Neither fibrin nor plasminogen activator inhibitor-1 deficiency protects lung function in a mouse model of acute lung injury

Abstract

Fibrin impairs surfactant function in vitro, and inhibition of fibrinolysis by plasminogen activator inhibitor (PAI-1) is thought to promote fibrin accumulation in acute lung injury (ALI). This has led to speculation that impaired PAI-1 and fibrin accumulation should protect lung function in ALI. We tested this hypothesis by investigating ALI severity in fibrinogen-deficient (Fgn−/−) and PAI-1-deficient (PAI-1−/−) mice. PAI-1−/−, C57BL/6, Fgn−/−, and Fgn+/− females were anesthetized and allowed to aspirate 4 μl/g of hydrochloric acid (pH 1.0) and then reanesthetized and connected to a ventilator 48 h later. Naive C57BL/6 and Fgn+/− females served as controls. Following deep inflation (DI), forced oscillations were delivered periodically over 8 min to measure changes in elastance (H) as a surrogate of lung derecruitment, at positive end-expiratory pressures (PEEP) of 6, 3, and 1 cmH2O. Increases in H following DI in acid-injured mice were greater than naive strain-matched controls. Increases in H were no different between injured PAI-1−/− and C57BL/6, or between injured Fgn−/− and +/− mice, at any PEEP. Pressure-volume curves were no different between injured groups. Total lung fibrin was lower in injured PAI-1−/− and Fgn−/− mice relative to injured C57BL/6 and Fgn+/− mice, respectively, but indices of permeability were no different between strains. Unexpectedly, neither fibrin nor PAI-1 deficiency protects lung mechanical function in mice with acid-induced ALI. We speculate that in vivo lung function may be more closely tied to permeability and alveolar protein in general, rather than being linked specifically to fibrin.

Acute lung injury (ALI) is a severe form of noncardiogenic pulmonary edema and hypoxemic respiratory failure stemming from numerous causes (60). Current treatment of ALI rests largely on supportive care with mechanical ventilation, and its prognosis remains poor with a mortality of 30–40% in the general population, and higher in the elderly (49). The pathology of ALI typically progresses through an initial exudative phase characterized by neutrophil infiltration, edema, and accumulation of hyaline membranes, the latter consisting primarily of necrotic debris and fibrin (58). In this regard, the coagulation pathway and its end product fibrin have excited particular interest, in part due to the ability of fibrin to inhibit surfactant function in vitro (52, 54), and the increasingly recognized interplay between coagulation and innate immunity (17, 63). Fibrin formation and clearance in the lung are governed by the relative quantity and activity of fibrinolysis promoters such as plasminogen activators and fibrinolysis inhibitors such as plasminogen activator inhibitor-1 (PAI-1) (28). The importance of PAI-1 in ALI pathogenesis is suggested by its upregulation in various ALI models (5, 7) and by the finding that PAI-1-deficient mice fail to accumulate alveolar fibrin and die less quickly in response to injury (8). The importance of PAI-1 in ALI is further underscored by the finding that elevated plasma and edema fluid levels of PAI-1 are associated with higher mortality in ALI patients (45, 61). In agreement with the widely speculated role for PAI-1 and fibrin in the impairment of lung function in ALI, we demonstrated that progressive derangement in lung mechanics over 48 h in mice with acid aspiration injury corresponds with an increase in air space PAI-1 and fibrin (5). Although implied by our findings, the direct roles of fibrin accumulation and its clearance, as governed by PAI-1, in the derangement of in vivo lung mechanical function has yet to be firmly established. We thus set out to ascertain the direct roles of PAI-1 and air space fibrin in the disruption of lung mechanical function in an acid-aspiration model of ALI among varying strains of mice with different capacities to generate fibrin or PAI-1. Among these different strains, and their respective controls, we examined the effects of acid aspiration on lung mechanical derangement, various markers of injury, and on the accumulation of fibrin and fibrin breakdown products within the lung.