Release of cytokines by brain endothelial cells: A polarized response to lipopolysaccharide

Brain endothelial cells (BECs) comprise the blood-brain barrier (BBB) and are an active part of the neuroimmune system, responding to and transporting cytokines. BECs also have the ability to secrete neuroimmune substances, including cytokines. A unique feature of the BEC is its polarization, with its luminal (blood-facing) and abluminal (brain-facing) cell membranes differing in their lipid, receptor, and transporter compositions. This polarization could have functional consequences for neuroimmune communication. We postulated (i) that cytokine secretion from the luminal or abluminal membranes could differ under baseline or stimulated conditions and (ii) that an immune challenge from one side of the BBB could result in cytokine release from the other. We used an in vitro BBB model of mouse BECs cultured as monolayers to investigate cytokine secretion into luminal and abluminal chambers. Our major findings in these studies were: (i) the first demonstration that interleukin (IL)-1alpha, IL-10, and granulocyte-macrophage colony-stimulating factor are secreted from BECs and confirmation of the secretions of IL-6 and tumor necrosis factor-alpha, (ii) that constitutive and lipopolysaccharide (LPS)-stimulated secretion of cytokines is polarized in favor of luminal secretion, and (iii) that response to neuroimmune stimulation is also polarized as exemplified by the finding that abluminal LPS more robustly induced secretion of IL-6 than did luminal LPS. Overall, these findings support the BBB as an important source of cytokines. Furthermore, the BBB can respond to immune challenges received from one side of the neuroimmune axis by releasing cytokines into the other.