Microenvironmental Sensing by Fibroblasts Controls Macrophage Population Size

Animal tissues are comprised of diverse cell types. However, the mechanisms controlling the number of each cell type within tissue compartments remain poorly understood. Here, we report that different cell types utilize distinct strategies to control population numbers. Proliferation of fibroblasts, stromal cells important for tissue integrity, is limited by space availability. In contrast, proliferation of macrophages, innate immune cells involved in defense, repair, and homeostasis, is constrained by growth factor availability. Examination of density-dependent gene expression in fibroblasts revealed that Hippo and TGF- target genes are both regulated by cell density. We found YAP1, the transcriptional co-activator of the Hippo signaling pathway, directly regulates expression of Csf1, the lineage-specific growth factor for macrophages, through an enhancer of Csf1 that is specifically active in fibroblasts. Activation of YAP1 in fibroblasts elevates Csf1 expression and is sufficient to increase the number of macrophages at steady state. Our data also suggest that expression programs in fibroblasts that change with density may result from sensing of mechanical force through actin-dependent mechanisms. Altogether, we demonstrate that two different modes of population control are connected and coordinated to regulate cell numbers of distinct cell types. Sensing of the tissue environment may serve as a general strategy to control tissue composition. For analyzing transcriptional response of MEFs in different stress conditions, 100,000/well (10,000/cm2) MEFs were plated in DMEM. For oxidative stress and osmotic stress, 10 µM sodium arsenite (Sigma Aldrich) or 100 mM NaCl (American Bio) were added to the cells after overnight culture for 3 hours. For ER stress, 1 µM thapsigargin (Cayman) was added for 10 hours after overnight culture. For glucose depletion, glutamine depletion and hypoxia, after overnight culture, cells were incubated in glucose-free DMEM (Gibco) or glutamine-free DMEM (Gibco), or placed in a hypoxia chamber set at 0.1% oxygen for 10 hours.