title: Spatial organization of the transforming MHC class II compartment.
abstract: BACKGROUND
  DC (dendritic cells) continuously capture pathogens and process them into small peptides within the endolysosomal compartment, the MIIC (MHC class II-containing compartment). In MIICs peptides are loaded on to MHC class II and rapidly redistributed to the cell surface. This redistribution is accompanied by profound changes of the MIICs into tubular structures. An emerging concept is that MIIC tubulation provides a means to transport MHC class II-peptide complexes to the cell surface, either directly or through vesicular intermediates. To obtain spatial information on the reorganization of the MIICs during DC maturation, we performed electron tomography on cryo-immobilized and freeze-substituted mouse DCs after stimulation with LPS (lipopolysaccharide).
  
  
  RESULTS
  In non-stimulated DCs, MIICs are mostly spherical. After 3 h of LPS stimulation, individual MIICs transform into tubular structures. Three-dimensional reconstruction showed that the MIICs frequently display fusion profiles and after 6 h of LPS stimulation, MIICs become more interconnected, thereby creating large MIIC reticula. Microtubules and microfilaments align these MIICs and reveal physical connections. In our tomograms we also identified a separate population of MIIC-like intermediates, particularly at extended ends of MIIC tubules and in close proximity to the trans-Golgi network. No fusion events were captured between reticular MIICs and the plasma membrane.
  
  
  CONCLUSIONS
  Our results indicate that MIICs have the capacity to fuse together, whereby the cytoskeleton possibly provides a scaffold for the MIIC shape change and directionality. MIIC-like intermediates may represent MHC class II carriers.
authors: Institute of Biomembranes, Padualaan 8, 3584 CH Utrecht, The Netherlands.
authors: Hezder E
authors: HE
authors: van Nispen tot Pannerden
authors: Willie J
authors: WJ
authors: Geerts
authors: Monique J
authors: MJ
authors: Kleijmeer
authors: Harry F G
authors: HF
authors: Heijnen
mesh_terms: D000818:Animals; D002459:Cell Fusion; D002462:Cell Membrane / Q000378:metabolism*; D003713:Dendritic Cells / Q000166:cytology* / Q000187:drug effects / Q000502:physiology*; D055032:Electron Microscope Tomography; D005802:Genes, MHC Class II; D008070:Lipopolysaccharides / Q000494:pharmacology; D051379:Mice; D016253:Microscopy, Immunoelectron; D021601:trans-Golgi Network
chemical_list: D008070:Lipopolysaccharides