Interferon regulatory factor 5 and nuclear factor kappa‐B exhibit cooperating but also divergent roles in the regulation of pro‐inflammatory cytokines important for the development of TH 1 and TH 17 responses

A large body of data demonstrates that interferon regulatory factor 5 ( IRF 5) and nuclear factor kappa B ( NF ‐κB) are the two major transcription factors in classically activated macrophages responsible for the transcriptional control of proinflammatory genes. Although recent evidence suggests that IRF 5 interacts with certain members of the nuclear factor kappa B pathway, the extent of cooperation and its implications in disease are ambiguous. Since both pathways are known for their strong contributions in TLR 8 signaling we used the human monocytic cell line THP ‐1.Dual, featuring gene reporters for NF ‐κB and IRF s, to simultaneously study the roles of IRF 5 and the NF ‐κB subunit p65 in TLR 8‐mediated gene reporter activities. Furthermore, we profiled from these cells the proinflammatory cytokines involved in the differentiation of TH 1 and TH 17 cells. After ablation of IRF 5 and/or p65 we activated the resultant cells with the TLR 8 agonists R848 or the psoriasis‐associated antimicrobial peptide LL ‐37 complexed with ss RNA and demonstrate that IRF 5 deficiency drastically impairs the secretion of IL ‐1β, IL ‐6, IL ‐12, IL ‐23 and TNF α. In contrast, the lack of p65 impaired only IL ‐6, IL ‐12, and IL ‐23 secretion. Furthermore, we discovered that upon TLR 8 stimulation, IRF 5 but not NF ‐κB signaling is essential to provide a cytokine milieu supporting TH 1 responses. Additionally, we demonstrate that IRF 5 and NF ‐κB cooperate to provide a cytokine milieu supporting TH 17 responses. Therefore, the distinct role of IRF 5 in the intricate signaling network downstream of TLR 8 may open new treatment options interfering with but not disrupting NF ‐κB signaling in human diseases.