CXCL4 signaling and gene induction in human monocytes involve a TLR4 response divergent from LPS

The chemokine CXCL4 activates myeloid cells and contributes to the pathogenesis of inflammatory and fibrotic diseases. One mechanism of CXCL4 action is binding of nucleic acids to promote their internalization and activation of endosomal TLRs. However, the signaling pathways and receptors that mediate myeloid cell responses to CXCL4 alone are not well characterized. Here, we report that in primary human monocytes, CXCL4 activated NF-κB and a TBK1-JNK signaling axis that drive the expression of inflammatory, fibrotic and neutrophil chemokine genes, and also RIPK3-dependent necroptosis. Surprisingly, six distinct lines of evidence targeting TLR4 expression and function suggested a role for TLR4 in CXCL4 responses. However, in contrast to the canonical TLR4 agonist LPS, CXCL4 did not effectively activate an autocrine IFN response or IL-12 family genes, even after IFN-γ priming of monocytes. In accord with its role in inducing chemokine and inflammatory gene expression, CXCL4 regulated neutrophil and macrophage infiltration into skin wounds, and promoted wound healing in a mouse model. Our findings suggest that CXCL4 modulates the profile of TLR4 responses to induce a macrophage phenotype divergent from LPS-activated cells, provide new insights into mechanisms by which CXCL4 activates human monocytes, and implicate CXCL4 in regulation of skin wound healing.