Identification of cellular and molecular mechanisms involved in leukocyte trafficking across the blood-brain-barrier in multiple sclerosis

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In multiple sclerosis and in its animal model, experimental autoimmune encephalomyelitis (EAE), autoaggressive T cells get access to the central nervous system (CNS), causing inflammation and demyelination. Immune cell recruitment into the CNS is controlled by the highly specialized endothelial cells forming the blood-brain barrier (BBB). Inhibition of immune cell trafficking across the BBB with the humanized anti-alpha4-integrin antibody natalizumab is a successful therapeutic regimen for the treatment of relapsing-remitting multiple sclerosis, but comes with a risk of developing progressive multifocal leukoencephalopathy (PML), an often fatal disease of the CNS caused by JC virus infection of oligodendrocytes. The general objective of the proposed research project is to identify novel molecular pathways involved in the multi-step recruitment of T cells that will allow the sole blockade of CNS recruitment of destructive immune cells, while leaving the migration of protective immune cells into the CNS untouched. Whereas the first molecules mediating T cell arrest, polarization and crawling have been described the molecular mechanisms involved in T cell diapedesis across the BBB remain almost unexplored. Here we will focus on investigating the individual contribution of the junctional adhesion molecule-B (JAM-B) as an alternative ligand for alpha4-integrins, as wall as the more recently described molecules such as ALCAM, MCAM and Ninjurin in T cell polarization, arrest, crawling and diapedesis across the BBB in vitro and in vivo. Due to the fact that JAM-B is an interaction partner of alpha4-integrin and therefore is an interesting target in EAE the first aim is dedicated to clarify the role of JAM-B in EAE pathogenesis by studying disease development in JAM-B-/- C57BL/6 and SJL mice. Infiltrated leukocytes will be isolated from the CNS and subtypes will be determined using flow cytometry. Furthermore, using an in vitro model for the BBB the contributions of JAM-B, ALCAM, MCAM and Ninjurin to the BBB integrity and T cell extravasation under flow conditions shall be analysed. The second aim is designed to delineate, if the special barrier characteristics of the BBB favor transcellular or paracellular diapedesis of T cell subsets and to determine, if JAM-B is involved in the respective processes. The interaction of different T cells with the CNS microvasculature will be investigated by means of intravital microscopy and 2-Photon intravital microscopy in JAM-B-/- mice. Moreover, using VE-cadherin-GFP knockin mice, in which endothelial junctions are visualized, the favoured diapedesis route can be evaluated in dependence to individual adhesion molecules. The third aim is to identify new adhesion molecules in specific cell types by transcriptional profiling using mouse TU tagging. Finally, the fourth point is to examine whether MK2 is involved in the biosynthesis of JAM-B and ALCAM.