All tissues are surrounded by a mixture of noncellular matrix components, that not only provide physical and mechanical support to cells, but also mediate biochemical signaling between cells. The extracellular matrix (ECM) of endothelial cells, also known as the perivascular matrix, forms an organ specific vascular niche that orchestrates mechano‐, growth factor, and angiocrine signaling required for tissue homeostasis and organ repair. This concise review describes how this perivascular ECM functions as a signaling platform and how this knowledge can impact the field of regenerative medicine, for example, when designing artificial matrices or using decellularized scaffolds from organs. Stem Cells Translational Medicine 2019;8:375–382

The bioengineering of a replacement kidney has been proposed as an approach to address the growing shortage of donor kidneys for the treatment of chronic kidney disease. One approach being investigated is the recellularization of kidney scaffolds. In this study, we present several key advances toward successful re‐endothelialization of whole kidney matrix scaffolds from both rodents and humans. Based on the presence of preserved glycosoaminoglycans within the decelullarized kidney scaffold, we show improved localization of delivered endothelial cells after preloading of the vascular matrix with vascular endothelial growth factor and angiopoietin 1. Using a novel simultaneous arteriovenous delivery system, we report the complete re‐endothelialization of the kidney vasculature, including the glomerular and peritubular capillaries, using human inducible pluripotent stem cell –derived endothelial cells. Using this source of endothelial cells, it was possible to generate sufficient endothelial cells to recellularize an entire human kidney scaffold, achieving efficient cell delivery, adherence, and endothelial cell proliferation and survival. Moreover, human re‐endothelialized scaffold could, in contrast to the non‐re‐endothelialized human scaffold, be fully perfused with whole blood. These major advances move the field closer to a human bioengineered kidney.

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Genetically tailored pigs to eliminate human immune rejection of xenografts is one promising solution to the global donor organ shortage. The development of xenograft transplantation has however been hampered by incomplete understanding of its immune rejection and the inability to test this in a human transplantation setting. Here we use an ex vivo organ perfusion system with human whole blood to assess the initial immune activation within the xenograft endothelium at single cell transcriptome level. Renal injury, complement deposition, coagulation and lymphocyte influx are all strongly reduced in genetically modified pig kidneys with porcine MHC class I and three xenoantigens (GGTA1, CMAH, B4GALNT2) eliminated (4KO) compared to wildtype (WT) pig kidneys after 6-hours human blood perfusion. Single cell RNA sequencing of endothelial cells (EC) from 4KO and WT pig kidneys respectively reveal that there is a compartment (cortex, glomeruli and medulla) specific endothelial activation, with cortical and glomeruli endothelial cells being more affected. Differential gene expression analysis shows a downregulation of endothelial transcriptome activation response to human blood perfusion in the 4KO ECs. Pathway enrichment analysis further identify the NF-kB pathway as strongly activated in human blood perfused WT ECs but diminished in the 4KO. In conclusion, the 4KO pig model has strongly reduced endothelial immune activation response when perfused with human whole blood, that goes beyond prevention of humoral rejection. Our data support further development of the 4KO for use in clinical transplantation.

The ability to preserve metabolically active kidneys ex vivo for multiple days may permit reconditioning, repair and regeneration of deceased donor kidneys. However, the kidneys high metabolic demand limits its functional preservation. Current approaches focus on normothermic machine perfusion (NMP) at 37C or hypothermic machine perfusion (HMP) at 4-8C. At normothermia, kidneys are metabolically active but ex vivo preservation is limited to hours. During hypothermia kidneys can be preserved up to 24 hours but are metabolically inactive and suffer cold-induced injury. Therefore, we revisited sub normothermic perfusion (at 25C) as an alternative approach to preserve human kidneys in a metabolically active state for extended periods of time. In a custom-made platform that includes a cell-free perfusate enriched with TCA cycle fuels, urine recirculation, and continuous hemofiltration we perfused discarded human kidneys up to 8 days. Using spatially resolved single cell resolution isotope tracing we demonstrate active metabolism in all the different renal cell types over this period. However, beyond 4 days cell composition of nephron segments assessed with spatial lipidomics changed substantially and injury markers such as NGAL and LDH increased in the perfusate. Up to 4 days, perfused human discarded donor kidneys maintained metabolic fluxes, functional parameters and allow for reperfusion using a porcine auto transplantation model. These data underpin that extended multi-day metabolic preservation of human kidneys is achievable using a sub normothermic perfusion platform.