A 2-event in vitro model of PMN-mediated pulmonary endothelial damage. HMVECs were grown to 80% to 90% confluence on 12-well plates and were incubated with buffer or LPS for 6 hours, followed by the addition of buffer or freshly isolated PMNs that were allowed to settle for 30 minutes at a target-effector ratio of 10:1. After the addition of PMNs or buffer, a range of human recombinant CD40L concentrations or buffer controls was added to the HMVECs. (dark gray bars) Control cells; buffer-treated HMVECs with no PMNs. (light gray bars) HMVECs pretreated with 2 μg/mL LPS for 6 hours at 37°C, followed by treatment with buffer or a range of sCD40L concentrations (as delineated on the x-axis). (black bars) HMVECs incubated with LPS for 6 hours, followed by the addition of PMNs and sCD40L. Incubation of the adherent PMNs with concentrations of sCD40L at 100 ng/mL, 10 ng/mL, and 1 μg/mL resulted in a significant decrease in the number of viable HMVECs. *P < .05 (n = 3) compared with buffer. These data demonstrate that sCD40L may cause concentration-dependent PMN-mediated damage of LPS-activated HMVECs. Importantly, HMVECs stimulated with LPS and exposed to PMNs without sCD40L did not demonstrate PMN-mediated damage but did evidence widespread PMN adherence. Data are presented as mean ± standard error of the mean.