P908 Aims: Discovery of liver transplant tolerance has led to an enthusiasm of investigating liver dendritic cells (DC) which play a key role in determining immunity and tolerance. Recent in vivo demonstration of myeloid CD11c+ hepatic DC with strong capacity to stimulate T cells certainly supports the notion that hepatic DC generate immunity. There is however, lacking of evidence of existence of liver tolerogenic DC in vivo. Methods and Results: We adopted, in this study, an aaproach to transfect gene(s) in the liver by tail vein rapid injection of naked plasmid carrying genes encoding IL-3, CD40L or GM-CSF, (GFP as reporter) into B6 (H-2b) mice, which enabled significant levels of transgene expression in the liver in 8h, reached the peak in 24 hr, decreased thereafter, and lasted for 5-7 days, as demonstrated either by detection of GFP with fluorescent microscopy or by serum levels of IL-3 with ELISA. While we confirmed the expansion of mixed population in the group of transfection with GM-CSF, most of them morphologically resembled myeloid DC, and located in portal and periportal areas. Another more homogeneous population was identified following transfection with IL-3 plus CD40L, not IL-3 alone, which is morphologically distinctive from myeloid DC and form big aggregates in expanded lobules as well as portal and periportal areas. FACS analysis revealed that in addition to common DC surface markers (MHC class II, CD80, CD86, and CD40), they were B220+, CD205+, but CD11b- and CD11c-, and expressed high TLR-9, but low TLR-4. We examined their response to bacterial stimuli. Not surprising, after LPS stimulation, the co-stimulatory molecule expression was upregulated, similar to myeloid DC, but what differ are their cytokine profiles. Demonstrated by RNAse protection assay, B220+/CD205+ DC expressed defective IL-12 mRNA with only p35 but not p40 mRNA, and up-regulated IL-10 but down-regulated IL-1 expression. In contrast, myeloid hepatic DC showed cross-board upregulation of inflammatory cytokine, including IL-1α, β, IL-6 and IL-12 (both p35 and p40). We studied in vitro DC-T cell interaction in an antigen-specific manner. Using both CD4 (DO10.11) and CD8 (Des) TCR transgenic T cells. In comparison to myeloid DC, B220+/CD205+ DC induced hyporesponsiveness of both CD4 and CD8 T cells measured by thymidine uptake with repressed levels of IL-2 (1.0 ng/ml vs 2.6 ng/ml), but similar level of IFN-γ (8.3 ng/ml vs 8.2 ng/ml). This hyporesponsiveness is not due to failure of T cell activation since CFSE-labeling showed T cell dividing similar to culture with myeloid DC. It appeared to result from shortened T cell survival. In contrst to myeloid DC administration of which exacerbated B6 cardiac allograft rejection in C3H (H-2k) recipients, B220+/CD205+ DC prolonged allograft survival. Conclusion: A tolerogenic DC population in the liver was differentiated in vivo, which may play an important role in balancing hepatic T cell immunity and tolerance.