Costimulation of TLR8 responses by CXCL4 in Human Monocytes Mediated by TBK1-IRF5 Signaling and Epigenomic Remodeling

Abstract

Regulation of endosomal TLR responses by the chemokine CXCL4 is implicated in inflammatory and fibrotic diseases. The current paradigm is that CXCL4 potentiates TLR responses by binding and facilitating endosomal delivery of nucleic acid TLR ligands. We report that in human monocytes/macrophages, CXCL4 initiates signaling cascades and downstream epigenomic reprogramming that change the profile of the TLR8 response by selectively and dramatically amplifying inflammatory gene transcription and IL-1{beta} production, while partially attenuating the IFN response. Mechanistically, costimulation by CXCL4 and TLR8 synergistically activated TBK1/IKK{varepsilon} and repurposed these kinases towards an inflammatory response via coupling with IRF5, and by activating the NLRP3 inflammasome without the need for an exogenous activator of a second signal for IL-1{beta} maturation. CXCL4 signaling strongly induced chromatin remodeling and de novo enhancers associated with inflammatory genes in a cooperative and synergistic manner with TLR8. These findings identify new mechanisms of regulation of TLR responses relevant for cytokine storm, and suggest targeting the TBK1/IKK{varepsilon}-IRF5 axis may be beneficial in inflammatory diseases.