Abstract
The infection of viruses to a neighboring cell is considered to be beneficial in terms of evasion from host anti-virus defense systems. There are two pathways for viral infection to “right next door”: one is the virus transmission through cell-cell fusion by forming syncytium without production of progeny virions, and the other is mediated by virions without virus diffusion, generally designated cell-to-cell transmission. Influenza viruses are believed to be transmitted as cell-free virus from infected cells to uninfected cells. Here, we demonstrated that influenza virus can utilize cell-to-cell transmission pathway through apical membranes, by handover of virions on the surface of an infected cell to adjacent host cells. Live cell imaging techniques showed that a recombinant influenza virus, in which the neuraminidase gene was replaced with the green fluorescence protein gene, spreads from an infected cell to adjacent cells forming infected cell clusters. This type of virus spreading requires HA activation by protease treatment. The cell-to-cell transmission was also blocked by amantadine, which inhibits the acidification of endosomes required for uncoating of influenza virus particles in endosomes, indicating that functional hemagglutinin and endosome acidification by M2 ion channel were essential for the cell-to-cell influenza virus transmission. Furthermore, in the cell-to-cell transmission of influenza virus, progeny virions could remain associated with the surface of infected cell even after budding, for the progeny virions to be passed on to adjacent uninfected cells. The evidence that cell-to-cell transmission occurs in influenza virus lead to the caution that local infection proceeds even when treated with neuraminidase inhibitors.
Highlights
It is generally accepted that viruses, released as cell-free virions from an infected cell, transmit to distant cells and tissues
Noted that a large number of single fluorescent foci caused by initial infection markedly expanded and formed cell clusters consisting of 5–10 infected cells in an Madin-Darby canine kidney (MDCK) cell monolayer (Figures 1B and S1), suggesting influenza virus can spread to some extent in the presence of oseltamivir
The NA activity is eliminated from the recombinant influenza virus, and enhanced green fluorescent protein (EGFP) can be utilized as a marker for viral infections
Summary
It is generally accepted that viruses, released as cell-free virions from an infected cell, transmit to distant cells and tissues. This spreading pathway contributes to wide-ranged diffusion of cell-free viruses. In this spreading pathway, viruses are exposed to host anti-virus defense systems. Direct infection to a neighboring cell is considered to be beneficial for the virus in terms of evasion from the host anti-virus defense. There are two typical manners in infection to ‘‘right door’’: one is the virus transmission through cell-cell fusion by forming syncytium without production of progeny virions, and the other is mediated by virions without virus diffusion, generally designated cell-to-cell transmission [1,2]. Viruses can spread from cell to cell without producing cell-free virus particles
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