A Dying Cell Leaves a “Footprint”—and Influenza Can Use It

When an attached cell activates its self-destruct program, it does not simply vanish. As the cell contracts and pulls away from the surrounding surface, it leaves behind a thin patch of membrane marking the place where it once sat.

The researchers named this structure FOOD, short for *FOotprint Of Death*. Using three-dimensional live-cell imaging, they watched the flat membrane remnants gradually round up into separate vesicles about two micrometres across. These particles were named FOOD-derived apoptotic extracellular vesicles, or F-ApoEVs.

Extracellular vesicles are membrane-bound packages that can carry proteins, lipids and other cellular material. Dying cells were already known to fragment into structures called apoptotic bodies. F-ApoEVs, however, form through a different route. Instead of breaking away from protrusions extending from the dying cell, they develop from material deposited on the underlying surface as the cell retracts.

The phenomenon was common across the cell models tested. Approximately 80–99% of apoptotic human and mouse cells produced a footprint, and a single cell generated a median of about 40 F-ApoEVs within four hours.

FOOD formation depended on ROCK1, a protein kinase involved in cellular contraction. During apoptosis, caspase enzymes activate ROCK1, which drives contraction of the actin–myosin cytoskeleton. Cells carrying a version of ROCK1 that could not be properly activated failed to retract normally and produced far fewer rounded vesicles.

F-ApoEVs also displayed phosphatidylserine on their outer surface. This lipid is normally kept on the inner side of a living cell’s membrane, but during apoptosis it moves outward and becomes an “eat-me” signal for macrophages and other phagocytic cells. In laboratory co-cultures, macrophages engulfed the new vesicles. Exposure to FOOD also increased their subsequent uptake of apoptotic cells, suggesting that the footprint may help flag a location where cellular debris needs to be cleared.

The same process may have a less helpful consequence during infection. When cultured human lung epithelial cells were infected with H1N1 influenza A virus, their F-ApoEVs contained viral proteins. Electron microscopy occasionally revealed complete viral particles inside the vesicles. Adding isolated F-ApoEVs from infected cells to uninfected cells led to the appearance of viral nucleoprotein in the recipient cells, consistent with transmission of infection.

The experiments were performed in cultured cells and used a high multiplicity of influenza infection. The study therefore does not establish how often this route operates in living tissue or how much it contributes to influenza spread compared with conventional viral release.

Cell death, in this case, is not a silent disappearance. A dying cell leaves behind a final message that can guide the clean-up response—while also creating a route that an infection may exploit.