CLL cells actively underwent transendothelial cell migration associated with the upregulation of MMP-9, CD49d, CD80, CD38, and CD69. CLL cells were circulated through the hollow fiber model system lined with HUVEC endothelial cells for 48 hours. Paired samples were taken from the circulating compartment of the model and from the EVS. (A) Scanning electron micrograph of the outside of a hollow fiber showing CLL cells that migrated from the inside of the fiber to the outside. Scale bar represents 10 microns. (B) CLL cells showed a significant increase in migration at 48 hours when compared with 1 hour (n = 21; P = .03). (C) Polystyrene beads were introduced into the circulating compartment of the circulating system and circulated for 1 hour. Aliquots of 100 μL were removed from both the circulating and extravascular compartment, and the number of beads in each 100-μL aliquot was measured by flow cytometry. (Ci) Ten-micrometer beads and (Cii) 2.5-μm fluorescent beads in the circulating compartment and below the dashed line in the extravascular compartment after 1 hour. No beads were recovered from the EVS in each of the 3 replicates of the experiment. In contrast, CLL cells were consistently recovered from the extravascular compartment at this time. Compared with CLL cells remaining in the circulating compartment, CLL cells recovered from the EVS had increased expression of MMP-9, CD49d, CD80, CD38, CD69, and CD19 when measured by flow cytometry after (D) 1-hour and (E) 48-hour circulation around the system. Error bars indicate ± 1 SD around the mean fluorescence intensity on a population of doublet-discriminated CD19⁺ lymphocytes. *P < .05.