ABSTRACT The pathophysiology of delirium and the mechanisms underlying its epidemiological association with long-term cognitive decline (LTCD) remain largely unknown. This important gap limits development of preventive and disease-modifying therapies. To address this gap, we conducted the ?Biomarker Discovery for Delirium? project within the P01 ?Interdisciplinary Study of Delirium and Its Long Term Outcomes?. Our results support a model for delirium in which a predisposing, pre-inflammatory state results in a heightened inflammatory response to surgery, leading to blood-brain barrier breakdown, microglial activation and neuro-inflammation, resulting in neuronal injury and death. This model is intriguing, as inflammation could be the mechanism underlying the epidemiological link between delirium, LTCD, and Alzheimer's Disease and Related Dementias (ADRD). For the P01 renewal, Delirium, Dementia, and the Vulnerable Brain: An Integrative Approach, our project will leverage banked specimens from the first cycle's Successful Aging after Elective Surgery study (SAGES I), which enrolled and followed 560 participants undergoing major scheduled surgery, and collected plasma at 4 time points relative to surgery. We add banked specimens from the Healthier Postoperative Recovery study (HiPOR), which enrolled 242 participants undergoing total joint replacements under spinal anesthesia using a similar protocol to SAGES I, with the additional collection of preoperative cerebrospinal fluid (CSF). Further, we will collect new blood and CSF samples from a probability sample of 128 SAGES I participants, and from a new cohort of 400 older patients undergoing total joint replacement under spinal anesthesia (SAGES II). We will use two state-of-the-art approaches, SOMAscan, a next generation proteomics platform, to discover new inflammatory proteins (Aim 1), and CyTOF, a single-cell mass cytometry platform, to characterize circulating immune cells that regulate inflammation (Aim 2). We will also extend our prior work by examining CSF in addition to plasma, and by quantifying a novel inflammatory index (Aim 3). Using these techniques, we will compare inflammatory proteins and cells in patients who do and do not develop delirium, and in those who have slower and faster rates of LTCD following delirium. Importantly, we will also independently validate all SOMAscan and CyTOF results using standard laboratory methods. Our current Project represents the next in a series of systematic studies extending important findings from the first P01 cycle, and leading to more detailed understanding of the full inflammatory protein profile associated with delirium and LTCD, including markers in the CSF, plus origins of the inflammatory response from immune cells. The Aims also represent an initial step toward development of blood and CSF protein, and cytometry-based biomarker panels to refine prediction of delirium and LTCD. Importantly, the proposed work will improve our understanding of the pathophysiology of delirium and its association with ADRD, ultimately leading to targeted interventions to improve outcomes of hospitalized older adults with vulnerable brains.