Ectopic Expression of <i>MUC5B</i> in the Respiratory Bronchiole Initiates Endoplasmic Reticulum Stress in the IPF Lung

We used spatial transcriptomics from idiopathic pulmonary fibrosis (IPF) and unaffected control lung tissue to further understand the pathogenesis of MUC5B-driven lung fibrosis. We captured 43 fields of view in 15 IPF and 13 controls with and without the <i>MUC5B</i> promoter variant using the CosMx^® platform and identified 19 cell types via semi-supervised clustering. <i>MUC5B</i> was ectopically expressed in AT2 cells in controls with the risk variant. We observed a decreased proportion of AT2 cells in controls and an increased proportion of aberrant basaloid cells in IPF associated with the <i>MUC5B</i> risk variant. We identified co-localized expression of <i>MUC5B</i> in respiratory bronchioles with 13 genes including the endoplasmic reticulum (ER) stress marker <i>XBP1</i> and distal secretory markers <i>SCGB3A1</i> and <i>SCGB1A1</i>. Experimentally, we demonstrated a direct relationship between <i>MUC5B</i> expression and ER stress in bronchiolar epithelia <i>in vitro</i> and validated the co-expression of <i>MUC5B</i> and <i>XBP1</i> in the IPF lung. Based on our results, we conclude that <i>MUC5B</i> injures alveolar and bronchiolar epithelia that results in loss of AT2 cells and an increase in aberrant basaloid cells which initiates ER stress and a secretory phenotype in the terminal respiratory bronchiole, establishing a persistently injured distal airspace.