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

Background: Traumatic 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 SIRP alpha (signal regulatory protein-alpha) on macrophages and microglia. Methods: CD47 and SIRP alpha expression was studied by confocal immunofluorescence microscopy, and myelin phagocytosis by ELISA. Results: We first document that myelin, oligodendrocytes and Schwann cells express CD47 without SIRP alpha and further confirm that microglia and macrophages express both CD47 and SIRP alpha. Thus, CD47 on myelin can bind to and subsequently activate SIRP alpha on phagocytes, a prerequisite for CD47/SIRP alpha-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 SIRP alpha on phagocytes is blocked by mAbs against CD47 and SIRP alpha, indicating that down-regulation of phagocytosis indeed depends on CD47-SIRP alpha binding. Further, phagocytosis in serum-free medium of CD47(+/+) myelin is augmented after knocking down SIRP alpha levels (SIRP alpha-KD) in phagocytes by lentiviral infection with SIRP alpha-shRNA, whereas phagocytosis of myelin that lacks CD47 (CD47(-/-)) is not. Thus, myelin CD47 produces SIRP alpha-dependent down-regulation of CD47(+/+) myelin phagocytosis in phagocytes. Unexpectedly, phagocytosis of CD47(-/-) myelin by SIRP alpha-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 SIRP alpha-dependent down-regulation of myelin phagocytosis irrespective of the other. Conclusions: Myelin down-regulates its own phagocytosis through CD47-SIRP alpha 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.