A transformation from blobs (top) to tubules (bottom) helps load antigen.

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 accessory molecule DM is found mostly in the MVB limiting membrane. When the cells are activated, the internal vesicles fuse with the limiting membrane, presumably allowing DM to load antigen onto the MHC. The MVB then elongates into a tubular structure that extends toward the plasma membrane. Vesicles containing both MHC II and DM bud from the tip of this structure, possibly ending up at the cell surface where antigens are displayed. ▪