Exogenous IL-15 in Combination With IL-15Rα Rescues Natural Killer Cells From Apoptosis Induced by Chronic Alcohol Consumption

NATURAL KILLER (NK) cells are a small population of lymphocytes that can kill virus-infected and tumor cells without prior immunization (Colucci et al., 2003; Yokoyama and Plougastel, 2003). By producing cytokines and chemokines and interacting with dendritic cells, NK cells also regulate T cell function (Zingoni et al., 2005). Thus, these cells play important roles in innate and adaptive immunity. More than 90% of NK cells in the periphery originate from the bone marrow (BM); however, a small population of NK cells are derived from the thymus in mice (Di Santo and Vosshenrich, 2006; Kim et al., 2002; Vosshenrich et al., 2006) and the lymph nodes in humans (Farag and Caligiuri, 2006). Most of the peripheral NK cells are found in the spleen, blood, and liver. Like T and B cells, the numbers of peripheral NK cells are tightly controlled, and equilibrium is maintained by homeostatic proliferation. Although the precise mechanism of NK cell homeostatic proliferation is still not completely understood, there is convincing evidence that IL-15 plays an essential role in this process (Ranson et al., 2003b). IL-15 is essential for NK cell commitment, development, differentiation, maturation, homeostatic proliferation, and survival (Prlic et al., 2003; Ranson et al., 2003b; Vosshenrich et al., 2005). This cytokine is expressed by multiple tissues (Waldmann and Tagaya, 1999), and it activates responding cells via its receptor. The IL-15 receptor (R) is composed of 3 subunits: IL-15Rα, β, and γ chains. IL-2 and IL-15 receptors share the same β and γ chains, and differences in the α chain make the 2 cytokines distinct in their activation of target cells (Waldmann et al., 2001). IL-2 binds the receptor with all 3 chains combined together on the target cell to activate downstream signaling. IL-15 first binds IL-15Rα, which is then trans-presented to the IL-2R β/γ chains on the target cell to induce activation (Dubois et al., 2002). Presence of the α receptor on target cells is not required for IL-15 induced target cell activation. IL-15Rα is synthesized by IL-15-producing cells and primes target cells by binding to IL-15 (Koka et al., 2003, 2004; Sandau et al., 2004). Because most of the IL-15 is bound to the α receptor on the IL-15 producing cell, it is difficult to detect this cytokine in serum or in tissue fluid via ELISA (Waldmann and Tagaya, 1999). Experiments with IL-15Rα knockout mice showed that the NK cells from these mice survive and proliferate in IL-15Rα+/+ hosts; while, NK cells from IL-15Rα+/+ mice do not survive in IL-15Rα-/- hosts (Koka et al., 2003). This study indicates the importance of trans-presentation of IL-15 by IL-15Rα to NK cell survival and proliferation. This observation is underscored by the fact that exogenous administration of IL-15 has limited effects on NK cell proliferation, while administration of IL-15/IL-15Rα strongly boosts proliferation (Rubinstein et al., 2006; Stoklasek et al., 2006). Chronic alcohol consumption decreases peripheral NK cells in humans (Perney et al., 2003) and in mice (Pan et al., 2006; Zhang and Meadows, 2005, 2008), and the underlying mechanism is still largely unknown. We recently found that chronic alcohol consumption compromises NK cell release from the BM (Zhang and Meadows, 2008), and this could contribute to decreased numbers of peripheral NK cells. Lymphopenia associated with decreased NK cells will invoke homeostatic proliferation to return NK cells to steady state levels (Jamieson et al., 2004; Prlic et al., 2003). It is not known whether the reduced availability of NK cells from the BM is the stimulus that invokes splenic NK cell homeostatic proliferation or if other events contribute to the loss of peripheral NK cells. Little is known regarding the effects of alcohol on IL-15. Acute alcohol intake inhibits IL-15 mRNA expression (Pruett etal., 2003). The effect of acute and chronic alcohol consumption on IL-15 protein expression, however, especially IL-15 expression on IL-15 presenting cells, is not known. In this study, we found that chronic alcohol consumption results in a self-unrecoverable splenic NK cell depletion in mice. Although the reduction in NK cells induces homeostatic proliferation, chronic alcohol consumption also increases apoptosis of NK cells in the spleen and BM. This is associated with decreased numbers of IL-15+ cells in the spleen, but not in the BM. In vivo treatment with IL-15 enhances NK cell proliferation in the spleen, and the percentage and number of NK cells return to control levels in alcohol-consuming mice. IL-15 treatment increases the percentage and number of NK cells in the BM. This treatment does not abrogate enhanced NK cell apoptosis associated with chronic alcohol consumption in either the spleen or BM. Combined treatment with IL-15/IL-15Rα further increases NK cells in the spleen and BM and also rescues these cells from the enhanced apoptosis induced by chronic alcohol consumption.