IFN-ß expression is directly activated in human neutrophils transfected with plasmid DNA and is further increased via TLR-4-mediated signaling.

Upon LPS binding, TLR4 activates a MyD88-dependent pathway leading to the transcriptional activation of proinflammatory genes, as well as a MyD88-independent/TRIF-dependent pathway, responsible for the transcriptional induction of IFN-ß. Previous findings delineated that human neutrophils are unable to induce the transcription of IFN-ß in response to TLR4 stimulation. Because neutrophils do not express protein kinase C ε, a molecule recently reported as essential for initiating the MyD88-independent/TRIF-dependent pathway, we optimized an electroporation method to transfect PKCε into neutrophils with very high efficiency. By doing so, a significant IFN-ß mRNA expression was induced, in the absence of LPS stimulation, not only in PKCε-overexpressing neutrophils but also in cells transfected with a series of empty DNA plasmids; however, LPS further upregulated the IFN-ß transcript levels in plasmid-transfected neutrophils, regardless of PKCε overexpression. Phosphoimmunoblotting studies, as well as chromatin immunoprecipitation assays targeting the IFN-ß promoter, revealed that IFN-ß mRNA induction occurred through the cooperative action of IRF3, activated by transfected DNA, and NF-κB, activated by LPS. Additional immunoblotting and coimmunoprecipitation studies revealed that neutrophils constitutively express various cytosolic DNA sensors, including IFN-inducible protein 16, leucine-rich repeat (in Flightless I) interacting protein-1, and DDX41, as well as that IFN-inducible protein 16 is the intracellular receptor recognizing transfected DNA. Consistently, infection of neutrophils with intracellular pathogens, such as Bartonella henselae, Listeria monocytogenes, Legionella pneumophila, or adenovirus type 5, promoted a marked induction of IFN-ß mRNA expression. Taken together, these data raise questions about the role of PKCε in driving the MyD88-independent/TRIF-dependent response and indicate that human neutrophils are able to recognize and respond to microbial cytosolic DNA.