Role of alveolar progenitor cells in lung regeneration upon infection of highly pathogenic avian influenza A/H5N1 virus

Abstract

Highly pathogenic avian influenza (HPAI) H5N1 virus continues to pose pandemic threats to human. Human infection of HPAI H5N1 virus has been associated with the dysregulated robust innate host response which leads to severe lung injury and high mortality rate. In contrast, infection with low pathogenic influenza virus strains, like the seasonal and pandemic influenza A virus show mild disease severity. I hypothesize that there is difference between the low pathogenic and HPAI in alveolar regeneration after infection, which is the critical factor governing the differential clinical outcome. In this study, alveolar regeneration upon injury induced by influenza A viruses, H1N1pdm and H5N1, was investigated by using alveolar progenitor cells (Krt5+ cells). The differential in cellular proliferation, expression of progenitor cell marker gene (Krt5), and apoptotic related genes were observed between the H1N1pdm and H5N1 infected alveolar progenitor cells. The significantly lower cellular proliferation but higher induction of apoptosis in H5N1 infected alveolar progenitor cells were observed, suggesting the cellular deterioration of alveolar progenitor cells upon HPAI H5N1 infection. Furthermore, in vivo study indicated the involvements of different types of alveolar progenitor cells upon H1N1pdm or H5N1 infection. Massive expansion of Krt5+ cells was observed at the injured region indicating the critical role in response to lung injury upon infection of H1N1pdm virus. Krt5+ cells differentiated into well organised “pods-like” structure and further differentiated into new hollow alveoli-like structure at day 20 post infection (p.i.). More importantly, Krt5+ hollow structure co-expressed with Aqp5, one of the markers of alveolar type I epithelial (AT1) cell, indicated the transition of Krt5+ alveolar progenitor cells to AT1-like cells. In contrast, Krt5 expression was significantly suppressed in H5N1 infected mice as evidenced by immunohistochemical staining and RNA sequencing analysis. Only scattered and limited expression of Krt5 at the injured region was observed upon H5N1 virus infection. Furthermore, Krt5+Aqp5+ cells were not observed in H5N1 infected mouse lungs. Instead, alveolar type II epithelial (AT2) cells emerged at the injured region at day 20 p.i. which indicated the differential progenitor cell populations employed during alveolar regeneration of H5N1/486 infected mice. Transcriptome analysis of influenza virus infected mice at the recovering stage was carried out and Wnt signaling pathway was proposed to be involved in the alveolar regeneration process. Activation of non-canonical Wnt/planar cell polarity (WNT/PCP) pathway via Cthrc1 but inhibition of canonical Wnt signaling pathway by Tfap2A were detected in H1N1pdm infected mouse lung RNA. Taken together, H5N1 infection favours alveolar regeneration via AT2 cells proliferation with the aid of activated canonical Wnt signaling pathway, while H1N1pdm infection utilises Krt5+ cells to regenerate Aqp5+ cells under the influence of Wnt/PCP signaling pathway. I believe that the investigation of these regeneration mechanisms will offer new insights and potentially lead to the development of new treatment options for patients suffering from acute respiratory distress syndromes caused by influenza H5N1 virus infection and other respiratory pathogens that cause severe lung injury.