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In order to study the effect of cell mediated immunity regulation of duck IFN-α eukaryon expression plasmid (pcDNA-SDIFN-α) on duck plague virus (DPV)attenuated vaccine in ducks,pcDNA-SDIFN-α was administered to 28-day-old ducks at doses of 1,3 and 6 μg per duck,respectively,by gene-gun.PBS and empty vector pcDNA were used as control.Fifteen days later,all ducks were injected with DPV attenuated vaccine and blood samples were collected at 3,7,14,21,28,35,49,63 and 84 days after injection.T-lymphocyte proliferation tests (MTT) were used to detect the T-lymphocyte proliferation in the peripheral blood (PBL) of ducks.Blood samples collected at 7,14,21,28,35 and 49 days after injection were detected by fluorescence-activated cell sorter (FACS) for recording the number of CD3+ T-lymphocytes of ducks.Results were as follows:(1) Reaction of T-lymphocytes in PBL to ConA (OD value) of ducks treated with pcDNA-SDIFN-α was higher than that of PBS and pcDNA control groups in 3-84 days.There were highly significant differences between the 1 μg per duck group and the two control groups in 3-84 days (P ≤ 0.01),between the 3 μg per duck group and the two control groups in 3-84 days (P ≤ 0.01,P ≤ 0.05),and between the 6 μg per duck group and the two control groups in 7-49 days (P ≤ 0.01,P ≤ 0.05).The significant difference was also present between the groups of 1,3 and 6 μg per duck in 3-35 days (P ≤ 0.05).However,there was no significant difference between the 3 and 6 μg per duck groups (P ≥ 0.05).The pcDNA control group was higher than PBS control group,but no difference was detected (P ≥ 0.05).(2) Change of the number of CD3+ T-lymphocytes in ducks administered with different doses of pcDNA-SDIFN-α was higher than that of PBS and pcDNA control groups in 7-49 days.The change in the 1 μg per duck group was significantly higher than that in PBS and pcDNA control groups in 14-49 days (P ≤ 0.01).There were significant differences between the 3 μg per duck group and the two control groups in 21-49 days (P ≤ 0.01,P ≤ 0.05)and between the 6 μg per duck group and the two control groups in 7-49 days (P ≤ 0.01,P ≤ 0.05).However,no significant differences among the groups of 1,3,and 6 μg per duck groups (P≥ 0.05) and between the two control groups (P≥ 0.05) were found.The results indicated that pcDNA-SDIFN-α administered 15 days before injection of DPV-attenuated vaccine could significantly enhance cellular immunity induced by DPV-attenuated vaccine.pcDNA-SDIFN-α is an excellent DPV-attenuated vaccine molecular adjuvant and the best result can be obtained with the dose of 1 μg per duck of pcDNA-SDIFN-α inoculated by gene-gun.