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Regulatory T cells (Treg) play a crucial role in controlling immunity and transplant rejection. Two main groups of Treg have been described: antigen-induced Treg (iTreg) and natural Treg (nTreg). The ways to induce and the mechanisms of action of Treg subsets remained ill defined, particularly for their effects on CD8(+) T cells. CD8(+) T cells are major agents in the rejection of allografts; the aim of this study is to investigate the effects exerted on CD8(+) T cells by human CD4(+) iTreg induced by mycophenolic acid-treated dendritic cells. iTreg suppress the proliferation of CD8(+) T cells by allogeneic cell-cell interaction with mature dendritic cells and irrespectively of the TCR specificity of the CD8(+) T cells and cell-cell contact of iTreg with CD8(+) T cells. In our model, this suppression is independent of the action of IL-10 and TGF-β1. iTreg were able to modify phenotype and inhibited IFN-γ and TNF-α secretion by CD8(+) T cells. Most interestingly, iTreg inhibit the synthesis of perforin and of granzymes A and B by CD8(+) T cells and impaired their cytotoxicity against allogeneic targets. In summary, our study showed the involvement of iTreg in the down-regulation of cytotoxic responses mediated by CD8(+) T cells in an allospecific context. Following studies that have shown the existence of a regulation control exerted by iTreg on CD4(+) T cells and dendritic cells, this work ultimately shows that this regulation can reach CD8(+) T-cell functions.

Mycophenolic acid (MPA) is an immunosuppressive drug which induces resistance to several maturation signals in human dendritic cells (DC) by unknown mechanisms. As mitogen-activated protein kinases (MAPK) are involved in the maturation process, we studied whether MPA affected p38MAPK and extracellular signal-regulated kinase (ERK1/2) in human DC. We first showed that MPA reduced TNFalpha-induced phenotype maturation, whereas it had no effect after LPS activation, suggesting that MPA preferentially affects the signaling pathway used by TNFalpha. We found that TNFalpha preferentially used p38MAPK to induce phenotype maturation in DC, whereas LPS preferentially activated NF-kappaB. Importantly, we showed that MPA more strongly inhibited p38MAPK phosphorylation induced by TNFalpha than by LPS. This difference in inhibition may therefore explain its different effect on DC phenotype. Interestingly, MPA inhibited the inflammatory cytokine synthesis and allostimulatory capacity induced by both stimuli. Exogenous guanosine antagonized the effect of MPA on the phenotype of TNFalpha-matured-DC as well as the IL-12p70 and IFN gamma secretion induced by both stimuli, without affecting p38MAPK phosphorylation. The action of MPA on human DC phenotype maturation appears mainly to be due to its ability to inhibit p38MAPK. Furthermore, the difference between LPS and TNFalpha emphasizes that the DC microenvironment strongly influences DC sensitivity to MPA., ((c) 2009 Elsevier Ltd. All rights reserved.)

Depending on their degree of maturation, costimulatory molecule expression, and cytokine secretion, dendritic cells (DC) can induce immunity or tolerance. DC treated with mycophenolic acid during their maturation (MPA-DC) have a regulatory phenotype and may therefore provide a new approach to induce allograft tolerance. Purified CD4(+) T cells stimulated in a human in vitro model of mixed culture by allogeneic MPA-DC displayed much weaker proliferation than T cells activated by mature DC and were anergic. This hyporesponsiveness was alloantigen-specific. Interestingly, T cells stimulated by MPA-DC during long-term coculture in four 7-day cycles displayed potent, suppressive activity, as revealed by marked inhibition of the proliferation of naive and preactivated control T cells. These regulatory T cells (Tregs) appeared to have antigen specificity and were contact-dependent. Tregs induced by MPA-DC were CD25(+)glucocorticoid-induced TNFR(+)CTLA-4(+)CD95(+), secreted IL-5 and large amounts of IL-10 and TGF-beta, and displayed enhanced forkhead box p3 expression. These results obtained in vitro demonstrate that human MPA-DC can induce allospecific Tregs that may be exploited in cell therapy to induce allograft tolerance.

Immature dendritic cells (DCs) can induce T-cell hyporesponsiveness, thus interfering with the process of DC maturation in a pro-inflammatory context, may therefore provide a novel approach to inducing allograft tolerance. We have studied the effects of mycophenolic acid (MPA), an immunosuppressive agent currently used in transplantation, using an in vitro model of a mixed human DC/alloreactive CD4(+) T lymphocyte culture. DCs differentiated from monocytes were exposed to MPA during maturation. MPA treatment affected the maturation of DCs, and this was reflected both in the impairment of the up-regulation of co-stimulatory molecule expression and the maintained endocytic capacity. However, MPA-DCs exhibited a distinctive microscopic morphology and secreted IL-10 and so could no longer be regarded as immature DC. Moreover, MPA-DCs had a mature phenotype for chemokine receptor expression, exhibiting down-regulation of CCR5 and up-regulation of CCR7. Interestingly, the abilities of the MPA-DCs to induce CD4(+) T-cell proliferation in response to alloantigens was impaired not only via direct but also via indirect pathways. The maintenance of endocytosis and the inhibition of syngeneic T-cell activation suggest that these cells could have a potential role to avoid chronic rejection. All these characteristics suggest that MPA-DCs may be used in cell therapy to induce allograft tolerance.