title: Human brain endothelial cells endeavor to immunoregulate CD8 T cells via PD-1 ligand expression in multiple sclerosis.
abstract: BACKGROUND
  Multiple sclerosis (MS), an inflammatory disease of the central nervous system (CNS), is characterized by blood-brain barrier (BBB) disruption and massive infiltration of activated immune cells. Engagement of programmed cell death-1 (PD-1) expressed on activated T cells with its ligands (PD-L1 and PD-L2) suppresses T cell responses. We recently demonstrated in MS lesions elevated PD-L1 expression by glial cells and absence of PD-1 on many infiltrating CD8 T cells. We have now investigated whether human brain endothelial cells (HBECs), which maintain the BBB, can express PD-L1 or PD-L2 and thereby modulate T cells.
  
  
  METHODS
  We used primary cultures of HBECs isolated from non-tumoral CNS tissue either under basal or inflamed conditions. We assessed the expression of PD-L1 and PD-L2 using qPCR and flow cytometry. Human CD8 T cells were isolated from peripheral blood of healthy donors and co-cultured with HBECs. Following co-culture with HBECs, proliferation and cytokine production by human CD8 T cells were measured by flow cytometry whereas transmigration was determined using a well established in vitro model of the BBB. The functional impact of PD-L1 and PD-L2 provided by HBECs was determined using blocking antibodies. We performed immunohistochemistry for the detection of PD-L1 or PD-L2 concurrently with caveolin-1 (a cell specific marker for endothelial cells) on post-mortem human brain tissues obtained from MS patients and normal controls.
  
  
  RESULTS
  Under basal culture conditions, PD-L2 is expressed on HBECs, whilst PD-L1 is not detected. Both ligands are up-regulated under inflammatory conditions. Blocking PD-L1 and PD-L2 leads to increased transmigration and enhanced responses by human CD8 T cells in co-culture assays. Similarly, PD-L1 and PD-L2 blockade significantly increases CD4 T cell transmigration. Brain endothelium in normal tissues and MS lesions does not express detectable PD-L1; in contrast, all blood vessels in normal brain tissues are PD-L2-positive, while only about 50% express PD-L2 in MS lesions.
  
  
  CONCLUSIONS
  Our observations suggest that brain endothelial cells contribute to control T cell transmigration into the CNS and immune responses via PD-L2 expression. However, such impact is impaired in MS lesions due to downregulation of endothelium PD-L2 levels.
authors: Department of Medicine, Université de Montréal, CRCHUM, Pavilion J,A, de Sève, 1560 Sherbrooke E, Montreal, QC, H2L 4M1, Canada.
authors: Camille L
authors: CL
authors: Pittet
authors: Jia
authors: J
authors: Newcombe
authors: Alexandre
authors: A
authors: Prat
authors: Nathalie
authors: N
authors: Arbour
mesh_terms: D000328:Adult; D000368:Aged; D060890:B7-H1 Antigen / Q000235:genetics / Q000378:metabolism*; D001812:Blood-Brain Barrier / Q000166:cytology* / Q000276:immunology / Q000473:pathology; D001921:Brain / Q000098:blood supply / Q000166:cytology / Q000276:immunology; D015496:CD4-Positive T-Lymphocytes / Q000276:immunology; D018414:CD8-Positive T-Lymphocytes / Q000166:cytology / Q000276:immunology*; D049109:Cell Proliferation; D002478:Cells, Cultured; D018920:Coculture Techniques; D016207:Cytokines / Q000276:immunology; D042783:Endothelial Cells / Q000166:cytology / Q000276:immunology*; D005260:Female; D006801:Humans; D008213:Lymphocyte Activation / Q000276:immunology; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000276:immunology* / Q000473:pathology*; D061027:Programmed Cell Death 1 Ligand 2 Protein / Q000235:genetics / Q000378:metabolism*; D061026:Programmed Cell Death 1 Receptor / Q000378:metabolism; D057705:Transendothelial and Transepithelial Migration
chemical_list: D060890:B7-H1 Antigen; C423236:CD274 protein, human; D016207:Cytokines; C105992:PDCD1 protein, human; C556977:PDCD1LG2 protein, human; D061027:Programmed Cell Death 1 Ligand 2 Protein; D061026:Programmed Cell Death 1 Receptor