XENOTRANSPLANTATION OF PORCINE AND BOVINE ISLETS WITHOUT IMMUNOSUPPRESSION USING UNCOATED ALGINATE MICROSPHERES

Uncoated spherical hydrogel microspheres (calcium alginate, nominal M( r) exclusion of >600 kD) 800-900 mu m in diameter were employed to pre vent immune rejection of discordant islet xenografts isolated from pig s and cows. The islets were immobilized in the microspheres and inject ed into the peritoneum of 14 non-immunosuppressed streptozotocin (STZ) -induced diabetic C57BL/6J mice. Four recipients received islet grafts from bovine calves, and 10 received islet grafts from pigs. In the co ntrol group of 15 diabetic mice implanted with nonencapsulated islets, 6 received i.p. porcine islets and 5 received i.p. bovine islets, whe reas the remaining 4 received porcine islets under the kidney capsule. Plasma glucose concentrations in recipients of the alginate-encapsula ted islets promptly dropped from a preimplantation value of 498+/-47 ( mean +/- SEM) to 142+/-6 (bovine) and 178+/-7 mg/dl (porcine) during t he first wk. All the animals sustained these levels for at least 1 mo. Two mice implanted with bovine islets subsequently reverted to diabet es (plasma glucose >250 mg/dl) at 43 days postimplantation. The remain ing grafts maintained function for >10 wk. In contrast, nonencapsulate d islets failed to function, or sustained euglycemia for <4 days. Mice receiving encapsulated islets showed a 23-38% gain in body weight dur ing the first mo after implantation, compared with <1% (P<0.002) and 3 2% (P=0.84) for the untreated diabetic (n=6) and normal control (n=6) groups. Immunohistochemical staining of long-term grafts (>10 wk) reve aled viable islets, with well-granulated alpha, beta, and delta cells; the external surfaces of the microreactors were free of fibrotic over growth and exhibited only occasional host cell adherence. Uptake studi es with IgG and thyroglobulin (M(r) of 669 kD) suggest that the micror eactors were permeable to molecules with a molecular weight of up to > 600 kD (including the various proteins of the complement system, M(r) of 24-570 kD). Spheres implanted in the peritoneum after only 1 wk sta ined positive for both IgG and for the C3 component of complement. The se findings suggest that prolonged survival of discordant xenografts o f porcine and bovine islets in the STZ diabetic mouse model can be ach ieved with uncoated alginate microspheres that are permeable to IgG an d complement, The question of whether similar results can be achieved with uncoated alginate microspheres in higher animals remains to be fu lly determined.(1)