The extrinsic pathway of apoptosis. Heat shock proteins are overproduced in many stressful conditions, including fasting. They are also involved in immune cell activation. In particular, extracellular HSP70 is involved in immune stimulation. HSP70 is expressed on the surface of some tumor cells and acts as a recognition structure for NK cells, promoting NK cell cytotoxicity. Furthermore, in some stressful situations, HSP70 is actively released in the extracellular space as a soluble protein or bound to exosomes to activate antigen-presenting cells or NK cells. It has also been shown that recombinant HSP70 can stimulate the proliferation and antitumor function of NK cells. Based on these studies, we hypothesized that acute starvation may lead to the GDC-0879 enhancement of NK cell activity against neoplastic cells by inducing the expression of HSP70. In this study, we show that both the proportion of TRAIL + NK cells and the expression of HSP70 were significantly elevated in the liver of fasted mice. Moreover, treatment of liver NK cells with recombinant HSP70 upregulated both TRAIL and CD69 expression, and neutralization of HSP70 in fasted mice by intraperitoneal injection of an anti-HSP70 monoclonal antibody downregulated TRAIL expression. Thus, our findings indicate that acute fasting enhances TRAIL-mediated liver NK cell activity against neoplastic cells through upregulating HSP70. Acute starvation is well known to induce physiological changes in the body. Consistent with previous studies, our study showed a decrease in body weight and liver weight as well as in the number of lymphocytes from various organs in fasted mice as compared to fed mice. Interestingly, we observed that, although the liver weight decreased proportionately with body weight, the lymphocyte number notably decreased under starvation. We previously reported that liver NK cells constitute a unique NK population characterized by high TRAIL expression and high production of perforin, granzymes, and cytokines and have the capacity to kill various kinds of cancer cells, virus-infected cells, or other transformed cells. The ligation of TRAIL with death receptor 4 or 5 on target cells induces NK cell activation. On the other hand, CD69, which is a type II transmembrane glycoprotein, is highly induced in many activated lymphocytes, in particular in NK cells. This study represents the first report showing that the proportion of liver-resident NK cells expressing TRAIL and CD69 is significantly higher in fasted mice than in fed mice. Liver NK cells from fasted mice have previously been demonstrated to have high antitumor activity. However, the mechanism underlying this activity has been entirely unknown. Our study indicates that liver lymphocytes from fasted mice showed high cytotoxicity against TRAIL-sensitive Hepa1-6 cells and related to the TRAIL-mediated apoptotic pathway.