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 fungusCandida albicansvia sugar polymers present in the cell wall, such as mannan and β-glucan. Here, we examined whether nucleic acids fromC. albicansactivate dendritic cells.C. albicansDNA 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 aC. albicans pmr1Δ 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-κB activation was detected in TLR9-expressing HEK293T cells stimulated withC. albicansDNA. Confocal microscopic analysis showed similar localization ofC. albicansDNA and CpG-oligodeoxynucleotide (CpG-ODN) in BM-DCs. Treatment ofC. albicansDNA 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 thatC. albicansDNA activated BM-DCs through a TLR9-mediated signaling pathway using a mechanism independent of the unmethylated CpG motif.