Neuroinflammation: The Devil is in the Details

Microglia: Central Players in Neuroinflammation

Microglia are the focal point for any discussion of neuroinflammation. This is because these innate immune cells perform the primary immune surveillance and macrophage-like activities of the CNS, including the production of cytokines and chemokines. Indeed, much of the innate immune capacity of the CNS is mediated by microglia. These cells are resident CNS cells that reside in both the white matter and gray matter of the brain and spinal cord. Overall, microglia comprise 10% of the CNS population. Microglia develop early in embryogenesis from myeloid precursor cells in the embryonic yolk sac, and migrate to the area of the CNS around embryonic day 8.5 (Ginhoux et al. 2010). In fact, microglia have the same progenitor as the rest of the long-lived tissues macrophages of the body (Alliot et al. 1999). Consistent with this idea, microglia are long-lived cells that have limited turnover from the myeloid cells of the bone marrow during the course of a lifetime (Ajami et al. 2007, Ginhoux et al. 2010). These cells are not replaced from myeloid cells of the bone marrow. Nonetheless, when depleted, microglia turnover is possible from a potential progenitor source within the CNS (Elmore et al. 2014). The rate at which microglia would normally be turned over in the absence of pharmacological or genetic depletion is still unclear, yet microglia appear to have a relatively low overall turnover rate (Ajami et al. 2007, Ginhoux et al. 2010). This low turnover would make them susceptible to the potentially pro-inflammatory effects of age, injury or stress.

Microglia have an active role in immune surveillance. For instance, elegant two photon imaging studies show that microglia use their processes to actively survey their microenvironment (Nimmerjahn et al. 2005, Davalos et al. 2005). Other immune related activities include the propagation of inflammatory signals that are initiated in the periphery (Dantzer et al. 2008). These responses are pivotal in the coordinated communication between the immune system and the brain. For example, in infection or disease, microglia become ‘activated’ and function as inflammatory cellular mediators. Activated microglia rapidly alter their transcriptional profile and produce inflammatory cytokines and chemokines. Depending on context, the production of cytokines and chemokines can facilitate the recruitment of leukocytes to the brain (Zhou et al. 2006). Active microglia also undergo cytoskeletal rearrangements that alter the pattern of receptor expression on the cell surface. In addition, these alterations allow for microglia to migrate towards sites of injury or infection (Russo & McGavern 2015) and potentially increase their phagocytic efficiency (Davalos et al. 2005). In general, microglial activation and the increased expression of cytokines are intended to protect the CNS and benefit the host organism. Nonetheless, amplified, exaggerated, or chronic microglial activation can lead to robust pathological changes and neurobehavioral complications such as depression and cognitive deficits (Norden & Godbout 2013).

Neuroinflammation after Traumatic Brain Injury

One example of CNS injury is traumatic brain injury. TBI is defined by a complex assortment of heterogeneous physical insults to the head, resulting in biochemical injury to the brain and surrounding structures. TBI includes biphasic injuries and are described by their primary and secondary components (Maas et al. 2008, Osier et al. 2014). In brief, the primary injury caused by mechanical disruption of macro- and microscopic structures within and associated with the brain, resulting in tissue damage. Depending on the type of injury this can include overt tissue damage from a penetration injury or diffuse axonal injury from accelerating and rotational forces. Secondary injury is associated with tissue pathology and cell dysfunction leading to progressive damage extending long after the initial primary injury. For instance, the secondary injury with TBI is associated with cell death, neurotransmitter excitotoxicity, electrolyte imbalances, mitochondrial dysfunction, and ischemic injury (Maas et al. 2008).

Myriad studies indicate that both focal (penetrating) and diffuse (non-penetrating) TBI induce significant inflammatory processes in the brain mediated, in part, by resident microglia and astrocytes (McCrea et al. 2003, Lifshitz et al. 2007b, Tang et al. 1997, Woodcock & Morganti-Kossmann 2013). Following TBI, microglia respond to damaged cells, other activated glia, and peripherally derived stimuli after the breakdown of the blood brain barrier (Abdul-Muneer et al. 2013, Blixt et al. 2015). Active microglia rapidly induce the expression and release of cytokines and chemokines after injury (Kumar et al. 2015, Witcher et al. 2015). Microglial activation after injury is also associated with alterations in morphology and redistribution of cell-surface markers. For instance, a recent study identified “jellyfish” microglia by two-photon microscopy (Corps et al. 2015) that were detected after TBI was induced via pressure on a thin skull preparation. These microglia were highly activated, moved to the area of damage, and phagocytosed particulate matter associated with tissue damage (Corps et al. 2015). Active microglia also increase surface expression of Iba1 and CD68 and have a de-ramified morphology after diffuse TBI induced by midline fluid percussion injury (mFPI) (Bachstetter et al. 2013). Another study showed that morphological changes in microglia following diffuse TBI were dependent on intact p38α MAPK signaling, a pathway that also induces pro-inflammatory cytokine production (Bachstetter et al. 2011). Diffuse TBI induced by mFPI results in transiently elevated IL-1β, TNFα, and CD14 in the cortex and hippocampus of mice within 4 hours (Fenn et al. 2014). Moreover, microglia have increased IL-1β, CD14, iNOS, and arginase expression 24 hours after diffuse TBI compared with sham-injured controls (Fenn et al. 2015). Therefore, TBI results in the induction of both inflammatory (M1) and repair (M2) cascades associated with macrophage and microglia profiles (Kumar et al. 2013). Similar induction of both inflammatory and repair related mediators including iNOS, CD86, CD68, CD11b, IL-10, chitinase, TGF-β, and arginase are induced after focal brain injury by controlled cortical injury (Loane et al. 2014, Turtzo et al. 2014, Wang et al. 2013). Thus, both diffuse and focal head injuries drive a concordant inflammatory and repair response by microglia.

Along with resident microglia, there are other myeloid cells within the injured CNS that contribute to acute neuroinflammation. Differentiating the contribution of resident microglia from infiltrating macrophages can be difficult, particularly in the context of penetrating brain injury. This is because once monocytes differentiate into brain macrophages they are nearly indistinguishable from microglia by histology due to similarities in surface marker expression. Nonetheless, activation of microglia following TBI results in production of chemokines including CCL2, which attract monocytes and granulocytes to the site of injury (Semple et al. 2010). Notably, antagonism of the CCL2 receptor, CCR2, reduces inflammation and improves cognitive function 1 month after TBI induced by controlled cortical impact injury (CCI) (Morganti et al. 2015). Penetrating injury causes microglia/macrophages to associate with the axon initial segment within 3 hours (Baalman et al. 2015) and diffuse brain injury induces microglia to form tight clusters and long rod-like structures in the cortex within 7 days (Ziebell et al. 2012). Both of these cell types are rapidly activated and have central roles in mediating neuroinflammatory responses after injury.

Clinical evidence supports the idea that microglial and macrophage activation occurs rapidly following TBI. In autopsy of patients who have died following severe TBI, activated microglial morphologies were detected 2-10 days after injury (Velazquez et al. 2015). In addition, increased cerebrospinal fluid (CSF) levels of ferritin and soluble CD163, markers of macrophage and microglial activation, were elevated in pediatric patients within 3 days of severe TBI (Newell et al. 2015). Additionally, plasma levels of microglial activation products MMP-9 and galectin-3 were elevated within 8 hours of mild TBI in adults (Shan et al. 2016). Clinically, the detection of inflammatory proteins in CSF or blood may be useful biomarkers of microglia/macrophage-mediated inflammation.

In summary, microglia are rapidly activated and have both pro-inflammatory and neuroprotective roles immediately after TBI. The majority of anti-inflammatory interventions in rodent models of head injury reduces inflammation and improves functional recovery. Therefore, the reduction in acute neuroinflammation in TBI is interpreted to be beneficial. In several examples of TBI in rodents, minocycline, an anti-inflammatory agent and purported microglia inhibitor, reduced inflammation and improved functional recovery after TBI (Haber et al. 2013, Kovesdi et al. 2012). In addition, intravenous administration of methylene blue (MB), an anti-inflammatory agent (Oz et al. 2011), reduced expression of inflammatory mediators in the brain 1 day post-injury (Fenn et al. 2014). Notably, this reduction blocked the development of a TBI-associated depressive-like phenotype 7 days later. Another paper showed that MB treatment increased neuronal survival in a controlled cortical impact model of injury (Talley Watts et al. 2016). There are many examples of therapies that reduce the activation profiles of microglia and improve functional recovery (Kumar & Loane 2012). Another is issue is the degree to which inflammation fails to resolve, instead persisting chronically after injury. Evidence suggests that such a prolonged neuroinflammatory response mediated by microglia and macrophages is significantly detrimental after CNS injury. Therefore treatment strategies that can lower acute inflammation after TBI may have a long term benefit.

Immune Conditioning or Euflammation

Another positive aspect of inflammation is immune conditioning (Chen et al. 2013, Tarr et al. 2014). One example of immune conditioning is euflammation, defined as the induction of peripheral inflammation in the absence of sickness behavior. This is induced by repeated subthreshold Escherichia coli i.p. injections on three consecutive days which create a euflammation induction locus (EIL). The doses of E.coli incrementally increase, and are administered at 2.0×10⁷, 25×10⁷, and 100×10⁷ CFUs. This low level challenge beneficially “conditions” the immune system and leukocytes within the EIL release less cytokines upon subsequent immune stimulation (Chen et al. 2013, Tarr et al. 2014). This reduction in inflammatory signal propagation is contrasted by increased phagocytic potential of macrophages within the EIL.

This immune condition of inflammation was associated with minimized sickness behavior compared to mice exposed to a single E.coli dose. Additionally, leukocytes plated from inside the EIL displayed decreased cytokine mRNA expression, and leukocytes from outside the EIL displayed increased cytokine mRNA expression, when stimulated with LPS (Tarr et al. 2014). Locomotor deficits and piloerection were absent after euflammation conditioning (Chen et al. 2013, Tarr et al. 2014). Additionally, anhedonia, anorexia, and adipsia, more subtle markers for sickness (Andonova et al. 1998, Borowski et al. 1998, Riediger et al. 2010), were all resolved faster in the pre-conditioned mice (Tarr et al. 2014). The difference in sickness behavior following euflammatory treatment was attributed to changes in the phenotype of myeloid cells and a reduction in their inflammatory profile. Cells associated with the immune response (CD11b⁺/MHC II⁺, CD11b⁺/TLR4⁺, CD11b⁺/CD86⁺) were reduced within the EIL, but are increased in the blood and the spleen (Tarr et al. 2014). In addition, cultured peritoneal leukocytes had reduced production of IL-1β, IL-6, and IL-10 mRNA when stimulated with LPS (Tarr et al. 2014). This is a phenotype similar to the development of endotoxin tolerance. Outside of the EIL, blood leukocytes instead increase their expression of IL-1β, TNFα, and IL-10, a phenotype similar to trained immunity. This duality is unique to euflammation, combining the profiles previously described as endotoxin tolerance (Biswas & Lopez-Collazo 2009) and trained immunity (Netea 2013).

Related to these points, there are other examples of immune conditioning relevant to CNS injury. In this context, studies have aimed to precondition the CNS and protect against traumatic CNS injury. This preconditioning can be induced in a wide variety of manners, such as via LPS injections, the use of anesthetics such as isofluorane, and exposure to hypo- and hyperthermia (Baughman et al. 1988, Ota et al. 2000, Tasaki et al. 1997). For example, repeated LPS exposures lead to preconditioning of the CNS. Conditioned microglial activation can lead to the ensheathing of cortical neurons and the subsequent reduction in inhibitory axosomatic synapses, a neuroprotective function after traumatic brain injury caused by cryo-injury (Chen et al. 2012). The mechanism of action for this function is still under investigation, but it is hypothesized to be similar to the protective benefits offered by the synaptic stimulation of NMDA receptors and their anti-apoptotic signaling pathway through CREB activation (Hardingham et al. 2002). This process is paralleled by the activated microglia transiently and preferentially reducing GABAergic signaling (Chen et al. 2012). Overall, immune preconditioning may drive a unique microglial profile that is more anti-inflammatory or repair that allow for protection from the hyper-inflammatory conditions associated with traumatic CNS injury.

Signaling in Memory

There are also example of the benefits of neuroinflammatory signaling in brain development and plasticity. As outlined in the introduction, this represents neuroinflammatory signaling as opposed to true neuroinflammation discussed for CNS injury. For instance, recent studies in learning and memory have detailed the involvement of a complex cytokine network during LTP (del Rey et al. 2013). The major cytokines involved in these studies, IL-1β, IL-6, and TNFα, are produced by several cell types, including immune cells. A similarly diverse cellular population responds to these immune signals. These cells include neurons, both within and outside the hippocampus, and astrocytes, which are involved in the maintenance of synapses (Elmariah et al. 2005, McAfoose & Baune 2009, Ullian et al. 2004). Both IL-1β and IL-6 are overexpressed in the hippocampus after LTP induction, and they produce opposing effects in that IL-1β bolsters learning while IL-6 inhibits it (Goshen & Yirmiya 2009, Schneider et al. 1998). In addition to these two primary cytokines, TNFα, IL-1 receptor antagonist (IL-1RA), and IL-18 are also implicated in learning and memory. All of these cytokines have important roles in learning processes, an often overlooked property of classically inflammatory molecules.

Cytokine expression in the hippocampus displays regional specificity. The hippocampal increases mentioned above are complimented by additional changes in expression in the pre-frontal cortex (PFC). The effect of IL-1 upon neuronal activity can be elucidated through the study of IL-1 receptor 3 (IL-1R3). This shortened protein is preferentially expressed in neurons, opposed to the classical IL-1R1 expression on immune cells (Qian et al. 2012). In conjunction with the IL-1 receptor accessory protein B (IL-1RACPb), both of which are found preferentially within tissue of the nervous system, IL-1R3 activation increases voltage-gated potassium current. The discovery of IL-1R3 has led to clearer explanations of how IL-1 signaling can induce neuronal activity in the absence of NF-κB pathway activation (Qian et al. 2012). In sum, IL-1R1 and IL-1R3 activity helps to illuminate the increasingly complex role of IL-1 signaling in learning and memory. Additionally, this demonstrates the importance of cytokine signaling in the CNS even during such basic, homeostatic conditions as LTP induction.

Another route to learning and memory alteration by cytokines may lie in their modulatory effect on neurotransmission. In this context the source for the cytokines, particularly IL-1β and IL-6, is glia including astrocytes (John et al. 2003). Astrocytes receive neuronal signals and reciprocating by activating NF-κB pathways to affect neuronal plasticity (del Rey et al. 2013, Freudenthal et al. 2005, Kaltschmidt et al. 2006, Meffert et al. 2003). IL-1 and IL-6 also exert opposite effects on molecular signaling: IL-1 enhances AMPA- or NMDA-mediated transmission, while IL-6 inhibits glutamate release and synaptic plasticity (Kawasaki et al. 2008). In addition, IL-1 stimulates aminergic release in the hippocampus (Kabiersch et al. 1988). There is also evidence that immune cells, especially T cells, communicate to resident cells and are critical to normal memory and learning processes (Ziv et al. 2006, Kipnis et al. 2012). Thus, neuroinflammation signaling in this context is interpreted have a beneficial role in brain development, memory, and learning processes.

Aging

The normal aging process provides a key example of the disruption in the communication and balance between the brain and the immune system. In this case, there is a myriad of alterations in both peripheral immune cells and brain cells. Notably, the number of microglia in the dentate gyrus of aged mice is inversely correlated with the number of stem/progenitor cells (Gebara et al. 2013). In the brain, a higher inflammatory profile of microglia in rodents, canines, non-human primates, and humans has been reported. Several mediators associated with immunity are increased including MHC II, CD68, CD11b and CD11c, TLRs, and CD86 (Frank et al. 2006, Godbout et al. 2005, Griffin et al. 2006, Lucin & Wyss-Coray 2009, Stichel & Luebbert 2007). Aged microglia also have altered morphological profiles associated with de-ramification, shorter processes, and fewer branching arbors (Choi et al. 2007). In addition, there is an increase in inflammatory genes for cytokines such as IL-1β and IL-6 (Godbout et al. 2005, Sierra et al. 2007, Xie et al. 2003), and a corresponding decrease in anti-inflammatory genes of cytokines including IL-10 and IL-4 (Maher et al. 2005, Ye & Johnson 2001). Recent transcriptome analysis of the aged brain (Youm et al. 2013) sorted aged microglia confirmed a higher inflammatory profile with an mRNA signature of less TGFβ-related responsiveness.

One interpretation of this increased inflammatory profile of microglia with age is that these cells are primed or sensitized. This ‘priming’ profile of microglia is represented threefold: 1) increased expression of markers and mediators of inflammation, 2) decreased threshold and time for activation, and 3) increased response and inflammation following this activation (Henry et al. 2009, Norden & Godbout 2013). It is clear that the inflammatory status increases with age (Chung et al. 2009). Recent studies have also used positron emission tomography (PET) to evaluate microglial activation in humans. The ligand PK [11C](R)PK11195 binds to translocator protein (TSPO) receptors that are expressed in mitochondria of activated microglia. PET imaging using this compound showed increased TSPO in older individuals, indicating that microglial activation was elevated with age (Schuitemaker et al. 2012). Taken together, the increase of inflammatory mediators and altered phenotype of microglia with age indicates that microglia develop a primed phenotype and that this may contribute to the heightened inflammatory status of the aged brain.

A consequence of an increased primed profile in the aged brain is the hyper-activation of microglia in response to immune challenge. For instance, peripheral immune challenge with lipopolysaccharide or E.coli causes an exaggerated and prolonged neuroinflammation in aged rodents compared to adults. In aged mice, peripheral LPS challenge results in elevated mRNA levels of IL-1β, IL-6, and TNFα (Abraham et al. 2008, Godbout et al. 2005), which remained elevated for 24 and 72 hours (Godbout et al. 2008b). The prolonged exaggeration of IL-1β in microglia coincides with a similarly prolonged downregulation of fractalkine receptor (CX3CR1) (Wynne et al. 2010). Evidence points to primed microglia as the major arbiters of this inflammation, as MHC II⁺ microglia had a robust increase IL-1β protein expression following LPS stimulation (Henry et al. 2009). Similarly, microglia isolated from aged mice display elevated TNFα, IL-1β, IL-6, IL-12, TLR2, and indoleamine 2,3-dioxygenase (IDO) mRNA after LPS when compared with adult controls (Henry et al. 2009, Sierra et al. 2007). These changes in cytokine production are also present in aged mice following minor surgery and mild psychological stress (Buchanan et al. 2008, Rosczyk et al. 2008). Furthermore, the induction of IL-1β after immune challenge was greater in MHC II⁺ microglia of aged mice compared to MHC II⁻ microglia (Henry et al. 2009). These studies indicate that these primed MHC II⁺ microglia produce inflammatory cytokines, to an exaggerated and prolonged degree, after peripheral immune stimulation.

As discussed above, cytokine-mediated sickness behavior is a necessary and beneficial response to infection. An unchecked, amplified, or prolonged response, however, affects behavioral and cognitive processes (Jurgens & Johnson 2010). In the studies discussed above, LPS stimulation, either peripheral or central, caused protracted neuroinflammation in the brain of aged rodents. This was accompanied by a protracted sickness response notably characterized by persistent anorexia, lethargy, and social withdrawal (Godbout et al. 2005, Abraham et al. 2008, Huang et al. 2008). An exaggerated sickness response was also detected in older rats following subcutaneous E.coli injection. The aged rats displayed an altered febrile response characterized by a blunted and delayed increase of core body temperature followed by a notable and protracted increase of inflammatory cytokines in the brain (Barrientos et al. 2009b). Similar to the extended sickness behaviors in aged BALB/c mice, the increase of inflammatory cytokines were driven by exaggerated microglial IL-1β (Henry et al. 2009). Indeed, i.c.v. infusion of IL-1 receptor antagonist (IL-1RA) reversed the extended LPS-induced sickness behavior in aged mice (Abraham & Johnson 2008). These findings indicate that the exaggerated sickness response in aged rodents was caused by the exaggerated and prolonged production of IL-1β by primed microglia.

Following an immune challenge, there is also evidence of the development of depressive-like behavioral complications and cognitive deficits in aged rodents. For example, in aged BALB/c mice, peripheral LPS stimulation caused prolonged depressive-like behavior 72 h after injection in aged mice after acute (Godbout et al. 2008a) and chronic (Kelley et al. 2013) immune challenge. In addition, injection of LPS caused an amplified cytokine response in the hippocampus of older mice that was paralleled by impaired hippocampal-dependent spatial memory (Chen et al. 2008). Moreover, infection by E.coli led to prolonged production of IL-1β in the hippocampus of aged rats (Barrientos et al. 2009a) and reduced long-term contextual memory examined by context-dependent fear conditioning and Morris water maze (Barrientos et al. 2006, Barrientos et al. 2009a). When aged mice were fed a diet supplemented with resveratrol, a potent anti-oxidant, LPS-induced neuroinflammation and working memory deficits were attenuated (Abraham & Johnson 2009). Increased cytokine production in the aged brain after peripheral innate immune challenge is also associated with impaired cognitive function. Taken together, when there is a primed or sensitized profile of microglia the normal communication between immune system and the brain is impaired. The normally acute neuroinflammatory signaling events are instead amplified and prolonged, and cognitive and behavioral deficits develop in aged mice that are not detected in healthy adults.

Chronic Microglial Activation after Traumatic Brain Injury

As discussed, acute neuroinflammation has both positive and negative influences of functional recovery and repair processes after injury. Thus, it can be difficult to state whether inflammation is negative or positive. The majority of clinical and experimental data, however, indicates that neuroinflammation has a negative effect when it is prolonged. For instance, a pro-inflammatory profile of microglia persists long after injury and is evident in diffuse, penetrating, and repeated head injuries. Furthermore, persistent changes in microglia and astrocytes after penetrating TBI, such as increased MHC II expression in rats 16 days post-injury, are caused by CCI (Holmin et al. 1995). CCI-injured rodents also display an increased inflammatory profile of microglia up to 1 year after injury (Loane et al. 2014). Microglia in the lesion border more highly express several pro-inflammatory mediators such as MHC II, CD68 and NADPH oxidase (NOX2) in the cortex and thalamus, persisting 12 months after injury. In the same brain regions, YM-1, an alternative activation marker (M2) was transiently up-regulated 1 week later, but was reduced over time and was undetected at 3 and 12 months after injury. This supports the notion that in the midst of heightened inflammatory status and progressing gliosis, reparative mechanisms are down-regulated. Additionally, CCI-initiated inflammatory marker expression elevation on microglia and macrophages is associated with increased lesion volume expansion and loss of neurons in the hippocampus (Loane et al. 2014). Such tissue damage and elevated inflammatory mediators are mediated by microglia and macrophages, persist over time, and are consistent with other models of focal or penetrating brain injury (Shultz et al. 2013, Huang et al. 2014).

There is evidence of a persistent, primed, pro-inflammatory microglial profile after diffuse head injury. Reliable experimental models of this non-penetrating, diffuse injury include mFPI, closed head impact (CHI), and blast injuries (Xiong et al. 2013). Injuries by mFPI are administered by a fluid-filled pulse directly at the brain midline without disrupting the dura, causing global undulation of brain tissue. This causes mild neuronal pathology including diffuse axonal injury (Lifshitz et al. 2007a) and transient neurological deficits (Morales et al. 2005) that mimic clinical complications after mild to moderate concussive head injuries in human patients (Lifshitz 2009). Moderate mFPI in rodents causes activated microglia and an altered microglial phenotype, such as increased MHC II and CD68 expression, that persists 1 week to 1 month post-injury. (Ziebell et al. 2012). Moreover, these microglia have a unique rod-like morphology and train arrangement along axon tracks and are predominantly detected in the primary sensory barrel fields of rats (Ziebell et al. 2012). A related study in mice has shown that mFPI increases mRNA and protein expression of MHC II on microglia in the brain up to 1 month post-injury (Fenn et al. 2014). Furthermore, there subtle changes in microglial morphology are evident in the parietal cortex and hippocampus, both of which are brain regions affected by the diffuse injury. These increases in MHC II expression on microglia 30 days post-injury were detected in mice that had returned to baseline behavior. Overall, these data provide evidence that there are pro-inflammatory microglial profiles of microglia that develop and are maintained after brain injury.

An inflammatory glial profile that persists after TBI is paralleled in human patients following moderate or severe TBI. For example, increased CD11b and CD68 microglial expression has been shown in blunt head trauma patients at short- and long-term time points, up to 16 years post-injury (Gentleman et al. 2004). Another study has demonstrated that densely-packed, reactive (CR3/43⁺ or CD68⁺) microglia are detectable 1-18 years after a single moderate or severe TBI (Johnson et al. 2013). Recent developments in magnetic resonance imaging (MRI) techniques have been used to examine microglial activation in human brains after TBI in vivo. As described above, PK [11C](R)PK11195 ligand is expressed on activated microglia. Higher PK binding was detected in TBI patients in the thalamus, occipital lobes, putamen and posterior limb of internal capsule (Ramlackhansingh et al. 2011). Notably, chronic microglial activation increases in cell populations further from a focal lesion, being most prominent within subcortical structures such as the thalamus and putamen. Furthermore, although the time post-injury for those studied ranged from 11 months to 17 years, no correlation was found between PK binding and the time since the head injury. These findings indicate that chronic microglial activation persists for months to years after the initial insult in human patients.

This research was supported by an NIA grant (R01-AG-033028 to J.P.G.).