Pulmonary fibroblast activation during infection enhances lung defense via immunomodulation and tissue remodeling.

is the etiologic agent of invasive aspergillosis, a life- threatening fungal pneumonia that is initiated by the inhalation of conidia (spores) into the lung. If the conidia are not cleared, they secrete large quantities of hydrolytic enzymes and toxins as they grow, resulting in extensive damage to pulmonary tissue. Stromal fibroblasts are central responders to tissue damage in many organs, but their functional response to pulmonary injury caused by has not been explored. In this study, we employed cell lineage tracing, targeted cell ablation, and single-cell RNA sequencing to monitor the dynamics of fibroblast behavior upon exposure to in both immunocompetent and immunosuppressed hosts. The results demonstrate that a subset of pulmonary fibroblasts becomes activated in an immunocompetent host in response to a challenge with conidia, acquiring a gene expression program reflecting the acquisition of new immunomodulatory properties as well as enhanced extracellular matrix (ECM)-secreting ability. Remarkably, through targeted ablation of fibroblasts that express the profibrotic activation marker periostin, we demonstrate that the progression of an invasive infection in an immunosuppressed host is accelerated by the absence of periostin lineage cells and is accompanied by severe alveolar hemorrhage and angioinvasion. These findings uncover a novel protective role for fibroblasts in limiting the severity of -induced pulmonary injury and emphasize the importance of the pulmonary stroma in host defense against this invasive fungal infection.