Scientists in Switzerland and Japan have captured, in real time and high resolution, the exact moment a flu virus enters a living human cell.

Cells routinely take in essential molecules such as hormones, cholesterol and iron through a process called endocytosis, allowing them to function correctly. Viruses like influenza exploit this mechanism to hide from the immune system and initiate infection. 

Until now, scientists could only capture this process using still images, rather than watching it unfold step by step. A new method, developed at ETH Zurich and Nagoya University, called virus-view dual confocal and AFM (ViViD-AFM),enables real-time tracking of the virus’s fine-scale movements as it enters the cell.  

Using this method, the team discovered that cells actively assist the virus during several steps of entry. They observed that the influenza virus first attaches and “surfs” the cell surface until it reaches a site rich in clathrin proteins, which form honeycomb-like structures that help pull molecules into the cell. The cell membrane then forms a pocket that wraps around the virus, enclosing it and drawing it inside. 

“The infection of our body cells is like a dance between virus and cell,” said Yohei Yamauchi from ETH Zurich and lead author of the study published in Proceedings of the National Academy of Sciences. 

By allowing scientists to watch infection unfold in real time, ViViD-AFM provides a valuable way for testing antiviral drug candidates directly in human cells. The technique could also be applied to study other viruses or even vaccines.