Effect of prestimulating HUVECs with IL-1 on transmigration of hematopoietic cells. Confluent HUVECs on Transwell filters were prestimulated with IL-1 (20 U/mL) for 4 hours and washed with warm medium before the start of transmigration assays. In further experiments with CD34⁺ cells, HUVECs were preincubated with IL-1 for up to 16 hours and/or IL-1 was present throughout the assay. The data from all experiments with CD34⁺ cells were similar and, hence, were pooled. Data are the mean ± SEM of seven experiments with neutrophils, four experiments with freshly isolated mobilized (quiescent) CD34⁺-selected cells, and four with growth factor-activated mobilized CD34⁺-selected cells.