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10.1126/science.aag2590

PERSPECTIVE

Inflammatory neuroprotectionfollowing traumatic brain injuryMatthew V. Russo and Dorian B. McGavern*

Traumatic brain injury (TBI) elicits an inflammatory response in the central nervous system(CNS) that involves both resident and peripheral immune cells. Neuroinflammation canpersist for years following a single TBI and may contribute to neurodegeneration. However,administration of anti-inflammatory drugs shortly after injury was not effective in the treatmentof TBI patients. Somecomponents of the neuroinflammatory response seem toplay a beneficialrole in the acute phase of TBI. Indeed, following CNS injury, early inflammation can set thestage for proper tissue regeneration and recovery, which can, perhaps, explain why generalimmunosuppression in TBI patients is disadvantageous. Here, we discuss some positiveattributes of neuroinflammation andpropose that inflammationbe therapeutically guided in TBIpatients rather than globally suppressed.

Traumatic brain injuries (TBIs) cause manyreactions; one of the most prominent isneuroinflammation. Damage to the CNSelicits inflammatory responses from resi-dent microglia and macrophages, as well

as peripheral immune cells, such as neutrophils,monocytes, and T cells. Microglia and residentmacrophages immediately respond to injury aftersensing damage-associated molecular patterns(DAMPs), such as the presence of adenosinetriphosphate (ATP) or intracellular proteins thatare released from damaged or dying cells. Sig-naling from DAMP receptors initiates local cyto-kine and chemokine production, which affectsthe immediate environment and provides a cuefor peripheral immune infiltration (1). A majorquestion in the field of TBI research is how theimmune response influences the pathogenesis ofbrain injury and recovery. Although a number ofstudies suggest that neuroinflammation is de-trimental and inhibitory to neural regenerationfollowing TBI, the failure of anti-inflammatorydrugs to achieve a therapeutic benefit in humanclinical trials supports a growing need to morecarefully interrogate the duality of TBI-inducedimmunity. Immune reactions do indeed havethe means to cause damage, but they also playa critical role in promoting tissue repair andrecovery following brain injury.

Pathogenic inflammation following TBI

Microglia are resident immune sentinels thatrespond to nearly all inflammatory events with-in the CNS. Their exact contribution to thepathogenesis of brain injuries is not entirelyunderstood, but studies have shown that mi-croglial activation can persist for years followingTBI in humans (2). For example, analysis ofmicroglia and associated pathology in TBI pa-tients revealed clusters of activated microglia(evidenced by CR3 and CD68 immunoreactivity)

in 28% of patients that survived for more than1 year after a single brain injury (2). These pa-tients also showed active signs of white matterdegeneration, indicative of a chronic patho-logical process. However, it is unclear whethermicroglia are active participants in this pro-longed degenerative process or are simply re-sponding to the pathology induced by othermechanisms. Investigators have attempted tointerrogate microglia in animal models of TBI,although the results are not definitive. Minocycline

is an antibiotic with anti-inflammatory proper-ties that is commonly used to suppress microg-lia and/or macrophage activation. This compoundshowed some therapeutic benefit (i.e., reducedmicroglia activation and brain lesion size) in aweight drop model of TBI (3), but the improve-ment cannot be linked exclusively to the effectof minocycline on microglia. Another study sim-ilarly concluded that microglia are pathogenicby studying cortical injury in the reduced formof nicotinamide adenine dinucleotide phosphate(NADPH) oxidase-2 (NOX2)−/− mice (4). NOX2 isa subunit of NADPH oxidase expressed by ac-tivated microglia and known to generate reactiveoxygen species (ROS). Both ROS production andlesion sizes were reduced in injured NOX2−/−

mice, which suggested that microglia-derived ROSexacerbates TBI damage (4). Because the mice inthis study were globally deficient in NOX2, it willbe important in future studies to link pathogenicNOX2 activity exclusively to microglia.

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“At least some inflammationmay be necessary in theacute stage of CNS injuryto clear damage and setthe stage for remodelingefforts.”

Viral Immunology and Intravital Imaging Section, NationalInstitutes of Neurological Disorders and Stroke, NationalInstitutes of Health, Bethesda, MD 20852, USA.*Corresponding author. Email: mcgavernd@mail.nih.gov

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Peripherally derived myeloid cells have alsobeen implicated in TBI pathogenesis. After CNSinjury, blood-derived monocytes migrate to thesite of damage, where they differentiate intomacrophages and persist as needed until the in-flammation subsides. Even though macrophagesaid wound-healing responses, many studies haveconcluded that monocytes are inherently path-ogenic after TBI. For example, a subset of cir-culating monocytes express C-C chemokinereceptor type 2 (CCR2), and macrophage numberswere reduced by >80% following cortical injuryof CCR2−/− mice. This resulted in improved hip-

pocampal neuronal survival and functional re-covery relative to wild-type controls (5). This studydemonstrates that peripheral CCR2+ monocytescan contribute to hippocampal damage after di-rect cortical injury. Additional studies are neededto address whether all injured brain regions suf-fer the same fate after invasion by CCR2+ mono-cytes, as well as how other monocyte subsetscontribute to TBI pathogenesis and recovery.

Immunosuppressive clinical trials

The association between neurodegeneration andneuroinflammation has stimulated a great deal

of interest in using immunosuppressive treat-ments for CNS injuries. Many drugs with anti-inflammatory properties—such as corticosteroids,nonsteroidals, statins, and specific cytokineinhibitors—have been tested in different TBIanimal models (6). Although many of these treat-ments were effective in lessening pathology andimproving neurological function, the resultsvaried according to the injury model and/or treat-ment window. On the basis of promising pre-clinical data, large phase III clinical trials wereconducted to assess the efficacy of immuno-suppressive treatments in the acute phase ofTBI. Corticosteroids had been used for decadesto treat head injuries without a substantialamount of clinical data to support their efficacy.In 2004, a definitive trial found that adminis-tration of methylprednisolone within 8 hoursof injury increased the risk of death at 6 monthspost TBI (7). The treatment group was also lesslikely to have a favorable recovery (based on theGlasgow Outcome Scale) at 6 months. Anotherclinical trial examined the effects of treating TBIpatients with progesterone, an anti-inflammatoryneurosteroid, within 4 hours of head injury (8).Evaluation at 6 months post injury revealed nosignificant difference between progesterone andthe placebo control groups in terms of mortalityor favorable outcomes. These trials demonstratethat suppressing the immune response acutelyafter a head injury at best has no effect andat worst actually increases the risk of death6 months later.

The benefit of CNS inflammation

Why have potent immunosuppressive drugs failedto benefit TBI patients in phase III stage clin-ical trials? At least some inflammation may benecessary in the acute stage of CNS injury toclear damage and set the stage for remodelingefforts. Immunity promotes regeneration inperipheral tissues, so it is counterintuitive tothink that neuroinflammation is universallyneurodegenerative. The immune system is usu-ally an active participant in wound healing.Within the CNS, positive aspects of immunecells that mobilize in response to damage in-clude clearing dead cells, supporting the barriersystem, and setting the stage for wound healing.Microglia, monocytes, macrophages, neutro-phils, and T cells can collectively orchestrate aresponse that preserves neural tissue and fostersregeneration.Because of their ubiquitous presence and abun-

dant DAMP sensors, microglia are usually amongthe first responders to brain damage. For ex-ample, microglia express several purinergic re-ceptors that allow them to quickly respond toextracellular purines, such as ATP. Detectionof extracellular ATP causes microglia to projectprocesses toward a site of damage within mi-nutes. In a meningeal contusion model, ATPreleased by surface-associated astrocytes in thedamaged glial limitans (a structure that separatesthe meninges and brain parenchyma) causedunderlying microglia to shift into honeycomb-and jellyfish-like morphologies that provided

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Fig. 1. Myeloid cell dynamics following brain injury. (A and B) Two-photon z-stacks capturedthrough a thinned skull (blue) window of CX3CR1gfp/+ mice show the morphology of macrophages andmicroglia (green) in the meninges and brain parenchyma, respectively. (A) Naïve microglia areramified, whereas meningeal macrophages have a wormlike appearance. (B) Within minutes to hoursafter a meningeal contusion injury, meningeal macrophages rapidly disappear (die) and activatedmicroglia migrate to the glia limitans and assume a “jellyfish” morphology that allows them to fillspaces previously occupied by dead astrocytes. These cells participate in phagocytosis and reduceleakage from themeninges into the underlying brain parenchyma. See correspondingmovie S1. (C andD) Lysozyme-Mgfp/+ myelomonocytic cells (neutrophils/monocytes in green) are recruited to sites ofmeningeal cell death (red) beginning ~1 hour after TBI. A two-photon z-stack captured at 24 hours postinjury demonstrates that nearly all the myelomonocytic cells responding to cell death are in the (C)meninges (top 30 mm of z-stack) as opposed to the (D) parenchyma (bottom 30 mm of the same z-stack). See corresponding movie S3.

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barrier support and debris clearance, respectively(Fig. 1 and movie S1) (1, 9). Acute inhibition ofthis response increased the amount of neuralcell death in the parenchyma. Microglia alsoparticipate in immediate CNS barrier supportfollowing damage of cerebral capillaries (10).Capillary damage caused by focal laser injuryinduces a purinergic receptor–dependent ex-tension of microglia processes toward the vas-cular wall in an effort to prevent leakage intothe parenchyma. Inhibition of this response re-sulted in capillary leakage that lasted three timesas long as that observed in control mice. Inaddition to providing barrier support, microgliaand macrophages can also clear debris from theinjured CNS (Fig. 2 and movie S2). After dam-age to the corpus callosum, impaired myelinclearance by microglia leads to a decreased re-cruitment of oligodendrocyte precursor cells,which results in disorganized and defective ax-onal remyelination (11). In general, microglia arecritical participants in the acute phase of a CNSinjury. Sealing barriers and clearing debris arejust two benefits of having these cells involvedin the response.Despite the various protective layers sepa-

rating the CNS from peripheral tissues, immunecells infiltrate the CNS in response to trauma.In a model of spinal cord injury, neutrophil re-cruitment was necessary for proper wound healingand repair (12). Mice treated with neutrophil-depleting antibodies exhibited decreased astro-cyte reactivity at the injury site, larger lesions,and worse neurological outcomes. Neutrophilsare also recruited to the meninges in a purinergicreceptor–dependent manner following acutebrain injury, and interference with this responseactually increased the amount of meningeal cell

death (Fig. 1 and movie S3) (9). It is presentlyunclear how neutrophils contribute to neuro-protective responses following CNS injury, butthese cells do have the ability to recruit pe-ripheral monocytes within a day or two. Spinalcord injury elicits a marked recruitment of pro-inflammatory macrophages that is followed sev-eral days later by wound-healing macrophages(13). When this wound-healing response wasblocked, mice were unable to properly repair thelesion and recover motor skills (13). Similar toperipheral tissues, innate immune cells are es-sential for tissue remodeling and repair in theCNS. The challenge is to develop strategies thatfoster wound-healing responses while impedingmaladaptive neuroinflammation.The adaptive immune system is usually asso-

ciated with the containment and clearance ofpathogens, but T cells can also play a positiverole in CNS injury responses. CNS damage canpromote the nonspecific recruitment of CD4+

T cells that produce interleukin-4 (IL-4) in amajor histocompatibility complex II–independentmanner (14). Release of IL-4 potentiates neuro-trophin signaling that helps stimulate axonalregrowth after injury. Mice lacking T cells orIL-4 demonstrated increased neuronal loss andneurological dysfunction following CNS injury.During CNS tissue repair, effector and regulatoryCD4+ T cells work in tandem. Regulatory T cells(Tregs) often keep immune responses in checkby modulating inflammatory mediators to pro-mote wound healing and remodeling. Depletionof Tregs before CNS injury increased the re-cruitment of effector CD4+ T cells and improvedneurological recovery (15). By contrast, Treg de-pletion several days after injury actually inter-fered with the tissue repair process. Similarly to

the transition from proinflammatory to wound-healing macrophages, these Treg data show thatthe neuroinflammatory response to injury changesas time progresses.

Concluding remarks

When preclinical and clinical data are combined,a clearer picture of TBI immunity emerges.Inflammation induced by a CNS injury shouldnot be viewed as inherently maladaptive or neu-rotoxic. In general, neuroinflammatory responsesduring the acute phase of TBI have a lot of pos-itive attributes that include barrier maintenance,debris clearance, cytokine and/or neurotrophinproduction, and immune regulation, amongothers. Inhibition of these responses will likelyenhance neural damage and impede the wound-healing response to TBI. This conclusion is sup-ported by the failure of immune-dampeningdrugs (e.g., methylprednisolone and progester-one) to achieve a clinical benefit in human TBIpatients when administered shortly after injury.Nevertheless, neuroinflammation can becomemaladaptive over time. This might occur duringthe chronic stages of TBI, especially when mac-rophages and microglia remain in an inflam-matory state in the CNS for months or yearsand acquire aberrant functions. Genetic pre-dispositions, environment variables, and the lo-cation and severity of the injury are also likelyto shape neuroinflammatory responses. Addi-tional research is required to explore all of thesevariables and to better define the temporalaspects of CNS inflammation after TBI. It isincredibly important to identify the critical timewindow when inflammatory cells participate intissue repair following TBI. Future therapiesshould focus on guiding CNS immunity towarda favorable outcome rather than suppressing itentirely.

REFERENCES AND NOTES

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(2014).9. T. L. Roth et al., Nature 505, 223–228 (2014).10. N. Lou et al., Proc. Natl. Acad. Sci. U.S.A. 113, 1074–1079

(2016).11. A. Lampron et al., J. Exp. Med. 212, 481–495 (2015).12. D. P. Stirling, S. Liu, P. Kubes, V. W. Yong, J. Neurosci. 29,

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ACKNOWLEDGMENTS

This work was supported by the NIH intramural program.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/353/6301/783/suppl/DC1Movies S1 to S3

10.1126/science.aaf6260

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Fig. 2. Scavenging macrophages after brain injury. (A and B) Meningeal macrophages repopulatethe meninges beginning ~2 days post injury, where they assume the role of scavengers. Time-lapsetwo-photonmovies of themeningeal space show (A) a singlemacrophage that had previously engulfeda dead cell extending a process to phagocytose another cell at 2 days post injury and (B) amacrophageprobing and sampling a dead cell at 4 days post injury. See corresponding movie S2.

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Inflammatory neuroprotection following traumatic brain injuryMatthew V. Russo and Dorian B. McGavern

DOI: 10.1126/science.aaf6260 (6301), 783-785.353Science 

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