title: Phagocytosis of neuronal debris by microglia is associated with neuronal damage in multiple sclerosis.
abstract: Neuroaxonal degeneration is a pathological hallmark of multiple sclerosis (MS) contributing to irreversible neurological disability. Pathological mechanisms leading to axonal damage include autoimmunity to neuronal antigens. In actively demyelinating lesions, myelin is phagocytosed by microglia and blood-borne macrophages, whereas the fate of degenerating or damaged axons is unclear. Phagocytosis is essential for clearing neuronal debris to allow repair and regeneration. However, phagocytosis may lead to antigen presentation and autoimmunity, as has been described for neuroaxonal antigens. Despite this notion, it is unknown whether phagocytosis of neuronal antigens occurs in MS. Here, we show using novel, well-characterized antibodies to axonal antigens, that axonal damage is associated with HLA-DR expressing microglia/macrophages engulfing axonal bulbs, indicative of axonal damage. Neuronal proteins were frequently observed inside HLA-DR(+) cells in areas of axonal damage. In vitro, phagocytosis of neurofilament light (NF-L), present in white and gray matter, was observed in human microglia. The number of NF-L or myelin basic protein (MBP) positive cells was quantified using the mouse macrophage cell line J774.2. Intracellular colocalization of NF-L with the lysosomal membrane protein LAMP1 was observed using confocal microscopy confirming that NF-L is taken up and degraded by the cell. In vivo, NF-L and MBP was observed in cerebrospinal fluid cells from patients with MS, suggesting neuronal debris is drained by this route after axonal damage. In summary, neuroaxonal debris is engulfed, phagocytosed, and degraded by HLA-DR(+) cells. Although uptake is essential for clearing neuronal debris, phagocytic cells could also play a role in augmenting autoimmunity to neuronal antigens.
authors.affiliation: Department of Immunology, Erasmus MC, University Medical Centre, Rotterdam, The Netherlands.
authors.forename: Ruth
authors.initials: R
authors.lastname: Huizinga
authors.forename: Baukje J
authors.initials: BJ
authors.lastname: van der Star
authors.forename: Markus
authors.initials: M
authors.lastname: Kipp
authors.forename: Rosa
authors.initials: R
authors.lastname: Jong
authors.forename: Wouter
authors.initials: W
authors.lastname: Gerritsen
authors.forename: Tim
authors.initials: T
authors.lastname: Clarner
authors.forename: Fabiola
authors.initials: F
authors.lastname: Puentes
authors.forename: Christine D
authors.initials: CD
authors.lastname: Dijkstra
authors.forename: Paul
authors.initials: P
authors.lastname: van der Valk
authors.forename: Sandra
authors.initials: S
authors.lastname: Amor
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D002402:Cathepsin D / Q000494:pharmacology; D002403:Cathepsins / Q000494:pharmacology; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005260:Female; D006684:HLA-DR Antigens / Q000378:metabolism; D006801:Humans; D008297:Male; D051379:Mice; D017628:Microglia / Q000187:drug effects / Q000502:physiology*; D018613:Microscopy, Confocal; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000134:cerebrospinal fluid / Q000473:pathology*; D004676:Myelin Basic Protein / Q000134:cerebrospinal fluid / Q000378:metabolism; D009413:Nerve Fibers, Myelinated / Q000378:metabolism / Q000473:pathology; D016900:Neurofilament Proteins / Q000134:cerebrospinal fluid / Q000187:drug effects / Q000378:metabolism; D009474:Neurons / Q000187:drug effects / Q000473:pathology*; D010587:Phagocytosis / Q000187:drug effects / Q000502:physiology*; D013997:Time Factors
chemical_list: D006684:HLA-DR Antigens; D004676:Myelin Basic Protein; D016900:Neurofilament Proteins; C044882:neurofilament protein L; D002403:Cathepsins; D002402:Cathepsin D