Switching of the Relative Dominance Between Feedback Mechanisms in Lipopolysaccharide-Induced NF-κB Signaling

Dueling feedback loops set the threshold for mounting an effective innate immune response to infection. Rewiring NF-κB Signaling The bacterial product lipopolysaccharide (LPS) stimulates nuclear factor κB (NF-κB) signaling, which results in the production of proinflammatory cytokines, such as tumor necrosis factor–α (TNF-α), as part of the immune response. NF-κB target genes also include those encoding proteins that inhibit NF-κB signaling through negative feedback loops. By simultaneously studying the dynamics of the nuclear translocation of the NF-κB subunit RelA and the activity of a Tnf-driven reporter in a mouse macrophage cell line, Sung et al. found that the gene encoding RelA was also a target of NF-κB. Synthesis of RelA occurred only at higher concentrations of LPS and constituted a positive feedback loop that dominated over existing negative feedback mechanisms. Genes expressed in response to a high concentration of LPS were enriched for those involved in innate immune responses. Together, these data suggest that the RelA-mediated positive feedback loop enables macrophages to mount an effective immune response only above a critical concentration of LPS. A fundamental goal in biology is to gain a quantitative understanding of how appropriate cell responses are achieved amid conflicting signals that work in parallel. Through live, single-cell imaging, we monitored both the dynamics of nuclear factor κB (NF-κB) signaling and inflammatory cytokine transcription in macrophages exposed to the bacterial product lipopolysaccharide (LPS). Our analysis revealed a previously uncharacterized positive feedback loop involving induction of the expression of Rela, which encodes the RelA (p65) NF-κB subunit. This positive feedback loop rewired the regulatory network when cells were exposed to LPS above a distinct concentration. Paradoxically, this rewiring of NF-κB signaling in macrophages (a myeloid cell type) required the transcription factor Ikaros, which promotes the development of lymphoid cells. Mathematical modeling and experimental validation showed that the RelA positive feedback overcame existing negative feedback loops and enabled cells to discriminate between different concentrations of LPS to mount an effective innate immune response only at higher concentrations. We suggest that this switching in the relative dominance of feedback loops (“feedback dominance switching”) may be a general mechanism in immune cells to integrate opposing feedback on a key transcriptional regulator and to set a response threshold for the host.