Plasminogen Degradation by Neutrophil Elastase in Pleural Infection, Not High Plasminogen Activator Inhibitor-1 (PAI-1), is the Cause of Intrapleural Lytic Failure

Background Complex pleural space infections often require treatment with multiple doses of intrapleural tissue-plasminogen activator(tPA) and deoxyribonuclease(DNase), with treatment failure frequently necessitating surgery. Pleural infections are rich in neutrophils, and neutrophil elastase degrades plasminogen, the target substrate of tPA, that is required to generate fibrinolysis. We hypothesized that pleural fluid from patients with pleural space infection would have high elastase activity, evidence of inflammatory plasminogen degradation, and low fibrinolytic potential in response to tPA that could be rescued with plasminogen supplementation. Research Question Does neutrophil elastase degradation of plasminogen contribute to intrapleural fibrinolytic failure? Study Design and Methods We obtained infected pleural fluid and circulating plasma from hospitalized adults(n=10) with IRB approval from a randomized trial evaluating intrapleural fibrinolytics versus surgery for initial management of pleural space infections. Samples were collected pre-intervention, post-intervention day-1(PID1), PID2, and PID3. Activity assays, enzyme-linked immunosorbent assays, and western blot(WB) analysis were performed, and turbidometric measurements of fibrinolysis were performed on pleural fluid +/- exogenous plasminogen supplementation. Results are reported as median(Q1, Q3) or n(%) as appropriate, with alpha set at 0.05. Results Pleural fluid elastase activity was >4-fold higher(p=0.02) and plasminogen antigen levels >3-fold lower(p=0.04) than their corresponding plasma. Pleural fluid WB analysis demonstrated abundant plasminogen degradation fragments consistent with elastase degradation patterns. We found that plasminogen-activator inhibitor-1(PAI-1), the native tPA inhibitor, had high antigen levels pre-intervention but the overwhelming majority of this PAI-1(82%) was not active(p=0.003), and all PAI-1 activity was lost by PID2 in patients receiving intrapleural tPA/DNase. Finally, using turbidity clot lysis assays we found that 9 of 10 patients’ pleural fluid was unable to generate a significant fibrinolytic response when challenged with tPA and that plasminogen supplementation rescued fibrinolysis in all patients. Interpretation Inflammatory plasminogen deficiency, not high PAI-1 activity, is a significant contributor to intrapleural fibrinolytic failure.