Introduction: Pluripotent stem cell-derived islets (SC-islets) now emerge as a new source for beta-cell replacement therapy. While human islet transplants are hampered by post-transplantation cell death, inflammation, insufficient revascularization and islet amyloid formation, there is a gap in knowledge on the engraftment process of the stem cell-derived islets. In this experimental study, we investigated the engraftment, more specifically innervation and vascularization, of SC-islets in comparison to human islets three months post-transplantation.

Methods: Human embryonic stem cells (H1 cell line) were differentiated into SC-islets by using a previously published seven-stage protocol (Balboa, Barsby et al., 2022). Immunodeficient NOD.Cg-Prkdc^scidI12rg^tm1Sug mice were transplanted with 700-800 islet equivalents of human islets or SC-islets beneath the kidney capsule. Three months post-transplantation, graft blood flow and oxygen tension measurements were performed. Grafts were immunostained for endocrine composition, proliferation, nerves, blood vessels and amyloid. The composition of cells and blood vessel and neuron density were analyzed before and after transplantation. RNA sequencing of human islets and SC-islets was performed before transplantation to identify differences in factors known to affect angiogenesis and neurogenesis.

Results: Three months post-transplantation the vascular density was similar in SC-islets to that of human islets. However, while the vascular network in human islets was a mixture of remnant donor endothelium and ingrowing blood vessels, SC-islets had ingrowing blood vessels only. The blood perfusion tended to be higher in transplanted SC-islets (p=0.067) while the oxygenation of the SC-islet grafts was twice as high (p>0.01) as human islets. Ingrowth of nerves and blood vessels were in close vicinity of each other, although in line with higher vascularization, also the reinnervation of the SC-islets was four- to five-fold higher than the human islets. Changes in cellular composition were observed with immunostaining three months after transplantation. Human islet transplants had an increased percentage of somatostatin cells while a decrease in the percentage of insulin producing cells and an increase in glucagon producing cells was seen in SC-islets compared to before transplantation. Both before and after transplantation, maturity markers were expressed and there was a low proliferation rate observed with immunostaining. Both SC-islets and human islets contained amyloid at three months post-transplantation.

Conclusion: We conclude that the vascular and neural engraftment of SC-islets is superior to human islets, which may be a useful future alternative to islet transplantation within beta-cell replacement strategies. However, grafts of both origins develop amyloid with potential long-term consequences post-transplantation.