Background OX40/OX40 ligand (OX40/OX40L) and programmed death-1/programmed death ligand-1 (PD-1/PD-L1) costimulatory signals play important roles in T cell-induced immune responses.The aim of this study was to investigate the roles of OX40/OX40L and PD-1/PD-L1 costimulatory pathways in mouse islet allograft rejection.Methods Lentiviral vectors containing OX40L siRNA sequences and an adenovirus vector containing the PD-L1 gene were constructed.The streptozotocin-induced model of diabetes was established in C57BL/6 (H-2b) mice.Diabetic C57BL/6 mice were randomly allocated into five groups:group 1,untreated control; group 2,Ad-EGFP treatment; group 3,Ad-PD-L1 treatment; group 4,OX40L-RNAi-LV treatment; group 5,OX40L-RNAi-LV combined with Ad-PD-L1 treatment.Lentiviral vector and the adenovirus vector were injected,singly or combined,into the caudal vein one day before islet transplantation.The islets of DBA/2 (H-2d) mice were transplanted into the renal subcapsular space of the diabetic recipients.Recipient blood glucose and the survival time of the allografts were monitored.Antigen-specific mixed lymphocyte reaction was also evaluated.Results The recombinant lentiviral RNA interference vector OX40L-RNAi-LV reduced OX40L protein expression by 70％.The recombinant adenovirus vector Ad-PD-L1 increased PD-L1 protein expression in vivo in C57BL/6 recipient mice.Combined OX40L-RNAi-LV/Ad-PD-L1 treatment induced a synergistic protective effect in pancreatic islet allografts.Allograft survival time in the combined treatment group was (92.27±9.65) days,not only longer than that of the control ((6.51±0.27) days) and Ad-EGFP groups ((7.09±0.13) days) (P ＜0.01),but also significantly longer than that of Ad-PD-L1 and OX40L-RNAi-LV single treatment groups ((40.64±3.95) days and (55.14±5.48) days respectively,P ＜0.01).The blood glucose concentration of recipient mice in the combined treatment group was also stable and kept within the normal range.Flow cytometry analysis showed that combined OX40L-RNAi-LV/Ad-PD-L1 treatment significantly decreased proliferation in an antigen-specific mixed lymphocyte reaction.After donor DBA/2 lymphocyte stimulation,89.71％ of lymphocytes from recipient combination treatment C57BL/6 mice were not split and proliferated.In contrast,after stimulation with third party Lewis rat lymphocytes,only 45.84％ lymphocytes of C57BL/6 mice were not split and proliferated.Conclusions This study demonstrates the successful construction of the recombinant lentivirus vector OX40L-RNAi-LV and adenovirus vector Ad-PD-L1 for the blockade of OX40/OX40L and activation of PD-1/PD-L1 costimulatory pathways simultaneously in pancreatic islet allografts in diabetic mice.Combination therapy with these two vectors resulted in inhibition of T cell activation,synergistically prolonging the survival time of pancreatic islet allografts.