The primary objectives are to validate that a previously identified gene variant influences the proportion of activated microglia (PAM) and the amount of TSPO binding on PET imaging, to identify novel loci that influence PAM and TSPO PET, and to understand the functional consequences of gene variants that drive microglial activation in Alzheimer's disease.

While activated microglia have been observed in the vicinity of neuritic amyloid plaques in Alzheimer's disease (AD), there have been no large-scale assessments of microglial activation in aging and neurodegenerative disease. The investigators seek to understand the genetic underpinning of microglial responses-particularly the proportion of microglia in a morphologically-defined state of activation-that increase susceptibility to AD, so the investigators can develop more targeted forms of immune-based therapies to prevent cognitive decline and progression to dementia. The objective is to refine the genetic architecture of microglial activation to validate a previously identified gene variant -- and to identify novel loci -- that influence the proportion of activated microglia. The investigators also seek to understand the functional consequences of variants driving microglial activation in AD.

The central hypothesis is that identifiable gene variants influence microglial activation and susceptibility to AD. The investigators will test this hypothesis by conducting genome-wide analysis and identifying associations between gene variants and microglial activation. Microglial activation will be measured in human autopsy tissue (ex vivo), living human brain using PET imaging (in vivo), and in monocyte-derived microglia-like cells (in situ and in vitro).

This genetic study is designed to validate a finding that was discovered in participants with self-reported European-Caucasian ancestry. Therefore, the study seeks to enroll participants who self-report as white, not Hispanic or Latino. However, if this study is successful, the investigators plan to use the methods in this protocol in a future study to identify new genetic variants associated with changes on TSPO PET in a more diverse participant population. The investigators intend to use the results from this study to eventually benefit individuals of all racial and ethnic groups.

Inclusion criteria:

1. Age 50 and older at time of study entry.
2. Meet criteria for either a) amnestic mild cognitive impairment (single or mixed domain) or Alzheimer's disease, or b) have no cognitive impairment, based on history, exam, and neuropsychological testing.
3. Patients must have Clinical Dementia Rating Scale score of 0.5 or 1 at enrollment. Controls must have Clinical Dementia Rating scale score of 0 at enrollment.
4. Subjects must have AD biomarker previously obtained for research or clinical purposes or undergo a 18F-florbetaben PET scan during the screening process. Patients must have positive amyloid PET scan or CSF results consistent with AD. Controls must have a negative amyloid PET scan or CSF results not consistent with AD.
5. Self-identify as white, non-Hispanic or Latino
6. Subjects must be ableto provide informed consent
7. Written and oral fluency in English
8. Able to participate in all scheduled evaluations and to complete all required tests and procedures.
9. In the opinion of the investigator, the subject must be considered likely to comply with the study protocol and to have a high probability of completing the study.

Exclusion Criteria:

1. Past or present history of certain brain disorders other than MCI or AD.
2. Certain significant medical conditions, which make study procedures of the current study unsafe. Such serious medical conditions include uncontrolled epilepsy and multiple serious injuries.
3. Contraindication to MRI scanning
4. Conditions precluding entry into the scanners (e.g. morbid obesity, claustrophobia, etc.).
5. Exposure to research related radiation in the past year that, when combined with this study, would place subjects above the allowable limits.
6. Participation in the last year in a clinical trial for a disease modifying drug for AD.
7. Inability to have a catheter in subject's vein for the injection of radioligand.
8. Inability to have blood drawn from subject's veins.
9. Taking anticoagulant (e.g., warfarin) or immunosuppressive/immunomodulatory medication. Nonsteroidal anti-inflammatory drugs (NSAIDs) are not exclusionary. Use of steroids in the 30 days preceding the PET scan.
10. Having a diagnosis of a chronic inflammatory disease (for example, multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis, psoriasis, systemic lupus erythematosus) or a chronic infectious disease such as H.I.V.

Individual participant data will be available upon reasonable request from a qualified investigator.

Up to two weeks after review and approval of request.

Investigator qualifications and previous work will be reviewed by PI. Subsequent email correspondence will relay technical criteria needed for access.