Objective      To determine the effect of persistent over-expression of IL-15 in the liver on immune system.       Methods      Recombinant plasmid pLIVE-IL-15 encoding IL-15 was constructed

and the plasmid of 10 μg was then delivered into the liver of C57BL/6 mice by hydrodynamic tail vein injection. Serum IL-15 was detected by ELISA in 1 and 3 d

and 1

2

3 and 4 weeks after injection. Spleen and liver lymphocytes were analyzed by flow cytometry.       Results      p-LIVE-IL-15 plasmid was successfully constructed

and hydrodynami

c injection of the plasmid pLIVE-IL-15 resulted in a persistent expression of IL-15 in the serum

reaching the peak in 1 week and keeping in a high level till in 4 weeks after injection. Compared with pLIVE-EGFP group

pLIVE-IL-15 treatment significantly increased the numbers of innate immune cells (NK cells and NKT cells) and acquired immune cells (CD4

+

T cells and CD8

+

T cells) significantly （

P

0.05）. In vivo over-expression of IL-15 enhanced the expression of CD69 and IFN-γ in NK cells

but had no effect on the activation status of NKT cells. IL-15 dramatically stimulated the production of IFN-γ by CD8

+

T cells（

P

0.05）

and mildly affected the intracellular expression of IFN-γ by CD4

+

T cells.       Conclusion      Hydrodynamic injection of the plasmid pLIVE-IL-15 results in a persistent and high-level expression of IL-15 in mouse serum

and modulation in the immune responses of innate and acquired immune cells.