ABSTRACT Impaired cognitive function after common surgical procedures is a growing concern especially among over 5 million people in the United States who suffer from dementia, including Alzheimer’s disease (AD), and thus have a 3-fold increased risk for fracture requiring surgical repair. After orthopedic surgery, acute changes in cognitive function, often referred to as postoperative delirium, occur in up to 89% of patients with preexisting dementia, and associate with poorer prognosis and even 2-fold greater risk for 1-year mortality compared to patients without dementia or delirium. Our long-term goal is to define the mechanisms that underlie surgery-induced cognitive dysfunction, and to provide safe and effective approaches to reduce this potentially devastating complication. From R01-AG057525 we discovered that orthopedic surgery triggers an acute deposition of amyloid-β (A) in the brains of transgenic Alzheimer’s (CVN‐AD) mice. This proposal continues to characterize the role of innate immune dysregulation following a common orthopedic surgery (tibial fracture) on the blood-brain barrier (BBB) and microglial activation using a model of postoperative delirium superimposed on dementia. The overall objectives are to characterize the immune heterogeneity of delirium superimposed on dementia, and to evaluate the efficacy of minimally-invasive percutaneous vagus nerve stimulation (pVNS) on microglial function and A levels in AD-like mice after surgery. Our central hypothesis is that dysregulation of immune cell clusters and elevated fibrinogen contribute to delirium-like behavior in AD mice after surgery, and that postoperative pVNS attenuates this effect. This hypothesis is based on preliminary data acquired in the applicants’ laboratories, and will be tested by pursuing 3 specific aims: 1) Develop a single-cell atlas of the mouse brain with delirium superimposed on dementia; 2) Analyze the mechanisms of microglial activation and A deposition after surgery in AD-like mice with delirium; and 3) Define the impact of pVNS on BBB opening and microglial-dependent A clearance after surgery. Feasibility for these models and techniques has been established in the applicants’ hands. In this innovative approach, we will employ functional transcriptomics with advanced computational approaches to identify the innate immune single-cell landscape in response to surgical trauma and delirium-like changes in the brain of dementia-prone mice. We will continue to characterize neuro-immune interactions and changes in blood-brain function using both animal models as well as organ-on-chip technology. We will also test the efficacy of pVNS to facilitate A clearance after surgery and limit the onset of postoperative delirium. The rationale for the proposed research is that successful completion will advance and expand our understanding of how surgery affects the innate immune system, and will provide new molecular mechanisms of relevance to delirium, neurodegener...