Human T cell leukemia virus type 1 (HTLV-1) is a lymphotropic retrovirus whose cell-to-cell transmission requires cell contacts. HTLV-1-infected T lymphocytes form 'virological synapses', but the mechanism of HTLV-1 transmission remains poorly understood. We show here that HTLV-1-infected T lymphocytes transiently store viral particles as carbohydrate-rich extracellular assemblies that are held together and attached to the cell surface by virally-induced extracellular matrix components, including collagen and agrin, and cellular linker proteins, such as tetherin and galectin-3. Extracellular viral assemblies rapidly adhere to other cells upon cell contact, allowing virus spread and infection of target cells. Their removal strongly reduces the ability of HTLV-1-producing cells to infect target cells. Our findings unveil a novel virus transmission mechanism based on the generation of extracellular viral particle assemblies whose structure, composition and function resemble those of bacterial biofilms. HTLV-1 biofilm-like structures represent a major route for virus transmission from cell to cell.

P.Drevot and C.Langlet contributed equally to this workRecent studies suggest that rafts are involved in numerous cell functions, including membrane traf®c and signaling. Here we demonstrate, using a polyoxyethylene ether Brij 98, that detergent-insoluble microdomains possessing the expected biochemical characteristics of rafts are present in the cell membrane at 37°C. After extraction, these microdomains are visualized as membrane vesicles with a mean diameter of~70 nm. These ®ndings provide further evidence for the existence of rafts under physiological conditions and are the basis of a new isolation method allowing more accurate analyses of raft structure. We found that main components of T cell receptor (TCR) signal initiation machinery, i.e. TCR±CD3 complex, Lck and ZAP-70 kinases, and CD4 co-receptor are constitutively partitioned into a subset of rafts. Functional studies in both intact cells and isolated rafts showed that upon ligation, TCR initiates the signaling in this specialized raft subset. Our data thus strongly indicate an important role of rafts in organizing TCR early signaling pathways within small membrane microdomains, both prior to and following receptor engagement, for ef®cient TCR signal initiation upon stimulation. Keywords: lipid raft/membrane domain/signal transduction/T cell receptor IntroductionMembrane rafts are found in all mammalian cell types as well as in Drosophila, Dictyostelium and yeast (Simons and Ikonen, 1997;Brown and London, 1998;Simons and Toomre, 2000). Not only are rafts enriched in sphingolipids (sphingomyelins and glycosphingolipids) and cholesterol, but these constituents are essential for the formation of rafts (Simons and Ikonen, 1997;Brown and London, 2000). An increasing amount of data suggest that rafts play fundamental roles in diverse cellular functions, particularly in signal transduction, by promoting a segregated arrangement of membrane proteins and lipids (Brown and London, 2000;Simons and Toomre, 2000).Studies in model membranes indicate that rafts correspond to a phase of the lipid bilayer, namely the liquidordered (lo) phase (Brown and London, 2000;Simons and Toomre, 2000). The formation of this lo phase is promoted by sphingolipids, the long saturated acyl chains of which allow tight molecular packing (Brown and London, 2000), and further facilitated by the presence of cholesterol (Simons and Ikonen, 1997;Brown and London, 2000). The use of GPI-anchored proteins and other raft markers to investigate the existence of rafts in living cells has revealed that they are usually very small in size (Simons and Toomre, 2000).Engagement of the T cell receptor (TCR) by its speci®c peptide-MHC (pMHC) ligand triggers intracellular signaling cascades that are required for T lymphocyte development and functions (for a review see Weiss and Littman, 1994). Such cascades are initiated by the activation of a signal transduction machinery, the main components of which include the TCRab heterodimer and the tightly associated CD3 e, g, d and z polypeptides, Lck and ZAP-70...