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

2 is an innate immune receptor expressed on the surface of microglia. Loss-of-function mutations of 2 are associated with increased risk of Alzheimer's disease (). 2 is a type-1 protein with an ectodomain that is proteolytically cleaved and released into the extracellular space as a soluble variant (2), which can be measured in the cerebrospinal fluid (). In this cross-sectional multicenter study, we investigated whether levels of 2 are changed during the clinical course of , and in cognitively normal individuals with suspected non- pathology (). 2 levels were higher in mild cognitive impairment due to than in all other groups and controls. individuals also had significantly increased 2 compared to controls. Moreover, increased 2 levels were associated with higher total tau and phospho-tau, which are markers of neuronal degeneration and tau pathology. Our data demonstrate that 2 levels are increased in the early symptomatic phase of , probably reflecting a corresponding change of the microglia activation status in response to neuronal degeneration