A common bacterium found in the mouth and airways may help the lungs better defend themselves against pneumonia, according to new research from the University of Colorado Anschutz.
The study, published in Proceedings of the National Academy of Sciences, found that exposure to Prevotella melaninogenica (P. melaninogenica), a type of bacteria commonly found in healthy airways, changed the behavior of immune cells in the lungs of mice. When the mice were later exposed to Streptococcus pneumoniae, a leading cause of bacterial pneumonia, their immune systems were better able to recognize, engulf and clear the bacteria.
The findings offer new insight into why some people are more resilient to respiratory infections than others and could eventually point toward new ways of strengthening the lungs’ natural defenses against pneumonia.
“Our lungs encounter bacteria from our mouths and upper airways all the time, and those exposures may actually be helping to educate the immune system,” said Sarah Clark, PhD, senior author and associate professor at the University of Colorado Anschutz School of Medicine. “We found that exposure to P. melaninogenica changes the way immune cells respond, essentially putting them in a better position to fight a bacterial infection.”
Preparing the immune system
Streptococcus pneumoniae is a major cause of pneumonia, but exposure to the bacterium does not always lead to serious illness. Researchers have increasingly suspected that the community of microorganisms living throughout the airway, known as the respiratory microbiome, may play an important role in determining how well someone fights an infection.
Prevotella is one of the most common groups of bacteria found in the healthy mouth and respiratory tract. Previous research has found that higher levels of Prevotella are associated with better outcomes in people with respiratory infections.
But scientists did not know exactly how the bacterium might help protect against pneumonia.
Study background
To investigate, Clark and her colleagues exposed mice to P. melaninogenica and then challenged them with S. pneumoniae. The researchers used single-cell RNA sequencing to examine thousands of individual cells in the lungs and determine how their activity changed.
The results showed that Prevotella exposure essentially reprogrammed several types of immune cells involved in fighting infection.
In mice exposed to Prevotella, immune cells called neutrophils and macrophages were better at engulfing and killing S. pneumoniae. The mice also had significantly lower amounts of the pneumonia-causing bacteria in their lungs.
“The immune cells were present in greater numbers and behaving differently,” Clark said. “They had been functionally reprogrammed to do a better job of clearing bacteria.”
First line of defense
The study adds to growing evidence that bacteria normally found in the respiratory tract are not simply passive passengers. Instead, they may actively influence how the immune system responds to disease.
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The researchers note that clinical studies have linked greater respiratory microbial diversity and the presence of anaerobic bacteria such as Prevotella with better outcomes in people with severe respiratory illness.
The new findings provide a possible biological explanation for those observations.
“We’re interested in understanding whether we can harness this natural interaction between the microbiome and the immune system,” Clark said. “If we can figure out how these bacteria educate immune cells to respond more effectively, it could open up entirely new approaches to preventing respiratory infections.”
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