The immune synapse is an exquisitely evolved means of communication between T cells and antigen-presenting cells (APCs) during antigen recognition. Recent evidence points to the transfer of RNA via exosomes as a novel mode of intercellular communication. Here we show that exosomes of T, B and dendritic immune cells contain microRNA (miRNA) repertoires that differ from those of their parent cells. We investigate whether miRNAs are exchanged during cognate immune interactions, and demonstrate the existence of antigen-driven unidirectional transfer of miRNAs from the T cell to the APC, mediated by the delivery of CD63+ exosomes on immune synapse formation. Inhibition of exosome production by targeting neutral sphingomyelinase-2 impairs transfer of miRNAs to APCs. Moreover, miRNAs transferred during immune synapsis are able to modulate gene expression in recipient cells. Thus, our results support a mechanism of cellular communication involving antigen-dependent, unidirectional intercellular transfer of miRNAs by exosomes during immune synapsis.

Human adult stem cells are being evaluated widely for various therapeutic approaches. Several recent clinical trials have reported their safety, showing them to be highly resistant to transformation. The clear similarities between stem cell and cancer stem cell genetic programs are nonetheless the basis of a recent proposal that some cancer stem cells could derive from human adult stem cells. Here we show that although they can be managed safely during the standard ex vivo expansion period (6-8 weeks), human mesenchymal stem cells can undergo spontaneous transformation following long-term in vitro culture (4-5 months). This is the first report of spontaneous transformation of human adult stem cells, supporting the hypothesis of cancer stem cell origin. Our findings indicate the importance of biosafety studies of mesenchymal stem cell biology to efficiently exploit their full clinical therapeutic potential. (Cancer Res 2005; 65(8): 3035-9)

Redistribution of specialized molecules in migrating cells develops asymmetry between two opposite cell poles, the leading edge and the uropod. We show that acquisition of a motile phenotype in T lymphocytes results in the asymmetric redistribution of ganglioside GM3-and GM1-enriched raft domains to the leading edge and to the uropod, respectively. This segregation to each cell pole parallels the specific redistribution of membrane proteins associated to each raft subfraction. Our data suggest that raft partitioning is a major determinant for protein redistribution in polarized T cells, as ectopic expression of raft-associated proteins results in their asymmetric redistribution, whereas non-raft-partitioned mutants of these proteins are distributed homogeneously in the polarized cell membrane. Both acquisition of a migratory phenotype and SDF-1␣-induced chemotaxis are cholesterol depletion-sensitive. Finally, GM3 and GM1 raft redistribution requires an intact actin cytoskeleton, but is insensitive to microtubule disruption. We propose that membrane protein segregation not only between raft and nonraft domains but also between distinct raft subdomains may be an organizational principle that mediates redistribution of specialized molecules needed for T cell migration. Cell movement across a two-dimensional substrate requires a dynamic interplay between attachment at the cell front and detachment at the rear cell edge, combined with a traction machinery that pulls the net cell body forward. As adhesion and detachment occur at opposite cell edges, the moving cell must acquire and maintain spatial and functional asymmetry, a process called polarization (1, 2). This asymmetry develops between two opposite cell edges-the leading edge, which protrudes, and the rear (termed uropod in lymphocytes), which retracts.Because of the specialized functions of these compartments, each pole in migrating cells is enriched in specific receptors and signaling molecules but lacks others. In fibroblast-like cells and lymphocytes, the leading edge contains chemokine receptors, several glycosylphosphatidylinositol-linked proteins, such as the urokinase plasminogen activator receptor (uPAR), as well as the machinery that senses the environment and induces localized actin polymerization (1). Whereas the rear edge in fibroblasts appears to be a passive tail, the lymphocyte uropod is a specialized pseudopod-like projection with important functions, including motility and recruitment of bystander cells. Several intercellular adhesion molecules (ICAMs) concentrate at the uropod, including ICAM-1, -2 and -3, CD43, CD44, as well as the actin-binding proteins of the ezrin-radixinmoesin family. In accordance with its importance in lymphocyte migration, crosslinking of molecules located in the uropod is sufficient to trigger neutrophil polarization and motility (3).To understand polarization and chemotaxis processes, the molecular mechanisms involved in the generation and maintenance of the asymmetric distribution of cell-surface components must be...