Lung epithelium releases growth differentiation factor 15 in response to pathogen-mediated injury
Rationale: Growth/differentiation factor 15 (GDF15) is a stress cytokine with numerous proposed roles including stress erythropoiesis. Higher circulating GDF15 levels are prognostic of mortality during acute respiratory distress syndrome, but the sources and downstream effects of GDF15 during pathogen-mediated injury remain unclear. Methods: We quantified GDF15 in plasma and lower respiratory tract (LRT) biospecimens from critically-ill humans with acute respiratory failure. Publicly available data from SARS-CoV-2 infection were re-analyzed. We utilized mouse models of acute lung injury mediated by P. aeruginosa exoproducts in wildtype mice and in mice genetically deficient for Gdf15 and its putative receptor, Gfral.Results: Plasma levels of GDF15 associated with worse outcomes and LRT GDF15 levels in critically-ill humans. Intra-tracheal P. aeruginosa Type 2 secretion system exoproducts (PA SN) were sufficient to induce airspace and plasma release of GDF15 in wildtype mice, which was attenuated during epithelial-specific Gdf15 deficiency. SARS-CoV-2 infection induced GDF15 expression in human lung epithelium. Mice with global Gdf15 deficiency had decreased airspace hemorrhage, an attenuated cytokine profile, and altered lung transcriptional profile during PA SN injury, which was not recapitulated in mice deficient for Gfral.  Airspace GDF15 reconstitution did not significantly modulate key lung cytokine levels but did increase circulating erythrocyte counts.Conclusions: Lung epithelium releases GDF15 during pathogen injury, which associates with plasma levels in both humans and mice and can increase erythrocyte counts in mice. Potential local and other extra-pulmonary roles for GDF15 remain undefined. Overall design: Wildtype or Gdf15-/- mice were intra-tracheally inoculated with the supernatant of P. aeruginosa (PA SN), or vehicle, under direct visualization after isoflurane anesthesia. At the time of necropsy approximately 20 hours post-inoculation, mice received a terminal dose of ketamine/xylazine and the left lung was clamped, removed, and snap-frozen in liquid nitrogen prior to storage at -80°C. Snap frozen lungs were homogenized in Trizol (Life Sciences) and RNA was isolated per manufacturer instructions. RNA cleanup was performed using Qiagen RNeasy Mini Kit per manufacturer instructions. Quality control and mRNA library preparation of isolated lung RNA were performed by Novogene (Sacramento, CA) followed by paired-end whole genome sequencing using an Illumina NovaSeq platform.