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.

REFERENCES

1.Taubenberger JK, Morens DM. Influenza: the once and future pandemic. Public Health Rep. Apr 2010;125 Suppl 3(Suppl 3):16-26.

2.Saunders-Hastings PR, Krewski D. Reviewing the history of pandemic influenza: understanding patterns of emergence and transmission. Pathogens. Dec 6 2016;5(4)doi:10.3390/pathogens5040066

3.Madhav NO, B.; Gallivan, M.; Mulembakani, P.; Rubin, E.; Wolfe, N. Pandemics: Risk, Impacts, and Mitigation. In: Jamison DTG, H.; Hortan, S.; Jha, P.; Laxminarayan, R.; Mock, C.N.; Nugent, R., ed. Disease Control Priorities: Improving Health and Reducing Poverty, Third Edition. The International Bank for Reconstruction and Development; 2017.

4.Iuliano AD, Roguski KM, Chang HH, et al. Estimates of global seasonal influenza-associated respiratory mortality: a modelling study. Lancet. Mar 31 2018;391(10127):1285-1300. doi:10.1016/S0140-6736(17)33293-2

5.Paget J, Spreeuwenberg P, Charu V, et al. Global mortality associated with seasonal influenza epidemics: New burden estimates and predictors from the GLaMOR Project. J Glob Health. Dec 2019;9(2):020421. doi:10.7189/jogh.09.020421

6.U.S. Centers for Disease Control and Prevention. Weekly US Influenza Surveillance Report: Key Updates for Week 6, Ending February 8, 2025. Accessed February 20, 2025. https://www.cdc.gov/fluview/surveillance/2025-week-06.html

7.Bartley PS, Deshpande A, Yu PC, et al. Bacterial coinfection in influenza pneumonia: Rates, pathogens, and outcomes. Infect Control Hosp Epidemiol. Apr 23 2021:1-6. doi:10.1017/ice.2021.96

8.Teng F, Liu X, Guo SB, et al. Community-acquired bacterial co-infection predicts severity and mortality in influenza-associated pneumonia admitted patients. J Infect Chemother. Feb 2019;25(2):129-136. doi:10.1016/j.jiac.2018.10.014

9.Chertow DS, Memoli MJ. Bacterial coinfection in influenza: a grand rounds review. JAMA. Jan 16 2013;309(3):275-82. doi:10.1001/jama.2012.194139

10.Randolph AG, Vaughn F, Sullivan R, et al. Critically ill children during the 2009-2010 influenza pandemic in the United States. Pediatrics. Dec 2011;128(6):e1450-8. doi:10.1542/peds.2011-0774

11.Nguyen T, Kyle UG, Jaimon N, et al. Coinfection with Staphylococcus aureus increases risk of severe coagulopathy in critically ill children with influenza A (H1N1) virus infection. Crit Care Med. Dec 2012;40(12):3246-50. doi:10.1097/CCM.0b013e318260c7f8

12.Finelli L, Fiore A, Dhara R, et al. Influenza-associated pediatric mortality in the United States: increase of Staphylococcus aureus coinfection. Pediatrics. Oct 2008;122(4):805-11. doi:10.1542/peds.2008-1336

13.Randolph AG, Xu R, Novak T, et al. Vancomycin monotherapy may be insufficient to treat methicillin-resistant Staphylococcus aureus coinfection in children with influenza-related critical illness. Clin Infect Dis. Jan 18 2019;68(3):365-372. doi:10.1093/cid/ciy495

14.Ploin D, Chidiac C, Carrat F, et al. Complications and factors associated with severity of influenza in hospitalized children and adults during the pandemic wave of A(H1N1)pdm2009 infections--the Fluco French cohort. J Clin Virol. Sep 2013;58(1):114-9. doi:10.1016/j.jcv.2013.05.025

15.Wang H, Xiao X, Lu J, et al. Factors associated with clinical outcome in 25 patients with avian influenza A (H7N9) infection in Guangzhou, China. BMC Infect Dis. Oct 3 2016;16(1):534. doi:10.1186/s12879-016-1840-4

16.Lobo SM, Watanabe ASA, Salomao MLM, et al. Excess mortality is associated with influenza A (H1N1) in patients with severe acute respiratory illness. J Clin Virol. Jul 2019;116:62-68. doi:10.1016/j.jcv.2019.05.003

17.Shi Y, Chen W, Zeng M, et al. Clinical features and risk factors for severe influenza in children: a study from multiple hospitals in Shanghai. Pediatr Neonatol. Jul 2021;62(4):428-436. doi:10.1016/j.pedneo.2021.05.002

18.Hussain M, Galvin HD, Haw TY, Nutsford AN, Husain M. Drug resistance in influenza A virus: the epidemiology and management. Infect Drug Resist. 2017;10:121-134. doi:10.2147/IDR.S105473

19.Hayden FG, de Jong MD. Emerging influenza antiviral resistance threats. J Infect Dis. Jan 1 2011;203(1):6-10. doi:10.1093/infdis/jiq012

20.Guo Y, Song G, Sun M, Wang J, Wang Y. Prevalence and therapies of antibiotic-resistance in Staphylococcus aureus. Front Cell Infect Microbiol. 2020;10:107. doi:10.3389/fcimb.2020.00107

21.Tang S, De Jesus AC, Chavez D, et al. Rescue of alveolar wall liquid secretion blocks fatal lung injury by influenza-staphylococcal coinfection. J Clin Invest. Aug 15 2023;133(19):e163402. doi:10.1172/JCI163402

22.Lindert J, Perlman CE, Parthasarathi K, Bhattacharya J. Chloride-dependent secretion of alveolar wall liquid determined by optical-sectioning microscopy. Am J Respir Cell Mol Biol. Jun 2007;36(6):688-96. doi:10.1165/rcmb.2006-0347OC

23.Huang Z, Tunnacliffe A. Response of human cells to desiccation: comparison with hyperosmotic stress response. J Physiol. Jul 1 2004;558(Pt 1):181-91. doi:10.1113/jphysiol.2004.065540

24.Fang X, Song Y, Hirsch J, et al. Contribution of CFTR to apical-basolateral fluid transport in cultured human alveolar epithelial type II cells. Am J Physiol Lung Cell Mol Physiol. Feb 2006;290(2):L242-9. doi:10.1152/ajplung.00178.2005

25.Bove PF, Grubb BR, Okada SF, et al. Human alveolar type II cells secrete and absorb liquid in response to local nucleotide signaling. J Biol Chem. Nov 5 2010;285(45):34939-49. doi:10.1074/jbc.M110.162933

26.Li X, Comellas AP, Karp PH, et al. CFTR is required for maximal transepithelial liquid transport in pig alveolar epithelia. Am J Physiol Lung Cell Mol Physiol. Jul 2012;303(2):L152-60. doi:10.1152/ajplung.00116.2012

27.Johnson MD, Bao HF, Helms MN, et al. Functional ion channels in pulmonary alveolar type I cells support a role for type I cells in lung ion transport. Proc Natl Acad Sci U S A. Mar 28 2006;103(13):4964-9. doi:10.1073/pnas.0600855103

28.Regnier A, Dannhoffer L, Blouquit-Laye S, Bakari M, Naline E, Chinet T. Expression of cystic fibrosis transmembrane conductance regulator in the human distal lung. Hum Pathol. Mar 2008;39(3):368-76. doi:10.1016/j.humpath.2007.06.020

29.Johnson M, Allen L, Dobbs L. Characteristics of Cl- uptake in rat alveolar type I cells. Am J Physiol Lung Cell Mol Physiol. Nov 2009;297(5):L816-27. doi:10.1152/ajplung.90466.2008

30.Haies DM, Gil J, Weibel ER. Morphometric study of rat lung cells. I. Numerical and dimensional characteristics of parenchymal cell population. Am Rev Respir Dis. May 1981;123(5):533-41. doi:10.1164/arrd.1981.123.5.533