Integrin Alpha E (CD103) Beta 7 Expression Marks A Subset of Human Bone Marrow-Derived CD207⁺ Dendritic Cells Which Induce T Regulatory Cells Via Indoleamine 2,3-Dioxygenase,

Abstract 3233

Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme in tryptophan catabolism along the kynurenine pathway. IDO expressed by different cell subsets inhibits T-cell activation, proliferation and survival and induces regulatory T cells (Tregs), thus mediating immunological tolerance. Although human monocyte-derived dendritic cells (DCs) have been shown to express IDO, little is known about its expression in other subsets of human DCs, including those generated from CD34⁺ hematopoietic progenitors (CD34⁺-derived DCs). In particular, no data are currently available for IDO expression in CD34⁺-derived DC subsets. CD34⁺-derived DCs were generated from healthy donors from purified CD34⁺ cells after 7 days of culture with GM-CSF and TNF-a. Then, DCs were separated into CD1a⁻CD14⁺ and CD1a⁺CD14⁻ cells. DCs subsets were analyzed for IDO expression by real-time PCR and western immunoblot, kynurenine production, inhibition of allogeneic proliferation and Tregs induction. CD34⁺ cells did not express IDO mRNA expression regardless of the progenitor cell sources (cord blood, mobilized peripheral blood, bone marrow). During DC differentiation, IDO expression and function, evaluated by enzymatic and immunological tests, was markedly induced at day 7. Interestingly, the expression of IDO was shown to be 10 times higher in the CD1a+ compartment as compared to CD1a- cell fraction. IDO expression resulted in increased production of kynurenine and in reduced allostimulatory capacity of T-cell proliferation. Moreover, CD1a⁺ cells were shown to induce a population of CD4⁺CD25⁺Foxp3⁺ which acted as Tregs by inhibiting allogeneic T cell proliferation. This effect was abrogated by the addition of the IDO inhibitor 1-methyl tryptophan. Phenotypically, IDO-expressing CD1a⁺ cells expressed typical marker of Langerhans cells such as CD207, CD11b, CD1c, as well as CD103, which has been recently identified as a marker for tolerogenic DCs. Importantly, IDO expression was mainly detected in the CD103⁺ CD207⁺ fraction, which induces Tregs through an IDO-dependent manner. In conclusion, DC differentiation of CD34⁺ cells results in the expression of a functionally active IDO protein in CD103-expressing DCs. Given the role of IDO in regulating immune tolerance, a subset of bone marrow-derived DCs, expressing CD103, may be intrinsically committed to function as regulatory DCs. These data point toward IDO expression as part of a tolerogenic signature during DC development.