Targeted Treatment Of Acute Chest Syndrome In Transgenic Sickle Mice

Acute chest syndrome (ACS) is a major cause of morbidity and mortality in sickle cell disease (SCD). The diagnosis, prevention and treatment of ACS pose major clinical concerns in SCD partly because the mechanism underlying the pathogenesis of this syndrome remains elusive. Our group first reported that excess intravascular hemin causes a lethal acute lung injury (ALI) in transgenic SCD mice reminiscent of ACS (Ghosh and Ofori-Acquah, Blood 116 Suppl 1:944, 2010). Subsequently, large-scale genomics studies by Bean et al., (Blood 120:3822-8, 2012) and Galarneau et al., (Blood, 122:434-42, 2013) have implicated hemin catabolism and inflammation in the pathogenesis of ACS. In addition, we have reported recently that raised plasma free hemin increases the odds of ACS in children with SCD (Adisa et al., Br J Haematol. 2013). Collectively, these studies support a new theme of ACS pathogenesis involving extracellular hemin. In the current study, we validated the respiratory dysfunction of this ACS model, tested the hypothesis that toll-like receptor 4 (TLR4) mediates the associated lung injury, and examined the efficacy of two strategies to treat the condition in mice. Arterial blood gas analysis of SS mice with the ACS-like disease confirmed severe hypoxemia (PaO₂; 40.23 ±3.85 mmHg, SO₂; 58.72±6.6%, p<0.001), and revealed worsening hypercapnia (PaCO₂; 56.08±3.64 mmHg) and acidosis (mean pH; 7.21), which are all typical of severe ALI. Blood gas parameters remained normal in control AA mice exposed to the same concentration of extracellular hemin. Next, we generated two types of sickle bone marrow chimeric mice that lack expression of TLR4 in non-hematopoietic cells (SS^NHTLR4-/-) or express TLR4 on all cell types (SS^TLR4+/+). Induction of extracellular hemin crisis with 70 micromoles/kg of hemin resulted in rapid oxygen desaturation, reduced breath rate, pulmonary infiltration, high lung wet/dry weight ratio and sudden death in the SS^TLR4+/+ chimeras (n=6). Although all the SS^NHTLR4-/- mice acquired a similar phenotype of hemin crisis as the SS^TLR4+/+ mice, defined by acute intravascular hemolysis (∼1g/dl drop in Hb, ∼1g/dl increase in cell-free Hb), exhaustion of plasma hemopexin, ∼3-fold increase in plasma free hemin, they did not develop any respiratory symptoms (n=7). Next, we tested two potential therapies, recombinant human hemopexin and TAK-242 a small molecule inhibitor of TLR4. Both agents protected SS mice from developing ACS when they were administered immediately after the induction of hemin crisis. When SS mice were treated after developing respiratory distress evident by significant reductions in oxygen saturation and breath rate, only the recombinant human hemopexin was effective in preventing respiratory failure. In conclusion, we show that TLR4 expressed by the vessel wall, most likely the endothelium, mediates the inflammatory response to excess free hemin associated with severe ACS in mice. Pharmacological inhibition of TLR4 signaling and hemin sequestration may be effective in preventing the development of ACS. In addition, hemin sequestration offers a targeted approach that may halt the progression of ACS in those with more advanced disease.