High-resolution intravital imaging reveals that blood-derived macrophages but not resident microglia facilitate secondary axonal dieback in traumatic spinal cord injury

Exp Neurol. 2014 Apr;254:109-20. doi: 10.1016/j.expneurol.2014.01.013. Epub 2014 Jan 24.


After traumatic spinal cord injury, functional deficits increase as axons die back from the center of the lesion and the glial scar forms. Axonal dieback occurs in two phases: an initial axon intrinsic stage that occurs over the first several hours and a secondary phase which takes place over the first few weeks after injury. Here, we examine the secondary phase, which is marked by infiltration of macrophages. Using powerful time-lapse multi-photon imaging, we captured images of interactions between Cx3cr1(+/GFP) macrophages and microglia and Thy-1(YFP) axons in a mouse dorsal column crush spinal cord injury model. Over the first few weeks after injury, axonal retraction bulbs within the lesion are static except when axonal fragments are lost by a blebbing mechanism in response to physical contact followed by phagocytosis by mobile Cx3Cr1(+/GFP) cells. Utilizing a radiation chimera model to distinguish marrow-derived cells from radio-resistant CNS-resident microglia, we determined that the vast majority of accumulated cells in the lesion are derived from the blood and only these are associated with axonal damage. Interestingly, CNS-resident Cx3Cr1(+/GFP) microglia did not increasingly accumulate nor participate in neuronal destruction in the lesion during this time period. Additionally, we found that the blood-derived cells consisted mainly of singly labeled Ccr2(+/RFP) macrophages, singly labeled Cx3Cr1(+/GFP) macrophages and a small population of double-labeled cells. Since all axon destructive events were seen in contact with a Cx3Cr1(+/GFP) cell, we infer that the CCR2 single positive subset is likely not robustly involved in axonal dieback. Finally, in our model, deletion of CCR2, a chemokine receptor, did not alter the position of axons after dieback. Understanding the in vivo cellular interactions involved in secondary axonal injury may lead to clinical treatment candidates involving modulation of destructive infiltrating blood monocytes.

Keywords: Axonal dieback; Bone marrow chimera mice; CCR2; CX3CR1; Macrophages; Microglia; Monocytes; Retraction bulb; Spinal cord injury; Two-photon microscopy.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Axons / pathology*
  • Bone Marrow Transplantation
  • CX3C Chemokine Receptor 1
  • Cell Communication / immunology
  • Female
  • Macrophages / immunology
  • Macrophages / pathology*
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Mice, Transgenic
  • Microglia / immunology
  • Microglia / pathology*
  • Microscopy / methods
  • Monocytes / immunology
  • Monocytes / pathology
  • Nerve Crush
  • Phagocytosis / immunology
  • Receptors, CCR2 / genetics
  • Receptors, Chemokine / genetics
  • Sciatic Nerve / pathology
  • Spinal Cord Injuries / immunology
  • Spinal Cord Injuries / pathology*
  • Transplantation Chimera


  • CX3C Chemokine Receptor 1
  • Ccr2 protein, mouse
  • Cx3cr1 protein, mouse
  • Receptors, CCR2
  • Receptors, Chemokine