Severe viral infections in the lungs can result in acute respiratory distress syndrome (ARDS), a potentially life-threatening inflammatory condition for which there is no effective therapy. Paul Thomas and colleagues have now characterized the distinct activation states that occur in lung fibroblasts during severe influenza virus infection. They report in Nature that excessive fibroblast activation characterized by upregulation of the protease ADAMTS4 is associated with immunopathology, reduced lung function and higher mortality in both mice and humans during influenza virus infection.

During respiratory viral infections, much of the damage in the lungs is caused by infiltrating immune cells that kill both infected and bystander cells. As remodelling of the extracellular matrix (ECM) facilitates immune cell entry, the authors set out to define the roles of stromal cell populations during an acute respiratory infection. They performed single-cell gene-expression profiling (scGEX) on CD45– stromal cells isolated from the lungs of mice infected with influenza A virus and found that fibroblasts showed particularly dynamic patterns of gene expression. Gene-set enrichment analysis identified three main functional fibroblast subsets: resting fibroblasts, ECM-synthesizing fibroblasts (ESFibs) and inflammatory fibroblasts.

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ESFibs expressed genes encoding ECM proteins but not inflammatory genes. The inflammatory fibroblasts showed high expression of genes associated with type I interferon, IL-6 or NF-κB signalling and could be further subdivided into two subsets, namely damage-responsive fibroblasts (DRFibs) and interferon-responsive fibroblasts (IRFibs). DRFibs were enriched for genes involved in tissue damage-response pathways and showed upregulation of Itga5 and downregulation of Cd9, whereas IRFibs were enriched in genes involved in type I interferon response pathways and showed upregulation of Bst2 and Cd9. Similar functional fibroblast subsets were also identified in human lung biopsy samples and, of note, the authors found ITGA5+CD9lo fibroblasts with a damage-responsive profile in patients who had died of respiratory failure but not in healthy donors. In vitro culture experiments suggested that IL-1α, IL-1β and TNF are important for fibroblast activation during influenza virus infection.

The authors next examined how proteases produced by activated fibroblasts affect the response to influenza virus infection. They found that ADAMTS4 (which degrades the ECM proteoglycan versican) is one of the earliest proteases to be induced in the lungs of infected mice, is expressed throughout the course of infection and is almost exclusively expressed by fibroblasts. A meta-analysis of publically available scGEX human datasets indicated that DRFib-like cells show highest levels of ADAMTS4 expression and that ADAMTS4 is markedly upregulated in inflammatory lung diseases compared with in healthy lungs.

Compared with controls, Adamts4–/– mice showed improved survival following a lethal dose of influenza virus, and this was associated with reduced inflammation and improved lung function. The lungs of infected Adamts4–/– mice also showed higher levels of intact versican, and a series of additional experiments suggested that degradation of versican by ADAMTS4-expressing DRFibs permits the recruitment of CD8+ T cells, which are one of the main drivers of immunopathology during influenza virus infection.

Finally, the authors measured the levels of ADAMTS4 in respiratory tract samples from cohorts of paediatric or adult patients with moderate or severe influenza virus infection. They found that, across all age groups, levels of ADAMTS4 in the lower respiratory tract strongly correlated with increased risk for respiratory failure and death following severe influenza virus infection.

targeting ADAMTS4 or other ECM proteases could improve clinical outcomes in patients who have developed ARDS in response to influenza viruses, SARS-CoV-2 or other respiratory infections

These findings indicate a crucial role for damage-responsive fibroblasts in regulating the magnitude of the immune response and the propensity to develop lung failure in response to severe respiratory infections. The authors propose that targeting ADAMTS4 or other ECM proteases could improve clinical outcomes in patients who have developed ARDS in response to influenza viruses, SARS-CoV-2 or other respiratory infections.