revealed binding of MGL-1 to the oral aperture of L3. Immunoblot experiments identified distinct protein fractions representing potential ligands for MGL-1 and MCL. To evaluate how these interactions influence the host immune response, bone marrow-derived dendritic cell (BMDC) assays were performed, showing MCL-dependent T. cati-mediated cytokine production. In conclusion, MGL-1 and MCL are promising candidates for immune modulation during Toxocara infection, deserving further investigation in the future.

Hemozoin Promotes Lung Inflammation via Host Epithelial Activation

Respiratory distress (RD) is a complication of severe malaria associated with a particularly high risk for death in African children infected with the parasite Plasmodium falciparum. The pathophysiology underlying RD remains poorly understood, and the condition is managed supportively.

Respiratory distress in severe malaria is associated with high mortality, but its pathogenesis remains unclear. The malaria pigment hemozoin (HZ) is abundant in target organs of severe malaria, including the lungs, and is known to be a potent innate immune activator of phagocytes. We hypothesized that HZ might also stimulate lung epithelial activation and thereby potentiate lung inflammation. We show here that airway epithelium stimulated with HZ undergoes global transcriptional reprogramming and changes in cell surface protein expression that comprise an epithelial activation phenotype. Proinflammatory signaling is induced, and key cytoadherence molecules are upregulated, including several associated with severe malaria, such as CD36 and ICAM1. Epithelial and extracellular matrix remodeling pathways are transformed, including induction of key metalloproteases and modulation of epithelial junctions. The overall program induced by HZ serves to promote inflammation and neutrophil transmigration, and is recapitulated in a murine model of HZ-induced acute pneumonitis. Together, our data demonstrate a direct role for hemozoin in stimulating epithelial activation that could potentiate lung inflammation in malaria.

Setting the stage: The initial immune response to blood-stage parasites

Individuals growing up in malaria endemic areas gradually develop protection against clinical malaria and passive transfer experiments in humans have demonstrated that this protection is mediated in part by protective antibodies. However, neither the target antigens, specific effector mechanisms, nor the role of continual parasite exposure have been elucidated, which complicates vaccine development. Progress has been made in defining the innate signaling pathways activated by parasite components, including DNA, RNA, hemozoin, and phospholipids, which initiate the immune response and will be the focus of this review. The challenge that remains within the field is to understand the role of these early responses in the development of protective adaptive responses that clear iRBC and block merozoite invasion so that optimal vaccines and therapeutics may be produced.

Sensing Tissue Damage by Myeloid C-Type Lectin Receptors

After both sterile and infectious insults, damage is inflicted on tissues leading to accidental or programmed cell death. In addition, events of programmed cell death also take place under homeostatic conditions, such as in embryo development or in the turnover of hematopoietic cells. Mammalian tissues are seeded with myeloid immune cells, which harbor a plethora of receptors that allow the detection of cell death, modulating immune responses. The myeloid C-type lectin receptors (CLRs) are one of the most prominent families of receptors involved in tailoring immunity after sensing dead cells. In this chapter, we will cover a diversity of signals arising from different forms of cell death and how they are recognized by myeloid CLRs. We will also explore how myeloid cells develop their sentinel function, exploring how some of these CLRs identify cell death and the type of responses triggered thereof. In particular, we will focus on DNGR-1 (CLEC9A), Mincle (CLEC4E), CLL-1 (CLEC12A), LOX-1 (OLR1), CD301 (CLEC10A) and DEC-205 (LY75) as paradigmatic death-sensing CLRs expressed by myeloid cells. The molecular processes triggered after cell death recognition by myeloid CLRs contribute to the regulation of immune responses in pathologies associated with tissue damage, such as infection, autoimmunity and cancer. A better understanding of these processes may help to improve the current approaches for therapeutic intervention.

Hemozoin in Malarial Complications: More Questions Than Answers

Plasmodium parasites contain various virulence factors that modulate the host immune response. Malarial pigment, or hemozoin (Hz), is an undegradable crystalline product of the hemoglobin degradation pathway in the parasite and possesses immunomodulatory properties. An association has been found between Hz accumulation and severe malaria, suggesting that the effects of Hz on the host immune response may contribute to the development of malarial complications. Although the immunomodulatory roles of Hz have been widely investigated, many conflicting data exist, likely due to the variability between experimental set-ups and technical limitations of Hz generation and isolation methods. Here, we critically assess the potential immunomodulatory effects of Hz, its role in malarial complications, and its potential effects after parasite clearance.

C-Type Lectins in Veterinary Species: Recent Advancements and Applications

C-type lectins (CTLs), a superfamily of glycan-binding receptors, play a pivotal role in the host defense against pathogens and the maintenance of immune homeostasis of higher animals and humans. CTLs in innate immunity serve as pattern recognition receptors and often bind to glycan structures in damage- and pathogen-associated molecular patterns. While CTLs are found throughout the whole animal kingdom, their ligand specificities and downstream signaling have mainly been studied in humans and in model organisms such as mice. In this review, recent advancements in CTL research in veterinary species as well as potential applications of CTL targeting in veterinary medicine are outlined.

Ovine C-type lectin receptor hFc-fusion protein library - A novel platform to screen for host-pathogen interactions

C-type lectin receptors (CTLRs) are pattern recognition receptors which are important constituents of the innate immunity. However, their role has mostly been studied in humans and in mouse models. To bridge the knowledge gap concerning CTLRs of veterinary relevant species, a novel ovine CTLR hFc-fusion protein library which allows in vitro ligand identification and pathogen binding studies has been established. Its utility was tested with known ligands of corresponding murine CTLRs in ELISA- and flow cytometry based binding studies. The ovine CTLR-hFc library was subsequently used in a proof-of-principle pathogen binding study with the ruminant pathogen Mycoplasma mycoides subsp. capri. Some ovine CTLRs, such as Dendritic Cell Immunoreceptor (DCIR, Clec4a), Macrophage C-Type Lectin (MCL, Clec4d) and Myeloid Inhibitory C-Type Lectin-Like Receptor (MICL, Clec12a) were identified as possible candidate receptors whose role in Mycoplasma recognition can now be unraveled in further studies. This study thus shows the utility of this novel ovine CTLR-hFc fusion protein library to screen for CTLR/pathogen interactions.

Innate immunity to malaria-The role of monocytes

Monocytes are innate immune cells essential for host protection against malaria. Upon activation, monocytes function to help reduce parasite burden through phagocytosis, cytokine production, and antigen presentation. However, monocytes have also been implicated in the pathogenesis of severe disease through production of damaging inflammatory cytokines, resulting in systemic inflammation and vascular dysfunction. Understanding the molecular pathways influencing the balance between protection and pathology is critical. In this review, we discuss recent data regarding the role of monocytes in human malaria, including studies of innate sensing of the parasite, immunometabolism, and innate immune training. Knowledge gained from these studies may guide rational development of novel antimalarial therapies and inform vaccine development.