Project Summary/Abstract Allogeneic hematopoietic cell transplantation (HCT) was developed in the 1950’s as a treatment option for life- threatening malignant and non-malignant diseases. The process involves high dose chemoradiotherapy followed by intravenous infusion of a donor allograft with the intention of correcting hematopoietic defects or achieving a graft-versus-malignancy effect. While overall survival has improved, HCT can lead to severe pulmonary injury due to infection, alloreactivity, or direct chemotherapy toxicity to the lung. Excessive or chronic injury can lead to pulmonary fibrosis (PF), which is associated with morbidity and premature mortality. The biology of PF involves inflammation, TGFβ signaling, and epithelial-mesenchymal transition, but additional mechanisms may contribute in the setting of HCT. An expanding array of molecules are being developed to target PF, thus opening the door to clinical testing in the pediatric HCT population. However, strategies to select ideal candidates for specific therapies are currently lacking, precluding children from advances in the field. Therefore, the goal of this R03 proposal is to enhance our ability to identify and treat pediatric HCT patients with or at-risk for PF. To do this, we will leverage bronchoalveolar lavage (BAL) fluid and paired blood samples obtained from pediatric HCT patients. In Specific Aim 1, we will determine which patients have PF at the time of BAL based on biopsy, pulmonary function tests, and chest imaging. We will then identify BAL transcriptomic and proteomic signatures that differ among patients with vs without PF while accounting for important clinical differences. We will repeat these comparisons using paired blood samples to see if BAL signatures can be identified using a more readily testable sample type. In Specific Aim 2, we will follow patients without PF at the time of BAL for 12 months. We will determine which patients go on to develop PF. We will then compare transcriptomic and proteomic signatures from the original BAL and blood samples to identify predictors of the future development of PF. Through these aims, we will gain insight into PF pathobiology in this high risk population. These data will then facilitate early identification of high-risk patients who may benefit from the rapidly growing pipeline of novel anti-fibrotic therapies. Overall, this work may increase the safety and accessibility of allogeneic HCT as a treatment for numerous life-threatening pediatric diseases.