The differentiation of natural killer (NK) cells is regulated by various factors including soluble growth factors and transcription factors. Here, we have demonstrated that tumor necrosis factor-alpha (TNF-alpha) is a positive regulator of NK cell differentiation. TNF-alpha augmented the IL-15-induced expression of NK1.1 and CD122 in mature NK cells, and TNF-alpha alone also induced NK cell maturation as well as IL-15. TNF-alpha also increased IFN-gamma production in NK cells in the presence of IL-15. Meanwhile, mRNA expression of several transcription factors, including T-bet and GATA-3, was increased by the addition of TNF-alpha and IL-15. In addition, TNF-alpha increased nuclear factor-kappa B (NF-kappa B) activity in NK cells and inhibition of NF-kappa B impeded TNF-alpha-enhanced NK cell maturation. Overall, these data suggest that TNF-alpha significantly increased IL-15-driven NK cell differentiation by increasing the expression of transcript ion factors that play crucial roles in NK cell maturation and inducing the NF-kappa B activity. (C) 2009 Elsevier Inc. Ail rights reserved.