Differential immobilization and hierarchical involvement of chemokines in monocyte arrest and transmigration on inflamed endothelium in shear flow

Monocyte extravasation into areas of inflammation involves sequential inter actions of multiple adhesion molecules. However, differential contribution of chemokines produced by cytokine-stimulated endothelium and their recepto rs to leukocyte attachment and transmigration under Row conditions remains to be elucidated. The activation of endothelial cells with TNF-alpha up-reg ulated mRNA and protein expression of the CXC chemokine GRO-alpha and the C C chemokine monocyte chemotactic protein (MCP)-1, which act through the rec eptors CXCR2 and CCR2 expressed on monocytes, respectively. Whereas GRO-alp ha was immobilized to endothelial cells via heparan sulfate proteoglycans, MCP-1 was secreted in a soluble form. Consequently, inhibition experiments with blocking peptide analogues and monoclonal antibodies revealed that GRO -alpha and its receptor CXCR2 but not MCP-I and its receptors substantially contributed to conversion of rolling into firm, shear-resistant arrest of monocytes to TNF-alpha-stimulated endothelium in physiological flow. In con trast, MCP-1 and CCR2 but not GRO-alpha and CXCR2 mediated spreading, shape change and subsequent transendothelial migration, which was evident in flo w but rarely in stasis and may thus require the establishment of a diffusib le MCP-1 gradient. Differential patterns of presentation may determine a fu nctional specialization and hierarchy of chemokines and their receptors in sequential steps of monocyte emigration on inflamed endothelium and shear f low.