Myelin down-regulates myelin phagocytosis by microglia and macrophages through interactions between CD47 on myelin and SIRPα (signal regulatory protein-α) on phagocytes

Abstract

BackgroundTraumatic injury to axons produces breakdown of axons and myelin at the site of the lesion and then further distal to this where Wallerian degeneration develops. The rapid removal of degenerated myelin by phagocytosis is advantageous for repair since molecules in myelin impede regeneration of severed axons. Thus, revealing mechanisms that regulate myelin phagocytosis by macrophages and microglia is important. We hypothesize that myelin regulates its own phagocytosis by simultaneous activation and down-regulation of microglial and macrophage responses. Activation follows myelin binding to receptors that mediate its phagocytosis (e.g. complement receptor-3), which has been previously studied. Down-regulation, which we test here, follows binding of myelin CD47 to the immune inhibitory receptor SIRPα (signal regulatory protein-α) on macrophages and microglia.MethodsCD47 and SIRPα expression was studied by confocal immunofluorescence microscopy, and myelin phagocytosis by ELISA.ResultsWe first document that myelin, oligodendrocytes and Schwann cells express CD47 without SIRPα and further confirm that microglia and macrophages express both CD47 and SIRPα. Thus, CD47 on myelin can bind to and subsequently activate SIRPα on phagocytes, a prerequisite for CD47/SIRPα-dependent down-regulation of CD47+/+ myelin phagocytosis by itself. We then demonstrate that phagocytosis of CD47+/+ myelin is augmented when binding between myelin CD47 and SIRPα on phagocytes is blocked by mAbs against CD47 and SIRPα, indicating that down-regulation of phagocytosis indeed depends on CD47-SIRPα binding. Further, phagocytosis in serum-free medium of CD47+/+ myelin is augmented after knocking down SIRPα levels (SIRPα-KD) in phagocytes by lentiviral infection with SIRPα-shRNA, whereas phagocytosis of myelin that lacks CD47 (CD47-/-) is not. Thus, myelin CD47 produces SIRPα-dependent down-regulation of CD47+/+ myelin phagocytosis in phagocytes. Unexpectedly, phagocytosis of CD47-/- myelin by SIRPα-KD phagocytes, which is not altered from normal when tested in serum-free medium, is augmented when serum is present. Therefore, both myelin CD47 and serum may each promote SIRPα-dependent down-regulation of myelin phagocytosis irrespective of the other.ConclusionsMyelin down-regulates its own phagocytosis through CD47-SIRPα interactions. It may further be argued that CD47 functions normally as a marker of "self" that helps protect intact myelin and myelin-forming oligodendrocytes and Schwann cells from activated microglia and macrophages. However, the very same mechanism that impedes phagocytosis may turn disadvantageous when rapid clearance of degenerated myelin is helpful.

IFN-γ ameliorates autoimmune encephalomyelitis by limiting myelin lipid peroxidation

“…Along these lines, it has been shown recently that CD47 is expressed by intact myelin and myelinforming oligodendrocytes and may protect them from phagocytosis by microglia and macrophages via interaction with CD172a. This mechanism might be detrimental when clearance of myelin debris is desired (45). Thus, the increased expression of CD172a on microglia, and to a lesser extent on macrophages, may contribute to the lack of myelin Ag uptake observed in IFN-γ-deficient EAE mice in our studies.…”

“…The failure to clear free myelin debris could inhibit the differentiation of oligodendrocyte precursors and could be toxic to neurons (20,21). Furthermore, myelin debris may contain inhibitory molecules that antagonize axonal regeneration (45), and the removal of axonal debris by microglia increases axonal regeneration (46,47). Additionally, microglial phagocytosis of myelin debris can promote resolution of the CNS inflammatory response (16,48), and myelin-phagocytosing microglia express genes involved in the activation, migration, proliferation, and differentiation of oligodendrocyte precursor cells (49).…”

MerTK Is a Functional Regulator of Myelin Phagocytosis by Human Myeloid Cells

“…M2c cells showed upregulation of FcgR1a, an FcR known to enhance myelin uptake by microglia, and downregulation of SIRPa, a molecule that inhibits phagocytosis through its interaction with CD47 (10,39,40). However, phagocytic activity by M2c cells was reduced by MerTK antagonism, albeit to a much lesser extent than what was seen in Mtgf cells, likely reflecting antagonism of the basal MerTK expressed in these cells.…”

“…Myelin debris is a major obstacle to the repair process both as a physical impediment and through the expression of axon growth inhibitory molecules, including CD47, which mediates the activation of SIRPa expressed on myeloid cells (10,11). The specific molecules regulating myelin phagocytosis are incompletely defined (for review, see Ref.…”

Characterisation of glioblastoma subtypes: implications for therapy resistance

“…Microglia are derived from macrophages that integrate into the central nervous system during development and like microglia express both the CD47 ligand and SIRPα receptor (Gitik, Liraz-Zaltsman, Oldenborg, Reichert, & Rotshenker, 2011). Microglia typically contain a small soma and long processes, which extend and retract to scan the local environment.…”