As illustrated in Fig. 3F, IFN-γ treatment inhibited the find more expression of α-SMA and TGF-β1 in 2-week CCl4 mice but not in 10- or 12-week CCl4 mice. STAT1 was phosphorylated in isolated HSCs of the IFN-γ–treated 2-week group, but not in HSCs of the IFN-γ–treated

10- or 12-week groups. Finally, expression of SOCS1 protein, a negative regulator of STAT1,16 in HSCs was up-regulated in 2-week CCl4 mice after IFN-γ treatment. HSCs isolated from 10- or 12-week CCl4 mice had higher basal levels of SOCS1 protein than those from 2-week CCl4 mice, which were not further up-regulated after IFN-γ treatment (Fig. 3F). To further understand the underlying mechanism of suppressed NK cell function observed in advanced liver fibrosis, day 4 (D4) (early activated) or day 8 (D8) (intermediately activated) cultured HSCs were cocultured with liver NK cells for 24 hours. After coculturing with HSCs, IFN-γ

production by NK cells was significantly ICG-001 manufacturer increased in coculturing with D4 HSCs or with D8 HSCs. Higher levels of IFN-γ were observed when cocultured with D4 HSCs than those with D8 HSCs (Fig. 4A). Coculture studies of IFN-γ–deficient cells suggest that the source of IFN-γ production is from NK cells (Fig. 4B). Furthermore, incubation with NKG2D neutralizing antibody diminished IFN-γ production in the coculture experiments (Fig. 4C), suggesting that activated HSCs induce IFN-γ production by NK cells through an NKG2D-dependent mechanism. Expression of TGF-β protein was significantly higher in D8 HSCs compared with D4 HSCs (Fig. 4D). Because TGF-β is a potent inhibitor for NK cells,7, 17 we hypothesized that TGF-β1 produced by cocultured HSCs may inhibit IFN-γ production and cytotoxicity of NK

cells. As illustrated in Fig. 4E, incubation with TGF-β neutralizing antibody markedly enhanced NK cell cytotoxicity against D8 HSCs as well as D4 HSCs (albeit to a lesser extent). In addition, Thiamet G TGF-β antibody treatment increased IFN-γ production by NK cells when cocultured with D8 HSCs but did not affect IFN-γ production in coculture experiment with D4 HSCs (Fig. 4F). Furthermore, the addition of TGF-β1 ligand suppressed the cytotoxicity of NK cells against D4 and D8 HSCs (Supporting Fig. 4). Although IFN-γ–mediated STAT1 activation has been well documented in HSCs,6, 11, 12, 18 the aforementioned experiments show that IFN-γ activation of STAT1 in HSCs from livers with advanced liver fibrosis appears to be disrupted (Fig. 3F). To study the underlying mechanisms responsible for the disruption, IFN-γ–mediated inhibitory cell proliferation and activation of STAT1 were compared on D4 and D8 HSCs. As shown in Fig. 5A, IFN-γ treatment suppressed cell proliferation of D4 HSCs, but not D8 HSCs. Western blotting showed that IFN-γ induced STAT1 activation (phosphorylated STAT1) in D4 HSCs, but this activation was markedly attenuated in D8 HSCs (Fig. 5B and Supporting Fig. 5A).