Title:
Splenic stroma-educated regulatory dendritic cells induce apoptosis of activated CD4 T cells via Fas ligand-enhanced IFN-γ and nitric oxide.

Abstract:
Stromal microenvironments of bone marrow, lymph nodes, and spleen have been shown to be able to regulate immune cell differentiation and function. Our previous studies demonstrate that splenic stroma could drive mature dendritic cells (DC) to further proliferate and differentiate into regulatory DC subset that could inhibit T cell response via NO. However, how splenic stroma-educated regulatory DC release NO and whether other molecules are involved in the suppression of T cell response remain unclear. In this study, we show that splenic stroma educates regulatory DC to express high level of Fas ligand (FasL) by TGF-β via ERK activation. The findings, that inhibition of CD4 T cell proliferation by regulatory DC required cell-to-cell contact and FasL deficiency impaired inhibitory effect of regulatory DC, indicate that regulatory DC inhibit CD4 T cell proliferation via FasL. Then, regulatory DC have been found to be able to induce apoptosis of activated CD4 T cells via FasL in caspase 8- and caspase 3-dependent manner. Interestingly, FasL on regulatory DC enhanced IFN-γ production from activated CD4 T cells, and in turn T cell-derived IFN-γ induced NO production from regulatory DC, working jointly to induce apoptosis of activated CD4 T cells. Blockade of IFN-γ and NO could reduce the apoptosis induction. Therefore, our results demonstrated that splenic stroma-educated regulatory DC induced T cell apoptosis via FasL-enhanced T cell IFN-γ and DC NO production, thus outlining a new way for negative regulation of T cell responses and maintenance of immune homeostasis by regulatory DC and splenic stromal microenvironment.