Research Article3 March 2016Open Access Transparent process sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers Marc Suárez-Calvet Marc Suárez-Calvet BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany German Center for Neurodegenerative Diseases (DZNE) Munich, Munich, Germany Search for more papers by this author Gernot Kleinberger Gernot Kleinberger BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany Munich Cluster for Systems Neurology (SyNergy), Munich, Germany Search for more papers by this author Miguel Ángel Araque Caballero Miguel Ángel Araque Caballero Institute for Stroke and Dementia Research, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Matthias Brendel Matthias Brendel Department of Nuclear Medicine, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Axel Rominger Axel Rominger Munich Cluster for Systems Neurology (SyNergy), Munich, Germany Department of Nuclear Medicine, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Daniel Alcolea Daniel Alcolea Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Juan Fortea Juan Fortea Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Alberto Lleó Alberto Lleó Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Rafael Blesa Rafael Blesa Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Juan Domingo Gispert Juan Domingo Gispert Clinical and Neuroimaging Departments, Barcelona Beta Brain Research Center, Pasqual Maragall Foundation, Barcelona, Spain Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona, Spain Search for more papers by this author Raquel Sánchez-Valle Raquel Sánchez-Valle Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author Anna Antonell Anna Antonell Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author Lorena Rami Lorena Rami Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author José L Molinuevo José L Molinuevo Clinical and Neuroimaging Departments, Barcelona Beta Brain Research Center, Pasqual Maragall Foundation, Barcelona, Spain Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona, Spain Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author Frederic Brosseron Frederic Brosseron German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany Search for more papers by this author Andreas Traschütz Andreas Traschütz Neurology Department, Universitätsklinikum Bonn, Bonn, Germany Search for more papers by this author Michael T Heneka Michael T Heneka German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany Neurology Department, Universitätsklinikum Bonn, Bonn, Germany Search for more papers by this author Hanne Struyfs Hanne Struyfs Reference Center for Biological Markers of Dementia (BIODEM), Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Department of Neurology and Memory Clinic, Hospital Network Antwerp (ZNA) Middelheim and Hoge Beuken, Antwerp, Belgium Search for more papers by this author Sebastiaan Engelborghs Sebastiaan Engelborghs Reference Center for Biological Markers of Dementia (BIODEM), Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Department of Neurology and Memory Clinic, Hospital Network Antwerp (ZNA) Middelheim and Hoge Beuken, Antwerp, Belgium Search for more papers by this author Kristel Sleegers Kristel Sleegers Neurodegenerative Brain Diseases Group, Department of Molecular Genetics, VIB, Antwerp, Belgium Laboratory of Neurogenetics, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Search for more papers by this author Christine Van Broeckhoven Christine Van Broeckhoven Neurodegenerative Brain Diseases Group, Department of Molecular Genetics, VIB, Antwerp, Belgium Laboratory of Neurogenetics, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Search for more papers by this author Henrik Zetterberg Henrik Zetterberg Clinical Neurochemistry Lab, Institute of Neuroscience and Physiology, The Sahlgrenska Academy at the University of Gothenburg, Mölndal, Sweden Reta Lila Weston Laboratories and Department of Molecular Neuroscience, UCL Institute of Neurology, London, UK Search for more papers by this author Bengt Nellgård Bengt Nellgård Department of Anaesthesiology and Intensive Care, Institute of Clinical Sciences, Sahlgrenska Academy, Gothenburg University, Gothenburg, Sweden Search for more papers by this author Kaj Blennow Kaj Blennow Clinical Neurochemistry Lab, Institute of Neuroscience and Physiology, The Sahlgrenska Academy at the University of Gothenburg, Mölndal, Sweden Search for more papers by this author Alexander Crispin Alexander Crispin Institute of Medical Informatics, Biometry, and Epidemiology, Munich, Germany Search for more papers by this author Michael Ewers Corresponding Author Michael Ewers Institute for Stroke and Dementia Research, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Christian Haass Corresponding Author Christian Haass BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany German Center for Neurodegenerative Diseases (DZNE) Munich, Munich, Germany Munich Cluster for Systems Neurology (SyNergy), Munich, Germany Search for more papers by this author Marc Suárez-Calvet Marc Suárez-Calvet BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany German Center for Neurodegenerative Diseases (DZNE) Munich, Munich, Germany Search for more papers by this author Gernot Kleinberger Gernot Kleinberger BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany Munich Cluster for Systems Neurology (SyNergy), Munich, Germany Search for more papers by this author Miguel Ángel Araque Caballero Miguel Ángel Araque Caballero Institute for Stroke and Dementia Research, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Matthias Brendel Matthias Brendel Department of Nuclear Medicine, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Axel Rominger Axel Rominger Munich Cluster for Systems Neurology (SyNergy), Munich, Germany Department of Nuclear Medicine, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Daniel Alcolea Daniel Alcolea Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Juan Fortea Juan Fortea Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Alberto Lleó Alberto Lleó Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Rafael Blesa Rafael Blesa Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain Search for more papers by this author Juan Domingo Gispert Juan Domingo Gispert Clinical and Neuroimaging Departments, Barcelona Beta Brain Research Center, Pasqual Maragall Foundation, Barcelona, Spain Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona, Spain Search for more papers by this author Raquel Sánchez-Valle Raquel Sánchez-Valle Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author Anna Antonell Anna Antonell Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author Lorena Rami Lorena Rami Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author José L Molinuevo José L Molinuevo Clinical and Neuroimaging Departments, Barcelona Beta Brain Research Center, Pasqual Maragall Foundation, Barcelona, Spain Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona, Spain Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain Search for more papers by this author Frederic Brosseron Frederic Brosseron German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany Search for more papers by this author Andreas Traschütz Andreas Traschütz Neurology Department, Universitätsklinikum Bonn, Bonn, Germany Search for more papers by this author Michael T Heneka Michael T Heneka German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany Neurology Department, Universitätsklinikum Bonn, Bonn, Germany Search for more papers by this author Hanne Struyfs Hanne Struyfs Reference Center for Biological Markers of Dementia (BIODEM), Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Department of Neurology and Memory Clinic, Hospital Network Antwerp (ZNA) Middelheim and Hoge Beuken, Antwerp, Belgium Search for more papers by this author Sebastiaan Engelborghs Sebastiaan Engelborghs Reference Center for Biological Markers of Dementia (BIODEM), Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Department of Neurology and Memory Clinic, Hospital Network Antwerp (ZNA) Middelheim and Hoge Beuken, Antwerp, Belgium Search for more papers by this author Kristel Sleegers Kristel Sleegers Neurodegenerative Brain Diseases Group, Department of Molecular Genetics, VIB, Antwerp, Belgium Laboratory of Neurogenetics, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Search for more papers by this author Christine Van Broeckhoven Christine Van Broeckhoven Neurodegenerative Brain Diseases Group, Department of Molecular Genetics, VIB, Antwerp, Belgium Laboratory of Neurogenetics, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium Search for more papers by this author Henrik Zetterberg Henrik Zetterberg Clinical Neurochemistry Lab, Institute of Neuroscience and Physiology, The Sahlgrenska Academy at the University of Gothenburg, Mölndal, Sweden Reta Lila Weston Laboratories and Department of Molecular Neuroscience, UCL Institute of Neurology, London, UK Search for more papers by this author Bengt Nellgård Bengt Nellgård Department of Anaesthesiology and Intensive Care, Institute of Clinical Sciences, Sahlgrenska Academy, Gothenburg University, Gothenburg, Sweden Search for more papers by this author Kaj Blennow Kaj Blennow Clinical Neurochemistry Lab, Institute of Neuroscience and Physiology, The Sahlgrenska Academy at the University of Gothenburg, Mölndal, Sweden Search for more papers by this author Alexander Crispin Alexander Crispin Institute of Medical Informatics, Biometry, and Epidemiology, Munich, Germany Search for more papers by this author Michael Ewers Corresponding Author Michael Ewers Institute for Stroke and Dementia Research, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany Search for more papers by this author Christian Haass Corresponding Author Christian Haass BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany German Center for Neurodegenerative Diseases (DZNE) Munich, Munich, Germany Munich Cluster for Systems Neurology (SyNergy), Munich, Germany Search for more papers by this author Author Information Marc Suárez-Calvet1,2, Gernot Kleinberger1,3, Miguel Ángel Araque Caballero4, Matthias Brendel5, Axel Rominger3,5, Daniel Alcolea6,7, Juan Fortea6,7, Alberto Lleó6,7, Rafael Blesa6,7, Juan Domingo Gispert8,9, Raquel Sánchez-Valle10,11, Anna Antonell10,11, Lorena Rami10,11, José L Molinuevo8,9,10,11, Frederic Brosseron12, Andreas Traschütz13, Michael T Heneka12,13, Hanne Struyfs14,15, Sebastiaan Engelborghs14,15, Kristel Sleegers16,17, Christine Van Broeckhoven16,17, Henrik Zetterberg18,19, Bengt Nellgård20, Kaj Blennow18, Alexander Crispin21, Michael Ewers 4,‡ and Christian Haass 1,2,3,‡ 1BioMedical Center (BMC), Biochemistry, Ludwig-Maximilians-University Munich, Munich, Germany 2German Center for Neurodegenerative Diseases (DZNE) Munich, Munich, Germany 3Munich Cluster for Systems Neurology (SyNergy), Munich, Germany 4Institute for Stroke and Dementia Research, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany 5Department of Nuclear Medicine, Klinikum der Universität München, Ludwig-Maximilians-University Munich, Munich, Germany 6Department of Neurology, Institut d'Investigacions Biomèdiques, Hospital de la Santa Creu i Sant Pau, Universitat Autònoma de Barcelona, Barcelona, Spain 7Center for Networked Biomedical Research for Neurodegenerative Diseases, CIBERNED, Madrid, Spain 8Clinical and Neuroimaging Departments, Barcelona Beta Brain Research Center, Pasqual Maragall Foundation, Barcelona, Spain 9Centro de Investigación Biomédica en Red de Bioingeniería, Biomateriales y Nanomedicina (CIBER-BBN), Barcelona, Spain 10Alzheimer's Disease and Other Cognitive Disorders Unit, Neurology Service, ICN Hospital Clinic i Universitari, Barcelona, Spain 11Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS), Barcelona, Spain 12German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany 13Neurology Department, Universitätsklinikum Bonn, Bonn, Germany 14Reference Center for Biological Markers of Dementia (BIODEM), Laboratory of Neurochemistry and Behavior, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium 15Department of Neurology and Memory Clinic, Hospital Network Antwerp (ZNA) Middelheim and Hoge Beuken, Antwerp, Belgium 16Neurodegenerative Brain Diseases Group, Department of Molecular Genetics, VIB, Antwerp, Belgium 17Laboratory of Neurogenetics, Institute Born-Bunge, University of Antwerp, Antwerp, Belgium 18Clinical Neurochemistry Lab, Institute of Neuroscience and Physiology, The Sahlgrenska Academy at the University of Gothenburg, Mölndal, Sweden 19Reta Lila Weston Laboratories and Department of Molecular Neuroscience, UCL Institute of Neurology, London, UK 20Department of Anaesthesiology and Intensive Care, Institute of Clinical Sciences, Sahlgrenska Academy, Gothenburg University, Gothenburg, Sweden 21Institute of Medical Informatics, Biometry, and Epidemiology, Munich, Germany ‡These authors contributed equally to this study *Corresponding author. Tel: +49 89 4400 46221; Fax: +49 89 4400 46113; E-mail: [email protected] *Corresponding author. Tel: +49 89 4400 46549; Fax: +49 89 4400 46546; E-mail: [email protected] EMBO Mol Med (2016)8:466-476https://doi.org/10.15252/emmm.201506123 See also: SE Schindler & DM Holtzman (May 2016) PDFDownload PDF of article text and main figures. Peer ReviewDownload a summary of the editorial decision process including editorial decision letters, reviewer comments and author responses to feedback. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Abstract TREM2 is an innate immune receptor expressed on the surface of microglia. Loss-of-function mutations of TREM2 are associated with increased risk of Alzheimer's disease (AD). TREM2 is a type-1 protein with an ectodomain that is proteolytically cleaved and released into the extracellular space as a soluble variant (sTREM2), which can be measured in the cerebrospinal fluid (CSF). In this cross-sectional multicenter study, we investigated whether CSF levels of sTREM2 are changed during the clinical course of AD, and in cognitively normal individuals with suspected non-AD pathology (SNAP). CSF sTREM2 levels were higher in mild cognitive impairment due to AD than in all other AD groups and controls. SNAP individuals also had significantly increased CSF sTREM2 compared to controls. Moreover, increased CSF sTREM2 levels were associated with higher CSF total tau and phospho-tau181P, which are markers of neuronal degeneration and tau pathology. Our data demonstrate that CSF sTREM2 levels are increased in the early symptomatic phase of AD, probably reflecting a corresponding change of the microglia activation status in response to neuronal degeneration. Synopsis TREM2 is an innate immune receptor selectively expressed by microglia in the brain. Measuring its soluble variant in the CSF (sTREM2) may be a candidate as a marker of microglial activity. This study aimed to investigate how CSF sTREM2 levels change during the course of Alzheimer's disease (AD). CSF sTREM2 levels are increased in the mild cognitive impairment (MCI) stage of AD compared to controls (P = 0.002), and to the preclinical (trend level, P = 0.062), and dementia stage of AD (P = 0.013). CSF sTREM2 levels are increased in individuals with suspected non-AD pathology (SNAP) compared to controls (P = 0.0004). CSF sTREM2 levels increase with aging. Increased CSF sTREM2 levels are associated with higher levels of T-tau and P-tau181P, markers of neuronal cell injury, and neurofibrillary tangles. Introduction Heterozygous missense mutations in the gene encoding the triggering receptor expressed on myeloid cells 2 (TREM2) have been recently described to significantly increase the risk of late onset Alzheimer's disease (AD) with an odds ratio similar to that of carrying an apolipoprotein E (APOE) ε4 allele (Guerreiro et al, 2013a; Jonsson et al, 2013). Heterozygous missense mutations in TREM2 also increase the risk for other neurodegenerative diseases (Borroni et al, 2013; Rayaprolu et al, 2013; Cuyvers et al, 2014), and homozygous loss-of-function mutations in TREM2 cause Nasu–Hakola disease (NHD) (Paloneva et al, 2002) and frontotemporal dementia (FTD)-like syndrome (Guerreiro et al, 2013b), which are both early-onset neurodegenerative diseases presenting as a frontal syndrome. Together, these findings indicate that TREM2 may be a common denominator in the pathogenesis of several different neurodegenerative diseases. TREM2 is a type-1 transmembrane glycoprotein with an immunoglobulin-like extracellular domain, one transmembrane domain and a short cytosolic tail (Klesney-Tait et al, 2006). It belongs to the TREM family of innate immune receptors and is expressed in cells of the monocytic lineage (Bouchon et al, 2001; Schmid et al, 2002; Paloneva et al, 2003; Kiialainen et al, 2005). In the central nervous system (CNS), it is selectively expressed by microglia and involved in regulating phagocytosis and removal of apoptotic neurons as well as in the inhibition of microglia proinflammatory response (Takahashi et al, 2005; Klesney-Tait et al, 2006; Hsieh et al, 2009; Wang et al, 2015). We have shown that loss-of-function of TREM2 impairs the phagocytic activity of microglial cells and reduces clearance of amyloid β-peptide (Aβ; Kleinberger et al, 2014), suggesting that TREM2 may play an important role in the development of AD pathology and neurodegeneration during the course of the disease. TREM2 undergoes proteolytic processing, releasing its ectodomain into the extracellular space as a soluble variant (sTREM2) via shedding by ADAM proteases (Wunderlich et al, 2013; Kleinberger et al, 2014), and can be detected in human plasma and cerebrospinal fluid (CSF) (Piccio et al, 2008; Kleinberger et al, 2014). Piccio et al found that CSF sTREM2 levels were increased in multiple sclerosis and other neurological inflammatory diseases (Piccio et al, 2008). We described that sTREM2 was almost undetectable in the CSF and plasma of a FTD-like patient carrying a homozygous TREM2 p.T66M mutation. This mutation leads to misfolding of the full-length protein, which accumulates within the endoplasmic reticulum. Due to the lack of cell surface transport, shedding is dramatically reduced, which explains the absence of sTREM2 in patients with the homozygous TREM2 p.T66M mutation (Kleinberger et al, 2014). In contrast, we observed a slight decrease in CSF sTREM2 levels in AD dementia patients compared to elderly cognitively normal subjects (Kleinberger et al, 2014). While our manuscript was under consideration, we learned that two groups independently found that CSF sTREM2 was increased in AD patients (Heslegrave et al, 2016; Piccio et al, 2016). Taken together, these results suggest that CSF sTREM2 levels are altered not only in subjects with TREM2 mutations but also in sporadic cases of neurodegenerative diseases. In AD, amyloid plaques and neurofibrillary tangles, the major pathological hallmarks of the disease, develop decades before the onset of clinical symptoms (Morris et al, 1996; Braak & Braak, 1997; Hulette et al, 1998; Price & Morris, 1999). Increased microglial activation and neuroinflammation frequently accompanies the early development of Aβ and tau pathology (Mosher & Wyss-Coray, 2014; Streit et al, 2014; Heneka et al, 2015; Tanzi, 2015). Since TREM2 is a key protein involved in the activation of microglia, the question arises whether TREM2 levels are pathologically altered in the early course of AD. If so, CSF sTREM2 would be an attractive biomarker candidate for tracking of the disease and as a potential outcome parameter for future clinical trials focusing on TREM2 and neuroinflammation. However, it is not known whether CSF sTREM2 levels change during the different stages of AD, a question that we addressed in the current study. The main aim of this cross-sectional multicenter study was to determine whether the levels of CSF sTREM2 change across the continuum of AD. We tested CSF TREM2 in subjects with preclinical AD, mild cognitive impairment (MCI) due to AD (MCI-AD), and AD dementia and controls, defined by clinical and CSF biomarker criteria as recommended by the National Institute on Aging-Alzheimer's Association (NIA-AA) criteria (Albert et al, 2011; McKhann et al, 2011; Sperling et al, 2011). We also tested whether CSF sTREM2 levels are associated with the core AD CSF biomarkers Aβ1–42, total tau (T-tau) and tau phosphorylated at threonine 181 (P-tau181P) (Blennow et al, 2010). As a secondary aim, we investigated whether CSF sTREM2 levels are altered in cognitively normal subjects with suspected non-AD pathology (SNAP) and MCI subjects without CSF biomarker evidence of AD pathology (MCI-noAD). SNAP is a recently defined diagnostic category that comprises those individuals with abnormal neurodegeneration biomarkers (T-tau and P-tau181P) but without evidence of underlying amyloidosis (Jack et al, 2012) and might thus represent neurodegenerative diseases different from AD. Results Study population The current study included 150 controls and 63 preclinical AD, 111 MCI-AD as well as 200 AD dementia subjects (Table 1). The diagnostic criteria of each group were defined according to the NIA-AA criteria, which uses a combination of clinical diagnosis and the CSF biomarker profile including Aβ1–42, T-tau, and P-tau181P (Albert et al, 2011; McKhann et al, 2011; Sperling et al, 2011). Decreased Aβ1–42 was a requisite for preclinical AD, and the combination of decreased Aβ1–42 and increased T-tau and/or P-tau181P for MCI-AD and AD dementia. The control group consisted of asymptomatic cognitively normal individuals with all three AD CSF core biomarkers within the normal range. The diagnostic criteria are described in more detail in the methods section. The cutoff values to define abnormal CSF values for each of the three AD CSF core biomarkers were defined for each center and are displayed in Appendix Table S1 (Antonell et al, 2011; Alcolea et al, 2014; Van der Mussele et al, 2014). Table 1. Demographic and clinical characteristics of the control and AD continuum groups Variable Control (n = 150) AD continuum (n = 374) P-value (group effect) Preclinical AD (n = 63) MCI-AD (n = 111) AD dementia (n = 200) Females, % 59 60 60 62 0.940 APOE ε4 carriers, % 21 58a 52a 62a <0.0001 Age, years 62.4 (11) 70.8 (11)a 74.3 (9)a 73.8 (10)a <0.0001 CSF biomarkers Aβ1–42, pg/ml 796 (159) 414 (98)a 426 (107)a 408 (113)a <0.0001 T-tau, pg/ml 218 (81) 450 (428)b 737 (410)a,c 920 (564)a,d,e <0.0001 P-tau181P, pg/ml 43 (12) 66 (39)a 95 (32)a,d 102 (44)a,d <0.0001 Aβ, amyloid β-peptide; AD, Alzheimer's disease; APOE, apolipoprotein E; CSF, cerebrospinal fluid; MCI-AD, MCI due to AD; P-tau181P, tau phosphorylated at threonine 181; T-tau, total tau. Data are expressed as percent (%) or mean (SD), as appropriate. Probability values (P) denote differences between groups. APOE genotype was available in 103 controls (69%), 39 preclinical AD (62%), 89 MCI-AD (80%), and 148 AD dementia (74%). Only Aβ1–42 values measured by the INNOTEST ELISA are included; Aβ1–42 values from Bonn group (measured with MSD platform) are excluded. Chi-square statistics were used for the group comparisons of gender and APOE ε4 carrier. One-way ANOVA was used to compare age and CSF biomarkers between groups. The P-values indicated in the last column refer to the group effects in these tests. Significant group effects were followed by Bonferroni-corrected pair-wise post hoc tests. a P < 0.0001 versus controls. b P = 0.002 versus controls. c P = 0.0001 versus preclinical AD. d P < 0.0001 versus preclinical AD. e P = 0.002 versus MCI-AD. The demographic and CSF core biomarkers values of control, preclinical AD, MCI-AD, and AD dementia subjects are shown in Table 1. All patients in the AD continuum group were older and had a higher frequency of APOE ε4 carriers than the control group. Age and APOE ε4 status did not differ between the three AD subcategories. As expected, groups differed with regard to their CSF biomarkers profiles. There were no differences in gender between groups. CSF sTREM2 is influenced by age Age was positively correlated with CSF sTREM2 in the pooled group of subjects (Pearson r = +0.391, P < 0.0001). The correlation was still significant when tested within each diagnostic group, including the control group (Pearson r = +0.177, P = 0.030), preclinical AD (Pearson r = +0.510, P < 0.0001), MCI-AD (Pearson r = +0.289, P = 0.002), and AD dementia (Pearson r = +0.310, P < 0.0001) (Fig 1). Levels of CSF sTREM2 were not significantly affected by gender (F1,521 = 0.1, P = 0.719) nor by APOE ε
