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Objective To investigate the effects of adoptive reinfusion of regulatory T cell (Treg) on the recovery of islet function and graft survival time after islet allograft transplantation. Methods The diabetic model was established using C57BL/6 mice as recipients, and Balb/c mice were chosen as donors for islet allografts transplantation beneath the renal capsule. The recipient mice were divided into 3 groups and 3 mice in each group according to different processing Methods: Treg experiment group (Treg group, 1×106 Treg cells were injected via tail vein at 1 d before operation), positive control group [sirolimus (SRL) group, SRL at a dose of 300 μg/(kg·d) was intragastrically given every day from 1 d before operation] and blank control group (control group, an equivalent volume of normal saline was intragastrically given every day from 1 d before operation). Enzyme-linked immune absorbent assay (ELISA) was used to detect the changes of blood glucose and C-peptide in mice within 14 days after transplantation. In vivo imaging technique was used to dynamically monitor the survival of mice within 14 days after transplantation. Results In each group, the blood glucose levels at postoperative 3 d were significantly decreased compared with those before transplantation (all P < 0.001). At postoperative 1 d, the C-peptide levels showed an explosive rise to varying degree in each group. At postoperative 3 d, the C-peptide levels in each group were significantly higher than that before operation (all P < 0.001). At the end of the observation period at 14 d after operation, the C-peptide levels in the SRL and Treg groups were (427±50) pmol/L and (833±57) pmol/L, relatively higher than that in the control group. But the blood glucose levels were (14.5±0.5) mmol/L and (12.1±0.6) mmol/L, significantly lower than that in the control group (all P < 0.001). Compared with the SRL group, the explosive release amount of C-peptide was significantly lower, the declining trend was more remarkably stable, and the C-peptide level was considerably higher in the Treg group at the end of the observation period (all P < 0.001). At postoperative 14 d, the grafts were almost completely apoptotic in the control group, over 50% of the grafts survived in the SRL group, and over 80% of the grafts survived in the Treg group. Conclusions Adoptive reinfusion of Treg cells can effectively protect islet grafts, prolong the survival time of grafts, and maintain the normal levels of blood glucose and C-peptide in the recipient mice.