Minimum number of islets required to maintain euglycemia and their reduced immunogenicity after transplantation into diabetic mice.

In streptozocin (SZ)-induced diabetic mice, 200 islets, but not 50 islets, consistently restore euglycemia within 1 week of transplantation. To determine the minimum number of islets sufficient to maintain euglycemia in a diabetic mouse, we first transplanted 50 and 150 syngeneic islets simultaneously into the right (RK) and left kidney (LK), respectively, and then removed the LK 1 week post-transplantation. The remaining 50 islets maintained euglycemia in 8 of 11 mice with normal intravenous glucose tolerance tests (IVGTT). Protection of 50 islets for at least 7 days was necessary because removal of the 150 islets at 5 or 3 days resulted in a much lower incidence of persistent euglycemia. Similarly, 25 islets were capable of maintaining euglycemia in 2 of 9 mice once hyperglycemia was reversed by split-transplantation of 25 (RK) and 175 (LK) islets. To examine if 50-islet allografts survive longer than 200-islet allografts, we split-transplanted 50 DBA/2 islets in the RK and 150 islets of either B6 (syngeneic), DBA/2 (allogeneic), or C3H/He (third party allogeneic) mouse origin in the LK in 3 groups of diabetic C57BL/6 (B6) mice. The survival of 50 DBA/2 islets in each group after removal of the LK on day 7 was compared to that of 200 DBA/2 islets in control B6 mice. Maximum prolongation of allograft survival was obtained with 50 DBA/2 islets that were split-transplanted with syngeneic B6 islets. These results clearly demonstrate that 50 islets are sufficient to maintain normal glucose tolerance once euglycemia is induced by transplantation of a larger number (i.e., 200) of islets and that 50 islet allografts are much less immunogenic than 200-islet allografts.