Human and mouse macrophage-inducible C-type lectin (Mincle) bind Candida albicans

Interplay between Candida albicans and the mammalian innate host defense

Candida albicans is both the most common fungal commensal microorganism in healthy individuals and the major fungal pathogen causing high mortality in at-risk populations, especially immunocompromised patients. In this review, we summarize the interplay between the host innate system and C. albicans, ranging from how the host recognizes, responds, and clears C. albicans infection to how C. albicans evades, dampens, and escapes from host innate immunity.

Signaling by myeloid C-type lectin receptors in immunity and homeostasis

Myeloid cells are key drivers of physiological responses to pathogen invasion or tissue damage. Members of the C-type lectin receptor (CLR) family stand out among the specialized receptors utilized by myeloid cells to orchestrate these responses. CLR ligands include carbohydrate, protein, and lipid components of both pathogens and self, which variably trigger endocytic, phagocytic, proinflammatory, or anti-inflammatory reactions. These varied outcomes rely on a versatile system for CLR signaling that includes tyrosine-based motifs that recruit kinases, phosphatases, or endocytic adaptors as well as nontyrosine-based signals that modulate the activation of other pathways or couple to the uptake machinery. Here, we review the signaling properties of myeloid CLRs and how they impact the role of myeloid cells in innate and adaptive immunity.

Fungal surface and innate immune recognition of filamentous fungi

The innate immune system performs specific detection of molecules from infectious agents through pattern recognition receptors. This recognition triggers inflammatory responses and activation of microbicidal mechanisms by leukocytes. Infections caused by filamentous fungi have increased in incidence and represent an important cause of mortality and morbidity especially in individuals with immunosuppression. This review will discuss the innate immune recognition of filamentous fungi molecules and its importance to infection control and disease.

A20 and ABIN-3 possibly promote regression of trehalose 6,6'-dimycolate (TDM)-induced granuloma by interacting with an NF-kappa B signaling protein, TAK-1

OBJECTIVE

The objective of this paper is to examine the role of NF-kappa B inhibitors A20 and ABIN-family proteins in the trehalose 6,6'-dimycolate (TDM)-induced model of tuberculous granulomatous lesions.

MATERIALS AND METHODS

BALB/c mice were twice injected i.p. with w/o/w emulsions that contain TDM at a 1 week-interval. The mice were killed at days 0, 3, 7, 14, or 21 after the last injection. The mRNA and protein levels of A20 and ABIN-family proteins were measured by real-time PCR using mRNA or protein extract from the lesions. The activation status of NF-kappa B was analyzed by Western blotting and immunohistochemistry. Finally, the protein extracts were immunoprecipitated by anti-ABIN-3 antibody to identify the protein that potentially interacts with ABIN-3.

RESULTS

The activation of NF-kappa B pathway coincided with granuloma development, while A20 and ABIN-3 increased in accordance with granuloma regression. TAK-1 protein was co-precipitated with ABIN-3 by immunoprecipitation using anti-ABIN-3 antibody.

CONCLUSION

The results suggest that ABIN-3 contributed to granuloma regression by interacting with TAK-1 and, as a consequence, inhibiting activation of NF-kappa B pathway.

The classical CD14⁺⁺ CD16⁻ monocytes, but not the patrolling CD14⁺ CD16⁺ monocytes, promote Th17 responses to Candida albicans

In the present study, we investigated the functional differences between cluster of differentiation (CD)14(++) CD16(-) and CD14(+) CD16(+) monocytes during anti-Candida host defense. CD14(++) CD16(-) are the "classical" monocytes and represent the majority of circulating monocytes in humans, while CD14(+) CD16(+) monocytes patrol the vasculature for maintenance of tissue integrity and repair. Both monocyte subsets inhibited the germination of live Candida albicans, and there was no difference in their capacity to phagocytose and kill Candida. Although production of IL-6 and IL-10 induced by C. albicans was found to be similar between monocyte subsets, IL-1β and prostaglandin E2 (PGE2) production was higher in CD14(++) CD16(-) compared with CD14(+) CD16(+) monocytes. In line with the increased production of IL-1β and PGE2, central mediators for inducing Th17 responses, CD14(++) CD16(-) monocytes induced greater Th17 responses upon stimulation with heat-killed C. albicans yeast. The percentage of cells that expressed mannose receptor (MR) was higher in the CD14(++) CD16(-) monocyte subset, and MR-specific stimulation induced higher Th17 responses only in co-cultures of CD14(++) CD16(-) monocytes and CD4 lymphocytes. In conclusion, both monocyte subsets have potent innate antifungal properties, but only CD14(++) CD16(-) monocytes are capable of inducing a potent Th17 response to C. albicans, an important component of antifungal host defense.

2011 ESCI Award for Excellence in Basic / Translational Research: innate regulation of adaptive immunity by dendritic cells

Dendritic cells (DC) play a key role in the initiation of adaptive immunity, and the manipulation and/or targeting of DC has great potential for immune intervention. However, clinical applications are hampered by the fact that we still know relatively little about how DC become 'activated' to stimulate and direct T-cell responses. Over the last decade, much emphasis has been placed on dissecting innate signalling pathways that can trigger DC activation and promote T-cell priming. Here, we review work from our laboratory aimed at helping define 'pattern-recognition pathways' involved in DC activation by potential pathogens. One pathway for sensing infection by RNA viruses involves recognition of viral genomes or virally infected cells in endosomal compartments and utilises members of the toll-like receptor (TLRs) family, including TLR9, 7, or 3. RNA virus genomes can additionally be recognised in the cytosol by DExD/H-box helicases such as MDA5 or RIG-I, the latter of which is activated by RNAs bearing 5' tri-phosphates. Finally, a distinct pathway involves cell surface and phagosomal recognition of fungi by C-type lectins, which signal via Syk kinase. Notably, some of these pathways are involved not only in direct sensing of pathogens but also in the recognition of self-alterations that might accompany infection, such as induction of cell death. These studies help build a global picture of the receptors and signalling pathways that regulate DC activation and have applications in immunotherapy of cancer and infectious diseases.

Myeloid C-type lectin receptors in pathogen recognition and host defense

C-type lectin receptors (CLRs) comprise a heterogeneous group of transmembrane proteins. Many of them are expressed in myeloid cells and signal in response to pathogen-derived or self ligands to initiate or regulate cell activation. Here, we review the properties of myeloid CLRs, highlighting how their signaling function is coordinated with that of other innate receptor families to control immunity to infection.

Syk-coupled C-type lectins in immunity

The Syk-coupled C-type lectin receptor Dectin-1 was the first non-Toll like receptor described that could mediate its own intracellular signalling. It was initially identified as important for the innate recognition of and response to fungal pathogens but later studies revealed that it is also involved in triggering adaptive immune responses. It subsequently emerged that Dectin-1 is one of a number of spleen tyrosine kinase-coupled C-type lectin receptors that have been implicated not just in fungal immunity, but also in viral, mycobacterial and helminth infections. Here, we consider the ability of these receptors to trigger different aspects of immunity and highlight their emerging roles in a number of infection scenarios.

The role of Syk/CARD9-coupled C-type lectin receptors in immunity to Mycobacterium tuberculosis infections

There is increasing interest in understanding the mechanisms underlying the interactions that occur between Mycobacterium tuberculosis and host innate immune cells. These cells express pattern recognition receptors (PRRs) which recognise mycobacterial pathogen-associated molecular patterns (PAMPs) and which can influence the host immune response to the infection. Although many of the PRRs appear to be redundant in the control of M. tuberculosis infection in vivo, recent discoveries have revealed a key, nonredundant, role of the Syk/CARD9 signalling pathway in antimycobacterial immunity. Here we review these discoveries, as well as recent data investigating the role of the Syk/CARD9-coupled PRRs that have been implicated in mycobacterial recognition, including Dectin-1 and Mincle.

Inhibitory C-type lectin receptors in myeloid cells

C-type lectin receptors encoded by the natural killer gene complex play critical roles in enabling NK cell discrimination between self and non-self. In recent years, additional genes at this locus have been identified with patterns of expression that extend to cells of the myeloid lineage where many of the encoded inhibitory receptors have equally important functions as regulators of immune homeostasis. In the present review we highlight the roles of some of these receptors including recent insights gained with regard to the identification of exogenous and endogenous ligands, mechanisms of cellular inhibition and activation, regulated expression within different cellular and immune contexts, as well as functions that include the regulation of bone homeostasis and involvement in autoimmunity.

Host responses to Candida albicans: Th17 cells and mucosal candidiasis

Candida albicans causes mucosal and disseminated candidiasis, which represent serious problems for the rapidly expanding immunocompromised population. Until recently, Th1-mediated immunity was thought to confer the primary protection, particularly for oral candidiasis. However, emerging data indicate that the newly-defined Th17 compartment appears to play the predominant role in mucosal candidiasis.

Epithelial cells and innate antifungal defense

The human pathogenic fungus Candida albicans is the predominant cause of both superficial and invasive forms of candidiasis. Clinical observations indicate that mucocutaneous Candida infections are commonly associated with defective cell-mediated immune responses. The importance of the innate immune system as a first-line defense against pathogenic challenge has long been recognized. Over the last decade, many key molecules mediating innate host defense have been identified. Central to these developments is the discovery of pattern recognition receptors such as Toll-like receptors and C-type lectin-receptors that induce innate immune responses and also modulate cellular and humoral adaptive immunity during Candida infections. Although a large amount of information is now available in systemic infections, little is known about localized infections. We address the most relevant pattern recognition receptors and their signaling mechanisms in oral epithelial cells, to gain a better understanding of their contributions to antifungal innate immunity.

Mincle is a long sought receptor for mycobacterial cord factor

Mycobacterium tuberculosis is a leading killer worldwide, yet the adjuvancy of its cell wall has proven to be a valuable therapeutic tool for vaccination and immunotherapy. Much research effort has focused on the mycobacterial glycolipid trehalose-6,6’-dimycolate (TDM), a potent immunostimulant that is also known as cord factor. Now, the identification of the monocyte-inducible C-type lectin (Mincle) as an essential receptor for TDM provides new insight into the formation of the characteristic granulomas in tuberculosis and an avenue for rational adjuvant design.

The expanding roles of ITAM adapters FcRgamma and DAP12 in myeloid cells

The adapter proteins DAP12 and FcRgamma associate with a wide spectrum of receptors in a variety of innate immune cells to mediate intracellular signaling pathways when their cognate receptor is engaged. These adapter proteins are coupled to their receptors through charged residues within the transmembrane regions of the adapter and receptor. DAP12 and FcRgamma contain specific protein domains (referred to as immunoreceptor tyrosine-based activation motifs) that serve as the substrates and docking sites for kinases, allowing amplification of intracellular signaling reactions. Recent research has broadened the repertoire of receptors that utilize these adapters for signaling to include not only novel immunoglobulin superfamily members but also cytokine receptors, integrins, and other adhesion molecules. There is abundant evidence that these multifunctional signaling adapters also mediate inhibitory activity, downmodulating signaling from Toll-like receptors and other heterologous receptors. In this review, we discuss the newly described receptors that utilize DAP12 and/or FcRgamma adapters to modulate innate immune responses.

A homozygous CARD9 mutation in a family with susceptibility to fungal infections

BACKGROUND

Chronic mucocutaneous candidiasis may be manifested as a primary immunodeficiency characterized by persistent or recurrent infections of the mucosa or the skin with candida species. Most cases are sporadic, but both autosomal dominant inheritance and autosomal recessive inheritance have been described.

METHODS

We performed genetic studies in 36 members of a large, consanguineous five-generation family, in which 4 members had recurrent fungal infections and an additional 3 members died during adolescence, 2 after invasive infection of the brain with candida species. All 36 family members were enrolled in the study, and 22 had blood samples taken for DNA analysis. Homozygosity mapping was used to locate the mutated gene. In the 4 affected family members (patients) and the 18 unaffected members we sequenced CARD9, the gene encoding the caspase recruitment domain-containing protein 9, carried out T-cell phenotyping, and performed functional studies, with the use of either leukocytes from the patients or a reconstituted murine model of the genetic defect.

RESULTS

We found linkage (lod score, 3.6) to a genomic interval on chromosome 9q, including CARD9. All four patients had a homozygous point mutation in CARD9, resulting in a premature termination codon (Q295X). Healthy family members had wild-type expression of the CARD9 protein; the four patients lacked wild-type expression, which was associated with low numbers of Th17 cells (helper T cells producing interleukin-17). Functional studies based on genetic reconstitution of myeloid cells from Card9(-/-) mice showed that the Q295X mutation impairs innate signaling from the antifungal pattern-recognition receptor dectin-1.

CONCLUSIONS

An autosomal recessive form of susceptibility to chronic mucocutaneous candidiasis is associated with homozygous mutations in CARD9.

Crosstalk between components of the innate immune system: promoting anti-microbial defenses and avoiding immunopathologies

Because it reaches full functional efficacy rapidly upon encounter with a pathogen, the innate immune system is considered as the first line of defense against infections. The sensing of microbes or of transformed or infected cells, through innate immune recognition receptors (referred to as activating I2R2), initiates pro-inflammatory responses and innate immune effector functions. Other I2R2 with inhibitory properties bind self-ligands constitutively expressed in host. However, this dichotomy in the recognition of foreign or induced self versus constitutive self by I2R2 is not always respected in certain non-infectious conditions reminiscent of immunopathologies. In this review, we discuss that immune mechanisms have evolved to avoid inappropriate inflammatory disorders in individuals. Molecular crossregulation exists between components of I2R2 signaling pathways, and intricate interactions between cells from both innate and adaptive immune systems set the bases of controlled immune responses. We also pinpoint that, like T or B cells, some cells of the innate immune system must go through education processes to prevent autoreactivity. In addition, we illustrate how gene expression profiling of immune cell types is a useful tool to find functional homologies between cell subsets of different species and to speculate about unidentified functions of these cells in the responses to pathogen infections.

Syk kinase signalling couples to the Nlrp3 inflammasome for anti-fungal host defence

Fungal infections represent a serious threat, particularly in immunocompromised patients. Interleukin-1beta (IL-1beta) is a key pro-inflammatory factor in innate antifungal immunity. The mechanism by which the mammalian immune system regulates IL-1beta production after fungal recognition is unclear. Two signals are generally required for IL-1beta production: an NF-kappaB-dependent signal that induces the synthesis of pro-IL-1beta (p35), and a second signal that triggers proteolytic pro-IL-1beta processing to produce bioactive IL-1beta (p17) via Caspase-1-containing multiprotein complexes called inflammasomes. Here we demonstrate that the tyrosine kinase Syk, operating downstream of several immunoreceptor tyrosine-based activation motif (ITAM)-coupled fungal pattern recognition receptors, controls both pro-IL-1beta synthesis and inflammasome activation after cell stimulation with Candida albicans. Whereas Syk signalling for pro-IL-1beta synthesis selectively uses the Card9 pathway, inflammasome activation by the fungus involves reactive oxygen species production and potassium efflux. Genetic deletion or pharmalogical inhibition of Syk selectively abrogated inflammasome activation by C. albicans but not by inflammasome activators such as Salmonella typhimurium or the bacterial toxin nigericin. Nlrp3 (also known as NALP3) was identified as the critical NOD-like receptor family member that transduces the fungal recognition signal to the inflammasome adaptor Asc (Pycard) for Caspase-1 (Casp1) activation and pro-IL-1beta processing. Consistent with an essential role for Nlrp3 inflammasomes in antifungal immunity, we show that Nlrp3-deficient mice are hypersusceptible to Candida albicans infection. Thus, our results demonstrate the molecular basis for IL-1beta production after fungal infection and identify a crucial function for the Nlrp3 inflammasome in mammalian host defence in vivo.