SUMMARY The overarching goal of our project is to understand how immune cells impact the brain during aging, with the objective of restoring old brain function. The brain has long been considered an ‘immuno-privileged organ’. However, emerging studies have shown that immune cells infiltrate the brain in neurodegenerative diseases such as Alzheimer’s disease and during aging. A key remaining challenge is to understand how immune cells impact the brain during aging and to determine how to use this knowledge to restore functionality of old brain and treat neurodegenerative diseases? We have been performing single cell transcriptomic studies to understand how interventions that affect inflammation and immune cell infiltration impact the function of cells in the brain. New experiments are currently underway, with studies of mice treated with engineered checkpoint inhibitors, anti-inflammatory cytokines, and injury stimuli, forming the initial foundations. Importantly, we have in parallel spearheaded new spatial transcriptomics studies. This recent work has revealed an unprecedented glimpse into how T cells impact their neighboring cells. These findings would not be possible to assess with single cell RNA-seq experiments, as those are from dissociated cells and lose spatial information. Thus, we are submitting this proposal for an Administrative Supplement to apply spatial transcriptomics to one cohort of mice to explore the power of this technology to drive future studies of spatial brain aging. This work should give a fundamental understanding of the mechanistic impact of T cell on different cell types in the brain. Knowledge from our study should pave the way for building transformative strategies, including new immunotherapies, for the restoration of a pristine tissue, which will be a critical step for improving brain function during aging and age- related diseases such as Alzheimer’s disease.