The goal of the current proposal is to determine the role of monocyte Delta like-4 (Dll4) in cerebral small vessel disease (CSVD) and its impact on cognition in people with HIV (PWH). Dll4 is a Notch ligand usually restricted in endothelial cells (ECs). However, monocytes can express DII4 under pro-inflammatory conditions. This is relevant to the current era of combination antiretroviral therapy (cART), where persistent systemic inflammation is a driving force for the progression of comorbidities. Our preliminary data strongly suggest that DII4-Notch signaling is implicated in altering cerebrovascular function. In vitro, we demonstrate that pro-inflammatory stimuli such as lipopolysaccharides (LPS) or tumor necrosis factor alpha (TNFα) (often increased in HIV infection) induce a robust increase of Dll4 expression in human monocytes and Dll4 secretion (extracellular Dll4, exDll4) from monocytes. Monocytes with high Dll4 expression trigger Notch1 activation to promote a pro-inflammatory status. In vivo, mice injected with exDll4 show impaired blood-brain barrier (BBB) and cerebrovascular remodeling. Of interest, exDll4 inhibits Notch1 activation in human brain microvascular endothelial cells (HBMECs) and Notch2/ 3 activation in human brain vascular smooth muscle cells (HBVSMCs). In clinical specimens, we have found sex differences in exDII4 plasma levels of PWH on cART, elevated in males but not in females compared to HIV uninfected study participants. However, membrane-bound DII4 levels were similar in males and females, suggesting novel pathways that may contribute to the previously reported sex differences in cerebrovascular disease. Based on the observations above, we hypothesize that monocyte DII4-Notch signaling contributes to a monocyte pro-inflammatory phenotype in both male and female PWH. Pro- inflammatory monocytes are more likely to interact with endothelial cells (ECs) and cross the BBB differentiating in perivascular macrophages, thus contributing to neuroinflammation and CSVD. In addition, male PWH have increased levels of exDII4 in circulation, which could further affect the BBB through inhibition of Notch1 signaling in ECs and Notch3 signaling in pericytes. exDII4 also increases vascular remodeling in small arteries and arterioles, leading to ischemia as reflected by lacunae and white matter hyperintensities, typical manifestations of CSVD. We will test these hypotheses by addressing the three aims listed below: Aim 1. To determine, in a longitudinal study, the role of monocytes derived DII4 in brain microcirculation and microstructure integrity. Aim 2. To determine, in an integrated approach, the effects of HIV status, imaging metrics, exDII4, and mDII4 levels on cognitive performance. Aim 3. To define the role of mDll4 on molecular mechanisms of monocyte transmigration and the mechanistic role of exDll4 on vascular cell function.