Tethered exosomes and their role in MHC-II presentation (360G-Wellcome-216370_Z_19_Z)

Peptide-loaded MHC-II (pMHC-II) is presented at the surface of antigen presenting cells and it has historically been assumed that pMHC-II is localised on the plasma membrane. However, immunofluorescence data reveals pMHC-II localised to patches rather than uniformly distributed. The loading of peptide onto MHC-II occurs within MVBs, specifically on ILVs. In order for pMHC-II to be subsequently incorporated into the plasma membrane, ILVs would be required to ‘back-fuse’ with the limiting membrane of the MVB. This back-fusion step represents an ambiguous fusion event. I have recently shown that exosomes can be physically tethered to the cell surface by tetherin. Tetherin is highly expressed in antigen presenting cells and these cells display clusters of exosomes at their surface. Using biochemistry, immuno-electron microscopy and functional assays I wish to test the hypothesis that surface-tethered exosomes present pMHC-II at the surface of antigen presenting cells. I will establish the molecular machinery which traffics tetherin to ILVs/exosomes, and define the residues on tetherin that are required for this trafficking. Finally, I will explore where and how exosomes fuse to recipient cells, and use a screening approach to identify molecules involved in exosomes – plasma membrane fusion.