Macrophages perform diverse functions within tissues during immune responses to pathogens and injury, but molecular mechanisms by which physical properties of the tissue regulate macrophage behavior are less well understood. Here, we examine the role of the mechanically activated cation channel Piezo1 in macrophage polarization and sensing of microenvironmental stiffness. We show that macrophages lacking Piezo1 exhibit reduced inflammation and enhanced wound healing responses. Additionally, macrophages expressing the transgenic Ca2+ reporter, Salsa6f, reveal that Ca2+ influx is dependent on Piezo1, modulated by soluble signals, and enhanced on stiff substrates. Furthermore, stiffness-dependent changes in macrophage function, both in vitro and in response to subcutaneous implantation of biomaterials in vivo, require Piezo1. Finally, we show that positive feedback between Piezo1 and actin drives macrophage activation. Together, our studies reveal that Piezo1 is a mechanosensor of stiffness in macrophages, and that its activity modulates polarization responses.

Macrophages are innate immune cells that adhere to the extracellular matrix within tissues. However, how matrix properties regulate their function remains poorly understood. Here, we report that the adhesive microenvironment tunes the macrophage inflammatory response through the transcriptional coactivator YAP. We find that adhesion to soft hydrogels reduces inflammation when compared to adhesion on stiff materials and is associated with reduced YAP expression and nuclear localization. Substrate stiffness and cytoskeletal polymerization, but not adhesive confinement nor contractility, regulate YAP localization. Furthermore, depletion of YAP inhibits macrophage inflammation, whereas overexpression of active YAP increases inflammation. Last, we show in vivo that soft materials reduce expression of inflammatory markers and YAP in surrounding macrophages when compared to stiff materials. Together, our studies identify YAP as a key molecule for controlling inflammation and sensing stiffness in macrophages and may have broad implications in the regulation of macrophages in health and disease.

Macrophages perform critical functions for homeostasis and immune defense in tissues throughout the body. These innate immune cells are capable of recognizing and clearing dead cells and pathogens, and orchestrating inflammatory and healing processes that occur in response to injury. In addition, macrophages are involved in the progression of many inflammatory diseases including cardiovascular disease, fibrosis, and cancer. Although it has long been known that macrophages respond dynamically to biochemical signals in their microenvironment, the role of biophysical cues has only recently emerged. Furthermore, many diseases that involve macrophages are also characterized by changes to the tissue biophysical environment. This review will discuss current knowledge about the effects of biophysical cues including matrix stiffness, material topography, and applied mechanical forces, on macrophage behavior. We will also describe the role of molecules that are known to be important for mechanotransduction, including adhesion molecules, ion channels, as well as nuclear mediators such as transcription factors, scaffolding proteins, and epigenetic regulators. Together, this review will illustrate a developing role of biophysical cues in macrophage biology, and also speculate upon molecular targets that may potentially be exploited therapeutically to treat disease.