A soluble gradient of endogenous monocyte chemoattractant protein-1 promotes the transendothelial migration of monocytes in vitro

Chemokines secreted by endothelium may promote diapedesis of leukocytes by a gradient-dependent chemotactic mechanism or by stimulating random motility so that leukocytes transmigrate in a gradient-independent manner. Alternatively, chemokines may bind to endothelium and extracellular matrix to stimulate haptotactic migration. We first analyzed the role of the chemokine monocyte chemoattractant protein-1 (MCP-1) in the migration of human monocytes across untreated or IL-1-stimulated HUVEC monolayers cultured on human amnion. Then we further examined whether MCP-1-dependent transmigration occurred through a chemokinetic, chemotactic, or haptotactic mechanism. A neutralizing mAb against MCP-1 inhibited passage of monocytes across untreated or IL-1-stimulated HUVEC by 74 ± 3% and 45 ± 4%, respectively. Addition of MCP-1 itself to the apical compartment of unstimulated HUVEC/amnion cultures also reduced the transmigration of monocytes, in this instance by 73 ± 9%. MCP-1 suppressed diapedesis only when present above the endothelium at a concentration equal to or greater than that endogenously deposited beneath the endothelium, and its inhibitory action could be overcome by addition of more concentrated MCP-1 below the HUVEC cultures. As much as 90% of the MCP-1 secreted into the underlying basement membrane and connective tissue could be washed out of HUVEC/amnion cultures; this procedure decreased transmigration by 69 ± 4%. These data indicate that MCP- 1 promotes transmigration of monocytes, but only when present in a gradient across endothelial monolayers. They further suggest that this gradient is predominantly soluble, rather than haplotactic.