Direct cell-to-cell transmission of respiratory viruses: The fast lanes

What are "fast lanes" and why are they needed in the respiratory tract?From the speed and efficiency perspective, we refer to the infrastructure that viruses use for rapid cell-to-cell spread as "fast lanes."Viruses do not build fast lanes.Instead, they modify or connect existing host-cell trafficking and cytoskeletal infrastructure.In this sense, different categories of fast lanes exist that vary in their complexity.One of the best characterized is the formation of giant multinucleated cells, called syncytia, which result from membrane fusion and cytoplasmic content mixing between infected cells and neighbor cells [1] (Fig 1B).A recently described intracellular fast lane consists of strategically located openings that allow intercellular exchange of cytoplasm and viral genomes [3].Exchange of viral components has also been recently shown to occur through filamentous structures that extend towards distant cells [4][5][6].Other fast lanes, while forming below the cell surface, push released particles to "surf" towards noninfected cells [7].Moreover, some viruses take advantage of structures built by circulating cells to communicate, such as the immunological synapse, to spread more rapidly [2].We focus here on the recently described fast lanes employed by different respiratory viruses that may allow rapid spread in the respiratory tract.Respiratory viruses comprise a large group of pathogens that spans different viral families.Among those, we refer to the orthomyxovirus influenza virus, paramyxoviruses such as