The Potential of Engineered Allogeneic Hypoimmune Stem Cell–derived Pancreatic Islets for Transplantation

Type 1 diabetes mellitus (T1DM), an autoimmune-driven destruction of pancreatic beta cells, leads to insulin deficiency and hyperglycemia with an incidence rate of 0.4%–0.5%.1,2 Although exogenous insulin administration has transformed this once-fatal disease into a medically manageable chronic condition, T1DM remains associated with a variety of comorbidities, including retinopathy, nephropathy, neuropathy, and cardiovascular disease. Severe hypoglycemic events and impaired awareness of hypoglycemia contribute significantly to the increased morbidity and mortality.3 Most recently, artificial pancreas device systems containing a continuous glucose monitor, an insulin pump, and a control algorithm have been developed, providing improved glycemic control.4 Nevertheless, these systems still face delays in subcutaneous glucose sensing and insulin delivery and do not achieve physiological beta-cell responses. Biological replacement of beta cells achieved by the allogeneic whole pancreas or isolated islet transplantation has been shown to restore physiologically regulated insulin secretion, provide glycemic control, restore hypoglycemia awareness, prevent severe hypoglycemic events, and improve quality of life.5 While physiological blood glucose control is achieved, chronic immunosuppression is necessary.6-9 Although islet cell transplants avoid surgical risks largely, repetitive transfers are necessary and graft survival times (median graft survival time 4–6 y) are limited,5,10 compared with a median of 10 y for whole pancreas graft.11 Induced pluripotent stem cells (iPSCs) derived from adult somatic cells provide an interesting alternative as a regenerative approach targeting diseases that are characterized by the loss of specific cell populations. iPSCs have enormous potential to provide unlimited progenitor cells that can differentiate into functional tissue cells.12 In contrast, the utilization of autologous iPSCs for patient-specific treatments has shown less favorable outcomes, being laborious, costly, and associated with uncertain quality and efficacy of individual cell products.13 The work by Hu et al14 contributes to another milestone in the genetic engineering of allogeneic cell therapeutics. The authors compared various genetic engineering approaches and generated hypoimmune pluripotent (HIP) stem cells by depleting HLA class I and class II molecules, overexpressing CD47 (B2M–/–CIITA–/–CD47+). The immune checkpoint inhibitor CD47, which binds to signal regulatory protein-alpha, inhibits HLA-independent inhibition of innate immune cells. Xenogeneic human iPSCs were used to assess the hypoimmune editing platform in a relevant clinical scenario. Human iPSCs underwent CRISPR-Cas9 (clustered regularly interspaced palindromic repeats-CRISPR-associated protein 9)-mediated inactivation of B2M and CIITA and were transduced with lentiviral particles carrying a transgene CD47 to generate HIP iPSCs. These cells were then injected into nonhuman primate rhesus macaques and avoided systemic immune activation. Engineered rhesus macaques HIP iPSCs survived for 16 wk in fully immunocompetent allogenic recipients, whereas wild-type cells were vigorously rejected. Importantly, human HIP iPSCs were able to differentiate into active pancreatic islet cells. Transplantation of these HIP islets into allogenic diabetic humanized mice ameliorated diabetes without activating alloimmune responses. HIP-edited rhesus macaques primary islets functioned for up to 40 wk in immunocompetent allogenic rhesus macaque recipients in the absence of immunosuppression. Because streptozotocin-induced diabetes cannot be reliably induced in rhesus monkeys,15 Hu et al16 conducted a follow-up study to validate the curative effect of HIP-edited rhesus macaque primary islets in an immunocompetent diabetic cynomolgus monkey. Diabetes was induced with a single injection of streptozotocin. This diabetic monkey received insulin treatment to maintain a steady, controlled state of blood glucose level. HIP-edited primary islets were injected intramuscularly, and insulin requirements were reduced by 50% compared with pretransplant requirements by day 6, to 25% by day 9, and then discontinued by day 12. The monkey showed tightly controlled blood glucose levels and stayed exogenous insulin independent for 6 mo in the absence of any immunosuppressive and other supportive medication. To prove that the monkey's insulin independence was fully dependent on the transplanted HIP islets, an anti-CD47 antibody was given to target grafted islets. The quick relapse of diabetes requiring insulin injection confirmed the dependence of the animal on its graft function. Achieving indefinite survival of allogeneic grafts in the absence of a life-long immunosuppressive regimen remains an elusive goal in clinical transplantation. Significant efforts have been devoted to developing reliable and tolerogenic islet cell transplants for T1DM (Figure 1). Lei et al17 reported that the cotransplantation of allogeneic islets with streptavidin–FasL–presenting microgels into the omentum under transient rapamycin monotherapy resulted in survival times of >6 mo with excellent glycemic control, although not achieving insulin independence in a nonhuman primate with diabetes model. Others developed a polyethylene glycol–based conformal coating encapsulation method to improve the cytocompatibility of transplanted islets in both rodent with diabetes and nonhuman primate with diabetes models.18FIGURE 1.: Current strategies to develop tolerogenic islet cell transplants for T1DM include the cotransplantation of primary allogeneic islets with streptavidin–FasL–presenting microgels17; a polyethylene glycol–based conformal coating encapsulation method of primary allogeneic islets18; HIP-edited primary allogeneic islets16; and HIP-edited iPSCs-derived islet.14 Created with Biorender.com. HIP, hypoimmune pluripotent; iPSC, induced pluripotent stem cell; T1DM, type 1 diabetes mellitus.Although the work by Hu et al16 is the most innovative and encouraging, there are some limitations that need to be addressed further. First, only 1 case with HIP-edited islet transplantation was reported. A larger-scale nonhuman primate study is required. Second, 4 donor pancreata were necessary for the generation of HIP-edited islets transferred into 1 recipient. This relatively low efficiency of genetic engineering needs to be improved for clinical applications using pancreata from deceased donors. Although the potential retention of pluripotency of iPSC-derived islets might challenge safety issues, iPSCs remain an ideal unlimited resource for HIP editing. Notably, transplantation of human iPSC-derived HIP-edited islets into allogeneic humanized mice with diabetes only improved glycemic control but did not achieve insulin independence. Further studies are required to improve the iPSC-derived islet function and develop a "switch" that can easily eliminate transplanted cells once they are no longer helpful in recipients. Third, all these studies rely on animal with chemically induced diabetes models, which do not simulate the associated autoimmune component of T1DM. The immunoevasive aspect of HIP-edited cells still needs to be validated under autoimmune experimental conditions. Overall, the study by Hu et al is the first piece of evidence showing that genetically engineered allogeneic islet transplantation can achieve long-term graft survival and insulin independence in the absence of immunosuppression, providing proof of concept for upcoming clinical trials.