*Purpose: Islet transplantation can cure Type 1 diabetes; however, multiple donors are needed due to extensive perioperative loss of islets when removed from their native blood supply. We recently discovered that CD34+45- vascular endothelial cells from parathyroid glands (PTG), another richly vascular endocrine organ, have an extraordinary ability to promote beta cell engraftment and function. This study aimed to generate a human embryonic stem-cell (hESC) derived source of CD34+45- vascular endothelial cells that could equitably protect islet grafts.

*Methods: hESC-derived β-cells (eBCs) and CD34+45- (scCD34+) cells were differentiated from the same stem cell line and were co-transplanted in the subcutaneous (SQ) and intramuscular (IM) sites of immunodeficient mice. scCD34+ cells were FACS purified.

*Results: Using the same hESC line that we have used to produce eBCs, we have succeeded in generating CD34+CD45- cells after 9 days of in-vitro differentiation. These scCD34+ cells lack the hematopoietic marker CD45 and contain the endothelial precursor marker CD146+ (Fig A). Moreover, these cells efficiently formed tubular structures in 4 hours in an in vitro angiogenesis assay, suggesting that these cells are vascular endothelial progenitor cells (Fig B). Transplantation of scCD34+ cells in an in-vivo angiogenesis model, using mouse skin flaps, resulted in increased vascular density and number of vessel junctions compared to sham and hematopoietic CD34+45+ cells differentiated simultaneously. Co-transplantation of scCD34+ with eBCs in SQ and IM sites resulted in significant improvement of graft mass preservation when compared with eBC transplanted alone. Moreover, we developed a novel co-clustering method to combine scCD34+ with eBCs prior to transplantation, creating vascularized beta cells, which showed further engraftment protection in the SQ and IM via luciferase imaging when compared to control.

*Conclusions: While the βετα-cell-protective activity of PTG CD34+ cells is remarkable, a renewable source of CD34+ cells that possess the same activities of PTG CD34+ cells is highly desired to enable wide application of βετα-cell replacement therapy. This study show that vascular progenitor cells can be generated from stem cells and used to support the engraftment of stem cell derived beta cells.

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