Our initial attempts to detect human PBMCs in blood or any organ in transplanted mice failed even after injecting 2 × 107 cells, which is sufficient to establish human PBMC chimerism in SCID mice.27 We assumed that failure to develop chimerism was the result of the activity of NK cells and macrophages because the activity of these cells in uPA-SCID mice

is higher than in SCID mice.28, 29 Therefore, we attempted to eliminate these effects by administering clodronate and anti–asialo GM1 antibody, which are known to effectively eliminate these cells.30, 31 This assumption appears to be valid, because we were able to establish human PBMC chimerism and massive hepatocyte degeneration by suppressing these cells (Fig. 1). HBV-specific CTLs have been reported H 89 to play an important role in eliminating the virus.32-34 Accordingly, we attempted to detect HBV-specific CTLs in mice with massive hepatocyte

degeneration. Unexpectedly, we failed to detect HBV-specific CTLs (Fig. 2A and Supporting Fig. 9) and instead found that infiltrating cells in the liver were CD3-negative NK cells (Fig. 2B,D and Supporting Fig. 10). The reason for the absence of CTLs in our experiment is unknown, but this suggests that massive hepatocyte degeneration resembling fulminant hepatitis can be caused by NK cells as a main player, and recent reports demonstrating that NK cells contribute to severe acute and chronic hepatitis B (CHB) support this assertion.11, 35 We attempted to collect Enzalutamide supplier Bay 11-7085 CTLs from HBV-infected patients and to establish hepatitis in chimeric mice. However, we rarely detected tetramer-positive CTLs in blood samples from chronically infected patients and were therefore unable to establish hepatitis using CD8-positive T cells. Consequently, a limitation of this study is that differential roles of NK cells and CTLs in massive liver cell death could not be examined. Although it is not clear in this study how profoundly DC and NK cell activity plays a role in patients with FHB, our results suggest that the immune system can trigger severe hepatocyte degeneration. The importance of the activation of NK cells

by DCs was evident, because depletion of DCs almost completely abolished the massive hepatocyte degeneration in this model (Supporting Fig. 10; Table 1). The interaction between NK cells and DCs is not well characterized, although it has been established that antigen-presenting accessory cells provide both indirect (i.e., soluble) and direct (i.e., contact-dependent) signals to T cells. Experiments in which NK cells are separated from pathogens and antigen-presenting cells by semipermeable membranes are cultured with supernatants from pathogen-activated DCs or in which cytokines are neutralized with blocking antibodies. These reports indicate that both soluble and contact-dependent signals may contribute to the activation of NK cells.