In their work, Fine et al. demonstrate that GEF-H1 is required for the spreading and crawling of neutrophils in response to intravascular blood flow. They uncover a novel mechanism that couples shear stress with Rho-dependent migratory behavior of neutrophils during inflammation.

The development of chronic liver diseases is mediated by sustained hepatic inflammation. Our objective was to characterize the molecular mechanisms responsible for the hepatic inflammatory response to time-limited TNF-α and IL-1β expression. C57Bl/6 mice were injected with 2 × 107plaque forming units intraperitoneally of an adenoviral vector containing TNF-α or IL-1β (AdTNF-α or AdIL-1β). A nonreplicating adenoviral vector served as control. Four days later, under ketamine and xylazine anesthesia, the liver microvasculature was examined by intravital microscopy. In the postsinusoidal venules, leukocyte rolling increased significantly in response to both AdTNF-α and AdIL-1β, compared with controls. This response was significantly reduced following injection of an anti-α4-integrin monoclonal antibody (MAb). Postsinusoidal rolling was further reduced to baseline following injection of an anti-P-selectin or anti-L-selectin MAb. Sinusoidal adhesion was greater in mice treated with AdIL-1β than with AdTNF-α. Blocking α4-integrin, P-selectin, or L-selectin had no significant effect on sinusoidal or postsinusoidal adhesion. In separate experiments, we administered AdTNF-α or AdIL-1β to mice deficient in ICAM-1. In ICAM-1−/− mice, postsinusoidal leukocyte rolling significantly increased following expression of IL-1β but not TNF-α. AdIL-1β- but not AdTNF-α-mediated sinusoidal adhesion was ICAM-1 dependent. AdTNF-α-induced sinusoidal adhesion was significantly reduced following 4 days of anti-MIP-2 MAb and anti-KC MAb. Prolonged expression of the cytokines TNF-α and IL-1β increases hepatic leukocyte-endothelial cell interactions. Interestingly, the mechanisms through which these cytokines bring about adhesion within the sinusoids differ; AdIL-1β sinusoidal adhesion uses an ICAM-1-dependent mechanism whereas AdTNF-α-mediated adhesion is ICAM-1 independent but CXC chemokine dependent.

Rapid signaling and structural adaptations to the actin cytoskeleton enable leukocytes to stabilize α4 integrin–mediated adhesion and resist detachment from inflamed endothelium.

Chemokine/chemoattractant G protein-coupled receptors trigger an inside–out signaling network that rapidly activates integrins, a key step in inflammatory leukocyte recruitment. Integrins mediate leukocyte arrest and adhesion to endothelium through multivalent binding, and they transmit outside–in signals to stabilize adhesion and coordinate cell spreading and migration. In the present study, we used RNA interference in the U937 monocytic cell line to investigate the role of talin-1, kindlin-3, and α-actinin-1 in the fMLF- and SDF-1α–induced upregulation of α4β1 integrin affinity and consequent adhesive events. Affinity upregulation of α4β1 integrin was not impaired by small interfering RNA knockdown of talin-1, kindlin-3, or α-actinin-1. Only kindlin-3 knockdown increased flow-induced detachment from VCAM-1–coated surfaces in response to fluid flow, whereas knockdown of either talin-1 or kindlin-3 increased detachment from ICAM-1–coated surfaces. Biochemical analyses revealed that α4β1 expression was highly enriched in U937 cell microridges and murine lymphocyte microvilli. Kindlin-3 was present throughout the cell, whereas talin-1 was largely excluded from microridges/microvilli. The subcellular colocalization of α4β1 and kindlin-3 in microridges may explain why kindlin-3 rapidly associates with α4β1 after G protein-coupled receptor signaling and contributes to adhesion strengthening. Talin-1 contributed to α4β1-dependent chemotaxis, suggesting that it participates in a later stage of the leukocyte adhesion cascade when the leukocyte cytoskeleton undergoes dramatic rearrangement.