Leukocyte Arrest During Cytokine-Dependent Inflammation In Vivo

Abstract

Leukocyte rolling along the walls of inflamed venules precedes their adhesion during inflammation. Rolling leukocytes are thought to arrest by engaging β2 integrins following cellular activation. In vitro studies suggest that chemoattractants may instantaneously activate and arrest rolling leukocytes. However, how leukocytes stop rolling and become adherent in inflamed venules in vivo has remained rather mysterious. In this paper we use a novel method of tracking individual leukocytes through the microcirculation to show that rolling neutrophils become progressively activated while rolling down the venular tree. On average, leukocytes in wild-type mice roll for 86 s (and cover 270 μm) before becoming adherent with an efficiency around 90%. These rolling leukocytes exhibit a gradual β2 integrin-dependent decrease in rolling velocity that correlates with an increase in intracellular free calcium concentration before arrest. Similar tracking analyses in gene-targeted mice demonstrate that the arrest of rolling leukocytes is very rare when β2 integrins are absent or blocked by a mAb. Arrest is ∼50% less efficient in the absence of E-selectin. These data suggest a model of leukocyte recruitment in which β2 integrins play a critical role in stabilizing leukocyte rolling during a protracted cellular activation period before arrest and firm adhesion.