6B). This was not due to the toxicity of the inhibitors, since cellular BVD-523 in vitro viability as measured with the dye MTT was not affected (Supporting Information Fig. 5A). CD1a expression was not altered (data not shown). The results so far indicated that IL-6 and IL-10 are important for the induction of the TLR-APC phenotype. Both cytokines
are known to signal via STAT-3. We therefore analyzed expression and phosphorylation of STAT molecules (STAT-1, -3, -5 and -6). The STAT activation pattern of iDCs and TLR-APCs differed significantly (Fig. 7): differentiation of DCs in the presence of R848 resulted in an almost constitutive activation of STAT-3. In contrast, STAT-1 tyrosine phosphorylation was much shorter compared to STAT-3 phosphorylation (1 h–day 1). Regarding STAT-6 activation no significant differences between TLR-APCs and iDCs were detected (data not shown). In contrast, during the whole differentiation process, STAT-5-activation dominated in iDCs and was much lower in TLR-APC. Hence, the comparison of the STAT activation pattern in iDCs and TLR-APCs revealed a prevailing STAT-5 activation in iDCs and a dominant STAT-3 activation in TLR-APCs. To further corroborate the link between STAT-3 activation and expression
of CD14 and PD-L1, we performed blocking experiments of STAT-3 with the chemical inhibitor JSI-124. After addition of JSI-124 expression of CD14 was not sustained (Fig. 8A) and upregulation of PD-L1 expression was RXDX-106 price prevented (Fig. 8B). CD1a expression was unaffected (data not shown). Treatment with the inhibitor JSI-124 did not
compromise cell viability (Supporting Information Fig. 5B). To close the link between STAT-3 activation and induction of PD-L1 expression we used chromatin immunoprecipitation (ChIP) assay to determine the ability of STAT-3 to bind to the PD-L1 promoter. We found that STAT-3 was rapidly recruited to the PD-L1 promoter (Fig. 8C). Since STAT-1 is known to be involved in PD-L1 expression too Thalidomide and since STAT-1 was also activated we checked the binding activity of STAT-1 to the PD-L1 promoter (Fig. 8D). However, we found that STAT-1 binding was minor compared to STAT-3 and nearly no differences in STAT-1 binding between iDCs and TLR-APCs were detectable. From the results so far, we concluded that STAT-3 has a central role for the formation of the TLR-APC phenotype. On the other hand, inhibition of MAPKs with the pharmacological inhibitor SB203580 (MAPK p38) and UO126 (MAPK p44/42) had the same effect as STAT-3 inhibition: the failure to sustain expression of CD14 and the prevention of PD-L1 expression. To link both effects with each other, we tested whether suppression of cytokine production (especially of IL-6 and IL-10) after MAPK inhibition influenced the status of STAT-3 activation. After combined blockade of p38 and p44/42 tyrosine phosphorylation of STAT-3 was reduced markedly. The same pattern was found when LPS instead of R848 was used to induce TLR-APC (Fig. 9A).