Mononuclear Phagocytes Egress f rom an In Vitro

Little is known about how mononuclear phagocytes (MP) are cleared from sites o f inflammation as inflammatory lesions resolve. In this study, the possibility that MP could be cleared from tissues by migrating across endothelium in the basal to apical direction was investigated. In an in vitro model o f a blood vessel wall consisting of human umbilical vein endothelial cells (HUVEC) grown on amniotic tissue, a majority o f MP that initially transmigrated into the amnion later exited by migrating back across the endothel ium in the basal to apical direction. MP that egressed from these cultures adhered to the apical surface of the endothelium or were found nonadherent in the med ium above the endothehum. Egression o f MP continued throughout the 4-d period examined, displaying higher than first order kinetics and a tl/2 of approximately 24 h. These kinetics were decreased by increasing the volume of medium bathing the cultures, suggesting that a soluble factor(s) regulates the rate of egression. In contrast, the kinetics were accelerated by pretreating the endothelium with IL-1. The initial phase of this increased rate of egression was inhibited by antibodies to intercellular adhesion molecule 1 (ICAM-1) or CD18 by 100 and 71%, respectively. Immunostaining revealed that ICAM-1 was present on the apical and basal surfaces of umbilical vein endothel ium in vitro and in situ. These data demonstrate that MP can traverse endothelium in the basal to apical direction, and lend insight into the mechanisms by which this process occurs.