P984 Aims: Liver allografts are spontaneously accepted in pigs, rats and mice, which is associated with enhanced activated T cell apoptosis. The underlying mechanisms are unclear. We have identified a novel subset of dendritic cells (DC) in mouse livers that were phenotypically mature, but stimulated poor [3H]TdR incorporation in allogeneic T cells due to apoptotic death of activated T cells. The aims of this stuy was to investigate the involved mechanisms. Methods and Results: Liver-derived B220+ DC were propagated from B10 mouse liver NPC in the presence of IL-3 and CD40L. Bone marrow-derived myeloid DC in GM-CSF and Il-4 were used as controls. Cell cycle analysis showed that dividing of T cells stimulated by liver B220+DC was similar to BM-derived myeloid DC. Low thymidine uptake was a result of enhanced activated T cell apoptosis. ∼30% of activated T cells were TUNEL positive in liver B220+ DC group (evenly distributed in CD4+ and CD8+ populations), only <10% in myeloid DC group. In contrast to myeloid DC that exacerbated allograft rejection, administration of B10 (H-2b) liver B220+ DC (2 x 106) dramatically prolonged survival of B10, but not third party (BALB/c; H-2d) heart allografts in C3H (H-2k) recipients (MST 37 days vs. 10 days in control and third party groups). This was associated with a higher incidence of apoptotic cells in draining lymph nodes and spleen. T cell thymidine uptake in a liver B220+DC/T culture was markedly restored by addition of z-VAD-fmk, a common caspase inhibitor peptide, suggesting an involvement of caspase cascades. To determine the involved pathways, proteins were isolated from sorted T cells at different time point of culture, and incubated with substrates (AC-DEVE-AFC for caspase 3, AC-IETD-AFC for caspase 8 and AC-LEHD-AFC for caspase 9). The caspase activity determined by an OD value showed that liver B220+DC, not myeloid DC, activated caspase 3 and caspase 8, but not caspase 9 in T cells. RNase protection assay data delineated that B220+ DC, not myeloid DC, inhibited T cell mRNA expression of Bcl-w and Bfl-1 (anti-apoptosi), but enhanced Bak and Bad (pro-apoptosis) expression. T cell apoptosis induced by gld (FasL deficient) B220+ DC was reduced about 30%, indicating a partial role of Fas ligation. Liver B220+ DC induced similar apoptosis in TNFR (either p55 or p75) deficient T cells, suggesting that TNF and lymphotoxin (LT)a may not be important ligands. We also examined the role of LTβ, encouraged by a RNase protection assay result that expression of LTβ in liver B220+ DC was extraordinary high, and found that liver B220+ DC from LTβ−/− mice were poor apoptosis inducers, and stimulated profound T cell proliferation, suggesting a critical role of LTβ in mediating the apoptosis. Conclusion: The data suggest an involvement of multiple apoptosis pathways.