Differential Detection and Distribution of Microglial and Hematogenous Macrophage Populations in the Injured Spinal Cord oflys-EGFP-kiTransgenic Mice

Abstract

The acute inflammatory response that follows spinal cord injury (SCI) contributes to secondary injury that results in the expansion of the lesion and further loss of neurologic function. A cascade of receptor-mediated signaling events after SCI leads to activation of innate immune responses including the migration of microglia and active recruitment of circulating leukocytes. Because conventional techniques do not always distinguish macrophages derived from CNS-resident microglia from blood-derived monocytes, the role that each macrophage type performs cannot be assessed unambiguously in these processes. We demonstrate that, in the normal and spinal cord-injured lys-EGFP-ki transgenic mouse, enhanced green fluorescent protein (EGFP) is expressed only in mature hematopoietic granulomyelomonocytic cells and not in microglia. This allowed us to assess the temporal and spatial relationships between microglia-derived and hematogenous macrophages as well as neutrophils during a period of 6 weeks after clip compression SCI. Within the lesion, EGFP-positive monocyte-derived macrophages were found at the epicenter surrounded by EGFP-negative-activated microglia and microglia-derived macrophages. Neutrophils were not present when EGFP-positive monocyte-derived macrophages were depleted, indicating that neutrophil persistence in the lesion depended on the presence of these monocytes. Thus, these 2 distinct macrophage populations can be independently identified and tracked, thereby allowing their roles in acute and chronic stages of SCI-associated inflammation to be defined.

Hematogenous macrophage depletion reduces the fibrotic scar and increases axonal growth after spinal cord injury

“…In order to differentiate between microglial and monocyte origin of macrophages associated with the fibrotic scar, we used chimeric mice to demonstrate that microglia was present mostly in the glial scar while the fibrotic scar was comprised almost solely of hMΦ. Our results are consistent with previous studies that demonstrated different spatial distribution between microglia and hMΦ (Evans et al, 2014; Fenrich et al, 2013; Mawhinney et al, 2012; Popovich and Hickey, 2001). However, our study is the first to demonstrate their distribution in distinct portions of the glial and fibrotic scar.…”

SIV Encephalitis Lesions Are Composed of CD163+ Macrophages Present in the Central Nervous System during Early SIV Infection and SIV-Positive Macrophages Recruited Terminally with AIDS

“…In active lesion formation, the transition from early active MS lesions to late/inactive MS lesions is thought to be mediated by macrophage activation and redistribution of CNS macrophages rather than accumulation of recruited hematogenous macrophages. 61,62 A redistribution of macrophages to CNS lesions was also reported in a mouse model of spinal cord injury, 63 where blood-derived monocytes and macrophages were present diffusely throughout the spinal cord at 1 and 3 days after injury but coalesced into a lesion 7 to 14 days after injury before resolution of the lesion. 63 These similarities between HIVE and SIVE, MS, and spinal cord suggest that recruitment of hematogenous macrophages and redistribution of local macrophages are involved in CNS lesion formation.…”

Alterations of protein composition along the rostro-caudal axis after spinal cord injury: proteomic, in vitro and in vivo analyses

“…Thus, this discrepancy could be accounted in part to the presence or absence of macrophages derived from CNS-residing microglia or from blood-derived monocytes invading the most damaged regions at the rostral and caudal spinal levels (Mawhinney et al, 2012). Previous work have shown that after SCI, pro-inflammatory cytokines are upregulated by microglia and macrophages in the first few days after injury.…”