Deficiency of Interleukin-15 Enhances Susceptibility to Acetaminophen-Induced Liver Injury in Mice Hsein-San Hou 1 , Ching-Len Liao1,2, Huey-Kang Sytwu1,2, Nan-Shih Liao1,3, Tien-Yu Huang4, Tsai- Yuan Hsieh4, Heng-Cheng Chu1,4* 1 Graduate Institute of Life Sciences, National Defense Medical Center, Taipei, Taiwan, 2 Department of Microbiology and Immunology, National Defense Medical Center, Taipei, Taiwan, 3 Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan, 4 Division of Gastroenterology and Hepatology, Department of Internal Medicine, Tri- Service General Hospital, National Defense Medical Center, Taipei, Taiwan Abstract Hepatocytes have a direct necrotic role in acetaminophen (APAP)-induced liver injury (AILI), prolonged secondary inflammatory response through innate immune cells and cytokines also significantly contributes to APAP hepatotoxicity. Interleukin 15 (IL-15), a multifunction cytokine, regulates the adaptive immune system and influences development and function of innate immune cells. To better understand the role of IL-15 in liver injury, we treated wild-type (WT) and IL-15- knockout (Il152/2) mice with a hepatotoxic dose of APAP to induce AILI and evaluated animal survival, liver damage, APAP metabolism in livers and the inflammatory response. Production of pro-inflammatory cytokines/chemokines was greater in Il152/2 than WT mice. Subanalysis of hepatic infiltrated monocytes revealed greater neutrophil influx, along with greater hepatic induction of inducible nitric oxide synthase (iNOS), in Il152/2 than WT mice. In addition, the level of hepatic hemeoxygenase 1 (HO-1) was partially suppressed in Il152/2 mice, but not in WT mice. Interestingly, elimination of Kupffer cells and neutrophils did not alter the vulnerability to excess APAP in Il152/2 mice. However, injection of galactosamine, a hepatic transcription inhibitor, significantly reduced the increased APAP sensitivity in Il152/2 mice but had minor effect on WT mice. We demonstrated that deficiency of IL-15 increased mouse susceptibility to AILI. Moreover, Kupffer cell might affect APAP hepatotoxicity through IL-15. Citation: Hou H-S, Liao C-L, Sytwu H-K, Liao N-S, Huang T-Y, et al. (2012) Deficiency of Interleukin-15 Enhances Susceptibility to Acetaminophen-Induced Liver Injury in Mice. PLoS ONE 7(9): e44880. doi:10.1371/journal.pone.0044880 Editor: Partha Mukhopadhyay, National Institutes of Health, United States of America Received April 4, 2012; Accepted August 9, 2012; Published September 18, 2012 Copyright: 2012 Hou et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study was supported in part by the National Science Council (NSC94-2314-B-016-047), Tri-Service General Hospital, National Defense Medical Center (TSGH-C97-44, TSGH-C98-48 and TSGH-C99-61) and the Department of Defense, Taiwan (P98-25 and I-3). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: dr.chu0803@gmail.com Introduction Acetaminophen (APAP) is an over-the-counter analgesic widely used worldwide. However, APAP-induced liver injury (AILI) represents the most common hepatogenous poisoning secondary to drug overdose. Excess APAP saturates the sulfation and glucuronidation of the metabolic pathway and results in genera- tion of toxic N-acetyl-p-benzoquinone imine (NAPQI) by cyto- chrome P450 (CYP) [1], thereby depleting hepatic glutathione (GSH) [2]. Residual unconjugated NAPQI induces covalent binding of intracellular proteins and causes further formation of reactive oxygen species (ROS) [3], thus resulting in apoptosis and necrosis of hepatocytes [4]. Induction of intracellular inflammation regulatory proteins such as hemeoxygenase 1 (HO-1) attenuates APAP toxicity [5]. In addition, the downstream innate immune response, by immune cells and associated cytokines, modulates the progression of liver injury [6]. Innate immune cells such as natural killer (NK) cells, natural killer T (NKT) cells [7], neutrophils [8], dendritic cells (DCs) [9], and Kupffer cells (KCs) [10,11] play important roles in AILI. Depletion of NK and NKT cells by an antibody retarded APAP toxicity in mouse liver [7]. However, Masson et al. showed an indefinite role of NK and NKT cells in AILI [12]. The uncertain role of neutrophils in AILI was shown in different studies [8,13]. Recently, increased APAP sensitivity was attributed to enhanced inflammation in mice lacking DCs, but the detailed mechanism remained speculative [9]. Depletion or inactivation of KCs by chemicals in an AILI model had controversial results, with a protective effect in one study [11] but a negative result in another [10]. Furthermore, mice lacking of cytokines such as interleukin 10 (IL-10) [14], IL-6 [15] or IL-13 [16] were found susceptible to APAP hepatotoxicity, whereas induction of pro-inflammatory mediators such as tumor necrosis factor alpha (TNFa) [17], interferon gamma (IFNc) [18], IL-18 or IL-1b [19] and nitric oxide (NO) enhanced AILI in mice. Collectively, the roles of innate immune cells, especially antigen-presenting cells, and cytokines in AILI are complicated and still unclear. IL-15, a multifunction cytokine mainly produced by antigen- presenting cells such as macrophages, DCs, B cells or endothelial cells, regulates the adaptive immune system and plays an important role in innate immunity [20,21]. IL-15 can direct the development of CD8+ memory T cells, NK and NKT cells [20] and modulate the function of macrophages and DCs [22]. In addition, IL-15 can inhibit apoptosis of neutrophils [23] and regulate the production of inflammatory cytokines such as TNFa, IL-6, IL-1b and IL-10 in macrophages in response to lipopoly- saccharide stimulation [24]. Synthetic IL-15 could moderately PLOS ONE | www.plosone.org 1 September 2012 | Volume 7 | Issue 9 | e44880