Retroviruses are enveloped viruses that form by pinching off from cellular membranes. Upon maturation, released particles are competent to infect receptor-expressing cells. During entry, virus and cell membranes fuse and capsids disassemble in the cytoplasm. Virus budding and virus entry are commonly studied as separate processes. In reality, they are coordinated as most viruses spread when cells contact each other, rather than relying on diffusion to reach target cells. Under conditions of cell-cell contact, virus spreading is 2-3 orders of magnitude more efficient. To understand why this is the case, we use the murine leukemia virus (MLV) and the avain leukosis virus (ALV) as model systems. Chronically infected cells producing fluorescently labeled virions are co-cultured with target cells expressing fluorescent receptor. Using this system, we have visualized cell-to-cell transmission of retroviruses in living cells. Our results reveal a novel mode of viral spreading via thin cellular bridges that are formed from actin-rich filopodia. These bridges originate from non-infected cells and are anchored to the infected cells. Viruses form and bud preferentially at the site of filopodial anchorage and move along the outer surface of filopodial bridges, surfing, toward neighboring cells to infect them. Thus, our time-lapse movies offer a strong visual explanation for the efficient viral spreading under conditions of cell-cell contact. This system provides an excellent tool to dissect the molecular mechanism of cell-to-cell transmission and a number of questions can now be addressed. How are filopodial bridges formed? Is virus assembly directed towards sites of cell-cell contact? And how does virus binding to target cell filopodia engage the underlying retrograde F-actin flow to move towards the target cells? The underlying molecular principles determined in this proposal will likely apply to other modes of cell-to-cell transmission and offer insights into the ability of cells to communicate with each other. This research proposal will lead to a better understanding as to why oncogenic retroviruses spread efficiently from cell to cell. Newly identified cellular factors involved in this process could serve as potential new targets for antiviral and anti-cancer strategies. [unreadable] [unreadable] [unreadable]