Are indoleamine-2,3-dioxygenase producing human dendritic cells a tool for suppression of allogeneic T-cell responses?

Background. Suppressive dendritic cells (DCs) are a promising tool for tolerance induction in transplantation. A human DCs subpopulation, which constitutively expresses indoleamine-2,3-dioxygenase (IDO), a molecule shown to prevent the rejection of fetus during pregnancy, has recently been described. This subset, characterized by nonadherence and CD123/CCR6 expression, exhibited sustained IDO production if exposed to interleukin (IL)-10. In the present work, we generated human nonadherent, CD123(+)/CCR6(+) DCs secreting IL-10. Methods. Monocytes were separated by plastic adherence and differentiated to DCs in the presecence of IL-3 and IL-4. Expression of IDO was determined by reverse-transcriptase polymerase chain reaction and enzyme activity by reverse-phased high-performance liquid chromatography. Mixed lymphocyte cultures were performed with allogeneic nylon wool-purified T-cells. Results. Contradicting previous findings, CD123+/CCR6+ DCs did not express IDO. Maturation of these cells with inducer-cytokines up-regulated IDO, but the allogeneic T-cell stimulatory capacity of these DCs was even stronger than that of immature IDO- DCs, and chemical abrogation of IDO activity did not increase T-cell proliferation. In parallel, we generated mature IDO- DCs, but these cells also did not induce stronger T-cell stimulation than their IDO+ counterpart. Conclusions. In conclusion, CD123(+)/CCR6(+) DCs do not constitutively express IDO and "induced" IDO+ DCs, even coexpressing anti-inflammatory IL-10, do not suppress allogeneic T-cell responses.