Monocytes use either CD11/CD18 or VLA-4 to migrate across human endothelium in vitro

Monocytes traverse the endothelial lining of blood vessels and migrate into both normal and inflamed tissues. An in vitro model of a vascular wall, consisting of HUVEC cultured on acellular human amniotic tissue, was employed to examine the roles of several adhesion molecules in diapedesis of monocytes. Approximately half of the monocytes added to this system traversed the endothelium in a time-dependent fashion, completing their migration within 2 h. Pretreatment of HUVEC with IL-1 beta for 4 h increased the rate of adhesion of monocytes, but did not alter the number that ultimately migrated. A mAb to CD18, ts1/18, greatly inhibited adhesion and migration of monocytes when the monocytes were incubated with unstimulated HUVEC monolayers for 20 min. Much less inhibition was observed when the incubation period was increased to 2 h or when HUVEC were pretreated with IL-1 beta. A mAb to VLA-4, HP1/2, had little or no inhibitory effect in all cases. The combination of ts1/18 and HP1/2 greatly inhibited (up to 98%) adhesion and migration of monocytes across both unstimulated and IL-1 beta-stimulated monolayers. Additional inhibition experiments indicated that VLA-4 interacted with unstimulated endothelium by binding to VCAM-1 and, to a much lesser extent, fibronectin. These results suggest that monocytes are capable of interacting with endothelium during diapedesis via either CD11/CD18- or VLA-4-dependent pathways.