
==== Front
J NeuroinflammationJournal of Neuroinflammation1742-2094BioMed Central London 1742-2094-1-141528580110.1186/1742-2094-1-14ReviewMicroglia and neuroinflammation: a pathological perspective Streit Wolfgang J 1streit@mbi.ufl.eduMrak Robert E 2MrakRobertE@uams.eduGriffin W Sue T 3griffinsuet@uams.edu1 Department of Neuroscience, University of Florida College of Medicine, P.O. Box 100244, Gainesville, Florida 32610, USA2 Department of Pathology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA3 Department of Geriatrics, University of Arkansas for Medical Sciences and GRECC/CAVHS, Little Rock, Arkansas 72205, USA2004 30 7 2004 1 14 14 8 7 2004 30 7 2004 Copyright © 2004 Streit et al; licensee BioMed Central Ltd.2004Streit et al; licensee BioMed Central Ltd.This is an open-access article distributed under the terms of the Creative Commons Attribution License (), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Microglia make up the innate immune system of the central nervous system and are key cellular mediators of neuroinflammatory processes. Their role in central nervous system diseases, including infections, is discussed in terms of a participation in both acute and chronic neuroinflammatory responses. Specific reference is made also to their involvement in Alzheimer's disease where microglial cell activation is thought to be critically important in the neurodegenerative process.
==== Body
Background
A role for immune responses, involving antigen presentation and immune-response-generating cytokines, in neurodegenerative diseases such as Alzheimer's disease was recognized for a decade before the term neuroinflammation came into widespread use [1,2]. A PubMed search using "neuroinflammation" as the only key word yields some 300 papers, none before 1995 [3]. While some chronic/remitting neurological diseases, such as multiple sclerosis, have long been recognized as inflammatory, the term neuroinflammation has come to denote chronic, CNS-specific, inflammation-like glial responses that do not reproduce the classic characteristics of inflammation in the periphery but that may engender neurodegenerative events; including plaque formation, dystrophic neurite growth, and excessive tau phosphorylation. In this way, neuroinflammation has been implicated in chronic unremitting neurodegenerative diseases such as Alzheimer's disease – diseases that historically have not been thought of as inflammatory diseases. This new understanding has come from rapid advances in the field of microglial and astrocytic neurobiology over the past fifteen to twenty years. These advances have led to the recognition that glia, particularly microglia, respond to tissue insult with a complex array of inflammatory cytokines and actions, and that these actions transcend the historical vision of phagocytosis and structural support that has long been enshrined in the term "reactive gliosis." Microglia are now recognized as the prime components of an intrinsic brain immune system [4], and as such they have become a main focus in cellular neuroimmunology and therefore in neuroinflammation. This is not the inflammation of the adaptive mammalian immune response, with its array of specialized T-cells and the made-to-order antibodies produced through complex gene rearrangements. This is, instead, the innate immune system, upon which adaptive immunity is built [5].

Many researchers now consider this innate immune response in the brain to be a potentially pathogenic factor in a number of CNS diseases that lack the prominent leukocytic infiltrates of adaptive immune responses, but that do have activated microglia and astrocytes, i.e., neuroinflammation.

The idea that neuroinflammation is detrimental implies that glial cell activation precedes and causes neuronal degeneration [2], a sequence of events that appears to be at odds with experimental models of neurodegeneration in which glial cell activation occurs secondary to neuronal damage. What is missing from this simple linear model is the understanding that chronic neurological diseases are just that – chronic, and that this chronicity introduces complex interactions and feedback loops between neurons and glia that render attempts to construct simple, linear cascades of cause and effect inelegant.

In the following, we provide some basic definitions and discussion to more precisely define the idea of neuroinflammation as a CNS tissue response to injury, and the notion of neuroinflammation as a pathogenic factor in neurodegenerative diseases.

Some basic definitions
Inflammation is a reaction of living tissues to injury [6]. The discipline of pathology makes a fundamental distinction between acute and chronic inflammation. Acute inflammation comprises the immediate and early response to an injurious agent and is basically a defensive response that paves the way for repair of the damaged site. Chronic inflammation results from stimuli that are persistent. In the periphery, inflammation consists of leukocytic infiltrates characterized by polymorphonuclear cells (neutrophils) in acute inflammation and mononuclear cells (macrophages, lymphocytes, plasma cells) in chronic inflammation. In order to validate these principles of general pathology within the context of neuroinflammation, one must obviously consider both acute and chronic neuroinflammation and, therefore, these are addressed separately in the following sections.

Acute neuroinflammation
Before "neuroinflammation" became a commonly used term, neuroscientists spoke of "reactive gliosis" in describing endogenous CNS tissue responses to injury. Reactive gliosis specifically referred to the accumulation of enlarged glial cells, notably microglia and astrocytes, appearing immediately after CNS injury has occurred. In contrast to glial reactivity, which suggests a largely passive response to injury; glial activation implies a more aggressive role in responding to activating stimuli: activated glial cells release factors that act on and engender responses in target cells analogous to the responses of activated immune cells in the periphery. Activation of immune cells in the periphery leads to leukocyte infiltration of tissues, but this is notably absent in the brain unless there has been destruction or compromise of the blood brain barrier [7,8]. In the presence of such destruction or compromise, peripheral leukocytes do enter the brain producing a scenario similar to that seen in inflammatory responses in the periphery.

In limited, acute reactions to injury, in the absence of blood-brain barrier breakdown, there is the subtler response of the brain's own immune system, composed largely of rapid activation of glial cells. These responses represent the other end of the spectrum of CNS injury, where limited neuronal insults trigger glial cell activation without breakdown of the blood brain barrier and without concomitant leukocytic infiltration. This form of "pure" glial response occurs in neuronal injury caused by either loss of afferents [9] or loss of efferents [10]. Axotomy, for instance, results in neuronal chromatolysis, the classic example of potentially reversible neuronal injury [9]. It is in these situations that microglial and astrocytic responses (like their peripheral counterparts) fulfill their evolutionarily programmed functions of a reparative response to the benefit of the organism as a whole.

Although such specific responses might, in a strict sense, be included in the term "neuroinflammation," neuroinflammation as generally used and understood applies to more chronic, sustained cycles of injury and response, in which the cumulative ill effects of immunological microglial and astrocytic activation contribute to and expand the initial neurodestructive effects, thus maintaining and worsening the disease process through their actions.

Chronic neuroinflammation
The concept of chronic inflammation (as opposed to acute inflammation) is more relevant in the context of understanding CNS disease (as opposed to CNS injury), as the very term "disease" implies chronicity. Chronic multiple sclerosis is, of course, an unequivocal and long-recognized example of an inflammatory brain disease. Although the underlying cause(s) of multiple sclerosis have not been elucidated, it is probably safe to say that the persistent injurious stimulus that accounts for neuroinflammation in multiple sclerosis is a myelin-related protein that has escaped self-tolerance and become an autoimmunogen. Consistent with the chronic persistence of this autoimmunogen is a persistent accumulation of blood-derived mononuclear leukocytes in the CNS parenchyma, a phenomenon that is similar to what is found in other autoimmune diseases such as rheumatoid arthritis or polymyositis.

Infections are another group of diseases that are classically recognized as inflammatory in nature, with meningeal, perivascular, or even parenchymal infiltrates of peripheral leukocytes. There are, however, exceptions. Rabies is a disease in which the peripheral immune response is slow and inadequate, and in which classic inflammatory changes are less striking than those found in other viral encephalidites. Babes, in 1897 [11], described microglial activation in rabies infection, although he did not recognize the nodules he found as clusters of activated microglia. Similar small collections of activated microglia were subsequently found to occur in a wide variety of viral brain infections.

Today, the most important example of a chronic brain infection is human immunodeficiency virus (HIV). Chronic HIV encephalitis is characterized by the same nodules of activated microglia that Babes described in rabies. HIV enters and persists in the CNS via myelomonocytic cells: monocytes, perivascular cells, and microglia [12]. HIV infection is uniquely different from most other infectious diseases affecting the CNS in that the virus targets and disables precisely those cells that are key players in neuroinflammation; microglia in the brain and T lymphocytes in the periphery. It therefore comes as no surprise that prominent T cell infiltrates do not occur in HIV encephalopathy.

Prion diseases represent another chronic infectious CNS disease that is not accompanied by leukocytic infiltrates. Microglial activation, again, appears to be the most prominent inflammatory component of prion diseases [13,14], although there are a few reports describing T cell infiltration as well [15,16]. Prion diseases share interesting parallels to rabies infection in that infected cells are unrecognized by peripheral immune responses. This may explain in part the unusual patterns of neuroinflammation in prion diseases – manifest not only in atypical cellular infiltrates but also in unusual cytokine profiles [17]. Both HIV and prion infections probably produce an altered microglial physiology that is likely to translate into cycles of neurodegeneration, which could be a contributing factor in the development of dementia that occurs in these conditions.

Chronic microglial neuroinflammation in neurodegenerative diseases
Neurodegenerative diseases – particularly Alzheimer's disease, but also amyotrophic lateral sclerosis, Parkinson's disease, and Huntington's disease – lack the prominent infiltrates of blood-derived mononuclear cells that characterize autoimmune diseases. On the other hand, there is abundant evidence that many substances involved in the promotion of inflammatory processes are present in the CNS of patients with such neurodegenerative diseases. By far the bulk of this body of evidence is related to studies in Alzheimer's disease [18]. What distinguishes Alzheimer's disease from other neurodegenerative diseases is the conspicuous presence of extracellular deposits of amyloid in senile plaques. Senile plaques in Alzheimer brain are present in different stages of maturity, ranging from diffuse to neuritic to dense core, but they all contain the amyloid beta protein (Aβ). Aβ is a peptide that forms insoluble and pathological extracellular aggregates that seem to attract microglial cells, as suggested by the clustering of microglia at sites of Aβ deposition (see [19] for a review). There is evidence from experimental studies in animals to support the idea that microglia can phagocytose and degrade amyloid [20,21], but such phagocytosis is apparently either ineffective or inadequate in Alzheimer's disease. A key question within the current context is: "Does the amyloid in Alzheimer brain by itself represent a persistent injurious stimulus that causes neuronal injury, or are additional factors involved in eliciting this outcome?" Direct injection of Aβ into the brain produces activation of microglia and loss of specific populations of neurons [21]. Furthermore, transgenic mice that overexpress human, mutant β-amyloid precursor protein (βAPP) do develop Aβ deposits with associated evidence of neuritic injury (although they do not develop Alzheimer-type neurofibrillary tangles unless they are also transgenic for human tau protein) [22]. These Aβ deposits, born of transgenic overexpression of mutant human amyloid precursor protein, invariably contain activated microglia [22,23].

β-Amyloid precursor protein βAPP functions as a neuronal acute-phase, injury-response protein. For instance, there is excessive expression of βAPP, accompanied by microglial activation and cytokine expression, after traumatic head injury [24]. With head injury, there is also Aβ deposition, both in experimental animals [25] and in humans – particularly in individuals genetically susceptible for AD (i.e. ApoE ε4-positive) [26]. These observations emphasize the complex interactions that underlie neurodegeneration in Alzheimer's disease.

Conclusions
Chronic microglial activation is an important component of neurodegenerative diseases, and this chronic neuroinflammatory component likely contributes to neuronal dysfunction, injury, and loss (and hence to disease progression) in these diseases. The recognition of microglia as the brain's intrinsic immune system, and the understanding that chronic activation of this system leads to pathologic sequelae, has led to the modern concept of neuroinflammation. This vision of microglia-driven neuroinflammatory responses, with neuropathological consequences, has extended the older vision of passive glial responses that are inherent in the concept of "reactive gliosis."

Abbreviations
Aβ: β-amyloid peptide

βAPP: Aβ precursor protein

CNS: central nervous system

HIV: human immunodeficiency virus

MS: multiple sclerosis

Competing interests
None declared

Authors' contributions
WJS conceived this review, wrote the initial draft, modified this with the comments of REM and WSTG, and wrote the final draft. REM and WSTG contributed particularly to the sections on infections and on Alzheimer's disease. All authors read and approved the final version.

Acknowledgments
Supported in part by NIH R21 NINDS 049185, NIH PO1 AG 12411, NIH P30 AG 19606, NIH RO1 AG 37989, and the McKnight Brain Research Foundation at the University of Florida.
==== Refs
McGeer PL Itagaki S Tago H McGeer EG  Reactive microglia in patients with senile dementia of the Alzheimer type are positive for the histocompatibility glycoprotein HLA-DR Neurosci Lett 1987 79 195 200 3670729 10.1016/0304-3940(87)90696-3 
Griffin WST Stanley LC Ling C White L MacLeod V Perrot LJ White CL III Araoz C  Brain interleukin 1 and S-100 immunoreactivity are elevated in Down syndrome and Alzheimer disease Proc Natl Acad Sci U S A 1989 86 7611 7615 2529544 
Issazadeh S Mustafa M Ljungdahl A Hojeberg B Dagerlind A Elde R Olsson T  Interferon gamma, interleukin 4 and transforming growth factor beta in experimental autoimmune encephalomyelitis in Lewis rats: dynamics of cellular mRNA expression in the central nervous system and lymphoid cells J Neurosci Res 1995 40 579 590 7602612 
Streit WJ Kincaid-Colton CA  The brain's immune system Sci Am 1995 273 54 61 8966536 
Medzhitov R Janeway C Jr Advances in immunology: innate immunity N Engl J Med 2000 343 338 344 10922424 10.1056/NEJM200008033430506 
Robbins SL Angell M Kumar V  Basic Pathology 1981 3 Philadelphia: W.B. Saunders 
Sroga JM Jones TB Kigerl KA McGaughy VM Popovich PG  Rats and mice exhibit distinct inflammatory reactions after spinal cord injury J Comp Neurol 2003 462 223 240 12794745 10.1002/cne.10736 
Streit WJ Semple-Rowland SL Hurley SD Miller RC Popovich PG Stokes BT  Cytokine mRNA profiles in contused spinal cord and axotomized facial nucleus suggest a beneficial role for inflammation and gliosis Exp Neurol 1998 152 74 87 9682014 10.1006/exnr.1998.6835 
Kreutzberg GW  Principles of neuronal regeneration Acta Neurochir Suppl (Wein) 1996 66 103 106 
Ito K Ishikawa Y Skinner RD Mrak RE Morrison-Bogorad M Mukawa J Griffin WST  Lesioning of the inferior olive using a ventral surgical approach: characterization of temporal and spatial responses at the lesion site and in cerebellum Mol Chem Neuropathol 1997 31 245 264 9336767 
Babes V  Sur certains caractères des lesions histologiques de la rage Ann Inst Pasteur Lille 1892 6 209 223 
Garden GA  Microglia in human immunodeficiency virus-associated neurodegeneration Glia 2002 40 240 251 12379911 10.1002/glia.10155 
Eikelenboom P Bate C Van Gool WA Hoozemans JJM Rozemuller JM Veerhuis R Williams A  Neuroinflammation in Alzheimer's disease and prion disease Glia 2002 40 232 239 12379910 10.1002/glia.10146 
Perry VH Cunningham C Boche D  Atypical inflammation in the central nervous system in prion disease Curr Opin Neurol 2002 15 349 354 12045736 10.1097/00019052-200206000-00020 
Betmouni S Perry VH Gordon JL  Evidence for an early inflammatory response in the central nervous system of mice with scrapie Neuroscience 1996 74 1 5 8843071 10.1016/0306-4522(96)00212-6 
Lewicki H Tishon A Homann D Mazarguil H Laval F Asensio VC Campbell IL DeArmond S Coon B Teng C Gairin JE Oldstone MB  T cells infiltrate the brain in murine and human transmissible spongiform encephalopathies J Virol 2003 77 3799 3808 12610154 10.1128/JVI.77.6.3799-3808.2003 
Baker CA Martin D Manuelidis L  Microglia from Creutzfeldt-Jakob disease-infected brains are infectious and show specific mRNA activation profiles J Virol 2002 76 10905 10913 12368333 10.1128/JVI.76.21.10905-10913.2002 
Akiyama H Barger S Barnum S Bradt B Bauer J Cole GM Cooper NR Eikelenboom P Emmerling M Fiebich BL Finch CE Frautschy S Griffin WS Hampel H Hull M Landreth G Lue L Mrak R Mackenzie IR McGeer PL O'Banion MK Pachter J Pasinetti G Plata-Salaman C Rogers J Rydel R Shen Y Streit W Strohmeyer R Tooyoma I Van Muiswinkel FL Veerhuis R Walker D Webster S Wegrzyniak B Wenk G Wyss-Coray T  Inflammation and Alzheimer's disease Neurobiol Aging 2000 21 383 421 10858586 10.1016/S0197-4580(00)00124-X 
Streit WJ  Microglia and Alzheimer's disease pathogenesis J Neurosci Res 2004 77 1 8 15197750 10.1002/jnr.20093 
Frautschy SA Cole GM Baird A  Phagocytosis and deposition of vascular β-amyloid in rat brains injected with Alzheimer β-amyloid Am J Pathol 1992 140 1389 1399 1376558 
Weldon DT Rogers SD Ghilardi JR Finke MP Cleary JP O'Hare E Esler WP Maggio JE Mantyh PW  Fibrillar beta-amyloid induces microglial phagocytosis, expression of inducible nitric oxide synthase, and loss of a select population of neurons in the rat CNS in vivo J Neurosci 1998 18 2161 73 9482801 
Irizarry MC McNamara M Fedorchak K Hsiao K Hyman BT  APPSW transgenic mice develop age-related Aβ deposits and neuropil abnormalities, but no neuronal loss in CA1 J Neuropathol Exp Neurol 1997 56 965 973 9291938 
Frautschy SA Yang F Irrizarry M Hyman B Saido TC Hsiao K Cole GM  Microglial response to amyloid plaques in APPsw transgenic mice Am J Pathol 1998 152 307 17 9422548 
Graham DI Gentleman SM Nicoll JA Royston MC McKenzie JE Roberts GW Griffin WS  Altered beta-APP metabolism after head injury and its relationship to the aetiology of Alzheimer's disease Acta Neurochir – Suppl (Wein) 1996 66 96 102 
Smith DH Uryu K Saatman KE Trojanowski JQ McIntosh TK  Protein accumulation in traumatic brain injury Neuromolecular Med 2003 4 59 72 14528053 10.1385/NMM:4:1-2:59 
Graham DI Gentleman SM Nicoll JA Royston MC McKenzie JE Roberts GW Mrak RE Griffin WS  Is there a genetic basis for the deposition of beta-amyloid after fatal head injury? Cell Molec Neurobiol 1999 19 19 30 10079962 10.1023/A:1006937205905

