Type 1 diabetes (T1D) is caused by islet autoimmunity followed by immune destruction of the β-cells. In 2015 the International Diabetes Federation reported that 36 million people suffer from T1D globally, while it is estimated that 1.4 millions of Americans have T1D. Although life expectancy of patients with T1D has much improved since the introduction of insulin therapy, chronic complications, including blindness and renal failure, are hampering the quality of life and represent a multi-billion dollar annual burden on the health care system of industrialized countries. Keeping blood glucose levels under tight control represents the most effective way either to prevent the onset or to reduce the progression of the chronic complications of T1D. At present, such a goal may be accomplished by treating patients with intensified therapy regimens consisting of multiple insulin injections, which involve accurate blood glucose monitoring. However, administration of subcutaneous insulin can never approximate pulsatile insulin secretory patterns of the normal β-cells, and rarely attains normal blood glucose levels without the risk of major hypoglycemic episodes. In addition, intensive insulin therapy is only suitable for selected patients.

Pancreas transplantation is an alternative therapeutic modality which can stop the progression of diabetic complications without increasing the incidence of hypoglycemic events. Unfortunately, this procedure, usually performed simultaneously with a kidney graft, has a high morbidity and a significant mortality rate. Pancreas transplantation, in spite of an important impact on the quality of life in successful cases, is often restricted to selected patients. In this context, islet transplantation offers and alternative treatment solution, normalizing glucose metabolism without the risk of hypoglycemia and avoiding the potentially life threatening complications of whole pancreas grafts.

Clinical islet transplantation has continuously advanced over the past two decades, with clear improvements in islet manufacturing and clinical outcomes, therefore restore insulin production and ameliorate glycemic instability in patients with T1D. Currently, the procedure is primarily indicated for patients with a history of life threatening severe hypoglycemia and hypoglycemia unawareness for which islet transplantation has been highly effective both in the short and long terms. According to the most recent public presentation from the collaborative islet transplant registry (CITR), 1055 allogeneic islet transplantations have been reported by 50 islet transplantation centers in North America, Europe, Australia, and South Korea. Of these cases, islet transplant alone was the most frequent procedure (n=858) followed by islet after kidney (IAK) and simultaneous islet and kidney transplantation (SIK) (n=197). CITR data has identified factors that predict the achievement and maintenance of insulin independence as recipient age over 35 years, more than half a million infused islet equivalent (IEQ), islet glucose stimulation index \>1.5, induction therapy with Tcell depletion, and TNF-α inhibitor and maintenance with calcineurin inhibitor and mTOR inhibition. The combination of these factors in 60 recipients resulted in stable insulin independence after 5 years in 60 % of the patients. Recipient age, IEQ, and calcineurin inhibitor (CNI) maintenance were also predictive of positive C-peptide levels (≥0.3 ng/ml; n=308) and HbA1c (\<6.5 % or drop ≥2 %; n=530) and age and IEQ predicted absence of severe hypoglycemic events (SHE) (\>90 % of patients at 5 years). As another indicator of improvements in the procedure, the number of adverse events has dropped significantly in the past 5 years, with 80 % free of any adverse events.