Background: In patients with non-resolvable (NR) COVID-19, lung injury can progress rapidly to the point that lung transplantation is the only viable option for survival. This fatal progression of lung injury involves a rapid fibroproliferative response and takes on average 15 weeks from initial symptom presentation.

Aims: To identify the cellular mechanisms that lead to fulminant lung fibrosis (FLF) in NR-COVID-19.

Methods: Using a pre-designed unbiased PCR array for fibrotic markers, we analyzed the fibrotic signature in a subset of NR-COVID-19 lungs. We compared the expression profile against control lungs (donor lungs discarded for transplantation), and explanted tissue from patients with idiopathic pulmonary fibrosis (IPF). Subsequently, using RT-qPCR, western blots and immunohistochemistry were performed to validate and localize selected pro-fibrotic targets. A total of 23 NR-COVID-19 lungs were used for RT-qPCR validation.

Results: We revealed a unique fibrotic gene signature in NR-COVID-19 that is dominated by a hyper-expression of pro-fibrotic genes, including collagens and periostin. Our results also show a dramatically increased expression of Collagen Triple Helix Repeat Containing 1(CTHRC1) which co-localized in areas rich in alpha smooth muscle expression, denoting myofibroblasts. We also show a dramatic increase in cytokeratin (KRT) 5 and 8 expressing cells adjacent to fibroblastic areas and in areas of apparent epithelial bronchiolization.

Conclusions: Fulminant lung fibrosis in NR-COVID-19 involves an aggressive deposition of extracellular matrix and the presence of CTHRC1 expressing fibroblasts and transitional KRT8 positive cells.