ABSTRACT
Following influenza A virus (IAV) infection, the risk for subsequent pneumococcal pneumonia caused by
Streptococcus pneumoniae is enhanced 100-fold. Host aging is associated with more severe outcomes, thus
novel approaches that boost resistance to infections are needed. Our goal is to elucidate how IAV infection
alters the responses of polymorphonuclear leukocytes (PMN) and the extracellular adenosine (EAD) pathway
across host age, during pneumococcal pneumonia. Background: Following primary S. pneumoniae infection,
PMNs, which are innate immune cells whose function declines with age, are required for controlling bacterial
numbers. EAD, which is produced from breakdown of ATP leaking from injured cells by CD73, controls PMN
responses during infection. We found that CD73 was crucial for host resistance and PMN pulmonary influx as
well as anti-microbial function during primary pneumococcal pneumonia. Importantly, old mice had significant
changes in expression of EAD enzymes and receptors and targeting this pathway reversed the age-driven
susceptibility to primary pneumococcal infection. In humans, pneumococcal carriage in the nasopharynx is a
prerequisite to invasive diseases. Further, IAV infection of the respiratory tract where it triggers epithelial
damage and impairs host antibacterial responses is key for the lethal synergy of co-infection. However, animal
models incorporating the evolution of S. pneumoniae from colonizer to pathogen during IAV co-infection while
mimicking the severe disease seen in humans, are lacking. Therefore, we established an optimized model
where mice are intra-nasally colonized with pneumococci followed by IAV infection of the nasopharynx and
lungs. This model resulted in obvious signs of illness and lethality in half of the young mice and importantly,
reproduced the susceptibility of aging, where significantly more old mice displayed severe clinical signs and
succumbed to lethality compared to young controls. Using this model, we strikingly found that the role of CD73
was reversed, where CD73 became detrimental to resistance of young hosts to co-infection. This highlighted
that IAV infection triggers profound changes in the interaction of the host with pneumococci. However, the
effect of polymicrobial infections on host responses and the mechanisms behind that remain a gap in the field.
Here we will use our improved model to test the Hypothesis that IAV infection alters CD73-mediated control of
PMN responses to pneumococcal infection across host age using two specific Aims: 1) Test the effect of age
and IAV infection on PMN responses during secondary pneumococcal pneumonia; 2) Test the role of CD73 in
PMN responses and host resistance to S. pneumoniae/IAV co-infection. Significance/innovation/ relevance:
Using this physiologically relevant model that mimics aspects of human disease to elucidate how PMN
responses change in mono vs polymicrobial infections across host age and the role of CD73 in that, has the
potential to uncover novel pathways that drive host susceptibly to infection. This work can lead to highly
innovative approaches incorporating adenosine-based drugs to combat this serious lung disease.
