Differences in the phagocytic response of microglia and peripheral macrophages after spinal cord injury and its effects on cell death

Macrophages in the injured spinal cord arise from resident microglia and infiltrating, peripherally derived monocytes. It is still not clear if macrophages derived from these two populations differ in their roles after CNS injury. The aims of this study are to investigate the phagocytic response and clearance of damaged axons and tissue debris by these distinct subsets of macrophages and assess their viability after spinal cord injury (SCI). The lysozyme M EGFP-knockin mouse tags hematogenous macrophages, but not microglia. Using a combination of immunofluorescence, flow cytometry, and neuronal tracing techniques, we show that microglia contact damaged axons early (24 h) after SCI and are the main type of macrophage to contain phagocytic material at 3 d. Thereafter, infiltrating macrophages become the predominant cell in contact with degenerating axons and contain more phagocytic material, which in contrast to microglia, persists for up to 42 d. Furthermore, after phagocytosis of myelin in vitro, bone marrow-derived macrophages are much more susceptible to apoptotic and necrotic cell death than CNS microglia, which is mirrored in vivo with apoptotic TUNEL-positive cells of infiltrating macrophage origin. This work suggests that microglia play a major role in the early response to SCI, by phagocytosing damaged and degenerating tissue, processing phagocytic material efficiently, and remaining viable. Later, macrophages of peripheral origin contribute predominantly to phagocytosis but are less efficient at processing CNS debris, and their death, in situ, may contribute to the secondary damage after CNS injury.