Toll-like receptor 9 (TLR9) senses microbial DNA and triggers type I IFN responses in plasmacytoid dendritic cells (pDCs). Previous studies suggest the presence of myeloid differentiation primary response gene 88 (MyD88)-dependent DNA sensors other than TLR9 in pDCs. Using MS, we investigated C-phosphate-G (CpG)-binding proteins from human pDCs, pDC-cell lines, and interferon regulatory factor 7 (IRF7)-expressing B-cell lines. CpG-A selectively bound the aspartate-glutamate-any amino acid-aspartate/histidine (DExD/H)-box helicase 36 (DHX36), whereas CpG-B selectively bound DExD/H-box helicase 9 (DHX9). Although the aspartateglutamate-alanine-histidine box motif (DEAH) domain of DHX36 was essential for CpG-A binding, the domain of unknown function 1605 (DUF1605 domain) of DHX9 was required for CpG-B binding. DHX36 is associated with IFN-α production and IRF7 nuclear translocation in response to CpG-A, but DHX9 is important for TNF-α and IL-6 production and NF-κB activation in response to CpG-B. Knocking down DHX9 or DHX36 significantly reduced the cytokine responses of pDCs to a DNA virus but had no effect on the cytokine responses to an RNA virus. We further showed that both DHX9 and DHX36 are localized within the cytosol and are directly bound to the Toll-interleukin receptor domain of MyD88 via their helicase-associated domain 2 and DUF domains. This study demonstrates that DHX9/DHX36 represent the MyD88-dependent DNA sensors in the cytosol of pDCs and suggests a much broader role for DHX helicases in viral sensing.cytosolic sensor | innate immunity

NLRP3 is a key component of caspase-activating macromolecular protein complexes called inflammasomes. It has been found that DHX33 is a cytosolic double-stranded RNA (dsRNA) sensor for the NLRP3 inflammasome, which induces caspase-1-dependent production of IL-1β and IL18 upon activation. However, how the cytosolic dsRNAs induce the interaction between DHX33 and the NLRP3 inflammasome remains unknown. Here, we report that TRIM33, a member of the tripartite motif (TRIM) family, can bind DHX33 directly and induce DHX33 ubiquitination via the lysine 218 upon dsRNA stimulation. Knocking down of TRIM33 abolished the dsRNA-induced NLRP3 inflammasome activation in both THP-1-derived macrophages and human monocyte-derived macrophages. The ubiquitination of DHX33 by TRIM33 is lysine 63-specific and is required for the formation of the DHX33-NLRP3 inflammasome complex.