Lesion response of long‐term and recently immigrated resident endoneurial macrophages in peripheral nerve explant cultures from bone marrow chimeric mice

Abstract

Resident macrophages of the peripheral nervous system have recently been shown to respond rapidly to Wallerian degeneration before the influx of blood-derived macrophages. Because resident endoneurial macrophages are slowly but incompletely exchanged from the blood within 3 months, they could potentially comprise a heterogenous cell population consisting of long-term resident cells and more mobile cells undergoing turnover. We used bone marrow chimeric mice created by transplanting bone marrow from green fluorescent protein-transgenic mice into irradiated wildtype recipients to selectively analyse the response of these two resident macrophage populations to Wallerian degeneration in sciatic nerve explant cultures. In such nerves, recently immigrated macrophages exhibit green fluorescence whereas long-term resident macrophages do not. Studies in cultures from wildtype controls revealed rapid morphological changes of resident macrophages towards a bloated phenotype, a proliferative response resulting in a 3.7-fold increase of macrophage numbers over 2 weeks, and phagocytosis of myelin basic protein-immunoreactive myelin debris. When chimeric mice were analysed, both populations of resident endoneurial macrophages participated in morphological transformation, proliferation and phagocytosis. Quantitative studies revealed a stronger proliferative and phagocytic response in long-term resident endoneurial macrophages compared with recently immigrated macrophages. Our results point towards subtle, but not principal, differences between the two macrophage populations, which might indicate different stages of macrophage differentiation rather than the existence of entirely distinct endoneurial macrophage populations. The results further underline the versatility of resident endoneurial macrophages following peripheral nerve injury, which is reminiscent of the lesion response of microglial cells within the brain.

Further evidence for a crucial role of resident endoneurial macrophages in peripheral nerve disorders: Lessons from acrylamide‐induced neuropathy

“…Spleens were used to further validate the chimeric status of the animals. Sections from chimeric spleens exhibited brightly green follicles, while stromal cells in the red pulp and vessels were GFP negative similar to previous studies in GFP bone marrow chimeras (Leonhard et al, 2002;Mueller et al, 2003).…”

“…CCR2 deficient mice (CCR2 2/2 ) were bred at local animal facility and were originally obtained from the Jackson Laboratory (Bar Harbor, ME). Generation of radiation bone marrow chimeric mice was described previously (Leonhard et al, 2002;Mueller et al, 2003). Wild-type C57/Bl6 mice were lethally irradiated with a radiation dose of five Gray.…”

The Membrane Attack Complex of the Complement System Is Essential for Rapid Wallerian Degeneration

“…The slight increase in CD68-positive cells in both WT and C6 Ϫ/Ϫ nerves as early as 2 d after injury is probably attributable to proliferation and differentiation of the endoneurial macrophage population that still retains the resident-like (Pan-positive) phenotype but starts to express the CD68 lysosomal marker. These macrophages have the potential to phagocytose myelin (Leonhard et al, 2002). At this time point, in WT nerve, macrophages express high levels of the CR3 receptor, which is essential for myelin phagocytosis Friede, 1990, 1991;Reichert and Rotshenker, 2003), whereas in the C6 Ϫ/Ϫ nerves CR3 expression is virtually undetectable, consistent with the small and round morphology typical of inactive macrophages.…”

“…After degeneration of the axon, the myelin sheath collapses and initially remains within the parent Schwann cell cytoplasm. The endoneurial macrophages proliferate, become activated, and initiate myelin phagocytosis (Leonhard et al, 2002). This population is later supplemented by monocytes/macrophages recruited from the bloodstream that participate in myelin phagocytosis and removal (Mueller et al, 2001;Hirata and Kawabuchi, 2002).…”

Autophagy and Microglia: Novel Partners in Neurodegeneration and Aging

“…Nevertheless, they do not discard the possibility that phagocytosis [127,128,129] and/or LAP may also play a role in Schwann cell-mediated myelin degradation. In addition, tissue resident macrophages including microglia may contribute to myelin clearance by phagocytosis [130,131]. Overall, the available data suggest that myelin may be digested by autophagy and/or phagocytosis depending on the cell type and its context.…”