IL-6 prevents T cell-mediated hepatitis via inhibition of NKT cells in CD4+ T cell- and STAT3-dependent manners.

The hepatoprotective effect of IL-6 on various forms of liver injury including T cell-mediated hepatitis has been well documented, and it is believed that induction of antiapoptotic proteins is an important mechanism. In this study, we provide evidence suggesting an additional mechanism involved in the protective role of IL-6 in T cell-mediated hepatitis. In NKT cell-depleted mice, Con A-induced liver injury is diminished; this can be restored by the adoptive transfer of liver mononuclear cells or NKT cells from wild-type mice, but not from IL-6-treated mice. In vitro IL-6 treatment inhibits the ability of mononuclear cells to restore Con A-induced liver injury in NKT-depleted mice, whereas the same treatment does not inhibit purified NKT cells from restoring the injury. The addition of CD3(+) T cells or CD4(+) T cells can restore the inhibitory effect of IL-6 on purified NKT cells, whereas the addition of CD3(+) T cells from CD4-deficient mice fails to restore this inhibitory effect. The expression of IL-6R was detected in 52.6% of hepatic CD3(+) T cells and 32.7% of hepatic CD4(+) T cells, but only in 3.9% of hepatic NK and 1.5% of hepatic NKT cells. Finally, treatment with IL-6 induces STAT3 activation in hepatic lymphocytes and hepatic T cells, and blocking such activation abolishes the inhibitory effect of IL-6 on hepatic lymphocytes to restore liver injury. Taken together, these findings suggest that in addition to its antiapoptotic abilities, as previously well documented, IL-6/STAT3 inhibits NKT cells via targeting CD4(+) T cells and consequently prevents T cell-mediated hepatitis.