This study examines whether the survival of allografted rat islet β-cells is influenced by the presence of other pancreatic donor cells. Grafts (RT1u/l) of different cellular composition were intraportally transplanted in streptozocin-induced diabetic rats (RT1n/n). All grafts corrected the diabetic state within 3 days. Implants of freshly isolated islets contained various endocrine and nonendocrine cell types; they became diffusely infiltrated within 1 wk and were completely destroyed within 2 wk. A 4-day culture period did not lead to major changes in the cellular composition of the islets or in their survival as allograft. Islet cell aggregates prepared after islet dissociation and cell purification were less acutely infiltrated and less rapidly rejected. Aggregates composed of sorted MHC class II–negative cells maintained basal normoglycemia in 3 of 5 recipients for 5 wk but only in 1 of 5 for 20 wk. Aggregates of purified islet β-cells remained relatively free of diffuse infiltrations during the 1st wk and preserved the normalized state in 7 of 13 recipients for 5 wk; after 20 wk, 6 of 13 were still aglucosuric, but 40% of the implants were diffusely infiltrated and depleted of insulin. Reaggregation of purified islet β-cells with purified islet endocrine non–²-cells promoted their long-term survival as allograft: 11 of 13 recipients of mixed islet endocrine cells maintained normal basal glycemia over 20 wk; their implants contained relatively constant insulin reserves and remained virtually devoid of diffuse infiltrations. These results demonstrate that techniques aiming at the elimination of surface MHC class II–positive cells are less successful in preparing rat islet allografts of low immunogenicity than methods of positive cell selection. Pure islet β-cells are immunogenic as an allograft but illicit a milder and less-acute immune attack than undissociated islet tissue. Nonendocrine and damaged islet cells are suspected of enhancing the rapidity and intensity of the cytotoxic reaction. Survival of allografted β-cells is markedly prolonged by the presence of islet endocrine non–²-cells within the graft. The mechanisms underlying this effect have not yet been elucidated; they may involve immune and metabolic interactions of the endocrine non–²-cells. We conclude that purification of islet endocrine cells represents a new and powerful method for preparing insulin-producing allografts that can survive in hosts without pharmacological immunosuppression.

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