Characterization of binding sites for chemokines MCP-1 and MIP-1alpha on human brain microvessels

The presence of binding sites for the beta chemokines monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha) has recently been identified on human brain microvessels. We extend these findings in this report to reveal that such sites exemplify characteristics of the recognized major receptors for MCP-1 and MIP-1alpha: CCR2, and CCR1 and CCR5, respectively. Specifically, labeled MCP-1 binding to isolated brain microvessels was inhibited by unlabeled MCP-1 and MCP-3, the latter another CCR2 ligand, but not by MIP-1alpha. Inhibition of labeled MIP-1alpha binding was achieved with unlabeled MIP-1alpha and RANTES, the latter a beta chemokine that binds to both CCR1 and CCR5, but not by MCP-1. Labeled MIP-1alpha binding was also antagonized by unlabeled MCP-3, which is also recognized by CCR1, and MIP-1beta, which is a ligand for CCR5. Labeled MCP-1 and MIP-1alpha were further observed to be internalized within the endothelial cells of brain microvessels, following their binding to the microvascular surface at 37 degrees C. Additionally, exposure of microvessels to unlabeled MCP-1 or MIP-1alpha was accompanied by the initial loss and subsequent recovery of surface binding sites for these chemokines, which occurred on a time scale consistent with ligand-induced endocytosis and recycling. These collective features bear striking similarity to those that characterize interactions of MCP-1 and MIP-1alpha with their receptors on leukocytes and underscore the concept of cognate chemokine receptors on brain microvascular endothelium.