The blood-brain barrier regulates the expression of a macrophage sialic acid-binding receptor on microglia

In vitro the expression of a sialic acid-binding receptor on murine macrophages, sialoadhesin, is regulated by exposure to an inducing agent present in serum. We have used immunocytochemistry to examine the macrophage populations of the nervous system in order to test whether this serum inducing agent (SIA) also regulates sialoadhesin expression in vivo and whether plasma proteins may influence the phenotype of macrophages of the nervous system. Microglia, the resident macrophages of the central nervous system, reside behind the blood-brain barrier and do not express sialoadhesin. Microglia and macrophage populations in the cicumventricular organs, choroid plexus and leptomeninges are exposed to plasma proteins and some macrophages express sialoadhesin at these sites. Injury to the CNS, which damages the blood-brain barrier, induces sialoadhesin expression on a proportion of macrophages and microglia within the parenchyma. The expression of sialoadhesin matches the temporal and spatial distribution of the plasma extravasation into the brain parenchyma. These experiments show that exposure to SIA is necessary for sialoadhesin expression and lend further support to the idea that the phenotype of microglia is in part regulated by the presence of the blood-brain barrier.