Transendothelial neutrophil migration - Role of neutrophil-derived proteases and relationship to transendothelial protein movement

During an acute inflammatory response polymorphonuclear leukocytes (PMNs) adhere to and emigrate across the venular microvasculature. There is general agreement on the mechanisms involved in PMN adhesive interactions. However, the mechanisms by which PMNs migrate across the endothelial lining remain controversial, particularly with respect to the role of elastase. In the present study, we used human umbilical vein endothelial cells (HUVECs) and PMNs to test the hypothesis that the relative role of PMN-derived elastase may be dependent on the degree of HUVEC retraction within monolayers. A high (10(-7) mol/L), but not a low (10(-10) mol/L), concentration of platelet-activating factor (PAF) caused HUVEC retraction of sufficient magnitude to increase transendothelial protein movement. Elastase inhibitors prevented PMN transendothelial migration in response to the low, but not the high, concentration of PAF. These findings suggest that PMN migration across confluent endothelial cells is elastase dependent, whereas PMN migration across retracted endothelial cells is elastase independent. However, under the latter condition (high concentration of PAF), the two endogenous proteases, alpha(2)-macroglobulin and alpha(1)-antitrypsin, could interfere with PAF-induced PMN transendothelial migration. Thus, as the concentration of PAF is increased, migrating PMNs use other proteases, in addition to elastase. We also noted that transendothelial protein movement is closely coupled to PMN migration.