The short in vivo half-life of IFN-gamma can prevent the cytokine from inducing immunological changes that are favorable for the treatment of Th2-dominant diseases, such as atopic dermatitis. To examine whether a sustained supply of IFN-gamma is effective in regulating the balance of Th lymphocyte subpopulations, plasmid vector encoding mouse IFN-gamma, pCpG-Mugamma, or pCMV-Mugamma was injected into the tail vein of NC/Nga mice, a model for human atopic dermatitis. A single hydrodynamic injection of a CpG motif reduced pCpG-Mugamma at a dose of 0.14 microg/mouse resulted in a sustained concentration of IFN-gamma in the serum, and the concentration was maintained at >300 pg/ml over 80 d. The pCpG-Mugamma-mediated IFN-gamma gene transfer was associated with an increase in the serum concentration of IL-12, reduced production of IgE, and inhibition of mRNA expression of IL-4, -5, -10, -13, and -17 and thymus and activation-regulated chemokine in the spleen. These immunological changes were not clearly observed in mice receiving two injections of 20 microg pCMV-Mugamma, a CpG-replete plasmid DNA, because of the transient nature of the expression from the vector. The mice receiving pCpG-Mugamma showed a significant reduction in the severity of skin lesions and in the intensity of their scratching behavior. Furthermore, high transepidermal water loss, epidermal thickening, and infiltration of lymphocytes and eosinophils, all of which were obvious in the untreated mice, were significantly inhibited. These results indicate that an extraordinary sustained IFN-gamma expression induces favorable immunological changes, leading to a Th1-dominant state in the atopic dermatitis model.