title: Endosomal localization of TLR8 confers distinctive proteolytic processing on human myeloid cells.
abstract: Nucleic acid-sensing TLRs are involved in both antimicrobial immune responses and autoimmune inflammation. TLR8 is phylogenetically and structurally related to TLR7 and TLR9, which undergo proteolytic processing in the endolysosomes to generate functional receptors. Recent structural analyses of human TLR8 ectodomain and its liganded form demonstrated that TLR8 is also cleaved, and both the N- and C-terminal halves contribute to ligand binding. However, the structures and ssRNA recognition mode of endogenous TLR8 in human primary cells are largely unknown. In this study, we show that proteolytic processing of TLR8 occurs in human monocytes and macrophages in a different manner compared with TLR7/9 cleavage. The insertion loop between leucine-rich repeats 14 and 15 in TLR8 is indispensable for the cleavage and stepwise processing that occurs in the N-terminal fragment. Both furin-like proprotein convertase and cathepsins contribute to TLR8 cleavage in the early/late endosomes. TLR8 recognizes viral ssRNA and endogenous RNA, such as microRNAs, resulting in the production of proinflammatory cytokines. Hence, localization sites of the receptors are crucial for the nucleic acid-sensing mode and downstream signaling.
authors.affiliation: Department of Microbiology and Immunology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan.
authors.forename: Noriko
authors.initials: N
authors.lastname: Ishii
authors.affiliation: Department of Microbiology and Immunology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Funami
authors.affiliation: Department of Microbiology and Immunology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan.
authors.forename: Megumi
authors.initials: M
authors.lastname: Tatematsu
authors.affiliation: Department of Microbiology and Immunology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan.
authors.forename: Tsukasa
authors.initials: T
authors.lastname: Seya
authors.affiliation: Department of Microbiology and Immunology, Hokkaido University Graduate School of Medicine, Sapporo 060-8638, Japan matumoto@pop.med.hokudai.ac.jp.
authors.forename: Misako
authors.initials: M
authors.lastname: Matsumoto
mesh_terms: D000595:Amino Acid Sequence; D002403:Cathepsins / Q000378:metabolism; D011992:Endosomes / Q000378:metabolism*; D015870:Gene Expression; D057809:HEK293 Cells; D006801:Humans; D008024:Ligands; D008247:Lysosomes / Q000378:metabolism; D008264:Macrophages / Q000737:chemistry / Q000166:cytology / Q000378:metabolism*; D035683:MicroRNAs / Q000737:chemistry / Q000378:metabolism; D008969:Molecular Sequence Data; D009000:Monocytes / Q000737:chemistry / Q000166:cytology / Q000378:metabolism*; D061251:Primary Cell Culture; D045664:Proprotein Convertase 1 / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D059748:Proteolysis; D012367:RNA, Viral / Q000737:chemistry / Q000378:metabolism*; D015398:Signal Transduction; D051199:Toll-Like Receptor 7 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D051200:Toll-Like Receptor 8 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D051217:Toll-Like Receptor 9 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D008024:Ligands; D035683:MicroRNAs; D012367:RNA, Viral; C495348:TLR7 protein, human; C495349:TLR8 protein, human; C495371:TLR9 protein, human; D051199:Toll-Like Receptor 7; D051200:Toll-Like Receptor 8; D051217:Toll-Like Receptor 9; D002403:Cathepsins; D045664:Proprotein Convertase 1