Complement Drives Acute Chest Syndrome in a Model of Sickle Cell Disease

Background: Despite being the first genetic disease described, sickle cell disease (SCD) continues to afflict millions of individuals worldwide. Patients with SCD can experience severe complications, including vasoocclusive crisis (VOC), which can progress to acute chest syndrome (ACS), a form of acute lung injury. While VOC can cause significant morbidity, ACS is the leading cause of mortality in patients with SCD. Despite the devastating consequences of ACS, no interventions exist that directly treat this complication. As a result, when patients present with ACS, options often remain limited to supportive care. The lack of optimal treatment options for patients with SCD experiencing ACS directly contributes to their morbidity and mortality. Using a combined approach of prospective observational studies in patients with SCD that experienced ACS and a novel preclinical model specifically designed to define the role of complement in SCD pathophysiology, we tested the hypothesis that complement plays a central role in the pathophysiology of ACS.

Methods: Samples from patients with SCD experiencing ACS were compared to baseline measurements for the same patient obtained at outpatient follow up 4-6 weeks later. ELISA based assays were used to measure Ba, Bb, C3a, C5a, C5b-9 (membrane attack complex (MAC)) in each sample. SCD mice (HbSS) and control mice (HbAA) were injected with 7.5 units of cobra venom factor (CVF), a potent complement activator, followed by evaluation of hematocrit (Hct) measured by a Sysmex veterinary hematology analyzer, free heme by calorimetric assay, and complement deposition on red blood cells (RBCs) by flow cytometry or pulmonary endothelial cells by confocal analysis. Respiratory rate and O₂ saturation were measured using a MouseOx pulse oximeter. HbSS or HbAA mice were crossed with complement component 3 (C3) knock out (KO) mice to generate HbSS x C3 KO and HbAA x C3 KO mice. Hemin (70 to 210 mmol/kg concentration) or CVF (7.5 units/mouse) were injected into HbSS, HbAA, HbSS x C3 KO or HbAA x C3 KO followed by evaluation of hemolysis, C3 deposition and pulmonary function. A one-way ANOVA with a Tukey’s post hoc with a p value <0.05 was considered significant.

Results: During ACS in patients, significant elevations in Ba and Bb, complement activation products unique to the alternative pathway, and downstream activation of complement, including C3a, C5a and MAC, were observed (p<0.0001). Changes in complement activation were accompanied by a drop in hemoglobin during ACS when compared to baseline (p<0.001), suggesting that complement activation results in hemolysis. To formally test this, the impact of complement activation in a preclinical model of SCD was defined. Injection of CVF or hemin resulted in rapid C3 deposition on RBCs and pulmonary endothelium in HbSS, but not HbAA, recipients. In addition, CVF or hemin injection was accompanied by increased hemolysis and ACS manifested by increased respiratory rate, drop in oxygen saturation and death within 2 hours in the majority of recipients (p<0.001). In contrast, control Hb AA mice were unaffected by CVF or hemin injection. Given the role of free heme in the development of ACS, we next defined the role of complement in hemin-induced lung injury. To accomplish this, SCD mice were crossed with C3 KO mice to generate HbSS x C3 KO. HbSS x C3 KO mice displayed a mild increase in Hb values at baseline (p<0.01), suggesting a role of C3 even in steady-state SCD-related hemolysis. Equally important, injection of HbSS x C3 KO mice with CVF or hemin failed to result in the same levels of C3 deposition, hemolysis, compromised pulmonary function, or mortality (p<0.001) as HbSS recipients.

Discussion: These results demonstrate a critical role for C3 in the development of acute lung injury in SCD. The sensitivity of HbSS RBCs to complement-induced hemolysis and C3 deposition observed following hemin injection in SCD recipients suggests that C3 plays a key role in decreased hematocrits and pulmonary injury that accompanies ACS. As treatment of ACS largely relies on supportive care, these results suggest that approaches aimed at targeting complement activation may represent a useful prophylactic or treatment strategy for ACS. In doing so, these results hold promise in providing a more direct approach at reducing one of the most severe complications of patients with SCD.