Effect of IL-15 on APAP Hepatitis Figure 9. Effects of exogenous IL-15 or IL-15 neutralizing antibody administration on AILI in Il152/2 or WT mice. Before APAP injection, twenty micrograms of recombinant murine IL-15 were subcutaneously (SC) or intraperatoneally (IP) injected into Il152/2 mice. (A) Serum ALT levels and (B) representative liver histopathological changes at 8 hr after APAP injection (H&E staining; magnification 64). One hundred and twenty micrograms of Rat anti-IL-15 neutralizing antibody or Rat IgG2a isotype control antibody were intraperitoneally injected into WT mice. (C) Serum ALT levels and (D) representative liver histopathological changes at 8 hr after APAP injection (H&E staining; magnification 64). n.s., not significant. Data represent means 6 SEM from 5,8 mice per group, and 3 mice in IL-15 SC and Rat anti-IL-15 neutralizing antibody only group. doi:10.1371/journal.pone.0044880.g009 HSPs [39], whereas enhanced hepatic inflammation was found in APAP-injected Il152/2 mice, with decreased HSP induction (Figure 5, 7, S3). Additionally, elimination of KCs by GdCl3 did not alter the severity of AILI in Il152/2 mice, but increased the APAP sensitivity of WT mice, leading to a similar susceptibility in both groups of mice (Figure 10B). Moreover, pretreatment of WT mice with GdCl3 diminished serum IL-15 elevation during AILI (Figure 10C) and showed a negative correlation between serum IL- 15 level and liver damage extent (Figure 10D). Furthermore, with GdCl3 pretreatment, hepatic TNFa, IL-1b and nitrotyrosine levels were up-regulated in WT mice, indicating the inflammation prevention by KCs in our model (Figure 10E,F). However, depletion of KCs did not affect the enhanced hepatic inflamma- tion in Il152/2 mice after APAP challenge (Figure 10E,F). These findings suggested the probable upstream and downstream relationship of KCs and IL-15 in the protection against AILI. IL-15 had been found to determine the activation level of macrophages in mouse rheumatoid arthritis model [48]. In addition, IL-15, at extremely low concentration, selectively suppresses pro- (e.g., TNFa, IL-6 and IL-1) but not anti- inflammatory (e.g., IL-10) cytokine production in LPS-activated macrophage [24]. Thus, IL-15 might potentially influence KC function and thereby modulate the inflammatory response during AILI. There were two isoforms of IL-15, short and long signal peptide isoforms, existing in human and murine cells [49,50,51,52]. After translation, the short (intracellular) IL-15 isoform was not secreted but rather distributed within the cytoplasm and the nucleus [52]. Although the biological function of intracellular IL-15 had not been fully elucidated in KCs, it could function as a histone deacetylase inhibitor regulating the expression of IL-12 receptor b1 in macrophage [53]. Moreover, intracellular IL-15 could indirectly regulate mMCP-2 induction by altering the expressions of transcription factors C/EBPb and YY1 in mouse mast cells [54]. Therefore, based on previous reports and our results, IL-15 might function uniquely within KCs in AILI. In previous studies, IL-15 was found to enhance severity of endotoxin shock liver injury [27], colitis [55] and virus infection [56]. In contrast, IL-15 protected against nephrotoxic serum nephritis [57] and experimental autoimmune encephalomyelitis [58]. Increased serum IL-15 level was also noted in patients with liver transplantation [59] or hepatitis C virus infection [60]. Here, we demonstrated an enhanced APAP susceptibility in Il152/2 mice, which might result from an overactive inflammatory response. In view of these results, further studies might be justified to elucidate the biological function of intracellular IL-15 within KCs during AILI. PLOS ONE | www.plosone.org 11 September 2012 | Volume 7 | Issue 9 | e44880