title: Endosome-mediated autophagy: an unconventional MIIC-driven autophagic pathway operational in dendritic cells.
abstract: Activation of TLR signaling has been shown to induce autophagy in antigen-presenting cells (APCs). Using high-resolution microscopy approaches, we show that in LPS-stimulated dendritic cells (DCs), autophagosomes emerge from MHC class II compartments (MIICs) and harbor both the molecular machinery for antigen processing and the autophagosome markers LC3 and ATG16L1. This ENdosome-Mediated Autophagy (ENMA) appears to be the major type of autophagy in DCs, as similar structures were observed upon established autophagy-inducing conditions (nutrient deprivation, rapamycin) and under basal conditions in the presence of bafilomycin A1. Autophagosome formation was not significantly affected in DCs expressing ATG4B (C74A) mutant and atg4b (-/-) bone marrow DCs, but the degradation of the autophagy substrate SQSTM1/p62 was largely impaired. Furthermore, we demonstrate that the previously described DC aggresome-like LPS-induced structures (DALIS) contain vesicular membranes, and in addition to SQSTM1 and ubiquitin, they are positive for LC3. LC3 localization on DALIS is independent of its lipidation. MIIC-driven autophagosomes preferentially engulf the LPS-induced SQSTM1-positive DALIS, which become later degraded in autolysosomes. DALIS-associated membranes also contain ATG16L1, ATG9 and the Q-SNARE VTI1B, suggesting that they may represent (at least in part) a membrane reservoir for autophagosome expansion. We propose that ENMA constitutes an unconventional, APC-specific type of autophagy, which mediates the processing and presentation of cytosolic antigens by MHC class II machinery, and/or the selective clearance of toxic by-products of elevated ROS/RNS production in activated DCs, thereby promoting their survival.
authors.affiliation: Cell Microscopy Center; Department of Cell Biology, University Medical Center Utrecht, Utrecht, The Netherlands, Institute of Biomembranes, Utrecht, The Netherlands.
authors.forename: Vangelis
authors.initials: V
authors.lastname: Kondylis
authors.forename: Hezder E
authors.initials: HE
authors.lastname: van Nispen Tot Pannerden
authors.forename: Suzanne
authors.initials: S
authors.lastname: van Dijk
authors.forename: Toine
authors.initials: T
authors.lastname: Ten Broeke
authors.forename: Richard
authors.initials: R
authors.lastname: Wubbolts
authors.forename: Willie J
authors.initials: WJ
authors.lastname: Geerts
authors.forename: Cor
authors.initials: C
authors.lastname: Seinen
authors.forename: Tuna
authors.initials: T
authors.lastname: Mutis
authors.forename: Harry F G
authors.initials: HF
authors.lastname: Heijnen
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D001343:Autophagy / Q000187:drug effects; D015415:Biomarkers / Q000378:metabolism; D021961:Cell Membrane Structures / Q000378:metabolism / Q000648:ultrastructure; D003713:Dendritic Cells / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism*; D055032:Electron Microscope Tomography; D011992:Endosomes / Q000187:drug effects / Q000378:metabolism*; D000949:Histocompatibility Antigens Class II / Q000378:metabolism*; D006801:Humans; D008070:Lipopolysaccharides / Q000494:pharmacology; D018942:Macrolides / Q000494:pharmacology; D051379:Mice; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008954:Models, Biological; D010588:Phagosomes / Q000378:metabolism / Q000648:ultrastructure; D020123:Sirolimus / Q000494:pharmacology; D054875:Ubiquitination / Q000187:drug effects
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D015415:Biomarkers; D000949:Histocompatibility Antigens Class II; D008070:Lipopolysaccharides; D018942:Macrolides; D008869:Microtubule-Associated Proteins; C040929:bafilomycin A1; D020123:Sirolimus
keywords: LC3; MHC class II; autophagy; dendritic cell; electron tomography