Deletion of CD47 from Schwann cells, macrophages and microglia hastens myelin disruption and scavenging in Schwann cells and augments myelin debris phagocytosis in macrophages and microglia

Abstract Background: Myelin that surrounds axons breaks in trauma and disease; e.g., PNI and SCI (peripheral nerve and spinal cord injuries) and MS (multiple sclerosis). Resulting myelin debris hinders repair if not effectively scavenged by Schwann cells and macrophages in PNI and by microglia in SCI and MS. We showed previously that myelin debris evades phagocytosis as CD47 on myelin ligates SIRPα (signal regulatory protein-α) on macrophages and microglia, triggering SIRPα to inhibit phagocytosis in phagocytes. Using PNI as a model, we tested the in-vivo significance of SIRPα-dependent phagocytosis inhibition in SIRPα null mice, showing that SIRPα deletion leads to accelerated myelin debris clearance, axon regeneration and recovery of function from PNI. Herein, we tested how deletion of CD47, a SIRPα ligand and a cell surface receptor on Schwann cells and phagocytes, affects recovery from PNI. Methods: Using CD47 null (CD47-/-) and wild type mice, we studied myelin disruption and debris clearance, axon regeneration and recovery of function from PNI. Results: As expected from CD47 on myelin acting as a SIRPα ligand that normally triggers SIRPα-dependent phagocytosis inhibition in phagocytes, myelin debris clearance, axon regeneration and function recovery were all faster in CD47-/- mice than in wild type mice. Unexpectedly compared with wild type mice, myelin debris clearance started sooner and CD47-deleted Schwann cells displayed enhanced disruption and scavenging of myelin in CD47-/- mice. Furthermore, CD47-deleted macrophages and CD47-deleted microglia from CD47-/- mice phagocytosed more than CD47-expressing phagocytes from wild type mice. Conclusions: This study reveals two novel normally occurring CD47-dependent mechanisms that impede myelin debris clearance. First, CD47 expressed on Schwann cells inhibits myelin disruption and debris scavenging in Schwann cells. Second, CD47 expressed on macrophages and microglia inhibits myelin debris phagocytosis in phagocytes. The two add to a third mechanism that we previously documented whereby CD47 on myelin ligates SIRPα on macrophages and microglia, triggering SIRPα-dependent phagocytosis inhibition in phagocytes. Thus, CD47 plays multiple inhibitory roles that combined impede myelin debris clearance, leading to delayed recovery from PNI. Similar inhibitory roles may hinder recovery from other pathologies in which repair depends on efficient phagocytosis (e.g., SCI and MS).