Identification of epithelial alarmins that promote activation of primary human lung fibroblasts

Abstract

Background Activation of the innate immune system plays a key role in exacerbations of chronic lung disease, yet the role of fibroblasts in innate immune responses and the identity of danger signals (alarmins) that may contribute to their activation are still to be unraveled. The objectives of this study were to identify epithelial alarmins released during environmental insults which induce innate immune responses in lung fibroblasts and dissect the mechanisms responsible for this. Methods Primary human lung fibroblasts (PHLF) were treated with conditioned media from primary bronchial epithelial cells (PBECs) exposed to oxidant injury or endoplasmic reticulum stress (ER stress) and fibroblast responses assessed. The relevance of alarmins in-vivo was assessed clinically by measurement of relevant alarmins longitudinally in patients developing Bronchiolitis Obliterans Syndrome (BOS) after lung transplantation. Results Conditioned media from PBEC cells subjected to oxidant injury and ER stress contained elevated levels of alarmins. Treatment of PHLFs with conditioned media from damaged cells significantly upregulated IL-6, IL-8, MCP-1, GM-CSF, IL-1α and IL-1β expression (p<0.05). This effect was reduced with anti-IL-1α or IL-1Ra but not anti-IL-1β antibody. Co-stimulation with Poly I:C significantly accentuated the IL-1α induced inflammatory phenotype in PHLFs. Clinically, IL-1α was increased in BAL of lung transplant recipients with infections and within 3 months of developing bronchiolitis obliterans syndrome (BOS) (p<0.001). Additionally, IL-1α levels positively correlated with elevated IL-8 (p<0.001) and neutrophil counts (p<0.001). Conclusions IL-1α plays a pivotal role in triggering proinflammatory responses in fibroblasts and this process is accentuated in the presence of viral pathogen associated molecular patterns. This novel pathway warrants further evaluation of its therapeutic potential to limit the repeated cycles of injury and exacerbation in chronic lung diseases.