Researchers have found a further reason for why flu and Streptococcus pneumonia are such a deadly combination, by a surface protein causing it to stick to dead or dying lung cells. The finding by University of Alabama at Birmingham (UAB) follows thirty years after the discovery of the surface protein, called pneumococcal surface protein A, or PspA.
This new mechanism had been overlooked because it facilitates bacterial adherence only to dead or dying lung epithelial cells, not to living cells. Previously, researchers typically used healthy lung cell monolayers to search for bacterial adhesins that aid infection. In flu, the virus killing off lung cells was found to set the stage for S. pneumonia attachment to the airway, thereby worsening disease and pneumonia.
Study leaders Carlos Orihuela, PhD, and David Briles, PhD, professor at UAB, said their findings provide further explanation for how an infection by influenza A flu virus — followed by S. pneumoniae superinfection — causes severe pneumonia and a high death rate. Understanding of this mechanism could also lead to improvements for disease treatment and vaccination.
A historical example of the deadly synergy of flu infection followed by S. pneumoniae superinfection is found in banked lung samples from the 1918 Spanish influenza pandemic that killed 40 million to 50 million people — the vast majority of these samples showed co-infection or secondary infection with S. pneumonia.
The UAB research on PspA began with puzzling results from experimental lung infections of mice with influenza A, followed by either wild-type S. pneumonia that has the intact PspA gene, or a mutant S. pneumoniae that lacks PspA. Lung homogenates from mice infected with the wild-type had much higher numbers of S. pneumonia bacteria than lungs infected with the mutant. However, when researchers washed the interiors of the lungs and collected that bronchoalveolar lavage fluid, they counted similar numbers of the wild-type S. pneumonia and the mutant.
“This unexpected result was interpreted to mean that wild-type S. pneumoniae were more resistant to dislodgement than S. pneumonia with a pspA gene deletion, and it served as rationale for further experimentation,” Dr Orihuela said.
From this, the researchers were then able to show that PspA functions as an adhesin to dying host cells, as well as its previously established virulence mechanisms. The researchers also detailed the molecular mechanism of this bacterial adherence.
Both influenza A infection and release of the S. pneumoniae toxin pneumolysin cause death of lung epithelial cells. As they are dying, cells’ phosphatidylserine residues wind up on the outer cell membrane, where they bind the host enzyme glyceraldehyde-3-phosphate dehydrogenase, or GAPDH. In turn, the S. pneumoniae PspA on the bacteria surface binds to the GAPDH. PspA-GAPDH-mediated binding to lung cells increased S. pneumoniae localisation in the lower airway, and this was enhanced by pneumolysin exposure or co-infection with influenza A virus.
One of the fragments of protein responsible for the binding was introduced into the lungs of influenza-infected mice and reduced the disease severity of S. pneumoniae superinfection, presumably through binding competition.
“Our findings support the targeting of regions of PspA for therapeutic and vaccine development against influenza A/Streptococcus pneumoniae superinfections,” Dr Orihuela said. “Importantly, and despite more than 30 years since its discovery, PspA was not previously shown to function as an adhesin. Thus, our finding of PspA’s role in adherence substantially advances our knowledge on the interactions of S. pneumoniae with its host.”
Source: University of Alabama at Birmingham
Journal information: Sang-Sang Park et al, Streptococcus pneumoniae binds to host GAPDH on dying lung epithelial cells worsening secondary infection following influenza, Cell Reports (2021). DOI: 10.1016/j.celrep.2021.109267
