For many patients with advanced lung disease, lung transplantation remains the only viable therapeutic option to extend life. Yet the 5-year survival for lung transplant patients is 54%, the worst among solid organ transplant recipients. Chronic lung allograft dysfunction (CLAD) is the leading cause of morbidity and late mortality after lung or heart-lung transplantation. Hence, identification of biomarkers and novel therapeutic targets is essential to prevent or treat CLAD and extend survival after lung transplantation. While multiple factors contribute to CLAD, early events after the lung transplant procedure play a crucial role in setting the stage for subsequent CLAD. These include surgical trauma and ischemia-reperfusion injury that activates circulating and resident immune cells followed by the endothelial injury and immune cell extravasation. We have reported that monocytes, recruited to the injured lung, can establish a long-term residency and differentiate into pathogenic monocyte-derived alveolar macrophages. We have causally linked monocyte-derived alveolar macrophages to tissue-remodeling and fibrosis (resembling CLAD) using a genetic deletion strategy. Moreover, using unbiased single-cell transcriptomic profiling (RNA-seq) of explanted lung tissue from the patients with pulmonary fibrosis and biopsies of the donor lung (both obtained during lung transplantation by lung transplant surgeon ? Ankit Bharat, key contributor to this proposal), we identified a distinct population of pathogenic alveolar macrophages exclusively present in patients with pulmonary fibrosis. Our computational analyses of single-cell RNA-seq data suggest that monocyte-derived alveolar macrophages are guided to their new pathogenic niches via plexin D1/signaling and are uniquely maintained by M-CSF/M- CSFR signaling. Consistent with this hypothesis, targeting M-CSF/M-CSFR signaling specifically eliminated monocyte-derived alveolar macrophages and ameliorated pathology. We present preliminary data from mouse models and patients with chronic lung allograft dysfunction supporting relevance of this mechanism for CLAD. We will thus use mouse models and samples from lung transplant patients to test the hypothesis that pathogenic monocyte-derived alveolar macrophages, recruited during the initial peri-transplant injury, establish long term residency via plexin D1/semaphorin signaling and are maintained via M-CSF/M- CSFR signaling to drive CLAD in three interrelated aims:
Aim 1 : To determine whether monocyte-derived alveolar macrophages recruited to the transplanted lung within day of the transplant are maintained by M-CSF/M-CSFR signaling.
Aim 2 : To determine whether pathogenic monocyte-derived alveolar macrophages are localized to regions of lung fibrosis lung via plexin D1/semaphorin signaling.
Aim 3 : To determine whether the emergence of aberrant alveolar macrophages with increased expression of PLXND1 and autocrine M-CSF/M-CSFR signaling can be identified in BAL fluid from patients with early CLAD.
Early events after the lung transplant procedure, such as surgical trauma and ischemia-reperfusion injury, play a crucial role in setting the stage for subsequent chronic lung allograft dysfunction. Monocytes, recruited during the lung injury, can differentiate into monocyte-derived alveolar macrophages and drive development of CLAD. In this application, using causal murine models and unique human material from patients with CLAD, we will dissect molecular signals responsible for maintenance of these pathogenic monocyte-derived alveolar macrophages in patients with CLAD.