Developing New Treatments for Influenza That Target the Lung – Not the Virus

Influenza is a major cause of global mortality and a frequent cause of pandemics. Pandemic influenza viruses have caused more than 50 million deaths in the past century alone¹,², and the risk of a new influenza pandemic may be as high as 3% per year³. Moreover, seasonal influenza infections cause nearly half a million deaths every winter⁴,⁵, and rates of hospitalizations and mortality in the current influenza season are higher than they have been in more than a decade⁶. Clearly, influenza remains a serious threat to public health and the global economy.

Death from influenza infection often results from secondary pneumonia by inhaled Staphylococcus aureus (SA)⁷⁻⁹, especially in children¹⁰⁻¹². Although antibiotic and antiviral drugs form the cornerstone of therapy, they fail to contain lung injury once it initiates¹⁰ ¹³⁻¹⁷, and they are increasingly hindered by viral and bacterial drug resistance¹⁸⁻²⁰. New approaches to therapy are urgently needed.

Our American Lung Association (ALA)-funded research aims to develop new therapeutic approaches for influenza lung infection that protect against fatal SA coinfection. Our findings show influenza promotes SA infection in lung alveoli by disrupting an innate alveolar defense²¹. In healthy alveoli, the alveolar epithelium continuously secretes liquid onto the alveolar surface that prevents cell desiccation stress²¹⁻²³. The secretion results from epithelial function of the cystic fibrosis transmembrane conductance regulator (CFTR) ion channel²¹,²², and it generates liquid flow along alveolar walls that convectively transports particles toward the small airways, clearing them from alveoli²². We found influenza inhibits CFTR function in the alveolar epithelium to block alveolar liquid secretion²¹. At the same time, influenza activates the alveolar epithelial Na⁺ channel (ENaC) to induce alveolar liquid absorption²¹. In this absorptive microenvironment, SA inhaled into alveoli can stabilize against alveolar walls, initiating alveolar coinfection and causing fatal lung injury²¹. But, treatment of influenza-infected mice with systemic injections of the CFTR potentiator drug, ivacaftor, rescues alveolar wall liquid secretion and protects against SA-induced lung injury and death²¹.

This ALA-funded work provides the first understanding of alveolar responses to influenza in situ and identifies a new role for alveolar liquid secretion in lung defense against inhaled pathogens. In addition, these findings pave the way for the clinical repurposing of ivacaftor – a drug that is safe, well-tolerated, and highly efficacious in people with cystic fibrosis – to protect against fatal SA coinfection in people with influenza. Importantly, therapies that target the lung, like ivacaftor, will retain drug efficacy in the face of influenza mutations that render antiviral drugs ineffective.

Ongoing aspects of our ALA-funded research seek to refine our recent discoveries. Specifically, we are working to identify the alveolar epithelial cell type that generates alveolar liquid secretion in health and that is disrupted by influenza infection. Reports indicate alveolar epithelial type 2 (AT2) cells express CFTR protein and likely drive alveolar liquid dynamics²⁴⁻²⁶. However, neighboring alveolar epithelial type 1 (AT1) cells also express CFTR²⁷⁻²⁹ and may contribute to liquid secretion. Since AT1 cells comprise 97% of the air-facing alveolar surface³⁰, AT1 cells might make critical contributions to alveolar liquid secretion and may be major targets of influenza-induced cell damage. Because AT1 cells are difficult to culture and study, their contributions to alveolar liquid dynamics are not yet known.

To identify which cell type drives alveolar liquid secretion, we generated transgenic mice that harbor AT2 and AT1 cell-specific expression of non-functional CFTR protein. Simultaneous expression of green fluorescent protein marks cells that express non-functional CFTR. We applied confocal microscopy to image live, intact, blood-perfused lungs of the transgenic mice (Figure 1). Using this method, we can visualize alveolar liquid secretion in intact alveoli in real time²¹,²².

Our new preliminary data show CFTR function in AT1 cells is critical to alveolar liquid secretion. Furthermore, AT1 cell CFTR function drives the alveolar clearance of particles and SA from alveoli. These findings reveal AT1 cells to be major contributors to alveolar homeostasis and defense. For next steps, we plan to evaluate the extent to which AT1 cells are targets of influenza infection and influenza-induced alveolar damage.

The findings we have generated with ALA support are the first, to our knowledge, to define physiological responses of intact alveoli to influenza infection. Key data show influenza inhibits CFTR to block defensive alveolar liquid secretion and promote SA coinfection, but systemic administration of the CFTR potentiator drug, ivacaftor is protective. New preliminary data show AT1 cell CFTR function is critical to alveolar homeostasis and defense and suggest AT1 cells are major targets of influenza-induced alveolar damage. These findings support CFTR potentiator therapy as a new therapeutic strategy for influenza-SA coinfection. In addition, they inform the development of new, CFTR-potentiating therapies that may have more precise alveolar sites of action, better drug efficacy, and fewer side effects.

Hook has no disclosures to report.

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