Disgorging the MVB

The multivesicular body (MVB), a cluster of vesicles surrounded by a limiting membrane, is a ubiquitous but poorly understood structure in eukaryotic cells. On page 53,Kleijmeer et al. present the first evidence that the MVB can be used as a temporary storage depot for membranes and membrane proteins, which can then be deployed rapidly to the surface of the cell. The work also helps to explain how certain antigens are presented to initiate a primary T cell response. Previous work on this problem had left a basic topological question unresolved: how does antigen-presenting MHC II move from the internal vesicles of the MVB to the exterior surface of the plasma membrane?

The authors found that in immature cultured dendritic cells, MHC II is concentrated in the internal vesicles of MVBs, while the peptide-loading...