Imaging of microglial activation in MS using PET: Research use and potential future clinical application

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1 674568MSJ / Multiple Sclerosis JournalL Airas, E Rissanen research-article2016 MULTIPLE SCLEROSIS JOURNAL MSJ Topical Review Imaging of microglial activation in MS using PET: Research use and potential future clinical application Laura Airas, Eero Rissanen and Juha Rinne Multiple Sclerosis Journal 2017, Vol. 23(4) DOI: / The Author(s), Reprints and permissions: journalspermissions.nav Abstract: Multiple sclerosis (MS) is a complex disease, where several processes can be selected as a target for positron emission topography (PET) imaging. Unlike magnetic resonance imaging (MRI), PET provides specific and quantitative information, and unlike neuropathology, it can be non-invasively applied to living patients, which enables longitudinal follow-up of the MS pathology. In the study of MS, PET can be useful for in vivo evaluation of specific pathological characteristics at various stages of the disease. Increased understanding of the progressive MS pathology will enhance the treatment options of this undertreated condition. The ultimate goal of developing and expanding PET in the study of MS is to have clinical non-invasive in vivo imaging biomarkers of neuroinflammation that will help to establish prognosis and accurately measure response to therapeutics. This topical review provides an overview of the promises and challenges of the use of PET in MS. Keywords: Microglia, positron emission tomography, imaging, translocator protein, A2A receptor, multiple sclerosis Correspondence to: L Airas Division of Clinical Neurosciences, Turku University Hospital and University of Turku, Kiinamyllynkatu 4-8, Turku 20521, Finland. laura.airas@utu.fi Laura Airas Eero Rissanen Juha Rinne Division of Clinical Neurosciences, Turku University Hospital and University of Turku, Turku, Finland; Turku PET Centre, Turku University Hospital and University of Turku, Turku, Finland Date received: 25 August 2016; revised: 6 September 2016; accepted: 8 September 2016 Introduction Neuroinflammation and neurodegeneration both contribute to pathobiology of multiple sclerosis (MS). 1 The disease most often starts with active inflammation, whence the adaptive immune system attacks the central nervous system (CNS) and forms focal inflammatory lesions leading to demyelination. The pathological cascade soon develops to contain features of neurodegeneration with glial activation, axonal damage and tissue atrophy. 2 Activation of the innate immune system within the CNS (both resident microglia and blood-derived macrophages) contributes to neuronal damage via release of proinflammatory cytokines and reactive oxygen species which induce mitochondrial injury, and lead to oligodendrocyte damage and neurodegeneration. 3 Axonal loss underlies especially the transition from relapsing to progressive MS, but is known to be present already early in the disease. 3 At progressive stages of the disease, activated microglial cells are present both at the edge of chronic focal lesions (called chronic active lesions), and throughout the normal-appearing white matter (NAWM), which likely represent important background for the clinical disease progression. 3,4 Conventional magnetic resonance imaging (MRI) is sensitive in demonstrating focal inflammatory lesions and gross atrophy and therefore forms the cornerstone of MS diagnostics and clinical follow-up today. MRI, however, fails to detect the diffuse pathology associated particularly with progressive MS. Positron emission topography (PET), on the other hand, provides a method to specifically quantitate microglial activation as an important manifestation of the diffuse pathology taking place outside the focal plaques in the NAWM. Thus, PET allows longitudinal follow-up of the above-described aspects of the progressive disease. In the future, PET might provide an important tool in attempts to develop treatments for this disease. PET studies on MS have focused on evaluating microglial activation using the 18-kDa translocator protein (TSPO) ligands and on developing ways to assess remyelination. 5 In this Topical Review, we describe the advances made in imaging microglial activation in the field of MS and discuss its challenges and future prospects. PET as a method in clinical MS imaging PET is a non-invasive imaging technique with high molecular sensitivity and specificity which allows 496 journals.sagepub.com/home/msj

2 L Airas, E Rissanen et al. vasculature and to the plasma proteins and the lack of a clear reference region require complicated modelling. 12 The so-called Super-PK methodology (supervised clustering of the reference region) has been successfully applied to several [ 11 C]PK11195 PET studies of MS Despite its shortcomings, [ 11 C] PK11195 has still been the most commonly used TSPO tracer in studies of MS. 16 However, over 80 second-generation TSPO ligands with higher affinity and specificity are presently at some stage of development. 7,17 20 Figure 1. The brain-dedicated high-resolution ECAT HRRT PET scanner (CTI/Siemens) used in most of the MS studies at Turku PET centre. accurate in vivo quantitation of the target molecules. 6 PET uses short-lived radioactive isotopes, such as 18 F and 11 C, bound to ligands which interact with their respective targets. 6 After administration, the bound ligands enter the CNS and can be detected by a PET camera (Figure 1). Challenges of MS PET lie within the lack of a suitable reference tissue in MS brain, the complex modelling demands and complicated and expensive technology including the need of an on-site cyclotron, due to the short half-lives of the tracers. Imaging microglial activation by TSPO radioligands Measuring microglial activation relies on TSPO radioligands. TSPO, earlier called the peripheral benzodiazepine receptor (PBR), is a protein structure expressed on the outer mitochondrial membrane of activated microglia, 7 and its upregulation is considered to be a sensitive real-time marker of neuroinflammation and neuronal injury. Several functions have been attributed to TSPO, either directly or indirectly, including immunomodulation and regulation of cholesterol transport. 8 In the resting or surveying microglia, TSPO is expressed at a low level, mainly in the grey matter (GM). In non-neoplastic CNS damage without blood brain barrier (BBB) breakdown, microglia are the main cell population expressing TSPO, but also other cells, such as macrophages, reactive astrocytes and vascular endothelial cells, can express TSPO. 9,10 The prototypic TSPO radioligand, [ 11 C]PK11195, was first used in in vivo human MS study in [ 11 C]PK11195 has high specificity for TSPO, but its short half-life and low signal-to-noise ratio hinder its use. The abundant radioligand binding to the TSPO PET findings according to MS pathology TSPO PET findings relative to focal inflammatory lesions There is a wide consensus that TSPO binding is increased in acute lesions, and most studies agree that chronic T1 black hole-type lesions have low TSPO binding. 11,23 T2 lesions have higher TSPO binding during relapse than during stable disease. 21 Neuropathology studies have identified subtypes of chronic (non-enhancing) T1 black hole-corresponding lesions that are particularly prevalent in progressive MS. These lesions can be of the chronic active, slowly expanding (smouldering) type, with macrophages and activated microglial cells at the plaque edge, or chronic inactive type, without active cells of the innate immunity at the plaque rim. 4 Importantly, PET can be used to differentiate the chronic active and chronic inactive plaques also in vivo, which is impossible by MRI. Among advanced secondary progressive multiple sclerosis (SPMS) patients, 57% of the plaques were of the chronic active type, with increased TSPO binding at the plaque edge demonstrating persisting inflammatory activity. 15,30 MRI is clearly the first choice in the clinic for detecting focal inflammatory lesions in MS. However, the ability to detect activated microglial cells using TSPO PET takes MS imaging a step forward from MRI in terms of specificity. TSPO PET could thus complement MRI by providing more specific immunological characterization of the lesions. PET could be used longitudinally to evaluate the lesion development in patients in vivo. It could bring specificity to the evaluation of T2 lesions at any given time point by allowing detection of various T2 lesion types. Then, one could investigate how the different lesion types affect disease prognosis and eventually design individual therapy accordingly. journals.sagepub.com/home/msj 497

3 Multiple Sclerosis Journal 23(4) Figure 2. MRI and [ 11 C]PK11195 images from a female patient with relapsing MS, age 48 years, disease duration 15 years, EDSS 2.5. Axial views of (a) FLAIR, (b) GdT1 and (c) parametric [ 11 C]PK11195 PET fused with GdT1 images. Red arrows demonstrate an active plaque with increased gadolinium uptake around the lesion and highly increased [ 11 C] PK11195 binding. White arrows denote a late active plaque with increased [ 11 C]PK11195 binding within and surrounding the lesion. In contrast, majority of the T1 hypointense plaques present with negligible radioligand binding in the lesion core. Yellow squares highlight a WM area appearing normal in MRI but showing patchy areas of slightly increased [ 11 C] PK11195 binding. In (c), the colour of each voxel represents the specific [ 11 C]PK11195 binding as distribution volume ratio (DVR) values according to the scale bar. TSPO PET of diffuse inflammation in progressive MS With advancing disease, pathology in MS spreads outside the focal lesions, with increasing microglial activation and axonal damage in the NAWM and GM. 4 This diffuse, compartmentalized inflammation cannot be detected using conventional MRI, but with careful modelling, it can be quantified using TSPO PET. Importantly, studies of progressive MS have demonstrated an increase in TSPO uptake which appears to correlate to disease severity and patient age. In primary progressive multiple sclerosis (PPMS), TSPO PET studies are still mostly lacking, but SPMS studies have clearly demonstrated that [ 11 C]PK11195 binding in the NAWM is significantly increased in SPMS compared to healthy controls. 14,15,21,22 In relapsing-remitting multiple sclerosis (RRMS), such increase is less pronounced. 14 However, when compared to controls, also clinically isolated syndrome (CIS) patients have higher TSPO binding in NAWM, which predicts the conversion to clinically definite MS. 25 In these studies, all focal inflammatory lesions have been carefully masked out, and the tissue of interest comprises areas appearing normal in MRI. Hence, in this application, TSPO PET provides truly complementary information to MRI. Figure 2 demonstrates TSPO imaging findings related to different types of lesions as well as to NAWM in an individual patient. Attempts to measure therapeutic effects using longitudinal TSPO PET Longitudinal studies using TSPO imaging are still scarce. Kreisl et al. 31 reported recently an increase in TSPO binding among patients with Alzheimer s disease over a period of 2.4 years. The only published longitudinal TSPO PET study evaluating microglial activation in MS is by Ratchford et al., 24 where RRMS patients were evaluated before and after 1 year of glatiramer acetate treatment. In this study, 1-year treatment with glatiramer acetate reduced TSPO binding significantly in both cortical GM and cerebral white matter (WM) when compared with cerebellum. The study by Sucksdorff et al. included serial PET imaging of MS patients and showed that 6 months fingolimod treatment did not reduce microglial activation in the WM or GM (submitted). This was somewhat surprising given the earlier work demonstrating clear reduction in microglial activation after fingolimod treatment in an EAE rat model. 32 A longitudinal study to evaluate the alteration in microglial activation in untreated MS patients is still awaited. It would be interesting to evaluate at a patient level the rate of the increase in microglial activation in relation to clinical progression and other imaging parameters. This would also give excellent comparative data for longitudinal TSPO PET studies evaluating therapeutics in progressive disease. Table 1 lists all MS studies performed so far using TSPO imaging. 498 journals.sagepub.com/home/msj

4 L Airas, E Rissanen et al. Table 1. Human in vivo TSPO PET imaging studies in multiple sclerosis. TSPO ligand Study design Study population (n) Method for estimating ligand uptake Main findings [ 11 C]PK11195 Vowinckel Cross-sectional RRMS (2) Visual assessment of static sum et al. 11 images Banati et al. 21 Cross-sectional HC (8) RRMS (8) SPMS (1) PPMS (3) Debruyne et al. 22 Cross-sectional HC (7) RRMS (13) SPMS (7) PPMS (2) Versijpt et al. 23 Cross-sectional HC (8) RRMS (13) SPMS (7) Ratchford Longitudinal et al. 24 treatment study PPMS (2) Politis et al. 14 Cross-sectional HC (8) RRMS (10) SPMS (8) Giannetti et al. 13 Cross-sectional a RRMS (10) PMS (9; of which 8 SPMS, 1 PPMS) Rissanen et al. 15 Cross-sectional HC (8) SPMS (10) Giannetti et al. 25 Cross-sectional a HC (8) CIS (18) ROI-specific BP ND from parametric images, cluster reference region Normalized specific uptake (ratio to whole brain) within ROIs from static sum images Normalized specific uptake (ratio to whole brain) within ROIs from static sum images RRMS (9) ROI-specific BP ND from parametric images, reference Logan with cerebellum as reference region ROI-specific BP ND from parametric images, BF-SRTM with grey matter cluster reference ROI-specific BP ND from parametric images, BF-SRTM with grey matter cluster reference ROI-specific DVR from dynamic images and calculation of parametric images for voxel wise group comparison using reference Logan with grey matter cluster reference ROI-specific BP ND from parametric images; BF-SRTM with grey matter cluster reference Increased uptake in resolving WM lesion Decreased uptake in chronic T1 lesions Increased uptake in T1 and T2 lesions and thalami of individual patients compared to HC Correlation of higher percentage of TSPO binding T1 lesion to higher EDSS Increased uptake in Gd-enhancing active lesions Uptake in T2 lesions increased at the time of clinical relapse Increased NAWM uptake associated with longer disease duration Increased uptake in NAWM associated with higher atrophy Decreased uptake in T2 lesions associated with higher atrophy Decrease in global cortical GM and cerebral WM uptake after 1 year of treatment with glatiramer acetate Increased cortical uptake in MS patients than in HC and wider areas in SPMS than in RRMS Total cortical binding correlated with EDSS, stronger association in SPMS than in RRMS Higher uptake in WM of SPMS and RRMS than in HC, but with no correlation to disability Uptake in T1 black holes correlates with EDSS in PMS Total binding in T1 black holes a significant disability predictor in PMS Higher uptake in NAWM in SPMS and thalami compared to HC Increased perilesional uptake in 57% of T1 hypointense lesions Uptake in NAWM higher in CIS than in HC Higher binding in deep but not in cortical GM in CIS versus HC Higher uptake in NAWM correlated to higher EDSS and higher risk for conversion to MS (continued) journals.sagepub.com/home/msj 499

5 Multiple Sclerosis Journal 23(4) Table 1. (Continued) TSPO ligand Study design Study population (n) Method for estimating ligand uptake Main findings [ 11 C]vinpocetine and [ 11 C] PK11195 Vas et al. 26 Cross-sectional MS (4; mainly b RRMS) [ 11 C]PBR28 Oh et al. 27 Crosssectional and longitudinal d Park et al. 28 Cross-sectional and test retest [ 18 F]PBR111 Colasanti et al. 19 Cross-sectional and test retest [ 11 C] FEDAA1106 HC (7) MS (11; mainly b RRMS) Follow-up imaging for 6 MS patients HC (4) RRMS (4) HC (11) RRMS (11) Colasanti et al. 29 HC (22) RRMS (11) Takano et al. 20 Cross-sectional HC (5) RRMS (9) ROI-specific BP ND from dynamic images; reference Logan with cerebellum as reference region Genotyping c : no ROI-specific V T from parametric V T images calculated with Logan and normalized with plasma free fraction Genotyping c : no SUV and V T with kinetic multilinear analysis within ROIs. Parametric SUV, V T and K 1 images for visual comparisons Genotyping c : yes ROI-specific V T from dynamic images and calculation of parametric images with Logan method Genotyping c : yes ROI-specific DVR with 2TC using cortical grey matter as pseudoreference region. Parametric V T images calculated with Logan method Genotyping c : yes ROI-specific V T and BP ND, 2TC (no information provided on reference region). Visual inspection of SUV images and VT images calculated with Logan equation Genotyping c : no Global uptake of [ 11 C]vinpocetine higher than with [ 11 C] PK11195 Lesional and perilesional uptake of [ 11 C]vinpocetine higher than with [ 11 C]PK11195 but with low overlap in areas of high uptake between ligands No difference in global uptake between MS and HC Higher WM/GM binding ratio in MS versus HC Increased uptake in Gd-enhancing WM lesions Focally increased binding preceding development of Gdenhancing lesions Correlation of higher global binding with longer disease duration, but not with EDSS No significant differences in global, regional or lesional uptake between MS and HC Good test retest reproducibility Significantly higher SUV but not V T in HABs versus MABs Higher uptake in lesional and perilesional WM in RRMS versus normal WM in HC group Positive correlation between higher lesional-tononlesional WM binding ratio and MS severity scores in RRMS Higher hippocampal binding in RRMS than among HC group Positive correlation of higher hippocampal uptake to higher BDI score in RRMS patients No difference in global or regional uptake between RRMS and HC groups Robust lesional BP ND or V T not obtainable due to noisy time activity curves One lesion in one patient with high uptake in visual inspection. Otherwise no areas of increased uptake among patients TSPO: 18-kDa translocator protein; HC: healthy control; RRMS: relapsing-remitting multiple sclerosis; SPMS: secondary progressive multiple sclerosis; PPMS: primary progressive multiple sclerosis; PMS: progressive multiple sclerosis; CIS: clinically isolated syndrome; ROI: region of interest; BP ND, V T : distribution volume; DVR: distribution volume ratio; BF-SRTM: basis function simplified reference tissue model; 2TC: two-tissue compartment model; SUV: standardized uptake value; NAWM: normal-appearing white matter; WM: white matter; Gd: gadolinium; EDSS: Expanded Disability Status Scale; HAB: high-affinity binder, mixed-affinity binder; BDI: Beck Depression Inventory; MS: multiple sclerosis; PET: positron emission topography. a Longitudinal follow-up for clinical parameters. b Exact disease type not reported. c Genotyping for single-nucleotide polymorphism in the TSPO gene (rs6971). d Longitudinal PET data reported only for one patient. 500 journals.sagepub.com/home/msj

6 L Airas, E Rissanen et al. Methodological challenges of TSPO imaging in MS Until today, the methods for modelling [ 11 C] PK11195 imaging have varied greatly from study to study. For instance, in some of the earlier studies using [ 11 C]PK11195, cortical GM has been used as a reference region, whereas in others it was the main target of interest. 22,23 The use of a clustered reference region has now established its position for estimation of specific ligand binding in clinical [ 11 C] PK11195 studies, and there is hope for better comparison between studies ,25 Nonetheless, subtle differences between the studies may still arise from differences in image reconstruction protocols and PET scanner features, as well as from the different methods used for kinetic modelling and region of interest (ROI) acquisition, and for calculating the parametric images. Development of methods for coregistration of MRI and PET scans and motion correction of PET scans will help to exclude unwanted signal and to detect the specific signal. Due to the rather poor BBB permeability of [ 11 C]PK11195, special care should be taken when performing direct comparisons of ligand uptake in Gd-enhancing lesions versus in non-enhancing lesions, as the BBB breakdown may predispose to a greater concentration of free ligand in the adjacent areas and lead to misinterpretation of the results. 33 Despite some studies showing promise for the second-generation TSPO ligands, 19 the anticipation of improved image quality related to these ligands has not been delivered, but instead, significant issues have emerged that may limit the application of these tracers. 33 First, there is genetically determined variation in the binding affinity for second-generation TSPO tracers at a population level. This is attributed to TSPO gene polymorphism, resulting in high-, medium- and low-affinity binding. 34 Meaningful evaluation of the imaging results can, however, be obtained by using simple genetic testing to determine the binder status of an individual. Moreover, the higher affinity of the second-generation ligands disproportionally increases the signal from the TSPO in the vasculature compared with that of the tissue, which necessitates advanced post-imaging modelling. Following the abundant tracer binding to the vasculature, the use of tissue reference approaches is difficult, and there is a need for further development of the quantification of the specific microglia-associated TSPO signal. To overcome some of the problems, the [ 11 C]PBR28 schizophrenia study used the whole-brain normalization of the TSPO signal with correction for endothelial binding, 35 but further validation is necessary before wider application of this method. 36 Other targets for evaluating neuroinflammation in MS Adenosine A2A receptors Using [ 11 C]TMSX PET, increased A2AR expression was demonstrated in the NAWM of SPMS patients. Importantly, this was associated with higher Expanded Disability Status Scale (EDSS) and increased brain tissue loss. 37,38 In vitro studies demonstrate that inflammatory stimuli lead to upregulation of A2AR on microglial cells and subsequently alter their morphology and behaviour. 39 Thus, adenosine A2A receptor (A2AR) seems to be a potential alternative target molecule for in vivo imaging of microglial activation. 37 Other A2AR-binding tracers with more favourable imaging properties and specific binding have been developed and will hopefully soon be evaluated for their usefulness in imaging of neuroinflammation. 40 Importantly, findings from EAE experiments suggest that A2AR might play a role in controlling inflammation in MS. 41,42 Other targets Several other promising targets that have not yet been utilized in MS imaging have emerged in the field of neuroinflammation imaging. Purinoreceptor P2X7 binds ATP and is expressed on activated microglia. Novel P2X7 antagonists have been developed and are promising candidates for PET tracers. 43 Numerous ligands have recently been developed and preclinically tested for cannabinoid receptor 2, which is expressed on activated microglia. 44 In addition, histamine 4, cyclo-oxygenase 2 receptors, matrix metalloproteinases 2 and 9, monoamine oxidase B and folate receptors are linked to many inflammatory and autoimmune diseases and appear as interesting future targets for PET of neuroinflammation Prospects of MS PET in clinical use Due to the technical requirements and the complex modelling, TSPO PET imaging will not likely be an imaging technique that would be widely available, but probably will continue being an investigative tool in specialized PET centres. With advancing methodology, standardization of modelling, availability of control cohorts and fluorinated ligands, it is, however, possible journals.sagepub.com/home/msj 501

7 Multiple Sclerosis Journal 23(4) that in the future TSPO imaging could be performed also in imaging centres with no on-site cyclotrons. It could answer specific questions perhaps related to differential diagnostics (MS lesion or tumour) 51 or prognostic questions (risk of progression). Conclusion TSPO PET has potential for the evaluation of MS pathology in vivo in a specific and quantifiable way at a molecular level and thus gives added value over conventional MRI. Detection of various pathological determinants in MS brain using PET will enable better understanding of the disease pathogenesis, provide alternative methods to monitor disease progression, aid in characterization of disease phenotypes and improve therapy evaluation particularly in progressive MS. TSPO PET could be used as a predictive tool to identify patients most at risk of progression and as an important surrogate marker in therapeutic studies of progressive MS. PET has potential for measuring the effects of new disease-modifying drugs aimed at reducing neurodegeneration and neuroinflammation in a non-invasive and clinically translational manner. Thus, PET could be included in the clinical trials of progressive MS in the future. There are, however, still technical challenges, and it is of importance to harmonize and validate the methodology used in post-processing and modelling of the TSPO PET signal in order to be able to perform multi-centre studies with larger cohorts. Further work is needed to understand the significance of the TSPO ligand uptake in terms of microglial phenotype, proinflammatory or neuroprotective, and to develop new and better ligands, and ones that can distinguish between different microglial phenotypes. Acknowledgements We wish to acknowledge all patients who have taken part in the PET studies and all co-workers who have contributed to the studies. Ms Salla Nuutinen is acknowledged for assisting with manuscript preparation. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article. Funding The author(s) disclosed receipt of the following financial support for the research, authorship and/or publication of this article: Our funding sources are acknowledged: Finnish Academy, Sigrid Juselius Foundation, Finnish MS Foundation and the European Union s Seventh Framework Programme (FP7/ ) under grant agreement no. HEALTH-F (INMiND). References 1. Ransohoff RM, Hafler DA and Lucchinetti CF. Multiple sclerosis A quiet revolution. Nat Rev Neurol 2015; 11: Moll NM, Rietsch AM, Thomas S, et al. Multiple sclerosis normal-appearing white matter: Pathologyimaging correlations. Ann Neurol 2011; 70: Lassmann H, van Horssen J and Mahad D. Progressive multiple sclerosis: Pathology and pathogenesis. Nat Rev Neurol 2012; 8: Frischer JM, Weigand SD, Guo Y, et al. Clinical and pathological insights into the dynamic nature of the white matter multiple sclerosis plaque. Ann Neurol 2015; 78: Bodini B, Veronese M, Garcia-Lorenzo D, et al. Dynamic imaging of individual remyelination profiles in multiple sclerosis. Ann Neurol. Epub ahead of print 18 February DOI: /ana Zanzonico P. Principles of nuclear medicine imaging: Planar, SPECT, PET, multi-modality, and autoradiography systems. Radiat Res 2012; 177: Ching AS, Kuhnast B, Damont A, et al. Current paradigm of the 18-kDa translocator protein (TSPO) as a molecular target for PET imaging in neuroinflammation and neurodegenerative diseases. Insights Imaging 2012; 3: Papadopoulos V, Baraldi M, Guilarte TR, et al. Translocator protein (18 kda): New nomenclature for the peripheral-type benzodiazepine receptor based on its structure and molecular function. Trends Pharmacol Sci 2006; 27: Venneti S, Lopresti BJ and Wiley CA. The peripheral benzodiazepine receptor (translocator protein 18 kda) in microglia: From pathology to imaging. Prog Neurobiol 2006; 80: Cosenza-Nashat M, Zhao ML, Suh HS, et al. Expression of the translocator protein of 18 kda by microglia, macrophages and astrocytes based on immunohistochemical localization in abnormal human brain. Neuropathol Appl Neurobiol 2009; 35: Vowinckel E, Reutens D, Becher B, et al. PK11195 binding to the peripheral benzodiazepine receptor as a marker of microglia activation in multiple sclerosis and experimental autoimmune encephalomyelitis. J Neurosci Res 1997; 50: journals.sagepub.com/home/msj

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