An in-depth look at the connection between flu infection and pneumonia has been taken by a team of scientists from multiple research institutions.
The study led by researchers from the Helmholtz-Centre for Infection Research (HZI) in Braunschweig, the Otto-von-Guericke-University in Magdeburg, and the Karolinska institute in Sweden appears to have disproven a common theory about flu-like pneumonia.
The researchers point out that some viral infections trigger a decrease of immune cells in the blood, a so-called "lymphopenia".
With a view to finding out whether this is the case with influenza, the researchers infected mice with flu viruses, and measured the amount of immune cells in the animal's blood every day.
Some days later, the flu-infected mice received a dosage of pneumonia bacteria usually harmless for healthy mice.
The researchers observed that the flu-infected mice, though developed a superinfection and subsequently died, were not suffering from lymphopenia.
They said that the healthy, non-flu-infected mice defeated the bacteria successfully and recovered.
The team then set out to find out whether a lack of immune cells encourages an infection with pneumonia bacteria in general, and established an artificial drug-induced lymphopenia in the mice.
Without infecting the animals with flu viruses, they received pneumonia bacteria.
Despite a severe lack of immune cells, according to the researchers, the mice recovered completely.
The researchers said that their findings suggested that influenza facilitates and intensifies an infection from pneumonia bacteria, while disproving the common idea that this is caused by a lack of immune cells.
"This result was an enormous surprise for us because it directly contradicts widespread assumptions," says Sabine Stegemann, researcher in the groups "Immune regulation" at the HZI and "Molecular Immunology" at the Otto-von-Guericke-University in Magdeburg.
"Now we want to understand the reasons for the increased susceptibility. It could be interplay of weakened mucous membranes and scavenger cells that induce ideal conditions for pneumonia bacteria to create a deadly lung infection.
Another reason may be a reaction of the host immune system: It disables hyperactive flu-fighting immune cells to inhibit destruction of healthy lung tissue. The immune system keeps itself under control and that makes it easy for pneumonia bacteria to infect the lung," says Matthias Gunzer, head of the group in Magdeburg.
A research article on this study has been published in the scientific journal PLoS One.