Cell-Type-Specific Interleukin 1 Receptor 1 Signaling in the Brain Regulates Distinct Neuroimmune Activities.

Interleukin-1 (IL-1) signaling is important for multiple potentially pathogenic processes in the central nervous system (CNS), but the cell-type-specific roles of IL-1 signaling are unclear. We used a genetic knockin reporter system in mice to track and reciprocally delete or express IL-1 receptor 1 (IL-1R1) in specific cell types, including endothelial cells, ventricular cells, peripheral myeloid cells, microglia, astrocytes, and neurons. We found that endothelial IL-1R1 was necessary and sufficient for mediating sickness behavior and drove leukocyte recruitment to the CNS and impaired neurogenesis, whereas ventricular IL-1R1 was critical for monocyte recruitment to the CNS. Although microglia did not express IL-1R1, IL-1 stimulation of endothelial cells led to the induction of IL-1 in microglia. Together, these findings describe the structure and functions of the brain's IL-1R1-expressing system and lay a foundation for the dissection and identification of IL-1R1 signaling pathways in the pathogenesis of CNS diseases.

Chronic Cortical Inflammation, Cognitive Impairment, and Immune Reactivity Associated with Diffuse Brain Injury Are Ameliorated by Forced Turnover of Microglia.

Traumatic brain injury (TBI) is associated with an increased risk of cognitive, psychiatric, and neurodegenerative complications that may develop after injury. Increased microglial reactivity following TBI may underlie chronic neuroinflammation, neuropathology, and exaggerated responses to immune challenges. Therefore, the goal of this study was to force turnover of trauma-associated microglia that develop after diffuse TBI and determine whether this alleviated chronic inflammation, improved functional recovery and attenuated reduced immune reactivity to lipopolysaccharide (LPS) challenge. Male mice received a midline fluid percussion injury (mFPI) and 7 d later were subjected to a forced microglia turnover paradigm using CSF1R antagonism (PLX5622). At 30 d postinjury (dpi), cortical gene expression, dendritic complexity, myelin content, neuronal connectivity, cognition, and immune reactivity were assessed. Myriad neuropathology-related genes were increased 30 dpi in the cortex, and 90% of these gene changes were reversed by microglial turnover. Reduced neuronal connectivity was evident 30 dpi and these deficits were attenuated by microglial turnover. TBI-associated dendritic remodeling and myelin alterations, however, remained 30 dpi independent of microglial turnover. In assessments of functional recovery, increased depressive-like behavior, and cognitive impairment 30 dpi were ameliorated by microglia turnover. To investigate microglial priming and reactivity 30 dpi, mice were injected intraperitoneally with LPS. This immune challenge caused prolonged lethargy, sickness behavior, and microglial reactivity in the TBI mice. These extended complications with LPS in TBI mice were prevented by microglia turnover. Collectively, microglial turnover 7 dpi alleviated behavioral and cognitive impairments associated with microglial priming and immune reactivity 30 dpi.SIGNIFICANCE STATEMENT A striking feature of traumatic brain injury (TBI), even mild injuries, is that over 70% of individuals have long-term neuropsychiatric complications. Chronic inflammatory processes are implicated in the pathology of these complications and these issues can be exaggerated by immune challenge. Therefore, our goal was to force the turnover of microglia 7 d after TBI. This subacute 7 d postinjury (dpi) time point is a critical transitional period in the shift toward chronic inflammatory processes and microglia priming. This forced microglia turnover intervention in mice attenuated the deficits in behavior and cognition 30 dpi. Moreover, microglia priming and immune reactivity after TBI were also reduced with microglia turnover. Therefore, microglia represent therapeutic targets after TBI to reduce persistent neuroinflammation and improve recovery.

Traumatic Brain Injury Causes Chronic Cortical Inflammation and Neuronal Dysfunction Mediated by Microglia.

Traumatic brain injury (TBI) can lead to significant neuropsychiatric problems and neurodegenerative pathologies, which develop and persist years after injury. Neuroinflammatory processes evolve over this same period. Therefore, we aimed to determine the contribution of microglia to neuropathology at acute [1 d postinjury (dpi)], subacute (7 dpi), and chronic (30 dpi) time points. Microglia were depleted with PLX5622, a CSF1R antagonist, before midline fluid percussion injury (FPI) in male mice and cortical neuropathology/inflammation was assessed using a neuropathology mRNA panel. Gene expression associated with inflammation and neuropathology were robustly increased acutely after injury (1 dpi) and the majority of this expression was microglia independent. At 7 and 30 dpi, however, microglial depletion reversed TBI-related expression of genes associated with inflammation, interferon signaling, and neuropathology. Myriad suppressed genes at subacute and chronic endpoints were attributed to neurons. To understand the relationship between microglia, neurons, and other glia, single-cell RNA sequencing was completed 7 dpi, a critical time point in the evolution from acute to chronic pathogenesis. Cortical microglia exhibited distinct TBI-associated clustering with increased type-1 interferon and neurodegenerative/damage-related genes. In cortical neurons, genes associated with dopamine signaling, long-term potentiation, calcium signaling, and synaptogenesis were suppressed. Microglial depletion reversed the majority of these neuronal alterations. Furthermore, there was reduced cortical dendritic complexity 7 dpi, reduced neuronal connectively 30 dpi, and cognitive impairment 30 dpi. All of these TBI-associated functional and behavioral impairments were prevented by microglial depletion. Collectively, these studies indicate that microglia promote persistent neuropathology and long-term functional impairments in neuronal homeostasis after TBI.SIGNIFICANCE STATEMENT Millions of traumatic brain injuries (TBIs) occur in the United States alone each year. Survivors face elevated rates of cognitive and psychiatric complications long after the inciting injury. Recent studies of human brain injury link chronic neuroinflammation to adverse neurologic outcomes, suggesting that evolving inflammatory processes may be an opportunity for intervention. Here, we eliminate microglia to compare the effects of diffuse TBI on neurons in the presence and absence of microglia and microglia-mediated inflammation. In the absence of microglia, neurons do not undergo TBI-induced changes in gene transcription or structure. Microglial elimination prevented TBI-induced cognitive changes 30 d postinjury (dpi). Therefore, microglia have a critical role in disrupting neuronal homeostasis after TBI, particularly at subacute and chronic timepoints.

Interleukin-1 receptor on hippocampal neurons drives social withdrawal and cognitive deficits after chronic social stress.

Chronic stress contributes to the development of psychiatric disorders including anxiety and depression. Several inflammatory-related effects of stress are associated with increased interleukin-1 (IL-1) signaling within the central nervous system and are mediated by IL-1 receptor 1 (IL-1R1) on several distinct cell types. Neuronal IL-1R1 is prominently expressed on the neurons of the dentate gyrus, but its role in mediating behavioral responses to stress is unknown. We hypothesize that IL-1 acts on this subset of hippocampal neurons to influence cognitive and mood alterations with stress. Here, mice subjected to psychosocial stress showed reduced social interaction and impaired working memory, and these deficits were prevented by global IL-1R1 knockout. Stress-induced monocyte trafficking to the brain was also blocked by IL-1R1 knockout. Selective deletion of IL-1R1 in glutamatergic neurons (nIL-1R1-/-) abrogated the stress-induced deficits in social interaction and working memory. In addition, viral-mediated selective IL-1R1 deletion in hippocampal neurons confirmed that IL-1 receptor in the hippocampus was critical for stress-induced behavioral deficits. Furthermore, selective restoration of IL-1R1 on glutamatergic neurons was sufficient to reestablish the impairments of social interaction and working memory after stress. RNA-sequencing of the hippocampus revealed that stress increased several canonical pathways (TREM1, NF-κB, complement, IL-6 signaling) and upstream regulators (INFγ, IL-1ß, NF-κB, MYD88) associated with inflammation. The inductions of TREM1 signaling, complement, and leukocyte extravasation with stress were reversed by nIL-1R1-/-. Collectively, stress-dependent IL-1R1 signaling in hippocampal neurons represents a novel mechanism by which inflammation is perpetuated and social interactivity and working memory are modulated.

Interleukin-1 causes CNS inflammatory cytokine expression via endothelia-microglia bi-cellular signaling.

As a major producer of the inflammatory cytokine interleukin-1 (IL-1), peripheral macrophages can augment IL-1 expression via type 1 IL-1 receptor (IL-1R1) mediated autocrine self-amplification. In the CNS, microglial cells are the major producers of inflammatory cytokines, but express negligible levels of IL-1R1. In the present study, we showed CNS IL-1 induced microglial proinflammatory cytokine expression was mediated by endothelial, not microglial, IL-1R1. This paracrine mechanism was further dissected in vitro. IL-1 was unable to stimulate inflammatory cytokine expression directly from the microglial cell line BV-2, but it stimulated the brain endothelial cell line bEnd.3 to produce a factor(s) in the culture supernatant, which was capable of inducing inflammatory cytokine expression in BV-2. We termed this factor IL-1-induced microglial activation factors (IMAF). BV-2 cytokine expression was inducible by extracellular ATP, but IL-1 did not stimulate the release of ATP from bEnd.3 cells. Filtration of IMAF by size-exclusion membranes showed IMAF activity resided in molecules larger than 50 kd and incubation of IMAF at 95 °C for 5 min did not alter its activity. Microglial inhibitor minocycline was unable to block IMAF activity, even though it blocked LPS induced cytokine expression in BV-2 cells. Adding NF-κB inhibitor to the bEnd.3 cells abolished IL-1 induced cytokine expression in this bi-cellular system, but adding NF-κB inhibitor after IMAF is already produced failed to abrogate IMAF induced cytokine expression in BV-2 cells. RNA sequencing of IL-1 stimulated endothelial cells revealed increased expression of genes involved in the production and processing of hyaluronic acid (HA), suggesting HA as a candidate of IMAF. Inhibition of hyaluronidase by ascorbyl palmitate (AP) abolished IMAF-induced cytokine expression in BV-2 cells. AP administration in vivo also inhibited ICV IL-1-induced IL-1 expression in the hippocampus and hypothalamus. In vitro, either TLR2 or TLR4 inhibitors blocked IMAF induced BV-2 cytokine expression. In vivo, however, IL-1 induced cytokine expression persisted in either TLR2 or TLR4 knockouts. These results demonstrate IL-1 induced inflammatory cytokine expression in the CNS requires a bi-cellular system and HA could be a candidate for IMAF.

Traumatic brain injury-induced neuronal damage in the somatosensory cortex causes formation of rod-shaped microglia that promote astrogliosis and persistent neuroinflammation.

Microglia undergo dynamic structural and transcriptional changes during the immune response to traumatic brain injury (TBI). For example, TBI causes microglia to form rod-shaped trains in the cerebral cortex, but their contribution to inflammation and pathophysiology is unclear. The purpose of this study was to determine the origin and alignment of rod microglia and to determine the role of microglia in propagating persistent cortical inflammation. Here, diffuse TBI in mice was modeled by midline fluid percussion injury (FPI). Bone marrow chimerism and BrdU pulse-chase experiments revealed that rod microglia derived from resident microglia with limited proliferation. Novel data also show that TBI-induced rod microglia were proximal to axotomized neurons, spatially overlapped with dense astrogliosis, and aligned with apical pyramidal dendrites. Furthermore, rod microglia formed adjacent to hypertrophied microglia, which clustered among layer V pyramidal neurons. To better understand the contribution of microglia to cortical inflammation and injury, microglia were eliminated prior to TBI by CSF1R antagonism (PLX5622). Microglial elimination did not affect cortical neuron axotomy induced by TBI, but attenuated rod microglial formation and astrogliosis. Analysis of 262 immune genes revealed that TBI caused profound cortical inflammation acutely (8 hr) that progressed in nature and complexity by 7 dpi. For instance, gene expression related to complement, phagocytosis, toll-like receptor signaling, and interferon response were increased 7 dpi. Critically, these acute and chronic inflammatory responses were prevented by microglial elimination. Taken together, TBI-induced neuronal injury causes microglia to structurally associate with neurons, augment astrogliosis, and propagate diverse and persistent inflammatory/immune signaling pathways.

Microglia promote maladaptive plasticity in autonomic circuitry after spinal cord injury in mice.

Robust structural remodeling and synaptic plasticity occurs within spinal autonomic circuitry after severe high-level spinal cord injury (SCI). As a result, normally innocuous visceral or somatic stimuli elicit uncontrolled activation of spinal sympathetic reflexes that contribute to systemic disease and organ-specific pathology. How hyperexcitable sympathetic circuitry forms is unknown, but local cues from neighboring glia likely help mold these maladaptive neuronal networks. Here, we used a mouse model of SCI to show that microglia surrounded active glutamatergic interneurons and subsequently coordinated multi-segmental excitatory synaptogenesis and expansion of sympathetic networks that control immune, neuroendocrine, and cardiovascular functions. Depleting microglia during critical periods of circuit remodeling after SCI prevented maladaptive synaptic and structural plasticity in autonomic networks, decreased the frequency and severity of autonomic dysreflexia, and prevented SCI-induced immunosuppression. Forced turnover of microglia in microglia-depleted mice restored structural and functional indices of pathological dysautonomia, providing further evidence that microglia are key effectors of autonomic plasticity. Additional data show that microglia-dependent autonomic plasticity required expression of triggering receptor expressed on myeloid cells 2 (Trem2) and α2δ-1-dependent synaptogenesis. These data suggest that microglia are primary effectors of autonomic neuroplasticity and dysautonomia after SCI in mice. Manipulating microglia may be a strategy to limit autonomic complications after SCI or other forms of neurologic disease.

Sleep fragmentation engages stress-responsive circuitry, enhances inflammation and compromises hippocampal function following traumatic brain injury.

Traumatic brain injury (TBI) impairs the ability to restore homeostasis in response to stress, indicating hypothalamic-pituitary-adrenal (HPA)-axis dysfunction. Many stressors result in sleep disturbances, thus mechanical sleep fragmentation (SF) provides a physiologically relevant approach to study the effects of stress after injury. We hypothesize SF stress engages the dysregulated HPA-axis after TBI to exacerbate post-injury neuroinflammation and compromise recovery. To test this, male and female mice were given moderate lateral fluid percussion TBI or sham-injury and left undisturbed or exposed to daily, transient SF for 7- or 30-days post-injury (DPI). Post-TBI SF increases cortical expression of interferon- and stress-associated genes characterized by inhibition of the upstream regulator NR3C1 that encodes glucocorticoid receptor (GR). Moreover, post-TBI SF increases neuronal activity in the hippocampus, a key intersection of the stress-immune axes. By 30 DPI, TBI SF enhances cortical microgliosis and increases expression of pro-inflammatory glial signaling genes characterized by persistent inhibition of the NR3C1 upstream regulator. Within the hippocampus, post-TBI SF exaggerates microgliosis and decreases CA1 neuronal activity. Downstream of the hippocampus, post-injury SF suppresses neuronal activity in the hypothalamic paraventricular nucleus indicating decreased HPA-axis reactivity. Direct application of GR agonist, dexamethasone, to the CA1 at 30 DPI increases GR activity in TBI animals, but not sham animals, indicating differential GR-mediated hippocampal action. Electrophysiological assessment revealed TBI and SF induces deficits in Schaffer collateral long-term potentiation associated with impaired acquisition of trace fear conditioning, reflecting dorsal hippocampal-dependent cognitive deficits. Together these data demonstrate that post-injury SF engages the dysfunctional post-injury HPA-axis, enhances inflammation, and compromises hippocampal function. Therefore, external stressors that disrupt sleep have an integral role in mediating outcome after brain injury.

Microglia coordinate cellular interactions during spinal cord repair in mice.

Traumatic spinal cord injury (SCI) triggers a neuro-inflammatory response dominated by tissue-resident microglia and monocyte derived macrophages (MDMs). Since activated microglia and MDMs are morphologically identical and express similar phenotypic markers in vivo, identifying injury responses specifically coordinated by microglia has historically been challenging. Here, we pharmacologically depleted microglia and use anatomical, histopathological, tract tracing, bulk and single cell RNA sequencing to reveal the cellular and molecular responses to SCI controlled by microglia. We show that microglia are vital for SCI recovery and coordinate injury responses in CNS-resident glia and infiltrating leukocytes. Depleting microglia exacerbates tissue damage and worsens functional recovery. Conversely, restoring select microglia-dependent signaling axes, identified through sequencing data, in microglia depleted mice prevents secondary damage and promotes recovery. Additional bioinformatics analyses reveal that optimal repair after SCI might be achieved by co-opting key ligand-receptor interactions between microglia, astrocytes and MDMs.

Comparison between midline and lateral fluid percussion injury in mice reveals prolonged but divergent cortical neuroinflammation.

Animal models are critical for determining the mechanisms mediating traumatic brain injury-induced (TBI) neuropathology. Fluid percussion injury (FPI) is a widely used model of brain injury typically applied either midline or parasagittally (lateral). Midline FPI induces a diffuse TBI, while lateral FPI induces both focal cortical injury (ipsilateral hemisphere) and diffuse injury (contralateral hemisphere). Nonetheless, discrete differences in neuroinflammation and neuropathology between these two versions of FPI remain unclear. The purpose of this study was to compare acute (4-72 h) and subacute (7 days) neuroinflammatory responses between midline and lateral FPI. Midline FPI resulted in longer righting reflex times than lateral FPI. At acute time points, the inflammatory responses to the two different injuries were similar. For instance, there was evidence of monocytes and cytokine mRNA expression in the brain with both injuries acutely. Midline FPI had the highest proportion of brain monocytes and highest IL-1ß/TNFα mRNA expression 24 h later. NanoString nCounter analysis 7 days post-injury revealed robust and prolonged expression of inflammatory-related genes in the cortex after midline FPI compared to lateral FPI; however, Iba-1 cortical immunoreactivity was increased with lateral FPI. Thus, midline and lateral FPI caused similar cortical neuroinflammatory responses acutely and mRNA expression of inflammatory genes was detectable in the brain 7 days later. The primary divergence was that inflammatory gene expression was greater and more diverse subacutely after midline FPI. These results provide novel insight to variations between midline and lateral FPI, which may recapitulate unique temporal pathogenesis.

Sleep Disruption Exacerbates and Prolongs the Inflammatory Response to Traumatic Brain Injury.

Traumatic brain injury (TBI) alters stress responses, which may influence neuroinflammation and behavioral outcome. Sleep disruption (SD) is an understudied post-injury environmental stressor that directly engages stress-immune pathways. Thus, we predicted that maladaptive changes in the hypothalamic-pituitary-adrenal (HPA) axis after TBI compromise the neuroendocrine response to SD and exacerbate neuroinflammation. To test this, we induced lateral fluid percussion TBI or sham injury in female and male C57BL/6 mice aged 8-10 weeks that were then left undisturbed or exposed to 3 days of transient SD. At 3 days post-injury (DPI) plasma corticosterone (CORT) was reduced in TBI compared with sham mice, indicating altered HPA-mediated stress response to SD. This response was associated with approach-avoid conflict behavior and exaggerated cortical neuroinflammation. Post-injury SD specifically enhanced neutrophil trafficking to the injured brain in conjunction with dysregulated aquaporin-4 (AQP4) polarization. Delayed and persistent effects of post-injury SD were determined 4 days after SD concluded at 7 DPI. SD prolonged anxiety-like behavior regardless of injury and was associated with increased cortical Iba1 labeling in both sham and TBI mice. Strikingly, TBI SD mice displayed an increased number of CD45+ cells near the site of injury, enhanced cortical glial fibrillary acidic protein (GFAP) immunolabeling, and persistent expression of Trem2 and Tlr4 7 DPI compared with TBI mice. These results support the hypothesis that post-injury SD alters stress-immune pathways and inflammatory outcomes after TBI. These data provide new insight to the dynamic interplay between TBI, stress, and inflammation.

Interleukin-6 Induced by Social Stress Promotes a Unique Transcriptional Signature in the Monocytes That Facilitate Anxiety.

BACKGROUND: Interleukin-6 (IL-6) is elevated in circulation with chronic stress and may contribute to neurobehavioral complications. We have reported that repeated social defeat stress in mice caused recruitment of proinflammatory monocytes to the brain and triggered the onset of anxiety-like behavior. Therefore, the purpose of this study was to determine the role of IL-6 signaling in the peripheral immune response, neuroinflammation, and anxiety following stress. METHODS: Wild-type and IL-6 knockout mice were subjected to repeated social defeat, and immune and behavioral parameters were determined 14 hours later. RESULTS: Although monocyte release and recruitment to the brain during stress were maintained in the IL-6 knockout mice, anxiety and social avoidance were prevented. NanoString analysis of fluorescence-activated cell-sorted blood monocytes (CD11b+/Ly6Chi) and brain monocytes (CD11b+/CD45hi) revealed a unique pattern of immune-related gene expression that was dependent on stress and IL-6. For instance, blood monocytes after stress had a transcriptional signature and immune profile consistent with priming, which was attenuated in monocytes from IL-6 knockout stress mice. Moreover, the monocytes recruited to the brain and associated with the development of anxiety had a transcriptional signature (enhanced IL-1ß, CD14, Mmp9, Myd88, Ager, and Stat3) that was dependent on IL-6. CONCLUSIONS: Here, we show the effects of IL-6 on the transcriptional signature of monocytes in circulation and brain after stress. Overall, robust increases in IL-6 after stress induced a primed profile in monocytes that were recruited to the brain and propagated IL-1-mediated inflammation and anxiety.

The Influence of Microglial Elimination and Repopulation on Stress Sensitization Induced by Repeated Social Defeat.

BACKGROUND: Stress is associated with an increased prevalence of anxiety and depression. Repeated social defeat (RSD) stress in mice increases the release of monocytes from the bone marrow that are recruited to the brain by microglia. These monocytes enhance inflammatory signaling and augment anxiety. Moreover, RSD promotes stress sensitization, in which exposure to acute stress 24 days after cessation of RSD causes anxiety recurrence. The purpose of this study was to determine whether microglia were critical to stress sensitization and exhibited increased reactivity to subsequent acute stress or immune challenge. METHODS: Mice were exposed to RSD, microglia were eliminated by colony-stimulating factor 1 receptor antagonism (PLX5622) and allowed to repopulate, and responses to acute stress or immune challenge (lipopolysaccharide) were determined 24 days after RSD sensitization. RESULTS: Microglia maintained a unique messenger RNA signature 24 days after RSD. Moreover, elimination of RSD-sensitized microglia prevented monocyte accumulation in the brain and blocked anxiety recurrence following acute stress (24 days). When microglia were eliminated prior to RSD and repopulated and mice were subjected to acute stress, there was monocyte accumulation in the brain and anxiety in RSD-sensitized mice. These responses were unaffected by microglial elimination/repopulation. This may be related to neuronal sensitization that persisted 24 days after RSD. Following immune challenge, there was robust microglial reactivity in RSD-sensitized mice associated with prolonged sickness behavior. Here, microglial elimination/repopulation prevented the amplified immune reactivity ex vivo and in vivo in RSD-sensitized mice. CONCLUSIONS: Microglia and neurons remain sensitized weeks after RSD, and only the immune reactivity component of RSD-sensitized microglia was prevented by elimination/repopulation.

Forced turnover of aged microglia induces an intermediate phenotype but does not rebalance CNS environmental cues driving priming to immune challenge.

Microglia are the resident innate immune cells of the central nervous system. Limited turnover throughout the lifespan leaves microglia susceptible to age-associated dysfunction. Indeed, we and others have reported microglia develop a pro-inflammatory or "primed" profile with age, characterized by increased expression of inflammatory mediators (e.g., MHC-II, CD68, IL-1ß). Moreover, immune challenge with lipopolysaccharide (LPS) causes an exaggerated and prolonged neuroinflammatory response mediated by primed microglia in the aged brain. Recent studies show colony-stimulating factor 1 receptor (CSF1R) antagonism results in rapid depletion of microglia without significant complications. Therefore, we hypothesized that CSF1R antagonist-mediated depletion of microglia in the aged brain would result in repopulation with new and unprimed microglia. Here we provide novel evidence that microglia in the brain of adult (6-8 weeks old) and aged (16-18 months old) BALB/c mice were depleted following 3-week oral PLX5622 administration. When CSF1R antagonism was stopped, microglia repopulated equally in the adult and aged brain. Microglial depletion and repopulation reversed age-associated increases in microglial CD68+ lysosome enlargement and lipofuscin accumulation. Microglia-specific RNA sequencing revealed 511 differentially expressed genes with age. Of these, 117 genes were reversed by microglial repopulation (e.g., Apoe, Tgfb2, Socs3). Nevertheless, LPS challenge still induced an exaggerated microglial inflammatory response in the aged brain compared to adults. RNA sequencing of whole-brain tissue revealed an age-induced inflammatory signature, including reactive astrocytes, that was not restored by microglial depletion and repopulation. Furthermore, the microenvironment of the aged brain produced soluble factors that influenced developing microglia ex vivo and induced a profile primed to LPS challenge. Thus, the aged brain microenvironment promotes microglial priming despite repopulation of new microglia. Collectively, aged microglia proliferate and repopulate the brain, but these new cells still adopt a pro-inflammatory profile in the aged brain.

Priming the inflammatory pump of the CNS after traumatic brain injury.

Traumatic brain injury (TBI) can lead to secondary neuropsychiatric problems that develop and persist years after injury. Mounting evidence indicates that neuroinflammatory processes progress after the initial head injury and worsen with time. Microglia contribute to this inflammation by maintaining a primed profile long after the acute effects of the injury have dissipated. This may set the stage for glial dysfunction and hyperactivity to challenges including subsequent head injury, stress, or induction of a peripheral immune response. This review discusses the evidence that microglia become primed following TBI and how this corresponds with vulnerability to a 'second hit' and subsequent neuropsychiatric and neurodegenerative complications.