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 TH1 and TH17 responses.

A large body of data demonstrates that interferon regulatory factor 5 (IRF5) 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 IRF5 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 TLR8 signaling we used the human monocytic cell line THP-1.Dual, featuring gene reporters for NF-κB and IRFs, to simultaneously study the roles of IRF5 and the NF-κB subunit p65 in TLR8-mediated gene reporter activities. Furthermore, we profiled from these cells the proinflammatory cytokines involved in the differentiation of TH1 and TH17 cells. After ablation of IRF5 and/or p65 we activated the resultant cells with the TLR8 agonists R848 or the psoriasis-associated antimicrobial peptide LL-37 complexed with ssRNA and demonstrate that IRF5 deficiency drastically impairs the secretion of IL-1β, IL-6, IL-12, IL-23 and TNFα. In contrast, the lack of p65 impaired only IL-6, IL-12, and IL-23 secretion. Furthermore, we discovered that upon TLR8 stimulation, IRF5 but not NF-κB signaling is essential to provide a cytokine milieu supporting TH1 responses. Additionally, we demonstrate that IRF5 and NF-κB cooperate to provide a cytokine milieu supporting TH17 responses. Therefore, the distinct role of IRF5 in the intricate signaling network downstream of TLR8 may open new treatment options interfering with but not disrupting NF-κB signaling in human diseases.