A Noisy Paracrine Signal Determines the Cellular NF-κB Response to Lipopolysaccharide

A low-concentration paracrine TNF-α signal contributes to the variability in NF-κB activation dynamics in the response to lipopolysaccharide. Prolonging NF-κB Activation Regulation of the activity of the transcription factor NF-κB, which plays key roles in immune responses, exhibits complicated cellular dynamics. Tumor necrosis factor–α (TNF-α), a proinflammatory cytokine that activates the death-domain receptor TNFR, and lipopolysaccharide (LPS), a pathogen-derived molecule that activates the Toll-like receptor TLR4, both activate NF-κB. Lee et al. provide a mechanism by which cells respond to these two ligands with different kinetics. Cells responding to TNF-α exhibit an oscillating translocation of NF-κB in and out of the nucleus, with all cells responding similarly. In contrast, cells responding to LPS showed two distinct modes, with one population exhibiting transient nuclear localization of NF-κB and a second exhibiting persistent nuclear localization. Lee et al. modified an existing computational model of the pathways that activate NF-κB and found that cells responding to LPS produce TNF-α in concentrations that are low enough that only a subset of neighboring cells responds. This paracrine TNF-α signal produces the population of LPS-responsive cells with persistent prolonged NF-κB activation. Nearly identical cells can exhibit substantially different responses to the same stimulus. We monitored the nuclear localization dynamics of nuclear factor κB (NF-κB) in single cells stimulated with tumor necrosis factor–α (TNF-α) and lipopolysaccharide (LPS). Cells stimulated with TNF-α have quantitative differences in NF-κB nuclear localization, whereas LPS-stimulated cells can be clustered into transient or persistent responders, representing two qualitatively different groups based on the NF-κB response. These distinct behaviors can be linked to a secondary paracrine signal secreted at low concentrations, such that not all cells undergo a second round of NF-κB activation. From our single-cell data, we built a computational model that captures cell variability, as well as population behaviors. Our findings show that mammalian cells can create “noisy” environments to produce diversified responses to stimuli.