Intercellular exchange of proteins: the immune cell habit of sharing.

The recent recognition of new types of cell-cell communication pathways challenges classic theories of cell autonomy. Evidence of functional "proteome mixing" among interacting cells, particularly immune cells, supports the notion that no cell is an island, and that even these "unsplittable" units are actually non-autonomous. We summarize various mechanisms of intercellular transfer of proteins--trans-endocytosis, trogocytosis, exosomal transport, shuttle through nanotubes, and cell-contact-dependent intercellular transfer of intracellular proteins including oncogenic Ras. These phenomena suggest exciting new possibilities for proteome research, focusing on system-level proteomics that characterize cell contents and functions in the context of intercellular protein transfer.