Neutrophil recruitment limited by high-affinity bent β₂ integrin binding ligand in cis

Neutrophils are essential for innate immunity and inflammation and many neutrophil functions are β₂ integrin-dependent. Integrins can extend (E⁺) and acquire a high-affinity conformation with an 'open' headpiece (H⁺). The canonical switchblade model of integrin activation proposes that the E⁺ conformation precedes H⁺, and the two are believed to be structurally linked. Here we show, using high-resolution quantitative dynamic footprinting (qDF) microscopy combined with a homogenous conformation-reporter binding assay in a microfluidic device, that a substantial fraction of β₂ integrins on human neutrophils acquire an unexpected E^-H⁺ conformation. E^-H⁺ β₂ integrins bind intercellular adhesion molecules (ICAMs) in cis, which inhibits leukocyte adhesion in vitro and in vivo. This endogenous anti-inflammatory mechanism inhibits neutrophil aggregation, accumulation and inflammation.