While most of us shake off the flu after a week or two, in more extreme cases the virus has been known to actually reshape the structure of the lungs. Now, researchers at the University of Pennsylvania have discovered a major part of that restructuring that until now had gone unnoticed – "taste bud cells" seem to grow in the lungs after a severe case of the flu.

The team first noticed something was up a few years ago, while studying the effects of severe lung infections. It's previously been found that the worst of these infections actually trigger a restructuring of the lung tissue, and in a 2015 study the researchers discovered a new class of cells called lineage negative epithelial progenitors, which seem to be the culprit. At the same time, however, they also noticed inflammation that seemed to linger long after the virus itself passed.

For the new study, the team looked closer at that inflammation, in mice that had been exposed to H1N1 influenza. This kind of reaction is normally part of a Type 2 response by the immune system, but the flu itself only triggers a Type 1 immune response. There was a missing piece of the puzzle – this kind of inflammation usually requires tuft cells. These cells, which are virtually identical to taste buds, have been found to be behind inflammation of the gut.

The team went looking for those cells in the lung – and sure enough, found them all over the place.

"It was just really weird to see, because these cells are not in the lung at baseline," says Andrew E. Vaughan, senior author of the study. "The closest they are normally is in the trachea. What we did was show where they're coming from and how this same rare cell type that gives you all this maladaptive remodeling of the lung after flu is also the source of these ectopic tuft cells."

The team found these tuft cells lining the airway and alveoli, the tiny balloon-like structures that allow gas exchange with the bloodstream. It turns out that the lineage epithelial progenitor cells lead to the tuft cells, as well as the other cells that damage the structure of the lung after infection.

Next, the team tested what these taste bud cells are actually doing there. They used bitter compounds to activate them, and found that their numbers expanded and acute inflammation followed. When they compared this to lungs that hadn't been infected and as a result had no tuft cells, there was no inflammation response.

The researchers say these cells could be implicated in allergies and diseases like asthma. As such, they might become a new target for treating these conditions. Next up, the team plans to investigate whether the same thing occurs in human lungs.

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