Size-Based Sorting Enhances Antigen Presentation by Microglia

Most cells continuously take up extracellular solutes via pinocytosis, a form of endocytosis inwhichtheplasmamembranenonselectively engulfs extracellular fluid and encapsulates it in small endocytic vesicles called pinosomes. Antigen-presenting immune cells—including microglia that have been activated by CNS injury or disease—use pinocytosis to ingest soluble proteins, which they degrade to peptide fragments. The fragments are then bound to major histocompatibility complex II (MHCII) molecules and returned to the plasma membrane, where they can be recognized by T cells to trigger an immune response. In multiple sclerosis (MS), a T-cell-mediated demyelinating disease, activated microglia present myelin-derived antigens, thus stimulating the activity of myelin-targeting T cells and exacerbating the disease. Understanding the steps that occur between pinocytosis and antigen presentation in microglia might therefore yield insights into potential treatments for MS. To better understand these steps, Chen et al. studied the movement of fluorescently labeledproteinsandpolysaccharides incultured ratmicroglia.Pinocytosedmoleculeswere initially colocalized in single pinosomes. Newly formed pinosomes quickly acidified and were surroundedbyMHCII-containinglysosomes. Within 15 min, pinocytosed proteins were broken down by proteinases in the acidified compartments, and the peptide fragments moved from the pinosome to lysosomes, leavinglargerpolysaccharidesbehind.Whensmall polysaccharides were pinocytosed with proteins, the protein fragments and polysaccharides were both transferred to the same lysosomal compartments, suggesting that the transfer depended on the size of molecules, rather than recognition of a specific sorting signal. Size-based sorting of pinocytosed molecules required ATP, GTP, proteins involved in fusion between late endosomes and lysosomes, and dynamin (a protein involved in membrane fission). Inhibiting dynamin disrupted size-based sorting by preventing the transfer of some small molecules to lysosomes and permitting the transfer of some large molecules. At the same time, inhibiting dynamin reduced both secretion of interferonand proliferation of T cells cocultured with microglia, indicating that it reducedtheefficiencyofantigenpresentationby microglia. Treatments that similarly interfere with the size-based movement of molecules from pinosomes to MHCII-containing lysosomes in microglia might therefore slow the progression of MS.