The tryptophan (Trp)-degrading enzyme indoleamine 2, 3-deoxygenase (IDO) is induced in a subset of human macrophages and dendritic cells by cytokines derived from activated T cells. In vitro, IDO can suppress T cell receptor (TcR)-mediated proliferation of human T cells by depriving them of Trp. However, it is not known how Trp deprivation affects functional properties of activating T cells, nor is it known which cells are responsible for IDO expression in vivo. In this study, we examine the effect of Trp depletion on antigen-specific activation and differentiation of murine T cells using a defined Trp-free culture medium (TFM). T cells from TcR-transgenic mice specific for H-2Kb were activated using congenic splenocytes transgenic for the target antigen. In comparison to control T cells activated in Trp-sufficient medium, T cells activated in TFM failed to proliferate, although they showed upregulation of early activation markers (CD25, CD69) and progression through the initial 12 hrs of Gl. However, T cells deprived of Trp failed to express the late marker CD71 or enter S phase, indicating a Trp-sensitive arrest point in the activation process. Functionally, those T cells did not express the activationassociated isoform of CD43 ( l B11 ), a marker for cytolytic effector CDS T cells. Consistent with this phenotype, s'Cr release assay showed that CD8 T cells activated in TFM failed to differentiate into cytotoxic effector cells. When Trp was added back to the TFM in conjunction with antigen restimulation, the arrested T cells were rescued, proliferated, differentiated into cytotoxic effector cells, and expressed the IB 11 antigen. In order to determine the biologically relevant cell expressing IDO in vivo, we separated subpopulations of murine splenocytes by collagenase D treatment, plastic adherence for enrichment of macrophages and dendritic cells, and magnetic cells sorting. Analysis by RT-PCR showed that IDO expression was exclusively confined to CD11C+ spleen cells, which in mice represents mature and immature myeloid dendritic cells. CD1 lb+ cells, which include most splenic macrophages, did not express IDO, nor did T and B cells, or other myeloid or stromal cells even after stimulation by LPS or IFN-y. We conclude that Trp deprivation prevents proliferation and functional maturation of antigen-stimulated murine T cells and induces a cell cycle arrest in mid-Gl phase. The cells expressing IDO in murine lymphoid organs are a subset of CD 11 c+ dendritic cells. In light of the fact that dendritic cells are potent antigen presenting cells, expression of the immunosuppressive enzyme IDO by a subset of these cells may help explain the importance of IDO in maintaining tolerance toward fetal alloantigens and solid organ allografts.
|Original language||English (US)|
|Issue number||11 PART I|
|State||Published - Dec 1 2000|
ASJC Scopus subject areas
- Cell Biology