This project will investigate microglial involvement in cognitive impairment after surgery and general anesthesia. Cognitive impairment is prevalent and persistent in older surgical patients and associated with higher morbidity and cost. The cause is unknown but CNS inflammation is implicated. Microglia, the resident immune competent cells of the CNS, are major effectors of CNS inflammation, but until recently it was difficult to distinguish between resident microglia and myeloid cells that infiltrate the CNS durin stress because there were no known microglia specific genes encoding cell surface proteins. This is critical because resident microglia and infiltrating macrophages have separate and distinct immune functions. We propose a transcriptome / proteomincs / bioinformatics approach using a recently discovered unique molecular signature for microglia to investigate age-dependent microglia biology during and after surgery and anesthesia. Using such tools, we found there are fewer microglia in the old brain, that they express lower levels of immune-related and homeostatic genes, and that isoflurane anesthesia affects microglia in young vs. old mice differently. Based on these data, we propose microglial attrition and dysregulation in the old CNS are responsible for vulnerability to cognitive impairment after surgery and that surgery. Here we will identify the impact of surgery and general anesthesia on the molecular and functional signature of microglia; investigate the contribution of microglia to postoperative CNS inflammation and cognitive impairment using microglial ablation / replenishment; and rescue age-dependent cognitive vulnerability to surgery and anesthesia by immunomodulating microglia. Upon completion of the project, we will understand age differences in the molecular and functional properties of microglia, the role of resident microglia vs. infiltrating myeloid cels in surgery- induced CNS inflammation and cognitive decline, and how general anesthesia affects the process and defined a strategy for specifically targeting microglia to restore their homeostatic molecular signature, reduce CNS inflammation, and improve postoperative cognitive outcome in older subjects. Importantly, we will also specifically target microglia to restore their homeostatic molecular signature in order to reduce CNS inflammation and improve postoperative cognitive outcome. The work proposed is innovative because it uses state-of- the art methods and technology to explore a novel microglial mechanism for postoperative cognitive decline in older patients and significant because of the scale of the clinical problem and potential to identify new ways of preventing it.