Apohemoglobin-Haptoglobin Complex Improves Cardiac and Pulmonary Function in Homozygous Townes (HbSS Townes) Mice

The constant ischemia-reperfusion injury and the release of hemoglobin and heme in patients with sickle cell disease (SCD) can affect the heart, lungs, and vasculature. While treatments have improved survival, cardiovascular complications are becoming more prevalent, including pulmonary hypertension and left ventricular diastolic dysfunction. Pulmonary hypertension is associated with increased mortality risk and linked to intravascular hemolysis, leg ulcers, renal insufficiency, iron overload, and liver dysfunction. Chronic anemia leads to heart chamber dilation and left ventricular mass increase, often accompanied by left ventricular diastolic dysfunction, both impacting exercise capacity. Additionally, it can cause Acute Chest Syndrome (ACS), which is characterized by inflammation and blockage of small blood vessels in the lungs. In some cases, the heart may enlarge (cardiomegaly) due to the increased workload it faces in pumping blood through narrowed or obstructed blood vessels. Sudden death is also emerging as a concern, necessitating further cardiac investigations and risk assessment for effective management. These issues are speculated to result from the high amount of hemoglobin and heme released into circulation.

In this study, a synthetic protein scavenger for heme and Hb, the apohemoglobin (apoHb) was stabilized with haptoglobin (Hp) to create the apoHb-Hp complex, was used to study the cardiac and pulmonary changes in SCD mice resulting from heme and Hb. Homozygous Townes (HbSS Townes) mice (B6; 129-Hbatm1(HBA)Tow Hbbtm2(HBG1,HBB)Tow/Hbbtm3(HBG1,HBB)Tow/J) were used as the murine model of SCD, and HbAA Townes mice (homozygous for Hbatm1(HBA)Tow and Hbbtm3(HBG1,HBB)Tow) were used as the phenotypically controls. The results conclusively illustrate the benefits of using apohb-Hb, as it improves cardiac output, decreases pulmonary hypertension, and alleviates the immune response that is expected.

In conclusion, our study provides strong evidence supporting the significant cardiovascular implication of heme and hemoglobin. We have demonstrated that apoHb-Hp complex effectively neutralizes these harmful activities, rendering these heme and free Hb nonreactive, leading to a marked reduction in hemolysis induced endothelial activation. These findings have important implications, suggesting that apoHb-Hp complex plays a protective role for cardiac and pulmonary cells exposed to elevated levels of heme and free Hb, a condition commonly seen in SCD. Furthermore, our study's consistency with prior research reinforces the role of apoHb-Hp complex in preventing P-selectin expression. Our SCD mouse model experiments reveal that apoHb-Hp complex effectively ameliorates vaso-occlusion, regardless of whether the vaso-occlusion was triggered by Hb or heme-independent factors like hypoxia-reoxygenation. Notably, the apoHb-Hp complex also demonstrated some efficacy, indicating that the protective effect extends beyond simple heme clearance and TLR4 signaling inhibition. While our results are promising, further experiments are required to gain a deeper understanding of the apoHb-Hp complex activity in both clearance-dependent and independent mechanisms of resolving vaso-occlusion. Overall, our research highlights the potential therapeutic value of the apoHb-Hp complex in combating the detrimental effects of Hb and heme, and offers valuable insights into its protective role in managing vaso-occlusive events associated with conditions like SCD.