Live cell dynamics of MHC class II transport from lysosomes to the plasma membrane in developing dendritic cells

MHC class II (MHCII) transport in dendritic cells (DCs) follows a unique intracellular trafficking pathway from synthesis in the endoplasmic reticulum, transit through the Golgi apparatus, accumulation in lysosomes, and finally appearance at the cell surface. Its compartmentalization at each transport step correlates with changes it must undergo for proper functioning as ligand to a T cell receptor. As a result, antigen is presented to the T cell in the form of a peptide-MHCII complex and activation of the T cell leads to initiation of an immune response.; The dendritic cell system is ideal for examining MHCII export since it is the principle antigen presenting cell type. Moreover, DCs regulate MHCII transport in a developmental fashion which can be reproduced in an experimental setting allowing the investigator to follow a synchronous pool of molecules from one site in the cell to another.; Here, we focus on the export of MHCII molecules from lysosomes to the cell surface. In live cells, we identify tubular transport compartments which go on to fuse directly with the cell surface. We then characterize the transient tubular transport compartment showing that it tracks along microtubules and contains lysosomal membrane proteins which are eventually sorted from MHCII near or at the cell surface.