Different TLR4 expression and microglia/macrophage activation induced by hemorrhage in the rat spinal cord after compressive injury

Abstract

BackgroundHemorrhage is a direct consequence of traumatic injury to the central nervous system and may cause innate immune reactions including cerebral Toll-like receptor (TLR) 4 upregulation which usually leads to poor outcome in the traumatic brain injury. In spinal cord injury (SCI), however, how hemorrhage induces innate immune reaction in spinal parenchyma remains unknown. The present study aimed to see whether blood component and/or other factor(s) induce TLR4 and microglia/macrophages involved innate immune reactions in the rat spinal cord after traumatic injury.MethodsUsing the compressive SCI model of the rat, hemorrhage in the spinal cord was identified by hematoxylin-eosin staining. Microglia/macrophage activation, TLR4 expression, and cell apoptosis were investigated by immunohistochemistry. Nuclear factor (NF)-κB p50 level of the two segments of the cord was detected by western blotting assay. With carbon powder injection, blood origination of the hematoma was explored. The blood-spinal cord barrier (BSCB) states of the lesion site and the hematoma were compared with immunohistochemistry and tannic acid-ferric chloride staining.ResultsHistological observation found blood accumulated in the center of compression lesion site (epicenter) and in the hematoma approximately 1.5 cm away from the epicenter. TLR4 expression, microglia//macrophage activation, and subsequent apoptosis in the area of far-away hematoma were late and weak in comparison to that in epicenter. In addition, TLR4 positive microglia/macrophages appeared to be phagocytotic in the far-away hematoma more obviously than that in the epicenter. Injected carbon powder indicated that accumulated blood of the far-away hematoma originated from the bleeding of the lesion epicenter, and the BSCB around the hematoma was not compromised in the early phase. Accordingly, at 3 days post injury, NF-κB p50 was upregulated based on the similar levels of blood component hemoglobin, and cell apoptosis was obvious in the epicenter but not in the far-away hematoma.ConclusionThese data suggest that besides blood component, BSCB compromise and the extent of tissue injury contribute more to TLR4 and microglia/macrophage responses to the spinal cord hemorrhage. Therefore, the innate immune environment is a necessary consideration for the SCI therapy targeting TLR4 and microglia/macrophages.

Up-Regulation of HMGB1 Exacerbates Renal Ischemia-Reperfusion Injury by Stimulating Inflammatory and Immune Responses through the TLR4 Signaling Pathway in Mice

“…Consistent with our study, Freitas et al showed that increased macrophage and neutrophil infiltration was found in the WT mice [37]. Zhang et al reported that the hematoma in the injury site would up-regulate TLR4 and activate macrophages after injury, which further initiated pro-inflammation cascade, indicating that TLR4 plays a positive role in inflammatory responses [38]. In our study, compared with the I/R groups, T lymphocytes, CD4 + and CD8 + T cells, and monocyte/macrophages, were increased markedly in the HMGB1 group.…”

Activation and Regulation of NLRP3 Inflammasome by Intrathecal Application of SDF-1a in a Spinal Cord Injury Model

“…More topical studies also suggest a protective and Bpositive^regulatory function. After SCI, microglia plays a decisive role in secondary injury, and the activation of a single macrophage phenotype can facilitate proper wound healing following SCI [15]. Successful orchestration of macrophage-mediated tissue repair requires fine tuning different macrophage phenotypes to harmoniously facilitate transitions among different wound healing phases [22].…”

“…On the other hand, microgliosis was observed by SDF-1a. After SCI, microglia which are decisive regulators of tissue damage after SCI accumulate within the epicenter and the hematoma of the injured spinal cord and are a putative source of IL-1b [15].…”

Pattern recognition receptors in chronic pain: Mechanisms and therapeutic implications

“…Of the nine TLRs, protein expression has been demonstrated for TLR2, 3, 4, 6, 7 and 9 in primary microglia culture from mouse brain and TLR 1 -4 from human brain (Olson and Miller, 2004;Yoon et al, 2008) (Table 2). In vivo, the expression levels of microglial TLRs seem to be kept low under normal conditions (Bsibsi et al, 2002;Casula et al, 2011;Shi et al, 2011;Zhang et al, 2013;Zhao et al, 2015). However, the upregulation of TLR2, 3 or 4 has been reported in the brain from a mouse model of Alzheimer's disease (AD) and in the spinal cord sections from a mouse model of spinal cord injury , as well as amyotrophic lateral sclerosis (ALS) patients (Casula et al, 2011) and multiple sclerosis (MS) patients (Bsibsi et al, 2002), suggesting the potential of microglial TLRs to participate in neuro-immune communication in the CNS.…”