Lipopolysaccharide (LPS) activates both MyD88-dependent and independent signaling pathways via TLR4, but the extent to which each cascade controls gene transcription in different cell types remains unclear. Even though, with the exception of TLR3 and TLR7, human neutrophils express all known TLRs, our knowledge on the molecular mechanisms that in these cells regulate cytokine gene expression, particularly via TLR4, is still incomplete. For instance, we recently uncovered that, in contrast to autologous monocytes, neutrophils are unable to activate the MyD88-independent/TRIF-dependent signaling. As a result, LPS-stimulated neutrophils are unable to transcribe IFNβ since they do not activate TRAF family associated NF-κB binding kinase [TANK]-binding kinase-1 (TBK1), an IRF3-phosphorylating kinase and, consequently, interferon regulatory factor-3 (IRF3), a critical transcription factor for IFNβ induction. In addition to IFNβ, also IL-6 and IL-10 (two cytokines with critical immune modulatory functions) are selectively induced in monocytes but not in neutrophils. Interestingly, both cytokines are induced in monocytes in a TRIF- and IFNβ-independent manner, indicating that the LPS-activated MyD88-dependent pathway is sufficient for their gene activation. By contrast, IL-6 and IL-10 are detectable neither at the protein level, nor at the mRNA or primary transcript level in highly purified neutrophils stimulated with LPS. We have therefore analyzed posttranslational modifications of histones associated with genes that are active, repressed or poised for transcriptional activation, by chromatin immunoprecipitation (ChIP) assays. Differently from autologous monocytes, none of the marks under evaluation (including H3K4me3, H4Ac, H3K27Ac and H3K4me1) was detected at the IL-6 or IL-10 loci of resting or activated neutrophils. Taken together, these results suggest that, the differential capacity of human neutrophils and monocytes to express IL-6 and IL-10 upon LPS activation is likely controlled by cell specific chromatin configurations at their gene loci.
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