Placenta Derived Adherent Cells (PDAC) are multipotent progenitor cells derived from human placental tissues. Previously we have reported that PDACs could suppress T-cell proliferation when added to in-vitro mixed lymphocyte reactions (PDAC-MLR) (Paludan C. et al, Blood. (ASH Annual Meeting Abstracts) 2006 108: Abstract 1737). Here we present aspects of the mechanism of this PDAC suppression property. We have found that PDACs modify cytokine production in the PDAC-MLR reaction in comparison to the MLR; TNF-α and IFN-γ are reduced 75% and 30% while TGF-β production is significantly increased. We have used a transwell assay system to investigate the cell-contact-dependency of the effects of PDACs on T-cell proliferation. The assay system comprised combinations of the MLR in the top chamber together with the PDAC-MLR, PDACs plus naive T cells or PDACs alone in the bottom chamber. Maximum inhibition of T cell proliferation of the MLR in the top insert could be achieved by placing the PDAC-MLR co-culture in the bottom chamber. Minimum suppression was obtained when placing PDACs plus naive T cells or PDACs alone in the bottom chamber. PDAC-MLR conditioned media could partially suppress the MLR reaction. Addition of L-tryptophan into the MLR with PDAC conditioned media completely abolished PDAC-induced suppression of T cell proliferation. Likewise, the addition of the 1-methyl tryptophan to the PDAC-MLR reaction could abolish the PDAC-induced suppression. These results suggested that the suppression of the MLR by PDACs was possibly due to the depletion of the essential amino acid L-tryptophan which could be due to up-regulation of indoleamine 2,3-dioxygenase (IDO). Quantitative RT-PCR analysis of IDO gene expression revealed that IDO was up-regulated by about 4000-fold when PDACs were co-cultured with activated T cells, but not when co-cultured with naive T cells. Experiments are ongoing to confirm the causative role of IDO, and other factors, in PDAC-suppression of T-cell proliferation. In summary, we believe that soluble factors including the production of pro-inflammatory cytokines may contribute to PDAC suppression of the MLR, that induction of soluble factors from PDACs is significantly augmented by T-cell activation and that IDO expression by PDACs during the PDAC-MLR reaction plays a significant and direct role in suppression of T cell proliferation by PDACs.
Disclosure:Employment: Bitao Liang, Casper Paludan, Matthew Downie, Craig Lewis, Hemlata Rana, Chris Lugo, Xiaohua Lu, Wei Wu and James Edinger are all employed by Celgene Corporation or its affiliates or subsidiaries. Ownership Interests:; All Celgene Corporate authors have stock options. Research Funding: There is an agreement in place between Celgene Corporation and PHRI.