Project Summary A major unmet goal in pulmonary medicine is to distinguish pneumonia sub-phenotypes responsive to host- directed therapies. We observe that some humans who died with pneumonia have abundant fibrin in their airspaces while others show little or none, correlating with neutrophils and necrosis. Similarly, some mouse models of pneumonia result in lungs with abundant airspace fibrin, while others do not; airspace fibrin associates with both neutrophils and necrosis in these models as well. While fibrin in the airspaces is a recognized pathological feature of pneumonia, this heterogeneity is not. Because fibrin can be pro- inflammatory and neutrophils can enhance fibrin accumulation, we propose the central hypothesis that fibrin and neutrophils in the airspaces form a positive feedback loop that causes damage in a subset of pneumonias. We will test this by pursuing three independent but mutually informative specific aims. In Aim 1, we will determine whether fibrin in the airspaces drives neutrophilic pulmonary inflammation and adverse outcomes, during high-fibrin pneumonia. Using genetic and pharmacologic approaches to interrupt key endogenous drivers or inhibitors of fibrin accumulation (tissue factor, thrombin, or plasmin), we will test whether loss or gain of fibrin decreases or increases, respectively, neutrophils and adverse outcomes like necrosis and lung injury during the high-fibrin but not low-fibrin type of pneumonia in mice. In Aim 2, we will determine whether neutrophils amplify fibrin accumulation in the airspaces, during high-fibrin pneumonia. We will use gene- targeted mice to test whether CD11b and neutrophil elastase are each essential for airspace fibrin accumulation during the high-fibrin but not low-fibrin sub-phenotype of pneumonia. We will also test whether pharmacologically reducing fibrin or elastase activity when pneumonia is already underway can sever the proposed positive feedback loop and improve outcomes during the high-fibrin pneumonia in mice. In Aim 3, we will determine whether high-fibrin pneumonias involve distinctive lung transcriptomes that include a pro-fibrin signature in airspace cells. We will profile lung transcriptomes of humans who died with high-fibrin vs. low-fibrin pneumonia, to identify differentially expressed genes and pathways. In mouse models, we will compare lung transcriptomes over the time-course of lethal high- and low-fibrin pneumonias, to reveal dynamics of changes as well as consistencies or differences between the human subjects and mouse models. From mouse lungs, airspace cells (alveolar macrophages, epithelial cells, and neutrophils) will be sorted from high- and low-fibrin pneumonias to identify cell-specific transcriptome differences between these pneumonia sub-phenotypes. The proposed studies will help elucidate the mechanisms and significance of high fibrin accumulation in the airspaces of infected lungs. Results will be informative for whether and which subsets of pneumon...