Project Summary Alzheimer's disease (AD) is the leading cause of dementia in older adults and affects over 5 million individuals in the United States alone. The more common form of AD, late-onset, is a multifactorial disorder in which ApoE4 is the most potent genetic risk. The ApoE4 isoform is also linked to midlife obesity which can cause cognitive decline in older adults. Obesity is a condition of chronic inflammation characterized by activated immune cells and increased inflammatory mediators in the blood. Recently, through genetic association studies, microglia and other myeloid origin immune cells have emerged as crucial players in AD pathogenesis. Single-cell RNA sequencing analysis in mouse and human AD brains has identified a distinct subset associated with amyloid plaques and the upregulation of ApoE in microglia. Therefore, further investigation of microglia and other immune cells is needed to understand how these cells are affected by ApoE4 isoform and obesity contributing to late- onset AD progression. Our objective in this R21 project is to explore the gene-environment interactions between ApoE4 and obesity in regulating phenotypic plasticity of microglia and peripheral immune cells. This project aims to identify molecular pathways that could restore healthy immune function to treat late-onset AD. Our Specific Aims are to (1) Assess dynamic changes of brain immune cell populations influenced by ApoE4 and diet-induced obesity in human ApoE isoform knock-in mice; (2) Identify altered lipid metabolism in ApoE4 microglia that modulates the phenotypic plasticity. The successful completion of this project will reveal the multifaceted roles of ApoE4 in modulating immunometabolism. Moreover, it will provide a strong rationale for lifestyle interventions or individualized therapeutic strategies for late-onset AD in ApoE4 carriers.