CLL cell migration correlated with CD49d expression and was inhibited by the CD49d antagonist natalizumab. CLL cells that had been circulated in a closed system containing hollow fibers lined with HUVEC cells were recovered from both the circulating compartment and the extravascular compartment. CD49d expression correlated with (A) MMP-9 (r², 0.5; n = 28) and (B) CD38 (r², 0.8; n = 27). In addition, (C) MMP-9 and CD38 expression correlated with each other (r², 0.5; n = 25) and (D) MMP-9 and CXCR4 expression levels correlated with each other (r², 0.2; n = 24). (E) CD49d expression on circulating CLL cells after 48 hours showed the strongest correlation with the percentage of migration of CLL cells into the EVS (r², 0.47; n = 12). (F) To establish a functional role for CD49d in CLL cell migration, CLL cells were pretreated with the anti-CD49d monoclonal antibody and then introduced into the circulating system. Natalizumab-treated CLL cells showed a significant reduction in their capacity to migrate into the EVS at 48 hours.