BACKGROUND Thromboembolic events within the pulmonary arterial vasculature are a troublesome complication of severe blunt thoracic trauma. Mechanisms underlying these events are currently in question as pulmonary thromboembolic events in this particular trauma population tend to be diagnosed more rapidly, more frequently and without an associated systemic thrombosis. This study investigates the role of P-selectin in thrombus formation through the use of in vivo blocking antibodies. We hypothesize that P-selectin plays a pivotal role in de novo pulmonary arterial thrombosis following blunt thoracic trauma. METHODS A murine weight-drop model of lateral blunt thoracic trauma was used. Wild-type mice in the experimental group were given blocking antibodies against P-selectin prior to the trauma. All mice were euthanized at 24 hours for evaluation with hematoxylin-eosin staining or immunofluorescent staining for fibrin and P-selectin. RESULTS Injured mice that did not receive the P-selectin antibody showed a robust fourfold to fivefold increase in fibrin accumulation in both coup and contrecoup tissues (fluorescence per um of arterial wall) compared to uninjured sham mice. In contrast, mice pretreated with P-selectin blocking antibody showed no significant increase in fibrin accumulation on either side of the lungs after blunt thoracic trauma. No difference in mean fibrin deposition was found between sham controls that received the P-selectin–blocking antibody and those that received an isotype control antibody. CONCLUSION P-selectin expression increases at the pulmonary arterial luminal surface following blunt thoracic trauma. In addition, P-selectin–blocking in vivo prevents pulmonary arterial fibrin accumulation after blunt thoracic trauma, confirming that P-selectin is necessary for de novo pulmonary arterial thrombosis after traumatic injury.

Recent studies show a substantial incidence of Pneumocystis jirovecii colonization and infection in patients with chronic inflammatory lung conditions. However, little is known about the impact of Pneumocystis upon the regulation of pulmonary immunity. We demonstrate here that Pneumocystis polarizes macrophages towards an alternatively activated macrophage-like phenotype. Genetically engineered mice that lack the ability to signal through IL-4 and IL-13 were used to show that Pneumocystis alternative macrophage activation is dependent upon signaling through these cytokines. To determine whether Pneumocystis-induced macrophage polarization would impact subsequent immune responses, we infected mice with Pneumocystis and then challenged them with Pseudomonas aeruginosa 14 days later. In co-infected animals, a higher proportion of macrophages in the alveolar and interstitial spaces expressed both classical and alternatively activated markers and produced the regulatory cytokines TGFβ and IL-10, as well as higher arginase levels than in mice infected with P. aeruginosa alone. Our results suggest that Pneumocystis reprograms the overall macrophage repertoire in the lung to that of a more alternatively-activated setpoint, thereby altering subsequent immune responses. These data may help to explain the association between Pneumocystis infection and decline in pulmonary function.

Pulmonary thromboembolic events cause significant morbidity and mortality after severe trauma. Clinically, these lesions are believed to be emboli arising secondary to deep venous thrombosis (DVT) in the lower extremities. Recently, this notion has been challenged by clinical studies, showing that pulmonary clots arise after trauma in the absence of DVT. This suggests that pulmonary blood clots arise in situ via de novo thrombosis. In the present study, we characterize a murine weight-drop model of lateral blunt thoracic trauma. Our model demonstrates severe unilateral lung contusion injury with low (10%) mortality in the absence of extrapulmonary injury, after impact with a 50-g weight dropped from 45 cm height (657 J/m). At 24 h after injury, immunofluorescence and histological evidence revealed early pulmonary arterial thrombosis in the form of eccentric accumulation of fibrin and CD41 positive eosinophilic proteinaceous material, on both coup and contrecoup lung lobes of injured mice, indicating early thrombotic events both within and outside of the area of primary lung injury. Our model is ideal in that lateral impact enables greater impact energy to be applied to achieve significant lung contusion without significant mortality or extrapulmonary injury, and the model has additional translational value in creating thrombosis analogous to pulmonary embolism observed clinically after blunt thoracic trauma. To our knowledge, this is the first demonstration of de novo pulmonary thrombosis in a clinically translational model of blunt thoracic trauma, and supports challenges to current assumptions about the origin of pulmonary blood clots in the wake of severe traumatic injury.