Acute Chest Syndrome In Transgenic Mouse Models of Sickle Cell Disease Triggered by Free Heme

Abstract 944

Acute chest syndrome (ACS) is the leading cause of death among patients with sickle cell disease (SCD). It is a process of devastating acute lung injury that evolves from multiple exacerbating events including vaso-occlusive pain crises, infection and fat emboli. ACS results in pulmonary infiltration, hypoxemia, and occlusions in the pulmonary microcirculation. Hitherto, experimental models of ACS have been lacking, and molecular targets of therapy remain to be identified. Clinical studies indicate that most patients diagnosed with ACS hemolyse during the acute phase of the syndrome, which highlights a role for circulating heme/hemin in this process. Since the deleterious effects of hemin are defined by increased vascular permeability, we tested the hypothesis that acute elevation of circulating hemin would increase pulmonary microvascular leakage sufficiently to trigger ACS. Adult transgenic mice expressing exclusively human sickle hemoglobin (Hb SS), and control Hb AS and Hb AA mice were intravenously injected with hemin (70 micromoles/kg body weight), and cardiopulmonary function assessed in real-time using a mouse pulse oximeter. Arterial oxygen saturation (SpO₂) in the SS mice reduced significantly (p = 0.02) to 84.1 ± 5.6 % from a normal baseline value of 98.6 ± 0.3 %, within 25 minutes of administration of i.v. hemin, while SpO₂ in control AS and AA mice remained unchanged. Consistent with changes in cardiopulmonary function, all the SS mice (n=14) succumbed to hemin, within 2 hours, while all control AS and AA mice survived, and remained alive several weeks after the experiment (log-rank survival test, p= <0.0001). We obtained identical results for survival in experiments using the Berkeley mouse model of SCD (Sickle 0/6, hemizygote 5/6, p=0.003). Post-mortem findings of gross pulmonary infiltration, alveolar flooding and microvascular occlusions, in the lungs of SS and Berkeley sickle mice that succumbed to hemin was consistent with respiratory distress associated sudden death. Younger SS mice aged 5–6 weeks were more resistant to i.v. hemin, with a survival rate of 80% (12/15), recapitulating the age-dependant mortality in human ACS. As expected, i.v. hemin raised the total plasma heme concentration to the same level in all mice, regardless of genotype. However, the concentration of protein-free plasma heme (PFPH) was increased by 6-fold in SS compared to AS mice (p = 0.001, n=12). The inability of SS mice to effectively scavenge excess free heme was likely because of very low plasma concentrations of hemopexin (SS: 0.17 ± 0.06 mg/ml, AS: 0.71 ± 0.14 mg/ml, p=0.002, n=8). We found a 10-fold higher concentration of heme oxygenase-1 (HO-1) in the plasma of SS mice, compared to AS mice (p=0.006, n=12), however, this enhanced capacity to degrade circulating heme, failed to protect the SS mice. This study demonstrates that acute elevation of plasma hemin triggers ACS in SCD mice. Infusion of hemopexin may prevent ACS during episodes of hemolytic crises in SCD.