Innate Immune Regulation of Lung Alveolar Stem Cell Renewal in Mouse and Man

The lung, along with the skin and gut are the three organs in perpetual contact with our environment. The lung has evolved mechanisms for repairing injury due to exogenous noxious agents. The timely repair of lung injury is essential and determines the outcome of life or death. The mechanisms that regulate mature alveolar epithelial stem cell renewal are unknown. Our preliminary studies have led to the hypothesis that interactions between components of the innate immune system expressed by lung alveolar epithelial cells and the cell surface glycosaminoglycan hyaluronan are essential for stem cell renewal and lung repair after injury, and loss of this interaction results in severe lung fibrosis. In this proposal, we will define the mechanisms by which innate immune components, as well as endogenous matrix in lung alveolar epithelial cells promote progenitor cell renewal and lung repair. This is the first link between innate immunity and lung stem cell renewal. Elucidating the mechanisms by which endogenous matrix and innate immune components interact to promote stem cell renewal could lead to novel therapeutic approaches to lung diseases. During the first year of funding, we generated necessary reagents in the lab for all the aims of the project. We have made progress in characterization of alveolar epithelial cells from the mice lacking either innate immune receptors or extrecellular matrix hyaluronan synthase. We have also identified necessary cytokines for progenitor cell repair. Furthermore, we observed a difference in lung progenitor cells, as well as cell surface hyaluronan, from patients with progressive lung fibrosis relative to normal donor lungs.

During the past year we have performed more experiments for the proposed studies. We have found that Tlr4 expression in alveolar type II cells was down regulated following bleomycin lung injury in a similar pattern to the damage and renewal of AEC2s after lung injury. Reduction of IL-6 production results from Tlr4 or Has2 deficiency on AEC2s. IL-6 recombinant protein treatment increased AEC2 proliferation, renewal, and reduced epithelium permeability of bleomycin injured Tlr4-/- mice in vivo. Exogenous HA (healon) promoted colony formation of AEC2 from SFTPC-Cre;Has2flox/flox mice and HA blocking peptide (pep 1) was able to block healon- promoted colony formation. Furthermore, we have make good progress on the study of AEC2 stem cells in IPF patient lungs. we observed decreased HAS2 expression in IPF AEC2s. TLR4 expression of AEC2s from IPF lung and normal donor lungs were at similar level. High dose of exogenous HA was able to increase colony formation of IPF AEC2s. Our data indicate that there is failure of AEC2 stem cell renewal due to loss cell surface HA in IPF lung.