Plasmin reduces human T cell arrest on endothelial-like cells by cleaving bound CCL21 from the cell surface

CCL21 is a key homeostatic chemokine best known for its role in lymphocyte homing and compartmentalization in the lymph node. CCL21 also plays a role in trans-endothelial migration and is known to be bound to the surface of endothelial cells in high endothelial venules and inflamed tissues. The effects of CCL21 are highly dependent on its form; full-length CCL21 can bind to the surface of endothelial cells and induce lymphocyte arrest and transendothelial migration, whereas truncated CCL21 cannot. Earlier literature indicates that plasmin can cleave CCL21 from the surface of immune cells, although the mechanism regulating this process on endothelial cells has not been studied. This study demonstrates that the human endothelial-like cell lines ECV304 (LS12) and HMEC-1 can bind the plasmin precursor plasminogen to their cell surface. Furthermore, ECV304 (LS12) cells could endogenously activate plasminogen, yielding plasmin that subsequently released cell surface CCL21. In contrast, cell-surface CCL21 was only released from HMEC-1 after exogenous tPA activated the surface-bound plasminogen. Finally, it was shown that plasmin reduced T cell adhesion to endothelial-like cells with cell surface CCL21 under shear stress conditions. Collectively, for the first time, these data demonstrate that plasmin can cleave endothelial cell surface CCL21, reducing T cell adhesion to endothelial cells under shear stress. Interestingly, this study also indicates that endothelial cells' differential expression of plasminogen activators may regulate plasmin availability and influence T-cell arrest.