Toll-Like Receptor 9-Dependent Activation of Myeloid Dendritic Cells by Deoxynucleic Acids fromCandida albicans

Abstract

The innate immune system of humans recognizes the human pathogenic fungus Candida albicans via sugar polymers present in the cell wall, such as mannan and ␤-glucan. Here, we examined whether nucleic acids from C. albicans activate dendritic cells. C. albicans DNA induced interleukin-12p40 (IL-12p40) production and CD40 expression by murine bone marrow-derived myeloid dendritic cells (BM-DCs) in a dose-dependent manner. BM-DCs that lacked Toll-like receptor 4 (TLR4), TLR2, and dectin-1, which are pattern recognition receptors for fungal cell wall components, produced IL-12p40 at levels comparable to the levels produced by BM-DCs from wild-type mice, and DNA from a C. albicans pmr1⌬ null mutant, which has a gross defect in mannosylation, retained the ability to activate BM-DCs. This stimulatory effect disappeared completely after DNase treatment. In contrast, RNase treatment increased production of the cytokine. A similar reduction in cytokine production was observed when BM-DCs from TLR9 ؊/؊ and MyD88 ؊/؊ mice were used. In a luciferase reporter assay, NF-B activation was detected in TLR9-expressing HEK293T cells stimulated with C. albicans DNA. Confocal microscopic analysis showed similar localization of C. albicans DNA and CpG-oligodeoxynucleotide (CpG-ODN) in BM-DCs. Treatment of C. albicans DNA with methylase did not affect its ability to induce IL-12p40 synthesis, whereas the same treatment completely eliminated the ability of CpG-ODN to induce IL-12p40 synthesis. Finally, impaired clearance of this fungal pathogen was not found in the kidneys of TLR9 ؊/؊ mice. These results suggested that C. albicans DNA activated BM-DCs through a TLR9-mediated signaling pathway using a mechanism independent of the unmethylated CpG motif.

The role of pattern recognition receptors in the innate recognition ofCandida albicans

“…27 Similarly, using high dose challenges, TLR9-/-mice showed no significant alteration in survival upon clinical C. albicans isolate infection. 25,27 However, increased susceptibility to systemic candidiasis and impaired fungal clearance in TLR9-/-mice were observed upon challenge with a lower dose of C. albicans. 27 Taylor PR et al 184 investigatedb-glucan recognition and control of fungal infection using Dectin1-/-mice, and revealed that cytokine release, phagocyte recruitment, phagocytosis and microbial killing are all impaired in the Dectin1-/-mice compared with wild-type mice in disseminated candidiasis model.…”

“…147 Moreover, it is suggested that C. albicans activates murine bone marrow-derived myeloid DCs through a TLR9-mediated signaling pathway. 25 Dectin-1 was well characterized on DCs. It was found that Dectin-1 was high expressed on the DC cell line XS52, but less on the macrophage cell line J774.…”

“…TLR9 mediate the sensing of C. albicans unmethylated genomic DNA. 25 By using TLR9 knockout (TLR9KO) macrophages, TLR9 is revealed to be activated by fungal DNA and modulate macrophage anti-fungal effect. 126 TLR7 is required for the recognition of single-stranded RNA from C. albicans and the mice lacking TLR7 were hypersusceptible to systemic C. albicans infection.…”

“…22 TLRs and CLRs play major roles for the recognition of C. albicans PAMPs; NLRs are also involved in the recognition; while little is known about the role of RLR in the recognition of C. albicans PAMPs up to now (Table 1). TLR2, TLR4, and TLR9 directly recognize phospholipomannan (PLM), 23 O-linked mannosyl residues, 24 and CpG DNA 25 respectively in C. albicans. TLR7 is required for the recognition of single-stranded RNA of C. albicans.…”

Recognition of yeast nucleic acids triggers a host‐protective type I interferon response

“…In these activities, b-glucan receptors cooperate, to different extents, with Toll-like receptor 2 (TLR2) [8]. Moreover, fungal mannans are detected by several PRRs [9], whereas fungal DNA is recognized by TLR9 [10,11].It is presently unclear whether fungal recognition induces activation of the type I interferon (IFN) system, which regulates the expression of many genes and is therefore considered a fundamental arm of the innate immune system [12,13]. The type I IFN family includes the single IFN-b and many IFN-a subtypes that activate a common receptor (IFN-a/b receptor or IFNAR), composed of two subunits, IFNAR-I and IFNAR-II.…”