Endothelial cell ICAM‐1‐dependent signaling negatively regulates MCP‐1 production

Rapid recruitment of neutrophils to the site of infection to kill bacteria is followed by monocyte recruitment to remove cellular debris after bacterial invasion. The mechanisms which orchestrate the sequential leukocyte recruitment are not well known. Endothelial cell surface ICAM‐1 mediates firm adhesion of neutrophils which facilitates their transmigration across the vascular barrier. Whether ICAM‐1 activation by neutrophil adhesion regulates the adhesion and migration of monocytes is not known. Monocyte chemo‐attractant protein‐1 (MCP‐1) released from several cell types in the lung including endothelial cells induces chemotaxis and transendothelial migration of monocytes at sites of inflammation. Here, we examined the role of Src‐dependent signaling downstream of activated ICAM‐1 in the regulation of MCP‐1 generation by endothelial cells. siRNA‐mediated depletion of ICAM‐1 in human endothelial cells increased basal and TNFα‐stimulated MCP‐1 secretion, whereas activation of ICAM‐1 by antibody cross‐linking inhibited TNFα‐induced MCP‐1 generation. Time‐course studies showed that inhibition of TNFα‐induced MCP‐1 generation by ICAM‐1 antibody cross‐linking was maximal after 4 hrs. Furthermore, transfection of HUVEC with WT ICAM‐1 and phospho‐mimicking ICAM‐1 mutant (Y518D) inhibited TNFα‐induced MCP‐1 generation compared to cells transduced with phosphorylation‐defective ICAM‐1 mutant (Y518F). Finally, we observed 7‐fold greater MCP‐1 production in ICAM‐1 knockout mouse lungs compared to control mouse lungs. Taken together, these results suggest that basal ICAM‐1 expression as well as ICAM‐1‐phosphorylation‐dependent signaling inhibits MCP‐1 secretion and thereby may inhibit monocyte recruitment during the first wave of inflammation.