S3). This suggests that modulation of DCs by B10 cells observed in other tissue compartments [17] does not occur in the liver. Having demonstrated that hepatic B cells comprise fewer regulatory subsets than splenic B cells, a question not addressed in this study is why Bregs appear not to contribute to the overall tolerogenic liver environment. One possibility may be to prevent overinhibition of immune responses in the liver. As shown in this report and by others [15-17], the TLR-4 ligand LPS, a normal constituent of portal venous blood, is a potent stimulator of B10 cells. The absence of B10 cells and the presence of B cells with proinflammatory potential in an overall tolerogenic liver environment

could help to balance the hepatic capacities of immune tolerance and immune stimulation. Our data presented here show that the absence of hepatic B cells compromises further the capacity of mDCs to respond to LPS (Fig. 3). To obtain sufficient numbers of liver Wnt inhibitor mDCs for

analysis, Flt3L-treated mice were used in Fig. 3 and Supplementary Figs S2 and S3. We are aware of the caveat that Flt3L might modify the composition of mDC subsets as well as other cells. Extended experiments using animal models are find more needed to confirm the positive regulation of liver mDCs and liver immune responses by hepatic B cells. Future research to understand more clearly the mechanisms underlying hepatic B cell activation and function is merited, and may lead to improved understanding and therapy of different liver-related pathological conditions. The authors thank Dr David Rothstein for the gift of IL-10 reporter mice and Thomson laboratory members for helpful discussion. The work was supported by NIH grant P01AI81678 (A.W.T.), SPTLC1 grant (874279717) from the Roche Organ Transplantation Research Foundation (A.W.T.) and by an American Society of Transplantation Basic Science Fellowship awarded to Hong Zhang. Hong Zhang did most of the experiments and wrote the manuscript, Donna

Beer Stoltz performed immunofluorescence, Geetha Chalasani provided direction for B cell subset analysis and Angus W. Thomson provided intellectual input and guided the preparation of the manuscript. The authors declare no financial or commercial conflicts of interest. Fig. S1. Expression of cell surface activation markers on murine B cells following in-vivo poly I:C administration. C57BL/6 (B6) mice were injected intraperitoneally (i.p.) with lipopolysaccharide (LPS). On days 0 and 1 post-injection, the mice were examined for the expression of the indicated surface molecules on spleen versus hepatic B cells; n = 4 mice per group. On day 1, both liver and splenic B cells up-regulated expression of CD39, CD40, CD80 and CD86; *P < 0·05. No significant difference was observed between the liver and spleen. Data are representative of two independent experiments. Fig. S2. Close proximity of B cells (CD19+) and dendritic cells (DCs) (CD11c+) in liver parenchyma.