Project Summary/Abstract Inflammation and immune responses are critical in the body’s response to inflammatory injuries such as infection or tissue damage. Leukocytes, cells of the immune system, are recruited to these sites of damage or insult via a process known as the inflammatory cascade. In the inflammatory cascade, the step where leukocytes migrate out of the blood and across the endothelium is known as transendothelial migration (TEM) and occurs via interactions between leukocyte and endothelial PECAM and CD99. While much is known about the inflammatory cascade and subsequent neutrophil function in the systemic circulation and tissue beds, the process remains more elusive in the lungs. Understanding how types of leukocytes like neutrophils traffic from their sites of synthesis through the vasculature, migrate through the endothelium into the lungs and perform various functions (both pro-inflammatory and resolving) is of extreme importance due to the role of leukocytes in inflammatory pathology. When in the tissue, neutrophils perform many functions as granulocytic cells, resolving infection and insult, and are critical in ameliorating disease. However, they can also be damaging mediators of disease in their own right, contributing to inflammatory damage and disruption of normal physiological function. Two major forms of acute inflammatory airspace diseases in the lungs, gastric aspiration pneumonia and bacterial bronchopneumonia, have high prevalence and mortality and currently have no preventative therapies. The symptoms of these diseases in part are due to the crowding of the alveolar space, impaired gas exchange, and damage by neutrophils, and as such, high neutrophil presence correlates with adverse outcomes. However, neutrophils can also produce molecules known as specially pro-resolving mediators (SPMs) that are anti-inflammatory. Previous work shows that recruitment and migration of leukocytes in the lungs differs from the systemic circulation and neutrophil function is highly important in disease progression. I aim to interrogate the mechanisms by which neutrophils undergo TEM in the lungs during acute airspace disease and the functions they perform both individually and in concert with macrophages in the tissue and vasculature. I hypothesize that transmigration can be used to better pathology in infectious bronchopneumonia (Aim 1) and that there are populations of neutrophils that produce SPMs and interact with macrophages during lung injury (Aim 2). This will provide key mechanistic insights on TEM and function of neutrophils in the lungs, while also providing potential for therapeutic intervention and characterization of neutrophil roles in acute inflammatory airspace diseases.