The mechanisms of tolerance induction by iDCs are unclear, but several possibilities exist. Recently, new studies have identified IL-10 as a differentiation factor for a novel subset of immune suppressive regulatory T cells (Tr1).
43 Tr1 cells are immunosuppressive in vitro and in vivo,
44 and it is likely that Tr1 cells exist naturally within the CD4
+CD45RB
low T cell population and are dependent on IL-10 and TGF-β. Jonuleit et al.
41 reported that IL-10-producing CD4
+ T cells with regulatory properties can be generated by repetitive stimulation with iDCs in vitro, whereas Dhodapkar et al.
39 observed the induction of antigen-specific IL-10-producing CD8
+ T cells in vivo. Feili-Hariri et al.
45 reported that BMDCs administered intravenously can prevent the development of spontaneous diabetes in the NOD mouse by inducing a specific Th2 response, and they further suggested that the balance between regulatory Th2 and effector Th1 cells may have been altered in these mice. In the present study we have shown that subcutaneous inoculation of peptide-loaded iDCs, as well as inhibiting EAU, induced a population of cells in the draining nodes that were high secretors of IL-10 and IL-5. This is in contrast to the effects of inoculation with peptide-loaded mDCs which induced DLN cells producing low levels of IL-10 and IL-5. These data are partially in agreement with the findings of Dhodapkar et al.,
39 who report that a defined phenotype of T regulatory cells may be generated after injection of iDCs. As indicated earlier, EAU is a Th1-dependent disease and Th1 proinflammatory cytokines are important in the induction and pathogenesis of uveitis in genetically unmanipulated animals and in patients with uveitis.
46 Studies of different strains of rats and mice indicate that the EAU-susceptible mouse is likely to be a high Th1 responder, whereas the EAU-resistant mouse is likely to be a low Th1 responder (low IFN-γ and IL-12) in which a dominant Th2 response (high IL-4 and IL-5) is induced after uveitogen immunization.
8 In addition, there is evidence showing that if the response to antigens becomes skewed toward a Th2 phenotype, protection from EAU and other tissue-specific autoimmune disease models can be achieved.
7 In the present study, we have detected higher levels of IL-4, IL-5, and IL-10 in mice primed by peptide-loaded iDCs, but not by peptide-loaded mDCs. This suggests a skewing of the cytokine profile to a Th2-dominant type of regulatory response after subsequent uveitogenic challenge, and this may explain the inhibition of EAU. In contrast, peptide-loaded mDCs failed to skew the immune response
(Fig. 7) . After injecting iDCs, Dhodapkar et al.
39 did not assay for IL-5 production level but suggested that both IL-10-dependent and -independent mechanisms play a role in the inhibition of CD8
+ T cell function, in agreement with Menges et al.,
47 although Dhodapkar et al. used differently defined DCs.