Bronchoalveolar lavage during RSV infection of the lower airway predominantly contains PMNs, however, the mechanisms of migration of PMNs into the airway lumen are unknown. We have shown that RSV-infected human alveolar epithelial cell line (A549) produces IL-8. IL-8 is known to modulate the expression of PMN adhesion molecule:Mac-1 (CD11b/CD18), which is utilized in the adhesion-based migration of PMNs. In the present study, we evaluated (i) the role of RSV-induced IL-8 in regulation of expression of Mac-1, and (ii) the role of Mac-1 in adhesion and transepithelial migration of PMNs during RSV infection. 500 μl of conditioned supernatants of RSV-infected and-uninfected (sham) A549 cultures, and 10 ng of rIL-8 (positive control) were incubated with 20 μg of anti-human IL-8 antibody (Ab) or goat Ig-G control Ab. 5 × 106 human PMNs were then incubated with these preparations for 45 min, washed and stained with phycoerythrin-conjugated anti-Mac-1 Ab and analyzed by flow cytometry. The results of triplicate experiments are expressed as relative intensity when compared with sham infection. Table

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For adhesion and migration studies, A549 monolayers were grown on cell culture inserts in a 24-well plate. 24 h after RSV infection, neutralizing Ab against Mac-1 or control Ab were mixed in the chambers above the monolayer, and incubated for 2 h. 1.5 × 106 human PMNs were then inoculated into the upper chambers. After 3 h incubation, the PMNs in the top and bottom chambers, and the fraction adherent to A549 monolayers were isolated and quantitated by manual counting. The anti-Mac-1 Ab significantly blocked PMN adhesion by 75% and migration by 42% when compared with the control Ab(P<0.05).

These results indicate that IL-8 released from RSV-infected epithelial cells is the predominant mediator that modulates the surface expression of Mac-1 which in turn facilitates the PMN adhesion to and migration through the alveolar epithelial cells.