Tolerogenic dendritic cells actively inhibit T cells through heme oxygenase-1 in rodents and in nonhuman primates

Clinical translation of dendritic cell (DC)-based cell therapy requires preclinical studies in nonhuman primates (NHPs). The aim of this work was to establish the in vitro conditions for generation of NHP tolerogenic DCs (Tol-DCs), as well as to analyze the molecular mechanisms by which these cells could control an immune response. Two populations of NHP bone marrow-derived DCs (BMDCs) were obtained: adherent and nonadherent. Although both populations displayed a quite similar phenotype, they were very different functionally. We characterized the adherent BMDCs as Tol-DCs that were poor stimulators of T cells and actively inhibited T-cell proliferation, whereas the nonadherent population displayed immunogenic properties in vitro. Interestingly, the anti-inflammatory and immunosuppressive enzyme heme oxygenase-1 (HO-1) was up-regulated in Tol-DCs, compared to the immunogenic BMDCs. We demonstrated that HO-1 mediates the immunosuppressive properties of Tol-DCs in vitro (in NHPs and rats) and that HO-1 is involved in the in vivo tolerogenic effect of Tol-DCs in a rat model of allotransplantation. In conclusion, here we characterized the in vitro generation of NHP Tol-DCs. Furthermore, we showed for the first time that HO-1 plays a role in the active inhibition of T-cell responses by rat and NHP Tol-DCs.