Quantitative analysis of competitive cytokine signaling predicts tissue thresholds for the propagation of macrophage activation

Toll-like receptor (TLR) signaling regulates macrophage activation and effector cytokine propagation in the constrained environment of a tissue. In macrophage populations, TLR4 stimulates the dose-dependent transcription of nuclear factor B (NF-B) target genes. However, using single-RNA counting, we found that individual cells exhibited a wide range (three orders of magnitude) of expression of the gene encoding the proinflammatory cytokine tumor necrosis factor  (TNF-). The TLR4-induced TNFA transcriptional response correlated with the extent of NF-B signaling in the cells and their size. We compared the rates of TNF- production and uptake in macrophages and mouse embryonic fibroblasts and generated a mathematical model to explore the heterogeneity in the response of macrophages to TLR4 stimulation and the propagation of the TNF- signal in the tissue. The model predicts that the local propagation of the TLR4-dependent TNF- response and cellular NF-B signaling are limited to small distances of a few cell diameters between neighboring tissue-resident macrophages. In our predictive model, TNF- propagation was constrained by competitive uptake of TNF- from the environment, rather than by heterogeneous production of the cytokine. We propose that the highly constrained architecture of tissues enables effective localized propagation of inflammatory cues, while avoiding out-of-context responses at longer distances.