title: The intracellular plasma membrane-connected compartment in the assembly of HIV-1 in human macrophages.
abstract: In HIV-infected macrophages, newly formed progeny virus particles accumulate in intracellular plasma membrane-connected compartments (IPMCs). Although the virus is usually seen in these compartments, it is unclear whether HIV assembly is specifically targeted to IPMCs or whether some viruses may also form at the cell surface but are not detected, as particles budding from the latter site will be released into the medium.
  
  
  
  To investigate the fidelity of HIV-1 targeting to IPMCs compared to the cell surface directly, we generated mutants defective in recruitment of the Endosomal Sorting Complexes Required for Transport (ESCRT) proteins required for virus scission. For mutants unable to bind the ESCRT-I component Tsg101, HIV release was inhibited and light and electron microscopy revealed that budding was arrested. When expressed in human monocyte-derived macrophages (MDM), these mutants formed budding-arrested, immature particles at their assembly sites, allowing us to capture virtually all of the virus budding events. A detailed morphological analysis of the distribution of the arrested viruses by immunofluorescence staining and confocal microscopy, and by electron microscopy, demonstrated that HIV assembly in MDMs is targeted primarily to IPMCs, with fewer than 5 % of budding events seen at the cell surface. Morphometric analysis of the relative membrane areas at the cell surface and IPMCs confirmed a large enrichment of virus assembly events in IPMCs. Serial block-face scanning electron microscopy of macrophages infected with a budding-defective HIV mutant revealed high-resolution 3D views of the complex organisation of IPMCs, with in excess of 15,000 associated HIV budding sites, and multiple connections between IPMCs and the cell surface.
  
  
  
  Using detailed quantitative analysis, we demonstrate that HIV assembly in MDMs is specifically targeted to IPMCs. Furthermore, 3D analysis shows, for the first time, the detailed ultrastructure of an IPMC within a large cell volume, at a resolution that allowed identification of individual virus assembly events, and potential portals through which virus may be released during cell-cell transfer. These studies provide new insights to the organisation of the HIV assembly compartments in macrophages, and show how HIV particles accumulating in these protected sites may function as a virus reservoir.
authors.affiliation: MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, London, WC1E 6BT, UK.|Present Address: Department of Biology and Biotechnological Sciences, College of Science, Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana.
authors.forename: David O
authors.initials: DO
authors.lastname: Nkwe
authors.affiliation: MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, London, WC1E 6BT, UK.
authors.forename: Annegret
authors.initials: A
authors.lastname: Pelchen-Matthews
authors.affiliation: MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, London, WC1E 6BT, UK.
authors.forename: Jemima J
authors.initials: JJ
authors.lastname: Burden
authors.affiliation: The Francis Crick Institute, Lincoln's Inn Fields Laboratories, 44 Lincoln's Inn Fields, London, WC2A 3LY, UK.
authors.forename: Lucy M
authors.initials: LM
authors.lastname: Collinson
authors.affiliation: MRC Laboratory for Molecular Cell Biology, University College London, Gower Street, London, WC1E 6BT, UK. m.marsh@ucl.ac.uk.
authors.forename: Mark
authors.identifier: 0000-0002-0213-3259
authors.initials: M
authors.lastname: Marsh
mesh_terms: D002451:Cell Compartmentation; D002462:Cell Membrane / Q000821:virology*; D020285:Cryoelectron Microscopy; D057809:HEK293 Cells; D015497:HIV-1 / Q000502:physiology* / Q000648:ultrastructure; D006801:Humans; D021621:Imaging, Three-Dimensional; D042541:Intracellular Space / Q000378:metabolism*; D008264:Macrophages / Q000473:pathology* / Q000821:virology*; D009000:Monocytes / Q000473:pathology; D009154:Mutation / Q000235:genetics; D011533:Proviruses / Q000502:physiology; D014162:Transfection; D019065:Virus Assembly / Q000502:physiology*
keywords: Assembly compartment; Endosomal sorting complexes required for transport; HIV-1; Intracellular plasma membrane-connected compartments; Macrophage; Volume electron microscopy