Dendritic cells as regulators of T regs
Immature DCs: Another way to generate suppressor T cells in vitro involves the stimulation of naive T cells with iDCs. Repetive stimulation of naive cord-blood T cells with allogeneic iDCs has reportedly generated a population of poorly growing T cells that primarily produce IL-10. Although these cells produced IL-10, their suppressor phenotype resembled that of CD25+ T cells, as it was contact dependent, antigen non-specific and APC independent. Furthermore, suppression could be overcome partially by the addition of IL-2. These cells differ from in that IL-10 is not required for their generation because iDCs do not produce IL-10. The precursors of these suppressor cells in cord blood do not express CD25 so it is unlikley that they are derived form a CD25+ T cell population that has not fully differentiated.
Immature DCs are the ideal population to prime regulatory T cells as they are deficient in co-stimulatory moleucles, and priming with antigen-iDC complees might even be able to downregulated preexisting antigen specific immune responses.
Induction of Antigen specific CD8T-IL-10 producing cells: Injection of iDCs into human subjects reportedly results in inhibition of MP-specific CD8+ T cell effector function in freshly isolated T cells. Before immunization, MP-specific IFNy producing T cells were detectable in both human subjects as expected because most adults have been exposed to the influenza virus. However, after DC immunization, there was a decline in MP specific IFNy producing cells. Moreover, this decline was associated with IL10 but not IL4 producing T cells. These postimmunization IL-10 producers were CD8+CD4- cells.
Reversal of Anergic state with Antigen-processing mature DCs:
CD25+CD4+ T cells can be expanded both in vitro and in vivo with antigen-loaded BM DCs from Balb/c mice.
CD4+CD25+ T cells proliferation vigorously to stimulation with anti-CD3 in the presence of mature bone marrow-derived DC (BMDC).
A number of studies have shown that CD4+CD25+ T cells from TCR transgenic mice will proliferate following transfer in vivo when stimulated with their cognate Ag presented by DC.
CD25-mediated suppression of T cell proliferation is also abrogated when BMDC are used as APC.
Treg anergy is dependent on TLR activaiton of BMDCs and involves the potentiation of Treg responsiveness to IL-2 by cooperative effects of IL-6 and IL-1, both of which are produced by TLR activated mature DCs.
Reversal of suppressive function with Mature DCs:
The suppressive function of Tregs is readily reversed by the maturation of DCs induced by GM-CSF and does not require TLR activation of either DCs or Tregs. Thus BMDCs derived by short-term culture (day 5-6) were permissive for suppression by CD25+CD4+ T regs, whereas BMDCs derived by long term culture (day 10-11) were not. Activation of day 10-11 BMDCs by a TLR ligand (LPS), however, does further enhance T cell proliferation.
Using splenic DCs, Il-6 was required to block the suppressive activity of Tregs. However, IL-6 was not required when using BMDCs.
Therapies Using iDCs
One possible therapy application relates to the use of iDCs to generate antigen specific regulatory CD8+T cells in vitro, which may then be used for adoptive immunotherapy in vivo. In this system, T cells are co-cultured with immature DCs in vitro at a DC to T cell ratio of about 1:10-100 and the resulting T cells are then injected for the purpose of suppressing an active immune responde.
Model of Tolerance
In one proposed model, the maintenance of self tolerance is dependent on the ratio of T4 cells to potentially pathogenic autoreactive T cells that respond to a given periopheral antigen. In a given lymph node this ratio fluctuates depending on the infectious status of the local tissue. In the steady-state, immature DCs may traffic through peripherotic debris arising form normal cell turnover in the tissue without becoming activated. Evein in the absence of inflammation, a few of these iDCs will migrate to the draining lymph nodes where they will present a panel of self peptides to both TR cells and Tpath cells. However, autoimmunity will not occur, perhaps because the autoreactive T cells are insufficiently activated by iDCs or, alternatively, because the iDC preferentially stimulate TR cells. Consistent with the latter hypothesis, TR cells can respond to much lower concnetrations of cognate peptide ligands than conventional naive CD4+T cells and iDCs express relatively low amounts of MHC class II and costimulatory molecules. Check effects of EGCG treated DCs In contrast, the presence of an infectious agent will induce DC activation and migration so that high numbers of mature DCs will arrive in the lymph node and present peptides derived from the pathogen and from self antigen. This stimulus may be potent enough to transiently override TR cell activity, permitting an expansion of anti-pathogen T cells but also allowing expansion of T path cells. This transient loss of TR cell activity will also be associated with proliferation of the TR cells themselves, analygous to the in vitro hyperstimulation of CD4+CD25+TR cells with anti-CD3 + IL-2 of anti-CD28, which results in proliferation of the TR ceels accompanied by a transient loss of suppressive activity.