PROJECT SUMMARY/ABSTRACT Acute respiratory distress syndrome (ARDS) is an important cause of morbidity and mortality resulting in over 74,000 intensive care unit deaths per year in the United States. Patients often present with rapidly progressive respiratory failure requiring prolonged mechanical ventilation which is costly and associated with long term disability. A more refined understanding of the pathophysiologic mechanisms of ARDS may guide more personalized approaches for prognostication and therapy. The immune “checkpoint” pathway includes the receptor Programmed cell death protein 1 (PD-1) and its ligand Programmed death-ligand 1 (PD-L1). The binding of PD-L1 to PD-1 leads to negative regulation of T cell receptor signaling. PD-1 and PD-L1 have been associated with defects in immune function in patients with sepsis which is a major risk factor for ARDS. For example, high PD-1 expression on circulating T cells in patients with sepsis is associated with decreased T cell interferon-gamma production. However, other studies suggest a role for this pathway in limiting tissue inflammation. Preliminary data in this proposal shows that patients with ARDS who have prolonged mechanical ventilation have lower expression of PD-L1 on alveolar macrophages. Further, a serious side effect of pharmaceutical inhibitors of this pathway termed “checkpoint inhibitors” is immune mediated tissue injury such as an ARDS-like pneumonitis. Thus the role of this pathway in modulating immune responses may be critical to our understanding of ARDS. The hypothesis of this study is that immune checkpoint proteins are protective against poor outcomes in ARDS by limiting tissue injury. This study will also investigate potential mechanisms of this association through promotion of regulatory T cell responses or limiting inflammatory T cell responses. Study subjects will be enrolled into a discovery cohort where measurements will be performed using novel high dimensional mass cytometry to identify expression of these proteins on a single cell basis. A validation cohort will be recruited externally to confirm these findings. Aim 1 will identify whether cell surface immune checkpoint protein expression is associated with severity of ARDS, Aim 2 will focus on mechanisms behind the association in modulating T cell responses, and Aim 3 will identify potential soluble measures of this pathway. This study aims to identify a biologically relevant immune signature of patients at risk for prolonged mechanical ventilation and death from ARDS.

PROJECT NARRATIVE Acute respiratory distress syndrome (ARDS) is a feared complication of critical illness that can frequently lead to prolonged mechanical ventilation and death. ARDS is a heterogeneous syndrome and treatment for patients is largely supportive. This project will determine whether the immune checkpoint proteins PD-1 and PD-L1 play a role in ARDS pathophysiology and can identify a biologic signature of patients at higher risk for poor outcomes.