Unconventional Endocytic Pathways and Receptor Plasticity Support FMDV Internalization and Replication in Porcine Mesenchymal Stromal Cells

Viruses are often thought of as specialists, creatures that know exactly which door to knock on and which key to use to get inside a cell. For the foot-and-mouth disease virus, which causes a devastating illness in livestock, that door has long been known to be the surface of epithelial cells, the thin layer of tissue that lines the mouth and skin of animals. To enter these cells, the virus typically uses a specific set of molecular keys called integrins, which act like handles on the cell surface, and it slips inside through a well-documented process where the cell membrane folds in to swallow the invader. This standard pathway is so well understood that it forms the basis of how scientists usually think about how the disease spreads and how it might be stopped. However, nature is rarely limited to a single route, and the virus may have other ways of getting in, especially when it encounters different types of cells that do not have the usual handles or doors.

A team of researchers at the National Institute for Agricultural and Food Research and Technology in Spain set out to see if this virus could also infect a very different kind of cell: mesenchymal stromal cells. These are versatile cells found in fat, skin, and blood that help repair tissues and calm down the immune system. The scientists wanted to know if these repair cells, which are not the usual targets for the virus, could actually be infected, and if so, how the virus managed to get inside them. They used two different strains of the virus, known as serotypes A and O, which are common in the field, and tested them on cells taken from the fat, skin, and blood of Iberian pigs. They also compared these results to the standard laboratory cells that are known to be highly susceptible to the virus.

The researchers found that the virus did indeed infect these stromal cells, causing them to round up and die, just as it does in the standard cells. This was surprising because these stromal cells were not expected to be a target. More importantly, the virus did not use the same entry method it uses in epithelial cells. In the fat and blood cells, the virus relied on a different set of molecular handles, specifically a protein called beta-6 integrin, which was found on the surface of these cells but not on the standard control cells. The virus also used a protein called JMJD6, which is present on all the stromal cells they tested. Crucially, the virus did not need the usual sugar-coated molecules called heparan sulfate proteoglycans to attach to these cells, which are often the first step for the virus in other cell types.

To figure out exactly how the virus entered, the scientists used chemical tools to block specific pathways the cell uses to swallow things. They discovered that the two virus strains used different routes to get inside the stromal cells. The A strain required the cell's internal skeleton, made of actin fibers, to be flexible and moving, and it also required signals from tyrosine kinases, which are enzymes that act like switches inside the cell. The O strain, however, was selectively sensitive to genistein, a blocker that inhibits caveolae-mediated endocytosis, a process involving small, flask-shaped pockets in the cell membrane. This showed that even within the same type of cell, the virus can change its strategy depending on which strain it is.

In a twist that challenged their expectations, the researchers found that when they blocked the standard entry route used by the virus in the laboratory control cells, the infection actually got stronger. This suggests that when the usual path is closed, the virus can be forced into an alternative, highly efficient route to get inside. The study also showed that the specific type of stromal cell mattered; cells from abdominal fat and blood were better at supporting the A strain, while cells from abdominal and subcutaneous fat were better for the O strain. This means that the virus's ability to spread is not just about the virus itself, but also about the specific mix of cells it encounters in the animal's body.

These findings suggest that foot-and-mouth disease virus is more adaptable than previously thought. It can infect cells that are not its usual targets and can use a variety of different entry mechanisms to do so. This flexibility might allow the virus to hide in different parts of the body, such as fat or blood, where it could replicate without being immediately detected by the immune system. Since these stromal cells are known to help regulate the immune response, their infection could create a quiet environment that helps the virus survive longer. The study does not prove that this is how the disease spreads in the wild, but it shows that the virus has the tools to infect these cells and that it does so using complex, non-standard methods. This adds a new layer to our understanding of how the virus behaves and highlights the need to consider these alternative cell types when trying to control the disease.