The MYD88-Independent Pathway Is Not Mobilized in Human Neutrophils Stimulated via TLR4

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Abstract
LPS activates both MyD88-dependent and -independent signaling via TLR4, but the extent to which each cascade is operative in different cell types remains unclear. This prompted us to revisit the intriguing issue of CXCL10 production, which we previously showed to be inducible in neutrophils stimulated with LPS and IFN-γ but not with either stimulus alone, contrary to other myeloid cells. We now report that in neutrophils the MyD88-independent pathway is not activated by LPS. Indeed, microarray and real-time PCR experiments showed that neither IFNβ nor IFNβ-dependent genes (including CXCL10) are inducible in LPS-treated neutrophils, in contrast to monocytes. Further investigation into the inability of LPS to promote IFNβ expression in neutrophils revealed that the transcription factors regulating the IFNβ enhanceosome, such as IFN-regulatory factor-3 and AP-1, are not activated in LPS-treated neutrophils as revealed by lack of dimerization, nuclear translocation, confocal microscopy, and inducible binding to DNA. Moreover, we show that the upstream TANK-binding kinase-1 is not activated by LPS in neutrophils. A lack of IFNβ/CXCL10 mRNA expression and IFN-regulatory factor 3 activation was also observed in myeloid leukemia HL60 cells differentiated to granulocytes and then stimulated with LPS, indicating that the inability of neutrophils to activate the MyD88-independent pathway represents a feature of their terminal maturation. These results identify a disconnected activation of the two signaling pathways downstream of TLR4 in key cellular components of the inflammatory and immune responses and help us to better understand the primordial role of neutrophils in host defense against nonviral infections.