Atsuyoshi Shimada, M.D., Ph.D.

Professor of Pathology, Kyorin University Faculty of Health Sciences

Background: Systemic inflammation have been reported to shift the brain microenvironment towards a proinflammatory state. However, how peripheral inflammation mediates changes in the brain remains to be clarified. We aimed to determine what cells at the brain-immune interface and of the brain parenchyma and what cytokines are involved in mediating time-dependent changes in the brain microenvironment in response to endotoxemia.

Methods: Mice received lipopolysaccharide (LPS) or saline intraperitoneally and were examined 1, 4 and 24 h after injection. Tissue cytokine concentrations in the spleens and hippocampi were determined by MILLIPLEX MAP Mouse Cytokine/Chemokine Magnetic Bead Panels. Another group of mice were fixed with 4% paraformaldehyde and frozen sections of the spleens, livers and brains containing the choroid plexus and leptomeniges were made. The expression of cytokines and their receptors were studied using immunohistochemical and immunofluorescence staining.

Results: Out of 15 cytokines we measured, tissue concentrations of 14 cytokines were elevated in the spleen. Most of them reached the peak 1 h after LPS injection. In contrast, tissue concentrations of 10 cytokines were elevated in the hippocampus and most of them reached the peak 4 or 24 h after LPS. Cytokines increased at 4 h [CCL2, CXCL1, CXCL2 and interleukin (IL)-6) were expressed by choroid plexus stromal cells, choroid plexus epithelial cells, leptomeningeal stromal cells and hippocampal vascular endothelial cells, all of which were located at the brain-immune interface. Receptors for these cytokines were expressed by astrocytic endfeet. Cytokines increased at 24 h [CCL11, CXCL10 and granulocyte-colony stimulating factor (G-CSF)] were expressed by astrocytes.

In addition, our immunohistological studies revealed that choroid plexus stromal macrophages expressed IL-1beta 1 h after LPS and some stromal cells expressed IL-1R1, an IL-1 receptor. These stromal cells expressed IL-6, CXCL2 and CCL2 4 h after LPS and the same cells bore CXCR2 (CXCL1/CXCL2 receptor). Furthermore, the choroid plexus epithelial cells expressed CXCL1 and CXCL2 4 h after LPS and bore CXCR2 and IL-6 receptor.

Conclusion: Therefore, the signaling using IL-1beta from choroid plexus macrophages and IL-1R1 on choroid plexus stromal cells is the immediate reaction to endotoxemia and triggers the cytokine-mediated cell-cell interactions between choroid plexus stromal and epithelial cells. Then, cells of the brain-immune interface such as choroid plexus stromal cells, choroid plexus epithelial cells, leptomeningeal stromal cells and brain vascular endothelial cells respond to endotoxemia with producing CCL2, CXCL1, CXCL2 and IL-6 earlier than brain parenchymal cells. In the parenchyma, astrocytes play a key role in responding to the signals by using endfeet located in close apposition to the interface cells via cytokine receptors. Thereafter, stimulated astrocytes produce CCL11, CXCL10 and G-CSF, resulting in changes in the brain microenvironment.