Transcellular transport of CCL2 across brain microvascular endothelial cells

The means by which the chemokine CCL2 produced in the brain parenchyma can recruit leukocytes lying behind the highly impervious endothelium of the blood-brain barrier (BBB) has remained a paradox. As other chemokines have been evidenced to stimulate their own synthesis and release by peripheral microvascular endothelial cells, and/or undergo transcytosis in the abluminal-to-luminal direction, we determined whether CCL2 experiences similar fates across brain microvascular endothelial cells (BMEC). Using cultured BMEC as a paradigm of the BBB, it was observed that exogenous unlabeled CCL2 actually depressed the release of endogenous CCL2, and further caused diminished CCL2 mRNA levels in these cells. On the other hand, exogenous (125)I-labeled CCL2 exhibited transport across BMEC in a manner that was sensitive to temperature, competition by excess unlabeled CCL2 but not unlabeled CCL3, knockdown of caveolin-1/caveolae, and elimination of the cognate CCL2 receptor CCR2. These results implied a facet of CCL2 transport by a transcellular mechanism partly involving binding of CCL2 to CCR2, and subsequent transfer to caveolae vesicles for transcytosis. This notion was supported by double-label immuno-electronmicroscopy, which revealed co-localization of caveolin-1 with exogenous CCL2, during this chemokine's transit across BMEC. Collectively, these findings provide a rationale by which CCL2, deposited on the abluminal side of the brain microvasculature during inflammatory episodes, can be relayed across the BBB to foster leukocyte recruitment.