*Purpose: Kidney transplantation is a life-saving treatment for many patients with end-stage renal failure. Recently, machine perfusion of kidneys prior to transplantation has emerged as a technique to improve the preservation of kidneys during storage. Kidney machine perfusion may also provide the opportunity to assess graft quality prior to transplantation, which could improve outcome predictions and tailored management for transplant patients. Recently, extracellular vesicles (EV) have emerged as important mediators of cell to cell communication. EV have the unique ability to co-transport many signals and provide a “snapshot” of their cell of origin. Here, we aim to characterize graft-derived kidney perfusion fluid EV and to determine their immunogenic potential.

*Methods: Perfusion fluid samples (n=20) were collected from deceased donor kidneys placed on the LifePort Kidney device following perfusion; EV were purified by sequential centrifugation (1500 g, 15’, followed by 150 000 g, 90’). EV were profiled by electron microscopy, nanoparticle tracking analysis, Cytoflex and miRNA sequencing (HiSeq 4000). Peripheral blood mononuclear cells (PBMC) were isolated from healthy controls by Ficoll. Purified EV from donors (n=12) were labelled and co-cultured with PBMC. EV internalization, cytokine expression and T cell activation were measured by flow cytometry. T-tests and ANOVAs were used to compare controls and EV-exposed samples; miRNA expression and in vitro findings were correlated with clinical parameters by linear regression.

*Results: We demonstrate that human kidneys release EV in their perfusion fluid, the majority being 100 to 300 nm in size. We show that these EV express CD9 and CD63 as well as donor-specific HLA markers and are selectively internalized by monocytes and B cells. EV stimulation of PBMC triggers an increase in pro-inflammatory cytokine expression (IL-4, IFN-γ, IL-17) . Additionally, EV exposure leads to a downregulation of T regulatory cells (CD4⁺CD25^hiCD127^loFoxP3⁺), which was further downregulated in patients that reject their grafts. We further show that a specific donor kidney EV miRNA signature can differentiate donor kidneys of recipients with delayed graft function as compared to immediate graft function, which correlates with recipient clinical parameters such as glomerular filtration rates at 7, 30 and 90 days.

*Conclusions: Taken together, we show that human kidneys release pro inflammatory EV in their perfusion fluid which may play a role in graft dysfunction. Further in-depth characterization of these EV may help to develop specific molecular markers to assess graft quality and predict renal transplant outcomes and lead to potential interventions at the time of transplantation.

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