sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early‐stage Alzheimer's disease and associate with neuronal injury markers

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.


Control group criteria for each center I. Gothenburg, Sweden
Controls subjects were cognitively normal individuals with a normal cognition (CDR=0), no history of neurological diseases and all three AD CSF core biomarkers were normal.

II. Bonn, Germany
Control subjects were recruited from patients undergoing routine lumbar punctures during neurological work up for diseases other than neurodegenerative or neuroinflammatory diseases or acute CNS injury by vascular or other causes. Neurological/psychiatric diagnoses reached were unspecified visual disturbance (n=1), non-inflammatory polyneuropathy (n=3), depression (n=7) and somatoform disorder (n=1). Controls had no record of subjective or objective cognitive impairment, and all three AD CSF core biomarkers were normal.

III. Antwerp, Belgium
The control group consisted of cognitively healthy elderly in whom cognitive deterioration was ruled out by means of neuropsychological screening. In addition to a nondisease control (n=1), the control group contained patients with the following neurological diseases: polyneuropathy (n=4), headache (n=2), atypical pain syndrome (n=1) and peripheral cranial nerve paralysis (n=3). In all these patients neurodegenerative disorders were ruled out by means of an extensive neurological work-up (Struyfs et al, 2015). All three AD CSF core biomarkers were normal.

IV. Clinic Barcelona, Spain
Control subjects were cognitively normal people defined according to the following criteria: Mini Mental State Examination (MMSE) scores above 24, objective cognitive performance within the normal range (performance within 1.5 SD) in all tests from a specific test battery, clinical dementia rating (CDR) scale score of 0, no significant psychiatric symptoms or previous neurological disease and all three AD CSF core biomarkers were normal.

V. Sant Pau Barcelona, Spain
Control subjects were recruited among patients' caregivers. All underwent formal cognitive evaluation using a previously published neuropsychological battery (Sala et al, 2008). Control subjects did not have cognitive complaints and the results of the neuropsychological evaluation were in the normal range for age and education . All three AD CSF core biomarkers were normal.

sTREM2 ELISA protocol
The sTREM2 ELISA was previously established by our group using the Mesoscale platform (Kleinberger et al, 2014). In brief, it consists of a biotinylated polyclonal goat anti-human TREM2 antibody (R&D Systems BAF1828) as capture antibody; a monoclonal mouse anti-human TREM2 antibody (Santa Cruz Biotechnology; B-3, sc373828) as a detection antibody; and a SULFO-TAG-labeled anti-mouse secondary antibody (Meso Scale Discovery). Recombinant human TREM2 protein (Hölzel Diagnostika) was used as a standard (4000 to 62.5 pg/ml).

APPENDIX TABLES Appendix
-<50yo: 300 -51-70yo: 450 ->70yo: 500 § Depending on the sample batch. The table contains the mean (SD) of CSF sTREM2 (relative to an internal standard) and the number of subjects (in square brackets) included per group and center. Abbreviations: AD, Alzheimer disease; CSF, cerebrospinal fluid; MCI, mild cognitive impairment; SNAPs: suspected non-AD pathology; NA, not applicable; SNAPs: suspected non-AD pathology.