Cell-Free Hemoglobin-mediated Increases in Vascular Permeability. A Novel Mechanism of Primary Graft Dysfunction and a New Therapeutic Target.

RATIONALE: Cell-free hemoglobin (CFH) is a potent oxidant associated with poor clinical outcomes in a variety of clinical settings. Recent studies suggest that acetaminophen (APAP), a specific hemoprotein reductant, can abrogate CFH-mediated oxidative injury and organ dysfunction. Preoperative plasma CFH levels are independently associated with primary graft dysfunction (PGD) after lung transplant ( 1 ).

OBJECTIVES: Our objectives were to determine whether CFH would increase lung vascular permeability in the isolated perfused human lung and whether APAP would limit these effects.

METHODS: Human lungs declined for transplant were inflated and perfused with Dulbecco's modified Eagle medium/5% albumin at a pulmonary artery pressure of 8-12 mm Hg. After steady state was achieved, CFH (100 mg/dl) was added to the perfusate ± APAP (15 μg/ml). Lung permeability was measured by continuous monitoring of lung weight gain and by extravasation of Evans blue dye-labeled albumin from the vasculature into bronchoalveolar lavage. To test the mechanism of increased permeability, human pulmonary microvascular endothelial cells were exposed to CFH (0.5 mg/ml) ± APAP (160 μM) for 24 hours and permeability was assessed by electrical cell-substrate impedance sensing.

MEASUREMENT AND MAIN RESULTS: In the isolated perfused human lung, CFH increased lung permeability over 2 hours compared with control lungs (12% vs. 2% weight gain from baseline, P = 0.03). Increased vascular permeability was confirmed by a 4.8-fold increase in Evans blue dye-labeled albumin in the airspace compared with control lungs. Pretreatment with APAP prevented lung weight gain (P = 0.06 vs. CFH). In human pulmonary microvascular endothelial cells, CFH increased monolayer permeability (P = 0.03 vs. control), and this was attenuated by APAP (P = 0.045 vs. CFH).

CONCLUSIONS: Circulating CFH increases vascular permeability in the isolated perfused human lung and paracellular permeability in lung microvascular endothelial cells. These effects may explain the association of plasma CFH levels with PGD. The hemoprotein reductant APAP attenuates the effects of CFH and merits further exploration as a potential therapy for PGD prevention.