Spleen Tyrosine Kinase Inhibition Mitigates Hemin-Induced Thromboinflammation in an Organ-Specific Manner in Sickle Cell Mice

Sickle cell disease (SCD) is a challenging genetic disorder characterized by hemolytic anemia, vaso-occlusive crises (VOC), and progressive organ damage. Despite its severity, effective treatments are limited. The recent withdrawal of promising therapies, such as the anti-P-selectin antibody Crizanlizumab and the hemoglobin polymerization inhibitor Voxelotor, highlights the urgent need for innovative approaches to alleviate VOC and thromboinflammation. In this study, we used advanced techniques, including intravital microscopy, laser speckle contrast imaging, and histological analysis, to examine the role of platelets and neutrophils in thrombo-inflammation, VOC, and blood perfusion in the lung, kidney, liver, and spleen. In the Berkeley SCD model, hemin-induced VOC and impairment in pulmonary blood perfusion were independent of red cell congestion and fibrin deposition. Hypoperfusion was driven by adhesion of neutrophils and platelets in the microcirculation and exacerbated by pulmonary emboli. Hemin-induced cell adhesion and hypoperfusion were also observed in the renal microcirculation, whereas it was absent in the liver and spleen of SCD mice, suggesting that organ-specific mechanisms drive hypoperfusion and VOC. To explore therapeutic options, we investigated the potential of Syk (spleen tyrosine kinase) inhibition in improving blood perfusion and reducing thrombo-inflammation. Syk is constitutively phosphorylated in the neutrophils and platelets of SCD mice. Selective Syk inhibition, using BI-1002494, reduced cellular adhesion in the pulmonary and renal microvasculature, effectively restoring blood perfusion and reducing thrombo-inflammation. Low-dose Syk inhibitor was effective in reducing neutrophil adhesion and improving blood perfusion without inducing bleeding. On the contrary, increasing the dose exacerbated hemin-induced bleeding in the lungs due to the complete abolishment of platelet adhesion in the pulmonary microcirculation. These findings underscore the critical role of Syk in vascular thrombo-inflammation and hypoperfusion in SCD, suggesting that Syk inhibition is a promising strategy to reduce VOC, improve renal and pulmonary perfusion, and reduce organ damage.