While G4-stimulated cells showed high Akt inhibitor expression, R848-APC had a reduced number of MHC class II molecules, which could explain their low stimulatory potency. However, since PD-L1 is correlated with tolerance induction 32, we also tested, whether

PD-L1-dependent signaling contributes to the weak T-cell proliferation observed. Blockade of PD-L1 was effective to enhance T-cell proliferation in the presence of R848-APCs (Fig. 3C). Thus, reduced MHC class II expression and upregulation of PD-L1 are characteristics for TLR-APCs and their changed functional capacities. To further analyze the mechanisms of induction of the tolerogenic APC phenotype, we next analyzed release of cytokines upon initial TLR trigger. APCs generated in the presence of R848 secreted high amounts of pro-inflammatory cytokines (IL-6, TNF and IL-12p40) as well as immunosuppressive cytokines (IL-10) (Fig. 4A–D). Secretion of IL-6 was remarkably high (Fig. 4A). In order to determine whether auto- or paracrine active cytokines directly mimic the effect of R848 we added cytokines alone or cytokine mixtures to G4-stimulated cell cultures. While single addition of cytokines (IL-6 or IL-10) only partially induced the TLR-APC phenotype, a combination of both was almost similar effective to stimulation

with R848 (Supporting Information Fig. 4). In order ABT737 to further define the signal requirement for induction of TLR-APCs, we analyzed the pattern of MAPKs, known to be involved in TLR-mediated cytokine release 33. MAPKs are in addition important for differentiation processes. It was striking that the pattern of MAPK activation was clearly different between R848-APCs and conventional iDCs. Each MAPK exhibited a special pattern of activation (Fig. 5A): differentiation of monocytes in the presence of G4 and R848 showed an early

and prolonged phosphorylation of p38, whereas in G4-generated cells p38 phosphorylation was only detectable within the first 30 min. The activation pattern of p44/42 differed completely from p38 phosphorylation. p44/42 phosphorylation was only visible during the initial 15 min in R848-APCs and in contrast for 24 h in iDCs. Phosphorylation of SAPK/JNK was only detectable in R848-APCs and only for a short period. Inhibition of the two MAPK pathways (p38, p44/42) with pharmacological p38 (SB203580, SB) and p44/42 inhibitors all (UO126, UO) resulted in markedly reduced secretion of IL-6 (Fig. 5B) and IL-10 (Fig. 5C), at least when both MAPKs p38 and p44/p42 were blocked. Similar results were obtained when the cells were stimulated with LPS plus G4 (data not shown). IL-12p40 release in contrast was not diminished (Fig. 5D) but even slightly increased. The reduced cytokine release after MAPK inhibition correlated with reduced surface expression of CD14 and PD-L1. FACS analyses revealed that preservation of CD14 expression was blocked almost completely by the addition of SB and UO (Fig. 6A).