Modulation of airway inflammation and bacterial clearance by epithelial cell ICAM-1

Many cell types in the airway express the adhesive glycoprotein for leukocytes intercellular adhesion molecule-1 (ICAM-1) constitutively and/or in response to inflammatory stimuli. In this study, we identified functions of ICAM-1 on airway epithelial cells in defense against infection with Haemophilus influenzae. Initial experiments using a mouse model of airway infection in which the bacterial inoculum was mixed with agar beads that localize inflammation in airways demonstrated that ICAM-1 expression was required for efficient clearance of H. influenzae. Airway epithelial cell ICAM-1 expression required few or no leukocytes, suggesting that epithelial cells could be activated directly by interaction with bacteria. Specific inhibition of ICAM-1 function on epithelial cells by orotracheal injection of blocking antibodies resulted in decreased leukocyte recruitment and H. influenzae clearance in the airway. Inhibition of endothelial cell ICAM-1 resulted in a similar decrease in leukocyte recruitment but did not affect bacterial clearance, indicating that epithelial cell ICAM-1 had an additional contribution to airway defense independent of effects on leukocyte migration. To assess this possibility, we used an in vitro model of neutrophil phagocytosis of bacteria and observed significantly greater engulfment of bacteria by neutrophils adherent to epithelial cells expressing ICAM-1 compared with nonadherent neutrophils. Furthermore, bacterial phagocytosis and killing by neutrophils after interaction with epithelial cells were decreased when a blocking antibody inhibited ICAM-1 function. The results indicate that epithelial cell ICAM-1 participates in neutrophil recruitment into the airway, but its most important role in clearance of H. influenzae may be assistance with neutrophil-dependent bacterial killing.