Neutrophil adhesion and chemotaxis depend on substrate mechanics

Abstract

Neutrophil adhesion to the vasculature and chemotaxis within tissues play critical roles inthe inflammatory response to injury and pathogens. Unregulated neutrophil activity hasbeen implicated in the progression of numerous chronic and acute diseases such asrheumatoid arthritis, asthma and sepsis. Cell migration of anchorage-dependent cells isknown to depend on both chemical and mechanical interactions. Although neutrophilresponses to chemical cues have been well characterized, little is known about the effect ofunderlying tissue mechanics on neutrophil adhesion and migration. To address thisquestion, we quantified neutrophil migration and traction stresses on compliant hydrogelsubstrates with varying elasticity in a micromachined gradient chamber in which wecould apply either a uniform concentration or a precise gradient of the bacterialchemoattractant fMLP. Neutrophils spread more extensively on substrates ofgreater stiffness. In addition, increasing the stiffness of the substrate leads to asignificant increase in the chemotactic index for each fMLP gradient tested. As thesubstrate becomes stiffer, neutrophils generate higher traction forces withoutsignificant changes in cell speed. These forces are often displayed in pairs andfocused in the uropod. Increases in the mean fMLP concentration beyond theKD ofthe receptor lead to a decrease in chemotactic index on all surfaces. Blocking with an antibody againstβ2-integrins leads to a significant reduction, but not an elimination, of directed motility onstiff materials, but no change in motility on soft materials, suggesting neutrophils candisplay both integrin-dependent and integrin-independent motility. These findings arecritical for understanding how neutrophil migration may change in different mechanicalenvironments in vivo and can be used to guide the design of migration inhibitors that moreefficiently target inflammation.