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Platelets appear to be indispensable in the protection of our respiratory tract against pathogens, such as the coronavirus. Until now, physicians thought they only played a role in blood coagulation and the development of thrombosis. Researchers from Utrecht University, UMC Utrecht and Erasmus MC have now discovered that they also play a role in our immune system.

“In patients with the flu, the platelets absorb virus particles from the lungs, after which they return to the bloodstream,” says lead researcher Erhard van der Vries. This insight is also important for research into coagulation and acute cardiovascular problems in corona patients, such as strokes and blood clots in the pulmonary vessels.

The researchers investigated how platelets absorb virus particles. The process appears to depend on the binding of flu viruses to sugar molecules on the blood platelets. Their research shows a direct relationship between the degree of virus binding and the severity of the flu: The stronger the virus binds to the blood platelets, the more severe the flu.

This discovery also explains why a respiratory infection, such as pneumonia, can lead to acute platelet loss (thrombocytopenia). This phenomenon, which until now had been misunderstood, is now appearing in a new light. The role of platelets in infections with a flu virus was not yet known. This knowledge may also have consequences for research into coagulation problems and acute cardiovascular problems in corona patients, such as blood clots in the pulmonary vessels or strokes.

Key role for platelets

It was already known that there is a relationship between respiratory tract infections and cardiovascular problems. For example, the weekly mortality rate linked to cardiovascular problems shows a seasonal pattern. The mortality peak also coincides with the annual flu season. Conversely, elderly people who get the flu shot are 20 percent less likely to end up in hospital because of cardiovascular problems or a stroke. The researchers expect to see the same relationship in corona patients. The double function of blood platelets – in blood coagulation and in the immune system – may play a key role in this.

We know that the flu shot greatly reduces the risk of acute cardiovascular disease, such as strokes. Principal investigator Erhard van der Vries comments: “We know from international research that the flu shot greatly reduces the risk of acute cardiovascular disease, such as strokes. Up until now, the mechanism behind this was unknown. This research shows that an early immune response of platelets plays an important role during the flu. However, the same immune response can also lead to complications in certain situations. This seems to be the case, for example, in corona-infected patients, where the immune response can also lead to a pulmonary embolism, causing patients to end up in the ICU. We now want to investigate this further in animals and humans with a multidisciplinary team of virologists, hematologists and immunologists.”¬† Van der Vries was a virologist at the University of Utrecht at the time and is now affiliated with the UMC Utrecht and the Animal Health Service.

Blood tests for lung infection

The role of platelets in the immune system requires further investigation. An important question the researchers still want to answer is when blood platelets end up in the lungs during the flu, where they subsequently go and how they share their recorded virological information with other immune cells. Hospitals can also look specifically at the relationship between respiratory tract infections and platelet information from blood tests. These data may possibly help in the diagnosis of pneumonia.

The findings of this research may also contribute to the development of new therapies for acute cardiovascular problems and the prediction of complications in respiratory tract infections. In the long term, they may also help in the development of better vaccination strategies.

The research was published last week in ‘Blood Advances’.

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