g. resident DC). In support of our hypothesis that regulatory CD4+CD25+ T cell eliminate hapten-presenting DC through Fas–FasL interactions, the majority of FasL-expressing T cells were detected within the CD4+CD25+FoxP3+ cell population

while constitutive expression of FasL on CD4+CD25− T cells was at low to undetectable levels. Furthermore, hapten-bearing DC expressed higher levels of Fas than did hapten-negative DC. Finally, hapten-presenting DC experienced increased apoptosis during culture with CD4+CD25+ T cells than with CD4+CD25− T cells and this apoptosis was blocked by anti-FasL mAb. It is worth noting that even high concentrations of anti-FasL mAb (25 μg/mL) did not completely inhibit the DC apoptosis mediated by CD4+CD25+ T see more cells in vitro, suggesting that cytotoxic LY294002 chemical structure mechanisms other than Fas–FasL may also be involved. Human regulatory CD4+CD25+ T cells activated in vitro have been reported to utilize granzyme A and perforin-dependent cytotoxicity to kill autologous target cells, including both mature and immature DC 21. Negative regulation of effector T-cell expansion and CHS responses by FasL-mediated apoptosis of DC has been

suggested by several studies. First, the clearance of hapten-bearing DC is delayed in the LN of sensitized gld and lpr mice 22. Second, the LC-derived cell line XS52 is eliminated by agonist anti-Fas mAb or by CD4+ T cells through Fas–FasL engagement in vitro2. Third, regulatory T cells induced in CHS by UV irradiation require Fas–FasL to down-regulate CHS responses and to induce DC apoptosis during in vitro culture 23. Finally, studies from this laboratory have indicated that the unregulated expansion of hapten-specific CD8+ T cells and CHS responses in FasL-defective gld DNA ligase mice was down-regulated by adoptively transferred CD4+ T cells from WT mice 1. It is worth noting that we did not observe increased hapten-specific CD8+ T-cell development or CHS responses

in Fas-defective lpr mice when compared with WT animals (A. Gorbachev, unpublished observations). One possible explanation is that Fas–FasL interactions play a dual role in immune responses. While functions of APC are negatively regulated by Fas-induced apoptosis, FasL expressed by T cells may deliver co-stimulatory signals during CD8+ T-cell activation 24, 25. To dissect the influence of Fas/FasL on DC and T-cell functions in CHS responses, the effector CD8+ T-cell and CHS responses were compared in naïve mice that had received transferred hpLC from sensitized WT or lpr donors. Consistent with previous findings suggesting negative regulation of hpLC functions through Fas–FasL interactions 1, 2, 22, the expansion of hapten-specific CD8+ T cells and CHS responses were markedly increased and prolonged in WT mice receiving Fas-defective lpr DC when compared with recipients of WT DC.