CR3 and Dectin-1 Collaborate in Macrophage Cytokine Response through Association on Lipid Rafts and Activation of Syk-JNK-AP-1 Pathway

Collaboration between heterogeneous pattern recognition receptors (PRRs) leading to synergistic coordination of immune response is important for the host to fight against invading pathogens. Although complement receptor 3 (CR3) and Dectin-1 are major PRRs to detect fungi, crosstalk between these two receptors in antifungal immunity is largely undefined. Here we took advantage of Histoplasma capsulatum which is known to interact with both CR3 and Dectin-1 and specific particulate ligands to study the collaboration of CR3 and Dectin-1 in macrophage cytokine response. By employing Micro-Western Array (MWA), genetic approach, and pharmacological inhibitors, we demonstrated that CR3 and Dectin-1 act collaboratively to trigger macrophage TNF and IL-6 response through signaling integration at Syk kinase, allowing subsequent enhanced activation of Syk-JNK-AP-1 pathway. Upon engagement, CR3 and Dectin-1 colocalize and form clusters on lipid raft microdomains which serve as a platform facilitating their cooperation in signaling activation and cytokine production. Furthermore, in vivo studies showed that CR3 and Dectin-1 cooperatively participate in host defense against disseminated histoplasmosis and instruct adaptive immune response. Taken together, our findings define the mechanism of receptor crosstalk between CR3 and Dectin-1 and demonstrate the importance of their collaboration in host defense against fungal infection.     

The carbohydrate-recognition domain of Dectin-2 is a C-type lectin with specificity for high mannose

We examined the carbohydrate-binding potential of the C-type lectin-like receptor Dectin-2 (Clecf4n). The carbohydrate-recognition domain (CRD) of Dectin-2 exhibited cation-dependent mannose/fucose-like lectin activity, with an IC(50) for mannose of approximately 20 mM compared to an IC(50) of 1.5 mM for the macrophage mannose receptor when assayed by similar methodology. The extracellular domain of Dectin-2 exhibited binding to live Candida albicans and the Saccharomyces-derived particle zymosan. This binding was completely abrogated by cation chelation and was competed by yeast mannans. We compared the lectin activity of Dectin-2 with that of two other C-type lectin receptors (mannose receptor and SIGNR1) known to bind fungal mannans. Both mannose receptor and SIGNR1 were able to bind bacterial capsular polysaccharides derived from Streptococcus pneumoniae, but interestingly they exhibited distinct binding profiles. The Dectin-2 CRD exhibited only weak interactions to some of these capsular polysaccharides, indicative of different structural or affinity requirements for binding, when compared with the other two lectins. Glycan array analysis of the carbohydrate recognition by Dectin-2 indicated specific recognition of high-mannose structures (Man(9)GlcNAc(2)). The differences in the specificity of these three mannose-specific lectins indicate that mannose recognition is mediated by distinct receptors, with unique specificity, that are expressed by discrete subpopulations of cells, and this further highlights the complex nature of carbohydrate recognition by immune cells.     

Stage-Specific Sampling by Pattern Recognition Receptors during Candida albicans Phagocytosis

Candida albicans is a medically important pathogen, and recognition by innate immune cells is critical for its clearance. Although a number of pattern recognition receptors have been shown to be involved in recognition and phagocytosis of this fungus, the relative role of these receptors has not been formally examined. In this paper, we have investigated the contribution of the mannose receptor, Dectin-1, and complement receptor 3; and we have demonstrated that Dectin-1 is the main non-opsonic receptor involved in fungal uptake. However, both Dectin-1 and complement receptor 3 were found to accumulate at the site of uptake, while mannose receptor accumulated on C. albicans phagosomes at later stages. These results suggest a potential role for MR in phagosome sampling; and, accordingly, MR deficiency led to a reduction in TNF-α and MCP-1 production in response to C. albicans uptake. Our data suggest that pattern recognition receptors sample the fungal phagosome in a sequential fashion.     

Pattern-recognition receptor signaling initiated from extracellular, membrane, and cytoplasmic space

Invading pathogens are recognized by diverse germline-encoded pattern-recognition receptors (PRRs) which are distributed in three different cellular compartments: extracellular, membrane, and cytoplasmic. In mammals, the major extracellular PRRs such as complements may first encounter the invading pathogens and opsonize them for clearance by phagocytosis which is mediated by membrane-associated phagocytic receptors including complement receptors. The major membrane-associated PRRs, Toll-like receptors, recognize diverse pathogens and generate inflammatory signals to coordinate innate immune responses and shape adaptive immune responses. Furthemore, certain membrane-associated PRRs such as Dectin-1 can mediate phagocytosis and also induce inflammatory response. When these more forefront detection systems are avoided by the pathogens, cytoplasmic PRRs may play major roles. Cytoplasmic caspase-recruiting domain (CARD) helicases such as retinoic acid-inducible protein I (RIG-I)melanoma differentiation-associated gene 5 (MDA5), mediate antiviral immunity by inducing the production of type I interferons. Certain members of nucleotide-binding oligomerization domain (NOD)-like receptors such as NALP3 present in the cytosol form inflammasomes to induce inflammatory responses upon ligand recognition. Thus, diverse families of PRRs coordinately mediate immune responses against diverse types of pathogens.     

C-type Lectin Receptor Dectin-3 Mediates Trehalose 6,6′-Dimycolate (TDM)-induced Mincle Expression through CARD9/Bcl10/MALT1-dependent Nuclear Factor (NF)-κB Activation

Previous studies indicate that both Dectin-3 (also called MCL or Clec4d) and Mincle (also called Clec4e), two C-type lectin receptors, can recognize trehalose 6,6′-dimycolate (TDM), a cell wall component from mycobacteria, and induce potent innate immune responses. Interestingly, stimulation of Dectin-3 by TDM can also induce Mincle expression, which may enhance the host innate immune system to sense Mycobacterium infection. However, the mechanism by which Dectin-3 induces Mincle expression is not fully defined. Here, we show that TDM-induced Mincle expression is dependent on Dectin-3-mediated NF-κB, but not nuclear factor of activated T-cells (NFAT), activation, and Dectin-3 induces NF-κB activation through the CARD9-BCL10-MALT1 complex. We found that bone marrow-derived macrophages from Dectin-3-deficient mice were severely defective in the induction of Mincle expression in response to TDM stimulation. This defect is correlated with the failure of TDM-induced NF-κB activation in Dectin-3-deficient bone marrow-derived macrophages. Consistently, inhibition of NF-κB, but not NFAT, impaired TDM-induced Mincle expression, whereas NF-κB, but not NFAT, binds to the Mincle promoter. Dectin-3-mediated NF-κB activation is dependent on the CARD9-Bcl10-MALT1 complex. Finally, mice deficient for Dectin-3 or CARD9 produced much less proinflammatory cytokines and keyhole limpet hemocyanin (KLH)-specific antibodies after immunization with an adjuvant containing TDM. Overall, this study provides the mechanism by which Dectin-3 induces Mincle expression in response to Mycobacterium infection, which will have significant impact to improve adjuvant and design vaccine for antimicrobial infection.     

The Syk/CARD9-coupled receptor Dectin-1 is not required for host resistance to Mycobacterium tuberculosis in mice

There is interest in identifying the pattern recognition receptors involved in initiating protective or non-protective host responses to Mycobacterium tuberculosis (Mtb). Here we explored the role of the Syk/CARD9-coupled receptor, Dectin-1, using an aerosol model of Mtb infection in wild-type and Dectin-1 deficient mice. We observed a reduction in pulmonary bacilli burdens in the Dectin-1 deficient animals, but this did not correlate with significant changes in pulmonary pathology, cytokine levels or ability of these animals to survive the infection. Thus Dectin-1 makes a minor contribution to susceptibility to Mtb infections in mice.     

Polymorphisms of Dectin-1 and TLR2 Predispose to Invasive Fungal Disease in Patients with Acute Myeloid Leukemia

Background

Patients with acute myeloid leukemia (AML) who undergo induction chemotherapy are at high risk for invasive fungal disease (IFD). Dectin-1, a C-type lectin family member represents one of the most important pattern recognition receptors of the innate immune system and single nucleotide polymorphisms (SNPs) in the Dectin-1 gene have been associated with an increased risk of infectious complications. We sought to investigate the impact of three different Dectin-1 SNPs and one TLR2 SNP on developing IFD in 186 adult patients with newly diagnosed AML following anthracycline-based induction chemotherapy.

Patients and methods

Genotyping of Dectin-1 SNPs (rs16910526, rs3901533 and rs7309123) and TLR2 SNP (rs5743708) was performed by TaqMan method and pyrosequencing. IFD was defined according to the EORTC/MSG consensus guidelines. Multiple logistic regression analyses were applied to evaluate the association between the polymorphisms and the occurrence of pulmonary infections. Dectin-1 expression studies with SNP genotyped human monocytes were performed to elucidate susceptibility to IFD following chemotherapy.

Results

We could demonstrate that patients carrying the Dectin-1 SNP rs7309123 G/G (n = 47) or G/G and C/G (n = 133) genotype revealed a significant higher risk for developing both pneumonia in general (adjusted odds ratio (OR): 2.5; p = 0.014 and OR: 3.0, p = 0.004) and pulmonary IFD (OR: 2.6; p = 0.012 and OR: 2.4, p = 0.041, respectively). Patients carrying the TLR2 SNP rs5743708 (R753Q, GA/AA genotype, n = 12) also revealed a significantly higher susceptibility to pneumonia including IFD. Furthermore, Dectin-1 mRNA expression in human monocytes was lower following chemotherapy.

Conclusion

To our best knowledge, this study represents the first analysis demonstrating that harbouring polymorphisms of Dectin-1 (rs7309123) or TLR2 (rs5743708) represents an independent risk factor of developing IFD in patients with AML undergoing induction chemotherapy.     

Phospholipase Cγ2 (PLCγ2) is key component in Dectin-2 signaling pathway, mediating anti-fungal innate immune responses

C-type lectin receptors (CLRs) such as Dectin-2 function as pattern recognition receptors to sense fungal infection. However, the signaling pathways induced by these receptors remain largely unknown. Previous studies suggest that the CLR-induced signaling pathway may utilize similar signaling components as the B cell receptor-induced signaling pathway. Phospholipase Cγ2 (PLCγ2) is a key component in B cell receptor signaling, but its role in other signaling pathways has not been fully characterized. Here, we show that PLCγ2 functions downstream of Dectin-2 in response to the stimulation by the hyphal form of Candida albicans, an opportunistic pathogenic fungus. Using PLCγ2- and PLCγ1-deficient macrophages, we found that the lack of PLCγ2, but not PLCγ1, impairs cytokine production in response to infection with C. albicans. PLCγ2 deficiency results in the defective activation of NF-κB and MAPK and a significantly reduced production of reactive oxygen species following fungal challenge. In addition, PLCγ2-deficient mice are defective in clearing C. albicans infection in vivo. Together, these findings demonstrate that PLCγ2 plays a critical role in CLR-induced signaling pathways, governing antifungal innate immune responses.     

Innate immune recognition of microbial cell wall components and microbial strategies to evade such recognitions

The innate immune system constitutes the first line of defence against invading microbes. The basis of this defence resides in the recognition of defined structural motifs of the microbes called “Microbial associated molecular patterns” that are absent in the host. Cell wall, the outer layer of both bacterial and fungal cells, a unique structure that is absent in the host and is recognized by the germ line encoded host receptors. Nucleotide oligomerization domain proteins, peptidoglycan recognition proteins and C-type lectins are host receptors that are involved in the recognition of bacterial cell wall (usually called peptidoglycan), whereas fungal cell wall components (N- and O-linked mannans, β-glucans etc.) are recognized by host receptors like C-type lectins (Dectin-1, Dectin-2, mannose receptor, DC-SIGN), Toll like receptors-2 and -4 (TLR-2 and TLR-4). These recognitions lead to activation of a variety of host signaling cascades and ultimate production of anti-microbial compounds including phospholipase A2, antimicrobial peptides, lysozyme, reactive oxygen and nitrogen species. These molecules act in cohort against the invading microbes to eradicate infections. Additionally pathogen recognition leads to the production of cytokines, which further activate the adaptive immune system. Both pathogenic and commensal bacteria and fungus use numerous strategies to subvert the host defence. These strategies include bacterial peptidoglycan glycan backbone modifications by O-acetylation, N-deacetylation, N-glycolylation and stem peptide modifications by amidation of meso-Diaminopimelic acid; fungal cell wall modifications by shielding the β-glucan layer with mannoproteins and α-1,3 glucan. This review focuses on the recent advances in understanding the role of bacterial and fungal cell wall in their innate immune recognition and evasion strategies.     

Dectin-1 and DC-SIGN polymorphisms associated with invasive pulmonary Aspergillosis infection

The recognition of pathogen-derived structures by C-type lectins and the chemotactic activity mediated by the CCL2/CCR2 axis are critical steps in determining the host immune response to fungi. The present study was designed to investigate whether the presence of single nucleotide polymorphisms (SNPs) within DC-SIGN, Dectin-1, Dectin-2, CCL2 and CCR2 genes influence the risk of developing Invasive Pulmonary Aspergillosis (IPA). Twenty-seven SNPs were selected using a hybrid functional/tagging approach and genotyped in 182 haematological patients, fifty-seven of them diagnosed with proven or probable IPA according to the 2008 EORTC/MSG criteria. Association analysis revealed that carriers of the Dectin-1(rs3901533 T/T) and Dectin-1(rs7309123 G/G) genotypes and DC-SIGN(rs4804800 G), DC-SIGN(rs11465384 T), DC-SIGN(7248637 A) and DC-SIGN(7252229 C) alleles had a significantly increased risk of IPA infection (OR = 5.59 95%CI 1.37-22.77; OR = 4.91 95%CI 1.52-15.89; OR = 2.75 95%CI 1.27-5.95; OR = 2.70 95%CI 1.24-5.90; OR = 2.39 95%CI 1.09-5.22 and OR = 2.05 95%CI 1.00-4.22, respectively). There was also a significantly increased frequency of galactomannan positivity among patients carrying the Dectin-1(rs3901533_T) allele and Dectin-1(rs7309123_G/G) genotype. In addition, healthy individuals with this latter genotype showed a significantly decreased level of Dectin-1 mRNA expression compared to C-allele carriers, suggesting a role of the Dectin-1(rs7309123) polymorphism in determining the levels of Dectin-1 and, consequently, the level of susceptibility to IPA infection. SNP-SNP interaction (epistasis) analysis revealed significant interactions models including SNPs in Dectin-1, Dectin-2, CCL2 and CCR2 genes, with synergistic genetic effects. Although these results need to be further validated in larger cohorts, they suggest that Dectin-1, DC-SIGN, Dectin-2, CCL2 and CCR2 genetic variants influence the risk of IPA infection and might be useful in developing a risk-adapted prophylaxis.     

Innate immune responses to Mycobacterium ulcerans via toll-like receptors and dectin-1 in human keratinocytes

Mycobacterium ulcerans (MU), an environmental pathogen, causes Buruli ulcer, a severe skin disease. We hypothesized that epidermal keratinocytes might not be a simple barrier, but play a role during MU infection through pattern-recognition receptors expressed in keratinocytes. We found that keratinocyte Toll-like receptors (TLRs) 2 and 4 and Dectin-1 actively participate in the innate immune response to MU, which includes the internalization of bacteria, the production of reactive oxygen species (ROS), and the expression of chemokines and LL-37. Human keratinocytes constitutively expressed TLRs 2 and 4 and induced Dectin-1 in response to MU. Exposing keratinocytes to MU resulted in rapid ROS production, which in turn contributed to the mRNA and protein expression of LL-37. In addition, TLR2, Dectin-1 and, to an extent, TLR4 are essential for the MU-mediated expression of CXCL8, CCL2 and LL-37 in keratinocytes. Furthermore, confocal analysis showed that the Dectin-1 is necessary for keratinocytes to internalize bacilli. Importantly, blockade of ROS and LL-37 significantly increased the intracellular MU growth in keratinocytes, suggesting an important role of these mediators for cutaneous innate immune responses. Our results demonstrate that TLR2, TLR4 and Dectin-1 actively sense, internalize and respond in an innate way to MU in human epidermal keratinocytes.     

The role of Dectin-1 in the host defence against fungal infections

Dectin-1 is an innate immune pattern recognition receptor (PRR) that, through its ability to bind β-glucans, is involved in the recognition of several pathogenic fungi. Dectin-1 can stimulate a variety of cellular responses via the Syk/CARD9 signalling pathway, including phagocytosis, cytokine production and the respiratory burst. Several advances in our understanding of Dectin-1 immunobiology have been made in recent years, including characterisation of additional signalling pathways and demonstration of its ability to directly induce the development of adaptive immunity. However, the physiological role of many of the functions of this receptor is still unclear. This review aims to provide an update on Dectin-1 and its role within antifungal immune responses, focussing on progress made in the last two years.     

Dectin-1-triggered Recruitment of Light Chain 3 Protein to Phagosomes Facilitates Major Histocompatibility Complex Class II Presentation of Fungal-derived Antigens

Dectin-1 is a pattern recognition receptor that is important for innate immune responses against fungi in humans and mice. Dectin-1 binds to β-glucans in fungal cell walls and triggers phagocytosis, production of reactive oxygen by the NADPH oxidase, and inflammatory cytokine production which all contribute to host immune responses against fungi. Although the autophagy pathway was originally characterized for its role in the formation of double-membrane compartments engulfing cytosolic organelles and debris, recent studies have suggested that components of the autophagy pathway may also participate in traditional phagocytosis. In this study, we show that Dectin-1 signaling in macrophages and bone marrow-derived dendritic cells triggers formation of LC3II, a major component of the autophagy machinery. Further, Dectin-1 directs the recruitment of LC3II to phagosomes, and this requires Syk, activation of reactive oxygen production by the NADPH oxidase, and ATG5. Using LC3-deficient dendritic cells we show that whereas LC3 recruitment to phagosomes is not important for triggering phagocytosis, killing or Dectin-1-mediated inflammatory cytokine production, it facilitates recruitment of MHC class II molecules to phagosomes and promotes presentation of fungal-derived antigens to CD4 T cells.     

The C-type lectin-like receptors of Dectin-1 cluster in natural killer gene complex

Natural killer gene complex (NKC) encodes a group of proteins with a single C-type lectin-like domain, (CTLD) which can be subdivided several subfamilies according to their structures and expression patterns. The receptors containing the conserved calcium binding sites in the CTLD fold belong to group II of C-type lectin superfamily and are expressed on myeloid cells and non- myeloid cells. The receptors lacking conserved calcium binding sites in the CTLD fold have evolved to bind ligands other than carbohydrates independently on calcium and thereby are named as C-type lectin-like receptors. The C-type lectin-like receptors are previously thought to be exclusively expressed on natural killer (NK) cells and enable NK cells to discriminate self, missing self or altered self. However, some C-type lectin-like receptors are identified in myeloid cells and are intensely investigated, recently. These myeloid C-type lectin-like receptors, especially Dectin-1 cluster, have a wide variety of ligands, including those of exogenous origin, and play important roles in the physiological functions and pathological processes including immune homeostasis, immune defenses, and immune surveillance. In this review, we summarize each member of the Dectin-1 cluster, including their structural profiles, expression patterns, signaling properties as well as known physiological functions.     

Dectin-1 stimulation induces suppressor of cytokine signaling 1, thereby modulating TLR signaling and T cell responses

Suppressor of cytokine signaling (SOCS) proteins serve as negative regulators of cytokine receptor signaling. However, SOCS proteins are not only induced via the JAK/STAT pathway, but are also transcribed on triggering of pattern recognition receptors such as TLRs. We now show that SOCS1 can also be induced by the non-TLR pattern recognition receptor Dectin-1 in murine bone marrow-derived dendritic cells and macrophages (BMMs). The C-type lectin Dectin-1 binds to yeasts and signals either in an autonomous manner or can be triggered in combination with TLRs. In our study, SOCS1 was expressed independently of any TLR engagement as a direct target gene of the Dectin-1 ligand Zymosan. Induction of SOCS1 was mediated by a novel pathway encompassing the tyrosine kinases Src and Syk that activated the downstream kinase proline-rich tyrosine kinase 2. Proline-rich tyrosine kinase 2, in turn, caused activation of the MAPK ERK, thereby triggering SOCS1 induction. SOCS1 did not modulate Dectin-1 signaling but affected TLR signaling, leading to decreased and abbreviated NF-κB activation in BMMs triggered by TLR9. Furthermore, IL-12 and IL-10 secretion were inhibited by SOCS1. We additionally observed that IL-17-producing Th cells were clearly increased by SOCS1 in BMMs. Our results show that SOCS1 is expressed via a new, NF-κB-independent pathway in Dectin-1-triggered murine BMMs and influences TLR cross talk and T cell priming.     

The effects of some Curdlan derivatives on Dectin-1 expression and cytokine production in human peripheral blood mononuclear cells

The cells of immune system such as monocytes and macrophages are in first line defence against dangerous signals. In the present paper the recognition of Dectin 1 receptors and the modulation of Interleukin-10 (IL-10) and Tumor Necrosis Factor-alpha (TNF-alpha) cytokine production by Curdlan and Curdlan derivatives in peripheral blood mononuclear cells (PBMCs) were studied. The effect of Curdlan or Curdlan derivatives on the expression of Dectin 1 receptors in PBMCs was revealed by flow-cytometry and the levels of IL-10 and TNFalpha were measured by ELISA kit in supernatants of PBMCs cultured in presence or absence of Curdlan, Curdlan derivatives and LPS. Our results suggested that Curdlan and Curdlan derivatives were able to increase the expression of Dectin-1 receptors on monocyte cells. The combined treatment of Curdlan/Curdlan derivatives and Pam3Cys produced an increase of CD14+ cells possessing Dectin-1 receptors. We demonstrated that Curdlan (at 20 microg unique dose) up-regulated TNF-alpha production and down-regulated IL-10 production in PBMCs. Conversely, Palm CM/SP-Curdlan (20 microg unique dose) was able to down-regulate TNF-alpha production and to up-regulate IL-10 production in PBMCs. For instance, Palm CM/SP-Curdlan determined a 5 times decrease of TNF-alpha production than Curdlan. Regarding the effect of Palm CM/SP-Curdlan on IL-10 production in PBMCs, we noticed that the level of IL-10 was about 4 times greater than Curdlan activity. We observed that a combined treatment of Curdlan/Curdlan derivatives and LPS induced about 5 times decrease in TNF-alpha production in PBMCs. IL-10 production induced by Palm CM/SP-Curdlan and LPS was about 6 times greater than the combined effect of Curdlan and LPS. The treatment of PBMCs with SP-Curdlan alone affected neither TNF-alpha production nor IL-10 production. Our results are in accordance with other studies demonstrating that Dectin-1 and TLR2/TLR6 signaling combine to enhance the responses triggered by each receptor and the signaling pathway induced by Dectin-1 could mediate the production of pro-inflammatory cytokines.     

Characterization of the human beta -glucan receptor and its alternatively spliced isoforms

beta-1,3-d-Glucans are biological response modifiers with potent effects on the immune system. A number of receptors are thought to play a role in mediating these responses, including murine Dectin-1, which we recently identified as a beta-glucan receptor. In this study we describe the characterization of the human homologue of this receptor and show that it is structurally and functionally similar to the mouse receptor. The human beta-glucan receptor is a type II transmembrane receptor with a single extracellular carbohydrate recognition domain and an immunoreceptor tyrosine activation motif in its cytoplasmic tail. The human beta-glucan receptor is widely expressed and functions as a pattern recognition receptor, recognizing a variety of beta-1,3- and/or beta-1,6-linked glucans as well as intact yeast. In contrast to the murine receptor, the human receptor mRNA is alternatively spliced, resulting in two major (A and B) and six minor isoforms. The two major isoforms differ by the presence of a stalk region separating the carbohydrate recognition domain from the transmembrane region and are the only isoforms that are functional for beta-glucan binding. The human receptor also binds T-lymphocytes at a site distinct from the beta-glucan binding site, indicating that this receptor can recognize both endogenous and exogenous ligands.     

The Expression of Dectin-1, Irak1 and Rip2 During the Host Response to <Emphasis Type="Italic">Aspergillus fumigatus</Emphasis>

Background

C-type lectin receptors (CLRs), Toll-like receptors (TLRs), and Nod-like receptors (NLRs) have the ability to recognize Aspergillus fumigatus (A. fumigates) and induce innate immune response. Dectin-1 is a well-described CLR, while interleukin-1 receptor-associated kinase 1 (Irak1) and receptor-interacting protein 2 (Rip2) are pivotal adaptor proteins of TLRs and NLRs signaling pathways, respectively.

Objectives

Our primary aim is to elucidate whether Dectin-1 regulates the expression of Irak1 and Rip2, and confirm that CLRs, TLRs, and NLRs pathways act synergistically in response to A. fumigatus infection.

Methods

Pulmonary infection mouse models were established. Myeloid cells were differentiated in cell culture and examined by inverted microscopy, flow cytometry, and scanning electron microscopy. The relative mRNA levels were determined by qRT-PCR. The protein expression levels were determined by immunohistochemistry and Western blot.

Results

The expression of Dectin-1, Irak1, Rip2, and phosphorylation level of nuclear factor (NF)-κB p65 were induced by conidia in immunocompetent mice, while their expression and phosphorylation level were inhibited in immunocompromised mice after the administration of conidia. Conidia increased the expression of Dectin-1, Irak1, and Rip2 in myeloid cells, while Dectin-1 silencing significantly reduced their expression.

Conclusion

Our findings demonstrate that Dectin-1, Irak1, and Rip2 are involved in response to A. fumigatus infection. Dectin-1 modulates the expression of Irak1 and Rip2. Additionally, these three signaling pathways are interconnected, and CLRs pathway plays a dominant role against A. fumigatus invasion.

     

The fungal pattern recognition receptor, Dectin-1, and the associated cluster of C-type lectin-like receptors

The mammalian natural killer gene complex (NKC) contains several families of type II transmembrane C-type lectin-like receptors (CLRs) that are best known for their involvement in the detection of virally infected or transformed cells, through the recognition of endogenous (or self) proteinacious ligands. However, certain CLR families within the NKC, particularly those expressed by myeloid cells, recognize structurally diverse ligands and perform a variety of other immune and homoeostatic functions. One such family is the 'Dectin-1 cluster' of CLRs, which includes MICL, CLEC-2, CLEC12B, CLEC9A, CLEC-1, Dectin-1 and LOX-1. Here, we review each of these CLRs, exploring our current understanding of their ligands and functions and highlighting where they have provided new insights into the underlying mechanisms of immunity and homeostasis.