Does Substrate Stiffness Guide Neutrophils During An Inflammation Response?

Neutrophils play a critical role in host response to infection and injury. To reach points of inflammation, they execute a series of adhesion and migration events that allow them to move from the blood stream, through the endothelial cells lining the blood vessels, and into the tissue and surrounding extracellular matrix. Within these few minutes of time, the neutrophil experiences drastically different physical environments, ranging from the viscous fluid in the blood vessel, to the elastic extracellular matrix, to the highly variable points of stiffness at sites of inflammation. Each of these physical environments offers its own unique mechanical cues which can affect a neutrophil's function and guide its behavior. Traditional studies have relied upon glass and plastic substrates, despite the fact that they are orders of magnitude stiffer than blood vessels and most tissues in the body. Turning instead to a physiologically relevant range of elasticity of 5 to 50 kPa in Young's modulus, we tested how neutrophil adhesion, spreading and migration were affected by substrate stiffness. We find that a dramatic and immediate difference is seen in the neutrophil's ability to spread on softer substrates. During migration we find that both speed and directionality are influenced by the substrate stiffness, with more efficient migration occurring on stiffer gels. We also find that these adherent neutrophils pull significantly harder on stiffer gels. These findings demonstrate that neutrophils respond and are sensitive to mechanical cues in the microenvironment, and suggest a possible novel mechanism for neutrophil guidance during injury and inflammation.