The role of PGE2 in mediating MSC suppressive effects on Th17 differentiation DNA/RNA Synthesis inhibitor cultures was confirmed by addition of specific antagonists and agonists for candidate PGE2 receptors. IL-17A secretion by CD4+ T cells re-purified from MSC/Th17 co-cultures was restored to the

same level as that of control Th17 cultures by the highly selective EP4 receptor antagonist L-161,982 (Fig. 6C). Similarly, EP4 antagonism reversed the inhibition by MSCs of CD25 up-regulation on CD4+ T cells (data not shown). That this observation was specifically attributable to PGE2 produced by MSCs during co-culture was confirmed by transfer of conditioned media from FACS-sorted co-culture populations and relevant controls to fresh Th17 cultures in the presence or absence of EP4 antagonist (Supplementary Figs. S5, S6 and S7B). In this case, only medium conditioned by MSCs sorted from Th17/MCS co-cultures transferred a

Th17 suppressive effect that was reversible by EP4 antagonism. Experiments carried out with antagonists of the EP1 and EP2 receptors (SC-51322 and AH 6809 respectively) yielded negative results (data not shown). As further evidence of a specific role for PGE2/EP4, the EP4 agonist L-902,688-mediated dose-dependent inhibition of the primary induction of Th17 cells (Fig. 6D). Up to this point, the experiments were carried out exclusively with primary naïve and/or memory CD4+ T cells undergoing activation in vitro under PD98059 solubility dmso short-term Th17-skewing conditions. Making use of a unilateral ureteral obstruction (UUO) model in which we have previously reported intra-renal accumulation of effector-memory phenotype Th17 cells 22, it was determined

whether MSCs exert a mechanistically-similar Orotidine 5′-phosphate decarboxylase suppressive effect on the re-activation of committed Th17 cells from an area of ongoing tissue inflammation. As shown in Fig. 7A, B6 mice underwent UUO for 72 h following which CD45+ cells were enriched from obstructed and contralateral (non-obstructed) kidneys and briefly stimulated through the T-cell receptor in the absence or presence of MSCs. In-line with our previous findings 22, anti-CD3ε-stimulation was associated with robust secretion of IL-17A by cells from obstructed kidneys (Fig. 7B). The presence of MSCs was associated with dose-dependent reduction in IL-17A concentration following either 24 or 48 h culture periods. Qualitatively similar results were observed in a total of seven similar experiments with median proportionate inhibition of IL-17A production being 56% (range 19–69%) at MSC:CD45+ cell ratio of 1:20. As we have previously reported 22, IL-17A secretion was absent from stimulated cultures of CD45+ cells from non-obstructed kidneys (data not shown). The suppressive effect of MSCs was reversed by indomethacin (Fig. 7C). Thus, naturally occurring effector-memory Th17 cells undergoing activation through the T-cell receptor signalling complex are amenable to suppression by MSCs via a similar COX-2-dependent mechanism.