Circulating monocytic cells infiltrate layers of anterograde axonal degeneration where they transform into microglia

Abstract

In this study, we demonstrate the infiltration of blood-derived monocytic cells and their morphologic transformation into microglia in zones of acute, anterograde (Wallerian) axonal degeneration induced by entorhinal cortex lesion (ECL). ECL was performed in mice which had received green fluorescent protein (GFP)-transduced bone marrow grafts allowing identification of blood-derived elements within the brain. While in the unlesioned hemisphere GFP+ cells were restricted to perivascular and leptomeningeal sites, many round fluorescent cells appeared in hippocampal zones of axonal degeneration at 24 h post lesion (hpl). Within 72 hpl, these GFP+ cells acquired ramified, microglia-like morphologies, which persisted for at least 7 days post ECL. Differentiation of GFP+ cells into glial fibrillary acidic protein (GFAP)+ astrocytes was never observed. To exclude that this recruitment is an artifact of irradiation or bone marrow transplantation, the fluorescent cell tracker 6-carboxylfluorescein diacetate (CFDA) was injected into spleens of normal mice 1 day before ECL. Again, fluorescent cells appeared at the lesion site and along the layers of axonal degeneration at 48 hpl and CFDA+/MAC-1+, cells exhibited amoeboid and ramified morphologies. Thus, blood-derived cells infiltrate not only the site of mechanical lesion, but also the layers of anterograde axonal degeneration, where they readily transform into microglia-like elements. A role for infiltrating leukocytes in facilitating or modulating postlesional plasticity, e.g., by phagocytosis of growth-inhibiting myelin should now be considered. Moreover, monocytic cells may serve as vehicles to transport therapeutic substances such as neurotrophic factors or caspase inhibitors to zones of axonal degeneration.

Toll-Like Receptor 2 Signaling in Response to Brain Injury: An Innate Bridge to Neuroinflammation

“…The kinetics of response suggested that TLR2 regulated proliferation, rather than in entry of circulating precursors (Hailer et al, 1999;Bechmann et al, 2005;Ladeby et al, 2005b;Wirenfeldt et al, 2005). Immunohistochemical studies in rats suggested that microglial proliferation begins at ϳ24 h (Fagan and Gage, 1994), and we showed that lesion-induced microglial proliferation was significantly reduced in TLR2-deficient mice at this time, compared with wild-type mice.…”

“…T cells have been stained in the lesion-reactive hippocampus by others (Bechmann et al, 2001), as well as in other lesion-reactive regions of the CNS (Raivich et al, 1998). Infiltrating cells enter the parenchyma in denervated zones by crossing the glia limitans that surrounds blood vessels (Bechmann et al, 2005).…”

“…A number of studies have shown that this results from proliferation by resident microglia, migration into denervated zones, and by differentiation of blood-derived cells into ramified or CD45 dim microglia (Jensen et al, 1994;Hailer et al, 1999;Rappert et al, 2004;Bechmann et al, 2005;Ladeby et al, 2005b;Wirenfeldt et al, 2005). Microglial expansion can be detected using flow cytometry to compare the proportion of CD45 dim microglia in the lesion-reactive hippocampus to that in the contralateral hippocampus (Fig.…”

Microglial Recruitment, Activation, and Proliferation in Response to Primary Demyelination

“…Microglia originating from bone marrow precursors have been shown to engraft the CNS after injury to the adult mouse CNS. 33,38,40,41 Here, we show that influx of microglia progenitors from blood to the corpus callosum is a major contributor to the increase in microglial numbers during primary demyelination. 3 H]thymidine incorporation.…”

“…It has been proposed that CCL2-CCR2 signaling recruits microglial progenitors to the CNS. 33,34 Total numbers of microglial cells were similar in CCL2-deficient mice and C57BL/6 control mice at 3 and 6 weeks after cuprizone treatment, whereas macrophage numbers were significantly reduced ( Figure 7, B and D). This selective impairment of macrophage recruitment but not microglial expansion was also seen in CCR2-deficient mice.…”