| Summary: | Brain inflammatory diseases are characterized by the entry of blood-borne leukocytes
into the brain across the blood-brain barrier (BBB) that normally restricts their entry into
the central nervous system. Recent studies indicate that infiltrating brain
antigen-specific CD8+ T cells clonally expand in multiple sclerosis (MS) lesions.
Previous studies from this laboratory showed that cytokines induce human brain
microvessel endothelial cells (HBMEC) to express or upregulate several endothelial cell
adhesion molecules (ICAM -1, VCAM-1, PECAM-1 and E-selectin), which mediate
leukocyte adhesion and transendothelial migration. In addition, cytokine stimulated
HBMEC are induced to express class II MHC and co-stimulatory molecules and provide
secondary signals for T cell proliferation. In the present study, we investigated the role
of the proteasome in the regulation of MHC class I expression by cerebral microvessel
endothelial cells and CD8+ T cell adhesion to the BBB endothelium. Primary cultures
of HBMEC were treated with: 1) IFN-y to increase the cellular population of the 20S
immunoproteasome, a protease that generates MHC class I specific peptides and 2)
TNF-a to upregulate adhesion molecules. Interferon-Y induced upregulation of the
immunoproteasome subunits (31 i and (35i at the mRNA and the protein levels. The
constitutive expression of class IMHC by HBMEC was upregulated after treatment with
IFN-Y for 48h. Treatment with the proteasome inhibitor lactacystin downregulated the
IFN-y-induced class I upregulation. The minimal adhesion of CD8+ T cells to HBMEC
was significantly upregulated by IFN-y treatment of HBMEC and downregulated by both
lactacystin and MHC class I blocking antibodies. In addition, both the expression of
ICAM-1, VCAM-1 and E-selectin and CD8+ T cell adhesion to HBMEC were upregulated
by TNF-a and downregulated by co-incubation with lactacystin. These studies indicate
that upregulation of the inducible proteasome subunits in HBMEC by proinflammatory
cytokines leads to upregulation of MHC class I expression and antigen non-specific
adhesion of CD8+ T cells to cerebral endothelium. Adhesion of CD8+ T cells is
mediated by adhesion molecules expressed by HBMEC, which are also downregulated
following proteasome inhibition. We therefore suggest that the proteasome plays an
important role in the regulation of CD8+ T cell adhesion to the blood-brain barrier
endothelium during brain inflammatory diseases. === Medicine, Faculty of === Pathology and Laboratory Medicine, Department of === Graduate
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