Pulmonary fibrosis after COVID-19 is characterized by airway abnormalities and elevated club cell secretory protein-16

Prior studies testing biomarkers of residual lung abnormalities after COVID-19 are limited by sampling within the first year after acute COVID-19 illness and lack of external validation of findings. In three independent, international, racially and ethnically diverse prospective cohorts of survivors of moderate to critical COVID-19, we systematically tested 18 circulating biomarkers of inflammation, aging, endothelial activation, pulmonary epithelial function, fibrosis, and fibrinolysis. We found that only higher club-cell secretory protein-16 (CC16) levels are consistently associated with persistent fibrotic lung abnormalities in cross-sectional and longitudinal analyses for up to 3 years after acute COVID-19. Histopathological and single-cell RNA sequencing analyses of transbronchial biopsies of fibrotic lung abnormalities in COVID-19 survivors sampled between 3 and 4 years after acute illness and of COVID-19 lung explants suggest that circulating CC16 reflects underlying deranged pulmonary epithelial progenitor proliferation and anomalous CC16/MUC5B-related pro-fibrotic signaling in the distal airways. CC16 should be investigated further as a potential blood biomarker that may facilitate screening of COVID-19 survivors for pulmonary fibrosis and its progression. Overall design: We compared single cell analyses of transbronchial lung biopsies of fibrotic-like abnormalities in study participants obtained between 3 and 4 years after acute COVID-19 to transbronchial biopsies from four asymptomatic lung transplant recipients done as part of routine surveillance during the first year after lung transplantation. Since there was no evidence of infection or rejection and normal appearing lung architecture in the lung transplant biopsies, these samples were used as healthy lung tissue controls. A thoracic pathologist (AS) reviewed all transbronchial biopsies and explant lung tissues.