CXCL4 is a driver of cytokine mRNA stability in monocyte-derived dendritic cells

The chemokine CXCL4 has been implicated in several immune diseases. Exposure of monocyte-derived dendritic cells (moDCs) to CXCL4 potentiates the production of inflammatory cytokines in the presence of TLR3 or TLR7/8 agonists. Here we investigated the transcriptional and post-transcriptional events underlying the augmented inflammatory responses in CXCL4-moDCs. Our results indicate that CXCL4-moDCs display an increased expression and secretion of IL-12, IL-23, IL-6 and TNF upon TLR3 activation. Analysis of the cytokine transcripts for the presence of AU-rich elements (ARE), motifs necessary for ARE-mediated mRNA decay, revealed that all these cytokine transcripts are, at least in silico, possibly regulated at the level of mRNA stability. In vitro assays confirmed that mRNA stability of IL6 and TNF, but not IL12B and IL23A, is increased in CXCL4-moDCs. We next screened the expression of ARE-binding proteins (ARE-BPs) and found that TLR stimulation of CXCL4-moDCs induced tristetraprolin (TTP or ZFP36). Increased TTP mRNA expression was found to be a consequence of TTP phospho-mediated inactivation, which over time causes the protein to degrade its own mRNA. Concomitantly with TTP inactivation, we observed increased MAPK p38 signalling, upstream of TTP, in stimulated CXCL4-moDCs. P38 inhibition restored TTP activation and subsequently reduced the production of inflammatory cytokines. Finally, TTP knockdown in moDCs resulted in an increased production of IL6 and TNF after TLR stimulation. Overall, our study shows that the pro-inflammatory phenotype of CXCL4-moDCs relies in part on enhanced cytokine mRNA stability dictated by TTP inactivation.