PROJECT SUMMARY In 2020, nearly 808,000 U.S. patients had end-stage kidney disease (ESKD) and the majority underwent hemodialysis (HD) using arteriovenous fistulas (AVFs). AVFs have ~62% 1-year patency due to venous neointimal hyperplasia (VNH) and venous stenosis (VS). The molecular mechanisms of VS/VNH after AVF are not understood. We completed a randomized, blinded phase 1 clinical trial where 21 patients (5 females, average 65 yo) underwent periadventitial delivery of autologous adipose derived mesenchymal stem cells (MSC, n=10) or placebo to the outflow vein at new upper extremity AVF creation. The primary endpoint was time to AVF maturation. It was significantly decreased in MSC treated patients versus controls (1.5 vs 2.5 m respectively, Log Rank: P<0.05). Several MSC- treated patients had AVFs with added procedures to maintain patency (‘non-responder MSCs’). Murine xenografts using human MSCs (hMSCs) from responders had PPARg reduction in CD68 (+) cells and this is protective for VS/VNH formation. Senescence alters cellular function leading to increased production of pro-inflammatory cytokines known as senescence-associated secretory phenotype (SASP). MSCs are anti-inflammatory and reduce VS/VNH by decreasing many of the SASP cytokines. MSCs isolated from patients with chronic kidney disease are senescent. We hypothesized that senescence is responsible for the dysfunction of MSCs from non-responders, obtunding the ability of MSCs to reduce inflammatory cues responsible for VS/VNH. Studies show senolytic drugs such as Dasatinib and Quercetin (D&Q) can decrease the senescent burden in cells and organs leading to improved function. So, we treated hMSCs from responder and non-responder with D&Q (D&Q hMSCs) and found it decreased Sensig scores (senescent gene expression) and increased proliferation. In vitro studies showed conditioned media (CM) from MSCs reduced PMA induced THP-1 (monocyte) cell differentiation with an increase in pSTAT3 with a decrease in PPARg expression. Next, D&Q hMSC xenografts in mice with AVFs were created and there was a decrease in Pparg gene expression, Fridman_Up (fibrosis gene), and Sensig scores accompanied with a reduction in inflammatory, SMCs, fibrosis, with increased endothelial cells (ECs), and less VS/VNH. Our central hypothesis is that arteriovenous fistulas treated with D&Q MSCs increase pSTAT3 and decrease PPARg in resident monocytes/ MΦs of AVFs leading to less pro- inflammatory cells resulting in positive vascular remodeling, less fibrosis, Sensig, Fridman-Up scores, and increased endothelization There are three specific aims: Aim 1. Investigate the role of CM from D&Q hMSCs from non-responders and responders on monocyte to macrophage differentiation, activation, pSTAT3/ PPARG signaling, functional changes, and Sensig score. Aim 2. Determine the effect of D&Q hMSC on pSTAT3/ PPARg signaling and VS/VNH formation in murine AVFs. Aim 3. Assess the efficacy of D&Q MSC in retarding VS/VNH in a pig AVF model...